CA1069665A - Method of fabricating a gas transmitting body - Google Patents
Method of fabricating a gas transmitting bodyInfo
- Publication number
- CA1069665A CA1069665A CA317,524A CA317524A CA1069665A CA 1069665 A CA1069665 A CA 1069665A CA 317524 A CA317524 A CA 317524A CA 1069665 A CA1069665 A CA 1069665A
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- gas
- liquid
- slot
- bubbles
- capillary
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- Physical Or Chemical Processes And Apparatus (AREA)
Abstract
ABSTRACT OF THE DISCLOSURE
Method and apparatus for bubble shearing are disclosed in which nascent bubbles are formed by flowing gas through the very small capillary openings (no larger than about 100 microns in diameter) of a gas diffusing surface into a moving liquid that shears the nascent bubbles off as it moves past the capillary openings. In the apparatus, the gas diffusing surface forms one wall of a liquid transmitting slot through which the shearing liquid flows as it shears off fine gas bubbles. A gas transmitting body or gas "bar" for use in the apparatus of the invention is disclosed, as well as a method of fabricating such a gas bar.
Method and apparatus for bubble shearing are disclosed in which nascent bubbles are formed by flowing gas through the very small capillary openings (no larger than about 100 microns in diameter) of a gas diffusing surface into a moving liquid that shears the nascent bubbles off as it moves past the capillary openings. In the apparatus, the gas diffusing surface forms one wall of a liquid transmitting slot through which the shearing liquid flows as it shears off fine gas bubbles. A gas transmitting body or gas "bar" for use in the apparatus of the invention is disclosed, as well as a method of fabricating such a gas bar.
Description
~06g6~S
This invention relates to apparatus and method for diffusing fine gas bubbles into a body of liquid, and to a method for fabricating such diffusion apparatus.
~ ....
A
, ' ~06~5 This application is a divisional application of Canadian Patent Application Serial No. 219,753, filed on February 10, 1975.
BACKGROUND OF THE INVENTION
It is important in various processes to diffuse a gas into a body of liquid in a manner that will disperse the gas uniformly through a large portion of the liquid and result in as rapid as possible absorption of the gas by the liquid.
Diffusion of a gas into a body of liquid in this way is useful, `
for example, in a large number of chemical and petrochemical processes. It is also important in certain sewage treatment processes. It is becoming increasingly important in the treatment of natural bodies of water with air, oxygen, or ozone in order to revive polluted rivers, lakes, bays, etc.
When, for purposes of economy or for any other reason, gases used in the treatment of a body of liquid should not be allowed to escape from the surface of the liquid, it is im-portant that the method and apparatus used be such that all, ¦ or nearly all, the gas dissolves in the liquid before any sub-stantial number of gas bubbles can rise to the surface. In every case, it is usually desirable that the absorption of the ~'i gas into the liquid proceed as rapidly as possible.
Effect of Bubble Size and Uniformity of Size There are a number of factors that affect the rate at which a par~icular gas can be dissolved in a given body of liquid. Two of the most important of these factors are the size ~ of the bubbles and the degree of uniformity in bubble size, l~ which affect the rate of absorption of gas into the body of liquid as follows:
1. Smallest bubble size possible One of the principal aims in any diffusion process in which the objective is rapid absorption of the gas in the liquid is to produce bubbles in as small a size as possible, because:
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1069665 "
(a) The smaller the bubbles, the slower they rise through the liquid, and thus the longer the period of time they have in which to be dissolved.
It is known that at a bubble size of about 200 microns, there is a marked increase in the rise rate of the bubbles through a body of water. Thus, the size of the bubbles to be dissolved in water should be less than this figure. Improved results are obtained with water if the maximum bubble diameter is no more than about 100 microns, and best results if sub-stantially all the bubbles are no larger than about 50 micronsin diameter.
(b) The smal er the bubble, the larger is the sur-face that is available through which a given volume of gas can dissolve into the surrounding liquid. (To express the converse relationship: As bubbles grow larger,the surface of the bubble increases only as the square of the diameter~ while the volume of gas in the bubble increases as the cube of the diameter.) (c) Because in smaller sized bubbles the surface undulations which tend to draw adjacent bubbles together are more heavily damped by viscous forces, such smaller sized bubbles are less likely to collide with other bubbles to coa-lesce to form new, larger bubbles.
(d) For very small bubbles, the internal pressure of the bubble is markedly higher due to surface tension forces, and thus the driving force for transfer of the gas from the 1. . .
~ ga~eous to the liquid phase is higher.
,2. Highest possible degree of uniformit~ in bubble size The ideal situation would be to have all the gas ~; bubbles the same size when they are diffused into the body of liquid. When all bubbles are uniform in siæe, they will have a , . .
.
1069665 ~ ~
uniform terminal rise rate as they move upward through the body of liquid, which in turn means:
(a) Local turbulence due to the so-called "chimney effect" -- which would tend to disturb the uniformity of dis-tribution of the gas bubbles through the body of liquid --will be avoided.
(b) Since all the bubbles will be moving at a uni-form rate, none will overtake other bubbles and tend to couple or coalesce with ~hem-. If two bu~bles couple together with a kind of membrane between them, the overall surace through which a given volume of gas can dissolve into the surrounding liquid is reduced. If ~wo bubbles coalesce to-form a wholly new, larger bubble, the overall surface through which a given volume of gas can dissolve into the surrounding liquid is 3 likewise reduced. In both cases, the result is further com-~', pounded by the fact that the new, larger bubbles will have an increased rise rate, ana therefore will tend to overtake and join with still more bubbles.
. .
Bubble Shearing ~ ~ The ideal situation is to produce large numbers of very cmall bubbles, as nearly uniform in size as possible, at as high a~rate as possible and with the lowest possible ex-¦~ penditure ~of energy. The present invention involves a novel and greatLy improved "bubble shearing" method and apparatus ;ky~which this ideal situation is achieved.
It has long been known that method and apparatus em-ploying the phenomenon known as "bubble shearing" are well adapted to the~production of quite small gas bubbles in a ;; liquid.~ ~In~this method, a gas is passed through the capillary 30 ~ passages of~ a foraminous material to be introduced into a stre~m of liquid on the other side of the material. The gas 1' ~
1, ~ :, , !-, .~ :
`` ~ iO69~;6S
emerges at the boundary surface of the foraminous material ina number of locations spaced from each other, where it starts to form a plurality of fine bubbles, or in other words forms "nascent" gas bubbles. The liquid into which the gas is thus introduced is caused to move more or less rapidly past the capillary openings of the foraminous material. As a result, the viscous shear forces exerted by the moving body of liquid shear off the partially formed or nascent gas bubbles before they can grow to such a size that their natural buoyancy in the liquid will cause them to break the surface tension that tends to hold them on the boundary surfaces of the gas trans-mitting oraminous material.
The novel and striking results from the use of the method and apparatus of the present bubble shearing invention are accomplished in ways that were previously either never considered, or were believed to be impossible, by those skilled in the art.
Disadvantages of the Prior Art Probably the oldest method of introducing gas bubbles ~0 into a body of liquid is the method in which a gas transmitting body having a plurality of small openings is immersed in the liquid, and gas is caused to flow out of those openings into the liquid, to form bubbles that break away and rise in the liquid when they have grown to a large enough size that their buoyancy overcomes the surface tension that tends to hold them on the surface on which they fvrm. The rate of formation of bubbles with this method is relatively lowO Moreover, since each bubble must grow to such a size that its buoyancy is great enough to cause it to break away from the surface on ~ -' ! . .
which it is formed, these bubbles are of a larger size than ; is desirable for many applications. Examples of method and . ... .
~ -6-; - . , ~
.. , ', ', ' " , " ;''' ~ ,.. ' ... '. ,. . " ' ' ~ '' ~
apparatus that rely solely or primarily on the buoyancy of the bubbles to break them away from the bubble forming surface in-clude the pa~ents to ~een No. 2,295,740, Nordell No. 2,555,201, Fischerstrom et al. No. 2,708,571, and Schnur No. 2,719,032.
Another method of diffusing bubbles into a body of liquid includes the flow of liquid past a plurality of openings ~preferably oriented in the direction of downstream flow of the liquid) out of which gas is caused to flow by reason of the low pressure region in the wake of the moving liquid. The rate of formation of gas bubbles with this type of method and apparatus is only som~what higher, and the median bubble size only somewhat smaller, than with the method and apparatus that rely primarily on the buoyancy of the bubbles to break them off the surface on which they are formed. Examples are dis-closed in British patent No. 942,754 and U.S. patents No.
3,489,396 to D'~ragon and No. 3,671,022 to Laird et al.
A more or less conventional form of bubble shearing apparatus is disclosed in Polish patent No. 48,942. In that ap-paratus, a stream of liquid from an inlet pipe produces circula-tory flow that moves past a cylindrical surface whose upper part is perforated to emit gas, and the turbulent moving liquid shears off gas bubbles that form at the perforations. Both the presence of turbulence and the large size of the perforations in the gas diffusing surface (estimated to be at least about 5 mm. in di-ameter) mean that the bubbles produced with the apparatus of this Polish patent cannot approach the small sizè~of the bubbles produced by the method and apparatus of the present invention.
; Apparatus that utilizes the bubble shearing principle in a special way is disclosed in Smith et al. patent No.
3,450,800. In that patent, the freé stream flow of a moving body of liquid shears off nascent bubbles at the openings of capillary tubes~ and carries them downstream in the moving ~ ~7~
:. . -.
. - .. . . . . ~ . .
liquid. The discrete capillary openings are arranged in a free-standing array of capillary tubes, ~hich would obviously be sub-ject to a high level of brea~.age from the moving shearing liquid.
In addition, thousands of capillary tubes would be required to disperse fine bubbles in any significant quantity, which would so fill up the liquid transmitting passage as to make the appa-ratus impractical.
McManus patent No. 3,5a5,731 points out that it has been conventional to impart a flow of relatively low velocity to a liquid medium in order to shear off bubbles from a for-aminous gas transmitting surface on which the bubbles form, but McManus then rejects bubble shearing in favor of creating such a high li~uid velocity flow that a turbulent boundary layer is formed in a region that is located virtually at the surface of the wall t with only a very thin laminar sublayer intervening between that region and the wall. As McManus ex-plains, the pressure fluctuations in this turbulent region "snap off" the bubbles before they can be sheared off by viscous shear forces~ The specification of the patent states that the bubbles thus formed are "of miscropic size, on the order o 5-10 microns in diameter," but no experimental evi-dence to support the statement(such as photomicrographs)-iS
included in the patent. The major disadvantage of the McManus method and apparatus is that a liquid pressure head of 75 p.s.i.
is required to create sufficient liquid flow to ~erate the , apparatus; by contrast~ the liquid pressure head required to operate the apparatus of the present invention is typically'~
only about 2 to 7 p.s.i.
- Several other prior art patents are addressed to the `I 30 production of gas bubbles in moving streams of liquids under various conditions, but they do not give any hint of the desira-bility of establishing a partially developed laminar boundary 8 ~/
~ .
- r 1069665 layer in li~uid flowing over capillary openings small enough to produce fine gas bubbles, ~hich applicant has found to be a very important flow regime to establish. On the contrary, they actu-ally disclose structural features that will produce either vena contracta (British patent ~o. 1,039,702) or turbulence (patents No. 2,695,710 to Gibbs and No. 3,256,802 to Karr, and British patents Nos. 694,918, 713,064 and 885,406) when water moving at what a~plicant has found to be its minimum necessary velocity is used as the shearing liquid.
SUMMARY OF THIS INVENTION
This invention utilizes a gas diffusing surface having very small capillary openings, no larger than about 100 microns in diameter, preferably in close proximity to the leading edge of the surface. Shearing liquid i5 caused to flow past the lead-ing edge of the gas diffusing surface to a discharge edge, to produce substantially parallel laminar flow, including a partial-ly developed laminar boundary layer immediately adjacent the gas diffusing surface, with a free stream above that layer, over at least a substantial number of the capillary openings. In the method, the partially developed layer is present over at least about one-quarter of the capillary openings. The liquid over any additional capillary openings present in the gas diffusing surface downstream of the capillary openings already referred to must be substantially free of'any type of flow (such as vena contracta or turbulence) other than fully developed substantially ,:
parallel laminar flow.
The term "capillary openings" is used herein to mean any openings small enough to produce fine gas bubbles, no matter ~ what the nature of the gas diffusing surface is on which the bubbles are formed, or the nature of the gas transmitting pas-sages through which gas flows to reach ~hose openings.
~he liquid in the partially developed laminar flow ~ -~, \ .
_ g _ ./
,~ -, '' . . . .
iO~S6;65 just described, either with or without the action of the free stream above the partially developed laminar boundary layer, shears nascent gas bubbles rom the capillarv openings. The moving liquid carries the fine gas bubbles thus produced away from the gas diffusing surface at its discharge edge, and into -the body of liquid being treated. The partially developed lamin-ar boundary layer may be transformed into fully developed laminar flow before the shearing liquid leaves the gas diffusing sur-face, and there may or may not be additional capillary openings beneath that fully developed flow. Some advantage may be de-rived from use of this invention even if some turbulence appears in the moving shearing liquid before it leaves the gas diffus-ing surface, but in that case there must be no capillary open-ings beneath the region of turbulence.
, , The bubble shearing apparatus of this invention in-'~ cludes a slot for transmittlng shearing liquid past the capil-lary openings of the gas diffusing surface at which the nascent I gas bubbles are formed before they are sheared off by the mov-ing liquid. The shape, dimensions and spacing of the members defining the slot are adapted to establish laminar flow as de-ii scribed above when water is employed with the apparatus as the shearing liquid under cçrtain specified operating conditions. In addition, the apparatus includes means for providing a flow of li-~uid ln the slot at a velocity to produce such parallel laminar flo~J. It is best that the very small capillary ope~n~ings in the ~; ga9 diffusing surface be located in close proximity to the inlet end of the liquid transmitting slot, and preferably the farthe~t upstream of the capillary openings should lie immediately ad-ac~ent that inlet end, as close to the inlet as is structurally ~30 ~ practicable. ~ .
The bubble shearing apparatus of this invention may be defined alternatively in terms of a figure of merit that ! ' ' ' ' .
'- ~ 10696~5 is the product of (a) the distance separating the inlet end of the liquid transmitting slot from the very small capillary openings that lie farthest downstream on the gas diffusing sur-face over which the shearing liquid flows, times (b) the aver-age width of the slot throughout the indicated distance. The units for this figure of'merit are square inches. 5atisfactory results may be obtained in some applications when the magni-tude of this figure of merit is no more than about 0.1. Some-what improved results are generally obtained when the figure of merit is no more than about 0.075, and still further improvement is obtained when it is no more than about 0.05. Very good re-sults are obtained when the figure is no more than about 0.025, and excellent results when it is 0.01. For some applications, the preferred product of the indicated width and distance is no more than about 0.005 square inches, or even less.
This invention may also be defined in terms of the maximum distance from the leading edge of the gas diffusing sur-face to the most remote very small capi~lary openings in that surface. Values of 1 inch, 0.5 inch and 0.1 inch for this dis- -tance produce'good, improved and preferred results, respectively. ' ' ' When distances of this order of magnitude have been used in prior i' art devices, it has only been with very large openings in the gas diffusing surface, such as the holes in British patent No.
~ 1,039,702',which are at least 20 times as large as the 100 micron `1 or smaller diameter capillary openings of the present invention.It has been universally believed by those skilled ~ the art of ' 1;.:
bubble shearing that with capillary openings as small as are em-i. ~ `: 'r,.
I ployed here, it is necessary to use a very much wider band of capillary openings than applicant has found to be required.
~30 The present invention also includes a method of fabri- - ' cating a gas transmitting body for use in the gas diffusion ap-' -paratus of the invention. This method includes the manufact~.r~ ' ':: ., \ , ,-. . ' -11- ' , ~` ~069665 ing of the foraminous portion of the wall of a gas plenum by embedding a plurality of hollow capillary strands, preferably as constituent parts of a plurality of lengths of roving, in a matrix of hardenable binder, all con-tained between two plates to form a "sandwich" that is incorporated in one wall of the gas plenum.
In accordance with this invention there is provided a method of fabricating a gas transmitting body for use in apparatus for diffusing fine gas bubbles into a body of liquid which comprises: positioning a plurality of hollow capillary strands and binder across one face of a first support plate having at least one side wall adapted to form a part of the interior wall of a gas plenum, and at least one side wall adapted to form a part of the exterior wall of said gas plenum, so that said hollow strands are aligned substantially normal to each of said side walls, and sald strands and binder extend beyond the plate on both of said sides of the plate; positioning a second support plate having a similar shape to that of said first support plate adjacent said plurality of hollow capillary strands and binder to form a sandwich of said two plates with said strands and binder therebetween compressing said sandwich to cause said binder to fill all the crevices between said strands and between the strands and the support plates; holding said sandwich compressed until said binder is hardened to embed said hollow capillary strands in the binder as à matrix, to provide gas transmitting passages extending through said matrix; trimming back generally to said side walls of said first and second support plates adapted to form a part of the exterior wall of a ~as plenum the portions of said hollow strands embedded in said matrix that extend outwardly beyond said side walls, to form a gas diffus-ing surface with capillary openings distributed across the same; and severing the portions of said hollow strands embedded in said matrix that extend in-wardly beyond said side walls of said first and second support plates adapted to form a part of the interior wall of said gas plenum, to form the inlet ends of said gas transmitting passages.
BRIEF DESCRIPTION OF DRAWINGS
Figure lA is a greatly enlarged schematic cross sectional view of ~`h ~ -12-substantially parallel laminar flow of liquid from left to right through a slot, becoming fully developed laminar flow at the right-hand side of the figure;
Figure lB is a fragmentary view of the indicated portion of Figure lA, still further enlarged laterally and showing a plurality of capil-lary openings in close proximity to the leading edge of the slot;
Figures 2A through 2F are graphs of the velocity distribution for laminar flow of shearing liquid through a liquid transmitting slot, at posi-tions located progressively downstream from the inlet end of the slot;
Figure 3 is a fragmentary schematic drawing of the laminar flow of shearing liquid represented by Figures 2A through 2F, showing only the bottom wall of the slot;
Figures 4A through 4F are schematic cross sectional viewsof various types of substantially parallel laminar flow of shearing liquid through a liquid transmitting slot, produced by the use of the method and apparatus of this invention;
Figures 5 through 10 are graphs showing how the bubble diameter of bubbles produced by use of the method and apparatus of this invention varies with changes in certain physical characteristics of the apparatus, operating parameters, and characteristics of the shearing liquid;
Figure 11 is an illustration of a diffuser assembly - 12 a -"` ~ 1069665 ~ ~
having a plurality of units or modules constructed in accordance with the present invention;
Figure 12 is an enlarged fragmentary cross sectional view, schematic in nature, taken along the line 12-12 of Figure 11, illustrating the relationship between gas dispersing as-semblies or bars, a water supply chamber, and a plurality of : discharge slots in one of the diffuser unitsi Figure 13 is a schematic crosssectional view of a gas transmitting body and a slot-defining member that are :
spaced to define an annular shearing liquid transmitting slot ~ in another embodiment of the apparatus of this invention;
! Figure 14 is a drawing made from a photomicrograph giving a fragmentary view of an actual gas diffusing surface employed in apparatus according to this invention;
Figures 15 and 16 are photomicrographs, at an en-largement of about 280 times in the original figure submitted as a part of this application, showing the formation of fine bubbles of air in water resulting from the use of the method and apparatus of this invention;
Figures 17A through 17D show a succession of steps in I the fabrication according to this invention of a gas transmit-. ting body for use in the diffusion apparatus of the invention;
Figure 18A is a plan view of a completed gas trans-: mitting body whose fabrication includes the steps illustrated :.
in Figures 17A through 17D;
Figure 18B is an enlarged sectional viet~ taken along the Line 18B-18B of Figure 18A; ~ ::
Figure 19 is a perspective view of a winding fixture on~whi~ch a plurality of the support plates of Figure 17A have 30:`~ : been~secured for positioning a plurality of lengths of roving comprised of hollow capillary str.ands across said plates in .. accordance with this invention;
. ,: ' . - ~, 13~
.,/
: ~ . ' , ~ ~ , .. . . .
: . .
1(~69665 Figure 20A is a fragmentary plan view of one of the support plates of Figure 19 showing a plurality of rovings of hollow capillary strands in position across said plate;
Figure 20B is a schematic cross sectional view taken through one of the rovings of Figure 20A;
Figures 21A an~ 21B are plan views of steps in the :~
fabrication of another sandwich of hollow capillary strands embedded in a matrix between two support plates that is incorpo-rated in a gas transmitting body for use in the bubble shearing apparatus of this invention;
Figure 22A is a plan view of the bottom cover plate of said gas transmitting body;
Figures 22B and 22C are cross sectional views taken along the lines 22B-22B and 22C-22Cr respectively, of Figure 22Aî
Figure 22D is an end view of the bottom cover plate of Figure 22A;
Figure 2 3A iS a plan view of the top cover plate of said gas transmitting body; :~
Figure 23B is a cross sectional view taken along the line 23B-23B of Figure 23~;
Figure 23C is an end view of the top cover plate of Figure 23A;
Figure 24A is a plan view of an assembled gas trans-mitting body made according to this form of the m~ethod of fab-,!` ~''' `
rication of such a body for use in the apparatus o this in-` vention;"
Figure 24B iS an enlarged cross sectional view taken ~.
along the line 24B-24B of Figure 24A; and Figure 24C iS a similarly enlarged end view of thè
assembled gas transmitting body of Figure 24A.
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~9665 ~-FURT~ER DESCRIPTION OF THE INVENTION
Contrary to the teaching of the prior art, applicant has discovered that in producing fine gas bubbles by flowing liquid over very small capillary openings in a gas diffusing surface, it is important to avoid all turbulence or vena con-tracta in the moving liquid but at the same time to establish and maintain no more than partially developed laminar flow over a significant portion of the capillar~ openings of the gas dif-fusing surface. ~he importance of positioning all the small capillary openings in close proximity to the leading edge of the surface on which the bubbles are formed is likewise not taught in the prior art. With the novel method and apparatus ~ased on these discoveries by applicant, it has been found that bubbles of very small diameter and very good uniformity of bubble size can be formed with a surprisingly low expenditure of energy.
Substantially Parallel Laminar Flow As just indicated, the aim in the method and apparatus of the present inven'_ion, contrary to the teaching of the prior art, is to achieve a steady but rapid liquid flow, with only minimal perturbations, past the very small capillary openings at which bubble formation takes place. A specific objective of , the invention is to produce a partially developed laminar bound-ary layer over a substantial number -- in the method o this ap-plication, at least about one-quarter -- of the capil7ary open-ings in the gas diffusing surface. ;~
Figure lA gives a schematic representation of substan-tially parallel laminar flow through slot 40 (which is formeà
by substantially planar, parallel walls 42 and 44) from inlet ~ ~ end 46 to-discharge end 48 of the slot. Slot 40 is shown as being long enough that, as will be explained below, ~\
-15- .
. . . . .. . . .
`" ~ 106966S ~
the substantially parallel laminar flow through the slot be-comes fully developed. The scale of the figure is greatly exaggerated for clarity, and thus slot 40 should not be thought of as being nearly as large as the expanded scale of Figure lA
might suggest. In one actual embodiment of the apparatus of this invention, for example, the width of slot 40 is only about : -0.040 inches and the slot length only about 0.080 inches.
St.reamlines 50 show the direction of movement of the liquid from left fo right through slot 40. As will be seenr the movement is substantially parallel to the respective contours of walls 42 and 44 which form the slot. There is no vena contracta flow, which would include one or more zones in which the liquid would circulate in closed paths and thus be prevented from flow-ing continuously through the available channel of slot 40. There is also no turbulentflow through the slot, which would include extreme perturbations resulting in great variations in liquid pressure within the region of turbulence.
In laminar flow, the liquid moves as if it were in layers with different velocities. This is illustrated in Figure lA by interface 52 between laminar boundary layer 54 and free stream 56 at the bottom of slot 40, and interface 58 between laminar boundary layer 60 and free stream 56 at the top of slot 40. The existence of these two interfaces can be verified by techniques known to those skilled in hydrodynamics.
, ., .
i Partially_Developed_Laminar Boundary LaYèr ;:;
In the flow illustrated in Figure lA, the~velocity of the liquid at inlet end 46 of slot 40 is uniform across the width of the slot. Because of viscous friction between the .
liquid and walls 42 and 44, the moving liquid immediately ad-jacent those walls is slowed~dow~ markedly as it passes thr~ugh slot 40~ This in turn reduces the velocity of the liquid at greater distances from walls 42 and 44. The result is that a .. .
,. :. : - . . ' ,' ''' ' .'. ' '.' ' - . ~
.-. . ~ . ~ . : . . , 106~665 (-laminar ~oundary layer forms on each ~all, with the velocity of the moving liquid in the layer ranging from its lo~est value at the wall to its maximum value where it joins the free stream flow at the outer portions of the layer.
As shown in Figure lA, laminar boundary layers 54 znd 60 increase in thickness from zero at leading edge 61 of the surface of wall 42, until they merge at point 62 near the end of slot 40. Laminar boundary layers 54 and 60 are character-ized as "partially developed laminar boundary layers" from inlet 46 of slot 40 to point 62. "Fully developed laminar flo~"
64 continues from point 62 to discharge end 48 of slot 40.
Fisure lB is a laterally expanded view of the indi-cated fragment from the left-hand side of Figure lA, with capil-lary openings 65 added to the drawing in schematic form. Gas passages 66 extend through wall 42, and terminate in capillary openings 65 in close proximity to leading edge 61 at inlet end 46 of slot 40, to form gas diffusing surface 68.
Partially developed laminar boundary layer 54, in contact with slot wall 42, is seen slowly building up in thick-ness from leading edge 61, and partially developed laminar bound-ary layer 60 is seen building up in a similar way in contact with upper wall 44. As in Figure lA, free stream 56 lies be-tween the partially developed laminar boundary layers with inter-face 52 separating them in the lower part of slot 40 and inter-face 58 separating them in the upper part of slot 40. Stream-lines 50 again represent the flow of liquid from lëft to right through the slot. With the expanded scale of Figure lB, it is seen that partially developed laminar boundary layer 54 is rela-, tively thin above the capillary openings sho~n in that figure.
`30 As already pointed out above, in an actual embodiment of the apparatus of this invention, the dimensions of slot ~0 are very ~uch smaller than is suggested by the expanded scales of~`
~' ~ '\.' .
,, ~
-17- , . .
106~6~;5 Figures lA and lB. In that particular actual embodiment, however, the capillary openings are located somewhat farther downstream from the leading edge of the gas diffusing surface than is sug-gested in Flgure lB, although still in close pro~imity thereto.
ree Stream Velocity -For reasons alread~ explained, the liquid in the por-tions of laminar boundary layers 54 and 60 in contact with their respective walls 42 and 44 moves more slowly than the li~uid en-tering inlet end 46 of slot 40. Since the quantity of liquid that leaves slot 40 at its discharge end 48 must be the same as that entering at inlet end 46, the liquid in free stream 56 must compensate for this slowdown by moving more rapidly than the li-quid entering slot 40. As the retarding effect of the viscous friction between the walls and the moving liquid is felt by a thicker and thicker layer of liquid as one moves downstream, the free stream velocity balances this effect by continuing to in-crease as the liquid flo~Js through the slot, until the velocity reaches a limiting maximum value at some point downstream.
Figures 2A through 2F, which are derived from Figure 9.16 of Schlichting, Boundar~-Layer Theory, Sixth Ed. (McGra~-Hill,~1968), page 177, illustrate how the velocity profile of the liquid flow through slot 40 changes as one moves downstream through the slot. In these figures, fragments of walls 42 and 44 are shown as defining successive portions of slot 40. The successive stations shown in these figures proceed downstream from inlet 46 in Fi~ure 2A to a location in Figure 2F where the velocity profile is approaching a parabolic form. At stations still farther downstream, the velocity profile assumes the form -o a parabola, and the flow has become similar to the fully de-veloped 10w indicated at 62 in Figure lA.
The successive stations in Figures 2A through 2F are - located at 0.1, 0.4, 1.0, 2.0, 4 and 10 units of distance down-.
.~ ' ~ ' ' .
` ~ ~Q69665 stream from inlet 46, at points 70, 72~, 74, 76, 78 and 80, re-spectively, The distances from the slot wall are measured in the vertical direction. The velocity of the moving liquid at the indicated distances from the wall is represented by the dis-placement of the points of the graph to the right in the horizon-tal direction. Thus, as will be seen, the liquid moves with a velocity increasing from zero at walls a2 and 44 to a maximum --substantially uniform -- velocity across free stream 56.
Points 82, 84, 86, 88, 90 and 92 represent the dis-tances from slot wall 42 at which liquid is moving with substan-tially the same velocity as ~he free stream velocity (which in-I; ' .
creases with increasing dis~ances downstream) at points 70, 72, 74, 76, 78 and 80, respectively. In Figure lA or lB, in ; other words, points 82 through 92 would fall on interface 52 which defines laminar boundary layer 54. These points derived from Figures 2A through 2F have also been plotted in Figure 3.
At the same time that viscous friction with the slotwall slows do~m the moving liquid in the lower portions of lami-nar boundary layer 54, the liquid in the upper portions of lami-nar boundary layer 54 speeds up to keep pace with the increasingvelocity o free stream 56. Points 83, 85, 87, 89, 91 and 93 represent the distances from ~lot wall 42 at which liquid moves with substantially the same velocity with which it entered slot 40 at inlet end 46. These points are plotted as dashed line 9S in Figure 3. Above line 95, liquid moves more rapidly than it did when it entered slot 40, while below line 95 it moves more slowly.
The viscous shearing force exerted by any moving li-quid is directly proportional to the velocity with which the liquid is moving. Thus, a nascent gas bubble that forms at a capillary opening in slot wall 42 below layer 54 will not extend very far above the more slowly moving liquid in the bottom por-tions of laminar boundary layer 54 -- and certainly not far into ' .
free stream 56 above the top portion of the laminar boundary layer -- before it will be sheared off. This is what keeps the diameter of the gas bubbles produced by use o~ this invention small. -~ydrodynamic Requirements Of This Invention ~ o requirements must be met when the method and appa-ratus of this invention are de~ined hydrodynamically:
1. A partially developed laminar boundary layer is established above a substantial num~er -- in the method, at least about one-quarter -- of the capillary openings in the gas diffusing surface at which bubbles are formed and sheared off.
