Self-sustained webs of polyvinylidene fluoride electrospun nanofibers at different electrospinning times: 2. Theoretical analysis, polarization effects and thermal efficiency

A novel theoretical model that considers the gas transport mechanisms through the inter-fiber space of self-sustained electrospun nanofibrous membranes (ENMs) is developed for direct contact membrane distillation (DCMD). The theoretical model involves the structural characteristics of the ENMs, the heat transfer mechanisms and the nature of mass transport through the self-sustained web. The permeate fluxes of different ENMs prepared with different electrospinning times and therefore different thicknesses were predicted for different feed temperatures and sodium chloride salt concentrations up to 60 g/L. The used ENMs exhibit different parameters such as liquid entry pressure of water, inter-fiber space, void volume fraction, thickness, etc. In contrast to what reported in other theoretical MD studies considering Bosanquet equation with equal mass transport contributions for Knudsen diffusion and ordinary molecular diffusion, in this study the contribution of each mass transport mechanism was determined. It was found that the Knudsen contribution increases with the increase of the ratio of the mean electrospun fiber diameter to the inter-fiber space. The predicted permeate fluxes were compared with the experimental ones and reasonably good agreements between them were found. The temperature polarization coefficient (theta) and the vapor pressure polarization coefficient (psi) both increase with the thickness of the ENMs, whereas the concentration polarization coefficient ([beta) decreases indicating the dominant effect of the temperature polarization effect. beta was found to be higher for the ENMs having higher permeate fluxes and for greater feed temperatures, whereas it decreases slightly with the increase of the feed salt concentration. The thermal efficiency (EE) is enhanced with the increase of the feed temperature being in all cases for all studied ENMs greater than 78.8% and the heat transfer by conduction less than 20% of the total heat transferred through the ENMs