Dynamics and Rheology of Fouling Cakes Formed During Ultrafil Tration

Abstract

The solute cake which forms on a membrane surface during ultrafiltration processes is well known for its fouling characteristics. The dynamics and rheology of the cake are investigated and observed under the action of cross-flow shear. Experiments with slurries having 300 nm diameter particles of titanium dioxide indicate average volume concentrations of 0.56 or 0.57 and show that this cake indeed ‘flows’ with a creeping velocity under applied shear. The cake thickness reaches a steady state when the solute advection towards the membrane balances the solute mass carried away at the trailing edge by the creeping cake. The viscosityshear rate dependence of this layer is determined experimentally and the ‘Creeping velocity’ of the cake is calculated assuming the transverse drag force is determined from the Kozeny-Carman equation. Upon instantaneous compression the cake compresses while maintaining its mass distribution. The change in cake resistance allows interpetation of the pressure modified concentration. The volume concentration, determined from the mathematical modeling, is shown to lie between 0.54 to 0.65. Observations show that the top few layers of this cake move freely with the shearing flow due to the lifting action of normal stresses in the cake under external shear. Volume concentrations up to 0.65 are indicated from the analysis.