J. L. ENGLESBERG
NEW TECHNIQUES IN FILTRATION
SJnthetic jibers provide better jilter medza,
pob-
propylene coatings improve corrosion resistance, and equipment becomes more automatzc
heory, even though seldom used in actual design,
Tis valuable for interpreting laboratory tests, finding optimum conditions, and predicting effects of changes in operating conditions. Four interesting articles on theory have appeared (7-4) : Three types of equipment-a permeability cell, a vacuum filter, and a centrifuge (4)-were used to measure specific resistance for a bed of Lucite spheres having a given size distribution. These are the first experimental data which have been reported for resistance offered by beds of uncompressible spheres for all three methods of operation. Certain assumptions have been used to derive conventional equations for constant pressure filtration. Some of these are erroneous-e.g., it has been shown that specific cake resistance and flow rate are not constant (2). For thick slurries, the error is significant. Filtration theory has been summarized and applied to three filtration problems-Le., variable pressure and rate, constant pressure, and concentrated slurries ( 3 ) . Also, differential equations ( 7 ) for a filter-cake wash cycle have been derived, based on the assumption that wash liquor executes plug flow in the pores of the cake, with continuous mass transfer between the liquor and a boundary film of filtrate. Filter Media and Aids
T h e outstanding advances in filtration have been Synthetic fibers have replaced natural fibers because better chemical, physical, and thermal resistance leads to lower production and service costs. T h e filter medium usually does not perform the actual filtering, but merely supports the deposited cake of solids which in turn performs the actual filtration. Nevertheless, in system design, selection of the medium in filter media rather than filter systems.
50
INDUSTRIA!
AND ENGINEERING CHEMISTRY
is often the most important factor. For hydraulic and pneumatic systems, media are evaluated extensively (44). Tabulated data for 21 types are given. Selection of media is essentially a trial and error procedure ( I A ) . Experience a t the Algom-Norden uranium oxide plant in Canada is described, where filter cloth is used on large rotary vacuum filters to separate insolubles from an ore slurry, p H 1.8 to 2.5, and temperature of 75" to 90" F. Results tabulated below show that nylon, which is usually recommended for alkaline applications, has good acid resistance. Experience at Sethco with acid copper electrolytes, p H 1.0 to 1.2, confirms these findings. Service, Mo.
Cotton Dyne1 Nylon Dacron (Terylene)
Cause of Failure
1-2 3-4
Acid attack Abrasion
7-9
Gradual blindingc caused by ore slimes or fungus
Excellent tables are available (2A) for selecting the best synthetic woven-filter media for specific applications, and 32 manufacturers are listed. A new Du Pont nylon fiber (QA),HT-1, shows promise as a filter medium. Unlike ordinary nylon, it does not burn, melt, or fuse, and can withstand temperatures up to 840' F. without carbonizing. Of the synthetics, polypropylene fabrics and alkaline are the most promising ( 5 4 7A)-acid resistance is excellent, cake discharges easily, temperature characteristics up to 90" C. are good as well as particle retention (10 to 20 microns). One of the most interesting developments is the arrangement of dust-filtering fabrics into a triboelectric series ( 3 A ) . This development compares to that of the table of electrode potentials of metals. Electrostatic requirements of the fabric are dictated by the opposite electrostatic charge and properties of the collected dust. In clarification of liquids, the suspended solids are usually small and highly compressible. As a result, the
J . L. Englesberg is President of Sethco Mfg. Corp., Merrick, L. I., N . Y.
AUTHOR
i i l t r ~ i . i i i ( , ( l i i i i i i I J ( ~ ~ ~ I I I I cloqqcd ~~s o r sliiiicd ( i \ c r , ' i i i d ttic ' i i x \ tlillicLili to clean uiilcss i i filter aid is atlcletl.
