CA1334822C - Method of aeration of liquids - Google Patents

Abstract

A method of aeration of liquids which consists of the following steps. Firstly, injecting a gas into a flowing liquid stream thereby forming a plurality of gas bubbles.
The gas being of lesser density than the liquid. Secondly, directing the liquid stream through a restriction whereby the gas bubbles pass more quickly through the restriction pneumatically accelerating the liquid.

Description

The present invention relates to a method for aeration of liquids.

BAG~ROU~D OF THE IDV~NTION
Aeration of liquids i5 required in numerous industrial processes. For example, water treatment systems use oxygenation to remove iron, manganese and gases from water. Oxidization forms iron into ferric oxide (Fe203), and triggers a further reaction with 10 water (H20) to form the precipitate ferric hydroxide lPe2(0~)3. Oxidization forms manganese into manganese dioxide (MnO2). Manganese dioxide acts as a catalyst to oxidize iron and M~ng~nese into their respective oxides.
The~e precipitates are then captured in a filter medium 15 and ~ ,v d from the water. The efficiency of the water treatment system is, therefore, dependent upon the amount of oxygen which the water i8 capable of absorbing.

There are four accepted approaches used to increase the absorption capacity of a liquid. One approach is known as the "gravity" method. With this method droplets of liquid are dropped through the air. Another approach is "mechanical" aeration. With this method the 25 liquid is violently agitated. Another approach is the "spray" method. With this method liquid is forced through a nozzle and sprayed through the air. Another approach is through the use of "diffusers", which mix the liquid by diffusion. All methods attempt to 30 decrease the interfacial films between the liquid and gas molecules.

SU~aRY OF T~E INV~NTION
What is required is a method of increasing the 35 efficiency of aeration.

According to the present invention there is provided a method of aeration of liquids which is comprised of the following steps. Firstly, injecting a gas into a flowing liquid stream whereby a plurality of bubbles are formed, the gas being of lesser density than the liquid. Secondly, directing the liquid stream through a restriction whereby the gas bubbles pass more quickly through the restriction pneumatically accelerating the liquid.
The described "pneumatic acceleration" occurs in two way~. The liquid in front of the bubbles is pushed through the restriction and the liquid following the bubbles accelerates due to a decrease in resisting 15 pressure.

Although beneficial results may be obtained through the use of the described method, the e~h~nced spray obtained by virtue of the pneumatic acceration can be 20 combined with mechanical aeration means. Even more beneficial results may therefore be obtained if the liquid stream is directed at an impaction target thereby creating a pneumatic hammering effect.

Although beneficial results may be obtained through the use of the described method, it is preferred that the gas bubbles be of a substantially consistent size.
Even more beneficial results may therefore be obtained if the liquid stream is directed through a conduit 30 having a bend, such that small gas bubbles adhere to the wall of the conduit in the vicinity of the bend and amalgamate to form bubbles substantially consistent in size prior to being swept by the liquid stream through the restriction.

~ 3 l 334822 Although beneficial results may be obtained through the use of the described method, even more beneficial results may be obtained if the bubbles are substantially the same size as or larger than the restriction.

Although beneficial results may be obtained through the use of the described method, it is preferred that there be a spacial separation between the bubbles as they go through the restriction. Even more beneficial 10 results may therefore be obtained if the liquid stream is directed d~ - rdly through a vertically aligned conduit prior to flowing through the restriction, such that a spacial separation of the bubble~ occurs.

Although beneficial results may be obtained through the u~e of the described method, by controlling the atmospheric pressure the absorbtion capacity of the liquid can be increased. Even more beneficial results may therefore be obtained if the restriction is housed 20 in a pressure vessel.

Although beneficial results may be obtained through the use of the described method, if the difference in density between the gas and the liquid can be used to 25 ~nh-n5e acceration. Even more beneficial results may therefore be obtained if the impaction target is at the top of the pressure vessel such that the buoyancy of the air bubbles results in an acceleration of the bubbles prior to impacting upon the target.

