FILTRATION S. A. MILLER‘ UNIVERSITY OF K A N S A S ,LAWRENCE, K A N .
A
booming year for filtration, symbolized by the opening of two large, new plants b y established manufacturers of clarifying filters and diatomite filter aids, respectively, is attested b y the greatest number of publications ever reported. Contributions to theory have been in the realms of cake washing, filter-medium filtration, and, indirectly, packed bed permeability. Equipment advances include new pilot filters, a pressure filter with retractable leaves and manifold, and vacuum filters of rubber- or plastic-coated construction. Performance data have been reported for coal dewatering, wood pulp thickening, water deoiling, and continuous precoat sirup clarification. Synthetic-fiber filter media are now available as clarifier cartridges. Acrilan has joined the family of synthetics and Teflon i s produced in microporous sheets, and porous stainless steel has been given a smooth Finish. A revised-edition monograph and an encyclopedia entry devoted to filtration have appeared, along with reviews of theory, filter selection, the variables affecting filtration rate, and the causes of filter fabric failure.
assurance, further study of packed beds and of their flow characteristics is needed to aid this attack of the filtration problem. Seven such investigations (24, 60, 64, 7’0,104, 17’7, 181) recently have been reported. If the particles comprising a bed are of simple shape and are regularly packed, the resistance of the bed and the ability t o effect mass transfer a t the solid surface (pertinent t o cake washing) may be estimated from the component particle properties, according to R a m (177). The method is successful even for fibrous bodies if their porosity is extremely high, say 90% or greater (104). Presumably the success of this approach is germane to the use of those relationships which involve a mean particle dimension; yet few actual filter cakes will satisfy the necessary conditions of simplicity or porosity. Analysis of the aggregate is difficult; the interstitial volume consists of a network of bottlenecks and larger connected voids, and foreknowledge of the pore-size distribution is essential to any general interpretation of flow data in terms of a pore or particle dimension (7‘0). Attempts t o arrive at better working correlations continue: Rose (181) based a correction factor on molecular flow and thereby rendered consistent the specific surface values estimated from gas permeability and from nitrogen adsorption; and Ergun (64) offered the hypothesis that kinetic ~ t 8well as viscous losses contribute t o the pressure drop through a bed and proposed a modification t o the Kozeny equation consistent with this h y p o t h e sis. B u t neither Rose’s nor Ergun’s correction is likely t o apply to liquid-filled filter cakes. Coulson (60) has decided, at the conclusion of an exceptionally fine paper, t h a t what is needed is a shape factor for bed voids instead of one for particles. Meanwhile, the Koseny-Blake coefficient remains an empirical index for a particular material in a particular configuration, roughly reproducibIe, as demonstrated for bone char beds (24), and is as reliable for design purposes as any other filtration-resistance index. Bresle ( 1 4 , filtering peat suspensions a t constant pressure in a laboratory vacuum filter, found Ruth’s parabolic law t o be valid, and used the filtration coefficient of the equation as a rate coefficient for evaluating temperature and pH effects. Fuhrmeister (71) illustrated the importance of the slurry’s being in proper condition for best filtering by citing data for the average rate of filtration as a function of temperature, slurry concentration, filtrate viscosity, cake-particle size, and amount of ferric chloride added to sewage-sludge prefilt. The results were qualitatively in the direction predicted by theory. The action of a continuous precoat vacuum drum filter was examined critically by Weymouth and Montgomery (216) in a careful pilot-scale study. The material filtered was phosphate defecated affination sirup. The filtration variables systematically investigated were drum speed (filtration rate nearly proportional between 0.5 and 1.0 r.p.m.), depth of cut (rate almost independent-and hence filter aid consumption inversely propor0.0025 and 0.005 inch), and grade of filter aid tional-between
.NCE 1948 there has been a steady increase in publications 011 filtration and related subjects. The year ending October, 1952, followed the trend, and this review covers a larger number of references than any preceding one, notwithstanding the deliberate omission of some items and the probable involuntary omission of others. The number reflects increased investigational activity in the field, particularly by industry, but it is also influenced by the extraordinary volume of patents and industrial literature issued during the year. The scope of this review is t h a t of previous ones of the series, excluding gas clarification, centrifugal and sedimentary separation, adsorptive percolation, and biofiltration. Three comprehensive summaries of the filtration field have appeared. A new 478-page edition