Computational fluid dynamics simulations of flow and concentration polarization in forward osmosis membrane systems

Forward osmosis is an osmotically driven membrane separation process that relies on the utilization of a large osmotic pressure differential generated across a semi-permeable membrane. In recent years forward osmosis has shown great promise in the areas of wastewater treatment, seawater/brackish water desalination, and power generation. Previous analytical and experimental investigations have demonstrated how characteristics of typical asymmetric membranes, especially a porous support layer, influence the water flux performance in osmotically driven systems. In order to advance the understanding of membrane systems, models that can accurately encapsulate all significant physical processes occurring in the systems are required. The present study demonstrates a computational fluid dynamics (CFD) model capable of simulating forward osmosis systems with asymmetric membranes. The model is inspired by previously published CFD models for pressure-driven systems and the general analytical theory for flux modeling in asymmetric membranes. Simulations reveal a non-negligible external concentration polarization on the porous support, even when accounting for high cross-flow velocity and slip velocity at the porous surface. Results confirm that the common assumption of insignificant external concentration polarization on the porous surface of asymmetric membranes used in current semi-analytical approaches may not be generally valid in realistic systems under certain conditions; specifically in systems without mass-transfer promoting spacers and low cross-flow velocities.