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Filtration

Filtration Equipment Selection Criteria

| Editor: Dr. Jörg Kempf

Fig. 1: Pusher Centrifuge (Pictures: KMPT)
Fig. 1: Pusher Centrifuge (Pictures: KMPT)

Filtration centrifuges provide a good method for mechanically separating suspended solids from liquids. Separation in a centrifugal field generated by equipment rotating at high speed has a number of advantages compared to simple vacuum and pressure based techniques.

There are two types of filtration centrifuges: continuous and batch. Both types exploit the same separation principle. A solid-liquid mixture is introduced into a rotating basket where centrifugal force presses the product against the shell of the basket. A filter medium on the basket holds back the solids, while the liquid filtrate is ejected to the outside.

The cake which forms on the filter medium passes through a series of phases during the separation process. Once the cake has formed in the solid/liquid mixture, the point arrives when the liquid on the surface is completely immersed in the cake. The moisture is then removed from the fully saturated cake. If necessary, the cake can also be washed to remove contaminants. The level of residual moisture in the cake depends on the centrifugal force applied, cake thickness, residence time and the physical properties of the product. The cake is then removed from the basket using one of a number of techniques.

The time sequence differs in the two types of centrifuge. On batch centrifuges, loading, cake formation, centrifugal drying and cake removal take place at different times, whereas these steps occur simultaneously in a continuous machine.

The horizontal peeler centrifuge

Fig. 5 shows the principle of operation of a horizontal peeler centrifuge. Chronologically, this batch machine belongs to the first generation of centrifuges. The product is introduced into the feed distributor, and it is removed again with a blade when centrifugal drying is complete. The use of a rotary siphon is a process refinement to the peeler centrifuge principle (Fig. 4). A basket with a solid shell is used instead of a perforated basket. The filtrate does not pass directly through the filter medium and drum shell. Instead, the filtrate moves towards the rear end of the basket and is forced through holes into the siphon chamber which is outside the rear end of the basket. Because the liquid levels in the drum and siphon chamber are not the same, negative pressure builds up, which enhances centrifugal pressure and increases the filtration rate.

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