Composite Palladium Membrane Reactor for the Water Gas Shift Reaction

            Palladium membranes supported on porous sintered metal substrates have the potential to separate hydrogen at high purity from industrial steam reforming and coal gasification streams.  The resultant syngas from these processes contain significant quantities of carbon monoxide which can be utilized to produce additional hydrogen via the water gas shift (WGS) reaction.  A packed bed reactor (PBR) incorporating a composite palladium membrane can both shift the remaining carbon monoxide and separate the hydrogen simultaneously.  The in situ separation of hydrogen also allows for the attainment of WGS conversion beyond equilibrium values based on Le Chatelier's principle.  In this work, the characteristics of a palladium catalytic membrane reactor (CMR) are studied both for pure carbon monoxide and steam as well as syngas-like feeds over a wide range of temperatures and pressures.

            A 1.3 cm OD tubular porous Inconel support (Media Grade 0.1 µm) was oxidized in air at 700°C and then graded with 1.0 µm γ,α-aluminum oxide via the method reported by Ma et al. [1].  A composite Pd/Ag barrier and a dense Pd layer were then prepared via the method reported by Ayturk et al. [2].  The final thickness of the membrane was 12.5 µm.  The membrane was mounted in a tubular reactor and the catalyst (40-48 mesh iron-chrome oxide) was packed in the annular space around the membrane.  A packed bed reactor with the same dimensions as those of the CMR was also setup as a baseline for comparison. The PBR and CMR experiments were conducted over a temperature range of 300-450°C, pressures of 4.4-14.6 atm, steam to CO ratios of 1.2-3.2, and GHSV's of 800-3300 hr^-1.

            Compared to that of the PBR, the CMR achieved higher conversions for all conditions tested and also exceeded the equilibrium conversion for many of the conditions tested.  The highest conversion for the CMR with a pure CO and steam feed was 99.0% with an 89.9% hydrogen recovery at 349°C, a total feed pressure of 14.6 atm, a steam to CO ratio of 1.44, and a GHSV of 860 hr^-1.  Under similar conditions, the PBR resulted in a conversion of 92.7%; which was in good agreement with the equilibrium conversion of 93% for the stated conditions.  The highest conversion for the CMR with a mixed gas feed of 18% CO and 82% H₂ was 96.2% with an 89.7% hydrogen recovery at 332°C, a total feed pressure of 14.6 atm, a steam to CO ratio of 1.44, and a GHSV of 2750 hr^-1.

            In addition, a one dimensional isothermal unsteady state model has been constructed to predict the behavior of the reactor during start-up and during changes in operating conditions.  The model is based on the CSTR's-in-series approximation of a tubular reactor. The model simulations will be coupled with the WGS experimental results and utilized to determine the optimum operating conditions for the Pd-based CMR.

[1]  Ma, Y. H.; Mardilovich, I. P.; Engwall, E. E., US Patent 7,255,726 (August 14, 2007)

[2]  Ayturk, M. E.; Mardilovich, I. P.; Engwall, E. E.; Ma, Y. H. “Synthesis of composite Pd-porous stainless steel (PSS) membranes with a Pd/Ag intermetallic diffusion barrier.” Journal of Membrane Science 285(2006), 385-394.