Numerical/Experimental Study of a Direct Contact Membrane Distillation System

Direct contact membrane distillation (DCMD) is a thermal process in which warm feed and cool distilled water flow on opposite sides of a hydrophobic membrane. The temperature difference causes water to evaporate from the feed, travel through the membrane, and condense in the distillate. To date, computational fluid dynamics (CFD) studies of DCMD focus primarily on the challenge of temperature polarization. For high concentration brines, however, concentration polarization is another major challenge that reduces system efficiency and leads to mineral scaling. To study coupled temperature and concentration polarization phenomena in the DCMD treatment of high concentration brines, we develop an in-house CFD method to simulate 2-D/3-D, unsteady heat and mass transport within plate-and-frame DCMD systems. The coupled momentum, energy, and mass transport equations are solved using a finite-volume method with an efficient non-iterative unsteady scheme. Numerical results are also validated with comparison to dedicated experiments.

For the 2-D simulations, we perform a parametric study of temperature and concentration polarization under a variety of operating conditions, including inlet velocity, temperature, and concentration. The numerical results are then used to investigate the spatial distribution of temperature, concentration, vapor flux and conductive heat loss along the membrane surface. We report that though permeate flux in DCMD is small compared to RO, concentration polarization is still significant. We also present 3D simulations demonstrating polarization occurring both in the downstream and lateral directions.