High Stability Oxygen Permeable Membranes And Membrane-based Process

Oxygen-permeable dense ceramic membranes can separate oxygen from air with high selectivity, and thus are promising for applications in oxygen-involved processes. For practical application, the membrane is required to possess high oxygen permeability, good chemical and mechanical stability. This thesis is to explore membrane materials with both good oxygen permeability and stability and investigate membrane-based processes including combustion of fuel and partial oxidation of methane to syngas (mixture of CO and H₂).Chapter 1 gives an introduction of oxygen-permeable membranes. The concepts and theories of oxygen permeation for dense ceramic membranes are reviewed, and research needs are identified especially with respect to the stability of the membranes. Chapter 2 presents a study on the oxygen permeation behavior of Fe-doped La_1-xSr_xGaO_3-δ under various gradients. La_1-xSr_xGaO_3-δ is reported to be a pure oxygen ionic conductor with desired stability, and substitution of part of Ga with Fe is to convert the material into a mixed oxygen ionic and electronic conductivity required for oxygen permeation. It is shown that the oxygen permeation rate for a La_0.7Sr_0.3Ga_0.3Fe_0.7O_3-δ (LSGF) membrane tube increased with time when its tube side was exposed to a reducing atmosphere (and its shell side to the atmospheric air). The time required for permeation to reach a steady value depended on the composition of the atmosphere, which had the following sequence H₂4. The treatment of LSGF membrane with 40% H₂ at 950℃for 16 hr resulted in, a formation of a porous layer of thickness ⁵00μm. The as-treated membrane exhibited a large increase in oxygen permeation flux (by a factor of 4.5 at 950℃and 10.5 at 800℃) and a significant reduction in the apparent activation energy, which was attributed to the increased porosity on the tube-side surface. It is suggested that the H₂ treatment altered the rate-limiting step from the surface oxygen exchange to the transport of oxygen in the bulk of the membrane. The H₂ treatment also led to an enhanced depletion of Ga from the membrane surface, which could affect the applicability of the membrane.According to the studies on solid oxide fuel cell (SOFC), both electrolyte Zr_0.8Y_0.2O_1.9 (YSZ) and interconnect material La_0.8Sr_0.2CrO_3-δ (LSC) show excellent stabilities under large oxygen partial pressure difference. In Chapter 3, the oxygen permeability of YSZ-LSC (60-40 vol%) composite was investigated. In this composite, oxygen ions and electrons can transport through YSZ and LSC phase, respectively. Densification of the composite was achieved by sintering at elevated temperatures. No significant reactions between the two constituting phases occurred during the sintering. A minor amount of impurity was found on the surface of the sintered membrane, however the impurity was not detected in the bulk. The formation of the impurities was believed to be caused by volatilization of Cr species in LSC. The composite membrane possessed appreciable oxygen permeability: a flux of 9.2×10^-9 mol cm^-2 s^-1 was observed for a 1.23mm thick membrane at 950℃and under an oxygen partial pressure gradient of 0.209atm/0.00031atm, and a larger flux of 3.2×10^-8 mol cm^-2 s^-1 was under air/(20%CO+80%He) gradient (at 930℃).. It is suggested that the overall oxygen permeation process for the composite is controlled by the surface oxygen exchange step, and thus modification and activation of the membrane surface can lead to an increased oxygen permeation rate.Chapter 4 presents a study on composite of Sm0.2Ce0.8O₂-δ(SDC) and La0.75Sr0.25Cr0.5Mn0.5O3-δ(LSCM). In this composite the SDC phase is to conduct oxygen ions and the LSC phase electrons. Both SDC and LSCM are known to be stable under reducing atmosphere. The SDC-LSCM (60-40 vol%) composite