Hydrogen transport through a selection of thin Pd-alloy membranes: Membrane stability, H2S inhibition, and flux recovery in hydrogen and simulated WGS mixtures

Abstract

Resistance to H2S is important for the use of Pd membranes in fossil-fuel power stations with integrated carbon capture. In the current article we present the H2S inhibition and subsequent H2 flux recovery of thin (∼2μm) pure Pd, Pd77Ag23, Pd85Au15 and Pd70Cu30 membranes. Upon H2S exposure in H2/N2 feed mixtures, all the investigated thin Pd-alloy membranes exhibited a sharp decrease of the H2 flux depending of the H2S concentration. At 2ppm, the H2 flux reduces to a value equal to around 20–40% compared to the H2 flux obtained in the absence of H2S; at 20 and 100ppm H2S in the feed the H2 flux only equals 10–20% of its original value. The pure Pd membrane shows an instant failure during the exposure to 100ppm H2S. After removal of 2ppm H2S from the feed stream the H2 flux through the Pd, Pd85Au15 and Pd70Cu30 membranes recovers within 2h. After the exposure to 20ppm H2S only the H2 flux through the pure Pd, Pd85Au15 and Pd70Cu30 membranes recovers to the same extent. As opposite to these three alloys, the H2 flux through the Pd77Ag23 alloy is recovering much more slowly. When exposed to a simulated sour WGS mixture, the Pd85Au15 membrane experiences a loss of H2 flux down to 10–15% of the value obtained applying 62% H2 in N2 as feed. The extent of H2S poisoning is thus of the same magnitude in WGS mixtures and H2/N2 mixtures. No sign of unselective transport is observed for this membrane after 100h of operation in a sour WGS mixture. Post-process characterisation show signs of surface roughening combined with grain growth, whilst multiple small pits are found primarily in the grain boundaries of the membrane feed surface after the long-term operation. This study demonstrates that thin Pd–Au and Pd–Cu membranes survive sour atmospheres, but at a cost of a significant loss of H2 flux.