Temperature as a factor affecting transmembrane water flux in forward osmosis: Steady-state modeling and experimental validation

Abstract

Forward osmosis (FO) represents a new membrane-based technology to desalinate seawater driven by osmotic pressure of solution spontaneously. Temperature is closely correlated to the solution physicochemical properties, and thus has a remarkable impact to transmembrane water flux of FO process. The primary goal of this study is to evaluate the dependence of FO performance on temperature. The water flux was predicted using steady-state models that incorporate temperature effect on osmotic pressure, hydrodynamics of boundary layer, transmembrane mass and heat flux processes. The experiments were performed in a FO module with sodium chloride serving as feed and draw solution. The results demonstrated a substantial positive correlation between water flux and bulk solution temperature in the range of 20–40°C. As indicated by both theoretical prediction and experimental validation, the improvement of mass diffusion kinetics, rather than solution osmotic pressure, dominated the FO performance in terms of water transmembrane flux when temperature was increased. This should be preliminarily a result of decrease in solution viscosity, which was consistent with the decrease in polarization resistances of boundary layer, supporting layer as well as active layer. With the increased flow rate, water flux was increased at all temperatures due to the improvement of mass transfer at boundary layer. The magnitude of heat flux was observed to positively relate to the flow rate and transmembrane temperature difference. Toward more efficient and energy-effective operation, this study not only provides an insight into the effect of temperature on transmembrane water flux, but also suggests a strategic importance of regulating temperature in FO process.