Dehydration of isopropanol and its comparison with dehydration of butanol isomers from thermodynamic and molecular aspects

By examining the dehydration of aqueous isopropanol (IPA) systems using two Sulzer commercially available polyvinyl alcohol (PVA)/polyacrylonitrile (PAN) membranes which were prepared from different cross-linking agents, we have (1) investigated the membrane performance from the standpoints of flux versus permeance and separation factor versus selectivity plots and (2) determined the effects of membrane properties such as degrees of cross-linking and hydrophilicity on water and IPA flux and permeance and their separation factor and membrane selectivity. PERVAP 2201 has higher degrees of cross-linking and chain packing than PERVAP 2510, while PERVAP 2510 has stronger hydrophilicity than PERVAP 2201. For PERVAP 2510, water permeance increases sharply with feed water concentration, but shows little dependence on temperature. For PERVAP 2201, water permeance shows flat relationship with feed water concentration, but increases with increasing temperature. Both IPA flux and permeance increase with increasing feed water concentration and their relationship with feed water content can be mathematically explained. It is noticed that the separation factor versus feed water content plots may mislead the analysis of water influence on membrane performance and exaggerate the plasticization phenomenon. Coupled transport has been observed between IPA and water. However, the degree of coupled transport is dependent on the membrane properties, such as the degree of cross-linking, affinity to water and structure responses on temperature rise. Compared with the dehydration of butanol systems, we found that the degree of coupled transport follows the order of 1-butanol > 2-butanol > IPA ≥ tert-butanol. After examining many physicochemical properties of penetrants and membrane materials, it is concluded that the molecular linearity (or the aspect ratio) of penetrant molecules and their solubility parameters (as well as polarity parameter) determine the magnitude of coupled transport, while the effects of other physicochemical properties seem to be not so important if coupled transports become dominant in the dehydration process.