Desalination Advancement by Membrane Water Heating in Vacuum Membrane Distillation

Abstract

Membrane distillation (MD) is a technology that can help address the global freshwater shortage by desalinating seawater or brine. However, traditional MD methods require a hot feed, which leads to temperature polarization (TP) and reduced flux. To overcome this limitation, we developed a vacuum MD system that transfers external heat to the membrane-water interface. In this study, we also investigated the inorganic scaling that can occur in this system. To optimize the system, we tested different materials for the thermal conducting layer and found that an aluminum shim provided stable performance with good corrosion resistance. By adding a spacer to the cell, we were able to increase the flux to 6 L/m²·h by promoting turbulence in the water vapor channel. In addition, the use of the shim resulted in a TP coefficient exceeding 1.0, indicating that the TP was reversed. After conducting desalination experiments using this system, we found that the salt rejection was above 99.98%. The flux decreased due to inorganic salt scaling, and calcium foulant incurred a faster flux drop rate than magnesium foulant. The formation of calcium sulfate was found to be critical to membrane fouling. We also discovered that higher thermal conducting layer temperature and feed concentration resulted in a faster crystallization process, which led to an earlier flux critical point. Fortunately, we were able to effectively recover the initial flux of the membrane by cleaning it with deionized water. This novel vacuum MD system has the potential to further advance thermal desalination technology.