Microscale simulation of water desalination in Direct Contact Membrane Distillation

Abstract

Direct Contact Membrane Distillation (DCMD) is a promising desalination technique that can utilize low-grade energy to desalinate water. This study presents a microscale approach to simulate evaporation and condensation inside hydrophobic fibrous membranes. The novelty of the proposed method is that it incorporates the microstructure of the membranes in the calculations and can potentially be used for optimizing membrane’s microstructure. In particular, the model predicts how feed or permeate pressures and temperatures impact the rate of freshwater production. The simulations were conducted in computational domains that mimic the internal geometry of fibrous DCMD membranes in 2-D. The air–water interfaces (AWIs) over the feed and permeate sides of the membrane were simulated using an in-house Pore Morphology Method (PMM) MATLAB code. The resulting wetting and non-wetting phases were then exported to ANSYS using a cell-marking method. The Schrage phase change model was coupled with the ANSYS’s volume of fluid (VOF) solver to simulate water evaporation at the feed AWI and condensation at the permeate AWI. The simulations revealed that increasing the feed or permeate pressure can, to some extent, improve the rate of freshwater production by bringing the feed and permeate AWIs closer to one another and by increasing their surface areas. It was also observed that increasing the feed and permeate temperatures, while keeping their temperature difference constant, can enhance the freshwater production rate significantly.