Influence of microporous membrane properties on the desalination performance in direct contact membrane distillation

• Direct contact membrane distillation was studied with PVDF and ePTFE membranes.
• Pore size, thickness and porosity were respectively 0.05–0.45 µm, 23–125 µm, 0.7–0.8.
• Membrane tortuosity was measured by gas diffusion through water-wet membrane pores.
• Membrane resistance and two boundary layer resistances were isolated by Wilson plot.
• Membrane resistances were predicted by Knudsen diffusion/transitional regime model.

To evaluate the influence of membrane properties on direct contact membrane distillation (DCMD) performance, a variety of microporous hydrophobic flat sheet membranes of polyvinylidene fluoride (PVDF) and expanded polytetrafluoroethylene (ePTFE) were employed in this study over a range of hot brine temperatures, 65–85 °C. The membrane thickness was varied between 23 µm and 125 µm; the nominal pore size was varied from 0.05 µm to 0.45 µm; the porosity was generally high in the range of 0.7–0.8. Experiments were done using two different flat test cells, a stainless steel cell and a chlorinated polyvinyl chloride (CPVC) cell. Boundary layer heat transfer resistances in the membrane cell on both sides of the membrane and the two membrane surface temperatures were determined from the experimental data over a range of hot brine and cold distillate flow rates by the Wilson plot technique. Membrane properties such as the maximum pore size and tortuosity were characterized and employed in checking out model assumptions and model results for water vapor transport in the Knudsen regime and the transition region. Good agreements (within 5% deviation) of the membrane mass transfer coefficient of water vapor and the observed water vapor fluxes were obtained between the experimental values and the simulated results predicted for either the Knudsen regime or the transition region.