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

This paper investigates the effect of cyclic variations of transmembrane pr essure (TMP), velocity and concentration on the particle cake formed on an organic membrane during the microfiltration of CaCO3 suspension. The steady -state permeate Bur 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 betw een 1 and 700 g/l. In these tests, the permeate flux was observed to exhibi t an important hysteresis due to the irreversibility of the cake formed. Wh en the change in operating conditions resulted in cake growth (TMP or conce ntration increase, velocity decrease), the permeate flux was found to be go verned simultaneously by the resistance of the cake layer and concentration polarization with shear-induced diffusion. When the TMP or concentration w ere reduced, the cake thickness remained constant because of particle cohes ion. When the velocity was increased, the cake thickness also remained cons tant until a critical velocity corresponding to the erosion shear stress fo r the cake was reached. Above this critical velocity, the cake was progress ively 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 st ress on the cake surface. The main conclusion is that, when a cake has been formed, the permeate Aux is governed not only by the present operating con ditions but also by the previous ones. (C) 2000 Elsevier Science B.V. All r ights reserved.