Enhanced performance of membrane distillation using radio-frequency induction heated thermally conducting feed spacers

In the last decade, there is a growing interest in membrane distillation (MD) technology for treating hypersaline water and wastewater. However, MD commercialization is still limited by technical challenges such as temperature polarization, high energy required for heating the bulk feed solution, and heat losses. We demonstrate a proof-of-concept approach to overcome these limitations using stainless steel thermally conducting feed spacers. Feed spacers were directly heated by induction heating, a contactless and instant heating method, using radio frequency (RF) altering electromagnetic fields. Results show that heat was efficiently transferred to the water-membrane interface by conduction and advection, thus, eliminating the need to continuously heat the bulk feed solution. Furthermore, the RF heated spacers were shown (experimentally and by numerical simulations) to reduce temperature polarization. The influence of operating conditions (i.e., flow velocity, vacuum pressure, and power) and the spacer's material and geometry were evaluated and results were compared to a conventional MD process using a polymeric spacer. Overall, results show that higher spacer mass and larger porosity led to an increase in distillate flux and that the use of thermally conducting spacers heated by RF significantly enhance the distillate flux while reducing the specific heating energy.