The problem of membrane characterization for the process of osmotic distillation

The membrane used in the process of osmotic distillation (OD) is often considered passive as it simply provides a support for the vapour-liquid interfaces where evaporation and condensation take place and does not introduce any selectivity. However, the membrane is a major component of the system, not 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 depend on the membrane structure, but the conventional characterization methods do not always provide adequate information. In this paper, scanning electronic microscopy, mercury porosimetry and contact angle measurements were used to observe the porous structure and to characterize the hydrophobicity of two commercial membranes. The two first methods gave access to the structural parameters of each individual layer of the composite material. The membrane thickness, volume porosity and pore diameters appeared to be quite different from the values specified by the manufacturer that are obviously referring to a microfiltration application. These results allowed understanding the membrane's contribution to the mass transfer resistance in the process. 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 designed to characterize the microfiltration membranes employed for OD applications.