This invention relates to apparatus and method for diffusing fine gas bubbles into a body of liquid, and to a method for fabricating such diffusion apparatus.
~ ....
A
, ' ~06~5 This application is a divisional application of Canadian Patent Application Serial No. 219,753, filed on February 10, 1975.
BACKGROUND OF THE INVENTION
It is important in various processes to diffuse a gas into a body of liquid in a manner that will disperse the gas uniformly through a large portion of the liquid and result in as rapid as possible absorption of the gas by the liquid.
Diffusion of a gas into a body of liquid in this way is useful, `
for example, in a large number of chemical and petrochemical processes. It is also important in certain sewage treatment processes. It is becoming increasingly important in the treatment of natural bodies of water with air, oxygen, or ozone in order to revive polluted rivers, lakes, bays, etc.
When, for purposes of economy or for any other reason, gases used in the treatment of a body of liquid should not be allowed to escape from the surface of the liquid, it is im-portant that the method and apparatus used be such that all, ¦ or nearly all, the gas dissolves in the liquid before any sub-stantial number of gas bubbles can rise to the surface. In every case, it is usually desirable that the absorption of the ~'i gas into the liquid proceed as rapidly as possible.
Effect of Bubble Size and Uniformity of Size There are a number of factors that affect the rate at which a par~icular gas can be dissolved in a given body of liquid. Two of the most important of these factors are the size ~ of the bubbles and the degree of uniformity in bubble size, l~ which affect the rate of absorption of gas into the body of liquid as follows:
1. Smallest bubble size possible One of the principal aims in any diffusion process in which the objective is rapid absorption of the gas in the liquid is to produce bubbles in as small a size as possible, because:
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1069665 "
(a) The smaller the bubbles, the slower they rise through the liquid, and thus the longer the period of time they have in which to be dissolved.
It is known that at a bubble size of about 200 microns, there is a marked increase in the rise rate of the bubbles through a body of water. Thus, the size of the bubbles to be dissolved in water should be less than this figure. Improved results are obtained with water if the maximum bubble diameter is no more than about 100 microns, and best results if sub-stantially all the bubbles are no larger than about 50 micronsin diameter.
(b) The smal er the bubble, the larger is the sur-face that is available through which a given volume of gas can dissolve into the surrounding liquid. (To express the converse relationship: As bubbles grow larger,the surface of the bubble increases only as the square of the diameter~ while the volume of gas in the bubble increases as the cube of the diameter.) (c) Because in smaller sized bubbles the surface undulations which tend to draw adjacent bubbles together are more heavily damped by viscous forces, such smaller sized bubbles are less likely to collide with other bubbles to coa-lesce to form new, larger bubbles.
(d) For very small bubbles, the internal pressure of the bubble is markedly higher due to surface tension forces, and thus the driving force for transfer of the gas from the 1. . .
~ ga~eous to the liquid phase is higher.
,2. Highest possible degree of uniformit~ in bubble size The ideal situation would be to have all the gas ~; bubbles the same size when they are diffused into the body of liquid. When all bubbles are uniform in siæe, they will have a , . .
.
1069665 ~ ~
uniform terminal rise rate as they move upward through the body of liquid, which in turn means:
(a) Local turbulence due to the so-called "chimney effect" -- which would tend to disturb the uniformity of dis-tribution of the gas bubbles through the body of liquid --will be avoided.
(b) Since all the bubbles will be moving at a uni-form rate, none will overtake other bubbles and tend to couple or coalesce with ~hem-. If two bu~bles couple together with a kind of membrane between them, the overall surace through which a given volume of gas can dissolve into the surrounding liquid is reduced. If ~wo bubbles coalesce to-form a wholly new, larger bubble, the overall surface through which a given volume of gas can dissolve into the surrounding liquid is 3 likewise reduced. In both cases, the result is further com-~', pounded by the fact that the new, larger bubbles will have an increased rise rate, ana therefore will tend to overtake and join with still more bubbles.
. .
Bubble Shearing ~ ~ The ideal situation is to produce large numbers of very cmall bubbles, as nearly uniform in size as possible, at as high a~rate as possible and with the lowest possible ex-¦~ penditure ~of energy. The present invention involves a novel and greatLy improved "bubble shearing" method and apparatus ;ky~which this ideal situation is achieved.
It has long been known that method and apparatus em-ploying the phenomenon known as "bubble shearing" are well adapted to the~production of quite small gas bubbles in a ;; liquid.~ ~In~this method, a gas is passed through the capillary 30 ~ passages of~ a foraminous material to be introduced into a stre~m of liquid on the other side of the material. The gas 1' ~
1, ~ :, , !-, .~ :
`` ~ iO69~;6S
emerges at the boundary surface of the foraminous material ina number of locations spaced from each other, where it starts to form a plurality of fine bubbles, or in other words forms "nascent" gas bubbles. The liquid into which the gas is thus introduced is caused to move more or less rapidly past the capillary openings of the foraminous material. As a result, the viscous shear forces exerted by the moving body of liquid shear off the partially formed or nascent gas bubbles before they can grow to such a size that their natural buoyancy in the liquid will cause them to break the surface tension that tends to hold them on the boundary surfaces of the gas trans-mitting oraminous material.
The novel and striking results from the use of the method and apparatus of the present bubble shearing invention are accomplished in ways that were previously either never considered, or were believed to be impossible, by those skilled in the art.
Disadvantages of the Prior Art Probably the oldest method of introducing gas bubbles ~0 into a body of liquid is the method in which a gas transmitting body having a plurality of small openings is immersed in the liquid, and gas is caused to flow out of those openings into the liquid, to form bubbles that break away and rise in the liquid when they have grown to a large enough size that their buoyancy overcomes the surface tension that tends to hold them on the surface on which they fvrm. The rate of formation of bubbles with this method is relatively lowO Moreover, since each bubble must grow to such a size that its buoyancy is great enough to cause it to break away from the surface on ~ -' ! . .
which it is formed, these bubbles are of a larger size than ; is desirable for many applications. Examples of method and . ... .
~ -6-; - . , ~
.. , ', ', ' " , " ;''' ~ ,.. ' ... '. ,. . " ' ' ~ '' ~
apparatus that rely solely or primarily on the buoyancy of the bubbles to break them away from the bubble forming surface in-clude the pa~ents to ~een No. 2,295,740, Nordell No. 2,555,201, Fischerstrom et al. No. 2,708,571, and Schnur No. 2,719,032.
Another method of diffusing bubbles into a body of liquid includes the flow of liquid past a plurality of openings ~preferably oriented in the direction of downstream flow of the liquid) out of which gas is caused to flow by reason of the low pressure region in the wake of the moving liquid. The rate of formation of gas bubbles with this type of method and apparatus is only som~what higher, and the median bubble size only somewhat smaller, than with the method and apparatus that rely primarily on the buoyancy of the bubbles to break them off the surface on which they are formed. Examples are dis-closed in British patent No. 942,754 and U.S. patents No.
3,489,396 to D'~ragon and No. 3,671,022 to Laird et al.
A more or less conventional form of bubble shearing apparatus is disclosed in Polish patent No. 48,942. In that ap-paratus, a stream of liquid from an inlet pipe produces circula-tory flow that moves past a cylindrical surface whose upper part is perforated to emit gas, and the turbulent moving liquid shears off gas bubbles that form at the perforations. Both the presence of turbulence and the large size of the perforations in the gas diffusing surface (estimated to be at least about 5 mm. in di-ameter) mean that the bubbles produced with the apparatus of this Polish patent cannot approach the small sizè~of the bubbles produced by the method and apparatus of the present invention.
; Apparatus that utilizes the bubble shearing principle in a special way is disclosed in Smith et al. patent No.
3,450,800. In that patent, the freé stream flow of a moving body of liquid shears off nascent bubbles at the openings of capillary tubes~ and carries them downstream in the moving ~ ~7~
:. . -.
. - .. . . . . ~ . .
liquid. The discrete capillary openings are arranged in a free-standing array of capillary tubes, ~hich would obviously be sub-ject to a high level of brea~.age from the moving shearing liquid.
In addition, thousands of capillary tubes would be required to disperse fine bubbles in any significant quantity, which would so fill up the liquid transmitting passage as to make the appa-ratus impractical.
McManus patent No. 3,5a5,731 points out that it has been conventional to impart a flow of relatively low velocity to a liquid medium in order to shear off bubbles from a for-aminous gas transmitting surface on which the bubbles form, but McManus then rejects bubble shearing in favor of creating such a high li~uid velocity flow that a turbulent boundary layer is formed in a region that is located virtually at the surface of the wall t with only a very thin laminar sublayer intervening between that region and the wall. As McManus ex-plains, the pressure fluctuations in this turbulent region "snap off" the bubbles before they can be sheared off by viscous shear forces~ The specification of the patent states that the bubbles thus formed are "of miscropic size, on the order o 5-10 microns in diameter," but no experimental evi-dence to support the statement(such as photomicrographs)-iS
included in the patent. The major disadvantage of the McManus method and apparatus is that a liquid pressure head of 75 p.s.i.
is required to create sufficient liquid flow to ~erate the , apparatus; by contrast~ the liquid pressure head required to operate the apparatus of the present invention is typically'~
only about 2 to 7 p.s.i.
- Several other prior art patents are addressed to the `I 30 production of gas bubbles in moving streams of liquids under various conditions, but they do not give any hint of the desira-bility of establishing a partially developed laminar boundary 8 ~/
~ .
- r 1069665 layer in li~uid flowing over capillary openings small enough to produce fine gas bubbles, ~hich applicant has found to be a very important flow regime to establish. On the contrary, they actu-ally disclose structural features that will produce either vena contracta (British patent ~o. 1,039,702) or turbulence (patents No. 2,695,710 to Gibbs and No. 3,256,802 to Karr, and British patents Nos. 694,918, 713,064 and 885,406) when water moving at what a~plicant has found to be its minimum necessary velocity is used as the shearing liquid.
SUMMARY OF THIS INVENTION
This invention utilizes a gas diffusing surface having very small capillary openings, no larger than about 100 microns in diameter, preferably in close proximity to the leading edge of the surface. Shearing liquid i5 caused to flow past the lead-ing edge of the gas diffusing surface to a discharge edge, to produce substantially parallel laminar flow, including a partial-ly developed laminar boundary layer immediately adjacent the gas diffusing surface, with a free stream above that layer, over at least a substantial number of the capillary openings. In the method, the partially developed layer is present over at least about one-quarter of the capillary openings. The liquid over any additional capillary openings present in the gas diffusing surface downstream of the capillary openings already referred to must be substantially free of'any type of flow (such as vena contracta or turbulence) other than fully developed substantially ,:
parallel laminar flow.
The term "capillary openings" is used herein to mean any openings small enough to produce fine gas bubbles, no matter ~ what the nature of the gas diffusing surface is on which the bubbles are formed, or the nature of the gas transmitting pas-sages through which gas flows to reach ~hose openings.
~he liquid in the partially developed laminar flow ~ -~, \ .
_ g _ ./
,~ -, '' . . . .
iO~S6;65 just described, either with or without the action of the free stream above the partially developed laminar boundary layer, shears nascent gas bubbles rom the capillarv openings. The moving liquid carries the fine gas bubbles thus produced away from the gas diffusing surface at its discharge edge, and into -the body of liquid being treated. The partially developed lamin-ar boundary layer may be transformed into fully developed laminar flow before the shearing liquid leaves the gas diffusing sur-face, and there may or may not be additional capillary openings beneath that fully developed flow. Some advantage may be de-rived from use of this invention even if some turbulence appears in the moving shearing liquid before it leaves the gas diffus-ing surface, but in that case there must be no capillary open-ings beneath the region of turbulence.
, , The bubble shearing apparatus of this invention in-'~ cludes a slot for transmittlng shearing liquid past the capil-lary openings of the gas diffusing surface at which the nascent I gas bubbles are formed before they are sheared off by the mov-ing liquid. The shape, dimensions and spacing of the members defining the slot are adapted to establish laminar flow as de-ii scribed above when water is employed with the apparatus as the shearing liquid under cçrtain specified operating conditions. In addition, the apparatus includes means for providing a flow of li-~uid ln the slot at a velocity to produce such parallel laminar flo~J. It is best that the very small capillary ope~n~ings in the ~; ga9 diffusing surface be located in close proximity to the inlet end of the liquid transmitting slot, and preferably the farthe~t upstream of the capillary openings should lie immediately ad-ac~ent that inlet end, as close to the inlet as is structurally ~30 ~ practicable. ~ .
The bubble shearing apparatus of this invention may be defined alternatively in terms of a figure of merit that ! ' ' ' ' .
'- ~ 10696~5 is the product of (a) the distance separating the inlet end of the liquid transmitting slot from the very small capillary openings that lie farthest downstream on the gas diffusing sur-face over which the shearing liquid flows, times (b) the aver-age width of the slot throughout the indicated distance. The units for this figure of'merit are square inches. 5atisfactory results may be obtained in some applications when the magni-tude of this figure of merit is no more than about 0.1. Some-what improved results are generally obtained when the figure of merit is no more than about 0.075, and still further improvement is obtained when it is no more than about 0.05. Very good re-sults are obtained when the figure is no more than about 0.025, and excellent results when it is 0.01. For some applications, the preferred product of the indicated width and distance is no more than about 0.005 square inches, or even less.
This invention may also be defined in terms of the maximum distance from the leading edge of the gas diffusing sur-face to the most remote very small capi~lary openings in that surface. Values of 1 inch, 0.5 inch and 0.1 inch for this dis- -tance produce'good, improved and preferred results, respectively. ' ' ' When distances of this order of magnitude have been used in prior i' art devices, it has only been with very large openings in the gas diffusing surface, such as the holes in British patent No.
~ 1,039,702',which are at least 20 times as large as the 100 micron `1 or smaller diameter capillary openings of the present invention.It has been universally believed by those skilled ~ the art of ' 1;.:
bubble shearing that with capillary openings as small as are em-i. ~ `: 'r,.
I ployed here, it is necessary to use a very much wider band of capillary openings than applicant has found to be required.
~30 The present invention also includes a method of fabri- - ' cating a gas transmitting body for use in the gas diffusion ap-' -paratus of the invention. This method includes the manufact~.r~ ' ':: ., \ , ,-. . ' -11- ' , ~` ~069665 ing of the foraminous portion of the wall of a gas plenum by embedding a plurality of hollow capillary strands, preferably as constituent parts of a plurality of lengths of roving, in a matrix of hardenable binder, all con-tained between two plates to form a "sandwich" that is incorporated in one wall of the gas plenum.
In accordance with this invention there is provided a method of fabricating a gas transmitting body for use in apparatus for diffusing fine gas bubbles into a body of liquid which comprises: positioning a plurality of hollow capillary strands and binder across one face of a first support plate having at least one side wall adapted to form a part of the interior wall of a gas plenum, and at least one side wall adapted to form a part of the exterior wall of said gas plenum, so that said hollow strands are aligned substantially normal to each of said side walls, and sald strands and binder extend beyond the plate on both of said sides of the plate; positioning a second support plate having a similar shape to that of said first support plate adjacent said plurality of hollow capillary strands and binder to form a sandwich of said two plates with said strands and binder therebetween compressing said sandwich to cause said binder to fill all the crevices between said strands and between the strands and the support plates; holding said sandwich compressed until said binder is hardened to embed said hollow capillary strands in the binder as à matrix, to provide gas transmitting passages extending through said matrix; trimming back generally to said side walls of said first and second support plates adapted to form a part of the exterior wall of a ~as plenum the portions of said hollow strands embedded in said matrix that extend outwardly beyond said side walls, to form a gas diffus-ing surface with capillary openings distributed across the same; and severing the portions of said hollow strands embedded in said matrix that extend in-wardly beyond said side walls of said first and second support plates adapted to form a part of the interior wall of said gas plenum, to form the inlet ends of said gas transmitting passages.
BRIEF DESCRIPTION OF DRAWINGS
Figure lA is a greatly enlarged schematic cross sectional view of ~`h ~ -12-substantially parallel laminar flow of liquid from left to right through a slot, becoming fully developed laminar flow at the right-hand side of the figure;
Figure lB is a fragmentary view of the indicated portion of Figure lA, still further enlarged laterally and showing a plurality of capil-lary openings in close proximity to the leading edge of the slot;
Figures 2A through 2F are graphs of the velocity distribution for laminar flow of shearing liquid through a liquid transmitting slot, at posi-tions located progressively downstream from the inlet end of the slot;
Figure 3 is a fragmentary schematic drawing of the laminar flow of shearing liquid represented by Figures 2A through 2F, showing only the bottom wall of the slot;
Figures 4A through 4F are schematic cross sectional viewsof various types of substantially parallel laminar flow of shearing liquid through a liquid transmitting slot, produced by the use of the method and apparatus of this invention;
Figures 5 through 10 are graphs showing how the bubble diameter of bubbles produced by use of the method and apparatus of this invention varies with changes in certain physical characteristics of the apparatus, operating parameters, and characteristics of the shearing liquid;
Figure 11 is an illustration of a diffuser assembly - 12 a -"` ~ 1069665 ~ ~
having a plurality of units or modules constructed in accordance with the present invention;
Figure 12 is an enlarged fragmentary cross sectional view, schematic in nature, taken along the line 12-12 of Figure 11, illustrating the relationship between gas dispersing as-semblies or bars, a water supply chamber, and a plurality of : discharge slots in one of the diffuser unitsi Figure 13 is a schematic crosssectional view of a gas transmitting body and a slot-defining member that are :
spaced to define an annular shearing liquid transmitting slot ~ in another embodiment of the apparatus of this invention;
! Figure 14 is a drawing made from a photomicrograph giving a fragmentary view of an actual gas diffusing surface employed in apparatus according to this invention;
Figures 15 and 16 are photomicrographs, at an en-largement of about 280 times in the original figure submitted as a part of this application, showing the formation of fine bubbles of air in water resulting from the use of the method and apparatus of this invention;
Figures 17A through 17D show a succession of steps in I the fabrication according to this invention of a gas transmit-. ting body for use in the diffusion apparatus of the invention;
Figure 18A is a plan view of a completed gas trans-: mitting body whose fabrication includes the steps illustrated :.
in Figures 17A through 17D;
Figure 18B is an enlarged sectional viet~ taken along the Line 18B-18B of Figure 18A; ~ ::
Figure 19 is a perspective view of a winding fixture on~whi~ch a plurality of the support plates of Figure 17A have 30:`~ : been~secured for positioning a plurality of lengths of roving comprised of hollow capillary str.ands across said plates in .. accordance with this invention;
. ,: ' . - ~, 13~
.,/
: ~ . ' , ~ ~ , .. . . .
: . .
1(~69665 Figure 20A is a fragmentary plan view of one of the support plates of Figure 19 showing a plurality of rovings of hollow capillary strands in position across said plate;
Figure 20B is a schematic cross sectional view taken through one of the rovings of Figure 20A;
Figures 21A an~ 21B are plan views of steps in the :~
fabrication of another sandwich of hollow capillary strands embedded in a matrix between two support plates that is incorpo-rated in a gas transmitting body for use in the bubble shearing apparatus of this invention;
Figure 22A is a plan view of the bottom cover plate of said gas transmitting body;
Figures 22B and 22C are cross sectional views taken along the lines 22B-22B and 22C-22Cr respectively, of Figure 22Aî
Figure 22D is an end view of the bottom cover plate of Figure 22A;
Figure 2 3A iS a plan view of the top cover plate of said gas transmitting body; :~
Figure 23B is a cross sectional view taken along the line 23B-23B of Figure 23~;
Figure 23C is an end view of the top cover plate of Figure 23A;
Figure 24A is a plan view of an assembled gas trans-mitting body made according to this form of the m~ethod of fab-,!` ~''' `
rication of such a body for use in the apparatus o this in-` vention;"
Figure 24B iS an enlarged cross sectional view taken ~.
along the line 24B-24B of Figure 24A; and Figure 24C iS a similarly enlarged end view of thè
assembled gas transmitting body of Figure 24A.
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~9665 ~-FURT~ER DESCRIPTION OF THE INVENTION
Contrary to the teaching of the prior art, applicant has discovered that in producing fine gas bubbles by flowing liquid over very small capillary openings in a gas diffusing surface, it is important to avoid all turbulence or vena con-tracta in the moving liquid but at the same time to establish and maintain no more than partially developed laminar flow over a significant portion of the capillar~ openings of the gas dif-fusing surface. ~he importance of positioning all the small capillary openings in close proximity to the leading edge of the surface on which the bubbles are formed is likewise not taught in the prior art. With the novel method and apparatus ~ased on these discoveries by applicant, it has been found that bubbles of very small diameter and very good uniformity of bubble size can be formed with a surprisingly low expenditure of energy.
Substantially Parallel Laminar Flow As just indicated, the aim in the method and apparatus of the present inven'_ion, contrary to the teaching of the prior art, is to achieve a steady but rapid liquid flow, with only minimal perturbations, past the very small capillary openings at which bubble formation takes place. A specific objective of , the invention is to produce a partially developed laminar bound-ary layer over a substantial number -- in the method o this ap-plication, at least about one-quarter -- of the capil7ary open-ings in the gas diffusing surface. ;~
Figure lA gives a schematic representation of substan-tially parallel laminar flow through slot 40 (which is formeà
by substantially planar, parallel walls 42 and 44) from inlet ~ ~ end 46 to-discharge end 48 of the slot. Slot 40 is shown as being long enough that, as will be explained below, ~\
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`" ~ 106966S ~
the substantially parallel laminar flow through the slot be-comes fully developed. The scale of the figure is greatly exaggerated for clarity, and thus slot 40 should not be thought of as being nearly as large as the expanded scale of Figure lA
might suggest. In one actual embodiment of the apparatus of this invention, for example, the width of slot 40 is only about : -0.040 inches and the slot length only about 0.080 inches.
St.reamlines 50 show the direction of movement of the liquid from left fo right through slot 40. As will be seenr the movement is substantially parallel to the respective contours of walls 42 and 44 which form the slot. There is no vena contracta flow, which would include one or more zones in which the liquid would circulate in closed paths and thus be prevented from flow-ing continuously through the available channel of slot 40. There is also no turbulentflow through the slot, which would include extreme perturbations resulting in great variations in liquid pressure within the region of turbulence.
In laminar flow, the liquid moves as if it were in layers with different velocities. This is illustrated in Figure lA by interface 52 between laminar boundary layer 54 and free stream 56 at the bottom of slot 40, and interface 58 between laminar boundary layer 60 and free stream 56 at the top of slot 40. The existence of these two interfaces can be verified by techniques known to those skilled in hydrodynamics.
, ., .
i Partially_Developed_Laminar Boundary LaYèr ;:;
In the flow illustrated in Figure lA, the~velocity of the liquid at inlet end 46 of slot 40 is uniform across the width of the slot. Because of viscous friction between the .
liquid and walls 42 and 44, the moving liquid immediately ad-jacent those walls is slowed~dow~ markedly as it passes thr~ugh slot 40~ This in turn reduces the velocity of the liquid at greater distances from walls 42 and 44. The result is that a .. .
,. :. : - . . ' ,' ''' ' .'. ' '.' ' - . ~
.-. . ~ . ~ . : . . , 106~665 (-laminar ~oundary layer forms on each ~all, with the velocity of the moving liquid in the layer ranging from its lo~est value at the wall to its maximum value where it joins the free stream flow at the outer portions of the layer.
As shown in Figure lA, laminar boundary layers 54 znd 60 increase in thickness from zero at leading edge 61 of the surface of wall 42, until they merge at point 62 near the end of slot 40. Laminar boundary layers 54 and 60 are character-ized as "partially developed laminar boundary layers" from inlet 46 of slot 40 to point 62. "Fully developed laminar flo~"
64 continues from point 62 to discharge end 48 of slot 40.
Fisure lB is a laterally expanded view of the indi-cated fragment from the left-hand side of Figure lA, with capil-lary openings 65 added to the drawing in schematic form. Gas passages 66 extend through wall 42, and terminate in capillary openings 65 in close proximity to leading edge 61 at inlet end 46 of slot 40, to form gas diffusing surface 68.
Partially developed laminar boundary layer 54, in contact with slot wall 42, is seen slowly building up in thick-ness from leading edge 61, and partially developed laminar bound-ary layer 60 is seen building up in a similar way in contact with upper wall 44. As in Figure lA, free stream 56 lies be-tween the partially developed laminar boundary layers with inter-face 52 separating them in the lower part of slot 40 and inter-face 58 separating them in the upper part of slot 40. Stream-lines 50 again represent the flow of liquid from lëft to right through the slot. With the expanded scale of Figure lB, it is seen that partially developed laminar boundary layer 54 is rela-, tively thin above the capillary openings sho~n in that figure.
`30 As already pointed out above, in an actual embodiment of the apparatus of this invention, the dimensions of slot ~0 are very ~uch smaller than is suggested by the expanded scales of~`
~' ~ '\.' .
,, ~
-17- , . .
106~6~;5 Figures lA and lB. In that particular actual embodiment, however, the capillary openings are located somewhat farther downstream from the leading edge of the gas diffusing surface than is sug-gested in Flgure lB, although still in close pro~imity thereto.
ree Stream Velocity -For reasons alread~ explained, the liquid in the por-tions of laminar boundary layers 54 and 60 in contact with their respective walls 42 and 44 moves more slowly than the li~uid en-tering inlet end 46 of slot 40. Since the quantity of liquid that leaves slot 40 at its discharge end 48 must be the same as that entering at inlet end 46, the liquid in free stream 56 must compensate for this slowdown by moving more rapidly than the li-quid entering slot 40. As the retarding effect of the viscous friction between the walls and the moving liquid is felt by a thicker and thicker layer of liquid as one moves downstream, the free stream velocity balances this effect by continuing to in-crease as the liquid flo~Js through the slot, until the velocity reaches a limiting maximum value at some point downstream.
Figures 2A through 2F, which are derived from Figure 9.16 of Schlichting, Boundar~-Layer Theory, Sixth Ed. (McGra~-Hill,~1968), page 177, illustrate how the velocity profile of the liquid flow through slot 40 changes as one moves downstream through the slot. In these figures, fragments of walls 42 and 44 are shown as defining successive portions of slot 40. The successive stations shown in these figures proceed downstream from inlet 46 in Fi~ure 2A to a location in Figure 2F where the velocity profile is approaching a parabolic form. At stations still farther downstream, the velocity profile assumes the form -o a parabola, and the flow has become similar to the fully de-veloped 10w indicated at 62 in Figure lA.
The successive stations in Figures 2A through 2F are - located at 0.1, 0.4, 1.0, 2.0, 4 and 10 units of distance down-.
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` ~ ~Q69665 stream from inlet 46, at points 70, 72~, 74, 76, 78 and 80, re-spectively, The distances from the slot wall are measured in the vertical direction. The velocity of the moving liquid at the indicated distances from the wall is represented by the dis-placement of the points of the graph to the right in the horizon-tal direction. Thus, as will be seen, the liquid moves with a velocity increasing from zero at walls a2 and 44 to a maximum --substantially uniform -- velocity across free stream 56.
Points 82, 84, 86, 88, 90 and 92 represent the dis-tances from slot wall 42 at which liquid is moving with substan-tially the same velocity as ~he free stream velocity (which in-I; ' .
creases with increasing dis~ances downstream) at points 70, 72, 74, 76, 78 and 80, respectively. In Figure lA or lB, in ; other words, points 82 through 92 would fall on interface 52 which defines laminar boundary layer 54. These points derived from Figures 2A through 2F have also been plotted in Figure 3.
At the same time that viscous friction with the slotwall slows do~m the moving liquid in the lower portions of lami-nar boundary layer 54, the liquid in the upper portions of lami-nar boundary layer 54 speeds up to keep pace with the increasingvelocity o free stream 56. Points 83, 85, 87, 89, 91 and 93 represent the distances from ~lot wall 42 at which liquid moves with substantially the same velocity with which it entered slot 40 at inlet end 46. These points are plotted as dashed line 9S in Figure 3. Above line 95, liquid moves more rapidly than it did when it entered slot 40, while below line 95 it moves more slowly.
The viscous shearing force exerted by any moving li-quid is directly proportional to the velocity with which the liquid is moving. Thus, a nascent gas bubble that forms at a capillary opening in slot wall 42 below layer 54 will not extend very far above the more slowly moving liquid in the bottom por-tions of laminar boundary layer 54 -- and certainly not far into ' .
free stream 56 above the top portion of the laminar boundary layer -- before it will be sheared off. This is what keeps the diameter of the gas bubbles produced by use o~ this invention small. -~ydrodynamic Requirements Of This Invention ~ o requirements must be met when the method and appa-ratus of this invention are de~ined hydrodynamically:
1. A partially developed laminar boundary layer is established above a substantial num~er -- in the method, at least about one-quarter -- of the capillary openings in the gas diffusing surface at which bubbles are formed and sheared off.
2. The liquid over all capillary openings that are present in the gas diffusing surface is substantially free of any type of flow other than partially or fully developed sub-stantially parallel laminar flow.
These two requirements must be met no matter what sheariny liquid is used in the method of this invention. In de-fining the apparatus, however, a specific shearing liquid and specific test conditions are employed, as discussed further below.
It is not required that the laminar flow be absolutely parallel,but only substantially so. Likewise, it is not re-quired that the liquid over all the capillary openings in the ~ -gas diffusing surface be absolutely free of any type of flow other than the desired flow, but only substantially free. As ;~ one skilled in the art will recognize, residual minimal per-turbations will almost always be present in the sheariny liquid because of its history prior to entering the liquid transmitting slot. Likewise, there will almost alway~ be minimal perturba-tions present along the walls of the shearing liquid transmitt-ing slot. The very presence of the nascent gas bubbles and the shearing of those bubbles will al50 produce (see Figure 4A be~
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., ~ . . ,~ .
