| Dense membranes made of ceramic materials that can conduct both oxygen ions and electrons allow oxygen to permeate in the presence of oxygen partial pressure gradient. These membranes hold promise to bring a step-change to the production of oxygen from air and oxygen-consuming industrial chemical process. For example, a membrane-based reactor integrating oxygen separation and partial oxidation of methane (POM) into a single space is expected to significantly reduce the cost of syngas, an intermediate for production of liquid fuels, hydrogen and other value-added chemicals. For practical applications, the membrane materials are required to possess high oxygen permeability and sufficient stability under stringent operation conditions. In the previous study, it has been demonstrated that the composite membrane made of oxygen ionic conductor yttria-stabilized zirconia Zro.84Y0.16O1.92 (YSZ) and electronic conductor La0.8Sr0.2Cr0.5Fe0.5O3-δ(LSCrF) exhibits the required stability and that fabrication of the composite membrane into an asymmetric structure comprising a thin dense separation layer and thick porous layer can increase the oxygen permeation flux significantly. This thesis is thus devoted to the study of the preparation of the planar supported membrane via phase-inversion tape casting and to the development of the POM membrane reactor.Chapter 1 introduces the background and the application of oxygen-permeable membrane. The principle and concepts of oxygen permeation for dense ceramic membranes are reviewed. A phase-inversion tape casting method for the fabrication of membrane is also introduced.In Chapter 2, the phase inversion tape casting method has been improved for preparation of YSZ-LSCrF supported planar membrane. The planar membrane prepared by the phase inversion method consists of a relatively thin dense separation layer and a thick finger-like porous support. It was noted that a low porosity skin layer is present at top of the support, hindering gaseous molecules to transport into/out the finger-like pores in the support. In the present study, a membrane free of the low-porosity top layer was prepared by using the graphite as scarifying agent. A slurry composed of YSZ and LSCrF was co-tape cast with a slurry of graphite, and solidified into a green tape by immersion in the water bath. The as-formed green tape comprised a relatively dense layer at the top derived from the graphite slurry, a finger-like porous layer in the middle and a sponge-like layer at the bottom both derived from the YSZ-LSCrF slurry. After firing at 1420℃ in the air, the graphite layer was eliminated, and the other two layers were converted into ceramics. The resulting membrane consisted of a dense layer of thickness ～150μm providing the oxygen separation function and a finger-like porous layer of thickness ～850μm providing mechanical strength. Due to the use of graphite sacrificing layer, the finger-like pores in the support was fully opened up to the outer surface, allowing fast mass transport. The oxygen permeability of the two samples was measured by exposing its dense side to the ambient air and the porous support side to flowing helium at elevated temperatures. Oxygen permeation rate for the membrane without skin layer is 1.08 x 10-8molcm-2s-1 which is much larger than the one with a skin layer (2.57× 10-9molcm-2s-1). Apparently, the opening-up of the finger-like pores in the support of the membrane to the outer surface is beneficial to the gas phase transport and thus the overall oxygen permeation process. Furthermore an oxygen permeation flux of about 2.26×10-7molcm-2s-1 was obtained at 850℃ for the membrane without the skin layer when CO was used as sweep gas. The results of this chapter demonstrated that the preparation of the asymmetric permeable membrane using the improved phase inversion casting technique can significantly reduce the concentration polarization and increase the oxygen permeability of the gas permeable membrane. The oxygen permeation rate of the membrane still need to be improved if applied in industry.In Chapter 3, the effect of surface modification on the planar membrane was investigated. Since the dense separation layer of the membrane prepared by phase inversion tape casting is relatively thin, the overall oxygen permeation process is not limited by the transport of oxide ions and electrons in it. Instead, the oxygen reduction/evolution reactions are likely the rate-limiting step, which in turn could be promoted by proper surface