Theoretical analysis of different membrane distillation modules

Abstract

Membrane distillation (MD) is a thermally driven separation process that uses hydrophobic membranes. Although MD has been considered as an alternative desalination technology, relatively little information is available on the design and optimization of MD modules. Accordingly, this study focused on the comparison of various MD modules and the optimization of their operation conditions. Direct contact MD, air gap MD, and vacuum MD were experimentally compared using laboratory-scale systems. Then, the results were theoretically analyzed using a simple transport model. Different mass transfer mechanisms inside the membrane were considered in the model, including molecular diffusion, the Knudsen diffusion, and viscous flow. Experimental results showed that the temperature dependences of MD modules were different. This suggests that the optimum feed temperature should be different for each MD module. The dominant mass transfer mechanisms were also identified using the theoretical model for better understanding the characteristics of the MD modules.