Ceramic Oxygen Permeable Membrane Reactor For Fuel Reforming Applications

Some ceramic materials can conduct both oxide ions and electrons at elevated temperatures,thus they are permeable to oxygen,and can be used as membranes to separate oxygen from the air.One potential application of these dense ceramic membranes is in the economic and compact production of syngas(mixture of H2 and CO)by integrating partial oxidation(POX)of methane and separation of oxygen in a single space.Research conducted in our group has demonstrated that the composite membrane made from Zr0.84Y0.16O1.92(yttria-stabilized zirconia,YSZ)and La0.8Sr0.2Cr0.5Fe0.5O3-?(LSCrF)exhibits excellent stability in CO2 and reducing atmosphere.The planar membrane prepared by the phase inversion tape casting method consists of a thin dense oxygen-permeable layer and a mechanical support with large straight open pores,showing a fairly high oxygen permeation flux under the conditions relevant to the POX reactor.A reactor configuration comprising a planar membrane and catalyst bed with a small slit between them shows satisfactory performance for conversion methane to syngas.It is suggested that the total oxidation reaction occurs on the membrane surface,while the reforming reaction takes place in the catalyst bed,enabling effective mass and heat transfer.This thesis is to extend the research on the membrane reactor from methane to compressible fuels(propane)and liquid fuels(ethanol).To advance the membrane technologies,planar and flat tube geometry are developed as well.In Chapter 1,the working principle of ceramic oxygen permeable membranes is described,and the progresses in the research and development of membrane reactors for syngas and hydrogen production is reviewed as well.In Chapter 2,a membrane reactor for conversion of propane to syngas was investigated.A YSZ-LSCrF planar membrane of area 2.6×2.6 cm2 was sealed to two stainless steel molds to form two chambers.Air was fed into the upper chamber,and propane to the lower chamber to react with the permeated oxygen in the presence of Ru-Ni/SDC catalyst.The effect of the operating temperature on the oxygen permeation of the membrane was examined.With temperature increasing from 825 to 875 ?,the oxygen permeation rate was raised from 5.1 to 6.6 mL cm-2 min-1.Then,the influence of the carbon-to-oxygen molar ratio(?=C/O)(on the permeate side)on the performance of the membrane reactor was investigated.At 850 ?,propane feed rate 15 mL min-1,air feed rate 250 mL min-1,the value of ? was?1,and the membrane reactor achieved the best performance with propane throughput conversion 94%,the hydrogen yield 89%,carbon monoxide yield 85%,and the syngas production rate 89 mL min-1(equivalent to 22.2 mL cm-2 min-1).Under the given conditions,the membrane reactor operated stably during the 8-hour testing.The as-reformed propane was used as fuel for SOFC,and compared with pure hydrogen.The fuel cell showed comparable maximum power density with the two different fuel.The fuel cell run stably with the reformed propane at 800 ? for 40 hours,but it failed within 2 hours with the un-reformed propane.In Chapter 3,a membrane-based POX process for conversion of ethanol to syngas was proposed and experimentally verified.The structure of the membrane reactor was the same as that described in the previous chapter,while a different catalyst Ni/Al2O3 was used.The oxygen permeation increased with temperature as expected.At 750 ?,the oxygen permeation rate was 2.1 mL cm-2 min-1,and increased to 3.4 mL cm-2 min-1 at 800 ?,and 5.6 mL cm-2 min-1 at 850 ?.The membrane reactor demonstrated satisfactory performance at 800 ?,ethanol feed rate 30 mL min-1 and air feed rate 250 mL min-1.The throughput conversion rate of ethanol was as high as 93%,and CO selectivity 95%and H2 selectivity 97%.Since the ethanol fuel usually contains a certain amount of water,and to investigate the effect of the water on the performance of the POX reactor,steam was co-fed into the reactor.It was found that with increasing the steam feed rate,the CO concentration decreased while CO2 concentration increased,due to the increased water-shift reaction.In Chapter 4,a membrane-based novel process was proposed and experimentally verified for co-production of nitrogen from air and syngas from methane.In this process,oxygen in air is extracted through a dense oxygen-permeable membrane,which is then reacted with methane.By optimizing the air and methane flow rate,a nearly pure nitrogen as well as a syngas was obtained.At 800?,the reactor produced nitrogen at a rate of 9.2 mL cm-2 min-1 with purity over 99%,and methane was reformed to syngas with CH4 throughput conversion over 90%,H2 selectivity 92%and CO selectivity 92%.The syngas could be burned to generate heat or used as intermediate chemicals for production of liquid fuels and hydrogen.Since the membrane reactor is driven by the energy released by the reaction and does not consume high grade energy electricity,it has a much higher overall energy efficiency than the current industrial nitrogen separation processes.In Chapter 5,a POX reactor comprising a planar membrane was investigated for conversion of methane to syngas.The planar YSZ-LSCrF membrane was prepared using the tape casting-lamination method.The membrane consisted of thin dense layer sandwiched by a thick porous support layer,showing few defects and high mechanical strength.The support layer possessed disordered pore structure,imposing a large resistance to the gas phase transport.Consequently,the oxygen permeation rate of the membrane was relatively low.At 800 ?,an oxygen permeation rate of 1 ml cm-2 min-1 was obtained under CH4/Air gradient.The membrane reactor was comprised of five 10 x 10 cm2 YSZ-LSCrF single planar membranes,the total effective area was 350 cm2.At that temperature and methane feed rate 458 mL min-1,air feed rate 1800 mL min-1,the reactor attained a fairly good POX performance with methane throughput conversion 97%,H2 selectivity 81%and CO selectivity 84%.The syngas concentration in the effluent was 93%,and the yield 1093 mL min-1.The syngas was fed into an SOFC with an effective area of 50 cm2 to generate electricity,and the maximum power output was 21.7 W.During the nearly fifteen hours of testing,the performance of the fuel cell remained stable.In Chapter 6,a flat tube membrane reactor was proposed and tested for POX reaction of methane.The flat tube membrane consisted of two planar membranes and a support in between;the support also acted as a gas channel.Owing to its symmetrical structure,the membrane tube demonstrated good thermo-mechanical properties.The membrane showed a fairly high oxygen permeation rate of 1.7 mL cm-2 min-1 under CH4/Air gradient at 800 ?.The flat tube membrane was used to construct a methane POX membrane reactor.At methane feed rate 17 mL min-1,air feed rate 100 mL min-1 and temperature 800 ?,the membrane reactor attained methane throughput conversion rate 49%,H2 selectivity 68%and CO selectivity 33%.It is expected that the POX performance of the membrane reactor will be improved by optimizing the catalyst packing and gas distribution.In Chapter 7,the research presented in this dissertation is summarized,and future research needs are suggested.