Investigation Stability And Oxygen Permeation Of La_0.6Sr_0.4Co_0.2Fe_0.8O_3-δ Hollow Fiber Membrane

Mixed conducting perovskite oxide ceramic membranes have the potentials for industrial oxygen production to replace the conventional expensive air separation units via cryogenic distillation, which is of particular significance to improve the viability of these clean energy technologies like Oxyfuel projects by lowering their cost. From an application point of view, membranes must possess sufficiently high oxygen permeability and good structural stability to withstand real process conditions. Therefore, in this work, we investigated the operational stability of LSCF hollow fiber membranes by simulating a real industrial application environment.In most of these applications, the steam presence is unavoidable thus requiring these membranes to be stable in the steam-containing atmosphere under high temperatures. In this work, the La0.6Sr0.4Co0.2Fe0.8O3-δ(LSCF) perovskite hollow fiber membrane was prepared and tested for oxygen permeation in the presence of steam in the sweep. Those membranes were tested at temperature of 900 oC using the sweep containing 3%, 12% and 100% steam(balance by helium) for 370, 120 and 500 hours, respectively. In the long run, the steam can etch the membrane surface evidenced by some elements leaching like the iron and the formation of strontium carbonate; but this etching process is too slowly to cause sufficient damage on the membrane performance. Using pure steam sweep, the LSCF membrane was operated at 900 oC for nearly 500 hours without any sign of oxygen flux decay. The presence of minor steam content(less than 12%) in the sweep is kinetically favorable for the surface oxygen exchange reactions thus lifting up the oxygen flux.In practical application, the LSCF hollow fiber will be stored in air for a long time, during which the LSCF can be reacted with CO2 in air. This is a quite important issue when studying the oxygen permeation membranes. This paper compared the performance of LSCF membranes that had been stored in air for nearly 5 years with the fresh membranes newly fabricated. The permeation flux of the 5-year-old LSCF membrane was only 0.17 mL·min-1·cm-2 due to the formation of SrCO3 on the surface area. After sintering at high temperature, these secondary phases on the surface area were basically eliminated and the O2 flux has been recovered to the normal level of up to 0.78 mL·min-1·cm-2.The feed and sweep gas flow modes during the oxygen permeation unit can also affect the oxygen flux of hollow fiber membranes and the membrane material structure. At high temperatures, LSCF membranes will soften, resulting in the decrease of inter-grain acting forces. Therefore, change the pressure differences between the inside and outside of the hollow fiber can damage the thinner dense layer and subsequently, reduce the stability of hollow fiber. After comparing different gas flowing conditions, we conclude that the flow directions of feed and sweep gas has negligible effect on the oxygen permeability; while the oxygen flux was increased by 6% when the sweep gas flowed through inside of the hollow fiber.