A mathematical model was developed to describe mass transfer in a slit flow channel formed by a thin supported membrane at the bottom and an impermeable stainless steel block at the top in a lab-scale flat sheet pervaporation membrane module. Boundary layer theory was employed to obtain the mass transfer equations governing mass transport in the liquid boundary layer and in the membrane matrix. Equations of the model were solved numerically with computational fluid dynamics (CFD) techniques. The extent of concentration polarization and its impact upon permeation flux rate under different conditions were examined.