possessed moderate oxygen permeability: a flux of 3.8×10-8 mol cm^-2 s^-1 was observed for a 1.12mm thick membrane at 950℃and under a small oxygen partial pressure gradient of 0.209atm/0.0090atm, and a larger flux of 4.7×10^-7 mol cm^-2 s^-1 at 950℃and air/CO gradient (CO feed rate of 40 cm3 min-1). The SDC-LSCM membrane showed excellent chemical and mechanical stability under large oxygen pressure gradient. After operation under a large oxygen gradient for 250 hr, the membrane tube was found to remain intact, and its microstructure were largely retained except a slight porosity in the LSCM phase. In Chapter 5, combustion of CH₄ in a membrane reactor was investigated. In this study, a LSGF membrane tube was used to supply the needed oxygen, The membrane reactor showed a good performance on the CH₄ combustion: CH₄ was completely conversed to CO₂ and H₂O in the O₂-rich case while in the CH₄-rich case the main product of CH₄ oxidation was also CO₂ and water with high H₂O selectivity and CO₂ selectivity over 85% and 90%, respectively. The LSGF membrane reactor took a long time to reach a steady state, and applying a H₂ treatment on the membrane could significantly reduce the required time. A fixed-bed reactor study showed that LSGF possessed catalytic activity towards the oxidation reaction of methane under oxygen-lean conditions.In Chapter 6, combustion of CO was conducted in a membrane reactor, in which a SDC-LSC membrane tube was used to supply oxygen, and Sr_0.3Ba_0.5La_0.2MnAl₁₁O_19-a (SBLMA) powder was packed inside the membrane tube to catalyze the oxidation reaction. At temperature of 950°C and CO fuel feed rate of 10 cm³ min^-1, the conversion of CO to CO₂ exceeded 99% with an equivalent oxygen permeation rate of 0.64 cm³ cm^-2 min^-1. Compared with the case without the oxidation catalyst, the reactor packed with the catalyst exhibited higher oxygen permeation rate and CO conversion. It is suggested that the reaction of CO with the permeated oxygen proceeded faster in the presence of the catalyst, thus the overall oxygen permeation process was not limited by the surface reaction but by the transport of oxygen in the bulk of the membrane. The surface morphology and phase composition of the membrane were found to be well retained in the presence of the oxidation catalyst, which is attributed to the change of the CO reaction sites from the membrane surface to the catalyst.In Chapter 7, the catalytic behavior of perovskite-type catalyst SrTiO₃ and La₄Sr₈Ti₁₂O₃₈ on partial oxidation of methane (POM) was studied with oxygen supplied by a SDC-LSC membrane tube. It was shown that both SrTiO₃ and La₄Sr₈Ti₁₂O₃₈ oxides possessed good catalytic activity towards POM reaction. In the case of La₄Sr₈Ti₁₂O₃₈, at temperature of 950℃and CH₄ feed rate of 30 cm3 min-1, the methane was mainly partial oxidized to H₂ and CO with the maxium production rate of syngas (2.24 ml min^-1 cm^-2), XCH₄=18.1%, SH₂=88.6%, SCO=84.6%, and H₂:CO ratio -1.99. Coke was found to form on the oxide catalyst, resulting in the decrease of the catalyst. The catalyst could recover its activity fully by feeding air into the membrane tube to burn the formed coke. The excellent durability of La₄Sr₈Ti₁₂O₃₈ catalyst was demonstrated by operating the catalytic membrane reactor for more than 900 hr. The La₄Sr₈Ti₁₂O₃₈ catalyst was also investigated with a fixed-bed reactor. The oxide catalyst was found to be able to catalyze the steam reforming and CO₂ reforming of methane (SMR, CMR). It is suggested that POM reaction in the membrane reactor might proceed via an oxidation-reforming mechanism. Moreover, both SrTiO₃ and La₄Sr₈Ti₁₂O₃₈ catalysts were found to increase the oxygen permeation of SDC-LSC membrane and help the membrane maintain its surface morphology.Chapter 8 summarizes the research conducted in this thesis, and presents recommendations for further research.