Wall shear stress: effective parameter for the characterisation of the cross-flow transport in turbulent regime during skimmed milk microfiltration

Abstract

Sorne experimental evidence is accumulating in the literature which shows that the mean cross-flow velocity is not an effective parame ter for the characterisation of the cross-flow transport in membrane separation processes. The aim of this work was to determine the most appro- priate parameters (among the wall shear stress, tw' the wall shear rate, y, the Reynolds number, Re, and the mean cross-flow velocity, v) that should be used to assess the limiting and critical fluxes in turbulent regime. The experiments were applied to skimmed milk microfiitration (MF) using a tubu- Jar ceramic membrane (0.1 IJ.m mean pore diameter) for the separation of casein micelles from the solluble proteins. These experiments were conducted with several membrane geometries (seven or 19 channel s, 40 or 85.6 cm long) and seve rai rigs. Unlike Re and v, tw made it possible to assess the Iimiting and critical permeation fluxes in milk MF (0.1 IJ.m),whatever the membrane geometry. It was consequently concluded to be an effective parameter to account for cake filtration and to study the cross- flow filtration scale-up. The wall shear rate, calculated by assuming the turbulence damped at the mem- brane surface and the flow laminar, was not suitable for the assessment of the Iimiting and critical fluxes in turbulent regime. © Inra/Elsevier, Paris. wall shear stress / hydrodynamics / critical permeation flux / cross-flow microfiltration / skimmed milk