ENGINEERING, DESIGN, AND PROCESS DEVELOPMENT
Aerosols from
Air Streams EDWARD M. BERLY, MELVIN W. FIRST,
AND
LESLIE SILVERMAN
Harvurd School of Public Health, Depurtmenl o f Industrial Hygiene, Boston 75, Muss.
E
'
4 NSIVE use of hydrogen fluoride in process metallurgy for beryllium, uranium, and ohher metals has stimulated the search for improved methods of removal of fluorides from stack gases. The primary aim of any such study is high efficiency removal of gaseous and particulate fluorides. Recovery from the absorbing fluid and concentration for re-use in the plant manufacturing process is desirable in most instances because of the large quantities involved. Hydrogen fluoride can be absorbed in conventional packed toners filled with Berl saddles, Raschig rings, 01 similar materials to provide intimate mixing and absorbing surface for the gas stream and the countercurrently flowing solvent. Hydrogen fluoride exhibits such a strong affinity for lvater that it may also be absorbed in simple spray chambers. Several such processes for the disposal of fluoride gases have been described in a recent article (11). The fluoride gases absorbed in the spray water in this process are sent to neutralizing lime vats before final disposal. Efficiencies of 95% or better are reported.
more transfer units per foot of height than do towers filled with conventional packings. Tests of the Harvard Air Cleaning Laboratory (6) showed that wetted glass fiber cells give high efficiencies with aerosols containing soluble acid mists and soluble gases with low vapor pressures. The large wetted area represented b y the fiber surface of even large diameter fiber packs presents a favorable situation for gas absorption. The use of glass fibers for absorbing hydrogen fluoride is inipractical. However, the benefits of high efficiency absorption by the use of fiber packs containing large surface area may be obtained by substituting fluoride-resistant plastic materials, such as saran (thermoplastic vinylidene chloride) or dyne1 (vinyl chloride-acrylonitrile copolymer) for glass. The removal of fine particles-Le., fluoride mists and fumeis not readily accomplished in ordinary spray chambers or in towers filled with conventional packings because effective separation requires fine droplets (50 microns) or small diameter fibers ( 5 microns). It has been shown (6) that coarse fiber cells, such as those commonly used in wet cell washers, are ineffective for the removal of fine Table I. Physical Characteristics of Gas Absorption Packings particles from an air stream. Surface PorosDecreasing the fiber diamApproximate Cost per Weight, Area ity, eter of the packing results Fract. Lb./ sq. Fd.1 sq. ft. of Voids Cu. Ft. Cu. Ft. Size CU. it. Pound surface Material in increased efficiency, but 45 0.68 ! / q jnch 141 $26 50 Porcelain Berl saddles $0.59 80.1s mechanical features limit the 1 inch 79 0.69 42 Porcelain Berl saddles 9 95 0.24 0.13 1 / ~ inch 65 114 0.53 13 50 Porcelain Raschig rings 0.21 0.12 use of fine fibers t o dry opera45 58 0.68 1 inch 4 25 Porcelain Raschig rings 0.10 0.07 tions. This i n d i c a t e s t h a t 13 0.58 3 inches 29 0 29 0.01 Coke 0.028 0.50 88 82 S/a inch 0 12 Crushed stone 0.0015 0.001 fluoride gas can be absorbed 3 / r inch 49 0.49 85 42 50 Ceramic balls 0.50 0.87 440 0.94 178 niiorons 6 8 6 80 Saran fibers, diam. 1.00 0.016 in spray chambers, in conven3 6 0.97 530 78 microns 3 60 Saran fibers, diam. 1.00 0.007 tional packed towers, or in 51 microns 3 2 940 0.96 Saran fibers, diam. 3 20 1.00 0.003 wetted fiber packs and that fine mists and dry fluoride fumes can be removed effiLandau and Itosen ( 9 ) described a method in which hydrogen ciently b y dry, small diameter fiber filters. fluoride and fluorine are continuously absorbed in a 6 to 10% fiodium hydroxide solution. The effluent liquor is treated with Essentials of Efficient Collection of Gas, a calcium oxide slurry to regenerate sodium hydroxide for reuse Vapor, Fume, and Particles Are Outlined and to precipitate fluorine as calcium fluoride which may be