(653d) High-Accuracy Numerical Simulations of Concentration Polarization in Reverse Osmosis Systems

Fluid flow and mass transport in reverse osmosis (RO) membrane filtration systems are difficult to simulate numerically due to the coupling between the transmembrane filtrate flow, pressure, and osmotic pressure. This coupling is particularly challenging for real-world systems in which the local filtrate flow varies both spatially and temporally. Consequently, previous simulations tend to focus on two dimensional steady cases. We present a new, high-accuracy numerical method specifically tailored to simulating RO. The method ensures that the transmembrane filtrate flow, pressure, and osmotic pressure satisfy Darcy’s law on the membrane surface at each time step. Using the method, we simulate the transient accumulation of rejected solutes at membrane surfaces and the resulting formation of concentration polarization layers. Specifically, we investigate polarization in plate and frame RO systems as a function of inlet Reynolds number, inlet solute concentration, and outlet transmembrane pressure. From the computed flow fields, we directly measure the importance of advection and diffusion of solutes both normal and tangential to the membrane surface. Contrary to the thin film approximation, we find that normal diffusion is balanced by both normal and tangential advection within the concentration polarization layer. Additionally, we investigate these competing mechanisms as a function of downstream distance from the inlet. The results from this study help further understanding of both transient and steady concentration polarization in RO systems.