Condensation, re-evaporation and associated heat transfer in membrane evaporation and sweeping gas membrane distillation

Abstract

Vapor transport through membranes is very important in many industrial processes such as membrane distillation (MD), membrane condensation and flue gas dehydration. In this study, we explore the condensation, re-evaporation and associated heat transfer in membrane evaporation and sweeping gas membrane distillation (SGMD) at low gas flow rates. It is found that both condensation and re-evaporation are closely related to the membrane properties. Condensation is more severe for the membrane with lower mass transfer resistance. The condensation layer takes place between the separation layer and the bulk gas in the module. The re-evaporation rate is determined by the total surface area of the droplets, the gas stripping velocity and the gas temperature. The overall mass transfer coefficient increases significantly with the increases of the gas flow rate, while the liquid flow rate has limited effect on the overall mass transfer coefficient. The overall mass transfer coefficient decreases with the increase in the evaporation temperature. The traditional way to quantify temperature polarization effect may not be applicable for SGMD at low gas flow rates because of vapor condensation. The increase in the gas flow rate or evaporation temperature can effectively improve evaporation efficiency (EE) in SGMD, while the liquid flow rate has limited effect on EE.