Studies in vacuum membrane distillation with flat membranes

How microporous membrane characteristics affect membrane desalination performance in vacuum membrane distillation (VMD) is of significant interest. Different microporous hydrophobic flat membranes of polyvinylidene fluoride (PVDF) and expanded polytetrafluoroethylene (ePTFE) were investigated to that end. The ranges of variation of the membrane thickness and the nominal pore size were respectively 23-125 mu m and 0.05-0.45 in. The porosity of all membranes was high around 0.7-0.8. Experiments were performed with hot brine temperature varying between 65 degrees C and 85 degrees C for different vacuum levels and various brine feed flow rates in a rectangular chlorinated polyvinyl chloride (CPVC) cell. The liquid entry pressure (LEP) of each membrane was experimentally determined. Water vapor fluxes predicted as a function of brine flow rate and the vacuum level were compared using two models: the Knudsen diffusion and the dusty-gas model (DGM). Knudsen diffusion is the dominant transport regime in VMD since Knudsen number, K-n, for all membranes was larger than 1 at all temperatures. The brine-side boundary layer heat transfer resistance in the membrane cell and the brine-side membrane surface temperature were determined from experimental data via Wilson plot. The predicted values of the membrane mass transfer coefficient and water vapor fluxes were found to be close to those obtained experimentally.