A 3D CFD study on the effects of feed spacer designs on membrane performance for high-permeance RO membranes

Abstract

Feed spacers can promote fluid mixing near membrane surfaces, thereby enhancing water permeation performance through reverse osmosis (RO) membranes. Their role becomes more important under high-flux conditions, where concentration polarisation (CP) near the membrane surfaces occurs more severely. However, flow stagnation formed between the spacer filaments and membrane walls might induce an increased risk of membrane fouling. In this work, computational fluid dynamics (CFD) simulation is performed for spacer-filled feed channels with different water permeances and spacer types to assess the efficiency of each spacer type in mitigating CP under a wide range of flux conditions. Also quantitative analyses are made for wall shear rates, indicative of flow stagnation and fouling, as well as for water and salt fluxes and CP moduli. A new design, called a spine-ramp configuration, is accounted for in addition to the conventional unwoven and fully woven designs, which is expected to reduce flow stagnation. Simulation results demonstrate that discrepancies in CP modulus between different spacers for high-flux conditions are larger than those for low-flux conditions, suggesting that the choice of spacer design would be more critical for high-permeance membranes. Furthermore, an obtuse contact angle between spacer filaments and membrane walls turns out to be beneficial in minimising flow stagnation, thereby potentially reducing the risk of fouling.