Synthesis, oxygen permeation study and membrane performance of a Ba_(0.5)Sr_(0.5)Co_(0.8)Fe_(0.2)O_(3-#delta#) oxygen-permeable dense ceramic reactor for partial oxidation of methoane to syngas

摘要

Ba_(0.5)Sr_(0.5)Co_(0.8)Fe_(0.2)O_(3-#delta#) (BSCFO) oxide was synthesized by a combined EDTA-citrate complexing method. The partial substitution of Sr in SrCo_(0.8)Fe_(0.2)O_(3-#delta#) by Ba led to obvious iprovement in the phase stability of the material at high temperatures. Under an air/helium oxygen gradient, the oxygen permeation fluxes of BSCFO were considerably high, a permeation flux approx=1.6 ml/cm~2 min was achieved for a 1.5 mm membrane at 950deg C, when P_(O_2)~'=0.21 atm and P_(O_2)~"=0.05 atm. the oxygen permeation was rate-determined mainly by oxygen bulk diffusion under non-reducing environment within the membrane thickness (1.5-1.8 mm) and temperature range (800-900deg C) investigated. the oxygen permeation flux kept at 1.1-1.2 ml/cm~2 min under ambient air/helium oxygen gradient during more than 1000 h operation. However, at lower temperatures, the permeation flux slowly decreased in an exponential relationship with time, which was posibley due to the phase decomposition of the material. Two membrane configurations and three cases were investigated for the partial oxidation of methane to syngas in a planar BSCFO reacor with LiLaNiO_x/#gamma#-Al_2O_3 as the reforming catalyst. The decrease in the distance between membrane surface and the reforming catalyst led to increase in the oxygen permeation fluxes and the role of surface exchange at the reaction membrane side in the oxygen permeation rate determination. High temperature treatmetn made some catalyst deposit onto the membrane surface, which led to a change of the oxygen exchange mechanism at the reacetion side membrane surface, and resulted in an increase in oxygen permeation flux and decrease in activation energy of oxygen surface exchange. the stable long-term performance of the membrane reactor demonstrated that the permeation was still controlled by surface oxygen exchange of the reaction side membrane surface after the high temperature treatment. At 875deg C, a permeation flux of 11.5 ml/cm~2 min, methane conversion of 97-98%, and CO selectivity of 95-97% were achieved.