Ind. Eng. Chem. Res. 2005, 44, 61-66
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MATERIALS AND INTERFACES Preparation of LSCF Ceramic Hollow-Fiber Membranes for Oxygen Production by a Phase-Inversion/Sintering Technique Xiaoyao Tan, Yutie Liu, and K. Li* Department of Chemical Engineering & Chemical Technology, Imperial College London, University of London, South Kensington, London SW7 2AZ, U.K.
A mixed oxide-electron conducting ceramic, La0.6Sr0.4Co0.2Fe0.8O3-R (LSCF), hollow-fiber membrane was prepared by a combined phase-inversion/sintering technique. The hollow-fiber membrane precursor was first spun from a starting solution containing 66.33 wt % LSCF powder, 6.63 wt % PESf binder, 0.5 wt % PVP additive, and 26.54 wt % NMP and then sintered at elevated temperatures between 1100 and 1280 °C for 4 h to obtain membranes with a gastight property. The hollow-fiber membranes prepared show an asymmetric structure in which a spongelike material is sandwiched by fingerlike structures located at the outer and inner walls of the fibers. Oxygen permeation fluxes through the hollow-fiber membranes were measured under different temperatures and downstream oxygen partial pressures. The results indicated that the oxygen flux obtained from the hollow-fiber membrane is higher than that obtained from conventional LSCF disk-shaped or tubular membranes, most probably because of the reduced thickness and porous inner surface of the membranes prepared. The operating temperature plays a more important role in determining the oxygen permeation flux of LSCF membranes. Once the operating temperature is over 700 °C, the oxygen permeation flux increases sharply as a result of an order-disorder transition of the oxygen vacancies. 1. Introduction Oxygen production by air separation is of great importance in both environmental and industrial processes. For example, if oxygen instead of air is used in power plants, the waste gas produced in the combustion process is pure carbon dioxide that can easily be reclaimed instead of being released as a greenhouse gas to the environment. Use of pure oxygen rather than air as the oxidant in chemical industrial processes would be able to decrease the cost remarkably as the separation of nitrogen from subsequent product streams becomes unnecessary. Currently, oxygen is usually produced by cryogenic distillation or pressure swing adsorption, which are very costly methods. Alternatively, ceramic membrane processes are economically more promising. In such processes, dense ceramic membranes, which conduct both oxygen ions and electrons simultaneously, i.e., the so-called mixed conductors [e.g., La1-xSrxCo1-yFeyO3-R (LSCF)] have been demonstrated to provide pure oxygen fluxes.1-3 When a gradient of oxygen chemical potential is imposed across these dense membranes at high temperatures, oxygen is transported in the form of oxygen ions from the high-partial-pressure side to the low-partial-pressure side and released downstream without the need of electrodes and external electrical loadings, which makes the system very simple. * To whom correspondence should be addressed. Tel.: 44 (0) 207-5945676. Fax: 44 (0) 207-5945629. E-mail: Kang.Li@ Imperial.ac.uk.
In most previous studies, disc-shaped membranes with only a limited membrane area (
