Modeling of heat and mass transfer in direct contact membrane distillation: effect of counter diffusion velocity

Membrane distillation is a non-isothermal separation process that uses a porous hydrophobic membrane. Water in the hot feed stream diffuses through the membrane in the form of vapor and condenses on the cold permeate side. Inside the porous membrane, the diffusion of water vapor is accompanied by the counter diffusion of air, which is often ignored in most studies. In this study, the role of counter diffusion velocity in a flat-sheet membrane contactor is analyzed using a two-dimensional model of direct-contact membrane distillation with a counter-flow configuration. Considering such a counter diffusion velocity, the simulation results of the total flux showed improved prediction accuracy in relation to experimental data in comparison with that in previous studies. The effects of different parameters, including feed inlet temperature and linear velocity, on gain output ratio (GOR), and transmembrane flux were investigated in detail. Our results indicate that an increase in the feed inlet temperature increases the total flux significantly. It is revealed that a higher linear velocity reduces the heat transfer resistance, which lowers the difference between the bulk and membrane interface temperatures. It was found that increases in both the feed inlet temperature and linear velocity enhanced the GOR. Using a sensitivity analysis, it was observed that membrane thickness had the strongest influence on the GOR and temperature polarization coefficient.