CA1124186A - Dry impact capture of aerosol particulates - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE
A dry method and apparatus for treating an effluent gas stream in order to facilitate the removal therefrom by conventional means of contaminating particulates, particularly those in the submicron range. Target particulates which are larger than the submicron contaminating particulates are dispersed in a secondary gas stream. The secondary gas stream is then introduced into the effluent gas stream. The manner of introduction is such that the contaminating particulates impact with and are captured on the larger target particulates.
The target particulates and the contaminating particulates inertially impaced thereon are then separated from the combined effluent and secondary gas streams by conventional gas cleaning equipment.

Description

v BACKGROUND OF T~IE INVE:NTION
1. ~'ield of the Invcntion This invention relates to a dry method and apparatus for treating an effluent gas stream in order to facilitate the removal therefrom by conventional means of contaminating particulates particularly those in the submicron range.

2. Des ription of the Prior Art In many countries throughout the world, emission standards have been or are being established to control the articulate content of effluent gases being exhausted to the atmosphere. Although these standards vary widely, most limit the particulate content of effluent gases to levels below 0.1 gr./sdcf. In many industrial applications, such as for example fiberglass furnaces, municipal incinerators, etc., such rigid standards can only be met by capturing and separating a large ~ercentage of the submicron particulates suspended in the effluent gas stream.
Conventional dry cleaning devices lack the ability to effectively and reliably se~arate submicron contaminating particulate suspended in effluent gas streams. For example, in the case of baghouses where fabric filter bags are employed, experience has indicated that the submicron particulates have a tendency to rapidly plug or "mask" the fabric interstices, thus requiring frequent disruptive bag shaking operations.
Where cyclone dust filters are employed, the submicron J/ ~ ~j ~ " L~

particulates have been found to lack the necessary mass required for efficient centrifugal separation.
Wet venturi scrubbers have also been employed for the purpose of separating contaminating particulates from effluent gas streams. Basically, a wet venturi scrubber consists of a constriction in the conduit carrying the contaminated effluent gas stream. The effluent gas stream is accelerated through the venturi constriction, and a liquid (usually water) is injected into the gas stream at the venturi throat. The high gas lO velocity atomizes the liquid and the relative velocities between the contaminating particulates and the liquid droplets result in a combination of one with the other through inertial impaction. The liquid droplets and their captured contaminating particulates are then separated from the effluent gas. While 15 this technique can lead to higher collection efflciencies, this advantage is offset to a considerable extent by other associatéd problems.
For example, it is known that the efficiency of the inertial impaction technique can be improved by reducing the 20 size of the target liquid droplets. This however requires igher gas velocities with accompanying pressure drops across the venturi of 30"-60" w.g. A pressure drop of 30" w.g. results ¦in an excessively high energy usage of 240 kWh per million cu. ft.
¦of gas cleaned. Attempts at reducing the pressure drop across 25 the venturi have not been successful, primarily because a high ~ u gas velocity is essential at the venturi throat in order to achieve optimum atomization of the injected liquid and still have a remaining differential velocity between the liquid droplets and the contaminating particulates which is sufficient to produce the desired inertial impaction. Some thought has been given to injecting a ~re-atomized liquid spray into the gas stream in order to accommodate reduced gas velocities through and reduced pressure dro~s across the venturi, but any advantage gained in this regard has been found to be offset by the power required ~o pre-atomize the liquid.
Another problem with wet venturi scrubbers is that the atomized liquid droplets combine with acid components of the effluent gas stream to produce a high corrosive medium.
This in turn makes it necessary to employ ducts and associated downstream equipment constructed of expensive exotic corrosion resistant materials. Even when this is done, however, corrosion related maintenance problems are encountered. Moreover, the resulting acid solutions must be neutralized, and even after this is done, disposal problems are encountered.
Other known gas cleaning arrangements have involved - the injection of solid material into the effluent gas stream.
An example of one such arrangement is shown in U.S. Patent No.
2,875,844. Such arranyements have resulted in little or no capture of submicron particulates because the solids have been dumped into the effluent gas stream at the outside diameter of the conveying duct as a dense agglomerate. By the time dispersion occurs, a condition which is essential for efficient capture, the solids have attained approximately the same velocity as that of the effluent gas and the contaminating particulates suspended therein. Without an adequate relative velocity between the contaminating particulates and the dispersed target particulates, effective particulate capture through inertion impaction is an impossibility.
According to U.S. patents 3,969,482 and 3,995,005, solid particulate material for sorbing and/or reacting with acid gases may be dispersed in a secondary air stream and radially injected through one or more points along an effluent gas carrying conduit. However, in the absence of adequate relative velocity between the gas streams together with means for inltially distributing the secondary stream into the effluent stream, little or no capture of contaminating particulates occurs, es~ecially at conduit ~ositions radially removed from the injection yoints. sy the time the added material is dispersed throughout the effluent gas, the relative velocity of the streams approaches zero.
While electrostatic preciyitators have met with B some success, their operation has been plagued by~ ~ }~, uildups of oils and fats where combustion practices are less han optimum, and variations in the conductivity of the ontaminating particulates.

