| The dense ceramic oxygen-permeable membrane holds promise to reduce the oxygen production cost by～30%compared to the traditional cryogenic distillation process. It also holds promise to bring a step-change to oxygen-consuming industrial chemical processes such as production of syngas (CO and H2mixture) through partial oxidation of methane (POM). Our preliminary research has shown that the dual-phase composite membranes in hollow fiber geoemtry exhibits satisfactory oxygen permeability and long-term stability under POM-relevant conditions. This thesis is mainly devoted to studying the dual-phase composite hollow fiber membranes and exploring their POM reactor applications.Chapter1introduces an overview of the background and state-of-the-art applications of dense ceramic oxygen-permeable membranes as well as the preparations of the hollow fiber membranes and low-temperature sintering technique of oxide ceramics. The concepts and theories of the oxygen permeation for dense ceramic oxygen-permeable membranes are reviewed, and the research progress is introduced as well.In Chapter2, the effects of sintering agent CuO on the sintering behavior, oxygen permeation properties and long-term stability of Zr0.84Y0.16O1.92-La0.8Sr0.2Cr0.5Fe0.5O3-δ (YSZ-LSCrF) hollow fiber are investigated. The sintering temperature of the hollow fiber membrane was decreased significantly from1430℃to1300℃due to the use of the sintering agent. This is attributed to the formation of an yttria-copper-oxide (Y2CU2O5) low-melting phase produced through the reaction of CuO and Y2O3. The sintered hollow fiber was checked to be gas-tight, The as-prepared membrane possessed an asymmetric structure with outer diameter of1.80mm, inner diameter of1.12mm. The oxygen permeability of the hollow fiber was measured by exposing its shell side to atmosphere air and feeding the lumen side with pure CO (30mL-min-1). An oxygen permeation flux of2.9mL·cm-2·min-1(STP) was obtained under air/CO gradient at950℃for a hollow fiber of length44.60mm. The membrane remained intact during650-hour continuous measurements including three thermal cycles cooling down to room temperature and then heating up to950℃. The oxygen permeation was also measured under a large chemical oxygen potential gradient by changing the sweep gas from CO to CH4, an oxygen permeation flux of2.0mL·cm-2·min-1was obtained under air/CH4gradient was obtained. It was noted that the hollow fiber membrane prepared with CuO sintering agent exhibited higher oxygen peremeation flux under air/CH4gradient than the one prepared without using the sintering agent, which is attributed to the catalytic effect of CuO on the methane oxidation reaction.In Chapter3, the effect of surface modification of oxygen ion-conducting nano-particles on YSZ-LSCrF dual-phase composite hollow fiber membranes is investigated. YSZ-LSCrF hollow fibers with outer diameter of1.80mm and inner diameter of1.12mm were prepared by a combined phase-inversion extrusion and sintering method. The shell side surface of the hollow fibers was modified with Ce0.8Sm0.2O1.9(SDC) nano-particles via a drop-coating technique. The oxygen permeability of the hollow fiber was measured by exposing its shell side to atmosphere air and sweeping the lumen side with pure helium (He) and pure CO. When0.93mg·cm-2SDC nano-particles was coated onto the shell surface of the hollow fiber of44.60mm length, the oxygen permeation rate enhancements up to110%and50%was observed, yielding a maximum oxygen flux of0.32and4.53mL·cm-2·min-1under air/He and air/CO gradients, respectively. This indicates that oxygen permeation for the unmodified membrane was partially limited by the surface oxygen exchange kinetics. This limitation could be alleviated by coating an appropriate amount of SDC nanoparticles on the shell surface (oxygen reduction side) of the hollow fiber. A model has been proposed to calculate the length of triple phase boundaries (TPBs) for the coated dual-phase composite membrane. The variation of the TPB length with SDC coverage generally matches with the variation of oxygen permeation. The model can be applied to explain the effect of coating of SDC nano-particles on the surface exchange reaction and the oxygen permeability.In Chapter4, a POM membrane reactor based on the YSZ-LSCrF hollow fiber membrane is studied. For a hollow fiber coated with33wt.