(355g) TR Polymers for Olefin/Paraffin Separation

Ethylene and propylene are currently the two largest, by volume, organic chemical feedstocks produced in the U.S [1].  These olefins must be purified for polyolefin synthesis by separating ethylene from ethane and propylene from propane, a thermally intensive process that requires approximately 0.12 Quads of energy per year and distillation columns containing nearly 200 trays [2].  This study focuses on determining the performance and plasticization resistance for a thermally rearranged (TR) polymer for olefin/paraffin separation. 

Polyimides containing reactive functional groups ortho-position to the diamine can be chemically transformed into TR polymers at elevated temperatures [3].  For this study, a polyimide was prepared from 3,3'-dihydroxy-4,4'-diamino-biphenyl (HAB) and 2,2'-bis-(3,4-dicarboxyphenyl) hexafluoropropane dianhydride (6FDA) via chemical, thermal, and solid-state imidization [4, 5].  Chemical imidization produces polyimides with ortho-functional acetate groups, while thermal and solid-state imidization produce polyimides with ortho-functional hydroxyl groups.  Furthermore, each imidization technique alters the non-equilibrium nature of the polymer film, thus producing polyimides with unique transport properties.  All films were characterized by ¹HNMR, DSC, FT-IR, TGA-MS, and pure gas permeation.

For ethylene, ethane, propylene, and propane, increasing polyimide conversion dramatically increased gas permeation.  For example, partial conversion of the chemically imidized HAB-6FDA polyimide to its corresponding TR polymer increased ethylene permeability by over two orders of magnitude.  Furthermore, ethylene/ethane selectivity decreased by less than 10%.  For ethylene/ethane and propylene/propane separation, HAB-6FDA TR polymers showed combinations of permeability and selectivity near the polymer upper bound [6, 7].  Propylene plasticization pressure curves were determined for TR polymers derived from all three synthesis routes, and the thermally imidized TR polymer had the highest plasticization pressure point of approximately 4 bar at 35°C.

1.         Facts & Figures: Output Declines in U.S., Europe, in Chemical & Engineering News. 2010. 54-62.

2.         Eldrige, R.B., Olefin/paraffin separation technology: A review. Industrial & Engineering Chemistry Research, 1993. 32(10), 2208-2212.

3.         Park, H.B., C.H. Jung, Y.M. Lee, A.J. Hill, S.J. Pas, S.T. Mudie, E. Van Wagner, B.D. Freeman, and D.J. Cookson, Polymers with cavities tuned for fast selective transport of small molecules and ions. Science, 2007. 318(5848), 254-258.

4.         Ghosh, M.K. and K.L. Mital, Polyimides: Fundamentals and applications. 1996, New York: Marcel.

5.         Ohya, H., V.V. Kudryavtsev, and S.I. Semenova, Polyimide membranes: Applications, fabrications, and properties. 1996, Amsterdam: Gordon and Breach Publishers.

6.         Burns, R.L. and W.J. Koros, Defining the challenges for C₃H₆/C₃H₈ separation using polymeric membranes. Journal of Membrane Science, 2003. 211(2), 299-309.

7.         Staudt-Bickel, C. and W.J. Koros, Olefin/paraffin gas separations with 6FDA-based polyimide membranes. Journal of Membrane Science, 2000. 170(2), 205-214.