Analysis of cake build-up and removal in cross-flow microfiltration of CaCO3 suspensions under varying conditions

Abstract This paper investigates the effect of cyclic variations of transmembrane pressure (TMP), velocity and concentration on the particle cake formed on an organic membrane during the microfiltration of CaCO 3 suspension. The steady-state permeate flux was measured in a series of tests in which the TMP was varied between 0.25 and 3 bar, the velocity was changed between 0.25 and 2 m/s and the particle concentration was successively raised and diluted between 1 and 700 g/l. In these tests, the permeate flux was observed to exhibit an important hysteresis due to the irreversibility of the cake formed. When the change in operating conditions resulted in cake growth (TMP or concentration increase, velocity decrease), the permeate flux was found to be governed simultaneously by the resistance of the cake layer and concentration polarization with shear-induced diffusion. When the TMP or concentration were reduced, the cake thickness remained constant because of particle cohesion. When the velocity was increased, the cake thickness also remained constant until a critical velocity corresponding to the erosion shear stress for the cake was reached. Above this critical velocity, the cake was progressively eroded, starting from the membrane outlet, and completely removed if the velocity became high enough. This critical velocity decreased when the concentration rose because the suspension viscosity and density increased. The erosion shear stress was found to be a linear function of the normal stress on the cake surface. The main conclusion is that, when a cake has been formed, the permeate flux is governed not only by the present operating conditions but also by the previous ones.