Factors contributing to flux improvement in vacuum-enhanced direct contact membrane distillation

• VEDCMD has higher water flux than DCMD and PEDCMD. • Higher flux with vacuum applied may be partially due to lower membrane compaction. • Higher flux with vacuum applied may be partially due to lower membrane air pressure. • Pressure differences due to vacuum or pressure enhancement has a minimal effect on flux. a b s t r a c t a r t i c l e i n f o Low water flux in membrane distillation (MD) is a concern for full-scale application. In the past decades, attempts have been made to improve water flux in MD and vacuum-enhanced direct-contact MD (VEDCMD) has been proven to be an effective configuration to achieve this. However, only qualitative assessments of the factors that might improve water flux have been reported in the literature. In this study, a mechanistic investigation of the factors contributing to higher water flux in VEDCMD was performed. Direct-contact MD (DCMD) and pressure-enhanced DCMD (PEDCMD) configurations were also investigated for comparison. Less membrane compaction was identified as one dominant factor contributing to improved water flux in VEDCMD as very little compaction occurred in VEDCMD compared to that which occurred in DCMD and PEDCMD. Lower air pressure inside the membrane pores was found to be the other dominant factor contributing to improved water flux in VEDCMD; the air pressure was calculated as the average of the feed and distillate pressures in VEDCMD and as the distillate pressure in DCMD and PEDCMD. Pressure difference, as is present in both PEDCMD and VEDCMD, was found to have a minimal effect on water flux. Membrane distillation (MD) is a thermally driven membrane process in which separation occurs through a phase change. The driving force in MD is the vapor pressure difference resulting from the temperature difference across the membrane. Because of the high latent heat of evaporation of water, MD is an energy-intensive process [1]. However, it can be combined with low-grade (" waste ") heat to reduce energy costs [2–8]. MD has advantages over conventional membrane processes (e.g., reverse osmosis) because the driving force of MD does not decrease significantly with increasing feed-water salinity [9]. This has inspired interest in MD for treatment of high-salinity brines [10–12] and complex feed waters such as flowback and produced waters in the oil and gas industry. More recent interest in MD results from the higher rejections of MD membranes over reverse osmosis …