Fruit juice concentration by membranes: effect of rheological properties on concentration polarization phenomena

Transport phenomena in membrane processes have been theoretically studied f or a lab-scale flat geometry membrane module, which was continuously fed wi th fruit juice solutions. The proposed model is based on the solution of co ntinuity, momentum and mass transfer equations, modified according to the b oundary layer theory coupled with the osmotic pressure model. The aim of th e present study is to determine the range of operating conditions in the la minar flow regime, where the concentration polarization phenomena, typical of membrane operation, are maintained within acceptable limits. Membrane hy draulic permeability, generalized Reynolds number and fluid rheological pro perties were shown to play a crucial role in the system performances. Both the diffusion coefficient and the free stream velocity are critical in eval uating membrane performances. As far as the permeate flux decay is concerne d, an opposite trend between the diffusion coefficient and axial velocity, is observed at a fixed Reynolds number. The presented model is general: it could be applied to characterize the concentration polarization phenomena w henever a solution, modeled by non-Newtonian constitutive equations, is fed to a membrane module. (C) 2001 Elsevier Science Ltd. All rights reserved.