I)[IIXY
cstreincl!~ fine filter surface \+,liicli 1)etivcciisolids a i i c l rlic filtcr iiicdiuiii, and cstciitls tlic c > d c lcngtli inany iiiiics. I n this art'ii%;in c w x l l c ~ n tarticlc (S.4) lists filter aids used i n typical industries. Given are floiv rates: packed cakr density, retention al)ility on coarse screens, so1uI)ility in alkali and acid solutions, and names of manufacturers. 'l'lics
< i i ( l loi.iiis ; i i i
r.r.t;iiiis ; i l l
h o I i ( 1 s . pi'e\~t'iitscontact
Filter Systems
Corrdsion-resistaiit materials are receiving. more attention, and several manufacturers are publicizing their experience in liquid-solids separation generally rather than \\it11 a particular type of filter. This trend ( 7 B ) is particularly helpful to the clesign engineer who must consider performance in relation to materials handling-, filtrate and cake quality, as well as economy. Plates and frames of presses are coated l>y a fluidized l)ed process ( 6 B ) de\doped hy the Polymer Corp. Penton, a chlorinated polyether, applied to cast iron components, results in a filter press that is GOYGcheaper than the least expensive stainless steel (D. R. Sperry and Co.). Press frames, inolded from polyester reinforced with ?lass filament, arc lizht Lveight (a 42-inch frame weighs only 60 pounds), strong, and clieinicall>.resistant. Made l)y Poly Fibre Associates, :l\.enel, N . these frames are used Iiy T. Schriver & Co. Dorr-Oliver makes a vacuuni drum filter ( 2 B ) with drum and tank of glass fii)er-reinforcecl resins. Drum piping is of unplasticized poly (vinyl chloride), and drainage decks and division strips are of inolded ruliber. Hastelloy C pumps and components Lvcre formerly used for filtering ferrous chloride electrolyte operatinq at a p H of 1 .0and a temperature of 200" F. Their useful life was aliout 30 dsys. Now, however, epoxy systems made ly Sethco can operate continuously for 1 2 months. The Graiiqer i-e\m-sil)le filter (iB)with injectionmolded plastic plates: used to clarify s\vimming pool ivatcrs, is no\v uscd in the chemical industry for solids concentrations of 0.01 to 1.07c, particularly where thin, slimy cakes arc formed which are hard to remove. In this filter, fccd passrs in m e direction until cake thickness s1on.s down flow rate. I h e n feed is reversed-i.e., it passes through the filrer in the opposite direction. Continuous, automatic filtering attracts consideratile attention. \\'here operations are periodic, efforts are made first to rncchanize the step and then the entire opcration. Timers, solenoids, pressure switches, control relays, or other devices are used. T o mechanize batch-type pressure leaf and tul>ular filters, various methods are used-e.g., stationary or traveling sluice, tubular jet, high frequency low amplitude vibration of leaves in a liquid-filled tank, shock hackwash, and then mechanical discharge. Selection and installation of filtration equipment is now considered more carefully by thoroughly studying the process, product, maintenance, operation, and finance ( S B ) . If a numerical evaluation is assigned to
Tubular Filters
T h e s filters Iiavc I i c c n used only to a liiiiirecl extent, they arc rccci\4iig more attcntion. They are low cost, adaptalile to highly specialized uses, and their surface is suitable for autoinatic sysrems. Also, they can be made of aliiiost any filter media, arid the enclosiiig pressure vessel can be of airnost any material. Depth-Type. In this type of tubular filter, particles, trapped after the); lia\,e entered the filter medium, prevent formation of a cake which increases resistance to flow and eventually forms its own filter medium. Thmc filters are useful for lubrication, hydraulic, and electroplating liquids. l ) u t no\v
4, horitontal c i r c d a r p a n rotary filtpr simiior to this provides eficient
reparution in
il
cylii,iier board poppi miil
r l
Filterpress with cast iron components coated with Penton costs only 4074 us much us the cheapest stainless steel unit ( 2 E )
VOL.
54
NO. 1 1
NOVEMBER 1962
51
ADDITIONAL IMPROVED EQUIPMENT Alantr/nclurrr 'I'yp~,Specifications
Tubular Filters Purolator Products, Nozzle-end hose, rated at 10 Inc., microns Rahway, N. J.
