Pressure filter aids

Addition of Inert Filter Aids. FUtet aids ate rigid, porous, and highly permeable powders added to feed suspensions to extend the appheabUity of surface filtration. Very dilute or very fine and slimy suspensions ate too difficult to filter by cake filtration due to fast pressure build-up and medium blinding addition of filter aids can alleviate such problems. Filter aids can be used in either or both of two modes of operation, ie, to form a precoat which then acts as a filter medium on a coarse support material called a septum, or to be mixed with the feed suspension as body feed to increase the permeabihty of the resulting cake. 

In the precoat and body feed mode, filter aids allow appHcation of surface filtration to clarification of Hquids, ie, filtration of very dilute suspensions of less than 0.1% by volume, such as those normally treated by deep bed filters or centrifugal clarifiers. Filter aids are used in this mode with pressure filters. A precoat is first formed by passing a suspension of the filter aid through the filter. This is followed by filtration of the feed Hquid, which may have the filter aid mixed with it as body feed in order to improve the permeabiUty of the resulting cake. The proportion of the filter aid to be added as body feed is of the same order as the amount of contaminant soHds in the feed Hquid this limits the appHcation of such systems to low concentrations. Recovery and regeneration of filter aids from the cakes normally is not practiced except in a few very large installations where it might become economical. 

Materials suitable as filter aids include diatomaceous earth, expanded perilitic rock, asbestos, ceUulose, nonactivated carbon, ashes, ground chalk, or mixtures of those materials. The amount of body feed is subject to optimisa tion, and the criterion for the optimisa tion depends on the purpose of the filtration. Maximum yield of filtrate per unit mass of filter aid is probably most common but longest cycle, fastest flow, or maximum utilisation of cake space are other criteria that requite a different rate of body feed addition. The tests to be carried out for such optimisation normally use laboratory or pilot-scale filters, and must include variation of the filtration parameters such as pressure or cake thickness in the optimisation. 

Liquid Dosage Forms. Simple aqueous solutions, symps, elixirs, and tinctures are prepared by dissolution of solutes in the appropriate solvent systems. Adjunct formulation ingredients include certified dyes, flavors, sweeteners, and antimicrobial preservatives. These solutions are filtered under pressure, often using selected filtering aid materials. The products are stored in large tanks, ready for filling into containers. QuaUty control analysis is then performed. 

In diatomaceous-earth filtration, the powdered filter aid is built upon a relatively loose septum to screen out suspended soHds. The filter becomes clogged, and pressure losses become excessive backwashing is then necessary. The smallest removable particle is 0.5—1 p.m (see Diatomite). 

Schweitzer, op. cit., sec. 4.2) and Hutto [Am. Tn.st. Chem. Eng. Symp. Ser, 73(171), 50 (1977)]. Figure 18-109 shows a flowsheet indicating arrangements for both precoat and body-feed applications. Most filter aid is used on a one-time basis, although some techniques have been demonstrated to reuse precoat filter aid on vertical-tube pressure filters. 

External-Cake Tubular Filters Several filter designs are available with vertical tubes supported by a filtrate-chamber tube sheet in a vertical cylindrical vessel (Fig. 18-115). The tubes may be made of wire cloth porous ceramic, carbon, plastic, or metal or closely wound wire. The tubes may have a filter cloth on the outside. Frequently a filter-aid precoat will be applied to the tubes. The prefilt slurry is fed near the bottom of the vertical vessel. The filtrate passes from the outside to the inside of the tubes and into a filtrate chamber at the top or the bottom of the vessel. The sohds form a cake on the outside ofthe tubes with the filter area actually increasing as the cake builds up, partially compensating for the increased flow resistance of the thicker cake. The filtration cycle continues until the differential pressure reaches a specified level, or until about 25 mm (1 in) of cake thickness is obtainea... 

Pressure leaf filters are used to separate much the same lands of slurries as are filter presses and are used much more extensively than filter presses for filter-aid filtrations. They should be seriously considered whenever uniformity of production permits long-time operation under essentially constant filtration conditions, when thorough washing with a minimum of hquor is desired, or when vapors or fumes make closed construction desirable. Under such conditions, if the filter medium does not require frequent changing, they may show a considerable advantage in cycle and labor economy over a filter press, which has a lower initial cost, and advantages of economy and flexibility over continuous vacuum filters, which have a higher first cost. 

To 40 g. of dry chitin in a 500-ml. beaker is added 200 ml. of concentrated hydrochloric acid (c.p., sp. gr. 1.18), and the mixture is heated on a boiling water bath for 2.5 hours with continuous mechanical agitation. At the end of this time solution is complete, and 200 ml. of water and 4 g. of Norite are added. The beaker is transferred to a hot plate, and the solution is maintained at a temperature of about 60° and is stirred continuously during the process of decolorization. After an hour the solution is filtered through a layer of a filter aid such as Filter-Cel. The filtrate is usually a pale straw color however, if an excessive color persists, the decolorization may be repeated until the solution becomes almost colorless. The filtrate is concentrated under diminished pressure at 50° until the volume of the solution is 10-15 ml. The white crystals of glucosamine hydrochloride are... 

Body-aid (i.e., the filter-aid) and precoating are often mentioned in connection with pressure filtration and the difference in their application is (1) Body aid is used when the slurry is low in solids content with fine and slimy particles that are difficult to filter. To enhance filtration coarse solids with large surface area are added to the slurry and serve as a body-aid that captures and traps in its interstices... 

