The problem of membrane characterization for the process of osmotic distillation

The membrane used in the process of osmotic distillation (OD) is often cons idered passive as it simply provides a support for the vapour-liquid interf aces where evaporation and condensation take place and does not introduce a ny selectivity. However, the membrane is a major component of the system, n ot only because it constitutes one of the resistances to mass transfer but also because the non-wettability of this component conditions the viability of the process. The mass transfer as well as the non-wetting condition dep end on the membrane structure, but the conventional characterization method s do not always provide adequate information. In this paper, scanning elect ronic microscopy, mercury porosimetry and contact angle measurements were u sed to observe the porous structure and to characterize the hydrophobicity of two commercial membranes. The two first methods gave access to the struc tural parameters of each individual layer of the composite material. The me mbrane thickness, volume porosity and pore diameters appeared to be quite d ifferent from the values specified by the manufacturer that are obviously r eferring to a microfiltration application. These results allowed understand ing the membrane's contribution to the mass transfer resistance in the proc ess. The contact angle theory originally designed for smooth ideal surfaces did not provide a satisfactory representation of the wetting behaviour of porous membranes. The wettability of such materials has to be characterized experimentally under the operating process conditions. The results of this study emphasize the lack of appropriate physical methods specifically desi gned to characterize the microfiltration membranes employed for OD applicat ions.