Flux behavior in clarification of chicory juice by high-shear membrane filtration: Evidence for threshold flux

Abstract

Membrane fouling and flux decline are important problems in chicory juice clarification by membrane process for inulin production. In this study, a rotating disk module (RDM) equipped with four micro- and ultra-filtration membranes was used to treat raw chicory root extract, and the permeate flux behavior at high-shear conditions was investigated. Increasing disk rotating speed can reduce concentration polarization and cake fouling, and thus improve permeate flux. However, a moderate rotating speed of 1000rpm was a good compromise between energy saving and polarization/cake layers control, while a higher disk rotating speed can be used at regular intervals for on-line membrane cleaning. Threshold and limiting fluxes were determined in chicory juice clarification at 1000rpm and 50°C by pressure stepping experiments, and they were almost independent of pore size, permeability and fouling extent, with ranges of 130–140Lm−2h−1 for threshold and 155–168Lm−2h−1 for limiting flux. Below threshold flux, filtration resistance is independent of flux, while above it, membrane fouling increases with increasing permeate flux. This work confirmed that threshold flux criterion was also applicable to high-shear microfiltration and ultrafiltration, and size-dependent shear-induced and Brownian back diffusions played an important role in such filtration resistance transition. Moreover, both limiting flux and flux fluctuations should be avoided in practical applications because they cause high fouling and flux decline.