Dynamic cross-flow filter systems

The cross-flow principle (Figure 3.76) began with the hollow fibres used in reverse osmosis, and has expanded to become one of the most important components of the filtration industry. In order to keep the surface free of deposit, high-shear conditions are employed, and these can be created either by a high suspension velocity across the medium, or by some sort of movement (rotation, vibration, etc.) of the medium with respect to the liquid flow or a nearby non-porous surface. This latter group, of movement promoted filtration, is often termed dynamic cross-flow filter systems. 

The design of a cross-flow filter system employs an inertial filter principle that allows the permeate or filtrate to flow radially through the porous media at a relatively low face velocity compared to that of the mainstream slurry flow in the axial direction, as shown schematically in Figure 15.1.9 Particles entrained in the high-velocity axial flow field are prevented from entering the porous media by the ballistic effect of particle inertia. It has been suggested that submicron particles penetrate the filter medium and form a dynamic membrane or submicron layer, as shown in... 

Mechanical Cake Removal. This method is used in the American version of the dynamic filter described under cross-flow filtration with rotating elements, where turbine-type rotors are used to limit the cake thickness at low speeds. The Exxflow filter, introduced in the United Kingdom, is described in more detail under cross-flow filtration in porous pipes. It uses, among other means, a roUer cleaning system which periodically roUs over a curtain of flexible pipes and dislodges any cake on the inside of the pipes. The cake is then flushed out of the curtain by the internal flow. 

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