modification. Therefore, three samples were prepared for the experiment, to promote the surface oxygen exchange rate, the surfaces of the phase-inversion derived planar membrane were modified by applying a thin porous layer of YSZ-LSCrF onto the surfaces of the dense separation layer and impregnating samarium doped ceria (SDC) nano-particles into the pores in the support. Oxygen permeation rates of 1.56×10-8molcm-2min-1,2.81×10-8 molcm-2min-1,3.83×10-8 molcm-2min-1 were obtained at 850℃ under air/He gradient for the membranes modified only at the dense side, support side or the both sides, respectively. Compared with the bare membrane, the respective increases were ～50%,～170%,-270%. When sweep gas was changed to CO, the oxygen permeation rate became one order of magnitudes higher than that obtained under the air/helium gradient. At 850℃, an oxygen permeation rate as large as 6.82×10-7 mol cm-2 s-1, equivalent to 1.0 ml (STP) cm-2 min-1, was obtained for the membrane modified on both sides. It is because that the permeated oxygen readily reacted with CO, reducing the oxygen partial pressure at the permeate side to a very low level and thus increasing the oxygen gradient across the membrane. No significant changes to the microstructure and phase composition of the membrane were observed for the membrane after the oxygen permeation test. The reasonably high oxygen permeability together with the satisfactory stability makes the membrane suitable for practical applications.In Chapter 4, the planar membrane reactor configuration was explored for partial oxidation of methane to syngas. A supported planar YSZ-LSCrF membrane of effective area 13cm2 was sealed to a stainless holder and a Ni/Al2O3 catalyst bed was placed under the membrane plane with a small slit between them. This reactor configuration would facilitate the POM reaction via oxidation-reforming mechanism: the oxidation reaction occurring at the membrane surface and the reforming reaction taking place in the catalyst bed. At 800℃ and methane feed rate of 32 ml/min, the reactor attained methane throughput conversion over 90%, CO and H2 selectivity both over 95%, and equivalent oxygen permeation rate 1.4 mlcm-2 min-1.The membrane and catalyst remained intact after the POM testing. The planar membrane reactor configuration explored in the present study may lead to the development of a compact reactor for syngas production.In Chapter 5, a novel membrane-based process is explored for co-generation of syngas and nitrogen from methane and air. In this process, an oxygen separation membrane is employed to construct a reactor integrating the separation of oxygen and nitrogen in the air and POM in a single space. The reactor comprised a planar membrane of YSZ-LSCrF with effective area 6.7 cm2 under which a Ni-based catalyst was packed. It was found that at 850℃ and methane feed rate of 22.8 ml/min and air feed rate of 55 ml/min, methane was transformed to syngas at a formation rate of 10mlcm-2min-1 and CH4 throughput conversion over 99%, CO selectivity 95%, H2 selectivity 99%; at the meantime, a nitrogen stream with concentration over 99% was obtained from the air side of the membrane. The co-generation of syngas and nitrogen may lead to development of a simpler and more energy-efficient process for production of ammonia and urea. The membrane reactor explored in this study may also be used to produce syngas with ideal hydrogen/CO ratio for synthesis of liquid fuels via the well-established Fischer-Tropsch process and at the same time to supply pure nitrogen for other uses.In Chapter 6, a POM membrane reactor short stack was constructed and examined. The stack consisted of two YSZ-LSCrF planar membranes with total effective area 16cm2 and a Ni/Al2O3 catalyst packed between the two membranes. At 850℃ and methane feed rate of 30 ml/min, oxygen permeation rate was 1.4 ml cm-2min-1.The reactor stack attained methane throughput conversion 76%, CO and H2 selectivity both over 75%. The POM performance of the short stack was somewhat poorer than the single membrane reactor, which was likely due to the unoptimized operating parameters such as the catalyst packing and the flow pattern in the stack. Nevertheless, this work has demonstrated the feasibility of the planar membrane reactor stack, paving the way for the scale-up of the membrane reactor.Chapter 7 summarizes the research conducted in this thesis, and presents recommendations of the membrane reactor for further research. |
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