U N I T OPERATIONS
FiItration 8
INTEREST
in the fundamental aspects of filtration continues to increase. While this comes mainly from those directly concerned with filtration, many significant contributions have been made by workers in related fields, such as geology, fluid hydrodynamics, colloid chemistry, and textile technology. T h e latter contributions must not be overlooked by those seriously interested in the development of sound filtration theory and practice. T h e time has arrived for better understanding between workers concerned with the mechanisms of particle retention and medium plugging in the respective fields of liquid clarification and aerosol filtration. Similarly, a mutual understanding should be developed between those interested in the design and development of textiles and other porous media for a multitude of uses and the engineer concerned with their use as a filter medium or bed.
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Significant advances were noted in several areas: Bed permeability and pore structure, particularly in connection with establishment of a definitive inter relat ionship. Theory and practice of cake washing, including extended statistical treatment of packed-bed theory. Aerosol filtration, especiaIIy the influence of wettability on collection efficiencies for liquid aerosols. Particle agglomeration and prefilt control, perhaps because of new, powerful nonionic flocculents. Effect of design variables in filter media construction on internal pore structure and performance.
Reviews and Books Three new books are musts for those seriously interested in the basic mechanisms of filtration. T h e first, by Carman ( 7 A ) ,is a concise, excellent review of the Kozeny-Carman approach to cake filtration. Scheidegger ( 6 A ) has expanded his statistical approach to the problem of flow through a packed bed of particles. H e proposes dispersivity as a third controlling characteristic, in addition to the usual porosity and specific surface characteristics. This third property describes the quality of pore structure and should be measurable i n terms of displacementmixing during a washing experiment. Green and Lane (4.4) have brought together most of the existing theory a n d practice relating to aerosol filtration which heretofore has been widely scat-
H. P. GRACE has been a member of the Engineering Research Laboratory, Engineering Department, E. I. du Pont d e Nemours & Co., Inc., since 1946, working primarily in the areas of mechanical separations and processing of particulate solids. He attended the University of Pennsylvania, obtaining a B.Ch.E. in 1941. Grace i s a member of the ACS and the AIChE, and received the AlChE Junior Award in 1954 for his publications concerning filtration.
354
tered throughout the literature. T h e treatment is critical, original, and timely. Two reviews of gas-filtration mechanisms and application have appeared. T h a t by Lunde and Lapple (5A) concerns the status of the field, the basic collection mechanisms, and the direction that new developments should take. Friedlander (2A) is concerned with basic collection mechanisms and their use to predict collection efficiencies for practical filter beds. The status of liquid filtration theory and practice has been surveyed in two recent revieivs. Suttle ( 7 A ) cited practical advances in cake filtration, filter aids, and filter media. Recent advances in theory and its application to both cake filtration and solution clarifications were described by Grace ( 3 A ) , Jvho included cake washing, cake dewatering, and filter medium design for predictable performance
Theory Permeability and Pore Structure. Irmay (70B) has reviewed the development of the Darcy and Forcheimer formulas and their limitations a t high Reynolds numbers. Tek (23B) has generalized the Darcy equation in a dimensionless form, including additional
INDUSTRIAL AND ENGINEERING CHEMISTRY
parameters relating to micropore structure. This generalized form predicts pressure drop with good agreement over all possible ranges of Reynolds numbers. S a g a r and others (79B) have investigated the limitations of the Darcy equation and suggest that flow velocity also depends on characteristics of liquid molecule structure as well as quality of bed pore structure. Gas flow through microporous solids in which adsorbed-layer flow is a major factor was studied by Gilliland and others (6B); they developed equations which correlate existing literature data. Jones ( 7 7B) has developed a method for taking internal porosity of solid particles into consideration in calculating gas flow through beds. Lord (74B) has further examined the application of the Kozeny-Carman relationship to flow through beds of textile fibers. I n a mathematical treatment based on two concentric spheres, Happel (8B) obtained closed solutions satisfying the Navier-Stokes equation, omitting inertia terms. These permit pressure drop to be predicted as a function of void volume for packed beds; predicted values agreed closely with those of the Kozeny-Carman equation for void fractions from 0.2 to 0.8. Ttvo investigations concerned transient flow behavior of packed beds. Turner (24B) analyzed the flow structure of a bed by means of a fluid flow containing a sinusoidally varying concentration of solute. This provided a histogram of distribution of structural parameters under investigation. Philip (27B)studied the transition from rest to steady motion on sudden application of a potential gradient to fluid contained in a saturated porous medium. T h e error in using Darcy’s equation, which neglects the transient phase, is unimportant \\hen a potential gradient is suddenly applied, but significant variations may occur Mhen the applied potential gradient is periodic, even in systems of low frequency. A4n important contribution to the theory of tito-phase Aoiv through beds has been made by Miller (78B) with his equations governing macroscopic flow through an unsaturated porous medium making no special pore-shape assumptions. A significant feature of the resulting equations is the prediction that liquid transmission and liquid hold-up proper ties of a n unsaturated porous medium will exhibit hysteresis in their dependence on the. liquid-gas pressure drop.
