High-Temperature Ethanol and Water Vapor Permeation in Thermally Rearranged (TR) Polymers

Production of fuel-grade ethanol from biological sources is conventionally accomplished through a combination of distillation and molecular sieve drying.  The process typically employs a beer still which converts the fermented product to a roughly 50%_wt mixture of ethanol and water.  A second column is used to increase the ethanol concentration to near the azeotrope, above 90%_wt.  Most of the remaining water is removed using molecular sieves, which must be regenerated periodically.  Polymer membranes have been proposed to replace the molecular sieves and the second column.  By performing the membrane separation using a vapor feed at elevated temperature, an energy savings of up to 43% is predicted compared with conventional production technology.

In this work, we present a high temperature permeation study of water and ethanol vapor in a thermally rearranged polyimide synthesized from 3,3'-dihydroxy-4,4'-diamino-biphenyl (HAB) and 2,2'-bis-(3,4-dicarboxyphenyl) Hexafluoropropane Dianhydride (6FDA).  Polyimide films were converted to polybenzoxazole (PBO) by heating at 450°C for 30 minutes.  Under these conditions, nearly 100% conversion to PBO occurred.  Chemical resistance of the PBO was investigated by exposing film samples to 50%_wt ethanol/water at 120°C and 3 atm for two weeks.  Light gas permeability and selectivity was measured before and after exposure, and FTIR-ATR was used to probe changes to the chemical structure.  Samples of the polyimide precursor exposed in this way were severely degraded, but fully converted PBO samples were little affected.  Permeation tests involving water and ethanol vapors were performed at temperatures ranging from 110-190°C and at pressures up to 6.4 atm.  At 110°C, water vapor permeability exceeded 3000 Barrer, and water/ethanol selectivity exceeded 60.  Water vapor permeability was found to decrease with increasing temperature, indicating that water sorption was affected more strongly by the changing temperature than was diffusivity.  On the other hand, ethanol permeability changed very little with temperature, indicating that the changes to sorption and diffusivity were more balanced for ethanol.  For elevated pressures, mild plasticization effects were apparent, causing permeability to increase and selectivity to decrease with pressure.