| The dense ceramic oxygen-permeable membranes hold promise to reduce the oxygen production cost by30%. The chemical reactors made from the membranes also hold huge economic potential through integration of oxygen separation and chemical reactions into a single space. For successful applications, the membranes are required to possess both high oxygen permeability and satisfactory stability under stringent operation conditions. It has been found that some perovskite transition metal oxides exhibit mixed oxide ionic and electronic conductivity and thus allow oxygen to permeate through, but the stability of these single-phase membranes is problematic especially for the membrane reactor applications. It has also been found that composite membranes made from an oxygen-ionic conductor and an electronic conductor demonstrate much better stability, but their oxygen permeability is poorer. The oxygen permeation performance of the dual-phase composite membranes can be improved significantly by fabrication of the membranes into an asymmetric planar structure in which a thin and dense oxygen separation layer is mechanically supported by a thick and porous layer. This planar membrane structure has another advantage in the membrane module can be built using the existing techniques developed for construction solid oxide fuel cells. This thesis is intended to investigate the dual-phase composite membrane materials, phase-inversion tape casting and oxygen permeation property of the supported membranes and their applications in membrane reactors.Charter1introduces the background of the dense ceramic oxygen-permeable membranes. A new variant of tape-casting method involving phase-inversion is also introduced for preparation of supported ceramic membranes.In Charter2, the stability and oxygen permeability of La0.8Sr0.2CrxFeo.5O3-δ (LSCrF) and Zro.84Y0.16O1.92-Lao.8Sro.2Cr0.5FeO.503.s (YSZ-LSCrF) are investigated. LSCrF powder exposed to diluted hydrogen was found to loss a weight of only-0.5%, corresponding to the formation of oxygen vacancies in the lattice.LSCrF powder exposed to flowing concentrated hydrogen for30h was found to decompose partially. The decomposition oxygen partial pressure of LSCrF was calculated to be6.3x10-28atm from the thermodynamic data. The stability of LSCrF under an oxygen chemical potential gradient was also examined by exposing a disk-shaped dense sample to air at one side and to reducing atmosphere (CO) at the other side at elevated temperatures. A thin, porous layer was formed on the low oxygen pressure side (CO). An oxygen permeation flux of0.37mL·cm-2·min-1was observed at950℃under given air/CO gradient. The occurrence of oxygen permeation revealed the presence of mixed oxygen ionic and electronic conductivity. The oxygen ionic conductivity was estimated to be-0.01S/cm at950℃, which is less10%of YSZ. The composite membrane made of LSCrF and YSZ exhibited smaller oxygen permeabiliy than the single-phase LSCrF but the stability was significantly improved.In Charter3, phase-inversion tape casting and oxygen permeation properties of Zro.84Y0.16O1.92-La0.8Sr0.2Cr0.5Fe0.5O3-δ (YSZ-LSCrF) are investigated. A supported planar membrane was prepared by phase-inversion tape-casting technique. The as-prepared membrane consists of a dense layer of thickness120μm supported by a porous layer of thickness880μm. The membrane with this asymmetric structure showed an improved oxygen permeation performance. An oxygen permeation flux of0.14mL·cm-2·min-1was obtained at900℃by exposing the porous support layer side to air and the dense layer to flowing helium. And the oxygen permeation flux was increased to0.41mL·cm-2·min-1when CO was used as sweep gas. Modification of permeate side of the membrane with a porous YSZ-LSCrF layer further raised the oxygen permeation flux to1.57mL·cm-2·min-1.In Charter4, the applicability of the YSZ-LSCrF membrane in partial oxidation of methane (POM) reaction is examined. The POM reaction was facilitated at elevated temperature with the porous side of the membrane exposed to air and the dense side methane. The membrane used had an area of4.5x4.5cm2, and to catalyze the POM reaction, the dense side of the membrane was coated with Ni-LSCrF catalyst. At875℃and CH4feed rate of30ml/min, the CH4conversion was92.3%, CO selectivity92.2%and H2selectivity93.9%, the H2/CO ratio2.0. The membrane reactor demonstrates excellent POM performance, and thus may find applications in methane pre-reforming for solid oxide fuel cells and conversion of methane to synthesis gas (a H2and CO mixture) required for production of hydrogen and liquid fuels.In Charter5, the fabrication method and oxygen permeation properties of the supported planar membranes composite with Zr0.84Y0.16O1.92-La0.8Sr0.2Mn03-δ (YSZ-LSM) dual phase are investigated. A supported planar membrane was prepared by phase-inversion tape casting technology, a dense layer with thickness of150μm supported on a thick porous layer with thickness of850μm. When the support layer was exposed to air and the dense layer was swept by helium, an oxygen permeation flux as large as0.26mL cm-2min-1was obtained at900℃, which is much larger than the supported membrane (0.05mL cm-2 min-1) prepared by conventional tape casting of the same composition and thickness. This may be due to the gas transport properties of the two support layers, the porous layer prepared by phase-inversion tape casting contains finger-like pores along the thickness direction, with an average diameter of a few tens of micrometers, and showed higher gas transport property. The pore of the other sample had a size of10micrometers, but pores were randomly distributed, the pore paths were tortuous, also unfavorable for gas transport.In Charter6, the phase-inversion tape casting and oxygen permeation properties of Ce0.8Sm0.2O1.9-La0.8Sr0.2MnO3-δ (SDC-LSM) composite planar membrane are reported. A supported planar membrane was prepared by phase-inversion tape casting technology, a dense layer with thickness of150μm supported on a thick porous layer with thickness of850μm.When the support layer was exposed to air and the dense layer was swept by helium, an oxygen permeation flux as large as0.188mL cm-2min-1was obtained at800℃, which is much larger than the supported YSZ-LSM membrane (0.07mL·cm-2·min-1).The supported planar SDC-LSM membrane shows appreciable oxygen permeability at intermediate temperatures, thus holds promises for application in production of oxygen.In Chapter7, the summary of this thesis is presented, and further research needs are identified including the optimization of the phase-inversion tape casting, characterization of the as-formed membrane,and the fabrication of membrane module. |
PDF Full Text Request