Use
Aircraft fuels as they enter tank Similar systems will be installed in gasoline service stations
Micronic element Jet and aircraft with coalescer for fuels; removes removing water particles 2-25 microns Enzinger Div., Duriron Co., Dayton, Ohio
Shock-type backHeavy duty washing; in nonfiltering metallics and allalloys
Pressure Filters, Plate and Frame D. R. Sperry & Co. Urethane- and Greater acid and N. Aurora, Ill. epoxy-coated; also alkali resistance phenolic- and furane-impregnated wood Niagara Filters Div., American Machine and Metals, Inc., E. Moline, 111.
Horizontal plate; self-centering drainage members
Horizontal tank, U. S. Filter Co., Los Angeles, Calif. vertical disk; new closing device gives tighter door sealing T. Shriver & Co., Harrison, N. J.
Horizontal tank, Hazardous horizontal tray in material and various sizes solvents
Enzinge,.
Horizontal tank, vertical rotating disks. New design. No bypassing; easy assembly of leaves.
The media are (3C) cotton, wool, cellulose acetate, viscose, Dynel, nylon, polypropylene, and glass fiber. Supporting cores, depending on the application, can be of steel, tinned copper, stainless steel, phenolics, polypropylene, or any material which can be fabricated into a perforated tube to support the fiber. Robert Sommers of L-S Plate & Wire Corp., Woodside, N. y . , reported a n interesting use of depth-type filters. His company purchased a Cincinnati 12 by 36 universal grinder for grinding the outside a n d inside diameters of cylinders down to 0.7 microinch. T h e cylinders are used on mills for rolling gold-filled plate. The original grinder was equipped with a pump, a simple wire mesh strainer, and a reservoir tank for holding the coolant. When the coolant was clean, no problems were encountered. However, when it became loaded with abrasive matter from the grinding wheel, fishtail marks appeared on the rolls. To eliminate this problem, a depth-type replaceable 1- to 3-micron cotton cartridge filter, rated at 16 to 22 gallons per minute, was used to circulate coolant from the reservoir through the filter system and then through nozzles over the cylinders being ground. No fishtails have appeared since, and cartridge replacement costs are only $50 per year. Surface-Type. I n this filter, particularly adaptable to automation, the tube may be of perforated metal, wire mesh, sintered metal, or porous materials such as ceramic, stone, carbon, or porous plastic. For automation, the tube is precoated with a filter aid, and various designs provide blowback to aid cake removal and backwashing the media. This is the most effective way to avoid down time; therefore, steel mills (7C) and industries using oils for fabricating or hydraulic purposes, and solvents for degreasing, are beginning to filter continuously, rather than reclaim the oil or solvent after it has become unfit for service.
Vertical tank, vertical leaf; completely automatic Sparkler Mfg. Co., Conroe, Tex.
