Helical strip-type spacers for enhanced multi-directional mixing and mitigation of temperature polarisation in membrane distillation – A numerical and experimental investigation

Abstract

Membrane distillation (MD) is a promising membrane separation process, capable of achieving low energy consumption and yielding high-purity products. However, temperature and concentration polarisation phenomena take place adjacent to membranes, resulting in diminished MD performance. The insertion of spacers between membrane sheets is thought to be a potential solution, enhancing fluid mixing. In this study, we present a new helical strip-type spacer designed for efficient mixing and polarisation mitigation and compare its performance with conventional double-layer spacers via computational simulations. Simulation results reveal that the helical strip-type spacers outperform conventional double-layer spacers by up to 27 % in terms of average water flux, which is attributed to multi-directionally disturbed flow patterns facilitated by the strip swirl design. However, it is important to note that the pressure drop increases by a maximum of 1.9 times. Also it is experimentally demonstrated that the unique shape of the proposed spacer can be practical in MD through 3D printing, showing the same trends of the helix spacers having higher water fluxes than the conventional design by up to 20 %. In conclusion, the proposed spacers can efficiently promote near-wall fluid mixing and be beneficial in high recovery MD where severe polarisation can lead to membrane scaling, albeit at the cost of increased pressure loss.