Air separation using a catalytically modified mixed conducting ceramic hollow fibre membrane module

A membrane reactor module consisting of four gas-tight hollow fibre membranes made of the mixed ionic-electronic conducting perovskite La0.6Sr0.4Co0.2Fe0.8O3-delta has been tested for oxygen permeation in the temperature range of 650-1000 degrees C with air on one side and helium on the other. The module was operated under gradientless gas-phase conditions and in the absence of mass transfer limitations at lower temperatures. Also at lower temperature the rate-determining step for oxygen permeation was found to be oxygen reduction with the oxygen permeation performance of the module being significantly enhanced by deposition of a porous platinum catalyst film onto the air-side of the membranes. A stable but mass transfer limited flux of 0.56 mu mol cm(-2) s(-1) (0.73 ml STP cm(-2) min(-1)) was achieved after a period of 4 days of operation at 1000 degrees C and continued at this rate for a further 4 days. A non-mass transfer limited flux of ca 0.14 mu mol cm(-2) s(-1) was obtained at 800 degrees C. The membrane module was operated for 500 h before the onset of significant leakage. Post operation qualitative analysis indicated localised changes in the stoichiometry of the membranes with no discernable trend, quantitative analysis on the other hand revealed that the perovskite had retained its starting composition. Post-operation analysis of the membranes revealed the presence of sulfur in the form of sulfates of strontium, cobalt and calcium and a change of surface microstructure for both the air- and lumen-side membrane surfaces. Possible sources of this sulfur contamination are: the glass-ceramic sealing material employed, the helium sweep gas and atmospheric sulfur dioxide. This contamination may contribute to the decrease in the longterm oxygen permeation rates. The metal cation stoichiometry of the perovskite showed changes in localised areas of the membranes following oxygen permeation studies. (c) 2006 Elsevier B.V. All rights reserved.