SUMMARY OF THE INVENTION
It is a general objective of the presen-t invention -to obviate or at least considerably reduce -the aforementioned problems and disadvantages.
According to one aspect of the present invention, there is provided in a dry method for -trea-ting a primary gas stream flowing in a conduit -to facili-tate the removal -therefrom of submicron contaminating particulates, wherein a secondary gas stream with targe-t particulates dispersed therein is intro-duced into the primary gas s-tream to promote inertial impaction between said contaminating and target particulates, the improve-men-t comprising: introducing said secondary gas stream counter-currently to said primary gas stream through an orifice circum-scribing an area within said conduit; the average particle size of the target particulates being between about 3-50 microns; and deflecting said primary gas stream away from said area and across said orifice while simul-taneously accel-erating said primary gas stream to achieve a relative velocity between said primary and secondary gas streams at said orifice of about 20-200 feet per second.
The target particulates are large enough so that after they combine with contaminating particulates, the resulting particle size and mass will ~e such as to permit efficient capture and separation by conventional gas cleaning equipment.
According to another aspect of the present invention, there is provided an apparatus for treating a primary gas stream flowing in a conduit to facilitate the removal there-from of submicron contaminating particulates, wherein a secondary gas stream with target particulates dispersed therein is introduced into the primary gas stream to promote inertial impaction between said contaminating and target particulates, the improvement comprising: distributor means for introducing said secondary gas stream countercurrently to said primary gas stream through an orifice circumscribiny an area within said conduit; and deflecting means for deflect-ing said primary gas stream away from said area and across said orifice while simultaneously accelerating said primary gas stream.

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The invention will be more clearly understood from the following description and the accompanying drawing~, both of which refer to preferred but nonlimiting embodiments of the inventi.on.
S BRIEF DESCRIPTION OF 'rHE DRAWINGS
Figure l is a schematic illustration of an industrial installation embodying the method and apparatus of the present invention;
Figure 2 is an enlarged side view, partially broken - 10 away, of the preferred embodiment of the injection apparatus of the present invention;
Figure 3 is an end view of the apparatus sl1own in Figure 2;
Figure 4 is a partially exploded view of the distributor .
means of the embodiment shown in :Figures 2 and 3;
Figure 5 is a side view, with portions broken away, of an alternate e~bodiment of the invention;
Figure 6 is a sectional view taken along the line 6-6 f Figure 5 and rotated 90;
Figure 7 is a side view, again with portions broken way, of a third embodiment of the invention;
Figure 8 is an end view of the embodiment of Figure 7;
nd, E~igure 9 is a sectional view of still ano-ther embodiment f the present invention.

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Referring lnit:ially to Figure 1, -there is shown an optional quench chamher 10 receiving an indus-trial effluent gas Sl from a conven-tional source (not shown) such as a municipal incincerator, glass furnace, or the like. The effluent gas S2 from chamber 10 is directed through first conduit 12 to conven-tional gas cleaning apparatus depicted at 14, which may comprise a fabric filter such as a baghouse, a cyclone separator, or -the like. At section 22 of conduit 12, a secondary air stream A2 is introduced through second conduit 18 and the resulting gas mixture S3 fed to cleaning apparatus 14 and exhausted to the atmosphere through stack 16 which may include a conventional exhaust blower (not shown).
The effluent gas Sl has solid particulates suspended therein, for example soot, salts, oxides or the like, a portion of which are typically of submicron size. Such gas also frequent-ly contains~acid gases such as CO2, SOx, HCl, HF, and the like.
Quench chamber 10 may be used to cool the gas by aqueous spray, or to remove a portion of the acid gases as salts by alkaline aqueous sprays as described, for e~ample, in U. S. Patents

