Water vapor permeation in polyimide membranes

The study of the influence of water vapor in the feed stream of a mixed gas membrane separation system is of considerable practical importance. Water vapor may plasticize the membrane, it may undergo competitive sorption with other gas species and it can form clusters as it permeates. In this work, a modified mixed gas permeation system was employed to accurately measure the permeation properties of the polyimides 2,2′-bis(3,4′-dicarboxyphenyl) hexafluoropropane dianhydrid-2,3,5,6-tetramethyl-1,4-phenylenediamine (6FDA-TMPDA) and poly(3,3′-4,4′-benzophenone tetracarboxylic – dianhydride diaminophenylindane) (Matrimid® 5218) under exposure to humidified methane at 35 °C. This approach was then applied to further evaluate the permeation properties of the polymers in humidified mixtures of carbon dioxide and methane at 35 °C at atmospheric and elevated feed pressures. Water vapor permeabilities obtained at 2 and 7.5 bar total feed pressure, increased from 3200 to 3900 Barrer and from 20000 to 27000 Barrer as the water level increased for Matrimid and 6FDA-TMPDA respectively, reflecting increases in water vapor solubility and possibly plasticization and clustering effects due to the presence of water vapor. Conversely, the permeabilities of CH4 and CO2 declined due to competitive sorption of water.