Gas flow enhanced mass transfer in vacuum membrane distillation

Developing cost-effective methods to enhance the mass transfer (i.e. water vapor flux) in vacuum membrane distillation (VMD) is of vital significance. In this study, we propose and demonstrate a simple but highly effective method to enhance the water vapor flux in VMD by introducing a gas flow on the vacuum side. The effect of the gas flow rate on the mass transfer performance in VMD was investigated by experimentation and computational fluid dynamics (CFD) simulation. Both experimental and modelling results confirm that gas flow on the vacuum side can effectively enhance mass transfer in VMD by minimising the boundary layer effect. A higher gas flow velocity results in a lesser boundary layer effect and reduced temperature polarization, thereby achieving a much higher water vapor flux. Specifically, when the gas flow rate is 10 L/min, the water vapor flux is eight times higher than that without gas flow (45.5 vs. 5.8 g/m2 h) on the vacuum side in VMD under the same vacuum pressure of 10 kPa. This study provides a cost-effective method to enhance the water vapor flux in VMD. It offers significant insights into understanding the mechanisms of vapor condensation and mass transfer in thermally-driven membrane processes.