| Oxygen production by air separation is of great importance in both environmental and industrial processes, as most large scale clean energy technologies require oxygen as feed gas. For example, if pure oxygen instead of air is used in power plants, the major constituent of the waste gas produced during the combustion process would be CO2, which can be readily and economically captured. Current tonnage O2production by cryogenic process is expensive and energy intensive. Coupling a cryogenic air separation unit at the front end of a coal gasifier or oxyfuel power plant is unpractical because of its lower power generation efficiencies. Dense mixed ionic and electronic conducting (MIEC) ceramic membranes can deliver100%pure O2under differential O2partial pressure gradient without the requirement of external power, offering the potential to tackle these energy penalties and improve the viability of CO2zero emission technology. In addition, these MIEC membranes are also of interest as membrane reactors for oxidative reactions to improve the product yield/selectivity beyond what the reaction equilibrium allows. Most previous work in this area employed membranes in the form of disks or tubes easily fabricated using conventional techniques.Recently, hollow fibers are attracting public attention because of their larger membrane area per unit volume. La0.6Sr0.4Co0.2Fe0.8O3(LSCF) exhibits good oxygen fluxes, high mechanical strength and thermal stability and therefore was chosen as the hollow fiber material for investigation in the present study.Lao6Sr0.4Co0.2Fe0.8O3-δ (LSCF) hollow-fiber membranes were successfully prepared through phase-inversion and the sintering technique. A mixture of N-methyl-2-pyrrolidone (NMP) and water as internal coagulant, tap water as external coagulant. Oxygen permeation fluxes through the obtained hollow fiber membranes were measured under air/He gradients at different conditions. The results indicate that the hollow-fiber membranes possess an oxygen permeation flux of0.11-0.25 mL·cm-2min-1in the temperature range of800-900℃and Oxygen permeation in the hollow fiber membranes is limited primarily by the surface exchange reactions at lower temperatures, but ionic bulk diffusion will have a rate-limiting effect at temperatures higher than900℃, the increase of the helium gas flow rate reduces the oxygen partial pressure (po2) on the core side and a higher oxygen permeation flux is observed.The oxygen permeability and stability of a perovskite oxide membrane of nominal composition LSCF were investigated in an atmosphere containing CO2and H2O. It was observed that, at a temperature of810℃, the oxygen permeation flux through the membrane decreased significantly with time when an air stream containing both CO2and H2O impurities was used as the feed gas and the membrane partially decomposed, whereas in air containing either CO2or H2O species alone, the oxygen flux decreased slightly and the membrane retained its phase composition and microstructure. It was also found that, at a higher temperature of900℃, the oxygen flux was slightly affected by the presence of these species and the membrane remained intact. The effects of CO2and H2O were explained in terms of formation of bicarbonate on the membrane surface, and optimal operation conditions for the membrane were proposed.In this paper, the stability of LSCF hollow fiber with different kinds of gas as sweep gas was observed. It was found that the oxygen transmission rate is a little lower when using CO2as sweep gas than He as sweep gas, but the stability of LSCF hollow fiber was not affected by the sweep gas containing CO2. However, the stability reduced when the sweep gas containing12%H2O,but when the water content is low about3%, the stability of hollow fiber did not reduce. Compared with other materials such as BSCF, the LSCF material is much easier to be used in industry. |
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