removed with excess lime. Concentration of hydrogen fluoride in When fluorides are present in air or gas streams as a mixed gas, the effluent gases 7va.s reported to be less than 3 p.p.m. Hignett niist, and fume, it is desirable t o collect them simultaneously in and Siege1 (8)devised a method in which hydrogen fluoride is a single unit. Even though it is desirable to combine the separaabsorbed in a bed of limestone a t temperatures above the dew tion of gas, vapor, mist, and fume from a carrier gas stream into point of the stack gas. Calcium fluoride, the reaction product, is a single collection unit, removal of gas and vapor must be conseparated from the limestone lumps as a fine dust. Portions of sidered separately from removal of particles because they are the bed are withdrawn from the towers a t intervals and screened essentially different processes. to remove the fines. Oversizes, with fresh make-up limestone, Gases and Vapors. The collcction of gases and vapors is usuare fed back to the tower. Fluoride recovery in a pilot plant ally accomplished by absorption processes involving a liquid using a &foot depth of packing is as high as 9G%. absorbent that reacts physically or chemically with the gases to I n the lattcr two methods the fluorides are converted to calbe collected. I n most absorption columns the flow through cium fluoride, a relatively cheap chemical. The product is many the packing is turbulent, while in a fibrous bed flow is transitional times more valuable when it is possible to concentrate it as hydro( 5 ) . Transitional flow used t o be considered detrimental to gen fluoride. good mass transfer because effective film thickness tends to be Williams and his coworkers (18) investigated the use of Fiberlarge in the absence of full turbulence. However, Sherwood and gias paclrings in conventional towers for gas absorption. Their Pigford ( l a ) point out that best results may be obtained from a studies indicated t>hat towers filled with fibrous material have September 1954
INDUSTRIAL AND ENGINEERING CHEMISTRY
1769
ENGINEERING, DESIGN, AND PROCESS DEVELOPMENT packing where there is just sufficient turbulence to keep the bulk of the stream well mixed. Table I indicates the relative packing surface area per cubic foot of absorber volume for various sizes of fibers, Raschig rings, Berl saddles, coke, balls, and gravel. Also shown in the table is the relative porosity of the packing materials. These figures indicate that packed fibers have a great advantage over more usual types of packings in relation to the amount of surface available for absorption per unit volume of fiber bed. Transitional-type flow (which takrs place through beds of coarse fibers) may increase the effective wetted area of the packing. The higher porosity of the fiber beds favors low rpsistance gas flow. Mist and Solids, The collection of mist and solid particles may be treated as a single process if the assumption (15 hich appears to be valid for small particles and mist droplets) is made that particulate matter that contacts the removal surface (fiber or other types of packing) is instantly and permanently removed from the air and gas stream. Liquid droplets and soluble solid particles such as sublimed fluoride fume are wetted or go into solution when captured bv the packing material and are washed into the sump. Inert particles which may accompany the mist or soluble fume in small or large concentration are similarly captured by the packing, but this material may adhere to the packing and eventually plug the bed. Therefore, provisions must always be made for removing inert particles early in the cleaning process, or for periodically cleaning or replacing those sections of packing that become plugged and useless. Conventional packings, on the ordrr of t o 2 inches, are poorly adapted to small particle removal. The reverse is true for small diameter fiber beds. The resistance characteristics of wet and dry fiber beds have been considered in detail in AEC Report STO-1581 ( 6 ) and by First and asPociates ( 6 ) .