Although beneficial results may be obtained through the use of the described method, it is preferred that aeration methods be combined to create a synergistic mixing effect. Even more beneficial re~ults may `~ 4 l 334822 therefore be obtained if the pressure vessel is a tower such that the liquid stream strikes the impaction target and then by force of gravity cascades through the gas in the pressure ves~el to further impact upon liquid accumulated in the pressure vessel.

BRIEF ~ PTPTION OF THE DRAWINGS
These and other features of the invention will become more apparent from the following description in 10 which reference is made to the appended drawings, wherein:
FIGURE 1 is a perspective view of a water treatment system con~tructed in accordance with the teachings of the preferred method.
FIGURE 2 is a longitudinal section view of an apparatus for the aeration of liquids taken along section lines 2-2 of FIGUR~ 1.
FIGURE 3 is a section view taken along section lines 3-3 of FIGURE 1.
FIGURE 4 is a detailed view of a portion of FIGURE
2.

~T~r~n D~SCRIPTION OF THE PREF~RR~D ~MBODIMENT
The preferred method will now be described with 25 reference to the apparatus illustrated in FIGURES 1 through ~. The apparatus are components of a water treatment system developed for intermixing oxygen with water, but the method described is equally applicable to the intermixing of other gases with other liquids.

Referring to FIGUR~ 1, the components of the water treatment system consist of a venturi air injector 10, a fluid conduit 12, a tower-like pressure vessel 14, a gas eliminator 16, and a filter 17. Although the described 35 components are well suited for use in accordance wi~h ~- 5 1 334822 the preferred method which will hereinafter be described, it must be recognized that other configurations could be adapted for use in accordance with the teachings of the preferred method. Referring to FIGURE 2, pressure vessel 14 has a top 18, a bottom 20, an interior cavity 22, interior side walls 23 and an exterior 24. Liquids have access to interior cavity 22 through an inlet 26 and C~r~ from interior cavity 22 through an outlet 28. Both inlet 26 and outlet 28 are 10 positioned adjacent bottom 20. A cylindrical conduit 30 extends substantially vertically in interior cavity 22.
Conduit 30 has a first end 32 coupled by a "T" joint 31 with inlet 26 and a second end 34 di~u_ed immediately adjacent top 18 forming a cylindrical flow restriction 15 gap 19. Referring to FIGURE 3, a plate 35 with a plurality of openings 37 i8 spaced from top 18 of pressure vessel 14. Referring to FIGURE 1, liquid flow conduit 12 has a substantially horizontal portion 36, a portion with a bend 38, and a substantially vertical 20 portion 40. Horizontal portion 36 has a first end 42 and a second end 44. Portion with a bend 38 has a first end 46 and a second end 48. Vertical portion 40 has a first end 50 and a second end 52. First end 42 of connecting portion 36 is secured to first end 46 of 25 portion with a bend 38. Second end 48 of connecting portion 36 is adapted for connection to a source of liquids under pressure ~not shown). First end 50 of vertical portion 40 is connected to second end 48 of portion with a bend 38 at a height of approximately 2/3 30 of the height of pressure vessel 14. Second end 52 of vertical portion 40 is connected by an 90 degree elbow 25 to inlet 26 of pressure vessel 14. Gas injector 10 is secured to horizontal portion 36 of liquid flow conduit 12.