New measurement and evaluation techniques make possible both the rapid expansion of filtration theory and the wider application of existing theory to practice
Sandberg and others (22B) have investigated the effect of flow rate and fluid viscosity on relative permeabilities with immiscible liquids ; relative permeabilities were solely a function of saturation provided there was no saturation gradient introduced by a boundary effect. The effect of bed structure on spreading a liquid by capillary action through an initially unsaturated bed of fibers has been studied by Gillespie (5B), Fatt (4B) used a network of pores as a \\orking model to study the effect of pore structure on flow characteristics. A difference between true and calculated pore size was found when capillary pressure curves were used to obtain pore-size distribution. Xfarshall (75B) has derived equations relating permeability to poresize distribution for isotropic materials. Calculated and measured values agreed well for flo~vof air through porous stones and the flow of water through saturated and unsaturated sands over a wide range of permeabilit).. Bucker (26’) and Cochran (3B) have described experimental techniques for measuring the poresize distribution of porous media. Matta (77B) has studied the flo~vdistribution in a network model of a porous medium by direct observation of pressure potentials at various points in the network pore structure. Petersen (2OB) has shown that the assumption of an expanding-contracting pore cross section can reasonably account for the observed ratio (ing gases from open hearth furnaces has been briefly described by Silverman ( 78M).
Filter Media and Filter Aids Recent studies concerned M irh design, development, properties, and performance of fibers, fabrics, porous metals, papers, and filter aids as filter media are summarized in Table 111. Progress is being made toward relating design variables to internal pore structure and performance characteristics such as permeability, plugging tendency, and particle retention. Backer (4.V) has defined the role of structural geometry of the yarns in controlling woven fabric properties. ) continued Honold and others ( 2 7 ~ Vhave their studies of the pore-size distribution of woven fabrics. T h e design of capillary filters of known and adjustable pore size has been described by Akobjanoff ( 7 N ) . The mechanisms of water transport through woven fabrics has been studied by Hollies and others (79.V, 20,V). The mechanisms of woven media plugging and the effect of yarn structure and twist have been the subject of several studies (38.V. 4O.V). Treatment of media to provide water repellency has been described bv Philips and others (28>V)and is of interest in view of the effect of such treatment o n aerosol filtration as described by Fairs (8H). Davis (74N) has studied design variables of felt structures as they affect air permeability. Zievers and others (4L.V) studied particle-retention characteristics of nonwoven media and papers. They suggest what appears to be a rather oversimplified but empirically verified correlation of this property with voidage and permeability for a selected group of such media. Four new fiber materials are finding increasing application as filter media in various forms. Fibers of Teflon tetrafluoroethylene fiber (75.Y, 78.V. 34,v)
