A study of flow field and concentration polarization evolution in membrane channels with two-dimensional spacers during water desalination

This study is a step toward integrated modeling of the evolving (in space and time) flow and concentration fields during water desalination in membrane spacer-filled channels. Such detailed model predictions are of practical significance, leading to improved understanding of spiral wound membrane (SWM) module operation. Submerged and zigzag spacer filament 2-dimentional configurations are employed, as they create geometric flow-channel features (contact lines, flow constrictions, etc) encountered in SWM modules. The numerical study, performed in a Reynolds number range typical for desalination modules, is focused on the evolution of local mass transfer coefficient k and concentration polarization, which significantly affect SWM module performance. A detailed quantitative prediction is obtained of these parameters, which are linked to the variation of wall shear stress, static pressure and permeation flux. By focusing on a typical region of the membrane channel (adequately capturing the evolving flow characteristics), it is predicted that increasing average k values (with the concomitant reduction of concentration polarization) are associated with the undesirable increase of pressure drop; these conflicting requirements can be balanced through the selection of appropriate spacer geometry. The negative effect of membrane-filament contact lines on concentration polarization is well documented. An assessment is made of the two spacer filament configurations, from this perspective, and directions for future research are outlined.