Analysis of heat and mass transfer in vacuum membrane distillation for water desalination using computational fluid dynamics (CFD)

Water shortage is a serious problem worldwide and will be even more critical in the next decades. Vacuum membrane distillation (VMD) is a separation process which has been widely studied for water desalination. The main merit of VMD over common thermal processes is its applicability in sub-boiling feed temperatures. A robust mathematical model can evaluate potentials of VMD industrial applications. A comprehensive model was developed in this study to investigate different VMD parameters in hollow fiber membrane modules. Due to its complexity, a numerical approach was considered for building the model. COMSOL multi-physics, which is a software package based on finite element method, was applied as the solver. A combination of Knudsen, free diffusion, and viscous flow were applied to obtain flow distribution in the membrane. Two vapor pressure models, the ideal solution model and the empirical model were applied to calculate vapor pressure. The effect of each model in evaluating transmembrane flux was investigated. Maximum error for empirical vapor pressure- based model was less than 15%. It was observed that at high temperature and for high concentration, ideal solution model exhibits significant deviation from the experimental results. The effect of concentration polarization on transmembrane flux was also investigated.