Large vacuum filters

This reduction in permeabiUty due to cake consoHdation or coUapse may be so large that it may nullify or even overtake the advantage of using high pressures in the first place and there is then no reason for using the generally more expensive pressure filtration hardware. While a simple Hquid pump may be cheaper than the vacuum pump needed with vacuum filters, if air displacement dewatering is to foUow filtration in pressure filters, an air compressor has to be used and is expensive. 

In apphcations where the fraction of fine particles in the soHds of the feed slurry is low, a simple and relatively cheap vacuum filter can yield cakes with moisture contents comparable to those discharged by pressure filters. Vacuum filters include the only truly continuous filters built in large sizes that can provide for washing, drying, and other process requirements. 

Eor vacuum filters, both the rate of filtration and the dryness of the cake may be important. The filter cake can be modeled as a porous soHd, and the best flocculants are the ones that can keep the pores open. The large, low density floes produced by high molecular weight polymers often coUapse and cause blinding of the filter. Low molecular weight synthetic polymers and natural products that give small but rigid floes are often found to be the best. 

The disk filter is similar to the dmm in operation, but filtration is conducted using a series of large diameter filter disks that carry the filter medium on both sides of the disk. They are connected to the main horizontal shaft and partly immersed in the feed slurry. The central shaft is connected by a set of valves which serve to provide vacuum and air as in dmm filters. As the disk sections submerge during rotation, vacuum is appHed to form a cake on both sides of the disk. The cycle of operation is similar to that in a dmm filter. One unit can have as many as 12 disks of up to 5-m diameter. Disk filters, both compact and cost effective, are used extensively in the iron ore industry to dewater magnetite concentrates. 

Both vacuum and pressure filters are used. Turbidity is more easily removed by vacuum filters, usually at 85% efficiency. Flow rates are low, ca 4 mL/(cm -min) [1 gal/(ft -min)] these filters are not practical for treating large volumes. 

Slurries of free-filtering sohds that are difficult to suspend are sometimes filtered on a top-feed drum filter or filter-diyer. An example application is in the production of table salt. An alternative for slurries of extremely coarse, dense solids is the internal drum filter. In the chemical-process industiy both top-feed and internal drums (which are described briefly by Emmett in Schweitzer, op. cit., p. 4-41) have largely been displaced by the horizontal vacuum filter (q.v). 

The cost of the filter station includes not only the installed cost of the filter itself but also that of all the accessories dedicated to the filtration operation. Examples are feed pumps and storage facihties, precoat tanks, vacuum systems (often a major cost factor for a vacuum filter station), and compressed-air systems. The dehvered cost of the accessories plus the cost of installation of filter and accessories generally is of the same order of magnitude as the dehvered filter cost and commonly is several times as large. Installation costs, of course, must be estimated with reference to local labor costs and site-specific considerations. 

This family of filters consist of a vertical pressure vessel with a horizontal filter plate at the bottom. The filtrate from this equipment flows out a nozzle on the bottom of the filter. These devises are usually used for slurries where large amounts of solids are being collected. Variations of this equipment include equipment with removable lower heads for easy cake removal, ability to pressure or vacuum filter, ability to wash the filter cake, an agitator to break-up and rewash the filter cake, and heating or cooling jackets for the whole vessel. The Nutsche filter is the industrial version of the well known laboratory scale Buchner Funnel with the exception that it is designed to operate under either on vacuum or pressure. 

The filtration area of large table filters is more than 200 m and having few moving parts can rotate at a cycle time of 1.5 minutes. These machines can handle thick cakes and may be operated at high vacuum levels. The major subassemblies of the machine include ... 

After the end of the fermentation (28 hours) the culture broth is filtered off by suction over a large suction filter. The mycel residue is washed with water several times. The filtrate is extracted three times, each time with 10 liters of methyl isobutyl ketone. The extract is concentrated under vacuum in a circulating evaporator and in a round flask carefully dried under vacuum. The residue is crystallized from acetone/isopropyl ether. The melting point is 157°-158°C (fermentation yield = 60%). The pure product yield obtained after a second crystallization and chromatography of the mother liquor on silica gel amounts to 53% of the theoretical. 

Whilst it may be possible to predict qualitatively the effect of the physical properties of the fluid and the solid on the filtration characteristics of a suspension, it is necessary in all cases to carry out a test on a sample before the large-scale plant can be designed. A simple vacuum filter with a filter area of 0.0065 m2 is used to obtain laboratory data, as illustrated in Figure 7.5. The information on filtration rates and specific resistance obtained in this way can be directly applied to industrial filters provided due account is taken of the compressibility of the filter cake. It cannot be stressed too... 

The disadvantages of cyclic operation, attributable to the need to remove the cake from batch filters, have been largely overcome by the development of a wide range of continuous vacuum filters as summarised in Table 7.2 which includes data on the selection for a given duty. This is further detailed in Table 7.3 and is based on the work of G. H. Duffield of Stockdale Engineering and E. Davies of ED ACS Ltd. 

Figure 7.18a shows a typical layout of a rotary drum installation and Figure 7.18Z shows the sequence of cake formation, washing and dewatering. A large rotary drum vacuum filter is shown in Figure 7.19. 

A baghouse can be thought of as a very large vacuum cleaner whose walls are lined with a semipermeable filter. Flue gases pass... 

The flask and contents are placed on a steam cone and heated for 2 hours under vacuum (furnished by a water aspirator) (Note 2) in order to remove the phosphorus oxychloride formed in this process as well as most of the unreacted phosphorus pentachloride. The dry cake is pulverized in a mortar and transferred to a 4-1. beaker. To this is added 750 ml. of distilled water and 2 1. of chloroform. The mixture is placed on a steam bath, heated to boiling, and stirred vigorously until nearly all the solid dissolves. By means of a separatory funnel the layers are separated while still hot. The chloroform solution is again heated to boiling and filtered through a large fluted filter into an Erlenmeyer flask. 

At the Elsnig factory the crystallization of cyclonite is accomplished as follows. About 110 kg of cyclonite are introduced into a closed tank, with a capacity of 10001., equipped with stirrers and lined with a woollen filter cloth. Approximately 900 1. of acetone heated to 50°C are run into the tank to dissolve the cyclonite, after which the solution filtering through the filter cloth is drained down into a 30001. tank. (The filter cloth is changed every 10 hr). Here ahout 13501. of water is added over a period of 5 min, while the temperature is maintained at 25°C, and cyclonite is precipitated from the acetone solution in the form of fairly large crystals approximately 90% of the total are longer than 0.1 mm. The precipitated cyclonite is separated on a vacuum filter. 

In the method described by Urbanski, the oxidized product was drowned, together with acid, in a large volume of ice water and the resulting mixture transferred to a vacuum filter... 

If there are remaining any large particles, vacuum filter the sample using a side-arm Erlenmeyer flask fitted with a Buchner funnel and Whatman no. 1 filter paper. 

A wide variety of filters are available for the cell recovery. There are generally two major types of filters pressure and vacuum filters. The detailed descriptions of those filter units can be found in Chemical Engineers Handbook (Perry and Chilton, 1973). The two types of filters most used for cell recovery are the filter press and rotary drum filters. A filter press is often employed for the small-scale separation of bacteria and fungi from broths. For large-scale filtration, rotary drum filters are usually used. A common filter medium is the cloth filter made of canvas, wool, synthetic fabrics, metal, or glass fiber. 

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