A suspension of the oxime 27 (475 g, 1.70 mol) arid 5% Ru/ C ( 9g) in fvleOH (8 L) was warmed to 50 C and stirred in a closed, evacuated autoclave. Hydrogen was admitted and the pressure inside the autoclave was maintained at 40 psi while the temperature was increased to 68-74 C. The mixture was stirred for 19-24h, cooled and filtered through Filter Aid and the solid was washed with MeOH (2x4 L). The combined filtrate and washings were concentrated to 4 L under reduced pressure, MeCN (4 L) was added and the mixture was again concentrated to 4L and the cycle was repeated twice more. The MeCN solution was cooled to 10 C and benzenesulfonic acid (260 g, 1.64 mol) in MeCN (1.6L) was added, whereupon the benzenesulfonate of the amine 28 separated yield 579 g (80%). The free amine had mp 110-112 C. 

Powdered resins systems (80-600 mesh) typically operate at lower pressure drops, generate less waste water, take up less space, and cost much less when compared to deep-bed polishers. However, they operate at only 3 to 4 gpm sq ft (although the filter element area is large) and are designed to be disposable, so that the powdered resin must be replaced whenever the bed is reconditioned. Sometimes inert resins or cellulose-based fibers are used either in place of powdered resins or as a premix, where they function as both filter aids and absorbents. 

Experimental results from the laboratory or pilot plant may often be scaled up by a factor of 100 times or more. However, to reduce errors in scale-up, a similar filter, the same slurry mixture, the same filter aid and approximately the same pressure drop should be used. 

Another method, which is even more successful in preventing binding of the septum, is the use of a precoat.53 Before filtration is begun a coating of 2-6 in (5-15 cm) of diatomaceous earth or perlite filter aid is deposited on the surface of the septum. During filtration operations the scraper is set so that it slowly removes the precoat and, of course, with it the materials that would have plugged the filter. Since the precoat causes a considerable pressure drop, the rate of filtration is slowed down. Flow rates may vary from 2 to 50 gal/hr/ft2 (0.025-0.60 m/hr). The precoat material costs around 3 or 40/lb and is used at the rate of 10- 15 lb/1,000 gal of feed (1,200 to 1,800 kg/m3). 

In case 4, the increasing pressure compresses the cake to such as extent that it actually squeezes off the flow so that as the pressure increases the flow rate decreases. This situation can be compensated for by adding a filter aid to the slurry. This is a rigid dispersed solid that forms an incompressible cake (diatomaceous earth, sand, etc.). This provides rigidity to the cake and enhances its permeability, thus increasing the filter capacity (it may seem like a paradox that adding more solids to the slurry feed actually increases the filter performance, but it works ). 

The apparatus is dismantled, and the reaction mixture is filtered quickly by suction through a 1.5-cm. layer of filter aid on an 11-cm. Buchner funnel placed on a 2-1. filter flask. The filter cake is pressed down well and washed with three 50-ml. portions of methylene chloride. The filtrate is immediately protected from moisture by calcium chloride tubes, and the solvent is removed by distillation from a 2-1. flask. After the solution has been concentrated to about 225 ml., the hot liquid is poured into a suitable container (Note 6), and the remaining solvent is removed under reduced pressure (Note 7). The yield is 192-199 g. (81-84%) of a white, crystalline solid which melts at 155-156°. 

A 2-L filter flask is equipped with an 11-cm Buchner funnel with filter paper. The funnel is charged with Celiteto a depth of 1 cm (Note 10) and 50.0 g of Florisilis spread evenly on top of the Celite. The above mixture is filtered through this pad of filter aid (Note 11). The filter cake is rinsed with ethyl acetate, 2 x 100 mL. The resulting pale yellow filtrate is concentrated under reduced pressure to yield 47.3 g (96.2%) of pale yellow crystals (Note 12). 

The combined dichloromethane solutions may be washed with dilute hydrochloric acid, sodium bicarbonate solution, and water to remove excess traces of pyridine and chromium salts, or they may be filtered directly through a filter aid or passed through a (ihromatographic column. The product is obtained by removal of dicihloromethaiU any pyridine that remains can oft m be removed under redtuied pressure. 

A 2-1. two-necked round-bottomed flask fitted with a mechanical stirrer and a reflux condenser is charged with 40.0 g. (0.22 mole) of p-cyanobenzenesulfonamide (Note 1), 600 ml. of 75% (v/v) formic acid, and 40 g. of Raney nickel alloy (Note 2). The stirred mixture is heated under reflux for 1 hour (Note 3). The mixture is filtered with suction through a Buchner funnel coated with a filter aid (Note 4), and the residue on the funnel is washed with two 160-ml. portions of 95% ethanol. The combined filtrates are evaporated under reduced pressure with a rotary evaporator (Note 5). The solid residue (Note 6) is heated in 400 ml. of boiling water and freed from a small amount of insoluble material by decantation through a plug of glass wool placed in a filter funnel. The filtrate is chilled in an ice bath and the precipitate is collected by filtration with miction, washed with a small amount of cold water and dried at 50°... 

Filtration is the separation of undissoived particulate solids from a mixture of fluid and solid. The separation is brought about by passage of the fluid thru a pervious septum (filter medium) in or on which the solids are retained. A driving force (gravity, vacuum, pressure, or centrifugal force) produces the flow. Filter aids may be added to the fluid before filtering to counterbalance the unfavorable characteristics of badly filtering materials... 

A filtered solution of Na[Mn(CO)5], prepared from 2.42g (6.21 mmol) of Mn2(CO)10 (Strem) in freshly distilled dry tetrahydrofuran (THF) (50mL),7 is treated with benzyl chloride (1.70g, 13.49mmol). The mixture is stirred for 2h at room temperature and then refluxed for lOmin. After cooling, the solution is filtered through a pad of filter aid (2cm), which is then washed with three 10-mL portions of dry diethyl ether. The solvent is then removed from the combined filtrate and washings under reduced pressure (rotary evaporator) until 5 mL remain. Silica gel (3 g, 200-325 mesh, Ajax) is then added and the... 

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