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Title: Oxygen flux through unmodified and modified La0.6Sr0.4Co0.2Fe0.8O3-8 hollow fibre membranes and application to methane oxidation
Author: Rodulfo-Baechler, Serbia
Awarding Body: University of Newcastle Upon Tyne
Current Institution: University of Newcastle upon Tyne
Date of Award: 2013
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Improved catalytic routes could help to transform the exploitation of the large worldwide natural gas reserves, whose principal component is methane. They transform methane into more valuable chemicals and fuels through carbon dioxide reforming of methane (CDRM), steam reforming of methane (SRM) and partial oxidation of methane (POM). These reactions facilitate the formation of syngas, which is subsequently converted to fuels through the Fischer-Tröpsch synthesis. Mixed Ionic and Electronic Conducting (MIEC) membrane reactors are of interest because they have the potential to produce high purity oxygen from air at lower costs and provide a continuous oxygen supply to reactions or/and industrial processes, and hence avoid sourcing the pure oxygen from air by conventional cryogenic separation technology. In addition, the MIEC ceramic membrane shows the ability to carry out simultaneous oxygen permeation and hydrocarbons oxidation into single compact ceramic membrane reactor at high temperature. This can reduce the capital investment for gas-to-liquid (GTL) plants and for distributing hydrogen. This study compares the oxygen release and oxygen uptake obtained through a LaSrCoFeO hollow fibre membrane (referred as LSCF6428-HFM) under anδ Air/He gradient at 850°C and 900°C. The separation and quantification of these two processes permitted the determination of the oxygen incorporated into LSCF6428 structure and the development of a model for apparent overall rate constant using the molar flow of the oxygen at the inlet and outlet in different side of membrane (i.e. shell side and lumen side). The results show that the oxygen flux is enhanced by rising helium flow rates, this is due to an increased driving force for oxygen migration across the membrane and also the air flow determines the oxygen amount that permeates across the membrane. In addition, the oxygen flux improves at higher temperatures, due to its dependence on bulk oxygen diffusion and the oxygen surface reaction rates. The temperature increase improves the mobility of the lattice oxygen vacancies and also the concentration of lattice oxygen vacancies in the perovskite. The impact of surface modification was also studied by coating CoO and 5%Ni-LSCF6428 34 catalysts on the shell side surface of the LSCF6428 hollow fibre membrane for oxygen permeation. It was found that the oxygen flux significantly improved under Air/He gradient for catalyst-coated LSCF6428-HFM. However, under continuous operation conditions over a long time both the unmodified and the modified perovskite LSCF6428-HFM reactors suffered segregation of metal oxides or redistribution of metal composition at the surface membrane, although the bulk LSCF6428 membrane stoichiometry did not change. The apparent overall rate constants for oxygen permeation of the CoO/LSCF6428-HFM and 34 5%Ni/LSCF6428-HFM were enhanced 3-4 fold compared to unmodified LSCF6428-HFM. Comparison of both modified HFM reactors revealed that the apparent overall rate constants for CoO/LSCF6428-HFM were 2 fold higher than those obtained for 5%Ni- 34 LSCF6428/HFM. According to the distribution of total oxygen permeation residence for unmodified and modified LSCF6428-HFM reactor, the oxygen permeation rate is limited by surface exchange on the oxygen lean side or lumen side (R) at 850°C and 900°C and the ex contribution of bulk diffusion on the oxygen permeation rate increased with a rise in the temperature (900°C). The methane oxidation reaction was studied in unmodified and modified 5%Ni- LSCF6428/LSCF6428 hollow fibre membrane in reactors at 850°C. The results suggest that catalytic pathways in methane oxidation depended upon flow operation modes, oxygen concentration, Htreatment and on the type of catalyst. The performances in methane conversion of LSCF6428-HFM and 5%Ni/LSCF6428-HFM modules facilitated the formation of SrCO3 because of the reaction of CO2 with segregated strontium oxide.
Supervisor: Not available Sponsor: EPSRC
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available