Long-Term Selectivity Stability and High-Pressure Mixed Gas Hydrogen Permeation Testing of Composite Pd/Inconel Membranes

The long-term selectivity stability is a key determinant towards the assessment of overall H₂ separation efficiency and successful utilization of Pd-based catalytic membrane reactors for the production of high-purity, fuel cell grade H₂ in industrial steam reformers and/or high temperature water-gas shift reactors integrated into the downstream of the industrial coal gasification units. For this purpose, a novel synthesis procedure was implemented to mitigate the high temperature leak growth and to improve the long-term selectivity stability of the composite Pd/Inconel membranes with a special emphasis on the formation of leak resistant deposition layers. Over the temperature range of 300-450°C, and at a DP range of 1-6.8 atm (P_Low=1 atm), an excellent H₂/He selectivity stability was achieved over a total testing period of ~3550 hours (>147 days). The He leak of the membrane was undetectable and maintained a stable H₂ flux of 51 m³/m²-h (at 450°C and a DP of ~7 atm). Under similar operating conditions, the long-term selectivity stability was successfully re-produced with several other 7-9 µm thick Pd/Inconel membranes, which were also tested over 58-90 days with numerous H₂/He cyclings.

Further characterization of the membrane performance was conducted at 400°C and a DP of 13.6 atm (P_High=14.6 atm & P_Low=1 atm), by utilizing simulated syngas streams of 61.7% H₂, 37.1% CO₂ and 1.2% CO and 50% H₂, 30% CO₂, 19% H₂O and 1% CO and over a total flow rate range of 300-20000 sccm. Compared to the pure H₂ flux, the lowering of the H₂ flux for the mixed gas testing was primarily attributed to the change of H₂ partial pressure along the length of the membrane module due to the in-situ removal of H₂. Similar results were also observed in the case of longer duration testing with 19% steam/H₂ mixture at a constant feed flow rate indicating no significant and/or additional inhibition of the permeate H₂ flux that can be directly attributed to the presence of steam in the mixed gas stream. Indeed, no other gases were detected in the permeate stream indicating the continuation of the long-term selectivity stability under mixed gas testing conditions.

In order to elucidate the factors affecting the H₂ flux under mixed gas testing conditions, additional test gas mixtures consisted of 38.3% He in H₂ and 38.3% CO₂ in H₂ were utilized to isolate and identify the effects such as the dilution of the H₂ concentration on the feed side due to the presence of other gases, gas phase mass transfer limitations due to the formation of a concentration boundary layer (concentration polarization), competitive adsorption of other gas components on the membrane surface and CO poisoning.