` 6 l 334822 The preferred method of aeration of liquids will now be described with reference to the cc ~ n~nts illustrated in FIGUReS 1 through 4. In its most fundamental form the method consists of only two steps.
Firstly, injecting a gas into a flowing liquid stream to form a plurality of gas bubbles in the flow stream. The gas should be of lesser density than the liquid.
Secondly, directing the liquid stream through a restriction gap whereby the gas bubbles pass more 10 quickly through the restriction pneumatically accelerating the liquid. In the preferred method those fundamental steps have been combined with additional steps which through experimentation it has been found e~hAnce the desired aeration. In addition some 15 conventional gravity, spray and mechanical aeration techniques have been incorporated in the preferred method. The inter- i~ing takes place in a pressure vessel to further increase the absorption capacity of the liquid. The preferred method, therefore, consists 20 of the following described steps.
Firstly, injecting a gas via gas injector 10 into a flowing liquid stream in horizontal portion 36 of liquid flow conduit 12 to form a plurality of gas bubbles (not shown). This is illustrated in FIGURE 1. The gas must 25 be of lesser density than the liquid. In the illustrated application air is being injected into water, however with other applications care must be taken to ensure the liquid is not so viscose as to prevent the desired pneumatic acceleration from 30 occurring.
Secondly, directiny the liquid stream through portion with a bend 38 of liquid flow conduit 12. When this is done it has been found that small gas bubbles adhere to the upper wall of portion with a bend 38 and 35 amalgamate to form a pocket of air. This pocket of air surrenders air bubbles which are relatively consistent in size, which are swept away by the liquid stream. The size of the bubbles produced is depe~Aent upon the size of the air pocket and the velocity of the liquid stream.
The bubbles enh~nce the desired pneumatic acceleration, as will be hereinafter explained. The bend in portion 38 is illustrated as being at 90 degrees, but it has been determined that a much less severe bend will also be operable. However, with a less severe bend it is more 10 difficult to produce bubbles of a consistent size.
Thirdly, directing the liquid stream do.. -~dly through vertical portion 40 of liquid flow conduit 12.
The air bubbles have a ten~ncy to rise in vertical portion 40. This te~ency keeps the air bubbles 15 accumulating to form an air pocket in portion with a bend 38, prior to bubbles being swept down vertical portion 40 by the liquid stream. As the bubbles descend down vertical portion 40 they become evenly spaced, which i~ important to avoid amalgamation. The height of 20 vertical portion 40 must be sufficient to achieve the desired spacing. A rule of thumb developed by the Applicant is that the height of vertical portion 40 should be approximately 2/3 of the height of pressure vessel 14, this will be hereinafter further explained in 25 relation to the studies made on the prototype.
Fourthly, directing the liquid stream through a cylindrical restriction gap 19 formed between second end 34 of conduit 30 and top 18 of pressure vessel 14. This is illustrated in FIGURE 4. The gas, being of lesser 30 density, passes more quickly through the restriction.
This pneumatically accelerates the liquid in front of the bubbles, as the bubbles "push" the liquid through the restriction. Liquid following the bubbles also accelerates due to a localized decrease in resisting 35 pressure. As the liquid exit restriction gap 19 it ` 8 l 334822 pneumatically hammers Ag~inst top 18 of pressure vessel 14 which serve as an impaction target and side walls 23.
The size of restriction gap 19 required is dependent upon the size of bubbles that are produced. The 5 variables on bubble Yize and the flow of the bubbles through restriction gap 19 will be hereinafter explained in relation to the studies made on the prototype. After striking top 18 and side walls 23, the liquid stream, by force of gravity, cascades through the gas in pressure 10 vessel 14 to further impact upon plate 35. The liquid stream then passes through openings 37 in plate 35 and by force of gravity ca~cades through the gas in pressure vessel 14 to further impact upon bottom 20 of pressure vessel 14. As pressure vessel 14 continues to be used 15 liquid accumulates at bottom 20 of pressure vessel 14, and it is primarily UpOA this liquid that the liquid stream impacts. Outlet 28 provides a means for removing liquid and gas from interior cavity 22 of pressure vessel 14, but interior cavity is always partially 20 filled during use. In this particular water treatment application the liquid stream continues on from outlet 28 to gas eliminator 16, where gases are vented, and then on to filter 1~ where the precipitates ferric hydroxide Fe2(OH)3 and m~ng~nese dioxide ~MnO2) are 25 removed.

