Modelling mass and heat transfers of Permeate Gap Membrane Distillation using hollow fibre membrane

Abstract

In this study, a set of mathematical models was used to simulate the heat and mass transfers of hollow fibre PGMD module for desalination applications. The developed model was firstly validated, and then utilized to study the impacts of different design parameters and operating conditions on the performance of hollow fibre PGMD. The simulation results show that coolant velocity and coolant temperature have less impacts on flux compared to those in DCMD, because in DCMD, coolant directly contacts the membrane but for PGMD it does not. The model also demonstrates that the higher cooling plate thermal conductivity will lead to higher flux and energy efficiency. However, when the cooling plate thermal conductivity is higher than 5 W/m.K, the temperature difference across the cooling plate is minimum and further increase of the cooling plate thermal conductivity has negligible impacts on flux and energy efficiency. A sensitivity analysis was undertaken to analyze the combined effects of gap channel inner/outer diameters and gap channel thermal conductivity on flux. It is concluded that the changes in gap channel of hollow fibre PGMD will lead to a more complex combination, and the gap channel thermal conductivity has a more significant effect on flux compared to the hydrodynamics within the permeate and coolant channels. The effect of multi-stage processes on energy efficiency is also evaluated. The results suggest that Gain Output Ratio (GOR) increases with number of stages, and reaches 2.4 with 20 stages. Finally, the roles of different parameters in PGMD optimization are discussed. The results suggest that cooling plate thermal conductivity plays the most important role in PGMD optimization compared to coolant velocity and coolant inlet temperature.