| The demand for energy is increasing due to the expansion in the global economy. Dense membranes made of ceramic materials that can conduct both oxide ions and electrons allow oxygen to permeate in the presence of oxygen partial pressure gradient. These membranes hold promise to bring about a step-change to the production of oxygen from air and oxygen-consuming industrial chemical process cleanly, efficiently and economically. In our previous work, Zr0.84Y0.16O1.92 (YSZ)-La0.8Sr0.2Cr0.5Fe0.5O3-δ (LSCrF) dual-phase composite membrane shows good stability and oxygen permeation flux under harsh reducing atmosphere at elevated temperatures. This thesis is mainly devoted to studying the structure optimization and preparation method of the supported planar YSZ-LSCrF composite membrane and exploring its oxygen permeability and membrane reactor applications.Chapter 1 introduces the concepts and theories of oxygen permeable membrane, the development of the materials, the fabrication and preparation methods and the applications. The proposal on this thesis is presented as well.In Chapter 2, a new variant of phase inversion tape casting method is explored for preparing supported planar membranes. Two slurries, one composed of YSZ and LSCrF powder, and the other composed also graphite, were used for preparation of the membrane. They were co-tape cast onto a Mylar sheet, and then immersed in a water bath for solidification into a green tape via phase inversion mechanism. After firing at 1450℃ in the air, the green tape was converted into a ceramic membrane. The sintered membrane possessed an asymmetric structure:a dense layer of thickness ～0 μm and a finger-like porous support of thickness ～1 mm. The oxygen permeability of the as-prepared membrane was measured by exposing its dense layer side to air and the support side to CO at elevated temperatures. An oxygen permeation flux as large as 1.3 ml·cm-2·min-1 was observed at 900℃. The membrane was further modified by applying a porous YSZ-LSCrF layer on its surface at the dense layer side and depositing Sm0.2Ce0.8O2 nano-particles on the inner surface of the support. The membrane exhibited desired oxygen permeability under the given measurement conditions. An oxygen permeation flux as large as 2.4 ml·cm-2·min-1 was observed at 900℃. Owing to its desired oxygen permeability and good stability, the supported planar YSZ-LSCrF membrane developed in the present study holds promise for applications in chemical reactors integrating oxygen separation and oxygen-consuming chemical reactions. The phase inversion tape casting method explored in the present study can be applied to the preparation of other ceramic membranes.In Chapter 3, the rate-determining steps of the oxygen permeation process for the supported YSZ-LSCrF membrane are investigated, and the method to promote the oxygen permeation process is explored. The thickness of the dense layer was varied in the range of 75-25μm by adjusting the blade gap for the tape casting. The oxygen permeability of the membranes was measured at elevated temperatures with the dense layer side exposed to air and the porous support side swept with CO to remove the permeated oxygen. At 850℃, oxygen permeation fluxes of 0.8,0.9,1.1 ml·cm-2·min-1 were observed for the membranes with a 75,50,25 μm thick dense layer, respectively. The oxygen permeation flux increased with decreasing the thickness of the dense layer appreciably, but the increment was much smaller than the extrapolated value assuming the linear dependence of the oxygen permeability on the reciprocal thickness, revealing that the overall oxygen permeation process was mainly controlled by the surface oxygen exchange step in the given thickness range. To enhance the surface oxygen exchange activity, the membrane was modified by applying a 10 μm thick porous YSZ-LSCrF layer on the dense layer side surface and depositing samarium doped ceria (SDC) nano-particles on the inner surface of the porous support. The membranes with modified surfaces exhibited much increased oxygen permeation flux. An increased flux of 2.1 ml·cm-2·min-1 was obtained at 850℃ for the membranes with a 25 μm thick dense layer, which were almost twice as large as that for the un-modified membrane. The membrane with a thin dense separation layer and modified surfaces shows much improved oxygen permeability, promising for practical applications.In Chapter 4, a POM membrane reactor based on the supported YSZ-LSCrF composite membrane prepared by phase inversion tape casting is studied. A supported planar YSZ-LSCrF membrane of effective area 6.8 cm2 was sealed to stainless holders and a Ni/Al2O3 catalyst bed was placed under the membrane. The POM performance of the planar reactor was tested by exposing its dense layer side to air and the support side to CH4. At 800℃, with air feeding rate of 200 ml/min and methane sweeping rate of 49 ml/min, the reactor attained methane throughput conversion over 88%, CO and H2 selectivity 94% and 93%, oxygen permeation rate 3.8 ml·cm-2·min-1 and syngas production rate 120 ml/min.In Chapter 5, a membrane reactor coupling CO2 decomposition and POM is studied. Supported YSZ-LSCrF dual-phase composite membrane was prepared by phase inversion tape casting and then surface modified. The reaction performance was tested by exposing its dense layer side to CO2 (and argon) and the support side to CH4 and argon using the same configuration in Chapter 4. At 850℃, with CO2 feeding rate of 30 ml/min and methane sweeping rate of 7.4 ml/min, the reactor attained methane throughput conversion 98%, CO and H2 selectivity 97% and 100%, oxygen permeation rate 0.46 ml·cm-2·min-1 and CO2 conversion 21%. The membrane reactor can facilitate the decomposition of CO2 into CO and O2 via shifting thermodynamic equilibrium, and effectively convert methane into syngas via POM reaction.In Chapter 6, a membrane reactor coupling the H2O splitting and POM reaction is studied. A supported YSZ-LSCrF dual-phase composite membrane was prepared by phase inversion tape casting. The reaction performance was tested by exposing its dense layer side to H2O(g) (and argon) and the support side to CH4 and argon. At 850 ℃, with H2O(g) feeding rate of 44 ml/min and methane sweeping rate of 6 ml/min, the reactor attained methane throughput conversion 98%, CO selectivity 96%, and H2 selectivity 100%. At the other side of the membrane,13% H2O was converted to hydrogen, and a H2 production rate of 0.86 ml·cm-2·min-1 was achieved. The membrane reactor concept explored in the present study is promising for practical applications.In Chapter 7, the summary of this dissertation is presented, and future research need indentified. |
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