cabinet, including the filter frames, was coated with protect'ive paint to reduce corrosion. The scrubbing liquor in the sump was recirculated, and the fluoride concentration was allovicd t o increase t o approximately 27,. -4ir was drawn into the unit by an exhauster on the exit side (washer under negative pressure). Increased capacity can be achieved by raising the number of filters in parallel in each stage. Commercial unit,s of this .type are available for treating several thousands of cubic fcvt of air or gas per minute. Filter Units. CHARACTERISTICS O F FIBERS.Previous work ( 5 , 6) with glass fibers has shoan that the following fiber properties are of importance in the construction of gas scrubbing filtt
1. Resistance to corrosion, erosion, and breakage 2. Bed stability over a long operational period (For xvett,ed fiber cells this means resilience and resistance t o matting under the act,ion of the scrubbing liquor) 3. Low air flow resistance 3. Large surface area per pound of packing 5 . Low bulking density-Le., low weight per cubic foot of packing For ahsorption of hydrogen fluoride the selection of materials from vihich to make chemically resistant' fibers is rather rostricted. 8everalplastics have excellent chemical resistanceand a r ~ produced in a large number of fiber sizes. The specific gravit,y of plastics used for the production of fibers is generally below 2.0 and relative costs are about $1.00 per pound. I n comparison with resistant metal fibers such as Monel, plastics have five t o six times as many fibers of a given size per pound, and on a fiber cost basis they are approximately '/Sjath the price of the metal, Saran (obtained from the Saran Warns Co., Odenton, Md.) and dynel (obtained from the Carbide and Carbon Chemicals Co., New York) plastic fibers were invest'iyated and a t the time of this study saran fibers were available in diameters greater than 27 microns and dynel in diameters greater bhan 10 microns.
"UIF
SCALE
Figure 1.
Efficiency. Cleaning requirements (degree of efficiency required) may be considered under t n o categories-removal of low concentrations from stack gas effluents, the primary purpose of which is the elimination or prevention of atmospheric contamination, and scrubbing of process gases or off-gases containing sufficient material to make recovery and re-utilization economically desirable. I n many instances it may be necessary to meet both requirements, although gases that only cause problems of atmospheric contamination are generally too lean for recovery, concentration, and purification. High collection efficiency is not always needed for the removal of offending material from stack gases. Generally, the intent is to reduce the effluent below a concentration that can produce a nuisance [or health hazard ( I ) ] . Multistage Washer with Changeable Filter Cells I s Used for Experiments
An experimental, multiple-stage xetted cell washer of the type described in detail ( 5 )was used in all the teets (Figure 1). The unit was built to receive a variety of fiber filters placed a t a 45 angle to the air steam. The coarse fiber wetted cells were followed by a droplet eliminator and fine fiber pads. The interior of the
1770
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a ,
Experimental Scrubber WET .%xuDRYFILTERS.Previous work ( 6 ) with Fiberglas filters indicat'ed that concurrent spraying permits higher gas velocities than are pract,ical with count,ercurrent operation and reduced channeling results in better netting of the packing. Wet,ting was accomplished with six suitably spaced flooding nozzles (Clarage Co., No. 0 flooding nozzles) per 20 X 20 inch cross section wet cell, placed 6 inches upstream of the cell face. It x a s thus possible t80 use a thin (-$-inch) pad with relatively large cross-sectional area and yet avoid channeling. I n convent'ional countercurrent toaers, by contrast, the height is often five times the diameter of the column to ensure proper distribution of gas and liquid t,hroughout' the packing. I n most tests the scrubbing liquor was recirculated at the rate of 8 to 9.6 gallons per minute per 20 X 20 inch cell. This quantity of liquid was large in comparison with the amount wf hydrogen fluoride to be absorbed; hence, 6he concentration of fluorides in the absorbing liquor remained substantially constant during passage through the wet cell. Because of the high liquid rate there mas no appreciable temperature change due to heat of solution of hydrogen fluoride in the scrubbing liquid. Fine-spray nozzles (Bet,e Co., Greenfield, Mass., nozzle No. P40) were subst,itutedfor the first Ret stage in order to determine
INDUSTRIAL AND ENGINEERING CHEMISTRY
Vol. 46, No. 9
ENGINEERING, DESIGN, AND PROCESS DEVELOPMENT
Table 11.
Fiber Packings and Gas Scrubbing Devices
(20 X 20 Inch face area) Cell Thick- Fiber ness Diam., Inch& Material p Curled saran 3 178 Curled saran 4 78 Curled saran 4 78 Curled saran5 1 78 4 51 5-Inch curled dynel Clarage No. 0 flooding nozzles .. .. Bete P40 fine-spray impingement .. .. nozzles Neva-Clog screen .. Saran 2 44 2 29 Saran 2 29 Saran l l / z inches thick 12 Cotton. I / % inch thick 2 29 3
Cell Designation 1W-wet 2W-wet 3W-wet 4W-wet 5W-wet 6W-wet 7W-wet
..
