Characterization of membranes for membrane distillation by atomic force microscopy and estimation of their water vapor transfer coefficients in vacuum membrane distillation process

Abstract

Flat-sheet membranes for membrane distillation (MD) were prepared from solutions of polyvinylidene fluoride (PVDF) in dimethylacetamide by the phase inversion technique. Various amounts of water were added to the solutions as a non-solvent additive. The surface of the membranes were investigated by tapping mode atomic force microscopy (TM-AFM), to obtain mean pore size, pore size distribution, nodule size, pore density, surface porosity and roughness parameters. Pore sizes fitted well to the log-normal distribution function for all membranes studied. The pore sizes, surface porosity, nodule sizes and roughness parameters were found to be higher for membranes prepared with higher water content in the casting solution. The mean pore sizes obtained with TM-AFM were 1.2 to 2.1 times larger than those determined from gas permeation test. To study the effect of pore size distribution on the water vapor transfer coefficient of MD membranes, a theoretical transport model that considers the membrane pore size distribution was developed for vacuum membrane distillation (VMD). The predicted water vapor transfer coefficients were compared to those determined from measured VMD water vapor flux. The pure water vapor flux of the prepared MD membranes increased exponentially with the water content in the casting solution. Calculations considering the TM-AFM pore size distribution resulted in 1.2–2.0 times larger water vapor transfer coefficients than those obtained with mean pore sizes. The predictions with mean pore size obtained with TM-AFM were closer to the measured values, while those obtained from gas permeation test gave the best predictions. TM-AFM technique is not suitable for prediction of the MD membrane water vapor transfer coefficients.