of Kufferath’s monograph (116) discusses theory, filter mediums, equipment and applications, gas Clarification, and materials of construction. It concludes with a number of patent abstracts. It appears t o be a worthy enlargement and modernization of the first edition. Brownell’s well-written contribution (18) to the Kirk-Othmer Encyclopedia covers cake (but not filter-medium) filtration theory, washing and air-blowing, and brief descriptions of filters, aids, and mediums. It seems t o this reviewer to underplay the importance of pressure-leaf filters. Streatfield (199) reviewed recent filtration publications, among them 23 British patents, and included brief descriptions and illustrations of 13 representative British filters. Miller (138) summarized theoretical developments since 1946 for both cake and filter-medium mechanisms, and described the significant experimental studies for the same period. An excellent and authoritative review of the advantages, restrictions, and fields of application of all types of cake and clarifying filters was prepared by Grace (79). Two revised editions of manufacturers’ brochures (73, 192) state briefly the problem of selecting a suitable filter and describe the construction and applications of the products they represent; one (192), limited to the filter press, is a concise manual of press selection, while the other (73)deals with rotary vacuum and rotary pressure-leaf filters. 4
T H E O R Y AND E X P E R I M E N T A L D A T A
Current proposals to characterize filter cakes by Kozeny-Blake coefficients are proving promising. Because the relationships are not rigorous, however, and cannot always be applied with 1 At present on leave t o Icing’s College, Durham University, Newcastleupon-Tyne, England
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January 1953
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INDUSTRIAL AND ENGINEERING CHEMISTRY
(optimum with J.-M. Hyflo or Celite 503). A pressureleaf filter with which the vacuum filter was compared was impractical, Coal sludge dewatering was the subject of three quantitative papers (199,128,178). After collecting many operating data on centrifugal dewaterers (law),Lyons turned a similar technique upon vacuum and pressure filters (123). His correlation of cake moisture content against average particle size undoubtedly is an over-simplification, for i t permits fine fractions to be balanced out by coarse ones and makes no attempt to analyze sub-sieve-size material (even when i t comprised 64% of the solids). But the data collected are interesting and welcome. Piros, Brusenback, and Dahlstrom (173)confined their attention to a single type of filter (Peterson top-feed rotary vacuum), comparing the operation of pilot and full-scale machines with a laboratory cell. They also selected cake moisture content as the key variable, correlating i t against a quantity related to blowing time, air rate, and thickness and texture of cake for various average particle sizes. Their correlation permitted correct scale-up from laboratory cell to 6 x 10 foot plant filter. Higher slurry temperature produced a drier cake, an observation in agreement with Fuhrmeister (71). Several investigators were concerned with cake washing. blondria (140)assumed perfect displacement up to 50% filtrate removal and an arbitrary wash to solvent ratio in the effluent thereafter and derived expressions for residual impurity of the cake from a vacuum drum filter. He calculated results for the whole range of practical conditions, and claimed agreement with unpublished data on lubricating oil dewaxing. Crosier and Brownell (61) conducted a detailed experimental research on liquid displacement from a granular bed by a miscible second liquid. Wash breakthrough did not occur until 90% of the filtrate had been displaced (unless the bed was less than 10 particle diameters thick), and the displacement was independent of particle size before breakthrough. Washing was most effective with liquid not less viscous than the filtrate. Jettmar and Roesler (108) reported that spontaneous mixing of a displacing and displaced liquid can occur in a capillary under certain conditions, an observation of possible relevance to cake washing. Atkinson’s study (4) of displacement is of little direct interest because i t was limited t o immiscible-liquid pairs. The investigation of viscose filterability and the significance of the “clogging constant” has continued. A quartet of valuable papers (209) and an errata note (211) by Vosters were completed. (Reference t o the first three of these was made in the last review (139),but the whole item is cited here because the first paper regrettably was omitted from the bibliography list last year). Led by difficulties of data interpretation to a thoughtful consideration of the filtration mechanism, Vosters elaborated the HermansBred6e filter-medium filtration theory, extended it to include constant rate, and collected many data to support it. Additional papers by Samuelson (186)and others (94,161) on the interpretation of filterability data in terms of viscose properties have been contributed to the expanding literature on the subject. Vosters (210)has suggested that the data should be evaluated statistically. The use of hydrodynamic principles in the design and operation of rapid sand filters has been discussed by Naumann (146). Michau (134)recommended a pressure-depth profile (practically rather difficult) as a guide to sand clogging as well as to the selection of design and operating variables. EQUIPMENT
