H 2 production from simulated coal syngas containing H 2S in multi-tubular Pd and 80 wt% Pd–20 wt% Cu membrane reactors at 1173 K

Abstract

99.7% conversion of CO in a simulated syngas feed containing 53% CO, 35% H 2 and 12% CO 2 was achieved via the water–gas shift (WGS) reaction in a counter-current Pd multi-tube membrane reactor (MR) at 1173 K and 2 s residence time. This conversion is significantly greater than the 32% equilibrium conversion associated with a conventional (non-membrane) reactor primarily due to the high rate of H 2 extraction from the reaction zone through the Pd membranes at elevated temperatures. Furthermore, nearly complete H 2 recovery was attained in the permeate, resulting in the simultaneous production of a high-pressure CO 2 (>99%) retentate stream after condensation of the steam. When Pd 80 wt% Cu tubes were used in the reactor, a significantly lower CO conversion of 68% was attained at comparable residence times, probably due to the lower H 2 permeance of the alloy. When H 2S was added to the syngas feed and the H 2S-to-H 2 ratio was maintained below the threshold required for thermodynamically stable sulfides to form, the Pd and Pd 80 wt% Cu MRs retained their mechanical integrity and H 2 selectivity, but a precipitous drop in CO conversion was observed due to deactivation of the catalytic surface. The Pd and Pd 80 wt% Cu MRs were observed to fail within minutes after increasing the H 2S-to-H 2 ratio to levels above that expected for thermodynamically stable sulfides to form, as evidenced by rupturing of the membrane tubes. SEM–EDS analyses of the membranes suggested that at high H 2S-to-H 2 ratios, the H 2S compromised the mechanical integrity of the MRs by preferentially attacking the grain boundary region.