The paper from Kim R. Kristiansen, Øivind Wilhelmsen and Signe Kjelstrup was accepted and published under “Desalination” (Volume 567, 1 December 2023, 116927), Congratulations!
Tittle: A systematic non-equilibrium thermodynamics approach for assessing transport mechanisms in membrane distillation
Membrane distillation (MD) is a promising technique for purifying volatile liquids at temperatures below their boiling points. This study presents a systematic non-equilibrium thermodynamics (NET) approach for analysis of transport mechanisms in direct-contact membrane distillation (DCMD). By incorporating the transport properties of the membrane, membrane interfaces, and temperature polarization layers, a unified framework is established to assess mass and heat transfer, including coupling effects in the composite membrane system. Explicit expressions for the transport properties are derived, and a numerical solution procedure is used to obtain temperature and partial pressure profiles through the system. The NET approach reveals that the temperature difference across the membrane is the true driving force of mass transfer, which after suitable approximations is equivalent to the saturation pressure difference. The commonly employed formula for distillate flux in MD literature was found to agree with the comprehensive NET approach within 3% when predicting the water flux through a DuraPore GVHP membrane. Incorporating a correction factor that accounts for kinetic heat-mass coupling effects improves the agreement to 0.5%. The simplified model disregards interfacial transport phenomena and mass-heat coupling. These effects are shown to be insignificant for the DuraPore GVHP membrane. However, it is crucial to account for temperature polarization to obtain agreement with experimental results. The resistance of the vapor-liquid interface is shown to be more important if the membrane has a Cassie-Baxter wetting state than a Wenzel wetting state, and this can enhance the water flux. When the pore-sizes approach the nanometer scale, we show that direct interactions between the molecules and the pore walls must be accounted for due to sorption effects. In applications where nanometer scale pores are important, such as in systems where the membrane must maintain a large pressure difference, it may be important to take such corrections into consideration. The presented NET approach provides a comprehensive toolkit for assessing and analyzing transport properties in membrane systems, which can be used to better understand how the properties of MD systems can be tailored to enhance performance.