A method is proposed that permits to evaluate the net membrane distill
ation coefficient. The method, which completes and is based on a model
previously reported, emphasizes the influence of the unstirred bounda
ry layers on the membrane distillation processes. The searched coeffic
ient is obtained from a set of measurements of thermally driven fluxes
, osmotically driven fluxes, and thermally and osmotically driven flux
es. The theory has been applied to the results obtained with two porou
s hydrophobic membranes, in different experimental conditions. The exp
eriments show that the net coefficient decreases with the mean tempera
ture 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.