-i ~ 1069665 t-low) minor departures from trul~ straight streamlines. In fact, the main streamlines themselves will not be wholly parallel (Figure 1~) until they reach the region of fully developed la-minar flow some distance downstream from the inlet end of the liquid transmitting slot. The skilled wor]cer will understand, however, that none of these minor variations from absolutely par-allel streamlines will be detrimental, and that substantial free-dom from unstable flow such as turbulence or vena contracta will be sufficient for the method and apparatus of this invention.
The presence or absence of the two hydrodynamic fea-tures listed above in the operation of any particular method and apparatus is readily ascertainable by those skilled in the art when the shearing ~iquid is transparent, or at least trans-lucent. In such systems the presence or absence of the defined laminar flo~ may be determined visually after release of a small amount of dye into the stream within the slot, or may be seen ~- ~ from photomicrographs which show alignments or "strings" of tiny : . . , : . .
bub~les running in the direction of the laminar flow, presumably from particular individual capillary openings.
With opaque liquids, such as sewage, the presence or absence of substantially parallel laminar flow including a partially developed laminar boundary layer is more difficult to determine, but techniques for making such a determination with some degree of reliability are known. In any event, this :
determination can be made, as a practical matter, by analogy to test~flow under similar conditions with transparent or translucent shearing liquids. A particular method or apparatus ma~y be tested, for example, by operation with a synthetic trans-parent or transLucent sewage fluid comprising an aqueous solu-~ ~tion oontaining a sufficient amount of a thickener, such as carboxymethylcellulose, to provide a viscosity similar to that :. . ' ' . ' ' -: ;
of the sewage being treated, and by the observance of the flow in such a liquid by dye, or by bubbles, as described above.
- Alternatively, one skilled in the art may, as a prac-tical matter, test a given apparatus with a readily available test liquid, such as water, to determine whether or not the de-fined laminar flo~ will prevail when the apparatus is utilized with a second liquid such as sewage. This may be important either because the liquid with which a particular piece of ap-paratus is designed to be used is opaque, or because it is not known for sure with what liquid the apparatus will be used.
'~, Demonstrating Laminar Flow, Including Partially Developed Laminar Boundary Layer, In One Liquid By Testing Another ~.......... . . ~ ..
; It is well known that laminar flow exists in any sys-tem in which the Reynolds number is below a specified critical level. In a flow system of given channel geometry and surface characteristics, the Reynolds number is directly proportional ~- ;
to the linear velocity of liquid flow and to the fluid density, and is inversely proportional to the fluid viscosity. The fluid densities of liquids of high water content are substantially .
identical and may, as a practical matter, be ignored; so that the threshhold velocity for conversion of laminar to turbulent flow in any designated apparatus is inversely proportional to the viscosity of liquid passing therethrough.
Utilizing this information, one skilled~n the art can test a given apparatus with water at 20C.,~-for example, `~ to ascertain whether the apparatus would be suitable for use , ~ ; with another liquid which has a higher viscosity, such as sewage.
, An apparàtus which provides lami~ar flow when water at 20C is , passed through the slot at a given linear velocity will also pro-vide laminar flow, including a partially developed laminar be~un--22- ~
.
. - -106g66S f ary layer, when se~Jage is passed through the same slot at that velocity because sewage has a higher viscosity and therefore a higher threshhold velocity for the beginning of turbulent flo~.
On the other hand, a velocitv which will produce tur-bulent flow with water in a given apparatus will not necessariily produce turbulent flo~ with sewage, because of the higher vis-cosity of the latter. For this reason, the test velocity at which water is passed through the slot of the apparatus in test-ing its suitability for use in the treatment of sewage may be a lower velocit~ than is to be used in the ~peration of the ap-paratus with sewage. For example, the apparatus may be tested to determine whether or not the defined laminar flow prevails when water at ~0C. is passed through the slot at a mean en-trance velocity of 10 feet per second. If laminar flow exists with watqr in a given apparatus at this flow rate, one skilled in the art can be assured, as a practical matter, that sewage containing about 1 per cent of suspended solids or about 6 per cent of suspended solids (both common forms of sewage) can pass - through the same slot, while still maintaining laminar flow, in-cluding a partially developed laminar boundary layer, at veloci-ties far above this figure, and in fact well above any practical level at which the apparatus of this invention would conceiva-bly be used.
- Accordingly, one way of defining the apparatus ^~ of this invention is to prescribe a specific test, invol-ving the use of water flowing a~ a stated minimùm velocity, that the particular construction must meet. The test pre-; scribed in the claims below is that the shape, dimensions and spacing of the gas transmitting bod~ and the slot-~efining mem-ber lying on both sides of the shearing liquid transmitting slot . .
must be adapted, when water at 20C. is flowing through the liquid transmitting slot with a mean entrance velocity of 1 . ,/
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- ~ iO69665 ~ ~
feet per second and no gas is flowing through said gas trans-mitting passages, to establish the defined substantially paral lel laminar flow. Higher shearing liquid velocities are also specified in the claims. The particular velocities specified have been found to be essential for ef~icient small bubble for-mation with water as the shearing liquid, and these results are nowhere disclosed in the prior art known to appliccant.
Apparatus that meets the test described falls within -the scope of the claimed apparatus invention, whether in its ultimate practical application it is used with ~Jater or with some other liquid as the shearing liquid. The validity of this method of definition of the apparatus of the present in-vention is supported by the "extrapolation," as it were, from one liquid (water) to a second, more viscous liquid (such as 6 per cent suspended solids sewage) in the manner that has been described above, to demonstrate that the defined laminar flow is present if the second liquld is employed, just as it is when the first liquid is used as the shearing liquid.
It has been found that the two hydrodynamic require-ments that are characteristic of the apparatus of thls inven-tion are present, and satisfactory results in terms of bubble size and uniformity of bubble size will be produced,when the average test velocity across the inlet end of a slot of typical .
width, with water at 20C. used as the shearing liquid, is 10 feet per second. Improved bubble shearing can be achieved with apparatus that produces the defined laminar flow when this test velocity is 15 feet per second, and still further improvement when the test velocity is 20 or 25 feet per second.
The same technique of proof by extrapolation may be used, if it is necessary to do so, to show that a particular bubble shearing method comes within the scope of this i~vention.
The presence or absence of the defined laminar flo~
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`~ ~ 1 ~ 5 in any given system may f desired by corroborated by secondary evidence, such as the efficiency of the gas absorption pro-cess resulting from use of the system. It has been found that for a given volume of gas introduced within a given time into a given volume of liquid, absorption is substantially more complete ~lhen the above described two hydrodynamic condi-tions are present in the method and apparatus by which gas bub-bles are formed. The converse is also true: The existence of these two conditions may be corroborated by demonstrating the greater effectiveness of the given system in comparison to the effectiveness which is obtained when the flow conditions through the slot are clearly turbulent flow conditions.
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Gas Flow Rate It is important in the method and apparatus of this invention not only that the two essential hydrodynamic reguire-; ments be present but also that the gas flow rate into the shear-ing liquid be sufficiently high. With too little gas flow, the volume of gas diffused into the liquid in a given period of time may be so small that even though the bubbles produced are of a desirable size and uniformity, the output of the apparatus is impractically inefficient and the necessary equipment for a given application prohibitively expensive.
The maintenance of desirably high gas flow rates may, however, tend to militate against the establishment of the two essential hydrodynamic characteristics of this invention. If the gas flow rate reaches too high a level, it will be impos-sible to establish the necessary parallel laminar flow and ~to ~, avoid~undesirable types of flow such as vena contracta or turbu-lence above the capillary openings in the gas diffusing surface.
The particular gas flow rate that is desirable for a given ap-paratus and for a given set of operating conditions can bes~ be determined empirically by one skilled in the art, keeplng in -~ ~
- , . - ~ . - : . . :, ~069665 mind all the many factors that affect bubble shearing.
In resolution of these conflicting objectives of high gas flow rate and substantially parallel laminar flow as des-cribed, it has been determined that with typical shape, dimen-sions and spacing for the members defining the shearing liquid transmitting slot of the apparatus of this invention, and water as the shearing liquid, the gas flow rate should be at least about 5 to 10 standard cubic feet per minute per square foot of active area in the gas diffusing surface. As a practical matter, this rate of oxygen flow employed with a water flow velocity of 20 feet per second and a water volumetric flow rate of 2.5 gal-lons per minute per inch of slot inlet length will produce a transfer of oxygen into water of about 0.5 to 1 lb.~HP/ hr., an oxygen transfer rate that is characteristic of a number of sewage treatment installations. Improved results are obtained if this gas flow figure for water as the test liquid is about 4~ scfm, and still better r~sults if it is about 70 scfm.
The term "active area" i5 used in this specification and claims to refer to the area defined by lines tangent to the outermost capillary openings at which bubbles are formed.
"~,~ , -Conventional Precautions To Achieve Substantially Parallel Laminar Flow It is also important in the practice of this invention I to take as many reasonable precautions -- in view of the width;l of the liquid transmitting slot, the length of flow through the slot, and the viscosity and flow velocity of the~`~hearing liquid with which the apparatus is designed to be operated -- as are necessary to avoid turbulence or vena contracta and achieve sub-I stan~ially parallel laminar flow in the slot.
With water flowing at a velocity of at least about 10 feet per second as the shearing liquid, for example, one s~illed ,\
' - , : , ' ' '~ 106g66i5 in the art will understand that if the length of flow through the liquid transmitting slot is more than about 1/2 inch, some attention must be paid to taking reasonable precautions to avoid the establishment of turbulent flow in the slot. The necessity to take care to avoid vena contracta increases when the liquid flow path is quite short. On the other hand, it is well known that greater care must be taken to avoid turbulence, the wider the liquid transmitting slot or the longer the liquid flow path through the slot. Increased care must also be taken when lower ; 10 viscosity shearing liquids or higher shearing liquid flow velo-cities are employed.
Conventional precautions against the occurrence of vena contracta include,in addition to avoiding too short a flow path, avoiding openings of too large size through which gas is intro-duced into the shearing liquid, and, most important of all, em-ploying rounded edges at the slot inlet. Conventional precau-tions against the ~ occurrence of turbulence include utilizing contoured inlets, avoiding anything more than minimal protuber-ances, avoiding rough channel walls, exercising special control of liquid flow into the inlet to minimize the initial level of perturbations, etc.
The roughness of the channel walls may be expressed in terms of (1) the height of the most prominent peaks and the depth of the lowest depressions, respectively, above and below the median plane of the channel wall, compared to (2) the dis-placement thic~ness of the laminar boundary layer in the liquid flowing along the wall. To minimize the tendency for the liquid flow to go turbulent, the former dimensionsshould ordinarily be only a small ~-~ction of the latter.
Conventional measures to control the liquid flow into -' . ' '.
,,~ \ :
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- ~ 1069665 the slot inlet in order to minimi~e the level of perturbations at that point include the use of a quietlng reservoir to dissi-pate any turbulence already present, avoiding abrupt corners in the feedJline leading to the slot inlet, using turning vanes when-ever turns are unavoidable, avoiding steps in the flow path, avoiding orifices in the flow path, using pumps and valves only ~ -when absolutely necessary, and similar measures designed to pro-duce as quiet a flow condition as possible.
Examples of Parallel Laminar Flow Covered By This Invention Best results are achieved with the method and appara~
tus o~ this invention when thése additional conditions obtain:
-- The partially developed laminar boundary layer ex-tends along the entire length of the gas diffusing surface --even those portions where there are no capillary openings --over which the shearing liquid flows; and -- All the nascent gas bubbles are sheared from the gas diffu~ing surface by the combined action of the liquid mov-ing in the partially developed laminar boundary layer and the li~uid moving in the free stream above that layer.
These particular conditions are illustrated schematic-, , .
I ally in Figure 4~, which represents a preferxed form of this in-j vention. This figure, like Figures lA and lB above and Figures 4B through 4F below, is greatly enlarged for clarity. In an ac-11 tual embodiment of the apparatus of this invention, the ~idth of ,~ slot 40 is about 0.040 inches, and the length aboht 0.080 inches.
I The capillary openings in that embodiment are much smaller and ¦ more numerous than are shown schematically in Figure 4A, al-though not located so close to leading edge 61.
Wall 42 is a gas transmitting body having a plurality ... . .
of gas transmitting passages 66 extending therethrough, each ~\
.~ '' '' ~'.: :
--~ 106S6f~5 of which terminat~s in a czpillary opening 65 at gas diffusing .: :
sur~ace 68 of the wall. The appar~tus includes means for pro- :
viding a f low of gas through gas transmitting passages 66 and ~:
out capillary openings 65 to form nascent gas bubbles 94.
Wall 44, spaced from gas diffusing surface 68, de-fines slot.40, which has an inlet end 46 and a discharge end 48.
The body of liquid 96 into which the fine bubbles produced by the use of this invention are diffused lies to the right of slot 40 in Figur~ 4A.
Laminar flow substantially parallel to the contour of gas diffusing surface 68 is established in the liquid flow-ing through slot 40. Because capillary openings 65 are lo-cated in close proximity to leading edge 61 at inlet end 46 of slot 40, they lie beneath partially developed laminar boundary layer 54. Free stream 56 is established above la- :~
minar boundary layer 54 over the capillary openings. As in .
the earlier figures, streamlines 50 illustrate the substan-tially parallel laminar flow from left to right.
:. The shape, dimensions and spacing of gas transmitting body 42 and slot defining member 44 are adapted, when water is : ~ employed as the shearing liquid under the conditions specified above, to establish laminar flow of the kind described above in :
I the water that flows through slot 40. In addition,.the appa-ratus includes means for providing a flow of liquid through slot 40 at a velocity that will produce in water that type of .
; substantially paràllel laminar flow.
i ~he result of use of this apparatus is to shear na-scent gas bubbles 94 from capillary openings 65, so thatYthe ~ :
flow of liquid through slot 40 carries the sheared gas bubbles . 30 out of the.slot at its discharge end 48 and into the body of ~r ~
.
~ -29- .~
, .
., , , , . -. . . : :
~069665 ~ ~:
liquid 96. As is shown in Figurc 4A, all bubbles sheared in this preferred embodiment of the apparatus of the invention are sheared from capillary openings 65 by the combined action of the liquid moving in partially developed laminar boundary layer 54 and the liquid moving in free stream 56 above the boundary layer. The surface tension at the juncture between the surface of the gas bubble and gas diffusing surface 68 surrounding each capillary opening 65 tends to cause each bubble to adhere to surface 68. However, the movement of liquid from left to right through slot 40 in Figure 4A shears the nascent gas bubbles off surface 68 against the force of the surface tension referred to, and in this manner produces gas bubbles of very much smaller diameter than would be formed if it was the buoyancy of the bubble as it reached its full growth that broke the surface tension holding the bubble to the gas diffusing surface.
Since capillary openings 65 are in close proximity to leading edge 61, partially developed laminar boundary layer 54 has little time in which to build up into a thick layer. When ~o the nascent bubbles g4 protrude above interface 52 between la-minar boundary layer 54 and free stream 56, they are exposed to the full force of free stream 56, which is moving with I greater velocity than the liquid in lamlnar boundary layer 54. For this reason, the nascent gas bubbles are sheared of before they can grow to any very large diameter. As an ~ example, the method and apparatus of this invention have been I used to produce bubbles of air in water some ~f which are ~n as small as 10 microns in diameter, with the median diameter being no more than about 25 to 30 microns. As illustrated schematically in Figure 4A,the gas bubbles grow slightly ;~
-30_ , ,, ,. . .
larger as one procee~ downstream and the partially developed laminar boundary layer increases in thickness.
10As is seen ~rom Figure 4A, partially developed lamin-ar boundary layer 54, with free stream 56 above it, extends be-yond all capillary openings 65 in gas diffusing surface 68 to discharge end 48 of slot 40, and free stream 56 extends still farther,into body of liquid 96. The same is true o~ the embodi-ments of the apparatus of this invention illustrated in Figures 4B and 4C, where the same designator numerals are employed for the various elements as in Figures lA, lB and 4A. .:
The bubble shearing achieved by the apparatus illus-trated schematically in Figure 4B is only slightly l~ss effec-tive than the bubble shearing achieved by the apparatus ofFigure 4A. In the apparatus of Figure 4B only some, but not all, of nascent gas bubbles 94 are sheared from capillary open-ings 65 in gas diffusing surface 68 by the combined action of the liquid moving in partially developed laminar boundary layer 54 and the liquid moving in ~ree stream 56. The remainder of nascent bubbles 94 are sheared from capillaries 65 solely by the action of the liquid m~ving in laminar boundary layer 54.
Because laminar boundary layer 54 is very thin, the embodiment of the apparatus of this invention shown schematical- : :
ly in Figure 4C likewise produces good results, although its use is somewhat less advantageous than use of the embodiments illus-trated in Figures 4A and 4B. In the apparatus of Figure 4C, all :
~, nascent gas bubbles 94 are sheared from capillary'openings 65 solely by the action of the liquid moving in pàrtially devel-: oped laminar boundary layer 54.
.
:Figures 4D and 4E illustrate schematically embodi-; ments of apparatus of this invention in which there are both .
: .
- - . : . . -. : .. - :
- ' ~
- ~ . . , ' .
~ t 1069665 ~_ partially developed and fully developed laminar flow wlthin the shearin~ liquid transmitting slot. The same numerals are employed to designate similar elements in these figures as in Figures 1~, lB and 4A through 4C.
' Figure 4D shows partially developed laminar boundary layer 54 as it merges with upper laminar boundary layer 60 at point 62 in a zone lying between capillary openings 65 and dis-charge end 48 of slot 40. In this embodiment, the surface of wall 42 lying beneath the fully developed laminar,flow to the right of point 62 in Figure 4D is free of any capillary openings.
Figure 4E shows a similar embodiment, except that there are addi-tional capillary openings lying beneath the fully developed la-mainar flow to the right of point 62, where the two partially developed laminar boundary layers 54 and 60 merge.
The embodiment of the apparatus of this invention ', shown schematically in Figure 4F displays some turbulence within the shearing liquid transmitting slot, but the basic advantage of having the capillary openings in close proximity to the leading edge of the gas diffusing surface to bring them under , 20 a partially developed laminar boundary layer still produces ~ useful results. In this embodiment, in which similar elements ', are identified by designator numerals similar to those employed ,1 in the preceding f igures, partially developed laminar boundary layer 54 lies immediately adjacent gas diffusing surface 68, with free stream 56 above it. Boundary layer 54 terminates ~ ~s" ~ .
-, in turbulent zone 98 adjacent dischaxge end 48 o'f~liquid trans-mitting slot 40 and within the slot. However, the surface of j wall 42 beneath turbulent zone 98 is free of any capillary ' openings. The same condition prevails in the upper part of slot 40, where partially developed laminar boundary layer 60 terminates in turbulent zone 100 short of discharge end 48 of ~, ' ' '\ . ' ~ , 3,2~
, . .. .
06~665 slot q0. The harmful effect of turbulent zones 98 and lO0 in tending to cause collision of ~he fine gas bubbles and their resulting coalescence into larger bubbles is minimized if free stream 56 extends, as shown in Figure 4F,to the end of the slot.
In the apparatus of Figures 4D and 4F, nascent gas bubbles 94 are all sheared from capillary openings 65 in gas diffusing surface 68 solely by the action of the liquid moving in partially developed laminar boundary layer 54. In the em-bodiment of Figure 4E, some of nascent gas bubbles 94 are sheared from capillary openings 65 solely by the action of the ;.
liquid moving in partially developed laminar boundary layer 54, while some are sheared solely by the action of the liquid moving in the fully developed laminar flow to the right of , point 62 near the discharge end 48 of slot 40.
' Apparatus Defined In Terms Of Product . . Of Maximum Distance From Leading Edge To Farthest Downstream Capillary Open-ings Times Average Slot Width The apparatus of this invention may also be defined in terms of a "figure of merit" that is the product of (a) the . distance separating the inlet end of the liquid transmitting slot from the very small capillary openings that lie farthest downstream on the gas diffusing surface over which the shearing . ~ - .
liquid flows, times (b) the average width of that slot through-out the indicated distance.
As already mentioned above, satisfactory results may :~
be obtained in some applications when this figure of merit is no more than about 0.1 square inches. Somewhat improved results : . are generally obtained when the figure of merit i9 no more than ,~ . .
about 0.075 square inches~ and still further improvement is ob- -:~ tained when it is no more than about 0.05 square inches. Very -. . .
~ . -33-~ , . . . , :
- , . .. ..
good results are obtained when the figure is no more than about 0.025 square inches, and excellent results when it is 0.01 square inches. For some applications, the preferred product of the indicated width and distance is no more than about 0.005 square inches, or even less.
Applicant has discovered, as has been explained, that contrary to the teaching of the prior art the best formation of fine bubbles is achieved if there is substantially no turbu-lence over any capillary openings in the gas diffusing surface over which the bubble prGducing liquid flows. This discovery makes it possible to formulate the figure of merit just defined.
The critical Reynolds number for the distance down-streàm from the leading edge of a flat plate at which transi-tion to turbulence will occur in liquid flowing across the plate at any given velocity is approximately 350,000. This number also applies to the flow through a parallel flat wall slot, so long as the slot width is at least as great as the fig-ure computed from a second critical Reynolds number, i.e., the crltical Reynolds number for slot width. The following 1~ 20 two equations, which include the two critical Reynolds numbers referred to, relate the velocity of liquid flow to the vis-cosity of the liquid and two important dimensions of the slot:
(1) y ~ 350,000; and (2~) ~^W ~ 2,pO~;
where U - the free stream velocity of the shearing liquid, x =
the distance~from the inlet end of the shearing liquid trans-mitting slot to the farthest downstream capillary openings above which~no turbulence wil~l occur, w = the critical slot wLdth for tur~bulent~flow, and Y = the kinematic viscosity of the shearing 30~ iquid.
1 :
~:: f ~ ~
~069665 (The first critlcal Reynolds number of 350,000 refer-red to above is indicated by Schlichting, ~oundary-Layer Theory, Sixth Ed. (McGra~-Hill, 1968), page 435, to be appropriate for apparatus in which only reasonably conventional precautions are taken -- as would usually be true in actual bubble shearing ap-paratus -- to achieve substantially parallel laminar flow. The second critical Reynolds number of 2,000 referred to above is based on experimental work reported in an article by D. Meksyn, "The Stability of Laminar Flow Between Parallel Planes for Two-and Three Dimensional Finite Disturbances," Zeitschrift fur Physik, vol. 17~, pp. 159-172 (1964).) These two equations, together with a third equation to be set forth below, were used to determine the critical dis-tance from the leading edge of the gas diffusing surface of the apparatus of this invention fox the most remote capillary open-ing over which no turbulence will occur when water is used as the shearing liquid and the shearing liquid transmitting slot is definea by parallel planar walls. In these determinations, since the goal o bubble shearing is to produce gas bubbles of very small diameter, capillaries of 20, 30 and 40 micron di-ameters were examined. The bubbles from these capillary open-ings were assumed to be 50 microns in diameter. Three typical widths of the shearing liquid transmitting slot were employed --0.5, 0.1 and 0.040 inches.
l The material defining the capillary openings is also I important. The bubble shearing ~chieved by the method and appa-~ ratus o~ this invention will be most effective when the material i surrounding the capillary openings in the gas diffusing surface has a low wettabilityl or in other words a high contact angle, ~30~ with the shearing liquid. Because the effectiveness of the in-vention in achieving very fine gas bubbles will vary consider-- - ., - ,.. ..
: .:: . : .
: :
--~~ 1069665 ably ~ith this factor, determinations were made for apparatus in which the material defining the capillary openings has a contact angle with the shearing liquid of 20 and 45, respec-tively. Under ordinary operating conditions, impurities con-tained in tap water form a coating on materials such as various metals and glass, and as a result such materials have a contact angle~of approximately 20 with the tap water. Contact angles -~
of 90 will give the best results with this invention, but no determinations were made for this figure. Materials having a contact angle with tap water of approximately 45 include, for example, polyvinyl chloride and polymethyl methacrylate. Ma-terials having a contact angle with tap water of approximately 90 include, for example, fluorosilicones and fluorocarbon plas-tics such as the material sold under the trademark Teflon.
.
- With the indicated assumptions, the velocities of shearing liquid required to produce bubbles of the predeter-1~ mined sizes from the given sized capillary openings were deter-1 mined as conservative estimates by the following equation:
~: ~, w~ ~d ~05 U t7æ ~ r2~ (3 vv~
where U = the free stream velocity of the shearing liquid, ~ -¦ ` the slot width, ~ = the surface tension of the shearing li-quid~ d =~the capillary opening diameter, ~ = the contact angle botween the shearing liquid and the material surrounding the capillary openings, ~ - the dynamic viscosLty of the sh~eari~ng~liquidj and r ~ the radius of the bubb ~ formed by use of the apparatus of this invention. j ~
, . ~ .
~ ~ With the resulting values from equation (3) for the ~ : :
velocity of the shearing liquid under the conditions indicated, equat~on~(l) above was employed to determine for each situation 30~ the~critical maximum distance from the gas diffusing surf~ce . ,/
~',~' ~ ` ' .
106g665 leading edge to the most remote capillary openingsover which no turbulence will occur. E~uation (2) was then used to confirm that for each situation the slot width was great enough under 211 the respective conditions to permit turbulence to occur just beyond the computed critical distances.
Table I below shows the maximum distance in inches from the leading edge of the gas diffusing surface to the farthest downstream capillary openingsover which no turbulence will be produced, for the indicated parameters and water as the shearLng liquid:
, ', `~, ., , ' . - .
-37~
' ' ,,/ ' , ' .
, . . : . : . : : :
` ~ 106~6~5 (-I ~ I U~ ~ o~
l oo o l o o o I ~ ~ o~ I ~ '` o I o o o o I o o o o ~ ., I ,.' 'u~ o o I ~ o o o o I o o o ~ I
o ,~ ~ ~ o I _, ~ o o o . . . U~ I o o I N ~ O O O ~ O = O
H ¦ . ~ O _i O ¦ O -i 0 ~1 I
~ l _l . l n ~ ~ ".
a: . O O O ¦ O
O O
~
~r O ~ ~. O O ~
O
. ~ ~
O U~ O . O 00 . - . .
O ~1 0 O N O
X - X
~U N ~ ~ a) N S: E3 ~ r~ ~I h (n ~ :
U~ ~ S,~
O~ aJ ~J O U U~ O~ O~
U ~ ~ ~ ~ U ~ t~
,I v 3 c~ 3 \.
o o' `~
~ ~ .
"
- ,, ' -' ' ' ' ,, ~ - :' - : :
.
.: . . : ' ~
.
1069665 ~ :
As the information in tiliS table confirms, the maximum distance from the inlet end of the shearing liquid transmitting slot to the farthest do~stream capillary openings that can be employed without producing turbulence over any capillary open-ings (referred to in the table as the distance "x") depends upon a ~ariety of factors, inc'iuding the slot width. The table indicates that satisfactor~ production of fine gas bubbles can generally be obtained even if the distance "x" is substantially more than 1 inch, if the product of the distance "x" times the ' 10 average width "w' of the liquid transmitting slot throughout that distance -- the defined "figure of merit" w~ -- is no more ~ ' than about 0.1 square inches. (For width "w" and distance "x,"
see Figure 12.) Thus, with a small enough slot width, small bubbles of 50 microns diameter can be produced with apparatus for which the distance "x" is more than 1 or even 2 inches, if the figure of merit is only slightly above 0.1, at 0.114 or 0.115, the contact angle between the material surrounding the capillary openings and water is 45, and the diameter of the capillary openings is 20 microns.
Improved shearing results are obtained when the figure of merit is reduced to about 0.75. Thus, production of the same 50 micron bubbles is possible with a figure of merit of 0.074 to 0.076 even though the capillary diameter is increased to 30 mi-crons. Similarly, it is possible to produce 50 micron bubbIes with a figure of merit of 0.063 to 0.065 even though the contact angle is reduced to the less effective value of ~0.
Still further improvement in bubble shearing is ob- ~
tained when the igure of merit of the apparatus used is sub- ~ ' stantially lower, i.e., no more than about 0.05. As'the table .
shows, bubbles of 50 microns diameter can still be produced when .
.
-39- ' \
.
: , , : , ' . , .
106~66S
the ~igure of merit is just about 0.05, at 0.055 to 0.057, even though the capillary size is increased to 40 microns. And, likewise, the size of the bubbles can be maintained at 50 microns with a figure o~ merit of 0.042 to 0.045 even though the contact angle is reduced to the less effective value of 20.
Very good results are obtained when the figure of merit is about 0.025. Thus, when the figure of merit is some-what above that figure, at 0.030 to 0.032, bubbles of 50 microns diameter can be produced without the necessity of using material 10 to define the capillary openings with a contact angle with water that is any better than 20, ~ven when the capillary opening di-ameter is 40 microns.
The trend of the data in the table indicates that the bubble shearing rasults described will be improved if the de-fined figure of merit is still further reduced to 0.01, or lower.
In one embodiment of the apparatus of this invention that pro-duces excellent bubble formation with sewage containing 6 per cent suspended sollds as the shearing liquid, the width o the liquid transmitting slot at its entrance end and throughout its 1~ ~20 length is about 0.125 inches, and the dis~ance from the inlet ~l end~to the farthest capillary opening is about 0.045 inches, making a figure of meri.t of about 0.0056 square inches. In another apparatus according to this inv~ntion that achieves equa-lly impressLve bubbLe shearing with water as the shearing liquid, the~slot width is about 0.04 inches and the distance "x" is a-bout 0~045 inches, making the figure of merit even less, about 0.0018 square inches, If apparatus constructed according to this invention has~a figure of merit as defined in this specifica~ion ("w" times ~h~
~30 ~ ";x"~) of O~.L~square inches, and the inlet end of the liquid trans-~ -40-: 1~:' ' , ' ' 1~ , ''`~ 1C1696f~5 mitting slct has a ~idth "w" of 0.1 inches, the distance "~" from the inlet end to the fartllest downstream capillary openings on the gas diffusing surface of the gas transmitting body is 1 inch. With a figure of merit of 0.05 square in-ches and the same 0.1 inch slot ~idth, the distance "x" is 0.5 inch. A figure of merit of 0.01 square inches and a slot width of 0.1 inches makes a distance "x" of 0.1 inch.
Wi~h the same slot width, a figure of merit of 0.005 makes a distance "x" o~ 0.05 inch.