have become commercially available in continuous filament and staple forms. They have been used for filtration in bulk as deep filter beds and as both woven filter media and Armalon nonwoven felts ( 8 5 ) . I n addition to being resistant to virtually all chemicals and useful to 550° F., these unique fibers possess a large built-in electrostatic charge which persists indefinitely and is unusually effective in filtering submicron-size aerosol particles. &'oven filter media of linear polyethylene have become commercially available ( 7 7Ar, 26.V, 32"). Their good chemical and thermal resistance combined with low cost can be expected to find increasing application. Fibrafax ceramic fiber, formerly available in bulk: can now be obtained in the form of broadwoven fabric (7UN) suitable for many high-temperature filtrations. Carbon fibers (9.V) of 5- to 50-micron diameter are now available and are finding application in high-temperature gas filtrations in reducing atmospheres. A process for making true felts from synthetic fibers by needle punching to promote fiber entanglement, followed hy chemical or thermal shrinkage, has been successfully developed (77N, 24V, 25X). Felts of Orlon acrylic fiber, Dacron polyester fiber, nylon, and Dyne1 are now commercially available (30,V). These have found immediate application in dust filtration on reverse jet, Pulsaire, and bag filters, permitting higher operating temperatures and better collection performance and life than previously used wool felts or woven media (37,V). The development and use of sintered metal filter media made from granular powders have continued (23S, 27N). These have been augmented by media compounded from woven metal fabrics by rolling and sintering to produce flexible, porous metallic sheets marketed as Rigimesh and Poroloy. A significant development has been the production of strong porous metal sheets of short metallic fibers by a fiber metallurgy technique (33N, 35X). These overcome the low-voidage limitation inherent in porous metals produced from granular particles. Strong media with voidage contents as high as 80 to 90% can be produced, with an attendant large increase in permeability and holding capacity for the same pore size. Porous sintered sheets of Teflon have also become available (2gAl7) and are finding increasing applications where chemical resistance of porous sintered metallic filters is inadequate. Development of nonwoven bonded fabrics as filter media (37N, 4 7 s ) has resulted in a new line of synthetics with mechanical properties intermediate between woven fabrics and paper. These possess many of the advantages of each and are finding increasing application to
Table Type of Filter or Treatment Diatomite filters Continuous pressure filter High-temperature filters Continuous vacuum filtration Filter thickener Pressure precoat filtration Various types Pressure filters Electrolytic, mechanical filter screens Commercial air, dust Pressure precoat High temperature Reverse jet and Fiberglas Continuous vacuum dewatering Granular bed pressure filters
I . Process Applications Application Water filtration Vegetable oils Metallic melts Oil extraction processes Sugar refining Molten sulfur Coolant filtration Paint, varnish Paint, varnish Ventilation, industrial dust collection Plating solutions Blast furnace gases Radioactive dusts Sewage sludge Water clarification
High-e5ciency gas filters Chemical flocculation of prefilts
Sulfuric acid mist Water, mining slimes, industrial wastes
Electrostatic filtration
Nonionic liquids
Table 111.
