CAKE FILTRATION MECHANISMS IN STEADY AND UNSTEADY FLOWS

The cake filtration kinetics of a neutrally buoyant suspension of approximately 150 mum diameter polyethylene particles have been studied in laminar and oscillatory flows. Simultaneous measurements of both filtrate flux and cake height have been made, together with direct in situ optical observations of particle behaviour at the cake-slurry interface. Mechanisms for the cake formation process have been identified, and under steady crossflow conditions, regimes of selective packing and decreasing particle capture probability have been observed. The effect on the filtration mechanisms of unsteadiness in the crossflow has been investigated. Sinusoidal oscillations have been imposed upon a constant net flow in a channel geometry and the effect of the introduction of baffles above the filter medium has been examined. Flow patterns showing the behaviour of particles in the vicinity of the filter have been obtained which show that eddy circulation, both upstream and downstream of the baffles, can cause filter cake erosion. This effect has been enhanced by periodic switching of the differential pressure across the filter cake and its applicability for filtrate flux improvement has been investigated.