Heat and mass transfer in sweeping gas membrane distillation

Membrane distillation (MD) has been widely investigated the last two decades as a novel and promising technique for desalination. Four main configurations are commonly used and developed. Sweeping gas membrane distillation (SGMD) which is not enough investigated in the literature, consists of a hot saline solution and a binary gas flow (generally dry air and water vapor) separated by a hydrophobic membrane allowing just water vapor to pass. In this work, a numerical analysis is conducted to investigate the details of the heat and mass transfer in the channel where the sweeping gas flows. The physical model considers also the transport phenomena in the feed solution and the hydrophobic membrane. The axisymmetric flow field is modeled using the two dimensional steady-state partial differential equations expressing conservation of mass (overall and species), energy and momentum using constant fluid properties. Viscous dissipation, thermal radiation and Soret and Dufour effects are neglected. Appropriate boundary conditions are applied. Results are expressed in terms of velocity and temperature profiles as well as heat transfer coefficients. Pure water production and performance of the overall process are also investigated. The results show in particular that pure water production and thermal efficiency are very sensitive to inlet temperatures and inlet velocities of both flows (saline solution and sweeping air). For instance, increasing the sweeping air inlet velocity tends to enhance the gas heat transfer mechanism and to reduce that of the saline solution.