Desalination by vacuum membrane distillation: a numerical study on the effect of heat transfer correlations and slip flow

This paper deals with a numerical study of heat and mass transfer in a vacuum membrane distillation unit used for desalination. The governing equations expressing the conservation of mass, momentum, energy, and species for the saline solution with coupled boundary conditions, as well as the mass and energy balances on the membrane sides, were solved numerically. The slip boundary condition due to the membrane's hydrophobicity was also taken into consideration. The numerical model was validated with available experimental data and found to be in good agreement. The effects of operational parameters on pure water production and temperature polarization were presented and discussed. A comparison between three flat-sheet membranes commonly used in vacuum membrane distillation was conducted under the same operating conditions. In addition, a comparison between the Nusselt and Sherwood numbers obtained in this study and correlations commonly used in the literature was conducted, which revealed remarkable deviations between models along the channel. The impact of the heat and mass transfer coefficients on the transmembrane permeate mass flux variation is found to be important.