The Effect of Spacer Orientations on Temperature Polarization in a Direct Contact Membrane Distillation Process Using 3-d CFD Modeling

Abstract

Membrane distillation is an emerging technology that uses hydrophobic membranes to separate nonvolatile solids from liquids. The vapor pressure gradient between the feed and the permeate sides drives the process. Low-grade thermal energy is used to heat feed water and create a pressure gradient. A large vapor pressure gradient across the membrane surfaces results in high permeation rates. The feed spacer is an important element of the membrane module that forms channels for feed and permeate flow. A good feed spacer design helps improve permeation. In this paper, 3-d CFD simulations are carried out for spacer-filled channels, and the effect of inlet velocity, filament orientation and spacing on heat transfer is studied. Temperature polarization is used as the parameter for heat transfer performance evaluation. Shear stress and temperature polarization index have been calculated for different spacer orientations in a direct contact membrane distillation process. The results show a major influence of the studied parameters on temperature polarization and shear stress. A comparison of 2-d and 3-d analyses reveals that the average shear stress in the two approaches is nearly the same, but the standard deviation of shear stress is lower for the 2-d case. Similarly, the average value and the standard deviation of temperature polarization index are lower than those obtained in the 3-d analysis. The findings also show that for staggered axial filaments, the temperature polarization index distribution is more uniform suggesting that such orientations are more suitable for enhancing heat transfer in a membrane distillation process.