Two new horizontal vacuum filters have been patented (167, 160), and an existing one has been improved (79). Several disclosures (7,69, 99, 112, 169, 904) are improvements to vacuum filters, among them a continuously renewable filter surface (112) a n d devices for application of a proper precoat (69, 904). An American (170)and an English firm (65) have issued clever cut-
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away views to show the construction of vacuum drum filters. A new vacuum continuous precoat filter is on the market (69). I n the pressure filtration field, a novel arrangement of Ieavea and manifold of a horizontal-shell filter permits them to be withdrawn on a trolley (162). “Plateless” filter-press plates, their grid surface replaced by porous slabs (68)or heavy screen (166)have been announced. Cartridge-type clarifiers are enjoying increasing general use: witness the expansion into new plant facilities of a leading manufacturer of such filter (168). A: package unit involving a strainer and cartridge clarifier is available for small batch handling (36). Other new in-line filters include a strainer with expandable apertures to facilitate backwashing (66), a unit with an automatic by-pass (W07),and a split-stream finecoarse combination filter for simultaneous straining and maintenance of a maximum fine-particle concentration (208). Filters particularly adapted to diatomite filtration have been described (91), and patents (48,124,157, 213) have been issued for several inventions in this field. The Metafilter, long popular in Europe as a precoat (or fiber cartridge) filter, is the subject of a pamphlet (133). Several pilot filters, all models of well-known plant machines, have been announced. Included are two competitive pressure-leaf a continuous precoat machine (69),and the inunits (W7,34,168), troduction t o England of the pilot model of a U. s. string-discharge vacuum filter (31). A number of items have been concerned with filters for a specific industry or product. An excellent impartial survey has been given of clarifiers for diesel lubricating oil (197), and an engine filter with a permanent porous-metal element has been announced (28). Several firms manufacturing oil-clarifying filters have released up-to-date bulletins (61, 96, 98, 100). Other new filters described are intended for hydraulic fluid (66), cutting coolant (106), milk (8, 162), and water (20,127, 141). Accessory improvements to filters include petcocks for filter presses (89),a poppet-type valve for water filters (102),and a chain drive to relieve the stress in conventional gear drives applied to low-temperature propane dewaxing filters (78). Pickard (171)described a simple injector for adding diatomaceous earth to a prefilt stream without use of a pump, and Fuhrmeister (71) mentioned an eductor for the same purpose. Advances in materials of construction for vacuum filters include the use of Ni-Resist for salt filter-dryers (166),Monel for fertilizergrade potassium chloride (176), and rubber-covered (60) and plastic-covered (60, 78) steel to replace expensive or unavailable rustless metals. Iron oxide paints, porcelain enamel, zinc metallizing, and metal plating have been suggested for protecting filter presses (96). PROCESS APPLICATIONS
Sugar. Filtration is a difiicult problem t o the sugar industry, and efforts are continued t o develop defecation procedures which will improve filtration without other sacrifices. Various methods of cane middle juice carbonation have been compared by Tan (201). Santen (187)suggested that limed juice from the last mill mixed with bagasse about to enter a preceding one results in a better filtration, with the bagasse assisting. Part of the filtration difficulty may result from hardness salts, for De Luzuriaga has found that electrochemical reduction of hardness resulted in better filtration efficiency (66). Progressive preliming of beet juice can improve filterability, but a t the expense of settling rate; Naveau (147) states that this suggests the advantage of continuous filtration without presettling. Return of a portion of first carbonation juice, over-carbonated, to the new juice makes filtration easier (206),b u t may result in lost quality (172);both filterability and quality depend on the ratio of returned to raw juice (220). Other innovations reported include flocculation of beet juice with acidified ethyl alcohol (206),destruction of reducing sugars
e
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INDUSTRIAL AND ENGINEERING CHEMISTRY
identified with darkening, followed by appropriate filtration (sometimes with filter aid) (184), and the production of invert sirup (involving filtration) from beet granulated sugar ( 2 2 ) . An industrial brochure (155) describes the use of pressurc- Ivaf filters in the sugar industry.
