Application of a porous composite hydrophobic/hydrophilic membrane in desalination by air gap and liquid gap membrane distillation: A comparative study

A first attempt was carried out comparing the two membrane distillation (MD) configurations, liquid gap (LGMD) and air gap (AGMD), using a porous composite hydrophobic/hydrophilic membrane, the same system and the same MD operating parameters. The surface modified membrane was prepared by the phase inversion technique in a single casting step using a fluorinated surface modifying macromolecule (SMM). Different membrane characterization techniques were applied. MD experiments were performed at different feed temperatures and sodium chloride aqueous solutions. The permeate fluxes were found to be slightly higher (2.2-6.5%) for LGMD compared to that of AGMD although the resistance to mass transfer in LGMD is higher due to the presence of the liquid permeate layer between the membrane and the cooling solid surface. This observed enhancement is attributed partly to the small established distance between the liquid/vapor interfaces at both side of the hydrophobic thin top-layer of the membrane in LGMD configuration, and the higher thermal conductivity of water, which is an order of magnitude higher than that of air, resulting in higher heat transfer coefficient of the permeate in LGMD. The salt rejection factors were found to be almost similar for both MD variants and higher than 99.61%. Compared to AGMD, the thermal efficiency is higher for LGMD, whereas the specific internal heat loss is lower. A linear increase of the thermal efficiency with the feed inlet temperature was observed for both MD configurations. The global heat transfer coefficient and the heat transfer of the permeate membrane side were also found to be greater for LGMD. The temperature polarization effect was found to be slightly higher for AGMD, whereas the concentration polarization effect was slightly higher for LGMD due to its higher permeate flux. In general, the LGMD proved to be more attractive than AGMD for desalination when using bi-layered hydrophobic/hydrophilic membranes.