( i G , 2G, GG. 1QL) ( 6 H ,8 H ) ( l J ,4J, 6J, 7 J ,
QJ,I O J ,
liJ,
Application and Design of Filter Media and Filter Aids Application or Design Feature Ref. Adjustable pore size (1s)
Type Capillary filters of synthetic fibers Woven fabrics Metal fiber papers Woven fabrics Synthetic fibers Woven fabrics Synthetic fibers Armalon felts of Teflon Carbon fibers Woven polyethylene fabrics Ceramic fiber Woven monofilament nylon fabrics Commercial, natural, synthetic fibers Synthetic fiber papers Teflon fibers, fabrics, porous sheets Wool felts Synthetic fiber felts Woven fabrics
Water repellency control Production, filtration characteristics Relation of structural geometry to physical properties Static electrification, unusual filtration properties Effect of twist on hairiness, cake adherence Heat, shrinkage resistance data Production, unusual characteristics Characteristics, filtration applications Physical, chemical, thermal characteristics Characteristics of woven media, batt forms of Fibrafax Characteristics of Nitex line Physical, chemical, thermal properties Physical, filtration properties Physical, chemical, thermal properties Effect of structure on air permeability, filtration Production, properties, filtration characteristics, application to taconite dusts
Cleaning woven fabrics
Mechanism of water transport through, pore size distribution of Properties, application in paint and varnish Corrosion resistant types, new designs, chemical applications Development, properties, potential advantages Production, physical properties, filtration Characteristics Mechanisms of solids retention, cleaning
Papers and nonwoven fabrics Slag wool Cellulose pulp Molecular sieves Ceramic fiber filter Sand bed
Particle retention characteristics High temperature gas filtration Filter aid for sulfur filtration Iodine-131 from air High-eaciency aerosol filter Performance, penetration for water
Millipore
Penetration through, application to water
Fiberglas and sand beds Diatomaceous filter aids
Aerosol filtration Variation in filtration characteristics with salt concentration
Diatomaceous filter aids Porous sintered metal (granular) Porous sintered metal (fibrous) Nonwoven fabrics
VOL. 51, NO. 3, PART II
MARCH 1959
359
UNIT OPERATIONS ___-_--__-______________________________.--.--.---.-~----. cake filtrations. T h e applicability of papers as commercial filter media promises to be extended by the recent development of papers of synthetic fibers (76-V), and of metallic fibers (3-V).
Bibliography Reviews (1A) Carman, P. C., “Flow of Gases through Porous Media,” .4cademic Press, New York, 1956. (2A) Friedlander, S. K., “Gas-Solids Separations,” A.1.Ch.E. 50th .Anniversary Meeting, Philadelphia, June 1958. (3A) Grace, H. P., “Art and Science of Liauid Filtration.” A.1.Ch.E. 50th Anniver‘sary Meeting, Philadelphia, June 20-25, 1958. (4-4) Green, H. L., Lane, M’. R., “Particulate Clouds: Dusts, Smokes, and Mists,” E. BL F. N.Spon, Ltd., London, 1957. (5‘4) Lunde. K. E.. LaDDle. C. E . Chem. Eng. Progr: 53, 385-91’ j1957j. (6A) Scheidegger, A . E., “Physics of Flow through Porous Media,” Macmillan, New York, 1957; Trans. .4m. Geophys. Union 39,929-32 (1958). (7A) Suttle, H. K., Chem. @ Process Eng. 39, 125-31 (1958). ’
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Theory Permeability and Pore Size (1B) Brooke, M., Chem. Eng. 64, 280 (1957). (2B) Bucker, H. P., Felsenthal, M., others, J . Petrol. Technol. 8, 65-6 (1956). (3B) Cochran, C. N., Cosgrove, L. A , , J . Phys. Chem. 61,1417-19 (1957). (4Bi Fatt, I., J . Petrol. Technol. 8, 144-81 (1956). (5B) Gillespie, T., J . CoNoid Sci. 13, 32-50 (1958). (6B) Gilliland, E. R., Baddour, R. F., others, -4.I.Ch.E. Journal 4, 90-6 (1958). 17Bi Hedlev. W. H.. Ph. D. thesis. Washington U&versity,’St. Louis, ~ d .1957. , (8BJ Happel, J., A.Z.Ch.E. Journal 4, 197-201 11958). (9B) Innes,‘W. B., -4nal. Chem. 29, 1069-73 (1957i ,__”.
