Drum filter

Drum Filters. The rotary dmm filter, also borrowed from vacuum filtration, makes relatively poor use of the space available in the pressure vessel, and the filtration areas and capacities of such filters cannot possibly match those of the disk pressure filters. In spite of this disadvantage, however, the pressure dmm filter has been extensively developed. 

TDF Drum Filter. This is a fairly conventional dmm filter housed in a vertical pressure vessel. Test data, obtained with the smallest model of only 0.75 m filtration area, is available (18). Larger models have also been announced, ranging up to the filtration area of 46 m and very large vessels. The operating pressures are moderate, up to 25 or 35 kPa, and the dmm speeds fairly conventional from 0.3 to 1.5 rpm. The range of dry cake production quoted is from 250 to 650 kg/m h for fine coal. 

Sealing of the belt along the edges of the filter drum is never perfect, and some leakage should be expected. If good clarity is essential, it may be preferable to use a drum filter with the cloth caulked in place and design the system to contend with the effects of bhnding. 

For most apphcations, the actual area of a drum filter will generally be no less than 94 to 97 percent of the nominal area, depending upon the size and number of sections. This variation is generally not accounted for separately and is assumed to be taken care of in the scale-up factor on filtration rate. 

Rotary Drum Filters The rotaiy drum filter is the most widely used of the continuous filters. There are many design variations, including operation as either a pressure filter or a vacuum filter. The major difference between designs is in the technique for cake discharge, to be discussed later. All the alternatives are characterized by a horizontal-axis drum covered on the cylindrical portion by filter medium over a grid support structure to allow drainage to manifolds. Basic materials of construc tion may be metals or plastics. Sizes (in terms of filter areas) range from 0.37 to 186 m (4 to 2000 ft ). 

All drum filters (except the single-compartment filter) utilize a rotary-valve arrangement in the drum-axis support trunnion to facih-tate removal of filtrate and wash hquid and to allow introduction of air or gas for cake blowback if needed. The valve controls the relative duration of each cycle as well as providing dead portions of the cycle through the use of bridge blocks. A typical valve design is shown in Fig. 18-121. Internal piping manifolds connect the valve with various sections of the drum. 

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). 

Drum filters commonly are classified according to the feeding arrangement and the cake-discharge technique. They are so treated in this subsection. The characteristics of the shiny and the filter cake usually dictate the cake-discharge method. 

Removable-Medium Filters Some drum filters provide for the filter medium to be removed and reapplied as the drum rotates. This feature permits the complete discharge of thin or sticky cake and provides the regenerative washing of the medium to reduce blinding. Higher filtration rates are possible because of the thinner cake and clean medium, but this is compromised by a less pure filtrate than normally produced by a nonremovable medium. 

Bird-Young fdter. This filter (Bird Machine Co.) differs from most drum filters in that the drum is not compartmented and there is no internal piping or rotaiy valve. The entire inside of the drum is sub-... 

FIG. 18-125 Cutaway of the single-compartment drum filter. (Bird Machine Co. )... 

Construction is similar to that of other drum filters, except that vacuum is applied to the entire rotation. Before feeding slurry a precoat layer of filter aid or other suitable solids, 75 to 125 mm (3 to 5 in) thick, is apphed. The feed slurry is introduced and trapped in the outer surface of the precoat, where it is removed by a progressively advancing doctor knife which trims a thin layer of solids plus precoat (Fig. 18-126). The blade advances 0.05 to 0.2 mm (0.002 to 0.008 in) per revolution of the drum. When the precoat has been cut to a predefined minimum thickness, the filter is taken out of service, was ned, and freshly precoated. This turnaround time may be 1 to 3 h. 

One of the oldest filters applied throughout the chemical processing industry is the rotary vacuum drum filter, which is illustrated in Figure 8. This machine belongs to the group of bottom feed configurations. Rotary drum filters are typically operated in the countercurrent mode of operation. The principle advantage of these machines is the continuity of their operation. 

Figure 13. Cycle sequence of a rotary drum filter.

Internal Rotary-Drum Filters An example of an internal rotary-drum filter is illustrated in Figure 14. The filter medium is contained on the inner periphery. This design is ideal for rapidly settling slurries that do not require a high degree of washing. Tankless filters of this design consist of multiple-compartment drum vacuum filters. 

In precoating, the prime objective is to prevent the filter medium from fouling. The volume of initial precoat normally applied should be 25 to 50 times greater than that necessary to fill the filter and connecting lines. This amounts to about 5-10 lb/100 fF of filter area, which typically results in a 1/16-in. to 1/8-in. precoat layer over the outer surface of the filter medium. An exception to this rule is in the precoating of continuous rotary drum filters where a 2-in. to 4-in. cake is deposited before filtration. The recommended application method is to mix the precoat material with clear liquor (which may consist of a portion of the filtrate). This mixture should be recycled until all the precoat has been deposited onto the filter medium. The... 

For a 50,000 GPD filtration operation with an average loading 50 mg/L TSS (Total Suspended Solids) determine the optimum flocculant to use in order to achieve at least an 85 % reduction in solids. Assume that a rotary drum filter unit is used. 

From the viewpoint of accommodation to the filter-supporting structure, some cloths cannot be used, even though the filtering characteristics are excellent. For rotary drum filters, for example, the cloth is pressed onto the drum by the caulking method, which uses cords that pass over the drum. In this case, the closely woven cloths manufactured from monofilament polyethylene or polypropylene fiber are less desirable than more flexible cloths of polyfilament fibers or staple cloths. 

Agitated/Monoplate Nutsche Filter/Dryer Horizontal Plate Filter/Dryer Continuous Equipment Rotary Drum Filter Centrifugal Filter Horizontal Belt Filter... 

When long drying time is required to reach asymptotic moisture in the cake. On Drum Filters, for example, the ratio of dry to form cannot normally exceed 1.5 since it is determined by its geometry and the number of circumferential compartments. 

The resistance of the filter medium is negligible. Determine the filter s capacity in filtrate and the velocity of the belt. To solve this problem take a look at the design criteria for a rotary drum filter - they are very similar. 

Figure 9. Precoat drum filter flow scheme for polishing operations.

Rotating Drum Filter Low space requirements Can be polymer dependent... 

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