The presence of plate 35 serves a dual purpose. It provides an impaction surface as described, but it also serves to prevent a portion of the liquid stream from 30 running down the side walls 23 of interior cavity 22, thereby avoiding the desired intermixing. The pressure level in pressure vessel 14 is maintained at 3 atmosphere. As a general rule the greater the atmospheric pressure the greater the ability of the liquid to absorb the gas; in this case the water to absorb the oxygen. The effect of a variation in the size of the restriction between second end 34 of conduit 30 and top 18 of pressure vessel 14 must be noted. If the distance is too large the impaction is diminished.
If the distance is too small, although there is good impaction, you lose your flow rate, open the possibility of clogging, and create too great of a pressure loss across the restriction. It will be noted the multiple impaction points in the described apparatus. The more 10 of these impaction points the thinner the interfacial films between the water and the air; the thinner the interfacial films the faster the transfer of oxygen.
When passing the liquid stream through the described apparatus the water should be cool as heat tends to 15 lower the maximum amount of gas soluble in the water, thereby preventing the desired aeration. The velocity of the liquid flow stream as it passes through the apparatus is approximately 9 feet per second. If the flow is too slow, it diminishes the force of the impact 20 upon top 18. If the flow is too fast the consistency of the size of bubbles downstream i5 adversely effected as excess turbulence tends to rip the bubbles apart. The applicant recommends A~ing to the water treatment ~ystem illustrated filter 17, preferably in the form of 25 a regenerating catalyst, to facilitate the removal of iron and m~ng~nese. The applicant also recommends adding to the water treatment system illustrated a magnetic or alloy based fluid stabilizing unit after filter 17, in order to prevent calcium scale build up on 30 equipment downstream.

As a general rule, the more turbulence created the greater the degree of gas transfer. The Applicant has ascertained that the presence of air bubbles of a 35 consistent size assists in e~ncing turbulence at restriction gap 19. However, as will be apparent from the description of the test apparatus which follows, in order for the preferred method to be operable the turbulence within liquid flow conduit 12 must be carefully controlled to ensure the bubbles are not ripped apart prior to reaching restriction gap 19.

The Applicant built a test apparatus out of transparent material in order that the method could be 10 studied and photographed. The flow in substantially horizontal portion 36 of conduit 12 was studied to determine the effect of the alteration of the distance between air injector 10 and bend 38 and other variables.
The Applicant discovered that air injected by air 15 injector 10 formed bubbles of relatively small random sizes. The rate at which these bubbles would amalgamate into larger bubbles in the flow stream and the size of the bubbles formed was dependent upon the size, length, and flow rate of liquid within horizontal portion 36.
20 Also of importance was the air to fluid ratio. As more air was injected into the flow stream, there was an increase in the rate at which the smaller bubbles amalgamated and the size of the bubbles formed. As the distance from air injector 10 to bend 38 was increased 25 the bubbles became larger due to increased time for the amalgamation. The flow rate effected the amalgamation rate as the slower the bubbles travelled along horizontal portion 36 the more time they had to amalgamate. As the speed of the flow stream was 30 increased, the turbulence in horizontal portion 36 similarly increased. The turbulence tended to rip the larger bubbles apart forming smaller bubbles and prevented the smaller bubbles from re-amalgamating.
Regardless of which variable was altered the bubbles 35 remained of random size.

In studying the flow around bend 38 it was determined that the smaller the radius of bend 38 the more mixing of the air and the fluid occurred. This is not, however, the primary purpose of bend 38. Bend 38 serves to collect most incoming bubbles into an air pocket located at the top of the bend. It is significant that the bubbles which are cut away by the fluid stream from the air pocket are of a relatively consistent size. The size of the pocket will vary, 10 largely dependent on the velocity of the fluid flow stream. The air pocket is continuously generated and as the air pocket increases in size it impinges upon the flow stream. The impingement eventually becomes so great that the flow stream separates and entrains a plurality 15 f air bubbles from the pocket. The pocket will then again increase its size to the point where another portion of the pocket will be separated and entrained.
There is a consistent pattern to this building and separating of a portion of the pocket. The faster the 20 velocity of the flow stream the smaller the bubbles formed will be. There is, however, a limiting factor in that there has to be enough air injected in the flow stream to accommodate pocket formation. If the air to fluid ratio is too low, the bubbles will have a te~ency 25 to stay entrained in the flowing stream.

