Computational fluid dynamics simulations of polarization phenomena in direct contact membrane distillation

Abstract

Direct Contact Membrane Distillation (DCMD) has emerged as a promising means of concentrating brines to their saturation limit. To date, computational fluid dynamics (CFD) studies of DCMD focus primarily on temperature polarization. However, when treating hypersaline brines, concentration polarization is an additional challenge that reduces system efficiency and leads to mineral scaling. To study coupled temperature and concentration polarization in the DCMD treatment of hypersaline brines, we develop an experimentally validated two-dimensional CFD code that simulates heat and mass transport in plate-and-frame DCMD systems. We then perform a comprehensive parametric study of polarization phenomena for a wide range of feed and distillate operating conditions, system length, and co-current versus counter-current operation. We also investigate the system-level performance by measuring the average permeate flux, single-pass water recovery, maximum concentration polarization coefficient, and gained output ratio of DCMD systems with heat recovery. Though the transmembrane vapor flux is small, we observe dramatic increases in solute concentration at the membrane surface, exceeding 1.6 times the feed value. The temperatures, concentration, and vapor flux vary considerably in the downstream direction, and are poorly approximated by common Nusselt and Sherwood correlations.