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

Some experimental evidence is accumulating in the literature which shows th at the mean cross-flow velocity ia not an effective parameter for the chara cterisation of the cross-flow transport in membrane separation processes. T he aim of this work was to determine the most appropriate parameters (among the wall shear stress, tau(w), the wall shear rate, gamma, the Reynolds nu mber, Re, and the mean cross-flow velocity, v) that should be used to asses s the limiting and critical fluxes in turbulent regime. The experiments wer e applied to skimmed milk microfiltration (MF) using a tubular ceramic memb rane (0.1 mu m mean pore diameter) for the separation of casein micelles fr om the solluble proteins. These experiments were conducted with several mem brane geometries (seven or 19 channels, 40 or 85.6 cm long) and several rig s. Unlike Re and v, tau(w) made it possible to assess the limiting and crit ical permeation fluxes in milk MF (0.1 mu m), whatever the membrane geometr y. It was consequently concluded to be an effective parameter to account fo r cake filtration and to study the crossflow filtration scale-up. The wall shear rate, calculated by assuming the turbulence damped at the membrane su rface and the flow laminar, was not suitable for the assessment of the limi ting and critical fluxes in turbulent regime. (C) Inra/Elsevier, Paris.