Elucidating the impact of temperature gradients across membranes during forward osmosis: Coupling heat and mass transfer models for better prediction of real osmotic systems

The forward osmosis (FO) process is an emerging desalination, reuse, and dewatering technology that can operate off of a thermally regenerated draw solution. Using thermal energy to regenerate the draw solution, however, can lead to a draw solution with elevated temperature. Likewise, certain feed streams, including wastewaters from the food, agriculture, and oil and gas industries, may be elevated in temperature. While each of these concepts has been discussed in the literature individually, lacking from the field is an understanding of how gradients of temperature can impact FO performance. This investigation elucidates how temperature gradients impact water flux, reverse solute flux, and intrinsic membrane properties using both experiments and model based prediction. A model was developed that couples the effect of simultaneous heat and mass transfer across the membrane. The model uses local solution conditions (temperature, concentration) to calculate local solution properties (diffusivity, viscosity, density) that are important for mass transfer. Results show that the direction of water flux is the critical factor when understanding the impact of a transmembrane temperature gradient. Specifically, when the draw and feed differ in temperature, the feed solution temperature has a more profound impact on performance (especially water flux) than the temperature of the draw solution due to the importance of convective heat transfer caused by the mass transfer.