Vertical tank, horizontal disk Internal sluicing system for cleaning without removing plates
Vacuum
Dorr-Oliver, Webtrol; rotary drum Stamford, Conn. bowed roll spreads and smooths fabric; belt positioned directly on drum by edge-position device Dorr-Oliver
52
Horizontal circular Short, fibrous mapan and disk; terials-e.g., cotton linters, pulp liquor and 3 wash liquors dischemical pulps charged separately
I N D U S T R I A L A N D E N G I N E E R I N G CHEMISTRY
The Bird- Young rotarv vacuum filter is redesigned for presswired operation
For filtering lubricating oils at 125 gallons per minute in a strip mill, the Hydromation Filter Co., Livonia, Mich., has developed an interesting automated tubular system (9C). A precoat combination of diatomaceous earth removes solid matter and fuller’s earth absorbs soluble impurities. The saturated cake is removed with compressed air. Although fuller’s earth (7C) is the best medium for restoring chemical properties of contaminated oils, it also removes rust inhibitors which should ‘ be restored. Tanks (6C) containing trichlorethylene and perchlorethylene for degreasing hardware in rolling mills for preannealing cleaning of bright metals build up solids which cause loss of heat transfer and costly downtime. Amerock Corp., Rockford, Ill., installed an automatic installed filtering system with vertical, stainless steel filter tubes having a diatomaceous earth precoat and a perforated basket centrifuge. The system will eliminate an estimated 250 hours of cleanout labor per year. T h e dry cleaning industry uses a flexible, stainless steel braided filter tube (8C)with diatomaceous earth. During reverse flow, the braided tube shortens and widens. This squirming action shears off the filter cake. Fuel for DC-8 jet air liners, fresh from the assembly line, is filtered with diatomaceous earth. Particulate matter in the raw fuel (1500 mg. per gallon) is reduced to 1 mg. Fine stainless steel-mesh tubular filters without filter aids are used for filtering jet bomber fuel (IOC), missile fuel (5C), and airplane hydraulic systems ( I C ) . Here, wire-mesh cartridge filters provide regular controllable pore size, resistance to vibration and rough handling, resistance to high temperature and differential pressure, compactness, and corrosion resistance. I n an electro-hydraulic servocontrol system (43, all contaminants down to 3-micron absolute must be removed. Rapid Magnetic Ltd., Birmingham, England, makes a filter (2C)which combines the advantage of a magnet and filter mesh for filtering lubricants, cut. ting oils and emulsions, and other coolants. Pressure Filters
Plate and Frame. Pit and Quarry describes how cost control played an important part (30) in filter selection by the Shoreham Cement Works, Brighton, England. Cement slurries can be partially dried by rotary vacuum filters, filter presses, or the costly process of drying in kilns. T o save money and improve efficiency, the Shoreham Works designed three filter presses, and resulting moisture extraction was 81.5% compared with 78y0 for rotary vacuum filters used in American cement plants. Although operation of the new filters is intermittent, the company claims that this is compensated for by the more efficient moisture extraction. T. Shriver & Co., Harrison, N. J., is equipping their recessed plate-type filter press with a chain-operated plate transportation mechanism and a double acting hydraulic closer. Thus, need for frames is eliminated. (Continued on next page) VOL. 5 4
NO. 1 1
NOVEMBER 1962
53
Horizontal Tank, Vertical Disk. An automatic pressure filter for wet or dry, open or closed cake discharge has been developed (Industrial Filter & Pump Mfg. Co., Cicero, Ill.). Novel features include a file drawer opening for easy removal of leaf assembly, a 180" oscillating sluice header lateral system, vibrator for cake discharge, and optional trough bottom with integral screw for closed chamber discharge of semidry cakes. Niagara Filters, East Moline, Ill., has developed a fully automatic filter with three controls for determining cycle length: one detects cake thickness, another is on a timed-cycle basis, and the third on a pressure basis. If one control fails, two remain operative. At the end of the cycle, feed is automatically shut off, the heel is blown down, and the cake can be washed and given a final blowdown with air or inert gas. The filter then automatically opens and the carriage rolls out with the leaves. A vibrator knocks the cake off into a hopper. If the carriage need not be opened, a screw conveyer for the bottom of the tank is available. Horizontal Tank, Vertical Rotating Disks. A filter which can be fully automated has been developed (Niagara Filters) which has an unique sluicing arrangement-a jet of cleaning fluid in close contact with thr cake. When sluicing is not permitted, sluicing arm shaves the cake. A screw conveyor at the bottom of the tank removes the cake. Vacuum Filters