The computations on which Table I above is based were made for a shearing liquid transmitting slot defined by parallel planar t~alls, but the values for the corresponding figure of merit for an annular slot are of comparable magnitude. Equations (1), (2) and (3) above may be used for an annular slot when the mean diameter of the slot is considerably larger than the slot width -- which will ordinarily be the case with bubble shearing apparatus -- since such a slot is equivalent to a slot that is first defined by parallel, planar walls and is then curved around upon itself.
- If the transverse cross section of the shearing liquid transmitting slot is circular, as for example in the case of a slot having the form of a right circular cylinder, the "average width" of the slot may be considered to be equal to the avera~e radius of the slot in the area of the capillary openings of the gas diffusing surface, tThis follows from the fact that the perpendicular distance between the walls of a parallel planar wall slot is hydraulically equivalent to the radius of a slot having a circular cross section.) The term "average width" of the li-quid transmitting slot is thus used with this meaning in this specification and claims for any slot of circular cross section, i.e., the average radius of the slot in the indicated area.
~.
.` ~1 , :
, ~069665 As already pointed out in some detail above in the dis-cussion of the definition of the apparatus of this invention by use of a specified hydrodynamic test, it will be understood by those skilled in the art that one practicing the present inven-tion as defined by the stated figure of merit should not only uti-lize the figure of merit, but should also take reasonable care to avoid including in the bub~le shearing apparatus any structural features(such as rough slot walls) that ~ould tend to produce turbulence -- or any other structural features (such as too low a ratio between the length and the width of the slot) that would tend to produce vena contracta -- with the particular shearing liquid and particular liquid velocity for which the apparatus is designed. It will be further kept in mind by the skilled worker that the effectiveness of a particular figure of merit in producing good bubble formation will always depend to some extent on the nature of the shearing liquid to be used t~ith the apparatus.
As has been pointed out above, prior to applicant's im-proved bubble shearing invention, the great advantages of utiliz-ing the very effective viscous shearing forces available in thetop portions of, and directly above, a partially developed lami-nar boundary layer in the shearing liquid flowing over a gas dif-fusing surface were ~holly unappreciated by those skilled in the art. Because o~ this, there was no recognition of the importance of using a narrow liquid transmitting slot (a low value for llt~JII) to establish a partially developed laminar boundary layer over a significant portion of the capillary openings in the gas diffus-ing surface, nor of the importance or even the possibility o lo-cating very small capillary openings in the gas diffusing surface in close proximity to the leading edge or inlet end of the slot - .
;42-- ~ ~Oti966S
(a low value for "x") in order to takeiadvant~ge of the thinnest portion of that partially developed boundary layer. ~ -As a consequence, no one knew the impor~ance of the low value of the "figure of merit" with very small capillary openings that provides one definition of the apparatus of this invention. Applicant knows of no one in the prior art who has, even accidentally, given any hint or suggestion of this con-c~pt.
. .:
Maximum Distance From Leading Edge To Most Remote Capillary Openings The information in Table I above also shows that this invention may be defined alternatively solely in terms of the maximum distance from the leading edge of the gas diffusing surface to the most remote capillary openings in that surface (distance "x"). This feature s~anding alone is an important aspect of the present invention.
The data relating to a contact angle of 20, for ex-ample, in the upper half of Table I shows that good bubble shearing can be obtained when the defined distance is no more ` 20 than 1 inch. Improved shearing results are obtainable when the distance is no more than about 0.5 inch, and still better if it is no more than about 0.1 inch, 0.05 inch, or even lower.
, V2rious Factors Affecting j _ Bubble Size A number of factors have been discussed that will affect the results achieved with the method and apparatus of the present invention. Some approximate data on various fac- ;
tors that affect the size of the gas bubbles produced in bubble shearing are presented in the form of graphs in Figures 5 through 10. In these graphs, bubble diameter is plotted against the following parameters:
:
-43- ~
-~ 1069665 ~ ~ :
TABL~
Figure Independent variable S Distance from leading edge of gas diffusing surface to most remote capillary openingS
6 Diameter of capillary openings 7 Contact angle between water and the material of which the gas diffusing surface is formed .~ ~''.
8 Water velocity through the shear-ing liquid transmitting slot 9 Surface tension between gas bubbles and various shearing liquids Viscosity of the shearing liquid The conditions for which the determinations on which the respective graphs are based were made are shown in the fol-~owi~g table:
~- TABLE III
Figure 5 Water velocity 20 ft./sec.
Capillary s~ze (median I.D.) 8 microns Sur~ace tension 50-60 dynes/cm.
Contact angle 20 (approx.) Viscosit~ ~.Ol i~p.
Figure 6 Water velocity 20 ft./sec.
Surface tension 50-60 dynes/cm.
Contact angle 20 (approx.) 30 Viscosity 0.01 cp.
Leading edge to most .045 inch remote capillaries ~~ ~~~~
~ ~ 106966S ~ ~
Fiaure 7 Water velocity 20 ft./sec~
Capillary size (median I.D.) 8 microns Surface tension 50-60 dynes~cm.
Viscosity 0.01 cp.
Leading edge to most .045 inch remote capillaries Figure 8 Capillary size (median I.D.) 8 microns Surace tension 50-60 dynes/cm.
Contact angle 20 (approx.) j. . Visc05 ity O.Ol cp.
~; Leading edge to most .045 inch remote capillaries s ; Figure 9 ` Water velocity 20 ft./sec.
, Capillary size (median I.D.) 8 microns Contact angle 20 (approx.) Viscosity O.Ol cp.
Leading edge to most .045 inch -remote capillaries Figure 10 i~ . :
Water velocity 20 ft./sec.
f Capillary size (median I.D.) 8 microns ~ Surface tension S0-60 dynesjcm.
1: :
Contact angle 20 (approx.) Leading edge to most .045 inch remote capillaries : 1 .
~ 45 .: .
-~r 1069665 ~-Specific Em~odiments Of Bubble Shearin~ ~pparatus Figures 11 thxough 14 illustrate specific emhodi-ments of the bubble shearing apparatus or gas diffuser of this invention.
Diffuser Asse~ly Although a diffuser assembly 110 constructed in ac-cordance w-th the present invention can be utilized in many different environments, the diffuser assembly is shown in Figure 11 at the bottom 112 of a lake or large body of polluted water 114. The polluted water is deficient in oxygen and this hampers normal life processes required to support fish life and to maintain the proper sanitary conditions in lake 114. By dissolving oxygen in the water, the natural processes of water purification are accelerated. A method and an apparatus for supplementing the natural process of water purification by the addition of oxygen to the water is described in United States Patent No. 3,505,213 to Anthony and Fulton and entitled "~lethod and Apparatus for Purifying a Natural Body of Water." Although it is contemplated that diffuser assem~ly 110 will advantageous-ly be utilized to promote the absorption of oxygen by bodies of water, it should be understood that it can be utilized to ; promote the abso-rption of other gases by other bodies of liquid as well.
Diffuser assembly 110 includes a plurality of units or , . .
` modules 118 having substantially vertical and parallel discharge i '~ openings or slots 120 from which a mixture of water and small bubbles 122 (Fig. 12) of oxygen flow into lake 114 to oxygenate the lake. Slots 120 may if desired be disposed at other angles to the lake bed, but the vertical orientation is preferred in ; order to permit fresh supplies of water for oxygenation to rise ,i , .
'~
.:. , , .. : , , . . :.
, . . , . , , . ::
069665 r between the slots most readily from the lower levels of the body of water.
Small bubbles of oxygen 122 are dispersed into a relatively large area of the lake and rise slowly toward the surface of the lake. As bubbles 122 rise, the oxygen within the bubbles is absorbed by the oxygen deficient waters of the lake. Since bubbles 122 of oxygen rise slowly and are very small, with a relatively large surface area per unit volume of oxygen contained~within the bubbles, substantially all of the oxygen is absorbed as the bubbles rise toward the surface of lake 114. If the bubbles were relatively large, they would ascend,quickly toward the surface of the lake so that there would not be sufficient time for the oxygen to be absorbed.
This could result in a "bubbling-off" or dissipation of the oxygen to the atmosphere. Of course, dissipation of oxygen ; into the atmosphere increases the cost of producing the de- sired oxygen content in the water of lake 114.
The modules or units 118 are connected with a common ~ource of oxygen under pressure by a gas main or line 124 (Fig-, ure 11). Gas main 124 is connected with gas disperser assem-blies or "bars" 128 in each of the units 118 by feeder lines 130. In addition, each of the units 118 is supplied with water under pressure by a common main or pipe 132 to which the dif-, ,fuser units 118 are connected by a base plate 134. A suitable pump with adjustable output velocity, schematically shown at 133, may be associated with main pipe 132 and opèr~tes to draw water from lake 114 and direct it at various velocities, as ,~
desired, along pipe 132. A suitable filter may be used to prevent solids from reaching the pump or the disperser as-... - . ........................... . .
~30 semblies 128. Thus, when the diffuser assemblies 118 are being , utilized to oxygenate lake 114, the diffuser assemblies arè
-~7-, : ~, ` ~ ~.06966S f continuously supplied with gaseous oxygen by gas line 124 and are continuously supplied ~ith wat~r by water ~ain 132. Al-though gas line 124 and water main 132 have been sho~m in Figure 11 resting on bottom 112 of the lake, it should be understood that they could be suspended or otherwise supported above the bottom of the la~e, if desired.
Extremelv fine bubbles 122 are formed on each side of slots 120 defined by the gas disperser assemblies or bars 128 (see Figure 12). Thus, small bubbles 122 are formed in groups of bubbles 142 and 144 which extend for the ull length of opposite longitudinally extending side walls 146 and 148 of slots 120, each of which slots is approximately 12 inches in ; vertical length in one embodiment of this apparatus. The bub-bles are swept away by water which flows in a continuous stream from chamber 149 through slots 120 into the lake. Water chamber 149 is connected in fluid communication with water line 132 by passages (not shown) extending through mounting plate 134 (~igure 1).
.. ..
Because substantially the only effective bubble forming force in this apparatus is the viscous shearing force of the liquid flow, and buoyancy plays essentially no role, the bubbles are swept away before they grow too large. The bubbles produced are so small that it is difficult or impossible for the observer to distinguish individual bubbles by the naked eye, and the bubbles thus take on the appearance of a "gas fog" or milky cloud of bubbles which extend out~qardly for a , substantial distance from-diffuser assembly 110. Since the ., 810wly rising bubbles 122 are dispersed over a relatively large area of lake 114, the gas in the bubbles can be absorbed by the water of the lake before the bubbles reach the surface . , ~ .
of the lake.
~ ~ .
., .. .. . . .
~ 106g6~5 Bubbles 122 are formed at open ends 154 of capillary passages 158, which extend from gas plenum 160 through side walls 146 and 148 of slot 120 tFigure 12). Each of the up-right gas cllambers 160 is connected in fluid communication with gas line 124,so that a continuous stream of gas under pressure flows from line 124 through feeder tubes 130 (Figure 1) to gas plenums 160, and then to open ends 154 of capillary passages 158.
It ahould be pointed out that this invention is use-n ful in applications, as for example some industrial applica-tions, in which an entire quantity of liquid into which gas bubbles are to be diffused is passed through pump 133, main liquid line 132, liquid chamber 149, and slots 120. In such a case, the term "body of liquid into which fine bubbles are to be diffused" refers to all the liquid that has already flowed through slots 120 of the gas diffusing apparatus and has been accumulated in a subsequent pipe, channel, or other vessel or further handling.
Shearing Liquid Transmitting Slot As water leaves chamber 149 to enter any given slot 120, it flows past rounded leading edges 150 on walls 146 and 148. When water is the shearing liquid ~as in Figure 12) the length of travel through slot 120 should preferably be at least about two times the width of the slot,although it may be less for more viscous liquids. These features, especially the rounded leading edges, will help to establish in slot 120 the substantially parallel laminar flow, including a partially ,de- -veloped laminar boundar~ layer, that is characteristic of this invention. The L/D ratio should not be made too large, however, or there will be too high a pressure drop and accompanying~
.
,, '~ "
., .
:, ' 065~ 5 encrgy requirement for the apparatus, and a clogging problem with some applications such as sewage treatment.
As illustrated in Figure 12, each slot 120 throughout its length should preferab]y have a substantially uniform width measured perpendicular to the direction of flow. In the em~
bodiment shown, slot 120 has a rectangular transverse cross section from its inlet end to its discharge end. ~s will be seen, a substantially straight, unimpeded path is provided for liquid flow through the slot, by utilizing side walls 146 and 148 that are substantially planar and parallel to each other. These features help to produce regularity and stability of flow of the shearing liquid through the liquid transmitting slot, t~hich encourages the establishment of the defined la-minar flow regime.
As already indicated above, the width of slot 120 may be varied depending upon a number of factors including,among other things, the nature of the shearing liquid. A slot width of 0.5 inches or even more may he used with shearing liquids of high viscosity, although with wider slots greater precautions may have to be taken to achieve substantially parallel laminar flow through the slot. A slot width of 0.125 inches has been used ~lith sewage contaîning 6 per cent suspended solids. A slot wi~th of 0.040 inches has been used with 1 per cent suspended solids sewage, and a slot ~Jidth of 0.030 inches or even as 10W
as 0~020 inches with water as the shearing liquid.
Other things being equal, the slot widt~ "w" (Figure 12) should be wider for larger capillary openi~gs,so that col-lision and coale.scence of fine bubbles to form larger bubbles ` can be minimized. The slot width may be made wider than would s 30 otherwise be feasible when the shearing liquid employed, such ~;~ as for example sewage, has a higher viscosity. This will have the advantage of helping to minimize clogging of the slot b~
: ~ \
. "
: .
, . . ~ . . ~ : , -` ~ iO6966S
particulate matter in suspension in the sewage. In the design of an apparatus according to this invention, the considerations mentioned must be balanced against the fact that the smaller the width of slot 120 is made, the lower the po~er input will be for a given velocity of shearing liquid.
It should be noted that each elongated opening or slot 120 of diffuser assembly 110 has a gap which can be easily set during manufacture of the diffuser assemblies to provide the desired relationship between the width of the stream of water flowing through the slots and the gas bubbles formed at the sides of the slots. Or, if desired, the width of the slot may be left adjustable -- as, for example, from zero to some pre-determined width -- so that it may be changed for various conditions of use of the apparatus. If the width of the slot is adjustable through a predetermined range, the present inven-tion concept is utilized if either the defined substantially parallel laminar flow or the specified "figure of merit" is pre-- sent for at least a portion of that range of slot widths.
Figure 13 is a schematic cross sectional view of another embodiment of a gas transmitting body and a slot-defin-ing member useful in the apparatus of this invention. In this ;~ embodiment, cylindrical gas transmitting body 172 has a plu-rality of gas transmitting passages 174 extending therethrough, each of which passages terminates in a capillary opening 176 at gas diffusing surface 178. Annular shearing liquid trans-mitting slot 180 is formed by cylindrical slot-~efining member 182, positioned concentrically with member 172. As with the embodiment of Figure 12, gas is introduced into gas plenum 160 within member 172 and fl~ws out through capillary passages ~30~ 174 to gas diffusing surface 178, where nascent gas bubbles are formed at capillary openings 176 and are sheared off by the shearing li~uid flowing longitudinally through slot 1~0.-' 10696~i5 Gas Transmitting Body Capillary openings 154 have substantially the same shape, size, and location in liquid transmitting slot 120 in both gas diffusing surfaces 146 and 148, on opposite sides of the slot (Figure 12). Gas transmitting passages 158 are prefer-ably substantially perpendicular to the planes tangent to gas di- -fusing surfaces 146 and 148, respectively. The latter surfaces are preferably generally planar, but need not be exactly so.
To achieve the smallest bubbles possible with this invention, the material surrounding capillary openings 154 in gas diffusing surfaces 146 and 148 should be one having a low wettability, or high contact angle, with the shearing liquid that flows through slot 120. Thus, the material is desirably one -- such as, for example, polystyrene or polyethylene --having a contact angle of at least 60 with tap water when that liquid is the shearing medium used with this invention.
Open ends 154 of capillary passages 158 must have a small diameter if small bubbles are to be formed as gas flows 20 from the ends of the capillary tubes. In the actual embodiment of which Figure 12 is a schematic representation, capillary passages 158 are defined by hollow fiber glass tubes having cylindrical internal passages with a diameter of from about
These two requirements must be met no matter what sheariny liquid is used in the method of this invention. In de-fining the apparatus, however, a specific shearing liquid and specific test conditions are employed, as discussed further below.
It is not required that the laminar flow be absolutely parallel,but only substantially so. Likewise, it is not re-quired that the liquid over all the capillary openings in the ~ -gas diffusing surface be absolutely free of any type of flow other than the desired flow, but only substantially free. As ;~ one skilled in the art will recognize, residual minimal per-turbations will almost always be present in the sheariny liquid because of its history prior to entering the liquid transmitting slot. Likewise, there will almost alway~ be minimal perturba-tions present along the walls of the shearing liquid transmitt-ing slot. The very presence of the nascent gas bubbles and the shearing of those bubbles will al50 produce (see Figure 4A be~
.
., ~ . . ,~ .
-i ~ 1069665 t-low) minor departures from trul~ straight streamlines. In fact, the main streamlines themselves will not be wholly parallel (Figure 1~) until they reach the region of fully developed la-minar flow some distance downstream from the inlet end of the liquid transmitting slot. The skilled wor]cer will understand, however, that none of these minor variations from absolutely par-allel streamlines will be detrimental, and that substantial free-dom from unstable flow such as turbulence or vena contracta will be sufficient for the method and apparatus of this invention.
The presence or absence of the two hydrodynamic fea-tures listed above in the operation of any particular method and apparatus is readily ascertainable by those skilled in the art when the shearing ~iquid is transparent, or at least trans-lucent. In such systems the presence or absence of the defined laminar flo~ may be determined visually after release of a small amount of dye into the stream within the slot, or may be seen ~- ~ from photomicrographs which show alignments or "strings" of tiny : . . , : . .
bub~les running in the direction of the laminar flow, presumably from particular individual capillary openings.
With opaque liquids, such as sewage, the presence or absence of substantially parallel laminar flow including a partially developed laminar boundary layer is more difficult to determine, but techniques for making such a determination with some degree of reliability are known. In any event, this :
determination can be made, as a practical matter, by analogy to test~flow under similar conditions with transparent or translucent shearing liquids. A particular method or apparatus ma~y be tested, for example, by operation with a synthetic trans-parent or transLucent sewage fluid comprising an aqueous solu-~ ~tion oontaining a sufficient amount of a thickener, such as carboxymethylcellulose, to provide a viscosity similar to that :. . ' ' . ' ' -: ;
of the sewage being treated, and by the observance of the flow in such a liquid by dye, or by bubbles, as described above.
- Alternatively, one skilled in the art may, as a prac-tical matter, test a given apparatus with a readily available test liquid, such as water, to determine whether or not the de-fined laminar flo~ will prevail when the apparatus is utilized with a second liquid such as sewage. This may be important either because the liquid with which a particular piece of ap-paratus is designed to be used is opaque, or because it is not known for sure with what liquid the apparatus will be used.
'~, Demonstrating Laminar Flow, Including Partially Developed Laminar Boundary Layer, In One Liquid By Testing Another ~.......... . . ~ ..
; It is well known that laminar flow exists in any sys-tem in which the Reynolds number is below a specified critical level. In a flow system of given channel geometry and surface characteristics, the Reynolds number is directly proportional ~- ;
to the linear velocity of liquid flow and to the fluid density, and is inversely proportional to the fluid viscosity. The fluid densities of liquids of high water content are substantially .
identical and may, as a practical matter, be ignored; so that the threshhold velocity for conversion of laminar to turbulent flow in any designated apparatus is inversely proportional to the viscosity of liquid passing therethrough.
Utilizing this information, one skilled~n the art can test a given apparatus with water at 20C.,~-for example, `~ to ascertain whether the apparatus would be suitable for use , ~ ; with another liquid which has a higher viscosity, such as sewage.
, An apparàtus which provides lami~ar flow when water at 20C is , passed through the slot at a given linear velocity will also pro-vide laminar flow, including a partially developed laminar be~un--22- ~
.
. - -106g66S f ary layer, when se~Jage is passed through the same slot at that velocity because sewage has a higher viscosity and therefore a higher threshhold velocity for the beginning of turbulent flo~.
On the other hand, a velocitv which will produce tur-bulent flow with water in a given apparatus will not necessariily produce turbulent flo~ with sewage, because of the higher vis-cosity of the latter. For this reason, the test velocity at which water is passed through the slot of the apparatus in test-ing its suitability for use in the treatment of sewage may be a lower velocit~ than is to be used in the ~peration of the ap-paratus with sewage. For example, the apparatus may be tested to determine whether or not the defined laminar flow prevails when water at ~0C. is passed through the slot at a mean en-trance velocity of 10 feet per second. If laminar flow exists with watqr in a given apparatus at this flow rate, one skilled in the art can be assured, as a practical matter, that sewage containing about 1 per cent of suspended solids or about 6 per cent of suspended solids (both common forms of sewage) can pass - through the same slot, while still maintaining laminar flow, in-cluding a partially developed laminar boundary layer, at veloci-ties far above this figure, and in fact well above any practical level at which the apparatus of this invention would conceiva-bly be used.
- Accordingly, one way of defining the apparatus ^~ of this invention is to prescribe a specific test, invol-ving the use of water flowing a~ a stated minimùm velocity, that the particular construction must meet. The test pre-; scribed in the claims below is that the shape, dimensions and spacing of the gas transmitting bod~ and the slot-~efining mem-ber lying on both sides of the shearing liquid transmitting slot . .
must be adapted, when water at 20C. is flowing through the liquid transmitting slot with a mean entrance velocity of 1 . ,/
, . . - . . . , : ,: - - : :
.
- ~ iO69665 ~ ~
feet per second and no gas is flowing through said gas trans-mitting passages, to establish the defined substantially paral lel laminar flow. Higher shearing liquid velocities are also specified in the claims. The particular velocities specified have been found to be essential for ef~icient small bubble for-mation with water as the shearing liquid, and these results are nowhere disclosed in the prior art known to appliccant.
Apparatus that meets the test described falls within -the scope of the claimed apparatus invention, whether in its ultimate practical application it is used with ~Jater or with some other liquid as the shearing liquid. The validity of this method of definition of the apparatus of the present in-vention is supported by the "extrapolation," as it were, from one liquid (water) to a second, more viscous liquid (such as 6 per cent suspended solids sewage) in the manner that has been described above, to demonstrate that the defined laminar flow is present if the second liquld is employed, just as it is when the first liquid is used as the shearing liquid.
It has been found that the two hydrodynamic require-ments that are characteristic of the apparatus of thls inven-tion are present, and satisfactory results in terms of bubble size and uniformity of bubble size will be produced,when the average test velocity across the inlet end of a slot of typical .
width, with water at 20C. used as the shearing liquid, is 10 feet per second. Improved bubble shearing can be achieved with apparatus that produces the defined laminar flow when this test velocity is 15 feet per second, and still further improvement when the test velocity is 20 or 25 feet per second.
The same technique of proof by extrapolation may be used, if it is necessary to do so, to show that a particular bubble shearing method comes within the scope of this i~vention.
The presence or absence of the defined laminar flo~
.
`~ ~ 1 ~ 5 in any given system may f desired by corroborated by secondary evidence, such as the efficiency of the gas absorption pro-cess resulting from use of the system. It has been found that for a given volume of gas introduced within a given time into a given volume of liquid, absorption is substantially more complete ~lhen the above described two hydrodynamic condi-tions are present in the method and apparatus by which gas bub-bles are formed. The converse is also true: The existence of these two conditions may be corroborated by demonstrating the greater effectiveness of the given system in comparison to the effectiveness which is obtained when the flow conditions through the slot are clearly turbulent flow conditions.
.
Gas Flow Rate It is important in the method and apparatus of this invention not only that the two essential hydrodynamic reguire-; ments be present but also that the gas flow rate into the shear-ing liquid be sufficiently high. With too little gas flow, the volume of gas diffused into the liquid in a given period of time may be so small that even though the bubbles produced are of a desirable size and uniformity, the output of the apparatus is impractically inefficient and the necessary equipment for a given application prohibitively expensive.
The maintenance of desirably high gas flow rates may, however, tend to militate against the establishment of the two essential hydrodynamic characteristics of this invention. If the gas flow rate reaches too high a level, it will be impos-sible to establish the necessary parallel laminar flow and ~to ~, avoid~undesirable types of flow such as vena contracta or turbu-lence above the capillary openings in the gas diffusing surface.
The particular gas flow rate that is desirable for a given ap-paratus and for a given set of operating conditions can bes~ be determined empirically by one skilled in the art, keeplng in -~ ~
- , . - ~ . - : . . :, ~069665 mind all the many factors that affect bubble shearing.
In resolution of these conflicting objectives of high gas flow rate and substantially parallel laminar flow as des-cribed, it has been determined that with typical shape, dimen-sions and spacing for the members defining the shearing liquid transmitting slot of the apparatus of this invention, and water as the shearing liquid, the gas flow rate should be at least about 5 to 10 standard cubic feet per minute per square foot of active area in the gas diffusing surface. As a practical matter, this rate of oxygen flow employed with a water flow velocity of 20 feet per second and a water volumetric flow rate of 2.5 gal-lons per minute per inch of slot inlet length will produce a transfer of oxygen into water of about 0.5 to 1 lb.~HP/ hr., an oxygen transfer rate that is characteristic of a number of sewage treatment installations. Improved results are obtained if this gas flow figure for water as the test liquid is about 4~ scfm, and still better r~sults if it is about 70 scfm.
The term "active area" i5 used in this specification and claims to refer to the area defined by lines tangent to the outermost capillary openings at which bubbles are formed.
"~,~ , -Conventional Precautions To Achieve Substantially Parallel Laminar Flow It is also important in the practice of this invention I to take as many reasonable precautions -- in view of the width;l of the liquid transmitting slot, the length of flow through the slot, and the viscosity and flow velocity of the~`~hearing liquid with which the apparatus is designed to be operated -- as are necessary to avoid turbulence or vena contracta and achieve sub-I stan~ially parallel laminar flow in the slot.
With water flowing at a velocity of at least about 10 feet per second as the shearing liquid, for example, one s~illed ,\
' - , : , ' ' '~ 106g66i5 in the art will understand that if the length of flow through the liquid transmitting slot is more than about 1/2 inch, some attention must be paid to taking reasonable precautions to avoid the establishment of turbulent flow in the slot. The necessity to take care to avoid vena contracta increases when the liquid flow path is quite short. On the other hand, it is well known that greater care must be taken to avoid turbulence, the wider the liquid transmitting slot or the longer the liquid flow path through the slot. Increased care must also be taken when lower ; 10 viscosity shearing liquids or higher shearing liquid flow velo-cities are employed.
Conventional precautions against the occurrence of vena contracta include,in addition to avoiding too short a flow path, avoiding openings of too large size through which gas is intro-duced into the shearing liquid, and, most important of all, em-ploying rounded edges at the slot inlet. Conventional precau-tions against the ~ occurrence of turbulence include utilizing contoured inlets, avoiding anything more than minimal protuber-ances, avoiding rough channel walls, exercising special control of liquid flow into the inlet to minimize the initial level of perturbations, etc.
The roughness of the channel walls may be expressed in terms of (1) the height of the most prominent peaks and the depth of the lowest depressions, respectively, above and below the median plane of the channel wall, compared to (2) the dis-placement thic~ness of the laminar boundary layer in the liquid flowing along the wall. To minimize the tendency for the liquid flow to go turbulent, the former dimensionsshould ordinarily be only a small ~-~ction of the latter.
Conventional measures to control the liquid flow into -' . ' '.
,,~ \ :
., .
- ~ 1069665 the slot inlet in order to minimi~e the level of perturbations at that point include the use of a quietlng reservoir to dissi-pate any turbulence already present, avoiding abrupt corners in the feedJline leading to the slot inlet, using turning vanes when-ever turns are unavoidable, avoiding steps in the flow path, avoiding orifices in the flow path, using pumps and valves only ~ -when absolutely necessary, and similar measures designed to pro-duce as quiet a flow condition as possible.
Examples of Parallel Laminar Flow Covered By This Invention Best results are achieved with the method and appara~
tus o~ this invention when thése additional conditions obtain:
-- The partially developed laminar boundary layer ex-tends along the entire length of the gas diffusing surface --even those portions where there are no capillary openings --over which the shearing liquid flows; and -- All the nascent gas bubbles are sheared from the gas diffu~ing surface by the combined action of the liquid mov-ing in the partially developed laminar boundary layer and the li~uid moving in the free stream above that layer.
These particular conditions are illustrated schematic-, , .
I ally in Figure 4~, which represents a preferxed form of this in-j vention. This figure, like Figures lA and lB above and Figures 4B through 4F below, is greatly enlarged for clarity. In an ac-11 tual embodiment of the apparatus of this invention, the ~idth of ,~ slot 40 is about 0.040 inches, and the length aboht 0.080 inches.
I The capillary openings in that embodiment are much smaller and ¦ more numerous than are shown schematically in Figure 4A, al-though not located so close to leading edge 61.
Wall 42 is a gas transmitting body having a plurality ... . .
of gas transmitting passages 66 extending therethrough, each ~\
.~ '' '' ~'.: :
--~ 106S6f~5 of which terminat~s in a czpillary opening 65 at gas diffusing .: :
sur~ace 68 of the wall. The appar~tus includes means for pro- :
viding a f low of gas through gas transmitting passages 66 and ~:
out capillary openings 65 to form nascent gas bubbles 94.
Wall 44, spaced from gas diffusing surface 68, de-fines slot.40, which has an inlet end 46 and a discharge end 48.
The body of liquid 96 into which the fine bubbles produced by the use of this invention are diffused lies to the right of slot 40 in Figur~ 4A.
Laminar flow substantially parallel to the contour of gas diffusing surface 68 is established in the liquid flow-ing through slot 40. Because capillary openings 65 are lo-cated in close proximity to leading edge 61 at inlet end 46 of slot 40, they lie beneath partially developed laminar boundary layer 54. Free stream 56 is established above la- :~
minar boundary layer 54 over the capillary openings. As in .
the earlier figures, streamlines 50 illustrate the substan-tially parallel laminar flow from left to right.