(10B) Irmay, S., Trans. Am. Geophys. Union 39, 702-7 (1958). (11B) Jones, W. hl., Bril. J . .4$pl. Phys. 7. 370-3 11956). (12B) Le Tourneau. B. LY.. Grimble. R. E., others, Tians. Am. ‘,Tot. Mech: Engrs. 79, 1751G8 (1957). (13B) Leva, M., Chem. Eng. 64, 263-4 (1957 ). (14Bj Lord, E., J . Textile Znsf. Trans. 47, T 635-49 (1956). (15B) Sfarshall, T. J., J . Soil Sci. 9, 1-8 11958). (l6B) hfarshall, T. J., .Tuture 180, 664-5 (19573. (17B) Matta, G., Compt. rend. 244, 2770-2 11957). (18B) Miller, E. E., J . Appl. Phys. 27, 324-32 11956). Bhattacharya, .A. K., Chem. (Leipzig) 208, ~I
E., A.1.Ch.E. Journal Australian J . Phys. IO,
R., Gournay, L. S., Terhnol. 10. 36-43 (23B) Tek, M. R., J . Petrol. Technol. 9, 45-7 (1957). (24B) Turner, G. A, Chem. Eng. Sci. 7, 156-65 (1958).
360
Cake Compression
(1C) Daniel, -A, W. T., Engineering 184, 45-6 11957). (2C) D k y , J., hfindlin, R. D., J . Appl. Mechanics 24, 585-93 (1957). (3C) Hutto, E. B., Jr., Chem. Eng. Progr. 53, 328-32 (1957). (4C) Jaffe, J., Ph.D. thesis, Univ. of Md., College Park, Md., 1957. (5C) Macrae. J. C.. Finlavson. P. C.. ~, Nature 179,’1365-6 ’(19 57). (6‘2) Roberts, J. E., Souza, J. M., Preprint 61st Ann. Meeting ASTM, Boston, June 1958. (7C) Train, D., Trans. Inst. Chem. Engrs. (London) 35, 258-66 (1957). ,
1
Cake Filtration (1D) Adcock, D. S., hfcDowall, I. C., J . Am. Ceram. Sor. 40, 355-62 (1957). (2D) Boone, E. T., M.S. thesis, Mass. Inst. Technol., Cambridge, Mass., 1958. (3D) Heertjes, P. M., Nijman, J., Chem. Eng. Sci. 7, 15-25 (1957). (4D) Horner, V., White, M. hf., others, J . Petrol. Technol. 9 , 183-9 (1957). (5D) Jaffard, P., Compt. rend. 243, 1383-5 (1956). (6D) Kottwitz, F. A , Boylan, D. R., A.Z.Ch.E. Journal 4. 175-80 11958). (7D) Satoh, Takao,‘ Chem. Enq. (Tokyo) 20, 541-6 (1956). (8D) Ibtd., 21, 481-5 (19571. (9D) Zbzd., 22, 25-7 (1958). (10D) Satoh, Takao, Mem. F a . .4gr., Tokyo Univ., 1-68 (March 1957). (11D) Tiller. F. M., A.Z.Ch.E. Journal 4, 170-4 (1958) Cake Washing (1E) h i s , R., .%mundson, N. R , A I.Ch. E. Journal 3, 380-2 (1957). (2E) Aronofsky, J. S., J . Petrol. Technol. 9, 345-9 (1957). (3E) Butler, R. hl.,Tiedje, J. L., Can. J . Technol. 34. 455-67 (1957,. (4E) Carberiy, J. J:: Bretton, R . H., A.I.Ch.E. Journal 4, 367-75 (1958). (5E) Choudhury, A. P. R., Dahlstrom, D. A,, Ibid., 3, 433-8 (1957). (6E) Craig, F. F., Sanderlin, J. L., others, J . Petrol. Technol. 9, 275-82 (1957). (7E) Day, P. R., Trans. .4m. Geophys. L’nion 37, 595-601 (19561. (8Ej Ebach, E. A, White, R.R., A.Z.Ch.E. Journal 4, 161-9 (19581. (9EI Erdos: E., Jiru, Z., Collection Czechoslor. Chem. Communs. 22,862-73 (1957). (l0E) Jong, G. d. 6 . de, Trans. .4m. Geophysical U n i o n 39, 67-74 (1958 1. (11E) Orlob, G. T., Radhakrishna, G. N., Trans. Am. Geophys. llnion 39, 648-59 (1958 ‘1. (12E) Prausnitz, d . hf.,A.1.Ch.E. Journal 4, 14M>22M (1958). (13Ei Richardson, J. G., J . Petrol. Techno/. 9, 64-6 (19571. (14E) Richardson, .J. G., Perkins, F. M., Jr., Ibid., 9, 14-24 (1957). J . r3ppl. Phyr. 29, 687-91 (15E) Rose, W., (1958). (16Ej Saffman, P. G., Taylor, G.: Proc. Roy. SOL.(London) A245,312-29 (1958). (17E) Stern, K. H., Shniad, H., J . Colloid Sei.13, 24-31 (1958). (18E) Templeton, C. C., Rushing, S. S., J . Petrol. Tech., 8, 211-14 (1956). (19E) Van Meurs, P., Zbid., 9, 295-301 (1957).