8W-wet 1D-dry 2aD-dry 2bD-dry 2cD-dry
2dD-dry 2eD-dry
Saran Glaas ( P F 105), inch thick Polystyrene Glass (PF lop), eight light */g-inch layers, resin bonded and compressed t o 2 inches 2-Inch curled dynel %Inch curled dynel
3D-dry
1
2
29 3
2
29 3 1 3
2
Total Packing Wt., Lb. 4.3 2.6 3.3 1.7 2.9
..
Packing Density Cu. F t . 6.8 2.8 3.6 2.9 3.2
Packing Porosity, Fract. Voids 0.94 0.97 0.97 0.97 0.96
..
.. ,.
4:3 4.8 6.3 4.3
0196
.. 2o: 2.2 2.2 0.6
2.2 0.2 0,055 2.2 0.2 2.2 0.1 0.6 0.9
Lb./
..
0.95 0.94 0.94
..
.. ..
Table 111.
.. ii25 1910 2500 5840
.. .. ..
,.
,.
.. ..
..
..
..
..
.
0: 98
i.'g
2 2
the coniparative efficiency of wetted fibers and fine spray droplets for the absorption of hydrogen fluoride gas. When gases contain large quantities of insoluble, inert partides, fiber filters plug rapidly and, in many cases, the scrubbing liquor cannot wash the fibers clean. When dirty gases were scrubbed, a wetted Neva-Clog screen (Multi-Metal Wire Cloth Co., Bronx, N. Y . ) was substituted for the first wetted fiber stage. Previous tests ( 4 ) using the Neva-Clog screen (a stainless steel perforated double plate screen with 125 staggered 0.045inch-diameter holes per square inch, sheet thickness of 0.010
..
..
18 1.3 2.8 0.97 0.75 3.2 0.96 18 0.5 2.2 15 0.97 2.9 5-Inch curled dynela 61 6D-dry 4 3.2 0.96 a 20 X 50 inch pad pleated into 4-inch-deep frame and placed between two l/la-inch mesh saran screens. 4aD-dry 4bD-dry
440 414 531 428 940
I .
..
I
Surface Area Sq.Ft:/ Cu. Ft.
.
I
4j80
2330 2790 2190 940
inch, and space between sheets, maintained by dimpling the sheets and spot welding, of 0.05 inch) showed a retention efficiency of 98.8% for 9-micron glass spheres and 81.7% for 0.7micron copper sulfate microspheres. I n addition to its ability to remove particles, i t seemed possible that the wetted screen might provide sufficient wetted surface to act a.s a partial absorber for hydrogen fluoride gas. Two wet stages were usually placed in series and followed by zigzag eliminator plates to remove coarse droplet carry-over. A dry filter composed of fibers considerably finer than those used
Test Aerosols
Characteristics Concn., mg./ c u . m. GAS
Composition Hydrogen fluoride
25,1-4690
MIST
Ammonium bifluoride Sulfuric acid Sulfuric acid
Sulfuric acid
Ammonium bifluoride Aluminum chloride Atmospheric dust
Talc Silica
September 1954
Std. Mass geometmedian ric diam., deviation IL
..
..
4.6-63
4.3
3
62-63
4.5
3
72-78
0.83
346-349
0.60
FUME 33-305
0.54
2.8
380-420
0 59
2 7
200-2000 100-1300
1.4 4 1
Sampling Method
Compressed anhydrous gas released into inlet air stream
Two fritted glass ab-
10% Solution atomized into inlet air stream 10,Yo Solution atomized into inlet air stream 10% Solution vaporized on hot plate and recondensed into droplets in inlet air stream Midget electrostatic 98Yo solution vaporized precipitatdr (4) on hot plate and recondensed into droplets in inlet air stream
3.1
3.1
INERTDUSTS 0 1-1 0 5 (estd.) (count)
iMethod of Generation
\
Sublimed on hot plate and recondensed as a solid in inlet air J stream I
Laboratory room air
1 8 2 1
} Dispersed stream
Bnalytical Method
)
t
Nephelometrically a s barium sulfate ( 1 7 )
Same as hydrogen fluoride Kephelometrically a s silver chloride (16) Filtration through 1- Determination of ininch disk oi Whattensity of dust stain man No. 41 filter with optical densipaper a t 1 cu. ft./ tometer ( 6 ) min.