:. The shape, dimensions and spacing of gas transmitting body 42 and slot defining member 44 are adapted, when water is : ~ employed as the shearing liquid under the conditions specified above, to establish laminar flow of the kind described above in :
I the water that flows through slot 40. In addition,.the appa-ratus includes means for providing a flow of liquid through slot 40 at a velocity that will produce in water that type of .
; substantially paràllel laminar flow.
i ~he result of use of this apparatus is to shear na-scent gas bubbles 94 from capillary openings 65, so thatYthe ~ :
flow of liquid through slot 40 carries the sheared gas bubbles . 30 out of the.slot at its discharge end 48 and into the body of ~r ~
.
~ -29- .~
, .
., , , , . -. . . : :
~069665 ~ ~:
liquid 96. As is shown in Figurc 4A, all bubbles sheared in this preferred embodiment of the apparatus of the invention are sheared from capillary openings 65 by the combined action of the liquid moving in partially developed laminar boundary layer 54 and the liquid moving in free stream 56 above the boundary layer. The surface tension at the juncture between the surface of the gas bubble and gas diffusing surface 68 surrounding each capillary opening 65 tends to cause each bubble to adhere to surface 68. However, the movement of liquid from left to right through slot 40 in Figure 4A shears the nascent gas bubbles off surface 68 against the force of the surface tension referred to, and in this manner produces gas bubbles of very much smaller diameter than would be formed if it was the buoyancy of the bubble as it reached its full growth that broke the surface tension holding the bubble to the gas diffusing surface.
Since capillary openings 65 are in close proximity to leading edge 61, partially developed laminar boundary layer 54 has little time in which to build up into a thick layer. When ~o the nascent bubbles g4 protrude above interface 52 between la-minar boundary layer 54 and free stream 56, they are exposed to the full force of free stream 56, which is moving with I greater velocity than the liquid in lamlnar boundary layer 54. For this reason, the nascent gas bubbles are sheared of before they can grow to any very large diameter. As an ~ example, the method and apparatus of this invention have been I used to produce bubbles of air in water some ~f which are ~n as small as 10 microns in diameter, with the median diameter being no more than about 25 to 30 microns. As illustrated schematically in Figure 4A,the gas bubbles grow slightly ;~
-30_ , ,, ,. . .
larger as one procee~ downstream and the partially developed laminar boundary layer increases in thickness.
10As is seen ~rom Figure 4A, partially developed lamin-ar boundary layer 54, with free stream 56 above it, extends be-yond all capillary openings 65 in gas diffusing surface 68 to discharge end 48 of slot 40, and free stream 56 extends still farther,into body of liquid 96. The same is true o~ the embodi-ments of the apparatus of this invention illustrated in Figures 4B and 4C, where the same designator numerals are employed for the various elements as in Figures lA, lB and 4A. .:
The bubble shearing achieved by the apparatus illus-trated schematically in Figure 4B is only slightly l~ss effec-tive than the bubble shearing achieved by the apparatus ofFigure 4A. In the apparatus of Figure 4B only some, but not all, of nascent gas bubbles 94 are sheared from capillary open-ings 65 in gas diffusing surface 68 by the combined action of the liquid moving in partially developed laminar boundary layer 54 and the liquid moving in ~ree stream 56. The remainder of nascent bubbles 94 are sheared from capillaries 65 solely by the action of the liquid m~ving in laminar boundary layer 54.
Because laminar boundary layer 54 is very thin, the embodiment of the apparatus of this invention shown schematical- : :
ly in Figure 4C likewise produces good results, although its use is somewhat less advantageous than use of the embodiments illus-trated in Figures 4A and 4B. In the apparatus of Figure 4C, all :
~, nascent gas bubbles 94 are sheared from capillary'openings 65 solely by the action of the liquid moving in pàrtially devel-: oped laminar boundary layer 54.
.
:Figures 4D and 4E illustrate schematically embodi-; ments of apparatus of this invention in which there are both .
: .
- - . : . . -. : .. - :
- ' ~
- ~ . . , ' .
~ t 1069665 ~_ partially developed and fully developed laminar flow wlthin the shearin~ liquid transmitting slot. The same numerals are employed to designate similar elements in these figures as in Figures 1~, lB and 4A through 4C.
' Figure 4D shows partially developed laminar boundary layer 54 as it merges with upper laminar boundary layer 60 at point 62 in a zone lying between capillary openings 65 and dis-charge end 48 of slot 40. In this embodiment, the surface of wall 42 lying beneath the fully developed laminar,flow to the right of point 62 in Figure 4D is free of any capillary openings.
Figure 4E shows a similar embodiment, except that there are addi-tional capillary openings lying beneath the fully developed la-mainar flow to the right of point 62, where the two partially developed laminar boundary layers 54 and 60 merge.
The embodiment of the apparatus of this invention ', shown schematically in Figure 4F displays some turbulence within the shearing liquid transmitting slot, but the basic advantage of having the capillary openings in close proximity to the leading edge of the gas diffusing surface to bring them under , 20 a partially developed laminar boundary layer still produces ~ useful results. In this embodiment, in which similar elements ', are identified by designator numerals similar to those employed ,1 in the preceding f igures, partially developed laminar boundary layer 54 lies immediately adjacent gas diffusing surface 68, with free stream 56 above it. Boundary layer 54 terminates ~ ~s" ~ .
-, in turbulent zone 98 adjacent dischaxge end 48 o'f~liquid trans-mitting slot 40 and within the slot. However, the surface of j wall 42 beneath turbulent zone 98 is free of any capillary ' openings. The same condition prevails in the upper part of slot 40, where partially developed laminar boundary layer 60 terminates in turbulent zone 100 short of discharge end 48 of ~, ' ' '\ . ' ~ , 3,2~
, . .. .
06~665 slot q0. The harmful effect of turbulent zones 98 and lO0 in tending to cause collision of ~he fine gas bubbles and their resulting coalescence into larger bubbles is minimized if free stream 56 extends, as shown in Figure 4F,to the end of the slot.
In the apparatus of Figures 4D and 4F, nascent gas bubbles 94 are all sheared from capillary openings 65 in gas diffusing surface 68 solely by the action of the liquid moving in partially developed laminar boundary layer 54. In the em-bodiment of Figure 4E, some of nascent gas bubbles 94 are sheared from capillary openings 65 solely by the action of the ;.
liquid moving in partially developed laminar boundary layer 54, while some are sheared solely by the action of the liquid moving in the fully developed laminar flow to the right of , point 62 near the discharge end 48 of slot 40.
' Apparatus Defined In Terms Of Product . . Of Maximum Distance From Leading Edge To Farthest Downstream Capillary Open-ings Times Average Slot Width The apparatus of this invention may also be defined in terms of a "figure of merit" that is the product of (a) the . distance separating the inlet end of the liquid transmitting slot from the very small capillary openings that lie farthest downstream on the gas diffusing surface over which the shearing . ~ - .
liquid flows, times (b) the average width of that slot through-out the indicated distance.
As already mentioned above, satisfactory results may :~
be obtained in some applications when this figure of merit is no more than about 0.1 square inches. Somewhat improved results : . are generally obtained when the figure of merit i9 no more than ,~ . .
about 0.075 square inches~ and still further improvement is ob- -:~ tained when it is no more than about 0.05 square inches. Very -. . .
~ . -33-~ , . . . , :
- , . .. ..
good results are obtained when the figure is no more than about 0.025 square inches, and excellent results when it is 0.01 square inches. For some applications, the preferred product of the indicated width and distance is no more than about 0.005 square inches, or even less.
Applicant has discovered, as has been explained, that contrary to the teaching of the prior art the best formation of fine bubbles is achieved if there is substantially no turbu-lence over any capillary openings in the gas diffusing surface over which the bubble prGducing liquid flows. This discovery makes it possible to formulate the figure of merit just defined.
The critical Reynolds number for the distance down-streàm from the leading edge of a flat plate at which transi-tion to turbulence will occur in liquid flowing across the plate at any given velocity is approximately 350,000. This number also applies to the flow through a parallel flat wall slot, so long as the slot width is at least as great as the fig-ure computed from a second critical Reynolds number, i.e., the crltical Reynolds number for slot width. The following 1~ 20 two equations, which include the two critical Reynolds numbers referred to, relate the velocity of liquid flow to the vis-cosity of the liquid and two important dimensions of the slot:
(1) y ~ 350,000; and (2~) ~^W ~ 2,pO~;
where U - the free stream velocity of the shearing liquid, x =
the distance~from the inlet end of the shearing liquid trans-mitting slot to the farthest downstream capillary openings above which~no turbulence wil~l occur, w = the critical slot wLdth for tur~bulent~flow, and Y = the kinematic viscosity of the shearing 30~ iquid.
1 :
~:: f ~ ~
~069665 (The first critlcal Reynolds number of 350,000 refer-red to above is indicated by Schlichting, ~oundary-Layer Theory, Sixth Ed. (McGra~-Hill, 1968), page 435, to be appropriate for apparatus in which only reasonably conventional precautions are taken -- as would usually be true in actual bubble shearing ap-paratus -- to achieve substantially parallel laminar flow. The second critical Reynolds number of 2,000 referred to above is based on experimental work reported in an article by D. Meksyn, "The Stability of Laminar Flow Between Parallel Planes for Two-and Three Dimensional Finite Disturbances," Zeitschrift fur Physik, vol. 17~, pp. 159-172 (1964).) These two equations, together with a third equation to be set forth below, were used to determine the critical dis-tance from the leading edge of the gas diffusing surface of the apparatus of this invention fox the most remote capillary open-ing over which no turbulence will occur when water is used as the shearing liquid and the shearing liquid transmitting slot is definea by parallel planar walls. In these determinations, since the goal o bubble shearing is to produce gas bubbles of very small diameter, capillaries of 20, 30 and 40 micron di-ameters were examined. The bubbles from these capillary open-ings were assumed to be 50 microns in diameter. Three typical widths of the shearing liquid transmitting slot were employed --0.5, 0.1 and 0.040 inches.
l The material defining the capillary openings is also I important. The bubble shearing ~chieved by the method and appa-~ ratus o~ this invention will be most effective when the material i surrounding the capillary openings in the gas diffusing surface has a low wettabilityl or in other words a high contact angle, ~30~ with the shearing liquid. Because the effectiveness of the in-vention in achieving very fine gas bubbles will vary consider-- - ., - ,.. ..
: .:: . : .
: :
--~~ 1069665 ably ~ith this factor, determinations were made for apparatus in which the material defining the capillary openings has a contact angle with the shearing liquid of 20 and 45, respec-tively. Under ordinary operating conditions, impurities con-tained in tap water form a coating on materials such as various metals and glass, and as a result such materials have a contact angle~of approximately 20 with the tap water. Contact angles -~
of 90 will give the best results with this invention, but no determinations were made for this figure. Materials having a contact angle with tap water of approximately 45 include, for example, polyvinyl chloride and polymethyl methacrylate. Ma-terials having a contact angle with tap water of approximately 90 include, for example, fluorosilicones and fluorocarbon plas-tics such as the material sold under the trademark Teflon.
.
- With the indicated assumptions, the velocities of shearing liquid required to produce bubbles of the predeter-1~ mined sizes from the given sized capillary openings were deter-1 mined as conservative estimates by the following equation:
~: ~, w~ ~d ~05 U t7æ ~ r2~ (3 vv~
where U = the free stream velocity of the shearing liquid, ~ -¦ ` the slot width, ~ = the surface tension of the shearing li-quid~ d =~the capillary opening diameter, ~ = the contact angle botween the shearing liquid and the material surrounding the capillary openings, ~ - the dynamic viscosLty of the sh~eari~ng~liquidj and r ~ the radius of the bubb ~ formed by use of the apparatus of this invention. j ~
, . ~ .
~ ~ With the resulting values from equation (3) for the ~ : :
velocity of the shearing liquid under the conditions indicated, equat~on~(l) above was employed to determine for each situation 30~ the~critical maximum distance from the gas diffusing surf~ce . ,/
~',~' ~ ` ' .
106g665 leading edge to the most remote capillary openingsover which no turbulence will occur. E~uation (2) was then used to confirm that for each situation the slot width was great enough under 211 the respective conditions to permit turbulence to occur just beyond the computed critical distances.
Table I below shows the maximum distance in inches from the leading edge of the gas diffusing surface to the farthest downstream capillary openingsover which no turbulence will be produced, for the indicated parameters and water as the shearLng liquid:
, ', `~, ., , ' . - .
-37~
' ' ,,/ ' , ' .
, . . : . : . : : :
` ~ 106~6~5 (-I ~ I U~ ~ o~
l oo o l o o o I ~ ~ o~ I ~ '` o I o o o o I o o o o ~ ., I ,.' 'u~ o o I ~ o o o o I o o o ~ I
o ,~ ~ ~ o I _, ~ o o o . . . U~ I o o I N ~ O O O ~ O = O
H ¦ . ~ O _i O ¦ O -i 0 ~1 I
~ l _l . l n ~ ~ ".
a: . O O O ¦ O
O O
~
~r O ~ ~. O O ~
O
. ~ ~
O U~ O . O 00 . - . .
O ~1 0 O N O
X - X
~U N ~ ~ a) N S: E3 ~ r~ ~I h (n ~ :
U~ ~ S,~
O~ aJ ~J O U U~ O~ O~
U ~ ~ ~ ~ U ~ t~
,I v 3 c~ 3 \.
o o' `~
~ ~ .
"
- ,, ' -' ' ' ' ,, ~ - :' - : :
.
.: . . : ' ~
.
1069665 ~ :
As the information in tiliS table confirms, the maximum distance from the inlet end of the shearing liquid transmitting slot to the farthest do~stream capillary openings that can be employed without producing turbulence over any capillary open-ings (referred to in the table as the distance "x") depends upon a ~ariety of factors, inc'iuding the slot width. The table indicates that satisfactor~ production of fine gas bubbles can generally be obtained even if the distance "x" is substantially more than 1 inch, if the product of the distance "x" times the ' 10 average width "w' of the liquid transmitting slot throughout that distance -- the defined "figure of merit" w~ -- is no more ~ ' than about 0.1 square inches. (For width "w" and distance "x,"
see Figure 12.) Thus, with a small enough slot width, small bubbles of 50 microns diameter can be produced with apparatus for which the distance "x" is more than 1 or even 2 inches, if the figure of merit is only slightly above 0.1, at 0.114 or 0.115, the contact angle between the material surrounding the capillary openings and water is 45, and the diameter of the capillary openings is 20 microns.
Improved shearing results are obtained when the figure of merit is reduced to about 0.75. Thus, production of the same 50 micron bubbles is possible with a figure of merit of 0.074 to 0.076 even though the capillary diameter is increased to 30 mi-crons. Similarly, it is possible to produce 50 micron bubbIes with a figure of merit of 0.063 to 0.065 even though the contact angle is reduced to the less effective value of ~0.
Still further improvement in bubble shearing is ob- ~
tained when the igure of merit of the apparatus used is sub- ~ ' stantially lower, i.e., no more than about 0.05. As'the table .
shows, bubbles of 50 microns diameter can still be produced when .
.
-39- ' \
.
: , , : , ' . , .
106~66S
the ~igure of merit is just about 0.05, at 0.055 to 0.057, even though the capillary size is increased to 40 microns. And, likewise, the size of the bubbles can be maintained at 50 microns with a figure o~ merit of 0.042 to 0.045 even though the contact angle is reduced to the less effective value of 20.
Very good results are obtained when the figure of merit is about 0.025. Thus, when the figure of merit is some-what above that figure, at 0.030 to 0.032, bubbles of 50 microns diameter can be produced without the necessity of using material 10 to define the capillary openings with a contact angle with water that is any better than 20, ~ven when the capillary opening di-ameter is 40 microns.
The trend of the data in the table indicates that the bubble shearing rasults described will be improved if the de-fined figure of merit is still further reduced to 0.01, or lower.
In one embodiment of the apparatus of this invention that pro-duces excellent bubble formation with sewage containing 6 per cent suspended sollds as the shearing liquid, the width o the liquid transmitting slot at its entrance end and throughout its 1~ ~20 length is about 0.125 inches, and the dis~ance from the inlet ~l end~to the farthest capillary opening is about 0.045 inches, making a figure of meri.t of about 0.0056 square inches. In another apparatus according to this inv~ntion that achieves equa-lly impressLve bubbLe shearing with water as the shearing liquid, the~slot width is about 0.04 inches and the distance "x" is a-bout 0~045 inches, making the figure of merit even less, about 0.0018 square inches, If apparatus constructed according to this invention has~a figure of merit as defined in this specifica~ion ("w" times ~h~
~30 ~ ";x"~) of O~.L~square inches, and the inlet end of the liquid trans-~ -40-: 1~:' ' , ' ' 1~ , ''`~ 1C1696f~5 mitting slct has a ~idth "w" of 0.1 inches, the distance "~" from the inlet end to the fartllest downstream capillary openings on the gas diffusing surface of the gas transmitting body is 1 inch. With a figure of merit of 0.05 square in-ches and the same 0.1 inch slot ~idth, the distance "x" is 0.5 inch. A figure of merit of 0.01 square inches and a slot width of 0.1 inches makes a distance "x" of 0.1 inch.
Wi~h the same slot width, a figure of merit of 0.005 makes a distance "x" o~ 0.05 inch.
The computations on which Table I above is based were made for a shearing liquid transmitting slot defined by parallel planar t~alls, but the values for the corresponding figure of merit for an annular slot are of comparable magnitude. Equations (1), (2) and (3) above may be used for an annular slot when the mean diameter of the slot is considerably larger than the slot width -- which will ordinarily be the case with bubble shearing apparatus -- since such a slot is equivalent to a slot that is first defined by parallel, planar walls and is then curved around upon itself.
- If the transverse cross section of the shearing liquid transmitting slot is circular, as for example in the case of a slot having the form of a right circular cylinder, the "average width" of the slot may be considered to be equal to the avera~e radius of the slot in the area of the capillary openings of the gas diffusing surface, tThis follows from the fact that the perpendicular distance between the walls of a parallel planar wall slot is hydraulically equivalent to the radius of a slot having a circular cross section.) The term "average width" of the li-quid transmitting slot is thus used with this meaning in this specification and claims for any slot of circular cross section, i.e., the average radius of the slot in the indicated area.
~.
.` ~1 , :
, ~069665 As already pointed out in some detail above in the dis-cussion of the definition of the apparatus of this invention by use of a specified hydrodynamic test, it will be understood by those skilled in the art that one practicing the present inven-tion as defined by the stated figure of merit should not only uti-lize the figure of merit, but should also take reasonable care to avoid including in the bub~le shearing apparatus any structural features(such as rough slot walls) that ~ould tend to produce turbulence -- or any other structural features (such as too low a ratio between the length and the width of the slot) that would tend to produce vena contracta -- with the particular shearing liquid and particular liquid velocity for which the apparatus is designed. It will be further kept in mind by the skilled worker that the effectiveness of a particular figure of merit in producing good bubble formation will always depend to some extent on the nature of the shearing liquid to be used t~ith the apparatus.
As has been pointed out above, prior to applicant's im-proved bubble shearing invention, the great advantages of utiliz-ing the very effective viscous shearing forces available in thetop portions of, and directly above, a partially developed lami-nar boundary layer in the shearing liquid flowing over a gas dif-fusing surface were ~holly unappreciated by those skilled in the art. Because o~ this, there was no recognition of the importance of using a narrow liquid transmitting slot (a low value for llt~JII) to establish a partially developed laminar boundary layer over a significant portion of the capillary openings in the gas diffus-ing surface, nor of the importance or even the possibility o lo-cating very small capillary openings in the gas diffusing surface in close proximity to the leading edge or inlet end of the slot - .
;42-- ~ ~Oti966S
(a low value for "x") in order to takeiadvant~ge of the thinnest portion of that partially developed boundary layer. ~ -As a consequence, no one knew the impor~ance of the low value of the "figure of merit" with very small capillary openings that provides one definition of the apparatus of this invention. Applicant knows of no one in the prior art who has, even accidentally, given any hint or suggestion of this con-c~pt.
. .:
Maximum Distance From Leading Edge To Most Remote Capillary Openings The information in Table I above also shows that this invention may be defined alternatively solely in terms of the maximum distance from the leading edge of the gas diffusing surface to the most remote capillary openings in that surface (distance "x"). This feature s~anding alone is an important aspect of the present invention.
The data relating to a contact angle of 20, for ex-ample, in the upper half of Table I shows that good bubble shearing can be obtained when the defined distance is no more ` 20 than 1 inch. Improved shearing results are obtainable when the distance is no more than about 0.5 inch, and still better if it is no more than about 0.1 inch, 0.05 inch, or even lower.
, V2rious Factors Affecting j _ Bubble Size A number of factors have been discussed that will affect the results achieved with the method and apparatus of the present invention. Some approximate data on various fac- ;
tors that affect the size of the gas bubbles produced in bubble shearing are presented in the form of graphs in Figures 5 through 10. In these graphs, bubble diameter is plotted against the following parameters:
:
-43- ~
-~ 1069665 ~ ~ :
TABL~
Figure Independent variable S Distance from leading edge of gas diffusing surface to most remote capillary openingS
6 Diameter of capillary openings 7 Contact angle between water and the material of which the gas diffusing surface is formed .~ ~''.
8 Water velocity through the shear-ing liquid transmitting slot 9 Surface tension between gas bubbles and various shearing liquids Viscosity of the shearing liquid The conditions for which the determinations on which the respective graphs are based were made are shown in the fol-~owi~g table:
~- TABLE III
Figure 5 Water velocity 20 ft./sec.
Capillary s~ze (median I.D.) 8 microns Sur~ace tension 50-60 dynes/cm.
Contact angle 20 (approx.) Viscosit~ ~.Ol i~p.
Figure 6 Water velocity 20 ft./sec.
Surface tension 50-60 dynes/cm.
Contact angle 20 (approx.) 30 Viscosity 0.01 cp.
Leading edge to most .045 inch remote capillaries ~~ ~~~~
~ ~ 106966S ~ ~
Fiaure 7 Water velocity 20 ft./sec~
Capillary size (median I.D.) 8 microns Surface tension 50-60 dynes~cm.
Viscosity 0.01 cp.
Leading edge to most .045 inch remote capillaries Figure 8 Capillary size (median I.D.) 8 microns Surace tension 50-60 dynes/cm.
Contact angle 20 (approx.) j. . Visc05 ity O.Ol cp.
~; Leading edge to most .045 inch remote capillaries s ; Figure 9 ` Water velocity 20 ft./sec.
, Capillary size (median I.D.) 8 microns Contact angle 20 (approx.) Viscosity O.Ol cp.
Leading edge to most .045 inch -remote capillaries Figure 10 i~ . :
Water velocity 20 ft./sec.
f Capillary size (median I.D.) 8 microns ~ Surface tension S0-60 dynesjcm.
1: :
Contact angle 20 (approx.) Leading edge to most .045 inch remote capillaries : 1 .
~ 45 .: .
-~r 1069665 ~-Specific Em~odiments Of Bubble Shearin~ ~pparatus Figures 11 thxough 14 illustrate specific emhodi-ments of the bubble shearing apparatus or gas diffuser of this invention.
Diffuser Asse~ly Although a diffuser assembly 110 constructed in ac-cordance w-th the present invention can be utilized in many different environments, the diffuser assembly is shown in Figure 11 at the bottom 112 of a lake or large body of polluted water 114. The polluted water is deficient in oxygen and this hampers normal life processes required to support fish life and to maintain the proper sanitary conditions in lake 114. By dissolving oxygen in the water, the natural processes of water purification are accelerated. A method and an apparatus for supplementing the natural process of water purification by the addition of oxygen to the water is described in United States Patent No. 3,505,213 to Anthony and Fulton and entitled "~lethod and Apparatus for Purifying a Natural Body of Water." Although it is contemplated that diffuser assem~ly 110 will advantageous-ly be utilized to promote the absorption of oxygen by bodies of water, it should be understood that it can be utilized to ; promote the abso-rption of other gases by other bodies of liquid as well.
Diffuser assembly 110 includes a plurality of units or , . .
` modules 118 having substantially vertical and parallel discharge i '~ openings or slots 120 from which a mixture of water and small bubbles 122 (Fig. 12) of oxygen flow into lake 114 to oxygenate the lake. Slots 120 may if desired be disposed at other angles to the lake bed, but the vertical orientation is preferred in ; order to permit fresh supplies of water for oxygenation to rise ,i , .
'~
.:. , , .. : , , . . :.
, . . , . , , . ::
069665 r between the slots most readily from the lower levels of the body of water.
Small bubbles of oxygen 122 are dispersed into a relatively large area of the lake and rise slowly toward the surface of the lake. As bubbles 122 rise, the oxygen within the bubbles is absorbed by the oxygen deficient waters of the lake. Since bubbles 122 of oxygen rise slowly and are very small, with a relatively large surface area per unit volume of oxygen contained~within the bubbles, substantially all of the oxygen is absorbed as the bubbles rise toward the surface of lake 114. If the bubbles were relatively large, they would ascend,quickly toward the surface of the lake so that there would not be sufficient time for the oxygen to be absorbed.
This could result in a "bubbling-off" or dissipation of the oxygen to the atmosphere. Of course, dissipation of oxygen ; into the atmosphere increases the cost of producing the de- sired oxygen content in the water of lake 114.
The modules or units 118 are connected with a common ~ource of oxygen under pressure by a gas main or line 124 (Fig-, ure 11). Gas main 124 is connected with gas disperser assem-blies or "bars" 128 in each of the units 118 by feeder lines 130. In addition, each of the units 118 is supplied with water under pressure by a common main or pipe 132 to which the dif-, ,fuser units 118 are connected by a base plate 134. A suitable pump with adjustable output velocity, schematically shown at 133, may be associated with main pipe 132 and opèr~tes to draw water from lake 114 and direct it at various velocities, as ,~
desired, along pipe 132. A suitable filter may be used to prevent solids from reaching the pump or the disperser as-... - . ........................... . .
~30 semblies 128. Thus, when the diffuser assemblies 118 are being , utilized to oxygenate lake 114, the diffuser assemblies arè
-~7-, : ~, ` ~ ~.06966S f continuously supplied with gaseous oxygen by gas line 124 and are continuously supplied ~ith wat~r by water ~ain 132. Al-though gas line 124 and water main 132 have been sho~m in Figure 11 resting on bottom 112 of the lake, it should be understood that they could be suspended or otherwise supported above the bottom of the la~e, if desired.
Extremelv fine bubbles 122 are formed on each side of slots 120 defined by the gas disperser assemblies or bars 128 (see Figure 12). Thus, small bubbles 122 are formed in groups of bubbles 142 and 144 which extend for the ull length of opposite longitudinally extending side walls 146 and 148 of slots 120, each of which slots is approximately 12 inches in ; vertical length in one embodiment of this apparatus. The bub-bles are swept away by water which flows in a continuous stream from chamber 149 through slots 120 into the lake. Water chamber 149 is connected in fluid communication with water line 132 by passages (not shown) extending through mounting plate 134 (~igure 1).
.. ..
Because substantially the only effective bubble forming force in this apparatus is the viscous shearing force of the liquid flow, and buoyancy plays essentially no role, the bubbles are swept away before they grow too large. The bubbles produced are so small that it is difficult or impossible for the observer to distinguish individual bubbles by the naked eye, and the bubbles thus take on the appearance of a "gas fog" or milky cloud of bubbles which extend out~qardly for a , substantial distance from-diffuser assembly 110. Since the ., 810wly rising bubbles 122 are dispersed over a relatively large area of lake 114, the gas in the bubbles can be absorbed by the water of the lake before the bubbles reach the surface . , ~ .
of the lake.
~ ~ .
., .. .. . . .
~ 106g6~5 Bubbles 122 are formed at open ends 154 of capillary passages 158, which extend from gas plenum 160 through side walls 146 and 148 of slot 120 tFigure 12). Each of the up-right gas cllambers 160 is connected in fluid communication with gas line 124,so that a continuous stream of gas under pressure flows from line 124 through feeder tubes 130 (Figure 1) to gas plenums 160, and then to open ends 154 of capillary passages 158.
It ahould be pointed out that this invention is use-n ful in applications, as for example some industrial applica-tions, in which an entire quantity of liquid into which gas bubbles are to be diffused is passed through pump 133, main liquid line 132, liquid chamber 149, and slots 120. In such a case, the term "body of liquid into which fine bubbles are to be diffused" refers to all the liquid that has already flowed through slots 120 of the gas diffusing apparatus and has been accumulated in a subsequent pipe, channel, or other vessel or further handling.
Shearing Liquid Transmitting Slot As water leaves chamber 149 to enter any given slot 120, it flows past rounded leading edges 150 on walls 146 and 148. When water is the shearing liquid ~as in Figure 12) the length of travel through slot 120 should preferably be at least about two times the width of the slot,although it may be less for more viscous liquids. These features, especially the rounded leading edges, will help to establish in slot 120 the substantially parallel laminar flow, including a partially ,de- -veloped laminar boundar~ layer, that is characteristic of this invention. The L/D ratio should not be made too large, however, or there will be too high a pressure drop and accompanying~
.
,, '~ "
., .
:, ' 065~ 5 encrgy requirement for the apparatus, and a clogging problem with some applications such as sewage treatment.
As illustrated in Figure 12, each slot 120 throughout its length should preferab]y have a substantially uniform width measured perpendicular to the direction of flow. In the em~
bodiment shown, slot 120 has a rectangular transverse cross section from its inlet end to its discharge end. ~s will be seen, a substantially straight, unimpeded path is provided for liquid flow through the slot, by utilizing side walls 146 and 148 that are substantially planar and parallel to each other. These features help to produce regularity and stability of flow of the shearing liquid through the liquid transmitting slot, t~hich encourages the establishment of the defined la-minar flow regime.
As already indicated above, the width of slot 120 may be varied depending upon a number of factors including,among other things, the nature of the shearing liquid. A slot width of 0.5 inches or even more may he used with shearing liquids of high viscosity, although with wider slots greater precautions may have to be taken to achieve substantially parallel laminar flow through the slot. A slot width of 0.125 inches has been used ~lith sewage contaîning 6 per cent suspended solids. A slot wi~th of 0.040 inches has been used with 1 per cent suspended solids sewage, and a slot ~Jidth of 0.030 inches or even as 10W
as 0~020 inches with water as the shearing liquid.
