A method to evaluate the net membrane distillation coefficient

A method is proposed that permits to evaluate the net membrane distillation coefficient. The method, which completes and is based on a model previously reported, emphasizes the influence of the unstirred boundary layers on the membrane distillation processes. The searched coefficient is obtained from a set of measurements of thermally driven fluxes, osmotically driven fluxes, and thermally and osmotically driven fluxes. The theory has been applied to the results obtained with two porous hydrophobic membranes, in different experimental conditions. The experiments show that the net coefficient decreases with the mean temperature and permits to quantify the temperature polarization effects. In addition, the heat of vaporization of water may be estimated. (C) 1998 Elsevier Science B.V.