| The membrane made of mixed oxygen ionic and electronic conductor possesses infinite selectivity to oxygen,and holds potential for production of high-purity oxygen from air and manipulation of oxygen-related energy and chemical industrial processes.The composite of oxygen ionic conductor Zr0.84Y0.16O1.92(YSZ)and electronic conductor La0.8Sr0.2Cr0.5Fe0.5O3-?(LSCrF)has been found by this laboratory to demonstrate both appreciable oxygen permeability and outstanding chemical and mechanical stability.This thesis is intended to further improve the oxygen permeability and robustness of the membrane by optimizing its geometry and pore structure,and to explore its applications in chemical reactors for reforming ethanol and methane into sysgas(a mixture of hydrogen and carbon dioxide).In Chapter 1,the working principle,materials composition,geometric configurations for the oxygen separation membrane and its applications in chemical reactors and processes are reviewed.The principle of synchrotron vacuum UV photoionization mass spectroscopy(SVUV-PIMS)and its applications in identification of chemical reaction mechanism are introduced as well.Finally,the scope of this thesis is described.In Chapter 2,the preparation method and the oxygen permeation properties of the YSZ-LSCrF membrane with symmetrically sandwiched geometry and an ultra-thin dense function layer was investigated.A slurry containing YSZ-LSCrF powders and a slurry containing graphite were co-tape casted onto a Mylar sheet,and transferred into water bath for solidification via the phase inversion mechanism.The resulting green tape possessed a three-layered structure:a relatively dense skin layer,a finger-like support layer and a sponge-like layer.Two green tapes were laminated with their skin layers facing each other,and sintered at 1420?.The as-prepared membranes consisted of a dense layer of thickness 5 ?m sandwiched by two porous layers of thickness 300?m,Note that the ultrathin dense layer in the membrane would present a much reduced resistance to the transport of oxide ions and electrons,and the two thick porous layers would provide mechanical support to the functional layer while allow fast gas phase transport.Note also that the sponge layers in the green tapes were composed of graphite which was removed during the high-temperature sintering process.As a result,the finger-like pores in the support layers were fully opened.The oxygen permeability of the membrane was measured by exposing one side of the membrane to air at flow rate 100 mL min-1 and the other side to CO at 45 mL min-1.At 900? an oxygen permeability of 1.95 mL(STP)cm-2 min-1 was observed.The membrane underwent over 400 times of thermal cycles(15? min-1),and no obvious air leakage was detected.The excellent thermal shock resistance of the membrane was ascribed to its symmetric geometric structure.The preparation method developed in this study could be applied to other ceramic membranes.In Chapter 3,the oxygen permeation process through the YSZ-LSCrF membrane was investigated.The permeation process consists of two steps,namely,exchange of oxygen crosses the gas/membrane surface and transport of oxide ions and electrons in the bulk of the membrane.In this study,the electrical conductivity relaxation(ECR)was adopted to study the surface process.At 800?,the surface oxygen exchange coefficient k was determined to be 9.0 × 104 cm s-1,which was raised to 22.5 × 10-4 cm S-1 after surface modification with Sm0.2Ce0.9802-?(SDC)nanoparticles.SDC modification significantly reduced the activation energy for the surface oxygen exchange:116.3 and 43.3 kJ mol-1 for the unmodified and SDC-modified membrane,respectively.The rate-limiting step for oxygen permeation through the membrane was identified by analyzing the dependence of oxygen permeation on the thickness of the dense layer in the membranes.For this purpose,membranes with a dense layer of thickness 30,60,90,130 ?m were prepared by phase-inversion tape casting/lamination/sintering process.Oxygen permeation through the membranes was facilitated by exposing one side to air and the other to helium.The oxygen permeation rate under the air/He gradient was found to be inversely proportional to the thickness of the dense layer in the range 130-60 ?m,indicating that the bulk transport step was the rate-limiting step in the given thickness range.As the thickness was further decreased to 30 ?m the increase in oxygen permeation rate was smaller than the extrapolated value,indicating that the oxygen permeation process was jointly controlled by the bulk transport and surface oxygen exchange steps.The oxygen permeation process under the air/CO gradient was also examined.It was revealed that when the thickness was decreased to 90 ?m,the overall oxygen permeation process was jointly limited by the bulk and surface steps.In Chapter 4,a membrane-based process for converting ethanol into syngas was explored.A reactor was constructed,comprising a sandwich-structured