(2F) Corey, .4.T., Rathjens, C. H.: others, J . Petrol. Technol. 8, 63-5 (1956). (3F) Henderson, A. F., Cornell, C . F., others, Mining Eng. 9, 349-53 (1957). (4F) Nelson, P. A,, Dahlstrom, D. . A , , Chem. Eng. Progr. 53, 320-27 (1957). (5F) Nenniger, E., Jr., Storrdw, J. .A,, A.1.Ch.E. Journal 4, 305-16 (1958). (6F) Probine, M. C., Brit. J . Appl. Phys. 2, 144-8 (1958). (7F) Yano, T., Kanise, I., others, Chem. Eng. (Tokyo) 21,426-33 (1957). Liquid Clarification b y Filtration (1G) Ghosh, G., J . Sanitary Eng. D Z L 84, . SAI, Pt. 1, Paper 1533 (February 1958). (2G) Ghash, G., Uhter and Water Eng. 62. 147-53 11958). 13G)‘ Hall, W. A . , J Sanitary En!. Dic. 83, SA3, Paper 1276. (June 1957) (4G) Heertjes, P. hi.. Chem. Eng. Srz. 6, 269-76 (195’). (5G) Ibtd., 6, 190-203 (1957). (6G) Krone, R. B., Orlob, G. T., Hodqkinson, C., Sewage and Znd. M‘astes 30, 1-13 (1958). Aerosol Filtration (1H) Amelin, A. G.: Belyakov, M. I., Colloid J . (U.S.S.R.r (Engl. Transl.; 18, 379-88 (1956). (2H) Belyaera, I. I., Mikororo, A. E., others, Zbid., 19, 25-7 (1957). (3H) Benton, D. P., Elton, G. A. H., Proc. 2nd Intern. Congr. Surface Xctivity (London) 2,88-96 (1957). (4H) Dawes, J. G., Greenough, G. K., others, Brit. J . AppI. Phys. 8, 236-41 (1957). (5H) Dawkins, G. S., Ph.D. thesis, Univ. Ill., Urbana, Ill., 1957. (6Hi Decker, H. M.,Harstad, J. B., Lense, F. T., J . .Air Pollution Conirol . ~ S S O C .7 , 15-16 (19571. (7H) Drozin, V. G., Ph.D. thesis, Columbia IJniversity, New York, 1957. (8H) Fairs, G. L., “High Efficiency Fiber Filters for Treatment of Fine Mists,” Inst. Chem. Engrs. London, .4pril 1958. !9H) Fitzgerald, J. J., Detwiler, C . G., ,4.M.A. Arch. Znd. Health 15, 3-8 (19j7). (10H) Fitzgerald, J. J.. Detwiler, C. G., Am. Ind. Hpg. Assoc. Quart. 18, 47--53 ( 1957). jllH’I Friedlander, S. K., IND. ENG CHEM.50, 1161-4 (1958). (12H) Friedlander, S. K., Johnstone, H. F.,Zbid., 49,1151-6 (1957). (13H) Gaillily, I., J . ColloidSci. 12, 161-72 (1957). (14H) Gunn, R., .Am. J . Phys. 25, 542-6 11057).
(15Hf Haase, H., Hardtke, B. H., Chem. In
