Hydrodynamics of sinusoidal spacers for improved reverse osmosis performance

Abstract

A key to maximizing flux and decreasing fouling in reverse osmosis (RO) is to decrease concentration polarization. This is currently partially accomplished by mesh feed spacers, but mesh spacers increase the longitudinal pressure drop and form dead zones where foulants accumulate. Alternative spacer geometry is presented here where sinusoidal flow patterns are created. Several models of RO spacer channels with varying amplitude and wavelength in their sinusoids were created for evaluation by three-dimensional computational fluid dynamics (CFD) simulations (COMSOL Multiphysics software) that included predictions of concentration polarization and flux. Results over a range of pressures and salt concentrations indicated that the more tortuous geometries (higher amplitude and shorter wavelength) induced greater local fluid velocity and decreased concentration polarization, which led to greater flux. Taylor–Goertler vortices generated in the peaks and valleys of the channels aided in mass transfer. The drawback to the sinusoidal geometry was an increased pressure drop, but one of the sinusoidal geometries tested had both a lower pressure drop and higher flux than a conventional mesh spacer. A subset of the spacer geometries was built and tested experimentally in a bench-scale RO unit. Experimental and modeling data were in good agreement, confirming the benefits of the sinusoidal spacer geometries and suggesting that CFD is an effective tool for predicting performance.