In order for the bubbles from the entrained pocket of gas to be swept down vertical portion 40, the velocity of the fluid has to be greater than the 30 buoyancy forces attempting to force the bubble in the opposite direction. As a bubble flows down vertical portion 40, a flattening of the bubble occurred. This causes a ixing and a transfer of gases from the water to the air and vice versa. The bubbles also separate ~_ 12 1 334822 and space themselves evenly as they flow downwardly.
The length of vertical portion 40 should be long enough to accommodate this bubble separation. Vertical portion 40 of the Applicant's prototype was 32 inches with a pipe having a 0.8 inch inside diameter. The velocity of the flow stream was maintained at 9 feet per second. As the Applicant increased the length of vertical portion 40, the contact time with the water was increased but the desired bubble separation was not substantially 10 effected. As the Applicant shortened vertical portion 40 the bubble separation diminished. The diminishing of the bubble separation was viewed as being undesirable, a~ this allowed for amalgamation of the bubbles into larger bubbles. Amalgamation is undesirable for reasons 15 which will be hereinafter explained.

When flowing through 90 d~ elbow 25 and "T"
joint 31, the bubbles generally did not amalgamate into larger bubbles. Thi~ was attributed to the separation 20 between the bubbles. Elbow 25 caused turbulence which increased oxygen transfer; the smaller the radius of the elbow the greater the turbulence. When a tee was substituted for elbow 25 for this redirection and turbulence function, the bubbles tended to amalgamate 25 and separate into more random sizes. The velocity of the water as it flowed through elbows 25 and 31 proved to be important. Slower moving water permitted the bubbles to amalgamate and faster flowing water broke up the bubbles prematurely. "T" joint 31 caused turbulence in the 30 fluid, much the same as the elbow 25. Howeverj more turbulence was created, due to the apparent short radius as compared to a typical elbow of the same size. This increased the oxygen transfer rate. The use of "T"
joint 31 to redirect the fluid flow from horizontal to 35 an upward flow did not have negative effects, unlike the ~_ 13 1 334822 substitution of a "T" joint for elbow 25.

As the bubbles flowed up conduit 30 they assumed a more spherical shape and raced towards the restriction and impaction target. The velocity of the bubbles was greater than that of the flowing water due to buoyancy.

As the bubbles reached top 18 of pressure vessel 14, they impacted upon top 18 with the water causing a 10 localized high pressure zone. This zone crushed the bubble to a smaller size. This caused an increase in bubble pressure and increased the oxygen transfer rate.
The greater the speed of the water the greater the crushing effect. These bubbles were then swept way by 15 following water causing the bubble to exr~n~ to its approximate original size. Some bubbles broke up due to this violent action. This is not viewed by the Applicant as being a desirable effect for reasons which will be hereinafter explained, however, it does cause 20 considerable gas transfer. The contracting and ~xpAn~ i ng of the bubble due to turbulence, causes a thi~ning of interfacial films on both the water and gas side.

The impacted bubbles and non impacted bubbles then travel over to restriction gap 19. Here the bubbles will begin to flow through restriction gap 19 in various areas. The water in front of the bubble is pushed forward at increased velocity by the following gas, 30 because of the ability of the gas to travel faster through restriction gap 19. This water sprays against interior walls 23 of interior cavity 22 of pressure vessel 14 causing intimate mixing with the gas contained in pressure vessel 14. The oxygen transfer rate is 35 e~hAnce due to a "misting" of the fluid. The fluid ` ~_ 14 1 334822 following the bubble also increases in velocity due a decrease in pressure caused by the faster travelling bubble. When the water reaches restriction gap 19 it then rapidly reduces its velocity and causes as localized high pressure zone. Any bubbles in the immediate vicinity are compressed, and then eYrAn~. The incrca3cd local pressure will cause spraying of water into the bubble travelling restriction gap 19. The compressing and expanding increase turbulence. This 10 will then equate to more gas transfer to and from the liquid. Restriction gap 19 used by the Applicant had a cylindrical peripheral edge. This cylindrical edge was preferred as the liquid sprayed in all directions. The greater the diameter of the cylindrical peripheral edge 15 the more area which was provided for spraying. However, the greater the diameter, the closer end 34 of conduit 30 had to be positioned to top 18 of pressure vessel 14.
The relationship between restriction gap 19 and bubble size is important. In order to maintain this 20 relationship it is required that the bubbles be of uniform size. The use of bend 38 to form bubbles into pockets is critical to the formation of bubbles of uniform size. The bubbles should be large enough so that they occupy the height of the restriction gap 19.
25 This is so that the restriction force is at a minimum, causing greater velocity changes in the liquid before and after the gap. The bubbles should also be numerous 80 that the effect can be spread around the cylindrical peripheral edge of restriction gap 19. This will have 30 the effect of creating numerous violent small jets of spray around restriction gap 19.