YSZ-LSCrF membrane(effective area 5.0 cm2)with a dense layer of 30 ?m and catalyst Ni-Ru/Al2O3.Air was fed at a rate of 350 mL min-1 on one side of the membrane while gaseous ethanol was fed at rate of 30 mL min-1 on the other.Ethanol was partially oxidized by the permeated oxygen,yielding syngas.At 800? the reactor attained ethanol conversion 83.5%,selectivities for H2,CO,CO2,and CH4 being 82.6%,81.7%,5.3%,and 13.0%,respectively.The corresponding syngas(H2+CO)production rate was 103.1 mL min-1,equivalent to 20.6 mL min-1 cm-2(membrane area).When ethanol and steam were co-fed into the reactor,the partial oxidation reaction of ethanol was coupled with steam reforming of ethanol.At the ethanol feed rate 30 mL min-1 and steam feed rate 54 mL min-1(H2O/Ethanol=1.8),the ethanol conversion was increased to 99.9%,the selectivities for H2,CO,CO2,and CH4 being 90.8%,74.6%17.5%and 8.0%,respectively.The corresponding syngas production rate increased to 138.7 mL min-1,equivalent to 27.7 mL min-1 cm-2(membrane area).The enthalpy for the overall reaction was examined with various H2O and ethanol total feed rate at fixed H2O/Ethanol ratio(=1.8).It was shown that at total feed rate 65.7 mL min-1,the enthalpy of the overall reaction was close to zero,thus the membrane reactor could be thermally self-sustained.A solid oxide fuel cell(SOFC)running on the effluents from the membrane reactor exhibited a maximum power density of 999 mW cm-2 at 750?,which was just slightly lower than the one running on pure hydrogen(1099 mW cm-2).This study demonstrates the feasibility of the use of ethanol reforming membrane reactor as fuel pre-reformer for SOFC.In Chapter 5,the membrane-based process for production of syngas from methane was investigated.A reactor was constructed,comprising a sandwich-structured YSZ-LSCrF membrane(effective area 7.8 cm2)with a dense layer thickness of 5?m and catalyst Ni-Ru/Al2O3.Air was fed into the reactor(at rate 350 mL min-1)on one side of the membrane,with methane on the other side,and partial oxidation of methane(POM)occurred to produce syngas.The POM reaction was examined by varying the O/C ratio in the reactant.As the methane feed rate increased from 20 mL min-1 to 60 mL min-1,the O/C in system decreased from 2.13 to 1.03.The methane throughput conversion decreased from 94.4%to 71.9%,and the selectivities for H2 and CO increased from 60.4%and 53.6%to 83.3%and 90.2%,respectively.The corresponding syngas production rate was raised from 33.2 mL min-1 to 110.0 mL min-1,equivalent to 4.3 and 14.1 mL min-1 cm-2(membrane area),respectively.The POM reaction coupled with steam reforming of methane(SRM)was explored with the membrane by co-feeding methane and steam into the membrane reactor.With methane and steam both at 60 mL min-1,the methane throughput conversion and hydrogen selectivity reached 96.1%and 98.4%,respectively.The membrane reactor operated stably for 120 hours,which was ascribed to the reduced coke formation due to the presence of steam in the reactants.Apparently,operating the membrane reactor in POM-SRM mode is not only beneficial for increasing the methane conversion and hydrogen selectivity,but also helpful for improving the durability of the catalyst and the reactor.In Chapter 6,SVUV-PIMS was applied to the study of POM reaction.First,non-catalytic POM reaction was investigated.Methane and oxygen were co-fed into the quartz tube at 700?,and the following species were detected:CH3·,HCHO,CH3OH,C3H4,C3H6,C2H2O,C2H5OH,OH·,H2O,CO and CO2.Then,catalytic POM reaction was investigated by packing Ni/Al2O3 catalysts in the quartz tube.The species were found to be the same as those detected in the non-catalytic POM reaction.Compared with the non-catalytic POM reaction,the concentrations for the carbonaceous species especially the HCHO were higher with smaller amounts of H2O.Moreover,the HCHO concentration increased with increasing CH4 to O2 ratio at a fixed total feed rate,while the concentration of CH3OH remained small.These observations suggest that the main reaction occurring in non-catalytic POM was cracking reaction of methane to carbon and hydrogen,while the main reaction for the catalytic POM was the dehydrogenation of CH3OH to HCHO followed by decomposition into CO and H2.The as-produced H2 could be further oxidized to H2O while CO might undergo oxidation reaction(generating CO2),disproportionation reaction(generating C and CO2)and water gas shift reaction(generating CO2 and H2).The side reactions were the decomposition of CH3OH(generating CH3· and OH·)and the reaction of CH3·and CH3OH(generating C2H5OH).The CH3· could be further dehydrogenated(generating CHx),leading to series of reactions(generating C,CH2CO,C3H6,C3H4).It was further suggested that the conversion of methane into syngas in the presence of catalyst was limited by the decomposition of HCHO.In Chapter 7,the summary of this dissertation was presented,and future research was proposed. |
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