Ceramic Oxygen-Permeable Membrane Materials And Processes

Mixed oxygen ion electrical conductor membranes show considerable oxygenpermeation at high temperature with a theoretical selectivity of 100%. Ceramicoxygen permeable membrane reactor has broad application for oxygen separation andoxygen-involved technology. This thesis is to study several aspects of ceramic oxygenpermeable membranes reactor.Chapter 1 gives an introduction to ceramic oxygen-permeable membranematerials and processes. In particular, the methods for studying the surface exchangestep of oxygen permeation process are reviewed.In Chapter 2, the surface exchange process forGd_xCe_1-xO_2-x/2-δ(x=0.1,0.2)wasinvestigated by the pulse isotopic exchange method. The overall surface oxygenexchange rate (?)₀ was measured, and oxygen dissociative adsorption rate(?)_a aandoxygen incorporation rate(?)_i iwas derived from it based on the two-step oxygenexchange mechanism. The effects of Gd doping and oxygen partial pressure on thesurface oxygen exchange process were discussed.At 700°C and oxygen partial pressure of 0.21bar, the overall exchange rate (?)₀,oxygen dissociative adsorption rate (?)_a, and oxygen incorporation rate (?)_iofGd_0.2Ce_1。8O_2-δwere determined to be 1.70×10^-3molo m^-2 s^-1,1.95×10^-3mol_o m^-2 s^-1and1.31×10^-2molo m^-2 s^-1, respectively. Oxygen dissociative adsorption is suggested tobe the rate-limit step for Gd doped ceria samples.The apparent activation energy associated to the overall oxygen exchange processis close to that for electronic conduction, indicating that the overall oxygen exchangeprocess is limited by the electric charge transfer step.The dependence of surface exchange coefficientk *on oxygen partial pressurecan be expressed as followsk *∝( PO₂)ⁿ. For Gd_0.2Ce_1。8O_2-δ, n ranged 0.22-0.48. Thevalue for n was found to decrease with increasing temperature.In Chapter 3, the surface oxygen exchange behavior of Sm_xCe_1-xO_2-δ(x=0.1,0.2,0.3,0.4) was also investigated by pulse isotopic exchange method.At 800°C and oxygen partial pressure of 0.21bar, the overall exchange rate(?)_O,oxygen dissociative adsorption rate a, and oxygen incorporation rate (?)_iofSm_0.2Ce_1。8O_2-δwas determined to be 3.03×10^-3molo m^-2 s^-1,5.03×10^-3mol_o m^-2 s^-1and7.61×10^-3mol_o m^-2 s^-1, respectively. Oxygen dissociative adsorption was also revealedto be the rate-limit step for overall surface oxygen exchange process.The oxygen partial pressure dependence of surface exchange coefficient k*also followsk *∝( PO₂)ⁿ.In Chapter4, Ce_0.8Sm_0.2O_2δ(SDC)-La_0.7Ca_0.3CrO_3-δ(LCC) dual-phasecomposite hollow fibres were explored for oxygen separation membrane applications.Hollow fibre precursors were prepared by the phase-inversion/extrusion method usingindividual metal oxides and carbonates as starting materials, and were converted togas-tight ceramic by sintering at 1560°C in N₂ for 10 h. XRD analysis revealed thatthe sintered hollow fibre comprised of cubic SDC and perovskite LCC phase. SEMobservation showed a homogeneous distribution of SDC and LCC grains in the fibre.An oxygen permeation of 2.3×10^-7mol·cm^-2·s^-1was observed with a 26.7mm-longhollow fibre by exposing its shell side to atmospheric air and feeding He at 30ml/mininto its lumen side at 950 oC. A much higher oxygen permeation flux of 3.9×10^-6mol·cm^-2·s^-1was observed when the sweep gas was changed from helium to95%CO+5%CO₂. The membrane remained intact after the oxygen permeation test atelevated temperatures under air/CO gradient. Since the hollow fibre membraneexhibited high oxygen permeation flux and satisfactory stability, it holds promise formembrane reactor applicationsIn Chapter5, the partial oxidation of methane to syngas over perovskite SrTiO₃catalyst was studied with Sm_0.2Ce_0.8O_2-δ La_0.8Sr_0.2CrO_3-δdual-phase compositemembrane reactor. The catalyst showed good performance and high resistance tocarbon deposition. The catalytic performance strongly depended on both reactiontemperature and CH4feed rate. At 950°C and CH₄feed rate of 20 ml/minapproximately 74% CO and 75% H₂selectivity at 17% CH₄conversion wereachieved under the optimized membrane reactor operating conditions. Comparativestudy with conventional fixed-bed reactor suggested a direct methane partial oxidationroute below 750°C and CO₂and steam reforming mechanism above 800°C.In Chapter6, perovskite-type oxides La_0.7Sr_0.3Fe_1-yGa_yO_3-δ(y=0,0.3,0.5,0.7,1)were investigated as catalysts for methane combustion. The structure and properties ofthese catalysts were characterized by XRD, BET, XPS and TPR techniques. Thecomposition La_0.7Sr_0.3Fe_0.7Ga_0.3O_3-δshowed the highest activity for methane catalyticcombustion. At 600°C, 90% methane conversion was attained withLa_0.7Sr_0.3Fe_0.7Ga_0.3O₍3-δ_, while the methane conversion was as low as 10% withLa_0.7Sr_0.3GaO_3-δ. The Fe content in the mixed oxides had strong influence on theirstructure and surface characteristics which are correlated with their surface oxygenspecies and redox properties. In Chapter7, a summary of this thesis and recommendations for further researchare presented.