The water leaving restriction gap 19 impinges on interior walls 23 and cascade down to baffle plate 35.
35 Water striking baffle plate 35 streams through openings ~ 15 l 33482~
37 the surface of water in vessel 14. The faster the water i`mpinges on the water surface the more the gases can transfer to and from the liquid. Also the faster the water impinges on the surface of the fluid in the vessel the higher the fluid level will become. This i8 due to the formation of very small bubbles which have very little buoyant forces acting upon them. This allows them to be swept away out the drain of the vessel, thus removing vessel gas. Having the vessel 10 under pressure also helps in oxygen transfer. This i8 due to an increase in partial pressure of each of the individual gases in the vessel and bubbles.

It will be apparent to one skilled in the art that modifications may be made to the preferred method and to the preferred apparatus without departing from the spirit and scope of the invention. For example, it is beneficial, but not essential, that the method and 20 apparatus also include other types of gravity and mechanical aeration techniques.

Claims (5)

1. A method of aeration of liquids, comprising the steps of:
a. firstly, injecting a gas into a flowing liquid stream thereby forming a plurality of gas bubbles;
b. secondly, directing the liquid stream upwardly through a vertically extending conduit against a transversely positioned impaction target at a top of a pressure vessel, the impaction target being closely spaced to a top edge of the vertically extending conduit thereby forming a restriction, such that buoyancy of gas bubbles in the liquid stream results in a pneumatic acceleration of the liquid stream through the restriction with a pneumatic hammering occurring as the liquid stream impacts upon the impaction target.

2. A method of aeration of liquids, comprising the steps of:
a. firstly, injecting a gas into a flowing liquid stream thereby forming a plurality of gas bubbles;
b. secondly, directing the liquid stream downwardly through a substantially vertically aligned conduit, such that a spacial separation of the bubbles occurs; and c. thirdly, directing the liquid stream substantially horizontally across a bottom of a pressure vessel, the horizontally flowing liquid stream being directed through a "T"
joint and upwardly through a vertically extending conduit against a transversely positioned impaction target at the top of the pressure vessel, the impaction target being closely spaced to a top edge of the vertically extending conduit thereby forming a restriction, such that buoyancy of gas bubbles in the liquid stream results in a pneumatic acceleration of the liquid stream through the restriction with a pneumatic hammering occurring as the liquid stream impacts upon the impaction target.

3. A method of aeration of liquids, comprising the steps of:
a. firstly, injecting a gas into a flowing liquid stream thereby forming a plurality of gas bubbles;
b. secondly, directing the liquid stream through a conduit having a bend, such that small gas bubbles adhere to walls of the conduit in the vicinity of the bend and amalgamate to form bubbles of substantially consistent size prior to being swept away by the liquid stream;
c. thirdly, directing the liquid stream downwardly through a substantially vertically aligned conduit, such that a spacial separation of the bubbles occurs; and d. fourthly, directing the liquid stream substantially horizontally across a bottom of a pressure vessel, the horizontally flowing liquid stream being directed through a "T"
joint and upwardly through a vertically extending conduit against an impaction target at the top of the pressure vessel, the impaction target being closely spaced to a top edge of the vertically extending conduit thereby forming a restriction, such that buoyancy of gas bubbles in the liquid stream results in a pneumatic acceleration of the liquid stream through the restriction with a pneumatic hammering occurring as the liquid stream impacts upon the impaction target.

4. The method as defined in Claim 2 and 3, the pressure vessel being a tower such that the liquid stream strikes the impaction target and then by force of gravity cascades through the gas in the pressure vessel to further impact upon liquid accumulated in the pressure vessel.

5. The method as defined in Claim 4, at least one plate with a plurality of openings being disposed in the path of the cascading liquid stream such that the liquid stream impacts upon the plate and passes through the openings in the at least one plate.