Revised external and internal concentration polarization models to improve flux prediction in forward osmosis process

Abstract

Previous modified external concentration polarization (ECP) model can only accurately account for the ECP effect of the forward osmosis (FO) process with NaCl or KCl as the draw solute, when Schmidt number (Sc) values are less than 800 and water fluxes are low. However, for other solutes (i.e., MgSO4, MgCl2, CaCl2 and glucose) that have considerable variation of solutes diffusivities with concentration and high Sc values, the previous modified ECP model under-predicted the effect of ECP on flux. To improve accuracy of flux prediction, the previous ECP model was revised to include dilution/suction and property (diffusivity) variation effects. The modified ICP model was also improved by considering a solute resistivity constant, Ks, that is specific to each membrane for each draw solute used, due to different degree of interactions of different solutes with the porous matrix membrane material. The revised ECP and ICP models, proposed in this study, improved the overall accuracy of flux prediction for the FO process when different draw solutions were considered. The overall FO model was verified by data obtained from laboratory-scale experiments and the appropriate FO models could be selected based on the feed and draw solutes under both pressure-retarded-osmosis and forward-osmosis mode of operation.