Partners and International Organizations
(English)
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A, B, CZ, DK, F, D, IRL, I, LV, LT, N, P, SI, E, S, CH, TR, GB
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Abstract
(English)
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The use of La0.6Ca0.4Fe0.75Co0.25O3-d perovskite-type membranes for the separation of oxygen in a partial oxidation (POX) reactor was previously reported and now published in 2004. Promising results were obtained with a tubular reactor in terms of oxygen flux (3 ml/min cm2) and catalytic properties (95% CH4 conversion, >90% CO selectivity) with pure methane inlet at 900°C. Furthermore, it operated stably over 1400 hours. However, post-operation examinations showed that the membrane material had started to demix under this strong oxygen chemical potential gradient. This was probably favoured by the fact that La0.6Ca0.4Fe0.75Co0.25O3-d, like other perovskite compounds, is composed of a complex coherent intergrowth of microdomains (50-500 Å in size) different in composition and structure. From this starting point, the composition of the membrane was modified to enhance its stability and avoid the occurrence of secondary phases. Therefore, both the Ca and Co content were reduced. The oxygen transport properties of this new composition were then characterised by permeation measurements between air and argon. Samples produced by different processes (cold isostatic compaction, injection moulding and extrusion) were used but the resulting fluxes were similar: 7×10-8 mol/cm2s at 900°C for a normalised thickness of 1 mm, and an apparent activation energy of 140 kJ/mol (0.1 ml/min.cm2, much weaker PO2 gradient between air and Argon (10-5 atm) than between air and methane (10-24 atm); flux increases > one order of magnitude between both gradients, see below). The thickness dependence of the flux showed clearly bulk limitation over the temperature range of 800 to 1000°C. Dense tubes of this material were then produced by the University of Brno (CZ) within the COST network, in order to be tested in a POX reactor. Dry methane, diluted in argon, is fed to the outer surface of the tube and reacts in the catalytic bed with the oxygen which has permeated through the tube. The methane inlet was gradually increased up to 100%, resulting in a 20-fold enhancement of the oxygen permeation flux to 1.5×10-6 mol/cm2s at 900°C (2 ml/min.cm2). The tubular POX reactor operated during ca. 2000 hours, with 80% methane conversion and 90% CO selectivity. The oxygen flux gradually decreased to 1×10-6 mol/cm2s. Post operation SEM examination of the cross-section of the membrane showed that the surface exposed to methane had started to decompose, indicating that the permeation flux was limited by bulk transport in the membrane. In addition, the growth of an extraneous porous layer was observed on the surface exposed to air. This means that the modification of the composition, even though it has improved the general stability of the membrane, has not fully avoided the problem of kinetical demixing in such a strong oxygen partial pressure gradient.
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