(223e) Long Term H2 Permeance and Selectivity Stability of Pd and Pd-Au Large Scale Membranes in Actual Syngas Atmospheres

The integration of composite Pd based membrane reactors (WGS or MSR reactions) into large scale H₂ producing units entails valuable cost savings for pre-combustion CO₂ sequestration. In addition, it was recently demonstrated that composite Pd and Pd-Au membranes were stable for more than 250 h in actual syngas atmosphere (450°C, 13 bara) and that H₂ permeate purity was higher than 99.9%. The Pd-Au membranes tested earlier were prepared by the deposition of a thin (0.15-0.2 μm) layer of Au on top of the dense Pd layer. The membranes showed some initial flux decline attributed to poisoning of the layer by unknown contaminants and a maximum recoverability of 77% after 300 h in pure H₂ at 450°C. The objective of the present work was to further investigate the stability of permeance and purity and the of Pd/Au membranes with a higher Au content on the Pd surface of 0.5 μm thickness.

Composite Pd and Pd/Au porous stainless steel membranes having an area of more than 200 cm² (2.54 cm OD, 25.4 cm L) were prepared by WPI’s patented technology and tested at the National Carbon Capture Center (NCCC) in Wilsonville, Alabama. The Syngas composition was slightly different from the previous study since in this second run, the syngas was shifted before entering the membrane module. Hence, CO concentrations were lower than 1% and H₂O/CO ratio was higher than 1. The operation conditions and H₂S concentration levels were kept the same as in the previous study. A Pd-0.5 μm/Au membrane showed a remarkably stable hydrogen flux for over 475 h albeit an initial hydrogen flux decline of ~40% after 20 hours of exposure to the syngas stream. The hydrogen purity, as high as 99.9%, was also stable for over 475 hours. Upon test at NCCC, the permeance of the Pd-0.5 μm/Au membrane was totally recovered after 160 h at 450°C in a pure H₂ atmosphere. Hence, a thicker Au layer enhanced drastically the recoverability of membranes exposed to actual syngas atmospheres which represents a breakthrough accomplishment in the development of composite Pd membranes for WGS-CMR applications.

In an attempt to understand the causes of the initial permeance flux decline, a preliminary XPS study of the surface of Pd coupons exposed to actual syngas was performed and showed the presence of carbon contamination in the Pd bulk. Further analysis would be needed to confirm this finding.