%NiO-LSCF catalyst at the lumen side, the POM performance was tested at elevated temperatures with the shell and lumen side exposed to the atmosphere air and methane, respectively. At a fixed ratio of injected methane to permeated O2of～2, the membrane reactor exhibited desired POM performance:CH4throughput conversion rate over88%, CO selectivity-97%and H2selectivity～96%. To improve the adherence of the catalyst to the membrane, a small amount of Na2SiO4·9H2O was added into the catalyst. As a result, the degradation of the CH4throughput conversion over the time was largely inhibited. The thickness of the Ni-based catalyst was also found to affect the performance of the POM reactor. With the thickness decreasing from～3.5μm to～20μm, the CH4throughput conversion decreased from～85%to～65%, while the degradation rate for the CH4throughput conversion reduced from0.92%/100h to0.12%/100h.In Chapter5, a novel dual-phase composite of SDC-LSCrF is explored for oxygen separation applications. The reason for choosing SDC is due to its higher oxygen ionic conductivity than that of YSZ. The SDC-LSCrF hollow fiber precursor was prepared by the phase-inversion and extrusion method and converted to a gas-tight hollow fiber after sintering at1550℃in the atmosphere air for20hours. The as-prepared hollow fiber with outer diameter of1.76mm, wall thickness of0.24mm exhibited a sandwich tri-layer structure, i.e. a central sponge-like layer of～140μm thickness between two finger-like layers of each thickness～50μm. The oxygen permeability of the hollow fiber was measured by exposing its shell side to atmosphere air and feeding the lumen side with pure CO2or CO at a rate of30mL-min-1. For a hollow fiber of48.00mm in length, the oxygen permeation fluxes of0.50and4.62mL·cm-2·min-1were achieved at950℃under air/CO2and air/CO gradient, respectively. Those values are higher than that of YSZ-LSCrF hollow fiber. During～650h continuous operation, the oxygen permeation fluxes kept stable, and the hollow fiber remained intact. Since the SDC-LSCrF membrane possesses satisfactory oxygen permeability and integrity under reactor-relevant operation conditions, it may find applications in O2/CO2oxyfuel and membrane reactor.In Chapter6, the manipulation methods of the structures (morphologies) and oxygen permeation performance of the dual-phase composithe hollow fiber are investigated. In particular, the rheology behavior of the initial slurry, structures and oxygen permeability of the hollow fibers were investigated in relation to the solid loading of the slurry, and for this purpose YSZ-La0.8Sr0.2MnO3-δ (LSM) dual-phase composite was used as a model system. It was shown that with increasing the solid loading in the slurry, initially the viscosity increased slowly. When the solid loading exceeds a critical value (Sl) of75.6wt.%, the viscosity increased rapidly. The hollow fiber (HF60) derived from the sluury of low solid loading of67.4wt.%exhibited a sandwich tri-layer structure:a20～30μm-thick central sponge-like layer between two～100μm-thick finger-like layers. For the hollow fiber prepared from a sluury of the critical solid loading, the sponge-like layer of the hollow fiber (HF90) became～100μm thicker, and the finger-like layer near lumen side became thicker while the finger-like layer near shell side became thinner. For the hollow fiber (HF120) prepared with a higher solid loading of80.5wt.%, the finger-like layer originating from the lumen side almost penetrated through the wall of the hollow fiber, while the finer-like layer near the shell side disappeared, and the sponge-like layer became as thin as～20μm. After sintering in atmosphere air at1380℃for10hours, all the hollow fibers were converted into gas-tight ceramic membrane, and the structures of the hollow fiber precursors were largely preserved. The porosities of the as-sintered hollow fibers, labeled as HF60, Hf90and HF120, are50.9±2.4%,49.4±0.8%and61.0±1.2%, respectively. And the corresponding mechanical strengthes for the three fibers were determined to be105.2±9.8,135.8113.5and39.8±5.7MPa, repectively. The oxygen permeability of the fibers was measured by exposing their shell side to atmosphere air and sweeping the lumen side with pure He (30mL·min-1). The oxygen permeation fluxes of0.23,0.28and0.37mL·cm-2·min-1were achieved under air/He gradient at950℃for HF60, HF90and HF120, respectively. The membrane HF90exhibited the best trade-off between the the oxygen permeability and the mechanical strength.In Chapter7, the summary of this dissertation is presented, and future research need indentified. |
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