3,969,482 and 3,995,005. The spray water is evaporated and raises the dew point of the gas which may be of assistance in the particulate capture hereinafter described. It is not necessary in all cases, however, and may be omitted. The spray additions, if used, should be limited to avoid raising the de~ point sufficiently hic3h to provide saturation in any part of the i~rocess, pa~ticularly in appara~us 14 where excess rnoisture can interfere with separation. Preferably a difference of at least about 40F or more betwcen dry and wet bulb temperature is maintained.
The secondary gas stream A2 is ~roduced in conduit 1~ by means of blower 20 and introduced into the primary effluent gas stream S2 at portion 22 of first conduit 12, between flanges 24 and 26. Target particulates P are fed through vibratory hopper 28 and feed screw 30 and injected into stream A2 at a point within conduit 18 remote from conduit 12 to assure dispersal prior to introduc:tion into the effluent stream S2. Target particulates P can comprise any suitable solid and should have an average particle size of at least bout 3 microns, preferable 3-50 microns, more preferably 3-20 icrons, and most preferably 10-20 microns. Particulate nepheline syenite or phonolite is preferred.
Figures 2-9 and the following description illustrate ethods and apparatus for introducing secondary gas stream A2 into ~rimary stream S2 with a relative velocity and distribution sufficient to capture smaller contaminating solid aerosol particulates by inertial impact with target particulates P.
The mechanis~ of the inelastic impact captur~ is not thoroughly understood but is believed to include mass attraction and mechanical effects between the roughened surfaces of the . , impacting solids. In some cases, electrostatic forces and h;;midity effects may be involved. Capture has been demonstrated by air elutriation studies in which the finer captured parti-culates were not separated.
The method and apparatus currently pre~erred are shown , in Fiyures 2-4. A curved upper section 34 of second conduit 18 extends through the wall of section 22 of first conduit 12 and has a flange ~2 for joining to the vertical portion of conduit 18 shown in Figure 1. The inner end of section 34 is fixed by supports 42a, 42b and 42c and is joined to a first right truncated cone 38 of sheet material with its larger base directed upstream with respect to the flow path of gas Sl.
. Located concentrically within and syaced from cone 38 is a second right cone 50 having its apex directed downstream with respect to the flow of S2 and upstream w.ith respect to the flow of gas A2. Cones 38 and 50 define therebetween an annular passageway for the secondary gas stream A2 which terminates in an annular orifice 40 through which the stream A2 is introduced and injected countercurrently into effluent gas S2. Together they constitute .
distributor means for introducing and injecting stream A2 into . stream S2 at a plurali.ty of positions about the axis of duct 12.
A third right cone 52 is provided with its base joined to the base of cone 50 and with its apex directed upstream l with respect to the flow of gas S2. Cone 52 serves both as accelerating means for the acceleration of gas stream S2 during ~3Z~

