Effective Hydrogen Separation Using Ion Beam Modified Polymeric Membranes
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Effective Hydrogen Separation Using Ion Beam Modified Polymeric Membranes M. R. Coleman, X. Xu, J. Ilconich, J. Ritchie, and L. Hu, Univ. of Toledo, Dept of Chem. & Env. Eng., Toledo, Ohio. Abstract High purity H2 gas streams are increasingly important for a variety of applications including feed gases for fuel cells. The potential of hydrogen gas as primary energy source has generated considerable interest in hydrogen separation technologies. We have been investigating ion beam irradiation as a method to modify polymeric membranes to enhance both hydrogen permeability and permselectivities. Combined high permeabilities and permselectivities are required to give high recoveries of high purity hydrogen. Ion irradiation typically results in the formation of numerous crosslinks within the polymer matrix that should enable these materials to maintain selectivities at high temperatures and to resist chemical attack. Helium separations over a range of temperatures of irradiated polyimides were used as a model of the hydrogen system. Finally, the impact of irradiation conditions on gas separations in these materials will be addressed. Introduction and Background The purity requirements for H2 feed streams to fuel cells depend upon the fuel cell membrane. While a PEM fuel cell requires a very high purity stream with no more than 10 ppm CO and a solid oxide fuel cell require lower purity feed streams. The goal of this work is to develop membrane materials to recover H2 from reforming streams. Based upon economic analysis, a membrane with a H2/CO selectivity of at least 100 to 150 at the operating temperature would be required to provide a high purity hydrogen stream with 90 % recovery of the H2[1]. In addition, the material must maintain high selectivities at elevated temperatures. The majority of commercial membrane are polymeric because of their low material costs, ease of processibility and generally attractive gas transport properties (i.e. combined high permeance and permselectivity). While considerable progress has been made in developing membranes for a wide range of gas separations at low temperatures, the primary limitations to existing commercial membranes are fine separations of species at elevated temperatures or from feed streams that contain highly soluble penetrants[2]. The development of advanced membrane materials relies on the ability to manipulate the material microstructure to fulfill the following conditions: (i) high fractional free volume (FFV); (ii) narrow free volume distribution which can precisely sieve the given gas pair; and (iii) a rigid microstructure. Since synthesis of new polymers is both time consuming and quite expensive, the focus of much recent work has been developing post-synthesis modification techniques. We are investigating ion beam irradiation as a post-synthesis method to modify the structure and transport properties of thin polymer surface layers. With an appropriate choice of irradiation conditions (i.e. ion type, energy, and fluence), energetic ions can modify the surf
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