Dense Oxide Membranes for Oxygen Separation and Methane Conversion
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DENSE OXIDE MEMBRANES FOR OXYGEN SEPARATION AND METHANE CONVERSION A. J. JACOBSON, S. KIM, A. MEDINA, Y. L.YANG AND B. ABELES, University of Houston, Department of Chemistry, Houston, TX 77204-5641. ABSTRACT Methane conversion to synthesis gas in membrane reactors that use dense mixed electronicionic conducting membranes for oxygen separation has received much recent attention. The oxygen flux achievable in these reactors depends on a combination of the bulk diffusion rate for oxygen transport and the surface reaction rate for oxygen activation and either recombination or reaction with methane. Here we compare recent oxygen permeation data for tubular membranes of Lao. 5 Sro.5Feo.8Gao.203-8 with our previous results for SrCo0. 8 Feo.203-8, Sm 0 .5Sr 0 .5CoO3-5, SrFeCo0.5 0 3 .25 -8. The pressure dependence of the oxygen permeation flux has been measured at different temperatures and used to determine the relative importance of bulk and surface kinetics for these oxides. INTRODUCTION Mixed-conducting oxides with high oxygen ion conductivities can form the basis for ceramic membranes that separate oxygen from air. Such membranes are also of interest for their potential use in membrane reactors that can produce synthesis gas (CO+H 2 ) by direct conversion of hydrocarbons such as methane. For this application, materials are required to have stability in reducing atmospheres, appropriate mechanical properties as well as high oxygen permeability. Electronic conductivity must also be maintained at the low oxygen partial pressures (_ 10-T7 atm) on the hydrocarbon side of the membrane. Since the early results of Teraoka et al. [ 1, 2] several groups have investigated the use of mixed electronic-ionic conducting perovskite oxides and related materials as membranes for these applications. [1-8] It is now well established that in general the oxygen flux achievable depends on a combination of the bulk diffusion rate for oxygen transport and the surface reaction rate for oxygen activation and either recombination or reaction with methane. The bulk diffusion rate depends on the coupled transport of oxygen vacancies and electron holes both of which depend on the specific metal atom substituents present in a particular oxide system and on the oxygen nonstoichiometry. The latter depends on the temperature and the oxygen partial pressures on either side of the membrane. The surface reaction rates also depend on the specific oxide composition and the oxygen stoichiometry but the surface kinetics are less well understood than the bulk diffusion. Kilner has shown [9] that the surface exchange coefficients (k) measured by isotope exchange under equilibrium conditions at high oxygen partial pressures (0.2 -1 atm) for both perovskite and fluorite structure oxides correlate with the diffusion coefficients (D) measured under the same conditions. For the fluorite oxides the slope of a plot of logk versus logD is close to 1 whereas for the perovskites the corresponding slope is 0.5. Little information is currently available about perovskite oxid
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