introduction of stream A2, and as deflector means for deflecting ; the flow of 52 toward the walls of conduit 12 transversely across the orifice 40.
Cone 50 is secured to cone 38 by means of bolts 54 and is spaced therefrom by means of four spacer members 58.
As shown in Figure 3, a hinged door 46 is provided upstream of flange 24 for providing access to the interior of conduit 12.
Also shown is a pipe stub 44 for connection to a fume hood (not shown) over hopper 28 to prevent escape of ~arget particulate to the atmosphere.
By the method and a~paratus shown in Figures 2-4, the gas stream A2 and dis~ersed target partic~lates are intro-duced into the interior of stream Slj with good distribution and high relative velocity. Relative velocity as used herein refers to the algebraic sum of the vectors of the flow of gas stream S2 and A2 parallel to the axis of duct 12 during intro-duction of A2 into S2. ~ relative velocity of about 20-200 feet per second is ~referred and from about 50-150 is more referred. By the means shown, an efficiency of cap~ure of ubmicron particulates equivalent to a liquid venturi having n inlet to throat pressure drop of from 50 to 150 inches water auge can be obtained with substantially reduced power, without ontaminated wash li~uid, and with reduced corrosion.
While circular sections for conduits and cones are referred as shown, other polygonal or oval sections can be usedO Preferably the same sections are used on all concentric parts to maintain uniformity of flow about the longitudinal axis of conduit section 22.
A second embodiment of the present invention is shown in Figures 5 and 6. A venturi is provided within conduit section 22 by means of truncated, conical member 60 fixed to its walls and having a narrowed throat opening 62. Concentric conical sheet metal members 64 and 66 are mounted between the outer wall of member 60 and the inner walls of section 22 and define between them a tapering circumferential passageway 67 which terminates in an annular orifice 68 surrounding throat . pening 62. Secondary gas stream A2 is fed through duct 70 tangentially into passageway 67, around member 60, and outwardly through orifice 68 where it is introduced cocurrently into the ccelerated effluent gas stream S2. Relative velocity is provided by feeding stream A2 at a different, preferably lower, velocity than stream S2. Both streams are turbulent with good istributed mixing and efficient impact capture of contaminating articulates is obtained Duct 70 is provi~ed with a flange 71 by which it is joined to the vertical portion of conduit 18 of Figure 1 and comprises the outlet portion of that conduit ithin the conduit 12.
Fro~ Figures 2-4 and Figures 5-6, it will be noted that while the flow is countercurrent and cocurrent, respectively, there are also radial components of flow as the streams S2 and A2 merge. As used herein, -the terms countercurrent and cocurrent include such flows where there is a substantial vector component of motion along or against the direction of flow of the primary effluent stream S2 parallel to the axis of conduit 12. Co-current and countercurrent injection of the dispersed target particles distributed interiorly of the walls of the conduit are preferred, with countercurrent flow being most preferred since it provides the greatest effective relative velocity.
A third embodiment is shown in Figures 7 and 8 in which duct 82 terminates within conduit section 22 with four nozzles 86 spaced even about the longitudinal axis of section 22.
Nozzles 86 are angled away from that axis toward the conduit walls and divides the secondary gas stream A2 into four sub-streams. Duct 82 has flange 84 for joining to vertical conduit 18 and constitutes the upper outlet end thereof within conduit 12. Secondary stream A2 is fed into effluent stream S2 either cocurrently or countercurrently, but is preferred countercurrent s shown. Conduit section 22 is provided with access door 80 and the end of duct 82 is supported within section 22 by eans of T-shaped support 88. Since the nozzles 86 and their associated structure significantly reduce the flow area ithin conduit section 22, gas stream S2 will be accelerated as the stream A2 is lntroduced therein.
While countercurrent or cocurrent flows are preferred, impact capture can also be obtained by radial injection from a plurality of circumferential positions as shown in the fourth embodiment of Figure 9, although with re~uced efficiency. As shown in Figure 9, the secondary gas stream ~2 with dispersed target particulates P is fed through conduit 18 into a circum-S ferential manifold 92 surrounding the wall of conduit section 22. Manifold 92 communicates with a plurality of openings 94 in the wall of section 22 which are preferably uniformly spaced therearound. Relatively high velocity for stream A2 and a relatively even distribution of flow through openings 94 is required.
~eferring again to Figure 1 t the gas streams S2 and A2 flow past flange 26 as combined stream S3 and into the cleaning apparatus 14. The effluent gas Sl has been cooled by admixture ~ith the ambient air stream A2 and by any quench liquid applied in chamber 10. If further cooling is desired, an additional air stream A3, controlled by a conventional damper or the like (not shownJ, may be admitted through the duct 100 prior to ntry into the apparatus 14.
While good results have been obtained without applying lectrostatic charge to the particles P,for some applications he efficiency of removal can be increased by providing a charge f either polarity. This can be accomplished, for example by assing the air stream A2 with entrained target particulates etween charged electrodes 102 and 104 within conduit 18.
lternatively, the target particulates can be triboelectrically ~ ~ d L?'~
I

charged by passing them in contact with a suitable triboelectric material such as ylass or the like located as a lining or as one or more collars within conduit 18. ~ venturi collar of such material can be used for increasing friction. The contam-inatiny particulates in stream Sl need not be charged, but can be similarly charged with the opposite polarity if desired.
EX~iPLE 1 The present invention has been tested in a pilot installation using a slip stream of effluent gas from a glass meltiny furnace used for the manufacture of glass fibers, employing the apparatus of Figures 1 and 5. Ninety percent of the contaminating ~articulates were less than 1 micron in size and the particulates were estimated to have an average ;~ particle size of about 1/2 micron. These particulates were omposed typically of the following salts and oxides: sodium fluoride, calcium fluoride, calcium oxides, silica, sodium ulfate and boron oxid~s. The gas also contained acid gas omponents, particularly oxides of sulfur and hydrogen fluoride s indicated in Table 1 below. ~etween about 10 and 35 pounds er hour of ne~heline syenite having an average particle size etween about 10 and 20 microns was metered into the secondary ir stream ~2 The incoming gas stream Sl was quenched in hamber 10 with a spray at the rate oE 2.5 gallons per minute ith a 2.5% by weight slurry of calcium hydroxide in water.
he results of this test are given below in Table 1 wherein CFI~ means actual cubic feet per minute, PPr~l means parts per nillion, and gr./SCF means grains per standard cubic Eoot.

