Heat and mass transfer analysis in direct contact membrane distillation

This work aims to provide a detailed analysis of the heat transfer in direct contact membrane distillation (DCMD). The influence of mass transfer on heat transfer flux was identified in the feed thermal boundary layer, across the membrane and through the permeate thermal boundary layer. A mathematical model was proposed to evaluate the experimental values of the thermal boundary layers' heat transfer coefficients, the membrane/liquid interface temperatures, the temperature polarization coefficient, the membrane mass transfer coefficient and the evaporation efficiency. This model was solved numerically using MATLAB (R) and it is based on direct experimental observations of the feed solution inlet/outlet temperatures and the mass fluxes. The obtained results showed that the mass transfer contribution to the overall heat transferred was significant only in the membrane region while it was negligible in both feed and permeate boundary layers. The membrane conductive heat transfer coefficient was estimated using trial and error procedure simultaneously with the developed model equations solution. The obtained values of the conductive heat transfer coefficient showed an agreement with experimental values reported in literature. The effect of feed temperature variation on the boundary layers' heat transfer coefficients, the temperature polarization coefficient, the evaporation efficiency, the membrane conductive heat transfer coefficient, the membrane mass transfer coefficient and the permeate flux was shown and fairly discussed.