Rotary Drum. In these filters, belt design has been changed so that filter medium is applied more smoothly to the drum, and belt tracking is more controlled and uniform. T h e Maxi-Belt (Filtration Engineers Division, Ametek, Inc.) has a grommet on each edge of the belt to keep the belt in alignment. Goslin-Birmingham Mfg. Co., Birmingham, Ala., has improved its wire-winder design-tension may be applied to the wire without chattering or jerking. Also, a tensiometer is available for checking tension during operation. T h e deflector blade has a reversiiile tip, permitting use of both edges. Two types of rotary vacuum filters, top and internal bottom feed, are used where stratification occurs because of rapidly settling crystalline materials in the feed liquor. For top feeders, operation, theory, and design ( I E ) are illustrated by actual examples in the sugar and metal industry. Broader potential uses are suggested. Atlantic Refining Co. has reduced downtime in the wastewater treatment plant of its Philadelphia refinery by using an hydraulic jet cleaning system (3E). Potablewater Filtration
Specialized equipment and automation has brought advances in treatment methods and reduction in costs for potable and municipal water supplies. Many of the advances are adaptable to the chemical industry. An excellent reference ( 6 F ) has appeared which contains valuable information on filtering water, industrial products, and wastes. I t discusses current design and construction of filters together with operating practices. 54
INDUSTRIAL AND ENGINEERING CHEMISTRY
Fully automatic gravity microstrainers, first used in Britain about 1946 and now manufactured in Scotland is an example of significant advances. About 40 installations are operating in this country ( I F , 2F, 7F, GF, 70F, 12F, 75F). Water is passed through a revolving drum strainer covered with fine stainless steel fabric. The flow is under gravity conditions and the mesh is 60, 35, or 23 microns, depending on the application. Total head loss through the machine varies from 12 to 18 inches. The revolving drum is submerged to about three fifths its depth, and at the top of its travel, i t is cleaned by wash-waterjets at 10 to 20 p.s.i. Development of a microphotometer to measure minute amounts of turbidity has resulted in significant advances-better contact and savings in alum treatment ( 7 4 9 , as well as basic understanding of filter back-wash under a variety of coagulation conditions (73F). Also, it has been instrumental in designing (5F) an automatic alum-feed water treatment plant. Following the pattern of soil conditioners, water conditioners are now available. One such installation saves one company $250,000 per year-at the Hanford Water Works ( 4 F ) , cold, raw water combined with fast filtering forced alum through the sand filter beds. Previous feeding of 1 to 4 p.p.m. of activated silica prevented this breakthrough. However, 1 to 20 p.p.b. of polyacrylamide gives better results than silica, costs less, and needs less alum to produce clear water. Sometimes, ordinarily dry chemicals such as sulfur and sugar are shipped in the liquid state. This custom has spread to water treatment chemicals. Liquid alum (SF) is now being delivered in tank trucks to the Cambridge, Mass., water treatment plant. This has not only saved $5700 per year, but also provides more intimate contact between alum and feed which in turn provides better flocculation of turbidity and higher quality water, and lower maintenance costs. Laboratory, pilot plant, and full-scale plant tests, over a period of 16 years, with a variety of filter types ( 3 F ) show that filters made of anthracite and sand together are superior to filters made of either material alone. Also, they have resulted in substantial savings in capital investment and operational costs. Chicago is building a sand filtration plant (71F) which will be by far the world’s largest filtration plant. Rated capacity is 960 million gallons per day at the nominal rating of 2 gallons per minute per square foot. Hydraulically, it will be capable of handling 1700 million gallons per day, and cost will be about $15 million. REFERENCES Filter Theory (1) Kuo, M. T., A.1.Ch.E. J . 6, 560-8 (December 1960). (2) Tiller, F. M., Cooper, H. R.,Zbid, 6 , 595-601 (December 1960). (3) Tiller, F. M., Guane, C. ,J., IND.ENG. CNEM.53, 529-37 (July 1561). . (4) , , Vallerov, V. V., Maioney, J. O., A.1.CIL.E. J . 6 , 382-90 (September 1960).
.,
I.
