Enhanced performance of direct contact membrane distillation via selected electrothermal heating of membrane surface

Abstract

Membrane distillation (MD) is a thermally driven separation process with great potential, but is currently limited by low energy efficiency. Heating of the entire circulating feed represents a major source of energy consumption in MD. Here, we present electrically conductive carbon nanostructure (CNS-) coated polypropylene (PP) membranes as a possible candidate to mitigate energy consumption through selected electrothermal heating of the membrane surface. A membrane for MD was coated with CNS using a tape casting technique. The resulting CNS-PP membrane is hydrophobic, and its smaller pore size and narrow pore size distribution resulted in a higher liquid entry pressure compared to the uncoated PP membrane. An increase in surface temperature was observed when a current was passed through the conductive CNS layer. The CNS layer on the PP membrane acts as an electrothermal heater when an AC potential is applied, and the rate of heating is proportional to the amplitude of applied AC potential. We applied electrothermal heating of these membranes to desalination by direct contact membrane distillation, in conjunction with heating of the circulating feed, and compared the performance with and without application of AC bias at three feed temperatures viz. 40, 50 and 60 °C. Applying a potential across the CNS layer increased permeate flux by 75, 76 and 61% at feed temperatures of 40, 50 and 60 °C respectively, while maintaining a salt rejection of >99%. This increase in flux is accompanied by a reduction in specific energy consumption of greater than 50% for all three feed temperatures. By combining electrothermal surface heating with MD, this study paves the way for smart, low-energy MD systems.