High purity hydrogen production in a Pd-Ag membrane reactor
Advancing the chemical engineering fundamentals
Chemical Reaction Engineering: Practical Applications (T2-2c)
Keywords: pure hydrogen, Pd-Ag membrane reactor, water gas shift
The use of hydrogen as an energetic carrier is a fundamental goal to be achieved. However, a critical stage in its realization is the possibility of having a hydrogen-pure stream with a CO content lower than 10 ppm for use in PEMFC.
CO + H2O = CO2 + H2 DH0-298 = -41 kJ/mol
Water gas shift reaction (WGS) is a necessary upgrading step in an integrated H2 production plant using light hydrocarbon, and its development in a Pd-Ag membrane reactor, where reaction and separation take place in the same vessel, presents several advantages, such as
• Hydrogen permeate pure stream production that can be directly fed to a PEMFC
• High CO conversion: the selective removal of a product from reaction side allows the TR thermodynamic equilibrium limits to be exceeded, shifting the reaction toward the formation of further products.
• Positive effect of the feed pressure that facilitates the permeation by pushing the reaction towards further product formation.
In this work WGS reaction was performed using a traditional reactor followed by a membrane reactor (TR+MR). A self-supported commercial Pd/Ag (23wt.%) membrane 60 microns thick was used. A commercial catalyst CuO/CeO2/Al2O3 was used in a temperature range of 280-320°C up to 600 kPa. In a simple MR the first part of the membrane is not exploited by the permeation, the H2 production by reaction being necessary in order to create a driving force promoting the permeation. The configuration TR+MR in series allows a better membrane exploitation, because an already partially converted current reaches the MR, therefore the permeation also involves the first part of the membrane . The space velocity and the reaction pressure effect on the CO conversion and H2 recovery were analysed with special attention paid to a significant performance improvement in the MR. CO conversion, always higher than that of a TR, also exceeds the TR equilibrium conversion (TREC) when a reaction pressure higher than 300 kPa is employed. In particular, 98.5% CO conversion was achieved at 320°C, 2000h-1 and 600 kPa with a gain of 13% with respect to the TREC and 20% to the TR, in the same operating conditions. In the meantime, 80% of the produced H2 (equal to 78% of the H2 extractable by the reaction) was recovered as pure stream. MR use also implies the possibility of working at high space velocity obtaining anyhow high CO conversion. In addition, the MR favours higher space velocity giving a very interesting performance. For instance, 96.7% and 93.5% CO conversion was measured at 3200 h-1 and 4500 h-1, respectively, operating at 320°C, 600 kPa. Furthermore, no drops in the electrical performance were observed when the hydrogen permeated stream was directly fed to a commercial PEMFC.
Acknowledgments
The Italian Ministry for Foreign affairs, Direzione generale per la promozione e la Cooperazione Culturale, Rome, Italy is gratefully acknowledged for co-funding this research.
Johnsonn Matthey and Oleg M. Ilinitch (Engelhard, USA) are gratefully acknowledged for the membrane and catalysts supplied.
[1] Barbieri, G.; Bernardo, P. Experimental evaluation of hydrogen production by membrane reaction. In Carbon Dioxide Capture for Storage in Deep Geologic Formations – Results from the CO2 Capture Project - Volume 1, Chapter 22, pp. 385-408. Elsevier, 2004.
Presented Tuesday 18, 15:40 to 16:00, in session Chemical Reaction Engineering: Practical Applications (T2-2c).