COURTESY COMMERICAL FILTERS CORP.
Figure 1.
Battery of Cartridge Type Clarifiers at a Petroleum Refinery
Pulp and Paper. Walsh (114) has tabulated the records of five different mills on groundwood pulp and broke handlihg, involving vacuum drum and disk filters, as Re11 as deckers, couch rolls, and pickup rolls. He listed extensive capacity and performance data. A rotary vacuum drum filter was investigated by Ottar (164) to determine the best operating conditions for groundwood thickening. According to Mannbro (126), sulfate and sulfite pulp may be washed advantageously by combining a rotary vacuum filter and a diffuser tank. Minerals. Batch vacuum filters and filter presses have been reported in use in the hydrometallurgy of zinc (49). Many kinds of vacuum filters are used for coal dewatering, and data on their operation have been given in two papers (123, 173), as mentioned earlier. The use of precoated pressure-leaf filters for sulfur clarification is the subject of a new vendor’s catalog (154). Filter presses, vacuum filters, and filter-dryer combinations used by the ceramic industry have been discussed briefly by Netzel (150). A Swiss patent covered the continuous purification of electrolyticcell mercury by its filtration through a glass fiber pack (114). Miscellaneous Processes. Writing under the title “Industrial Filtration of Paint and Varnish,” Genin (74) has surveyed the centrifugal filters and settlers used by the paint industry. The Turbo-filter which he describes in detail is actually a.type of centrifuge. Schmidt (190) concentrated latex by means of a tube-type vacuum thickener. The protection of steam turbines by circulating 5 t o 10% of the lubricating oil per hour through a clarifying filter has been recommended by Morrow (143). I n diesel and gas engines, on the other hand, the trend is said to be toward full-flow filters (97).
Vol. 45, No. 1.
Solids precipitated from a n alkaline protein solution have been removed b y a, pipeline strainer with a scraper-cleaner (32). Three novel applications of filters were reported. I n a gravel-bed filter, crystals are trapped from saturated solution t o guarantee the solution’s remaining saturated (103). Baker’s cheese can be manufactured in a plate-and-frame press by filling the press with rennetted milk, flushing with water, and air blowing (111). A horizontal vacuum filter has been explored by Southern Regional Research Laboratory as a continuous extractor of cottonseed or other meal (33). Water, Sewage, and Industrial Wastes. Methods of filteiing water were reviewed (143, 612). Boucher (12) described the operation of his patented fine-strainer. Grombach (84) studied the penetration of a rapid sand filter. Two papers (90, 120) reported the expansion of municipal water filtering capacity, in one case (90) by the addition of pressure filters. Lake plankton have been removed by rapid sand (19) and diatomite (186) filters. Truck mounted diatomite filters for emergency or other mobile use were discussed by Lowe (121) and by Martin (128) The surface washing of filter sand was discussed by Hopkins (101) and a special backwashing device was reported by Lava1 (117). The removal of iron from U. S. waters usually is easily accomplished by aeration (196) or by simple alkaline precipitation (54). If only small amounts of manganese are present, they can be removed simultaneously (92). I n one Dutch installation, oxidation removal is effected by spraying the water onto the surface of an evacuated sand filter (119). If manganese content is too high, as is often true in Europe, ordinary treatment will not remove it (93), and measures such as superchlorination followed by filtration through a n alkaline granular material must be taken (133). .4 filter bed of a special granular dolomite, described by Alter ( 1 ) and by Borner ( I O ) , is satisfactory. A tendency to set can be combatted by backwashing with air as well as with water ( f 7 4 ) . Bacteria can be quantitatively ultrafiltered from water by a cellulosic membrane, a basis for bacterial analysis proposed by Clark and coworkers (41, 4%’)and by Goetz and Tsuneishi (76). Investigations of a variety of filter mediums for the filtration of raw v a t e r (118), sewage (118, 182), and sewage sludge (110) have been reported. On the basis of cost and performance, Joseph (110) rated napped cotton best for sewage sludge. Rudolfs and Balmat (182) concluded that no filter medium except a membrane will quantitatively remove colloidal (