TABLE I
Sl S2 ~2 ~A3 S~
GAS VOL.
ACFM: 7000 5600 1000 1400 7000 TEI~IPE~TURE, F:
Dry Bulb: 700 235 Ambient Ambient 170 Wet ~ulb: 95 134 Ambient Ambient 117 Velocity, fps: ~ 50 50-80*
SO~, ppm:100~200 20-30 F , ppm: 90 B , gr./SCF0.25 0.003 ther Particulates gr./SCF: 0.2 0.01 *at injection into S2 Capture of the fine contaminating ~articulates in the effluent gas stream by the target particulates injected ith the stream A2 was verified by gas elutriation tests.
sample of the separated mixture of particulates shaken from the bags in the baghouse was placed in a column. A stream of air at various velocities was passed upwardly through the particulate sample. Particulates entrained with the air were seyarated, tested, and compared with like tests from the 2S riginal material. The tests were substantially the same indicati.ng that the fine particulates were bound to the heavier target particulatcs.
EX~PLE 2 A typical flue gas from a municipal incinerator containing a mixture of solid particulates of oxides and salts and treated according to the method and apparatus of Figures 2-4 will give results substantially as follows, when quenched with 60 gallons per minute of water in chamber 10 and supplied with 100 to 150 pounds per hour of target particulates of 3-15 micron nepheline syenite in stream A2:
TABLE II
Sl ' s2 A2 S~l as Vol., ACFM (OOO's) 250 150 5 156 emperature, ~F
Dry Bulb: 1600 400 80 380 Wet Bulb: 100 164 60 162 elocity, f~s, about: 50-80 50-80*
articulates, gr./SCF: 2.0 0.03 HC1, PPMo 200 5 HF, PP~: 10 3 . *at injection It should bc understood that the foregoing description nd e~amples are given for the purpose of illustration and that he invention includes all modifications and equivalents within the scope of the appended claims.

Claims (8)

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. In a method of treating a primary gas stream flowing in a conduit to facilitate the removal therefrom of submicron contaminating particulates, wherein a secondary gas stream with target particulates dispersed therein is intro-duced into the primary gas stream to promote inertial impaction between said contaminating and target particulates, the improve-ment comprising:
introducing said secondary gas stream countercurrently to said primary gas stream through an orifice circumscribing an area within said conduit; the average particle size of the target particulates being between about 3-50 microns; and deflecting said primary gas stream away from said area and across said orifice while simultaneously accelerating said primary gas stream to achieve a relative velocity between said primary and secondary gas streams at said orifice of about 20-200 feet per second.

2. The method of Claim 1 wherein the relative velocity is greater than about 50 feet per second.

3. The method of Claim 1 or 2 comprising the further step of passing the combined gas streams with sus-pended captured and target particulates through means for separating the particulates from the gas.

4. The method of Claim 1 wherein during intro-duction of the secondary gas stream into the primary gas stream, both gas streams are deflected outwardly away from their respective flow axes towards the conduit wall.

5. Apparatus for treating a primary gas stream flowing in a conduit to facilitate the removal therefrom of submicron contaminating particulates, wherein a secondary gas stream with target particulates dispersed therein is intro-duced into the primary gas stream to promote inertial impaction between said contaminating and target particulates, the improvement comprising:
distributor means for introducing said secondary gas stream countercurrently to said primary gas stream through an orifice circumscribing an area within said conduit; and deflecting means for deflecting said primary gas stream away from said area and across said orifice while simultaneously accelerating said primary gas stream.

6. The apparatus of Claim 5 wherein said orifice is annular.

7. The apparatus of Claim 6 wherein said deflecting means comprises a right circular cone supported coaxially with-in said conduit, with the apex of said cone pointing upstream with reference to the flow direction of said primary gas stream.

8. The apparatus of Claim 5 wherein said distrib-utor means comprises: a truncated right circular first cone supported coaxially within said conduit, the small diameter end of said first cone being in communication with a source for said secondary gas stream and with the base of said first cone facing upstream with reference to the flow direction of the primary gas stream, a second right circular cone received within and supported in spaced relationship relative to said first cone, the apex of said second cone pointing upstream with reference to the flow direction of said secondary gas stream, the base sections of said first and second cones co-operating in radially spaced relationship to define said orifice; said deflecting means comprising a third right circular cone connected at its base to the base of said second cone and having its apex pointing upstream with reference to the flow direction of the primary gas stream.