Filters discontinuous vacuum

Cake filters separate relatively large amounts of solids by forming a cake on the surface of the filtration medium. Cake filters may be operated by applying pressure on the upstream section of the filter medium or vacuum on the downstream section of the filter medium. The operation of the filters may be continuous or discontinuous. However, most pressure filters are discontinuous since the operation of the filter under positive pressure needs to be stopped to facilitate the removal and discharge of solids. Examples of cake filters include filter press, vacuum filter, and centrifugal separator. 

DISCONTINUOUS VACUUM FILTERS. Pressure filters are usually discontinuous vacuum filters are usually continuous. A discontinuous vacuum filter, however, is sometimes a useful tool. A vacuum nutsch is little more than a large Buchner funnel, 1 to 3 m (3 to 10 ft) in diameter and forming a layer of solids 100 to 300 mm (4 to 12 in.) thick. Because of its simplicity, a nutsch can readily be made of corrosion-resistant material and is valuable where experimental batches of a variety of corrosive materials are to be filtered. Nutsches are uncommon in large-scale processes because of the labor involved in digging out the cake they are, however, useful as pressure filters in combination filter-dryers for certain kinds of batch operations. ... 

A variation of the same principle is the DDS-vacuum pressure filter which has a number of small disks mounted on a shaft which rotates discontinuously. The cake is formed on both sides of the disks when they are at the bottom position, dipped into the slurry. When the disks come out of the slurry and reach the top position, hydrauhcaHy driven pistons squee2e the cake and the extra Hquid then drains from both sides of the cake. The cake is removed by blowback with compressed air. 

Catalyst A mixture of 5.26 g of rhodium chloride trihydrate, 0.34 g of palladium chloride, 18 g of carbon (Darco G-60), and 200 ml of water is rapidly stirred and heated to 80°. A solution of lithium hydroxide hydrate (2.7 g) in 10 ml of water is added in one portion and the heating discontinued. Stirring is continued overnight, after which the mixture is filtered and washed with 100 ml of 0.5 % aqueous acetic acid. The product is dried in a vacuum oven at 65°. About 20 g of the catalyst is thus obtained. 

A mixture of acetamide (30 g. = 0-5 mole) and bromine (80 g. = 26 c.c.) in a half-litre flask is kept well cooled with water while enough of a solution of 50 g. of potassium hydroxide in 350 c.c. of water is added to change the initially red-brown colour into a pale yellow this requires most of the alkali. The solution is now run from a dropping funnel in an unbroken jet into a solution of 80 g. of potassium hydroxide in 150 c.c. of water, maintained at 70°-75° in a litre flask. The operation lasts for several minutes. Until the reaction mixture becomes colourless (one quarter to half an hour) the temperature is maintained at 70°-75°, and then the methylamine is distilled with steam. An adapter is fixed to the lower end of the condenser and dips 1 cm. below the surface of the liquid in the receiver (100 c.c. of approximately 5 N-hydrochloric acid2). As soon as the liquid which forms in the condenser is no longer alkaline the distillation is discontinued and the contents of the receiver are evaporated to dryness in a porcelain basin on the water bath. The last traces of water are removed by allowing the basin to stand over night in a vacuum desiccator. The dried material is boiled with absolute alcohol, which dissolves the methylamine hydrochloride but not the ammonium chloride with which it is mixed. The clear filtrate obtained when the ammonium chloride is removed by filtration is concentrated to a small volume and the methylamine hydrochloride is allowed to crystallise out in the cold. The salt is filtered with suction, washed with a little alcohol, and dried in a desiccator. Yield 15-20 g. 

C. (R,R)-N,N -Bis(3,5-di-tert-butylsalicylidene)-1,2-cyclohexanediamine. A 2-L, three-necked, round-bottomed flask equipped with a mechanical overhead stirrer, reflux condenser, and an addition funnel is charged with 29.7 g of (R,R)-1,2-diammoniumcyclohexane mono-(+)-tartrate salt (0.112 mol), 31.2 g of potassium carbonate (0.225 mol, 2 eq), and 150 mL of water. The mixture is stirred until dissolution is achieved, and 600 mL of ethanol is added. The cloudy mixture is heated to reflux with a heating mantle and a solution of 53.7 g (0.229 mol, 2.0 eq) of 3,5-di-tert-butylsalicylaldehyde in 250 mL of ethanol is then added in a slow stream over 30 min (Note 19). The addition funnel is rinsed with 50 mL of ethanol and the mixture is stirred at reflux for 2 hr before heating is discontinued. Water, 150 mL, is added and the stirred mixture is cooled to <5°C over 2 hr and maintained at that temperature for another hour. The yellow solid is collected by vacuum filtration and washed with 100 mL of ethanol. After the solid is air dried, it is dissolved in 500 mL of methylene chloride. The organic solution is washed with 2 x 300 mL of water, followed by 300 mL of saturated aqueous sodium chloride. The organic layer is dried over sodium sulfate, and filtered to remove the drying agent. The solvent is removed by rotary evaporation to yield the product as a yellow solid, mp 200-203°C (Notes 20 and 21). 

It may be noticed that some of the filters discussed are operated continuously and some are not. For example, the rapid sand filter, the slow sand filter, the pressure filter, and the rotary vacuum filter are all operated continuously. The plate-and-frame press is operated as a batch. Thus, filters may also be classified as continuous and discontinuous. Only the plate-and-frame press is discussed in this chapter as a representation of the discontinuous type, but others are used, such as the shell-and-leaf filters and the cartridge filters. The first operates in a mode that a leaf assembly is inserted into a shell while operating and retracted out from the shell when it is time to remove the cake. The second looks like a cartridge in outward appearance with the filter medium inside it. The medium could be thin circular plates or disks stacked on top of each other. The clearance between disks serves to filter out the solids. 

The heating is discontinued after 24 hours, and the excess sodium metal is removed mechanically. After removal of the sodium, 20 ml. of methanol and then 50 ml. of water are added. The tetrahydrofuran and methanol are removed by vacuum rotary evaporation. The remaining aqueous solution is neutralized and gravity-filtered. The filtrate is treated with a saturated solution of cesium or tetramethylammonium chloride. The crude cesium salt is recrystallized as white plates from hot water in 80% yield based on the C-(trimethyIamine)carbaundec-aborane(12). The tetramethylammonium salt is recrystallized from methanol-acetone. 

At the start of filtration in a cake filter some solid particles enter the pores of the medium and are immobilized, but soon others begin to collect on the septum surface. After this brief initial period the cake of solids does the filtration, not the septum a visible cake of appreciable thickness builds up on the surface and must be periodically removed. Except as noted under bag filters for gas cleaning, cake filters are used almost entirely for liquid-solid separations. As with other filters they may operate with above-atmospheric pressure upstream from the filter medium or with vacuum applied downstream. Either type can be continuous or discontinuous, but because of the difficulty of discharging the solids against a positive pressure, most pressure filters are discontinuous. 

Ammoniation is complete when a 1 % solution of a portion of the solid product (which has been washed thoroughly with fresh ethanol) has a pH of approximately 8.2. (For complete reaction, ammonia must be added for 2 to 4 hours.) When conversion to the tetraammonium salt has been completed, passage of ammonia is discontinued, but stirring is maintained until the temperature of the reaction mixture reaches 20°. The product is then filtered under suction onto a Buchner funnel and air is pulled through the filter cake for one hour. The product is placed on a large dish and vacuum-dried at 25°. Yields of 90 to 95 % are obtained. Anal. Calcd. for (NH4)4P207 P, 25.2 N, 22.8. Found P, 25.6 N, 22.8. 

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