Journal of Membrane Science, Vol.580, 143-153, 2019
A modeling investigation on the thermal effect in osmosis with gap-filled vertically aligned carbon nanotube membranes
In recent decades, various nanoporous materials have attracted extensive interest in separation processes including osmosis-related phenomena due to their high permeability and significant screening ability. Osmosis is a ubiquitous transmembrane process defined in isothermal conditions, and the influence of solution temperature on the mass transfer process has been reported. However, temperature gradient as a driving force does not receive enough attention. In this work, we model the coupled heat and mass transfer across a gap-filled vertically aligned carbon nanotube (VACNT) membrane, considering the boundary layers and thermo-osmotic theory. The results show that the spontaneous temperature difference due to the mass transfer is small, even if the flux is very large. Therefore, this effect may not be measurable in practical applications. Meanwhile, because solvent can be driven down the temperature gradient through carbon nanotubes, we discuss the influence of solution temperature on flux and the possibility of desalination by heating the high-concentration solution. The result suggests that the following conditions are favorable for the purpose: a large thermo-osmotic coefficient, a large equivalent thermal resistance, a large temperature difference and a small osmotic pressure difference across the membrane This effect is almost independent on the membrane permeability. This work may help to explore the applications of membranes in desalination and separation processes with thermo-osmotic effect and ubiquitous low-grade thermal energy.
Keywords:Nanoporous material;Vertically aligned carbon nanotube membrane;Thermo-osmosis;Thermal effect