in inlet air
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ENGINEERING, DESIGN, AND PROCESS DEVELOPMENT circulated at 8 e0 10 gallons per minute per Gel1 and 300 to 600 cubic feet per minute. Samplm were taken befow the first wet cell, after the second, and after the dry f&er. When possible, stage samples were taken between the two wet cells. Hydrogen Fluoride Gas. Loadings of hydrogen fluoride ( a b fluoride ion) ranging from 25 to 4000 mg. per cubic meter of air were absorbed in water containing 0 to 2% of fluoride as tht: scrubbing liquor. The results are summarized in Table IV. Hydrogen fluoride gas was absorbed at greater than 99% efficiency by two Tyetted fiber cells in series (test A ) and efficiency appeared to be independent of velocity oYer the range tested (superficial face velocity 108 to 216 feet per minute), The final dry pad increased over-all collection efficiency only a fcn. tenths of 1%. since the dry filter collected only droplets small enough to penetrate the six-bend zigzag eliminator. Substitution of a concurrent water spray (six No. 0 Claragc* flooding nozzles without the fiber cell) for the first v e t cell ( t w t B) restilted in a significant decrease in total \tedfibers is the very large wetted surface
INDUSTRIAL AND ENGINEERING CHEMISTRY
Vol. 46, No. 9'
ENGINEERING, DESIGN, AND PROCESS DEVELOPMENT 1
Summary
015
0 3
0.2
O l
RESSTIIVCE
- IllCHEB HLlTER
06
08
I
GAGE
Figure 4. Effect of Water Rate on Wet Cell Resistance of 4-Inch Pad of 78-Micron Curled Saran Fibers Cell 3 W of Table II Water Rate, Gal./(Min.)(Sq. Ft.)
W a t e r Rate, GaI./(Min.)(Sq. Ft.)
Dry
1 1
1.6
3
2.3
Table IX.
5
2.9
3.4 4.0
Absorption Efficiency of Packing Materials
Packing 78-Micron curled saran fibers (packed 3.6 lb./cu. Et.) Rerl saddles 1/1 inch 1 inch 1I/* inch Raschig rings 1 inch 11/a inch Q
4 5 6
H.T.U., F t . Q 0.14b (calcd.) 0 . 8 1 (18) 0 . 8 5 (1%) 1.17 (1%)
Based on gas r a t e of 400 and liquid rate of 1500 lb./(hr.) (8s.Et.) Based on gas rate of 480 and liquid rate of 1680 lb./(hr.j (sq. f t . ) .
High efficiency absorption of soluble or reactive gases was obtained with wetted fiber beds. Wetted fibers were five to 10 times more efficient than Raschig rings or Berl saddles, compared on the basis of equal volumes. When compared on the basis of weight of packing, 1 pound of 78-micron-diameter %ran fibers mas 75 times more effective for the absorption of hydrogen fluoride gas than 1 pound of ‘/%-inch Berl saddles. This reduction in weight and bulk can be utilized to realize important savings in construction and maintenance of gas absorbing systems I n addition to hydrogen fluoride gas, cleaning efficiency for sulfuric acid and ammonium bifluoride mists, ammonium bifluoride and aluminum chloride fumes, and silica, talc, m d atmospheric dusts was investigated. High efficiency collection (greater than 99.9%) generally required the addition of a droplet eliminator composed of a 1- to 2-inch depth of dry fibers less than 5 microns in diameter. Although absorption of the gas M as complete, a significant quantity of fluorides passed the scrubber in the form of fine (less than 10 microns) mist droplets formed from condensation of hydiogen fluoride gas in the humid atmosphere of the scrubber or from fine droplets formed by the sprays. For some atmospheric pollution problems the small diameter drying fiber stage may be unnecessary. When gas streams containing inert particles were treated, thc absorbing stages were protected from fouling and plugging bT the use of an impingement device such as a Neva-Clog screen as a prefilter. Over-all resistance of the scrubber was proportional to the f l o i i rate. For gas flows of 200 cubic feet per (minute) (square foot) of scrubber face area, high efficiency scrubbing of gas and suhmicron particulate matter was obtained Kith resistances not exceeding 6 inches of water gage. For atmospheric pollution control of stack gas, emissions resistances less than half this mav be adequate. literature Cited
