Microfiltration and ultrafiltration of UHT skim milk with a vibrating membrane module

摘要

Dairy applications of a vibratory shear-enhanced (VSEP) filtration system were investigated using a 500 dm~2 membrane area pilot. Commercial UHT skim milk was ultrafiltered with a 50 kDa PES membrane for concentrating total proteins. The effect of mean membrane shear rate gamma_w was investigated by lowering the vibration frequency. The permeate flux was found to vary as (gamma_w)~0.533 at initial concentration and as (gamma_w)~0.567 at a volume reduction ratio (VRR) of 1.8. The permeate flux decayed with the logarithm of concentratio and the maximum VRR calculated by extrapolaton to zero flux was 8.66. The critical flux for stable filtratio was found to be 76 1 h~(-1) m~(-2) at the maxiumu frequency of 60.75 Hz, corresponding to a mean shear stress on the membrane of 50 Pa. Microfiltration tests were carried out with a 0.1 mum pores Teflon membrane for separating casein micelles. The permeate flux increased with gamma_w, but with a lower exponent than in UF, especially at high frequency. The critical flux at 60.75 Hz and initial concentration was 64 1 h~(-1) m~~(-2), thus lower than in UF. A comparison with critical fluxes reported in the literature for UF and UF ox skim milk indicates a larger domain of filtration stability for the VSEP than for cross-flow filtration with tubular membranes.