(Continued on next Page)
Filter Media; ‘Filter Aids (1A) Can.Chem. Proc. 44,59-62 (October 1960). (2A) Ehlers, S.,IND.ENC.CHEM.53,552-6 (August 1961). (3A) Frederick, E. R., Chem. Met. Eng. 68, 107-14 (June 26, 1961). (4A) Harper, C . H., Prod. Eng. 31,29-40 (Nov. 28, 1960). (5A) Hercules Powder Co., Bull. FD-5, May 21, 1962. (6A) Hooper, G. S., in “Polypropylene Expanding Position in Plastics,” Regional Tech. Conf., Philadelphia Section, SOC. Plastic Engrs. 69-77, April 17, 1962. (7A) IND. ENG.CHEM. 54, 74 (August 1962). (8A) ZLeppla, P. Zbid., 54,40-3 (May 1962). (9A) Snow, C., Plant Eng. 15, 133-5 (October 1961).
w.,
Filter Systems (1B) Chem. Eng. 68, 76 (October 30, 1961). (2B) Chem. Eng. Cat., pp. 367-82, Reinhold, New York (1962). (3B) Grimm, R. T., Chem. Eng. Progr. 57, 104 (June 1961). (4B) IND.ENC. CHEM. 53, 32A (May 1961). (5B) Meinhold, T. F., others, Chem. Process. 24, 62 (June 1961). (6B) Plastics Technol. 8, 66 (April 1962). (7B) Smith, W. C., Giesse, R. C., IND. ENG.CHEM.53, 538-45 (July 1961). (8B) Van Note, R. H., Weems, F. T., IND.ENG.CHEM.54, 54651 (July 1961). Tubular Filters (1C) Davis, H. D., Hydraulics B Pneumatics 13, 106 (November 1960). (2C) Engineer 212, 411 (Sept. 8, 1961). (3C) Englesberg, J. L., Metal Finishing Guidebook, Metals & Plastics Publication, 30th ed., pp. 424-34, 1962. (4C) Farris, A., Lubrication Eng. 17, 235-43 (May 1961). (5C) Gruner, F. R., Forman, H. L., S.A.E. J. 69, 126 (March 1961). (6C) Kearney, T. J., iron Age 189, 134-6 (May 31, 1962). (7C) McCoy, J. R., Iron Steel Eng. 37, 148-54 (December 1960). (8C) Prod. Eng. 33,72 (March 5, 1962). (9C) Steel 148, 76-7 (Jan. 9, 1961). (1OC) Steel 148, 107 (June 19, 1961). (11C) Tygret, J. M., Ulrich, R., Oil C? Gas J . 59, 106-10 (Sept. 11, 1961). Pressure Filters (1D) Chsm. Eng. 68, 64 (October 2: 1961). (2D) Chem. Proc. 25, 21 (April 23, 1962). (3D) Pit and Quarry 54, 40 (August 1961). Vacuum Filters (1E) Emmett, R. C., Dahlstrom, D. A., Chem. Eng. Progr. 57, 63-7 (July 1961). (2E) Lawless, R. T., Tappi 44, Suppl. 210A-211A (October 1961). (3E) Oil B Gas J. 59, 195 (Octobcr 2, 1961). Filtration of Potable Water (1F) Armstrong, C. G. R., Ibid., 92, 111-15. (2F) Berry, A. E., Am. Water Work Assoc. J . 53, 1473-8 (December 1961). (3F) Conky, W. R., Ibad., 53, 1473-8 (December 1961). (4F) Conky, W. R., Pitman, R. W., Water d Sewage Works 108,201-2 (May 1961). (5F) Conky, W. R., Pitman, R. W., Water Works Eng. 114, 28-30 (January 1961). (6F) Dickey, G. A., “Filtration,” Reinhold, New York, 1961. (7F) Evans, G. R., New Hampshire Water Works Assoc. J . (December 1961). (8F) McGinness, Wm. A,, Water Works Eng. 114, 408-9 (May 1961). (9F) Madsen, M. M., Znd. Water B Wastes, pp. 52-54 (MarchApril 1961). (10F) Nelson, F., Water @ Sewage Works 108, 192-3 (May 1961). (11F) Salter, G. S., Public Works 92, 101-4 (June 1961). (12F) Turre, G. J., Water Works En,