available for absorption. For example, eeveral important characteristics of a bed of 78-micron saran fibers (as used in this study) are compared with 1/2-inch Berl saddles and other packings in Table I. Not only is a given height of fibers five to 10 times more effective for absorption, but transfer unit for transfer unit, the total weight of a l/a-inch Berl saddle bed is 75 to 150 times as great as is required for fibers, thereby necessitating larger, stronger, and more expensive housings or towers. Applications. This method of gas absorption is of general utility for the absorption of other gases as well as those discussed in this paper since it has operating features usually unobtainable in conventional absorption installations. By recirculation of liquid from final to initial stages this system can not only incorporate the principle of recycling, which is necessary for high concentration of the gas in the solvent, but also the advantages of concurrent spraying and stage operation. As high as 70% acid can be attained by this procedure. Concurrent spraying on coarse fiber packing permits high capacity without flooding, low gas-flow resistance, and no channeling. Recirculation of the sump liquid a t each stage, in combination with overflow weirs to control the flow of liquor through the unit, permits high liquor rates to ensure thorough wetting of the fibers and prevent significant increase in concentration and temperature (from heat of solution) during a single pass. I n this manner, a maximum partial pressure driving force is maintained on each stage. These features would be useful, for example, in the design of hydrogen chloride absorption equipment where the important consideration is dissipation of the heat of solution. Modern hydrogen chloride towers are actually complicated heat exchangers, and the wetted fiber scrubber would greatly simplify their construction. September 1954
(1) American Conference of Governmental Industrial Hygienists. Arch. I n d . H y g . Occupalional M e d . , 6, 178 (1952). (2) American Society of Mechanical Engineers, New York, Power Test Codes, Suppl. on Instruments and Spparstus, Pt. 2,
1945. (3) Chemical Engineers’ Handbook (J. H. Perry, editor), 3rd ecl.. McGraw-Hill, New York, 1950. (4) Drinker, P., and Hatch, T.. “Industrial Dust,” McGraw-Hill, New York, 1938. (5) First, M. W., Rloschella, R., and associates, IND. Exo. CHZM., 43, 1363 (1951). (6) First, M. W., and associates, Harvard School of Public Health, Boston, U.S.A.E.C., Rept. NYO-1581, 1952. (7) Gilliland, E. R., IND. ENG.CHEM.,26, 681 (1934). (8) Hignett, T. P., and Siegel, M . R., Ibid., 41, 2493 (1949). (9) Landau, R., and Rosen, R., Ibid., 40, 1389 (1948). (IO) May, K. R., J . Sci. Instr., 22, 187 (1945). (11) Petit, A. B., Chem. Eng., 58, KO.8, 250 (1951). (12) Sherwood, T. K., and Pigford, R. L., “Absorption and Extraction,” McGraw-Hill, New York, 1952. (13) Silverman, L., “Industrial Air Sampling and Analysis,” Industrial Hygiene Foundation, Pittsburgh, 1947. (14) Silverman, L., First, hl. W., and associates, Harvard School of Public Health, Boston, U.S.A.E.C., Rept. NYO-1527, 1950. (15) Snell, F. O., and Snell, C. T., “Colorimetric Methods of Analgsis,” Val. 1, p. 531, D. Van Nostrand, New York, 1939. (16) Ibid., D. 574. i17) Ibid., 611. (18) Williams, G. C., Akell, R. B., and Talbott, C. P., Chem. Eng. Progr., 43, 585 (1947).
6.
RECEIVED for review J a n u a r y 29, 1954. ACCEPTEDM a y 24, 1B5.4. This s t u d y was made under Contract No. AT(30-1)841 between t h e U. S. Atomic Energy Commission and Harvard University. T h e opinions eypressed in this paper are those of t h e authors and do not necessarily reflect t h e views of t h e U. S. Atomic Energy Commission.
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