A 3D CFD comparative study between torsioned and non-torsioned net-type feed spacer in reverse osmosis

Abstract

Spacer-filled channels are incorporated in membrane modules in reverse osmosis applications where the role of feed spacer is to promote turbulence, improve mass transport and mitigate concentration polarization. The objective of this study is to emulate a realistic spacer, due to manufacturing conditions, spacers tend to have a special complex geometry (deformations) which is usually not adopted in computational fluid dynamics (CFD) studies, this deformation tends to be of a torsioned shape. A total of 18 simulations were conducted for torsioned and non-torsioned spacers evaluating concentration polarization and mass transfer across membrane walls at different inlet salinities (co = 400, 500 and 600 mol/m3) and different Reynolds number (Re = 100, 192 and 384) based on the inlet velocity using fully-coupled three dimensional CFD, where the solution-diffusion model was incorporated for the calculations of water and solute fluxes across reverse osmosis membrane. The current comparison illustrates that the torsioned spacers have an overall enhanced performance compared to non-torsioned spacers. It is found that concentration polarization is mitigated in torsioned spacers which is the contrary with non-torsioned spacers having a tendency to concentration polarization, consequently, the water flux in torsioned spacers was found higher than those in non-torsioned spacers. Generally speaking, all membrane modules are found to have a higher performance at higher Reynolds number and at lower inlet salinity. This study proves that spacer geometric details are critical to predict accurately the RO membrane performance.