Other things being equal, the slot widt~ "w" (Figure 12) should be wider for larger capillary openi~gs,so that col-lision and coale.scence of fine bubbles to form larger bubbles ` can be minimized. The slot width may be made wider than would s 30 otherwise be feasible when the shearing liquid employed, such ~;~ as for example sewage, has a higher viscosity. This will have the advantage of helping to minimize clogging of the slot b~
: ~ \
. "
: .
, . . ~ . . ~ : , -` ~ iO6966S
particulate matter in suspension in the sewage. In the design of an apparatus according to this invention, the considerations mentioned must be balanced against the fact that the smaller the width of slot 120 is made, the lower the po~er input will be for a given velocity of shearing liquid.
It should be noted that each elongated opening or slot 120 of diffuser assembly 110 has a gap which can be easily set during manufacture of the diffuser assemblies to provide the desired relationship between the width of the stream of water flowing through the slots and the gas bubbles formed at the sides of the slots. Or, if desired, the width of the slot may be left adjustable -- as, for example, from zero to some pre-determined width -- so that it may be changed for various conditions of use of the apparatus. If the width of the slot is adjustable through a predetermined range, the present inven-tion concept is utilized if either the defined substantially parallel laminar flow or the specified "figure of merit" is pre-- sent for at least a portion of that range of slot widths.
Figure 13 is a schematic cross sectional view of another embodiment of a gas transmitting body and a slot-defin-ing member useful in the apparatus of this invention. In this ;~ embodiment, cylindrical gas transmitting body 172 has a plu-rality of gas transmitting passages 174 extending therethrough, each of which passages terminates in a capillary opening 176 at gas diffusing surface 178. Annular shearing liquid trans-mitting slot 180 is formed by cylindrical slot-~efining member 182, positioned concentrically with member 172. As with the embodiment of Figure 12, gas is introduced into gas plenum 160 within member 172 and fl~ws out through capillary passages ~30~ 174 to gas diffusing surface 178, where nascent gas bubbles are formed at capillary openings 176 and are sheared off by the shearing li~uid flowing longitudinally through slot 1~0.-' 10696~i5 Gas Transmitting Body Capillary openings 154 have substantially the same shape, size, and location in liquid transmitting slot 120 in both gas diffusing surfaces 146 and 148, on opposite sides of the slot (Figure 12). Gas transmitting passages 158 are prefer-ably substantially perpendicular to the planes tangent to gas di- -fusing surfaces 146 and 148, respectively. The latter surfaces are preferably generally planar, but need not be exactly so.
To achieve the smallest bubbles possible with this invention, the material surrounding capillary openings 154 in gas diffusing surfaces 146 and 148 should be one having a low wettability, or high contact angle, with the shearing liquid that flows through slot 120. Thus, the material is desirably one -- such as, for example, polystyrene or polyethylene --having a contact angle of at least 60 with tap water when that liquid is the shearing medium used with this invention.
Open ends 154 of capillary passages 158 must have a small diameter if small bubbles are to be formed as gas flows 20 from the ends of the capillary tubes. In the actual embodiment of which Figure 12 is a schematic representation, capillary passages 158 are defined by hollow fiber glass tubes having cylindrical internal passages with a diameter of from about
3 to about 30 microns, with their median diameter being about 8 microns. Hollow fiber glass tubes with a diameter from about 6 microns to about 12 microns may also be used~
As disclosed in my Canadian patent 967,687 issued May 13, 1975, these straight fiber glass tubes are relatively easy , to embed in support bars 166 in parallel relationship with each 30 other and in a perpendicular relationship with the central plane of slot 120. This embedding of capillary tubes 158 may be accomplished, for example, by positioning the tubes in the de-~ ~ ~9665 sired relationship and ~lowing a suitab1e sealing material 1~2 around the tubes. I~hen sealing material or binder 162 solidi-fies, it sealingly interconnects capillary tubes 158 to form the bars 166 and prevent fluid flow around the tubes.
Other gas transmitting bodies may be used, if desired, with the method and apparatus of this invention. The gas trans-mitting body may be formed, for example, by drilling holes in member 166 or may be formed from porous sintered metal, porous ceramic material, metal or plastic screen or mesh material, various other woven materials, a porous sheet or membrane, or any other porous body having capillary openings in a gas dif-fusing surface, As explained above, the term "capillary openings"
is used in this specification and claims to mean any openings small enough to produce fine gas bubbles, no matter what the nature of the gas diffusing surface is on which the bubhles are formed, or the nature of the gas transmitting passages through which gas flows to reach those openings.
; Other porous composite materials besides hollow capil-lary strands 158 embedded in sealing material or binder 162 may also be used. In addition, solid filaments of glass, plastic or metal may be fused or sintered together in bundles to define gas transmitting passages in the form of the interstices between ad-jacent, substantially parallel filaments. Solid glass or plastic filaments are available, for example, with outside diameters of about 10 to 20 microns, and solid metal wire with outside dia-meters of about 40 to 80 microns. ~en such filaments are pressed together and fused or sintered, the resulting interstices between.filaments will provide quite small gas transmitting pas-: ~ :
sages, which terminate in capillary openings at which fine gas ~30 ~ bubbles can be formed and sheared off in accordance with the . ' , ', . ~ !., .
' ' " ," '" '' ,~ ' ' . ' ' .: :
6t~S
teaching of this invention.
I~atever the material may be of which the gas trans-mitting body is formed, the gas transmitting body and its gas diffusing surface should preferably have the characteristics discussed below in connection with the illustrative showing of Figure 14.
Since capillary tubes 158 in the embodiment shown are relatively small and closely spaced along the entire length of the slot 120, a large number of bubbles of a small diameter can be formed along sides 146 and 148 of slot 120. Bubbles of air in water ranging up to about 100 or 200 microns in diameter are readily o~tained. As already mentioned above, with a preferred embodiment of the apparatus of this invention, bubbles of air can be formed in water which have a median diameter of only about 25 to 30 microns. With higher viscosity liquids such as sewage, the majority of the bubbles will be even smaller.
As has been pointed out, the showing in Flgure 12 is a schematic one. In particular, gas transmitting passages 158 which extend through the wall of gas transmitting body or gas "bar" 166 to form capillary openings 154 in gas diffusing sur-faces 146 and 148 are shown schematically embedded in hardened binder 162 as a matrix. As shown, capillary tubes 158 and binder 162 are secured on both sides of the assemblage to sup-port plates 170 to form gas bar 166 enclosing gas plenum 160.
Figure 14, on the other hand, is drawn from a photo-micrograph made of an actual gas diffusing surface 146 or 148 in a gas bar 166. Capillary openings 154 are the open ends of discrete hoIlow fiber glass capillary strands or tubes 158 em-bedded in hardened epoxy plastic 162 as a matrix. Support plates 170, which complete gas diffusing surface 146, are in-, .
.
~06g6~s dicated by broken lines on either side of Figure 14. The dis-tance between the two plates is about 0.020 inches.
The flow of shearing li~uid across gas diffusing surface 146 (or 148) in Figure 14 is from left to right in the figure, while the gas flo~ is out of the paper. Nascent gas bubbles formed at capillary openings 154 are thus sheared off as extremely fine bubbles,and are swept to the right towards the body of liquid into which they are to be diffused.
The maximum bubble size produced ~ith the method and apparatus of this invention, as has been stressed above, is affected by many factors. One of the most important of these is the size of capillary openings 154 in gas diffusing surface 14G or 148. It is considered that to take full advantage of the benefits produced by the other important features of the pre-sent invention, substantially all the capillary openings in the gas diffusing surface should have a diameter no larser than ahout 100 microns. Considerably improved results are obtained if substantially all the capillary openings have a di-ameter no larger than about 50 microns, with further improve-ment when that figure is about 25 microns. For preferred re-sults, the maximum capillary diameter should be about 5 microns.
Another measure of the size of the capillary open-ings is related to the bubble size that is desired from use of this invention. ~len the apparatus of this invention is de-signed to diffuse into a body of liquid gas bubbles of a pre-determined size, the maximum diameter of the capillary openings in the gas diffusing surface should be no greater than about that p,redetermined size. For better results, the maximum cap-illary diameter should be no more than approximately 1/2 the 3C predetermined maximum bubble diameter, and for best resultsno more than about 1/4 that dimension.
In general, the capillary diameters should preferably : - :
10696f~5 be made as small as is practicable without making the gas pres-sure drop through the capillaries too high.
Each fiber glass hollow capillary strand or tube 158 has a cross section of substantially uniform size and shape throughout its length, ~eing substantially a right circular cylinder in shape. As a consequence, each of the hollo~J capil-lary strands provides a straight, unimpeded path for gas flow through the gas transmitting body past which the shearing li-quid flows. The passage provided by each hollow capillary strand 158 should preferably extend, adjacent its respective capillary opening, in a direction substantially perpendicular to the plane tangent to gas diffusing surface 146. It is be-lieved that regularity in all these aspects of the size, shape and orientation of gas passages 158, by tending to reduce tur-bulence within those passages, will contribute to greater uni-formity o~ bubble size in the fine bubbles resulting from the use of this invention.
Gas turbulence within capillary passages 15~ should also be minimized by selecting a length for each passage that is at least 10 times the diameter of the passage. It is be-.; .
lieved that an L/D ratio of at least 20/1 will give still better results.
The gas transmitting capillary passages 158 should also have a minimum L/D ratio to minimize backing up by the shearing liquid after a gas bubble has been sheared from cap-illary opening 154. Backing up of liquid can result in clog-ging, when the liquid evaporates and precipitates out dissolved salts after the gas again fills the capillary passage. These advantages of a minimum L/D ratio for the gas passages must of ; 30 course be balanced against the disadvantage of any increase in i the pressure drop through the passages.
. .
: `
..
- ~ ~96t;5 In Fiqure 12, capillary openings 154 are arranged in a plurality of straight rows perpendicular to the flow of shear-ing liquid through slot 120. In Figure 14,the capillary open-ings are randomly located across gas diffusing surface 146.
Although the benefits of the present invention can be obtained with only a few capillary openings disposed along the direction of flow, there should be an average of at least about 5 open-ings in that direction in substantially all portions of the sur-face. If a straight edge rule is laid across Figure 14 in the direction of shearing liquid flow (from left to right across the page), it will be seen that in this embodiment of a gas diffus-ing surface there are on the average more than 10 capillary openings distributed across that surface from its inlet end to its discharge end. An average of as many as 20 to 40 openings in the direction of shearing liquid flow have been used to ad-vantage, and still more could be employed if desired.
To take the greatest advantage of the fine bubble formation resulting from the use of this ir.vention, the land areas in the gas diffusing surface should not be too narrow, or in other words capillary openings 154 should not be too closely spaced to each other. To this end, each opening 154 is prefer-ably surrounded by a minimum land area on all sides that is suf-ficiently extensive in a plane generally tangent to the outer-most points on said gas diffusing surface to substantially avoid coalescence or collision of a gas bubble formed at that opening with a bubble formed at any of the other single open-ings to produce a bubble having a diameter larger than the predetermined maximum bubble diameter. Some such combining of gas bubbles either at their point of formation or downstream is acceptable, but this effect should be minimized as much as is practicable.
, -57-; ~ :
.. . .
: ' . . ' : ' r' .0696~;5 In a preferred embodiment, each capillary opening 154 is surrounded by a minimum land area elliptical in shape that extends on each side of the opening in the direction perpen-dicular to the direction of liquid flow through the liquid trans-mitting slot by a distance at least ahout equal to the maximum bubble diameter, and on each side of the opening in the di-rection of liquid flow by a distance equal to at least about three times the maximum bubble diameter, measured in a plane generally tangent to the outermost points on the gas diffusing surface.
Gas diffusing surface 146 shown in Figure 14 is gen-erally planar in configuration, to produce greater stability in the laminar flow through slot 120, and thus the best fine ; bubble formation. A planar gas diffusing surface will produce sharper edges for capillary openings 154, which should result in smaller bubbles more nearly uniform in size. In apparatus for use in sewage treatment, it will also help reduce clogging of slots 120 by solids suspended in the sewage that passes through the slots as the shearing liquid.
The size of the bubbles can be further decreased if the wettability of the material surrounding the capillary open-ings in the gas diffusing surface with the shearing liquid is low, or in other words the contact angle is high. Both hollow ; l fiber glass strands 158 (which terminate in capillary openings 154) and epoxy matrix material 162 have a contact angle with tap water of approximately 20. The preferred contact angle when tap water is the shearing liquid is at least about 60;
E~ PLES
.
Figures 15 and 16 are photomicrographs, at an en-largement of about 280 times in the original figure submit~ed as a part of this application, which show the formation of very -58- ;
06966S f fine ~ubbles of air resulting from the use of the present in-vention.
These photomicrographs show an identical area in a gas diffusing surface and generally similar bubble formations, but were taken at different times so that they show different groups of bubbles in the respective bubble formations. One of the parallel planar sides of the shearing liquid transmitting slot ~as a gas diffusing surface of the type shown in Figure 14.
The other side was a glass plate through which the photomicro-graphs were taken. The shearing liquid was tap water.
The dark portions on the left-hand side of the photo-micrographs in Figures 15 and 16 represent hollow capillary strands 158 embedded in epoxy binder 162 as a matrix, but be- ~ -cause of the lighting problem, capillary openings 154 cannot be distinguished. A few ~as bubbles 122 sheared from open-ings 154 b~ the advancing shearing liqui~ can be made out faintly over these dark portions of gas diffusing surface 146 where they join the medium dark, narrow, vertical band 163 (which may represent a layer of epoxy binder without any capil-lary strands 158 embedded therein) in the central part of each photomicrograph.
The bright portions on the right-hand side of Figures 15 and 16 represent the edges of metal suppoxt plates 170. ~e-cause there is more reflected light here, small air bubbles 122 can be seen quite clearly. ~he presence of identifiable groups of gas bubbles in relatively well alignedlis~rings,ll all of the bubbles in a given string being very nearly of the same size, shows the presence of the substantially parallel laminar flow that is characterized above as one of the essential features of this invention. Strings of bubbles 122a, 122b and 122c in Figure lS, as well as strings of bubbles 122d, 122e and 122f in Figure 16, all provide clear evidence of substan~-' A _59_ -., ' . .
106~665 tially parallel laminar flow, including a partially developed laminar boundary layer, The strings of bubbles are not entirely straight because o~ minimal "detours," such as might be caused by tiny local irregularities in the gas diffusing surface, but such minor deviations are always to be expected even in highly stable laminar flow. Presumably each separately identifiable string of bubbles originated at a different capillary opening.
If the few atypically large or small bubbles in Fig-ures 15 and 16 are omitted from consideration, it is seen that the great bulk of the bubbles are quite uniform in size. Thus, the diameter of the smallest bubbles that fall within normal limits of size appear to have diameters approximately two-thirds as large as the diameters of the largest bubbles that fall with-in those normal limits.
Photomicrographs of somewhat larger fields showing similar air bubble formation (which were made at an enlargement of about 200 times, and include the partial views seen in Fig-ures 15 and 16, respectively) were examined with the aid o~ a sizing grid in the form of a clear plastic overlay. In this manner, the estimated average bubble sizes and maximum bubble sizes given below were determined for the two larger photographs.
Similar determinations were also made for the other examples given below.
For all the photomicrographs examined (including Fig-ures 15 and 16), the following conditions were held constant:
Shearing liquid Tap waté~r Water pressure 3.0 psig Median capillary size 8 microns (approxOj Active gas diffusing 0.00042 square ~eet surface area Slot width 0.040 inches ~nlet end of slot to most 0.045 inches !
remote capillary opening . .
Arrangement of capillary As illustrated in openings Figure 14 Exam~le 1 A.
Six high speed frames were exposed under the follo~7-in operating conditions:
Liquid velocity 8.3 ft./sec.
Gas pressure 20 psig Gas flo~ for 3 inch slot 0.00175 ~cfm Gas bar Unsanded*
* Note -- In Examples 1 through 3, the gas transmittin~ bodies or "bars" were used as usually fabricated in accordance with this invention. In Examples 4 through 6, the edges of the gas bars that constitute the gas diffusing surfaces were sanded, in an effort to make those surfaces more uniform and thereby im-prove the bubble shearing action of the apparatus. As the bub-ble sizes obtained show, however, the sanding did not achieve the results desired, and actually appears to have produced lar-ger bubbles than were produced with the unsanded gas bars.
The followlng results were observed when the photo-.. . , . .. . _ .. , . . . ..... , . . _ .. . .. .. . _ . _ . . . .
micrograpps taken under the indicated conditions were examined as explained above:
Bubble size for all 37 microns = median*~ -frames 200 microns = maximum**
Bubble size for 4th 3g microns = median 'I frame (of which 120 microns = maximum Figure 15 is a ' portion) Bubble size for 7th 39 microns = median 3n . frame ~of which 200 microns ~ maximum -~
Figure 16 is a portion) -~
, .... . . . .....
f 10f;~665 ** Note -- In determining both median and maximum bubl~le size, all bubbles were considered, including any atypical bubble~
that fall at the extremes of size.
.
B. .
Seven high speed frames were exposed under the fol-lowing operating conditions:
Liquid velocity 8.3 ft./sec.
Gas pressure 30 psig.
Gas flow for 3 inch slot 0.0061 scfm Gas bar Unsanded ~:
The following results were observed when the photo- :~
micrographs taken under the indicated conditions wer2 examined .
as explained above: .
. ~ubble size for all 35 microns = median frames . 225 microns = maximum ~ ;
Example 2 A
Seven high speed frames were exposed ~nder the follow-i~g operating conditions: .
i 20 Liquid velocity 16 ft./sec.
Gas pressure 30 psig Gas flow for 3 inch slot 0.0061 scfm Gas bar Unsanded ., The following results were observed when the photo-micrographs taken under the indicated conditions were examined as explained above: -Bubble size for all 28 microns = median frames 100 microns = maximum B.
Seven high speed frames were exposed under the fol- :
lowing ~perating conditions:
-62- .
-, `` ~ 10~9665 Liquid velocity 4.4 ft./sec.
Gas pressure 20 psig Gas flow for 3 inch slot 0.00175 scfm Gas bar Unsanded The following results were observed when the photo-micrographs taken under the indicated conditions were examined as explained above:
Bubble size for all 43 microns = median frames 200 microns = maximum Example 3 A.
! Five high speed frames were exposed under the follow-ing operating conditions:
Liquid velocity 22.5 ft./sec.
Gas pressure 30 psi~
Gas flow for 3 inch slot 0.0061 scfm Gas bar Unsanded The following results were observed when the photo-micrographs taken under the indicated conditions were examined as explained above:
Bubble size for all 26 microns = median frames 75 microns = maximum ., . .
B.
Seven high speed frames were exposed under the fol-lowing operating conditions:
Liquid velocity 22.5 ft./sec.
Gas pressure 20 psig ; .
Gas flow for 3 inch slot 0.00175 scfm ; Gas bar Unsanded ~-~he following results were observed when the photo-micrographs taken under the indicated conditions were-e~a,nined ,i . ' , ~ ~63-- . : . . ,: . . .: : . ~ :
069665 ~-as explained abcve: ~
Bubble size for all 25 microns = median frames 50 microns = maximum Example 4 A.
Five hi~h speed frames were exposed under the follow-ing operating conditions:
Liquid velocity 7.9 ft./sec.
Gas pressure 20 psig Gas flow for 3 inch slot 0.0032 scfm Gas bar Sanded The following results were observed when the photo-micrographs taken under the indicated conditions were examined as explained above:
Bubble size for all 76 microns - median frames 200 microns - maximum B.
Seven high speed frames were exposed under the fol-I lowing operating conditions:
; 20 Liquid velocity 7.9 ft./sec.
Gas pressure 30 psig Gas flow for 3 inch slot 0.012 scfm ,~ G,as bar Sanded The following results were observed when the photo-micrographs taken undar the indicated conditions were examined as explained above:
Bubble size for all 98 microns = median frames 300 microns = maximum Example 5 A.
Six high speed frames were exposed under the following operating conditions:
Liquid velocitv 16 ft./sec.
Gas pressure 30 psig Gas flow for 3 inch slot 0.012 scfm Gas bar Sanded The following results were observed when the photo-micrographs taken under the indicated conditions were examined as explained above:
Bubble size for all 52 microns = median frames 100 microns - maximum :.:
B-Seven high speed frames were exposed under the fol-lowing conditions:
Liquid velocity 20.7 ft./sec, Gas pressure 30 psig Gas flow for 3 inch slot 0.012 scfm Gas bar Sanded The following results were observed when the photo-micrographs taken under the indicated conditions were examined 20 as explained above:
Bubble size for all 40 microns = median frames 75 microns = maximum ~ -Ex~ple 6 A.
Six high speed frames were exposed undèr the follow-ing operating conditions:
Liquid velocity 20.7 ft./sec.
Gas pressure 40 psig , ; Gas flow for 3 inch slot 0.027 scfm Gas bar Sanded .. .......
-65- ~, ' ' - - .
-' ' : :- .: ' .. :
The following results were observed when the photo-micrographs taken under the indicated conditions were examined as explained above:
Bubble size for all 75 microns = median frames 125 microns = maximum B.
Eight high speed frames were exposed under the fol-lowing conditions- -Liquid velocity 20.7 ft./sec.
Gas pressure 50 psig Gas flow for 3 inch slot 0.052 scfm Gas bar $anded The following results were observed when the photo- ' micrographs taken under the indicated conditions were examined as explained above:
, ~; Bubble size for all 100 microns ~ median frames 150 microns = maximum ~.
'~ As is seen from the above results, the smallest bub-bles were obtained with the unsanded bar under the operating conditions of Example 3, the next smallest bubbles with that bar under the~conditions of Example 2, and the next smallest ' ' ln Example l. The same gas bar and target area were used in all these three examples.
With the sanded gas bar, the best bubble shearing was~obtained in Example ~, and the next best in E ~mple 6.
Example~4 gave the poorest results. The bar with the sanded ~ gas diffusing surfaae was in all other respects the same typ~
!~ f bar~as~the~unsanded bar.
TheEbubble~shearlng of Examples 1 through 6, as ex-30 ~ plained~above,~was carried out with one wall of the shearin~ligUId~;transmi~ti-g slot formed by a flat glass plat-, which~
~ . : ,. - , . :. ,: . :
~069665 ~ ~
made it possible to photograph sample gas bubbles produced unde~
the indicated conditions. In Examples 7 through 9 below, the ap-. ~ .
paratus used was not modified in this way, but was operated as it would ha~e been operated in actual use, with a gas diffusing surface similar to the surface illustrated in Figure 14 on both sides of the shearing liquid transmitting slot.
Example 7 Excellent oxygen bubble shearing was visually observed from use of one embodiment of the apparatus of this invention, with tap water as the shearing liquid, under substantially the following conditions:
Sl~t width 0.040 inches Volumetric flow rate 2 gallons/min./inch for shearing liquid of slot Linear flow rate for 15 ft./sec.
` shearing liquid j Gas pressure 30 psig Gas flow rate 50 scfm/sq. ft. of active gas diffusing area Median capillary diameter 8 microns Example 8 I
Excellent oxygen absorption, more than 90 per cent - of the oxygen supplied to the body of liquid, was obtained with the use of the same apparatus, with 1 per cent suspended "
solids sewage as the shearing liquid, under subs~antially the following conditions:
l ~ Slot width 0.040 inches 1`~` : , ; Volumetric flow rate 2.5 gallons~min./inch , for shearing liquid of slot Linear flow rate for 20 ft./sec.
., ~ .
shearing liquid Gas pressure 30 psig . , .. .. .. , , . ~ . . . . . . . .
. . : .: . ~ : . . ~
Gas flow rate 45 scfm/sq. ft. of active gas diffusing area ~ledian capillary diameter 8 microns Example 9 Excellent oxygen absorption, again more than 90 per ' cent, was also obtained with use of the same apparatus, except that the slot width was set at 0.125 inches, with 6 per cent suspended solids se~lage as the shearing liquid, under substan-tially the foll'owing conditions:
Slot width 0.125 inches Volumetric flow rate 2.5 gallons/min./inch for shearing liquid of slot Linear flow rate ~or 7-8 ft./sec.
shearing liquid Gas pressure 30 psig ~' Gas flow rate 15 scfm/sq. ft. of active ' gas diffusing area Median capillary diameter 8 microns Because of the higher viscosity of the 6 per cent suspended solids sewage and the resulting improved bubble shearing, approximately the same oxygen absorption efficiency was obtained with 6 per cent solids sewage as with 1 per cent solids sewage, at about one-third the linear flow rate and approximately the same energy expended for pumping.
Specific Embodiments :of Gas Bari I Because of their elongated shape '(best seen in Figures 18A and 24 A), gas transmitting bodies 166 such as illustrated ' '~
in Figure 12 are for convenience usually referred to as gas "bars."
~30 Figures 18A and 18B show a gas bar 166a that may'be incorporated in the gas diffusing apparatus of this invention in the manner illustrated schematically in Figure 12. Gas .
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69~65 ~ ~
bar lG6a is an enclos~d container with a gas inlet opening 184 provided through elbow joint 186, suitably threaded for attach~
ment to a gas feeder line 130 (Figure 12). This embodiment of the gas transmittin~ body has two walls 1~6 and 148 with a plu-rality of gas transmitting passages extending therethrough, to permit gas to escape from interior enclosed chamber 160. Walls 146 and 148 are the gas diffusing surfaces of gas bar 166a.
Figures 24A through 24C show another embodiment of a gas bar 166b for use with this invention. This embodiment has only one gas diffusing surface, wall 148, to permit gas to es-cape ~rom interior enclosed chamber 160. As in gas bar 166a, inlet opening 184 and fitting 186 are provided in gas bar 166b.
As will be explained in more detail below, each gas diffusing surface 146 and 148 in specific gas bars 166a and 166b comprises a sandwich of two thin metal support plates --members 188 and 190 for gas bar 166a, and 192 and 194 for gas bar 166b -- on both sides of a plurality of hollow fiber glass I capillary strands 206 embedded in epoxy 162 as a matrix. Be-cause the support plates are so thin (Figures 18B, 24B and 24C), capillary openings 154 lie in close proximity to boundary de-fining edges 196 and 198 of the gas diffusing surfaces of gas bars 166a and 166b, respectively, ~^~hich become the leading edges of slots 120 when the gas bars are in place in the gas diffus-ing apparatus of this invention. (The flow of the shearing li- -~uid in the bubble shearing apparatus in which gas bar 166a or , ~....................... .
166b is installed is in the upward direction in Figures 18B, 24B and 24C.~ !
As used in this specification and claims, the term "gas diffusing surface" includes the sîdes of the two support plates but, as already indicated above, the "active area" o~
that surface includes only the area defined by the boundary lines tangent to the outermost capillary openings. (See Fig~re .. ' ~, \ .'" .
. ~ ~ 106966S
14, for example.) In the emhodiments shown in Figures 18 and 24, each support plate is about 0.025 inches thick, which brings the capillary openings as close to the leading edge of the gas dif-fusing surface as is structurally practicable in these e~odi-ments. Since the layer of hollow capillary strands 206 e~ed-ed in matrix 162 is about 0.020 inches thick, this positions the most remote capillary openings a total of about 0.045 inches from leading edgés 196 and 198, respectively.
Good results can be obtained under some operating conditions with the distance from the indicated boundary defin-; ing edge of the gas bar of this invention to the most remote capillary openings being about 1.0 inch. Improved results are obtained if that distance is about 0.5 inch, and still better results if it is about 0.1 inch, 0.05 inch, or even smaller.
Leading edges 196 and 198 are rounded to improve the stability of flow through slot 120. This feature helps produce stable laminar flow,including a partially developed laminar boundar~ layer, in the shearing liquid transmitting slot~ The rounded edges are shown in Figures 18B, 24B and 24C as accurate-ly as is possible at the scale of those drawings.
Gas diffusing surfaces 146 and 148 in gas bars 166a and 166b are similar to the surfaces shown in Figure 14 and described above in this specification. The surfaces are gener-ally planar, and the gas transmitting bars of which théy are a part are constructed in a manner similar to that described above in connection with Figure 14. This includes capillary passages of a shape, size ana orientation similar to the cap-illary passages indicated in Figure 14, as well as capillar~ -o~enings of an arrangement and location similar to the open-ings shown in Figure 14.
-7~-' 6g66S
Method of Fabrication Of ~as Bars Figures 17 through 24 illustrate two methods of fab-ricating gas bars for use in the apparatus of this invention designed for the oxygenation of sewage in the form either of~
so-called "mixed liquor" having about 1 per cent suspended solids or aerobic digester sludge having about 6 per cent sus-pended solids.
Figure 17A is a plan view of thin bottom support plate 188, which is rectangular in over-all shape and has a generally rectangular opening 200 in the interior portion there-of to provide an elongated O-shaped member. This member has two inner side ~Jalls 202 that are adapted to form a part of the interior wall of gas plenum 160, and two outer side walls 204 adapted to form a part of the exterior wall of the gas plenum.
As shown in Figure 17B, the first step of this method of fabricating gas bar 166a is to position a plurality of hollo~t capillary strands 206 and a supply of binder 162 across the top face of thin support plate 188 so that the hollow strands are aligned substantially normal to side walls 202 and 204. In this position, the strands extend beyond plate 188 on both the inner and outer sides of the plate~
The binder applied in this first step may be already present upon the fiber glass as received from the manufacturer of the hollow capillary strands, or it may be added by the ` fabricator of the gas bar either before or after the strands are positioned across bottom support plate 188. ~en desir~d, additional binder may be added to the hollow capillary strands in position upon plate 188, before thin top support plate 190, similar in all respects to bottom support plate 188, is added (Figure 18C) to produce the "sandwich" of two plates with hollow capillary strands and binder between them.
This sandwich is compressed to cause binder 162 to --71 ~
: ~ , .. , ' ';
106~665 fill all the crevices between strands 206 and between the strands and support plates 188 and 190. The sandwich is then ~eld compressed until the binder is hardened to embed the hollow capillary strands in the hardened binder as a matrix.
Binder 162 may if desired be a heat curable binder, and in that case heat is applied while the sandwich is compressed, to cure the binder.
