(485c) Influence of Microporous Membrane Properties on VMD and DCMD: Experimental and Modeling Studies

A variety of microporous hydrophobic flat sheet membranes of polyvinylidene fluoride (PVDF) and expanded-polytetrafluoroethylene (e-PTFE) were studied to evaluate the influence of membrane properties on their performance in desalination by vacuum membrane distillation (VMD) and direct contact membrane distillation (DCMD) processes. The membrane thickness was varied between 23 μm to 125 μm; the 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. VMD experiments were performed over a hot brine temperature range of 65 °C to 85 °C for various feed flow rates and various vacuum levels in a rectangular chlorinated polyvinyl chloride (CPVC) cell. Membrane properties, such as the maximum pore size, tortuosity as well as the liquid entry pressure (LEP) were experimentally determined. Water vapor fluxes were predicted and compared using two models: the Knudsen diffusion and the dusty-gas model (DGM). The deviation between two models is within 1.3 %. Knudsen diffusion is the dominant regime in VMD transport since Knudsen number, K_n, for all membrane were larger than 1 at all temperatures. The boundary layer heat transfer resistance in the membrane cell and the membrane surface temperature were determined from experimental data via Wilson plot. Good agreements of membrane mass transfer coefficients and water vapor fluxes were found between the DGM simulations and the experimental results (deviation within 5%). Same membranes were studied in a circular stainless steel cell and the CPVC cell for DCMD behavior using the Wilson plot method. Experimental and predicted values of the membrane mass transfer coefficients and water vapor fluxes have been compared for brine temperature range of 65°C - 85°C and distillate temperature at 20°C. Pore size distribution (PSD) did not matter if the membrane pore size variation falls entirely in the Knudsen diffusion or the transition regime. However, for membranes having nominal pore size â?? 0.1 μm, K_n â?? 1, PSD played an important role in DCMD. The effect of membrane thickness on water vapor flux and thermal efficiency were also simulated and compared with the experimental results.