Review of thermal efficiency and heat recycling in membrane distillation processes

Membrane distillation (MD) is a promising desalination technology for water recovery from high-salinity solutions such as RO brine. Despite its great potential, MD has not been industrially realized because this technique generates a lower flux than reverse osmosis processes and exhibits a lower thermal efficiency than multi-stage flash. Enhancing the flux and thermal efficiency of MD is a major research focus for membrane researchers. In this study, the principles, transfer mechanism and thermal efficiency characterization of the MD process were introduced. Various factors affecting the thermal efficiency were analyzed. Several heat recovery techniques in the MD field that have been developed and widely employed were reviewed. © 2015 Elsevier B.V. All rights reserved.

In summary, to industrialize the membrane distillation process, the membrane flux and the thermal efficiency should be improved to decrease the required energy consumption per unit of distilled water. This paper presented the heat transfer mechanisms of membrane distillation, an in-depth analysis of various factors affecting the thermal efficiency of the membrane distillation process and a discussion of methods to improve the thermal efficiency. Additionally, membrane distillation studies on heat recovery from internal and external heat exchangers, multi-effect distillations and heat pumps were reviewed. The membrane structure of a membrane distillation process significantly influences the thermal efficiency. Increases in the porosity and the average pore size and decreases in the transmembrane path improve the thermal efficiency. Using a membrane with a low thermal conductivity can also reduce the transmembrane conductive loss.