Surface hydrophobicity based heat and mass transfer mechanism in membrane distillation

The investigations of how the membrane surface hydrophobicity influences the heat and mass transfer in direct contact membrane distillation (DCMD) and vacuum membrane distillation (VMD) were carried out theoretically and experimentally. Based on the Wenzel model and the Cassie-Baxter model, the models about the relationship between the real evaporation area and the apparent contact angle (a-CA) were deduced theoretically and then checked by the experimental values. The results show that the Wenzel model based theory could be applied to predict the evaporation area in both DCMD and VMD processes when the membrane surface hydrophobicity is poor, while for the high hydrophobicity membrane surface the Cassie-Baxter model based theory is more suitable and the real evaporation area is remarkably larger than the poor hydrophobicity membranes in the same operation conditions. Moreover, the improvement of the surface hydrophobicity can significantly influence the temperature and velocity distribution near the membrane surface and higher temperature polarization coefficient (TPC) and vapour polarization coefficient (VPC) values could be achieved with high hydrophobicity membranes than the poor ones, which indicates that the heat and mass transfer could be enhanced by improving the surface hydrophobicity.