Internal Concentration Polarization in Forward Osmosis: Influence of Membrane Orientation and Operating Conditions on Flux Performance
Jeffrey R. McCutcheon, Department of Chemical, Materials and Biomolecular Engineering, University of Connecticut, United Technologies Engineering Building, 191 Auditorium Drive, Room 286, Storrs, CT 06269-3222 and Menachem Elimelech, Yale University, P.O. Box 208286, New Haven, CT 06520-8286.
Forward osmosis has recently received a good deal of attention concerning its use as a viable membrane-based desalination and water treatment process. The process utilizes a concentrated draw solution which causes the natural transport of water across a semipermeable membrane that is impermeable to salt. The draw solute is then either utilized or removed and recycled. Previous studies on FO, however, come to the conclusion that current generation polymeric membranes do not perform well in the FO process. These membranes, designed for pressure-driven flow, have a thin separating (active) layer supported by a porous polymer support layer cast upon fabric. The culprit for the poor flux performance with these membranes is the prevalence of internal concentration polarization (ICP). This phenomenon is similar to concentration polarization except that it takes place within the protective confines of the porous support and fabric layers. Here, the boundary layer thickness is not controlled by crossflow and hence significantly impacts the effective osmotic driving force. Using a commercially available asymmetric FO membrane which lacks a fabric backing layer, this study elucidates the ICP phenomenon by observing flux behavior when the membrane is oriented both with the draw solution on the porous support or on the active layer. It was found that both orientations reduced the osmotic driving force, particularly at higher water fluxes. The porous support layer of this FO membrane was characterized by determining the solute resistance for diffusion, K, which is a function of the support layer thickness, porosity, and tortuosity as well as the solute diffusion coefficient. This term was incorporated into a preexisting model to predict the water flux of this membrane at different temperatures for a variety of feed and draw solution concentrations. The model was found to correspond closely with experimental data, suggesting that it may be reliable as a stand-alone tool for predicting water flux at different experimental conditions. The model also was useful in determining how changing membrane structural properties, and hence K, would affect water flux performance.