In the next step of this method o~ fabricating gas bars, the portions 208 of hollow capillary strands 206 em-bedded in matrix 162 that extend outwardly beyond exteriorside walls 204 of support plates 188 and 190 are trimmed back generally to walls 204 t~ form gas diffusing surf2ces 146 and 148. The surface of the exterior side wall of the gas bar that is comprised of a first wall 204, a plurality o c~pillary openings 154 surrounded by epoxy 162, and a second wall 204 is preferably made as nearly flat as possible, but the extent of minor departures from the desired planar surface will be deter-mined by the cutting tool and trimming method employed.
The portions 210 of hollow capillary strands 206 em-bedded in matrix 162 that extend inwardly beyond interior sidewalls 202 of support plates 188 and 190 are severed to form the inlet ends of gas transmitting passages 158. It is not essential that these ends be trimmed back to wa~ls 202, but only that they provide passages for gas to leave gas plenum 1604 .
Gastight cover plates 212 and 214 are next affixèd to the sandwich, on opposite sides thereof, to form gas plenum 160 into which gas can be introduced through inlet opening 184, and out of which gas can flow through hollow capillary strands 15~. A layer of binder, in the form of liquid or in the form of a flat sheet of appropriate dimensions as desired, can be interposed between bottom support plate 188 and its as-fiOCiate~ c~ver plate 212 and between top support plate 190 .nd ..
':
.. , .... , .: . . . . .... . . .
~069665 its associated cover plate 214. A11 the elements of the gas bar are held compressed until ~he binder has hardened, with the additional use of heat if the binder is heat curable.
Chamfers 216 are preferably provided on the outer edges of upper bac}ing plate 214 that lie adjacent the flow path of the shearing liquid as the liquid flows out from the discharge end of the liquid transmitting slot (in an up~ard .. .~:
direction in Figures 18B, 24B and 24C), when the gas bar is ~.
installed in place in ~ubble shearing apparatus constructed ac-cording to this invention. The reason for these eh mfers is.
to avoid the "attachment" of the discharge flow from the æhear-.ing liquid transmitting slot to walls 218 of top backing plate 214 that would occur if those walls were not cut away as indi~ :
cated. Any such flow attachment would tend to cause the shear-ing liquid jet stream and the fine gas bubbles contained there-in to turn the right angled corner that would be presented by the full wall, and thus cause the jet stream to widen out I as it entered the body of liquid into which the gas bubbles , were being dispersed. Such a widening of the jet stream would i 20 reduce the effectiveness of the distribution of fine gas bub-¦ bles throughout the liquid into which they were being intro-duced; it is preferred to keep the jet stream narrow enough, if possible, that its total angle of spread in the initial stages is no more than about 15.
~: By use of a special winding fixture, the fabrication steps illustrated in Figures 17A through 17C may ~e carried out ¦~ : on a number of units at one.~ime. With winding fixture 220 as illustrated in Figure 19, eight bottom support plates 188 ;may be secured to the fixture, two on each vertical`wall of il~: .:
;;30 ~ the fixture, and hollow fiber glass capillary strands 206 ~: wrapped around all eight plates at the same time. With the .~ necessary binder 162 in place, top support plates 190 may be' ~., \~ , ':: . /
: .
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-` ~ 106966S
positioned to form eight sandwiches, and the necessary compres-sion and heating steps completed to produce eight separate units such as illustrated in Figure 17C.
Figure 20A illustrates the positioning on bottom sup-port plate 188 of a plurality of lengths of roving 221, in much the same manner as individual capillary strands are positioned in Figure 17B. Each length of roving 221, as seen in the sche-matic cross-sectional view of Figure 20B, is formed of a plu-rality of hollow capillary strands 206. The capillary strands in substantially all said lengths of roving 221 are generall~
parallel to each other.
Figures 21A through 24C illustrate another type of gas bar 166b, which is fabricated in a manner very similar to the method of fabricating gas bar 166a already described. Hol-low fiber glass capillary strands 206 and a quantity of binder 162 are sandwiched between bottom plate 192a-192b and top plate 194a-194b, and the necessary compressing and curing steps are carried out. Outer portions 222 of capillary strands 206 embedded in binder 162 are trimmed back to form a gas dif-fusing surface, ar.d inner portions 224 of the strands and I binder are severed. In addition, support plates 192 and 194 are severed along lines 226 to form two separate sandwiches, each with a single gas diffusing surface 148.
Bottom cover plate 228 has a groove 230 adapted toreceive an assembled sandwich of capillary strands and binder between two support plates. Cover plate 228 hà~`a second groove 232 surrounded by an elongated C-shaped wall 234. The cross sectional views of Figures 22B and 22C, and the end viéw of Flgure 22D (taken from the bottom of Figure 22A), show the 3~0~ general configuration of cover plate 228.
Top cc.ver plate 236, as is seen from the plan view I
of Figure 23A and the end view of Figure 23C (taken from th~
1~)69665 bottom of Figure 23A), has a flat planar surfâce. When top cover plate 236 is placed upon bottom cover ~late 228, the asse~bled plates perform t~;o functions. First, they secure between them the sandwich of two support plates on each side of a layer of hollow capillary strands embedded in binder as a matrix (Figures 24B and 24C). Second, between them they define gas plenum 160, surrounded on three sides by wall 234 (Figure 22C~ and on the fourth side by the sandwich (Figure 24B). Binder layexs 238 and 240 complete the gastight assemblage of gas 10 bar 166b.
The Method Of This Invention ~ he method of this invention involves the same sub-stantially parallel laminar flow with a partially developed ; laminar boundary layer over very small capillary openings pre-ferably located in close proximity to the leading edge of a gas diffusing surface, any remaining portion of the capillary openings on the surface being free of any type of flow but fully developed laminar flow, that has been described above. The method is not limited to the particular configuration of ele-ments, including a shearing liquid transmitting slot, that com-prises the apparatus of this invention. However~ it is limited I to the establishment of a partially developed laminar boundary layer over at least about one-quarter of the capillary openings of the gas diffusing surface, in order that the layer can have a significant effect on the bubble formation resulting from use ~ of the method.
I In the method of the present invention, gas is flowed through a gas diffusing surface with capillary openings substan-tially all of which have a diameter no larger than about 100 mi-crons, and a shearing liquid is flowed past a leading edge of .
,- . - . ~ .
.
~. ' ., ~ , ~ .
06966S ff the gas diffusing surface and over that surface to a discharge edge thereof. The flow is carried out with minimal perturbation, and at a velocity to produce the substantially parallel laminar flo~l that is characteristic of this invention, including a par-tially developed laminar boundary layer over at least about the first quarter of the capillary openings in the gas diffusing surface. Best results are obtained if the partially developed layer extends over a majority of the capillary openings.
The method is preerably carried out with the capil-lary openings located immediately adjacent the leading edge ofthe gas diffusing surface. Good results are obtain~d with this method if the capillary openings most remote from the leading ;
edge are located no more than about 1 inch from that edge. Im-proved results are obtained if that distance is no more than about 0,5 inch, and best results if it is no more than about 0.1 inch, 0.05 inch, or even less.
The method provides the best bubble shearing if the gas diffusing surface is generally planar. ~he size of the sheared bubbles is smaller the larger the contact angle of the shearing liquid is with the material that is selected to define or surround the capillary openings in the gas diffusing surface.
That contact angle is preferably at least about 60 when the `shearing liquid used is tap water.
Satisfactory bubble shearing results are obtained with the method of this invention with a shearing liquid velocity of about 10 feet per second when water is employed as the shearing liquid. Improved results are obtained with a shearing liquid velocity of about 15 feet per second, and still more improvement is achieved if the method employs a shearing liquid velocity of ~' 30~ about 20 feet per second. A velocity of at least about 25 feet per second is preferred. -.
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069665 ~
Use of this method with gas being supplied to the gas transmitting passages at a rate to produce a gas flow through the passages of about 10 standard cubic feet per minute per square foot of diffusing surface will ordinarily produce good bubble shearing results. As already explained above, increas-ing the gas flow rate affects bubble shearing in ways known to those skilled in the art, hut generaily speaking a gas flow rate ; of 40 standard cubic feet per minute per square foot of active diffusing area gives improved results, and increasing this `
figure to 70 or somewhat more gives still better results.
- As the---shearing liquid flows-~wa~ fxom the discharge edge of the gas diffusing surface it carries with it, and into the body of liquid being treated, the many small gas bubbles 'I sheared off the gas difusing surface by the moving shearing j liquid, The presence of the hydrodynamic characteristics that characterize the method of this invention can be determined in the same manner as explained above in the description of the I apparatus of the invention. The shearing liquid may be trans-lucent or transparent, as for example water, if desired.
1~ ~ Effect Of Various Parameters ¦~ On Fine Bubble Formation In the above description of the method and apparatus of this invention, a number of factors have been discussed that determine the effectiveness of bubble shearing in achieving gas : .
bubbles of the smallest possible size, with as great unifor-mity of size as possible, and at the lowest possible expendi-ture of energy. There are many other factors affecting the dlffus~ion of gas bubbles into a body of liquid, some of which ~30~ oan~be~va~ried~and some of which are fixed, but all of which should~be taken into account in the practice of the present in-vention.
i . ~ .
' -77-~069665 ~
The parameters which are ordinarily fixed in any process involving diffusion of ~as into a body of liquid in-clude the ambient conditions, the nature of the gas diffused, and the nature of the liquid into which it is diffused.
At least three ambient conditions affect the rate at which gas bubbles can be dissolved into a body of liquid:
(1) Ordinarily, the colder the body of li~uid, the greater will be the solubility of the gas in the liquid. The viscosity of the liquid may also be increased somewhat by a 1~ reduced temperature~ which would tend to improve bubble she~r-ing, and thus make smaller bubbles, by increasing the viscous shear force. It may also decrease the rise rate of the gas bubbles and therefore give them a longer period of time in which to be dissolved in the liquid. At the same time, it may decrease the extent to which the gas bubbles can initially be laterally dispersed as a part of a fluid stream directed into the bo~ly of liquid. Finally, by increasing the surace tension of the surfaces between the gas bubbles and the liquid, a lower ambient temperature may tend to increase the average bubble size by making shearing more difficult, at the same time that it tends ~o affect, in the other ways just indicated, the manner ; in which bubbles are formed and absorbed into the liquid after they are formed.
(2) The type of mo~ement in the body of liquid will affect the degree of dispersion of the bubbles throughout the liquid. It may also affect the extent of convective diffusion, i.e., the rate at ~hich fresh liquid is presented to the bubble surface for acceptance of gas by absorption from the bubble.
The body of liquid, for example, may be still, it may have a smooth, generally horizontal current, it may have a smooth, generally ~ertical current, or it may be turbulent.
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~06g665 - ~
(3) The partial pressure o~ the gas being dissolved, as measured at the surface of the body of liquid into which the gas is introduced, will affect the rate of absorption by the liquid. The higher the partial pressure, the more rapidly the diffused gas will be absorbed into the liquid.
A number of factors affecting the rate of absorption of the gas will be determined by the particular liquid and gas that are utilized in the process in question. These include:
(1) ~hé solubilitv of the gas in the liquid.
(2) The rate of diffusion of the gas into the liquid through the surface of a bubble of a given size in a still body of the liquid.
(3) The characteristic surface tension at surfaces between the gas and the liquid, including the effect of any sur-factants or other contaminants present.
As disclosed in my Canadian patent 967,687 issued May 13, 1975, these straight fiber glass tubes are relatively easy , to embed in support bars 166 in parallel relationship with each 30 other and in a perpendicular relationship with the central plane of slot 120. This embedding of capillary tubes 158 may be accomplished, for example, by positioning the tubes in the de-~ ~ ~9665 sired relationship and ~lowing a suitab1e sealing material 1~2 around the tubes. I~hen sealing material or binder 162 solidi-fies, it sealingly interconnects capillary tubes 158 to form the bars 166 and prevent fluid flow around the tubes.
Other gas transmitting bodies may be used, if desired, with the method and apparatus of this invention. The gas trans-mitting body may be formed, for example, by drilling holes in member 166 or may be formed from porous sintered metal, porous ceramic material, metal or plastic screen or mesh material, various other woven materials, a porous sheet or membrane, or any other porous body having capillary openings in a gas dif-fusing surface, As explained above, the term "capillary openings"
is used in this specification and claims to mean any openings small enough to produce fine gas bubbles, no matter what the nature of the gas diffusing surface is on which the bubhles are formed, or the nature of the gas transmitting passages through which gas flows to reach those openings.
; Other porous composite materials besides hollow capil-lary strands 158 embedded in sealing material or binder 162 may also be used. In addition, solid filaments of glass, plastic or metal may be fused or sintered together in bundles to define gas transmitting passages in the form of the interstices between ad-jacent, substantially parallel filaments. Solid glass or plastic filaments are available, for example, with outside diameters of about 10 to 20 microns, and solid metal wire with outside dia-meters of about 40 to 80 microns. ~en such filaments are pressed together and fused or sintered, the resulting interstices between.filaments will provide quite small gas transmitting pas-: ~ :
sages, which terminate in capillary openings at which fine gas ~30 ~ bubbles can be formed and sheared off in accordance with the . ' , ', . ~ !., .
' ' " ," '" '' ,~ ' ' . ' ' .: :
6t~S
teaching of this invention.
I~atever the material may be of which the gas trans-mitting body is formed, the gas transmitting body and its gas diffusing surface should preferably have the characteristics discussed below in connection with the illustrative showing of Figure 14.
Since capillary tubes 158 in the embodiment shown are relatively small and closely spaced along the entire length of the slot 120, a large number of bubbles of a small diameter can be formed along sides 146 and 148 of slot 120. Bubbles of air in water ranging up to about 100 or 200 microns in diameter are readily o~tained. As already mentioned above, with a preferred embodiment of the apparatus of this invention, bubbles of air can be formed in water which have a median diameter of only about 25 to 30 microns. With higher viscosity liquids such as sewage, the majority of the bubbles will be even smaller.
As has been pointed out, the showing in Flgure 12 is a schematic one. In particular, gas transmitting passages 158 which extend through the wall of gas transmitting body or gas "bar" 166 to form capillary openings 154 in gas diffusing sur-faces 146 and 148 are shown schematically embedded in hardened binder 162 as a matrix. As shown, capillary tubes 158 and binder 162 are secured on both sides of the assemblage to sup-port plates 170 to form gas bar 166 enclosing gas plenum 160.
Figure 14, on the other hand, is drawn from a photo-micrograph made of an actual gas diffusing surface 146 or 148 in a gas bar 166. Capillary openings 154 are the open ends of discrete hoIlow fiber glass capillary strands or tubes 158 em-bedded in hardened epoxy plastic 162 as a matrix. Support plates 170, which complete gas diffusing surface 146, are in-, .
.
~06g6~s dicated by broken lines on either side of Figure 14. The dis-tance between the two plates is about 0.020 inches.
The flow of shearing li~uid across gas diffusing surface 146 (or 148) in Figure 14 is from left to right in the figure, while the gas flo~ is out of the paper. Nascent gas bubbles formed at capillary openings 154 are thus sheared off as extremely fine bubbles,and are swept to the right towards the body of liquid into which they are to be diffused.
The maximum bubble size produced ~ith the method and apparatus of this invention, as has been stressed above, is affected by many factors. One of the most important of these is the size of capillary openings 154 in gas diffusing surface 14G or 148. It is considered that to take full advantage of the benefits produced by the other important features of the pre-sent invention, substantially all the capillary openings in the gas diffusing surface should have a diameter no larser than ahout 100 microns. Considerably improved results are obtained if substantially all the capillary openings have a di-ameter no larger than about 50 microns, with further improve-ment when that figure is about 25 microns. For preferred re-sults, the maximum capillary diameter should be about 5 microns.
Another measure of the size of the capillary open-ings is related to the bubble size that is desired from use of this invention. ~len the apparatus of this invention is de-signed to diffuse into a body of liquid gas bubbles of a pre-determined size, the maximum diameter of the capillary openings in the gas diffusing surface should be no greater than about that p,redetermined size. For better results, the maximum cap-illary diameter should be no more than approximately 1/2 the 3C predetermined maximum bubble diameter, and for best resultsno more than about 1/4 that dimension.
In general, the capillary diameters should preferably : - :
10696f~5 be made as small as is practicable without making the gas pres-sure drop through the capillaries too high.
Each fiber glass hollow capillary strand or tube 158 has a cross section of substantially uniform size and shape throughout its length, ~eing substantially a right circular cylinder in shape. As a consequence, each of the hollo~J capil-lary strands provides a straight, unimpeded path for gas flow through the gas transmitting body past which the shearing li-quid flows. The passage provided by each hollow capillary strand 158 should preferably extend, adjacent its respective capillary opening, in a direction substantially perpendicular to the plane tangent to gas diffusing surface 146. It is be-lieved that regularity in all these aspects of the size, shape and orientation of gas passages 158, by tending to reduce tur-bulence within those passages, will contribute to greater uni-formity o~ bubble size in the fine bubbles resulting from the use of this invention.
Gas turbulence within capillary passages 15~ should also be minimized by selecting a length for each passage that is at least 10 times the diameter of the passage. It is be-.; .
lieved that an L/D ratio of at least 20/1 will give still better results.
The gas transmitting capillary passages 158 should also have a minimum L/D ratio to minimize backing up by the shearing liquid after a gas bubble has been sheared from cap-illary opening 154. Backing up of liquid can result in clog-ging, when the liquid evaporates and precipitates out dissolved salts after the gas again fills the capillary passage. These advantages of a minimum L/D ratio for the gas passages must of ; 30 course be balanced against the disadvantage of any increase in i the pressure drop through the passages.
. .
: `
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- ~ ~96t;5 In Fiqure 12, capillary openings 154 are arranged in a plurality of straight rows perpendicular to the flow of shear-ing liquid through slot 120. In Figure 14,the capillary open-ings are randomly located across gas diffusing surface 146.
Although the benefits of the present invention can be obtained with only a few capillary openings disposed along the direction of flow, there should be an average of at least about 5 open-ings in that direction in substantially all portions of the sur-face. If a straight edge rule is laid across Figure 14 in the direction of shearing liquid flow (from left to right across the page), it will be seen that in this embodiment of a gas diffus-ing surface there are on the average more than 10 capillary openings distributed across that surface from its inlet end to its discharge end. An average of as many as 20 to 40 openings in the direction of shearing liquid flow have been used to ad-vantage, and still more could be employed if desired.
To take the greatest advantage of the fine bubble formation resulting from the use of this ir.vention, the land areas in the gas diffusing surface should not be too narrow, or in other words capillary openings 154 should not be too closely spaced to each other. To this end, each opening 154 is prefer-ably surrounded by a minimum land area on all sides that is suf-ficiently extensive in a plane generally tangent to the outer-most points on said gas diffusing surface to substantially avoid coalescence or collision of a gas bubble formed at that opening with a bubble formed at any of the other single open-ings to produce a bubble having a diameter larger than the predetermined maximum bubble diameter. Some such combining of gas bubbles either at their point of formation or downstream is acceptable, but this effect should be minimized as much as is practicable.
, -57-; ~ :
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: ' . . ' : ' r' .0696~;5 In a preferred embodiment, each capillary opening 154 is surrounded by a minimum land area elliptical in shape that extends on each side of the opening in the direction perpen-dicular to the direction of liquid flow through the liquid trans-mitting slot by a distance at least ahout equal to the maximum bubble diameter, and on each side of the opening in the di-rection of liquid flow by a distance equal to at least about three times the maximum bubble diameter, measured in a plane generally tangent to the outermost points on the gas diffusing surface.
Gas diffusing surface 146 shown in Figure 14 is gen-erally planar in configuration, to produce greater stability in the laminar flow through slot 120, and thus the best fine ; bubble formation. A planar gas diffusing surface will produce sharper edges for capillary openings 154, which should result in smaller bubbles more nearly uniform in size. In apparatus for use in sewage treatment, it will also help reduce clogging of slots 120 by solids suspended in the sewage that passes through the slots as the shearing liquid.
The size of the bubbles can be further decreased if the wettability of the material surrounding the capillary open-ings in the gas diffusing surface with the shearing liquid is low, or in other words the contact angle is high. Both hollow ; l fiber glass strands 158 (which terminate in capillary openings 154) and epoxy matrix material 162 have a contact angle with tap water of approximately 20. The preferred contact angle when tap water is the shearing liquid is at least about 60;
E~ PLES
.
Figures 15 and 16 are photomicrographs, at an en-largement of about 280 times in the original figure submit~ed as a part of this application, which show the formation of very -58- ;
06966S f fine ~ubbles of air resulting from the use of the present in-vention.
These photomicrographs show an identical area in a gas diffusing surface and generally similar bubble formations, but were taken at different times so that they show different groups of bubbles in the respective bubble formations. One of the parallel planar sides of the shearing liquid transmitting slot ~as a gas diffusing surface of the type shown in Figure 14.
The other side was a glass plate through which the photomicro-graphs were taken. The shearing liquid was tap water.
The dark portions on the left-hand side of the photo-micrographs in Figures 15 and 16 represent hollow capillary strands 158 embedded in epoxy binder 162 as a matrix, but be- ~ -cause of the lighting problem, capillary openings 154 cannot be distinguished. A few ~as bubbles 122 sheared from open-ings 154 b~ the advancing shearing liqui~ can be made out faintly over these dark portions of gas diffusing surface 146 where they join the medium dark, narrow, vertical band 163 (which may represent a layer of epoxy binder without any capil-lary strands 158 embedded therein) in the central part of each photomicrograph.
The bright portions on the right-hand side of Figures 15 and 16 represent the edges of metal suppoxt plates 170. ~e-cause there is more reflected light here, small air bubbles 122 can be seen quite clearly. ~he presence of identifiable groups of gas bubbles in relatively well alignedlis~rings,ll all of the bubbles in a given string being very nearly of the same size, shows the presence of the substantially parallel laminar flow that is characterized above as one of the essential features of this invention. Strings of bubbles 122a, 122b and 122c in Figure lS, as well as strings of bubbles 122d, 122e and 122f in Figure 16, all provide clear evidence of substan~-' A _59_ -., ' . .
106~665 tially parallel laminar flow, including a partially developed laminar boundary layer, The strings of bubbles are not entirely straight because o~ minimal "detours," such as might be caused by tiny local irregularities in the gas diffusing surface, but such minor deviations are always to be expected even in highly stable laminar flow. Presumably each separately identifiable string of bubbles originated at a different capillary opening.
If the few atypically large or small bubbles in Fig-ures 15 and 16 are omitted from consideration, it is seen that the great bulk of the bubbles are quite uniform in size. Thus, the diameter of the smallest bubbles that fall within normal limits of size appear to have diameters approximately two-thirds as large as the diameters of the largest bubbles that fall with-in those normal limits.
Photomicrographs of somewhat larger fields showing similar air bubble formation (which were made at an enlargement of about 200 times, and include the partial views seen in Fig-ures 15 and 16, respectively) were examined with the aid o~ a sizing grid in the form of a clear plastic overlay. In this manner, the estimated average bubble sizes and maximum bubble sizes given below were determined for the two larger photographs.
Similar determinations were also made for the other examples given below.
For all the photomicrographs examined (including Fig-ures 15 and 16), the following conditions were held constant:
Shearing liquid Tap waté~r Water pressure 3.0 psig Median capillary size 8 microns (approxOj Active gas diffusing 0.00042 square ~eet surface area Slot width 0.040 inches ~nlet end of slot to most 0.045 inches !
remote capillary opening . .
Arrangement of capillary As illustrated in openings Figure 14 Exam~le 1 A.
Six high speed frames were exposed under the follo~7-in operating conditions:
Liquid velocity 8.3 ft./sec.
Gas pressure 20 psig Gas flo~ for 3 inch slot 0.00175 ~cfm Gas bar Unsanded*
* Note -- In Examples 1 through 3, the gas transmittin~ bodies or "bars" were used as usually fabricated in accordance with this invention. In Examples 4 through 6, the edges of the gas bars that constitute the gas diffusing surfaces were sanded, in an effort to make those surfaces more uniform and thereby im-prove the bubble shearing action of the apparatus. As the bub-ble sizes obtained show, however, the sanding did not achieve the results desired, and actually appears to have produced lar-ger bubbles than were produced with the unsanded gas bars.
The followlng results were observed when the photo-.. . , . .. . _ .. , . . . ..... , . . _ .. . .. .. . _ . _ . . . .
micrograpps taken under the indicated conditions were examined as explained above:
Bubble size for all 37 microns = median*~ -frames 200 microns = maximum**
Bubble size for 4th 3g microns = median 'I frame (of which 120 microns = maximum Figure 15 is a ' portion) Bubble size for 7th 39 microns = median 3n . frame ~of which 200 microns ~ maximum -~
Figure 16 is a portion) -~
, .... . . . .....
f 10f;~665 ** Note -- In determining both median and maximum bubl~le size, all bubbles were considered, including any atypical bubble~
that fall at the extremes of size.
.
B. .
Seven high speed frames were exposed under the fol-lowing operating conditions:
Liquid velocity 8.3 ft./sec.
Gas pressure 30 psig.
Gas flow for 3 inch slot 0.0061 scfm Gas bar Unsanded ~:
The following results were observed when the photo- :~
micrographs taken under the indicated conditions wer2 examined .
as explained above: .
. ~ubble size for all 35 microns = median frames . 225 microns = maximum ~ ;
Example 2 A
Seven high speed frames were exposed ~nder the follow-i~g operating conditions: .
i 20 Liquid velocity 16 ft./sec.
Gas pressure 30 psig Gas flow for 3 inch slot 0.0061 scfm Gas bar Unsanded ., The following results were observed when the photo-micrographs taken under the indicated conditions were examined as explained above: -Bubble size for all 28 microns = median frames 100 microns = maximum B.
Seven high speed frames were exposed under the fol- :
lowing ~perating conditions:
-62- .
-, `` ~ 10~9665 Liquid velocity 4.4 ft./sec.
Gas pressure 20 psig Gas flow for 3 inch slot 0.00175 scfm Gas bar Unsanded The following results were observed when the photo-micrographs taken under the indicated conditions were examined as explained above:
Bubble size for all 43 microns = median frames 200 microns = maximum Example 3 A.
! Five high speed frames were exposed under the follow-ing operating conditions:
Liquid velocity 22.5 ft./sec.
Gas pressure 30 psi~
Gas flow for 3 inch slot 0.0061 scfm Gas bar Unsanded The following results were observed when the photo-micrographs taken under the indicated conditions were examined as explained above:
Bubble size for all 26 microns = median frames 75 microns = maximum ., . .
B.
Seven high speed frames were exposed under the fol-lowing operating conditions:
Liquid velocity 22.5 ft./sec.
Gas pressure 20 psig ; .
Gas flow for 3 inch slot 0.00175 scfm ; Gas bar Unsanded ~-~he following results were observed when the photo-micrographs taken under the indicated conditions were-e~a,nined ,i . ' , ~ ~63-- . : . . ,: . . .: : . ~ :
069665 ~-as explained abcve: ~
Bubble size for all 25 microns = median frames 50 microns = maximum Example 4 A.
Five hi~h speed frames were exposed under the follow-ing operating conditions:
Liquid velocity 7.9 ft./sec.
Gas pressure 20 psig Gas flow for 3 inch slot 0.0032 scfm Gas bar Sanded The following results were observed when the photo-micrographs taken under the indicated conditions were examined as explained above:
Bubble size for all 76 microns - median frames 200 microns - maximum B.
Seven high speed frames were exposed under the fol-I lowing operating conditions:
; 20 Liquid velocity 7.9 ft./sec.
Gas pressure 30 psig Gas flow for 3 inch slot 0.012 scfm ,~ G,as bar Sanded The following results were observed when the photo-micrographs taken undar the indicated conditions were examined as explained above:
Bubble size for all 98 microns = median frames 300 microns = maximum Example 5 A.
Six high speed frames were exposed under the following operating conditions:
Liquid velocitv 16 ft./sec.
Gas pressure 30 psig Gas flow for 3 inch slot 0.012 scfm Gas bar Sanded The following results were observed when the photo-micrographs taken under the indicated conditions were examined as explained above:
Bubble size for all 52 microns = median frames 100 microns - maximum :.:
B-Seven high speed frames were exposed under the fol-lowing conditions:
Liquid velocity 20.7 ft./sec, Gas pressure 30 psig Gas flow for 3 inch slot 0.012 scfm Gas bar Sanded The following results were observed when the photo-micrographs taken under the indicated conditions were examined 20 as explained above:
Bubble size for all 40 microns = median frames 75 microns = maximum ~ -Ex~ple 6 A.
Six high speed frames were exposed undèr the follow-ing operating conditions:
Liquid velocity 20.7 ft./sec.
Gas pressure 40 psig , ; Gas flow for 3 inch slot 0.027 scfm Gas bar Sanded .. .......
-65- ~, ' ' - - .
-' ' : :- .: ' .. :
The following results were observed when the photo-micrographs taken under the indicated conditions were examined as explained above:
Bubble size for all 75 microns = median frames 125 microns = maximum B.
Eight high speed frames were exposed under the fol-lowing conditions- -Liquid velocity 20.7 ft./sec.
Gas pressure 50 psig Gas flow for 3 inch slot 0.052 scfm Gas bar $anded The following results were observed when the photo- ' micrographs taken under the indicated conditions were examined as explained above:
, ~; Bubble size for all 100 microns ~ median frames 150 microns = maximum ~.
'~ As is seen from the above results, the smallest bub-bles were obtained with the unsanded bar under the operating conditions of Example 3, the next smallest bubbles with that bar under the~conditions of Example 2, and the next smallest ' ' ln Example l. The same gas bar and target area were used in all these three examples.
With the sanded gas bar, the best bubble shearing was~obtained in Example ~, and the next best in E ~mple 6.
Example~4 gave the poorest results. The bar with the sanded ~ gas diffusing surfaae was in all other respects the same typ~
!~ f bar~as~the~unsanded bar.
TheEbubble~shearlng of Examples 1 through 6, as ex-30 ~ plained~above,~was carried out with one wall of the shearin~ligUId~;transmi~ti-g slot formed by a flat glass plat-, which~
~ . : ,. - , . :. ,: . :
~069665 ~ ~
made it possible to photograph sample gas bubbles produced unde~
the indicated conditions. In Examples 7 through 9 below, the ap-. ~ .
paratus used was not modified in this way, but was operated as it would ha~e been operated in actual use, with a gas diffusing surface similar to the surface illustrated in Figure 14 on both sides of the shearing liquid transmitting slot.
