CROSS-FLOW MICROFILTRATION OF YEAST SUSPENSIONS IN TUBULAR FILTERS

Crossflow microfiltration experiments were performed on yeast suspensions through 0.2-mum pore size ceramic and polypropylene tubes at various operating conditions. The initial transient flux decline follows dead-end filtration theory, with the membrane resistance determined from the initial flux and the specific cake resistance determined from the rate of flux decline due to cake buildup. For long times, the observed fluxes reach steady or nearly steady values, presumably as a result of the cake growth being arrested by the shear exerted at its surface. The steady-state fluxes increase with increasing shear rate and decreasing feed concentration, and they are nearly independent of transmembrane pressure. The steady-state fluxes for unwashed yeast in deionized water or fermentation media are typically 2-4 times lower than those predicted by a model based on the properties of nonadhesive, rigid spheres undergoing shear-induced back-diffusion. In contrast, the steady-state fluxes observed for washed yeast cells in deionized water are only 10-30 % below the predicted values. The washed yeast cells also exhibited specific cake resistances that are an order of magnitude lower than those for the unwashed yeast. The differences are due to the presence of extracellular proteins and other macromolecules in the unwashed yeast suspensions. These biopolymers cause higher cell adhesion and resistance in the cake layer, so that the cells at the top edge are not free to diffuse away. This is manifested as a concentration jump from the edge of the cake layer to the sheared suspension adjacent to it. The shear-induced diffusion model includes a dimensionless parameter (referred to as the crossflow integral) that accounts for the concentration jump as well as the viscosity and diffusivity properties of a given suspension, and this parameter was determined for the unwashed yeast suspensions by fitting the experimental flux data to the model. The resulting values of this parameter are significantly lower than those for rigid spheres but show relatively little variation between experiments at different operating conditions for a given filter, indicating that the parameter is an intrinsic property of the suspension/membrane system. However, a significant difference in the parameter values was obtained between the ceramic and polypropylene membranes, indicating that the adhesive and fouling properties of the cake depend on the membrane material with which it is in contact. The resulting fluxes obtained from the model using single values for the membrane resistance, specific cake resistance, and crossflow integral for each data set (ceramic and polypropylene filters) are in good agreement with the measured fluxes. Inertial lift theory with no adjustable parameters was also compared to the data. It was found to overpredict the steady-state fluxes at high shear rates and underpredict the fluxes at low shear rates.