Improved efficiency in conductive heating vacuum membrane distillation with polyurethane based hydrogels

Abstract

Membrane distillation is an emerging desalination technique that utilizes thermal membrane coupling; however, traditional processes experience temperature polarization due to heated feed, which diminishes flux. Recent advancements in local heating technologies, such as thermally conductive vacuum membrane distillation (CH-VMD), have demonstrated potential in enhancing flux. Nevertheless, the stable structure of water molecules requires substantial heat energy for evaporation, limiting improvements in flux. To tackle this, we integrated tailored hydrogels at the thermal conducting layer-feed interface within the CH-VMD system, optimizing water evaporation and reducing its enthalpy. Three polyurethane-based hydrogels were developed and characterized. The effect of these hydrogels on water evaporation efficiency in the CH-VMD system was examined, alongside variations in flux and salt rejection across different brine concentrations, while also assessing thermal efficiency and specific energy consumption. Raman spectroscopy confirmed the presence of intermediate water in the hydrogel. Differential scanning calorimetry indicated that the hydrogel decreased water’s evaporation enthalpy by 51 %. The CH-VMD system employing hydrogels made from polyvinyl alcohol, sodium alginate, and carboxylated carbon nanotubes achieved a permeation flux of 21.4 L/m²·h when treating 3.5 wt% high-salinity water, marking a 96.2 % increase in flux, an 8.3 % rise in thermal efficiency, and a 48.2 % reduction in specific energy consumption compared to conventional CH-VMD. Even with brine concentration raised to 10 wt%, the permeate flux was 19.6 L/m²·h, achieving a rejection exceeding 99.5 %. The mechanism employs hydrogels to increase the proportion of intermediate water with weak hydrogen bonds, facilitating its escape. Additionally, the –COOH group bonds with –OH, reducing thermal resistance and enhancing heat transfer, allowing for greater heat absorption for evaporation and higher flux. These findings demonstrate that integrating hydrogels significantly enhances the CH-VMD system’s efficiency for treating high-salinity water.