Dense metal membranes have been identified as a promising technology for post-gasifier forward water-gas shift membrane reactors. Unfortunately, the impurities present in the gasifier effluent streams, such as H2S, can have adverse effects on the mechanical and chemical stability of potential metal membranes in the form of corrosion or catalyst deactivation, respectively. Thus, this study has focused on the identification and characterization of dense metal membranes that can tolerate the harsh environments encountered in the gasification process without significant detrimental effects on permeability.Pd-Cu alloys have been of interest in recent years due to its catalytic activity for hydrogen dissociation, high permeability relative to Pd, suppression of the hydride-phase transition and limited reports of stability in the presence of H2S. Initially, the permeability of pure palladium, pure copper and palladium-copper alloys containing 80wt%, 60wt% and 53wt% Pd was evaluated with hydrogen retentate streams at temperatures and pressures ranging from 350 to 900oC and 0.1 to 2.86 MPa, respectively. Results indicate that crystalline phase plays a significant role in membrane performance, with the B2, 60wt%Pd-Cu alloy exhibiting the highest permeability of the alloys tested at temperatures below approximately 500oC. However, at temperatures corresponding to an fcc crystalline temperature for all of the alloys, membrane performance increased with increasing palladium content of the alloy.Additionally, the permeability the above mentioned alloys, along with pure palladium, have been evaluated in a H2S containing retentate gas mixture at temperatures of 350, 450 and 635oC. Permeability measurements coupled with SEM and XRD analysis of post tested membranes indicate that the mechanisms influencing performance is strongly dependent on operating temperature, alloy composition, crystalline structure, and exposure time. Gravimetric analysis of the growth rate of the metals sulfides observed on the palladium-copper alloys and pure palladium during the transient period of flux measurements was conducted. A model of hydrogen transport through a composite membrane with a Pd base and a growing palladium tetra-sulfide film was then developed and fit to the transient flux results and sulfide growth rate. The optimization of the model resulted in the first reported values of the permeability of palladium tetra-sulfide.
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