Example 7 Excellent oxygen bubble shearing was visually observed from use of one embodiment of the apparatus of this invention, with tap water as the shearing liquid, under substantially the following conditions:
Sl~t width 0.040 inches Volumetric flow rate 2 gallons/min./inch for shearing liquid of slot Linear flow rate for 15 ft./sec.
` shearing liquid j Gas pressure 30 psig Gas flow rate 50 scfm/sq. ft. of active gas diffusing area Median capillary diameter 8 microns Example 8 I
Excellent oxygen absorption, more than 90 per cent - of the oxygen supplied to the body of liquid, was obtained with the use of the same apparatus, with 1 per cent suspended "
solids sewage as the shearing liquid, under subs~antially the following conditions:
l ~ Slot width 0.040 inches 1`~` : , ; Volumetric flow rate 2.5 gallons~min./inch , for shearing liquid of slot Linear flow rate for 20 ft./sec.
., ~ .
shearing liquid Gas pressure 30 psig . , .. .. .. , , . ~ . . . . . . . .
. . : .: . ~ : . . ~
Gas flow rate 45 scfm/sq. ft. of active gas diffusing area ~ledian capillary diameter 8 microns Example 9 Excellent oxygen absorption, again more than 90 per ' cent, was also obtained with use of the same apparatus, except that the slot width was set at 0.125 inches, with 6 per cent suspended solids se~lage as the shearing liquid, under substan-tially the foll'owing conditions:
Slot width 0.125 inches Volumetric flow rate 2.5 gallons/min./inch for shearing liquid of slot Linear flow rate ~or 7-8 ft./sec.
shearing liquid Gas pressure 30 psig ~' Gas flow rate 15 scfm/sq. ft. of active ' gas diffusing area Median capillary diameter 8 microns Because of the higher viscosity of the 6 per cent suspended solids sewage and the resulting improved bubble shearing, approximately the same oxygen absorption efficiency was obtained with 6 per cent solids sewage as with 1 per cent solids sewage, at about one-third the linear flow rate and approximately the same energy expended for pumping.
Specific Embodiments :of Gas Bari I Because of their elongated shape '(best seen in Figures 18A and 24 A), gas transmitting bodies 166 such as illustrated ' '~
in Figure 12 are for convenience usually referred to as gas "bars."
~30 Figures 18A and 18B show a gas bar 166a that may'be incorporated in the gas diffusing apparatus of this invention in the manner illustrated schematically in Figure 12. Gas .
- - ~ ., . .: . . . : . ~ :
.. . . . . .
69~65 ~ ~
bar lG6a is an enclos~d container with a gas inlet opening 184 provided through elbow joint 186, suitably threaded for attach~
ment to a gas feeder line 130 (Figure 12). This embodiment of the gas transmittin~ body has two walls 1~6 and 148 with a plu-rality of gas transmitting passages extending therethrough, to permit gas to escape from interior enclosed chamber 160. Walls 146 and 148 are the gas diffusing surfaces of gas bar 166a.
Figures 24A through 24C show another embodiment of a gas bar 166b for use with this invention. This embodiment has only one gas diffusing surface, wall 148, to permit gas to es-cape ~rom interior enclosed chamber 160. As in gas bar 166a, inlet opening 184 and fitting 186 are provided in gas bar 166b.
As will be explained in more detail below, each gas diffusing surface 146 and 148 in specific gas bars 166a and 166b comprises a sandwich of two thin metal support plates --members 188 and 190 for gas bar 166a, and 192 and 194 for gas bar 166b -- on both sides of a plurality of hollow fiber glass I capillary strands 206 embedded in epoxy 162 as a matrix. Be-cause the support plates are so thin (Figures 18B, 24B and 24C), capillary openings 154 lie in close proximity to boundary de-fining edges 196 and 198 of the gas diffusing surfaces of gas bars 166a and 166b, respectively, ~^~hich become the leading edges of slots 120 when the gas bars are in place in the gas diffus-ing apparatus of this invention. (The flow of the shearing li- -~uid in the bubble shearing apparatus in which gas bar 166a or , ~....................... .
166b is installed is in the upward direction in Figures 18B, 24B and 24C.~ !
As used in this specification and claims, the term "gas diffusing surface" includes the sîdes of the two support plates but, as already indicated above, the "active area" o~
that surface includes only the area defined by the boundary lines tangent to the outermost capillary openings. (See Fig~re .. ' ~, \ .'" .
. ~ ~ 106966S
14, for example.) In the emhodiments shown in Figures 18 and 24, each support plate is about 0.025 inches thick, which brings the capillary openings as close to the leading edge of the gas dif-fusing surface as is structurally practicable in these e~odi-ments. Since the layer of hollow capillary strands 206 e~ed-ed in matrix 162 is about 0.020 inches thick, this positions the most remote capillary openings a total of about 0.045 inches from leading edgés 196 and 198, respectively.
Good results can be obtained under some operating conditions with the distance from the indicated boundary defin-; ing edge of the gas bar of this invention to the most remote capillary openings being about 1.0 inch. Improved results are obtained if that distance is about 0.5 inch, and still better results if it is about 0.1 inch, 0.05 inch, or even smaller.
Leading edges 196 and 198 are rounded to improve the stability of flow through slot 120. This feature helps produce stable laminar flow,including a partially developed laminar boundar~ layer, in the shearing liquid transmitting slot~ The rounded edges are shown in Figures 18B, 24B and 24C as accurate-ly as is possible at the scale of those drawings.
Gas diffusing surfaces 146 and 148 in gas bars 166a and 166b are similar to the surfaces shown in Figure 14 and described above in this specification. The surfaces are gener-ally planar, and the gas transmitting bars of which théy are a part are constructed in a manner similar to that described above in connection with Figure 14. This includes capillary passages of a shape, size ana orientation similar to the cap-illary passages indicated in Figure 14, as well as capillar~ -o~enings of an arrangement and location similar to the open-ings shown in Figure 14.
-7~-' 6g66S
Method of Fabrication Of ~as Bars Figures 17 through 24 illustrate two methods of fab-ricating gas bars for use in the apparatus of this invention designed for the oxygenation of sewage in the form either of~
so-called "mixed liquor" having about 1 per cent suspended solids or aerobic digester sludge having about 6 per cent sus-pended solids.
Figure 17A is a plan view of thin bottom support plate 188, which is rectangular in over-all shape and has a generally rectangular opening 200 in the interior portion there-of to provide an elongated O-shaped member. This member has two inner side ~Jalls 202 that are adapted to form a part of the interior wall of gas plenum 160, and two outer side walls 204 adapted to form a part of the exterior wall of the gas plenum.
As shown in Figure 17B, the first step of this method of fabricating gas bar 166a is to position a plurality of hollo~t capillary strands 206 and a supply of binder 162 across the top face of thin support plate 188 so that the hollow strands are aligned substantially normal to side walls 202 and 204. In this position, the strands extend beyond plate 188 on both the inner and outer sides of the plate~
The binder applied in this first step may be already present upon the fiber glass as received from the manufacturer of the hollow capillary strands, or it may be added by the ` fabricator of the gas bar either before or after the strands are positioned across bottom support plate 188. ~en desir~d, additional binder may be added to the hollow capillary strands in position upon plate 188, before thin top support plate 190, similar in all respects to bottom support plate 188, is added (Figure 18C) to produce the "sandwich" of two plates with hollow capillary strands and binder between them.
This sandwich is compressed to cause binder 162 to --71 ~
: ~ , .. , ' ';
106~665 fill all the crevices between strands 206 and between the strands and support plates 188 and 190. The sandwich is then ~eld compressed until the binder is hardened to embed the hollow capillary strands in the hardened binder as a matrix.
Binder 162 may if desired be a heat curable binder, and in that case heat is applied while the sandwich is compressed, to cure the binder.
In the next step of this method o~ fabricating gas bars, the portions 208 of hollow capillary strands 206 em-bedded in matrix 162 that extend outwardly beyond exteriorside walls 204 of support plates 188 and 190 are trimmed back generally to walls 204 t~ form gas diffusing surf2ces 146 and 148. The surface of the exterior side wall of the gas bar that is comprised of a first wall 204, a plurality o c~pillary openings 154 surrounded by epoxy 162, and a second wall 204 is preferably made as nearly flat as possible, but the extent of minor departures from the desired planar surface will be deter-mined by the cutting tool and trimming method employed.
The portions 210 of hollow capillary strands 206 em-bedded in matrix 162 that extend inwardly beyond interior sidewalls 202 of support plates 188 and 190 are severed to form the inlet ends of gas transmitting passages 158. It is not essential that these ends be trimmed back to wa~ls 202, but only that they provide passages for gas to leave gas plenum 1604 .
Gastight cover plates 212 and 214 are next affixèd to the sandwich, on opposite sides thereof, to form gas plenum 160 into which gas can be introduced through inlet opening 184, and out of which gas can flow through hollow capillary strands 15~. A layer of binder, in the form of liquid or in the form of a flat sheet of appropriate dimensions as desired, can be interposed between bottom support plate 188 and its as-fiOCiate~ c~ver plate 212 and between top support plate 190 .nd ..
':
.. , .... , .: . . . . .... . . .
~069665 its associated cover plate 214. A11 the elements of the gas bar are held compressed until ~he binder has hardened, with the additional use of heat if the binder is heat curable.
Chamfers 216 are preferably provided on the outer edges of upper bac}ing plate 214 that lie adjacent the flow path of the shearing liquid as the liquid flows out from the discharge end of the liquid transmitting slot (in an up~ard .. .~:
direction in Figures 18B, 24B and 24C), when the gas bar is ~.
installed in place in ~ubble shearing apparatus constructed ac-cording to this invention. The reason for these eh mfers is.
to avoid the "attachment" of the discharge flow from the æhear-.ing liquid transmitting slot to walls 218 of top backing plate 214 that would occur if those walls were not cut away as indi~ :
cated. Any such flow attachment would tend to cause the shear-ing liquid jet stream and the fine gas bubbles contained there-in to turn the right angled corner that would be presented by the full wall, and thus cause the jet stream to widen out I as it entered the body of liquid into which the gas bubbles , were being dispersed. Such a widening of the jet stream would i 20 reduce the effectiveness of the distribution of fine gas bub-¦ bles throughout the liquid into which they were being intro-duced; it is preferred to keep the jet stream narrow enough, if possible, that its total angle of spread in the initial stages is no more than about 15.
~: By use of a special winding fixture, the fabrication steps illustrated in Figures 17A through 17C may ~e carried out ¦~ : on a number of units at one.~ime. With winding fixture 220 as illustrated in Figure 19, eight bottom support plates 188 ;may be secured to the fixture, two on each vertical`wall of il~: .:
;;30 ~ the fixture, and hollow fiber glass capillary strands 206 ~: wrapped around all eight plates at the same time. With the .~ necessary binder 162 in place, top support plates 190 may be' ~., \~ , ':: . /
: .
- . . ~ : . . :
-` ~ 106966S
positioned to form eight sandwiches, and the necessary compres-sion and heating steps completed to produce eight separate units such as illustrated in Figure 17C.
Figure 20A illustrates the positioning on bottom sup-port plate 188 of a plurality of lengths of roving 221, in much the same manner as individual capillary strands are positioned in Figure 17B. Each length of roving 221, as seen in the sche-matic cross-sectional view of Figure 20B, is formed of a plu-rality of hollow capillary strands 206. The capillary strands in substantially all said lengths of roving 221 are generall~
parallel to each other.
Figures 21A through 24C illustrate another type of gas bar 166b, which is fabricated in a manner very similar to the method of fabricating gas bar 166a already described. Hol-low fiber glass capillary strands 206 and a quantity of binder 162 are sandwiched between bottom plate 192a-192b and top plate 194a-194b, and the necessary compressing and curing steps are carried out. Outer portions 222 of capillary strands 206 embedded in binder 162 are trimmed back to form a gas dif-fusing surface, ar.d inner portions 224 of the strands and I binder are severed. In addition, support plates 192 and 194 are severed along lines 226 to form two separate sandwiches, each with a single gas diffusing surface 148.
Bottom cover plate 228 has a groove 230 adapted toreceive an assembled sandwich of capillary strands and binder between two support plates. Cover plate 228 hà~`a second groove 232 surrounded by an elongated C-shaped wall 234. The cross sectional views of Figures 22B and 22C, and the end viéw of Flgure 22D (taken from the bottom of Figure 22A), show the 3~0~ general configuration of cover plate 228.
Top cc.ver plate 236, as is seen from the plan view I
of Figure 23A and the end view of Figure 23C (taken from th~
1~)69665 bottom of Figure 23A), has a flat planar surfâce. When top cover plate 236 is placed upon bottom cover ~late 228, the asse~bled plates perform t~;o functions. First, they secure between them the sandwich of two support plates on each side of a layer of hollow capillary strands embedded in binder as a matrix (Figures 24B and 24C). Second, between them they define gas plenum 160, surrounded on three sides by wall 234 (Figure 22C~ and on the fourth side by the sandwich (Figure 24B). Binder layexs 238 and 240 complete the gastight assemblage of gas 10 bar 166b.
The Method Of This Invention ~ he method of this invention involves the same sub-stantially parallel laminar flow with a partially developed ; laminar boundary layer over very small capillary openings pre-ferably located in close proximity to the leading edge of a gas diffusing surface, any remaining portion of the capillary openings on the surface being free of any type of flow but fully developed laminar flow, that has been described above. The method is not limited to the particular configuration of ele-ments, including a shearing liquid transmitting slot, that com-prises the apparatus of this invention. However~ it is limited I to the establishment of a partially developed laminar boundary layer over at least about one-quarter of the capillary openings of the gas diffusing surface, in order that the layer can have a significant effect on the bubble formation resulting from use ~ of the method.
I In the method of the present invention, gas is flowed through a gas diffusing surface with capillary openings substan-tially all of which have a diameter no larger than about 100 mi-crons, and a shearing liquid is flowed past a leading edge of .
,- . - . ~ .
.
~. ' ., ~ , ~ .
06966S ff the gas diffusing surface and over that surface to a discharge edge thereof. The flow is carried out with minimal perturbation, and at a velocity to produce the substantially parallel laminar flo~l that is characteristic of this invention, including a par-tially developed laminar boundary layer over at least about the first quarter of the capillary openings in the gas diffusing surface. Best results are obtained if the partially developed layer extends over a majority of the capillary openings.
The method is preerably carried out with the capil-lary openings located immediately adjacent the leading edge ofthe gas diffusing surface. Good results are obtain~d with this method if the capillary openings most remote from the leading ;
edge are located no more than about 1 inch from that edge. Im-proved results are obtained if that distance is no more than about 0,5 inch, and best results if it is no more than about 0.1 inch, 0.05 inch, or even less.
The method provides the best bubble shearing if the gas diffusing surface is generally planar. ~he size of the sheared bubbles is smaller the larger the contact angle of the shearing liquid is with the material that is selected to define or surround the capillary openings in the gas diffusing surface.
That contact angle is preferably at least about 60 when the `shearing liquid used is tap water.
Satisfactory bubble shearing results are obtained with the method of this invention with a shearing liquid velocity of about 10 feet per second when water is employed as the shearing liquid. Improved results are obtained with a shearing liquid velocity of about 15 feet per second, and still more improvement is achieved if the method employs a shearing liquid velocity of ~' 30~ about 20 feet per second. A velocity of at least about 25 feet per second is preferred. -.
- .. , . . ., . - . .
069665 ~
Use of this method with gas being supplied to the gas transmitting passages at a rate to produce a gas flow through the passages of about 10 standard cubic feet per minute per square foot of diffusing surface will ordinarily produce good bubble shearing results. As already explained above, increas-ing the gas flow rate affects bubble shearing in ways known to those skilled in the art, hut generaily speaking a gas flow rate ; of 40 standard cubic feet per minute per square foot of active diffusing area gives improved results, and increasing this `
figure to 70 or somewhat more gives still better results.
- As the---shearing liquid flows-~wa~ fxom the discharge edge of the gas diffusing surface it carries with it, and into the body of liquid being treated, the many small gas bubbles 'I sheared off the gas difusing surface by the moving shearing j liquid, The presence of the hydrodynamic characteristics that characterize the method of this invention can be determined in the same manner as explained above in the description of the I apparatus of the invention. The shearing liquid may be trans-lucent or transparent, as for example water, if desired.
1~ ~ Effect Of Various Parameters ¦~ On Fine Bubble Formation In the above description of the method and apparatus of this invention, a number of factors have been discussed that determine the effectiveness of bubble shearing in achieving gas : .
bubbles of the smallest possible size, with as great unifor-mity of size as possible, and at the lowest possible expendi-ture of energy. There are many other factors affecting the dlffus~ion of gas bubbles into a body of liquid, some of which ~30~ oan~be~va~ried~and some of which are fixed, but all of which should~be taken into account in the practice of the present in-vention.
i . ~ .
' -77-~069665 ~
The parameters which are ordinarily fixed in any process involving diffusion of ~as into a body of liquid in-clude the ambient conditions, the nature of the gas diffused, and the nature of the liquid into which it is diffused.
At least three ambient conditions affect the rate at which gas bubbles can be dissolved into a body of liquid:
(1) Ordinarily, the colder the body of li~uid, the greater will be the solubility of the gas in the liquid. The viscosity of the liquid may also be increased somewhat by a 1~ reduced temperature~ which would tend to improve bubble she~r-ing, and thus make smaller bubbles, by increasing the viscous shear force. It may also decrease the rise rate of the gas bubbles and therefore give them a longer period of time in which to be dissolved in the liquid. At the same time, it may decrease the extent to which the gas bubbles can initially be laterally dispersed as a part of a fluid stream directed into the bo~ly of liquid. Finally, by increasing the surace tension of the surfaces between the gas bubbles and the liquid, a lower ambient temperature may tend to increase the average bubble size by making shearing more difficult, at the same time that it tends ~o affect, in the other ways just indicated, the manner ; in which bubbles are formed and absorbed into the liquid after they are formed.
(2) The type of mo~ement in the body of liquid will affect the degree of dispersion of the bubbles throughout the liquid. It may also affect the extent of convective diffusion, i.e., the rate at ~hich fresh liquid is presented to the bubble surface for acceptance of gas by absorption from the bubble.
The body of liquid, for example, may be still, it may have a smooth, generally horizontal current, it may have a smooth, generally ~ertical current, or it may be turbulent.
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~06g665 - ~
(3) The partial pressure o~ the gas being dissolved, as measured at the surface of the body of liquid into which the gas is introduced, will affect the rate of absorption by the liquid. The higher the partial pressure, the more rapidly the diffused gas will be absorbed into the liquid.
A number of factors affecting the rate of absorption of the gas will be determined by the particular liquid and gas that are utilized in the process in question. These include:
(1) ~hé solubilitv of the gas in the liquid.
(2) The rate of diffusion of the gas into the liquid through the surface of a bubble of a given size in a still body of the liquid.
(3) The characteristic surface tension at surfaces between the gas and the liquid, including the effect of any sur-factants or other contaminants present.
(4) The pressure exerted by the liquid -- because of its particular density -- at various depths from the feed point of the gas bubbles up to the surface of the body of liquid.
This pressure will have an effect, to some degree at least, upon the size of the bubbles as they rise through the body of the liquid.
(S) The difference between the specific gravity of ; the body of liquid and the gas. The greater this difference, the greater will be the rise rate of the gas bubbles.
(6) The viscosity of the liquid in the body of li-quid into which the bubbles are diffused. The greater the viscosity, the slower the rise xate of the bubbles, but also the more limited is the initial lateral dispersion of the bubbles through the body of liquid.
(7) The ultimate shape of the bubbles, which may be substantially spherical, or somewhat flattened, in form.
~ . . . . , . ~- - . , :
~Q69665 ~ :
(8) Witll every set of circumstances, there ~
be a different tendency on the part of bubbles of a particu~
lar gas in a particular liquid to couple or coalesce if the bubbles have non-uniform terminal rise rates and therefore tend to collide.
The factors that can be varied in bubble shearing apparatus, several of which have been discussed above, include at least the following:
(1) The size, shape and orientation of the gas-transmitting capillary openings all affect the size and uni-formity of the gas bubbles formed.
(2) The initial shape of the bubble immediately after formation -- as affected by the shape of the orifice from which it is discharged, the type of movement in the body of liquid being treated, etc. -- affects its rate of absorption into the li~uid. (This factor~is usuzlly of minor importance~ because the shape of the bubble changes relatively rapidly from its initial form.) (3) The size, shape and orientation of the liquid-transmitting passage affects the size and uniformity of the ; resulting gas bubbles.
(4) The size, shape and direction of flow of the stream of shearing liquid affects the degree of initial dis-persion (especially lateral dispersion) of gas bubbles through the body of liquid.
; The process conditions that may be varied to in fluence the rate of absor~tion of gas bubbles by a body of liquid in which they are diffused include at least the fol-lowing:
(1) The depth of the feed point of the gas bubbles - determines the length of travel the bubbles are permitted be-fore they must be fully dissolved in the body of liquid in ' . . .
1069665 ~,, order not to break the surface of the liquid and escape.
(2) The ~-elocity with which the gas is caused to flow through the capillary passages of the foraminous gas transmitting material affects the rate at ~Jhich gas bubbles are introduced into the body of liquid.
(3) The velocity with which the shearing liquid is caused to flo~ past the capillary openings of the gas~trans-mitting foraminous material affects principally the size of the resulting bubble.s, but may also affect the uniformity of lu those bubbles.
Many of the above factors are interrelated; both in ways that have been indicated above in this specification and in other ways. A person skilled in the art who applies the teaching of this disclosure will, of course, take into consi-deration in the practice of this invention all pertinent factors that may affect his attempt to achieve rapid absorption of dif-fused gas bubbles into a body of liquid.
The above detailed description of this invention has been given for clarity of understanding only. No unnecessary limitations should be understood therefrom, as modifications w-ill be ^bvious to those skilled in the art.
, ,~:
, ' . ' ' - ' .
This pressure will have an effect, to some degree at least, upon the size of the bubbles as they rise through the body of the liquid.
(S) The difference between the specific gravity of ; the body of liquid and the gas. The greater this difference, the greater will be the rise rate of the gas bubbles.
(6) The viscosity of the liquid in the body of li-quid into which the bubbles are diffused. The greater the viscosity, the slower the rise xate of the bubbles, but also the more limited is the initial lateral dispersion of the bubbles through the body of liquid.
(7) The ultimate shape of the bubbles, which may be substantially spherical, or somewhat flattened, in form.
~ . . . . , . ~- - . , :
~Q69665 ~ :
(8) Witll every set of circumstances, there ~
be a different tendency on the part of bubbles of a particu~
lar gas in a particular liquid to couple or coalesce if the bubbles have non-uniform terminal rise rates and therefore tend to collide.
The factors that can be varied in bubble shearing apparatus, several of which have been discussed above, include at least the following:
(1) The size, shape and orientation of the gas-transmitting capillary openings all affect the size and uni-formity of the gas bubbles formed.
(2) The initial shape of the bubble immediately after formation -- as affected by the shape of the orifice from which it is discharged, the type of movement in the body of liquid being treated, etc. -- affects its rate of absorption into the li~uid. (This factor~is usuzlly of minor importance~ because the shape of the bubble changes relatively rapidly from its initial form.) (3) The size, shape and orientation of the liquid-transmitting passage affects the size and uniformity of the ; resulting gas bubbles.
(4) The size, shape and direction of flow of the stream of shearing liquid affects the degree of initial dis-persion (especially lateral dispersion) of gas bubbles through the body of liquid.
; The process conditions that may be varied to in fluence the rate of absor~tion of gas bubbles by a body of liquid in which they are diffused include at least the fol-lowing:
(1) The depth of the feed point of the gas bubbles - determines the length of travel the bubbles are permitted be-fore they must be fully dissolved in the body of liquid in ' . . .
1069665 ~,, order not to break the surface of the liquid and escape.
(2) The ~-elocity with which the gas is caused to flow through the capillary passages of the foraminous gas transmitting material affects the rate at ~Jhich gas bubbles are introduced into the body of liquid.
(3) The velocity with which the shearing liquid is caused to flo~ past the capillary openings of the gas~trans-mitting foraminous material affects principally the size of the resulting bubble.s, but may also affect the uniformity of lu those bubbles.
Many of the above factors are interrelated; both in ways that have been indicated above in this specification and in other ways. A person skilled in the art who applies the teaching of this disclosure will, of course, take into consi-deration in the practice of this invention all pertinent factors that may affect his attempt to achieve rapid absorption of dif-fused gas bubbles into a body of liquid.
The above detailed description of this invention has been given for clarity of understanding only. No unnecessary limitations should be understood therefrom, as modifications w-ill be ^bvious to those skilled in the art.
, ,~:
, ' . ' ' - ' .
Claims (10)
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:
1. A method of fabricating a gas transmitting body for use in apparatus for diffusing fine gas bubbles into a body of liquid which comprises:
positioning a plurality of hollow capillary strands and binder across one face of a first support plate having at least one side wall adapted to form a part of the interior wall of a gas plenum, and at least one side wall adapt-ed to form a part of the exterior wall of said gas plenum, so that said hollow strands are aligned substantially normal to each of said side walls, and said strands and binder extend beyond the plate on both of said sides of the plate;
positioning a second support plate having a similar shape to that of said first support plate adjacent said plurality of hollow capillary strands and binder to form a sandwich of said two plates with said strands and binder therebetween compressing said sandwich to cause said binder to fill all the crevices between said strands and between the strands and the support plates;
holding said sandwich compressed until said binder is hardened to embed said hollow capillary strands in the binder as a matrix, to provide gas transmitting passages extending through said matrix; trimming back generally to said side walls of said first and second support plates adapted to form a part of the exterior wall of a gas plenum the portions of said hollow strands embedded in said matrix that extend outwardly beyond said side walls, to form a gas diffusing surface with capillary openings distributed across the same; and severing the portions of said hollow strands embedded in said matrix that extend inwardly beyond said side walls of said first and second support plates adapted to form a part of the interior wall of said gas plenum, to form the inlet ends of said gas transmitting passages.
positioning a plurality of hollow capillary strands and binder across one face of a first support plate having at least one side wall adapted to form a part of the interior wall of a gas plenum, and at least one side wall adapt-ed to form a part of the exterior wall of said gas plenum, so that said hollow strands are aligned substantially normal to each of said side walls, and said strands and binder extend beyond the plate on both of said sides of the plate;
positioning a second support plate having a similar shape to that of said first support plate adjacent said plurality of hollow capillary strands and binder to form a sandwich of said two plates with said strands and binder therebetween compressing said sandwich to cause said binder to fill all the crevices between said strands and between the strands and the support plates;
holding said sandwich compressed until said binder is hardened to embed said hollow capillary strands in the binder as a matrix, to provide gas transmitting passages extending through said matrix; trimming back generally to said side walls of said first and second support plates adapted to form a part of the exterior wall of a gas plenum the portions of said hollow strands embedded in said matrix that extend outwardly beyond said side walls, to form a gas diffusing surface with capillary openings distributed across the same; and severing the portions of said hollow strands embedded in said matrix that extend inwardly beyond said side walls of said first and second support plates adapted to form a part of the interior wall of said gas plenum, to form the inlet ends of said gas transmitting passages.
2. The method of claim 1 in which the hollow capillary strands employed in said first step have been precoated with binder prior to being positioned across said first support plate.
3. The method of claim 1 in which said support plates have an elonga-ted shape.
4. The method of claim 1 in which each of said support plates is rectangular in shape and has a generally rectangular opening in the interior portion thereof to provide an elongated O-shaped member having two sides adapted to form a part of the interior wall of a gas plenum and two sides adapted to form a part of the exterior wall of said gas plenum, and in which said hollow strands are positioned in said first step substantially normal to the longtidudinal axis of said support plate to extend across said rectangular opening and beyond the support plate on both exterior sides thereof.
5. The method of claim 4 which includes the step of severing the top and bottom portions of said two O-shaped support plates, after the binder between said two O-shaped plates has hardened to form a matrix in which said capillary strands are embedded, to form two generally rectangular gas transmitting bodies.
6. The method of claim 1 which includes the step of positioning a plurality of lengths of roving, each of which lengths of roving is formed of a plurality of hollow capillary strands, across said one face of said first support plate.
7. The method of claim 6 in which said hollow capillary strands in substantially all said lengths of roving positioned across said one face of support plate are generally parallel to each other.
8. The method of claim 1 which includes the steps of securing a plurality of said first support plates to a winding fixture, and winding said plurality of hollow capillary strands successively around said plurality of support plates to position a strand first across one support plate and then across the next plate.
9. The method of claim 1 which includes the steps of affixing a gastight cover plate to one side of said sandwich with said hollow capillary strand ends trimmed and severed as described, and affixing a gastight cover plate with an inlet opening therein to the other side of said sandwich, to form a gas plenum into which gas can be introduced under pressure through said inlet, and out of which said gas can flow through said plurality of hollow capillary strands.
10. The methods of claim 9 which includes positioning a layer of binder between each of said support plates and its associated cover plate.
Priority Applications (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| CA317,524A CA1069665A (en) | 1974-02-11 | 1978-12-06 | Method of fabricating a gas transmitting body |
Applications Claiming Priority (3)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| US05441573 US3927152A (en) | 1971-03-12 | 1974-02-11 | Method and apparatus for bubble shearing |
| CA219,753A CA1048173A (en) | 1974-02-11 | 1975-02-10 | Method and apparatus for bubble shearing and method of fabricating the apparatus |
| CA317,524A CA1069665A (en) | 1974-02-11 | 1978-12-06 | Method of fabricating a gas transmitting body |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| CA1069665A true CA1069665A (en) | 1980-01-15 |
Family
ID=27163806
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA317,524A Expired CA1069665A (en) | 1974-02-11 | 1978-12-06 | Method of fabricating a gas transmitting body |
Country Status (1)
| Country | Link |
|---|---|
| CA (1) | CA1069665A (en) |
-
1978
- 1978-12-06 CA CA317,524A patent/CA1069665A/en not_active Expired
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