(569c) 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.