A ballistic transport model for vacuum membrane distillation

Vacuum Membrane Distillation (VMD) as other membrane distillation (MD) techniques is receiving much attention for its ability to produce drinkable water at low cost. Aiming to analyze the VMD process and particularly mass transfer through porous membranes, a ballistic flow model is developed to predict fluxes out from high aspect ratio cylindrical pores. The model is based on the estimation of direct and indirect transmission probabilities by direct integration of species flux. A power law that uses the pore aspect ratio is developed to enable a quick estimation of the flux fraction exiting the pore. The flux entering the pore is calculated using Hertz–Knudsen–Schrage equation and depends on the values of the condensation and evaporation coefficients of the transferred species. Simulation results are compared to classical Knudsen diffusion predictions as well as to experimental data.