Dean Vortex Membrane Microfiltration Non-Newtonian Viscosity Effects

Abstract

Many industrial feeds behave as non-Newtonian fluids, and little understanding exists as to their influence on cross-flow microfiltration (CMF) performance. The viscosity effects of a model non-Newtonian shear-thickening fluid were investigated in CMF with and without suspended silica particles in the feed. The goals were to compare the performance of Dean vortex filtration for non-Newtonian fluids with that for Newtonian fluids and, using a shear-thickening fluid, to establish, in a negative control experiment for equivalent axial flow rates, that the mean wall shear rates is higher in helical tube flow than in linear tube flow. Hence, the effect of viscosity on the formation of controlled centrifugal instabilities (Dean vortices) was studied. Non-Newtonian behavior was imparted to the fluid through the addition of a cationic surfactant, tetradecyltrimethylammonium salicylate (TTASal), that could freely permeate the membrane and that exhibited little noticeable fouling. Because the membrane did not retain the micelles and freely passed the surfactant monomer, fouling behavior could be separately studied by adding silica particles to the micellar solution. Results were compared with a previously studied Newtonian system for both linear and helical CMF. Two new mass transfer correlations were obtained for a shear-thickening surfactant solution in the presence of a 0.1 w/w % silica concentration for laminar flow in linear (no vortices) and helical (with vortices) membrane pipes. The viscosity was described as a function of surfactant concentration and velocity. The correlation for Dean vortex laminar flow incorporated the effect of increasing viscosity on the stability of Dean vortices by adjusting the exponent of the Reynolds number in the expression Shhel = 0.19(a/rc)0.07Re[0.55(η(v=0)/η(v=v))]Sc0.33 for Re < 800.