(562f) One-Stage Process for Water Gas Shift in a Pd-Based Membrane Reactor

The water gas shift (WGS) reaction is the upgrading stage in the cycles of hydrogen production by, for example, steam reforming of light hydrocarbons from fossil or renewable sources. This reaction limited by thermodynamics often carried out in industry to convert CO, producing more hydrogen. A WGS industrial process consists of two reactors in series: the first one operates at a high temperature (300-400°C), exploiting the advantages offered by a fast kinetics of FeCr based catalysts; the second one operating in a low temperature range (200-300°C) benefits of the higher thermodynamic conversion.

This work proposes the use of one Pd-based membrane reactor (MR) [1], operating in the same temperature range of the high temperature WGS reactor as suitable alternative to the whole traditional reactor (TR) process. The Pd-based membrane allows the selective removal of H₂ from the reaction volume, thus, favouring the increase of the equilibrium conversion close to the total value. Operated in the high temperature range, the MR can exploit the faster kinetics offered by Fe-Cr based catalyst, together with the higher permeation rate, obtaining CO conversion and H₂ recovery significantly higher than the whole TR system.

The values of gas hourly space velocity (GHSV), temperature, H₂O/CO feed molar ratio, feed composition, etc. used in the simulations are those typical of an industrial application of a WGS upgrading stage. A feed pressure of 15 bar was assumed as reference value, this values being the strength limit of self supported Pd-Ag membranes currently available at market level. However, a feed pressure of 30 bar was also considered as it is the typical pressure used in industrial processes of hydrogen production. The pressure demonstrates to be one of the most interesting variables of MR processing.

MR always exceeded a CO conversion of 80% and 95% for H₂O/CO feed molar ratios of 1 and 3, respectively. These conversion values are always higher than those of the whole traditional process and also exceed significantly the TR equilibrium conversion (TREC). As a first important result, this comparison leads to conclude that only one MR can replace the two reactors of the traditional process. The MR operating at the higher temperature regime allows the kinetically faster Fe-Cr based catalyst to be used, implying, as a consequence, a very large (ca. 10 fold) reduction of the total catalyst required by the traditional process for a feed molar ratio of 1. The low temperature reactor present in the traditional process, where the Cu-Zn catalyst requires a space velocity ca. 10 times lower than the high temperature stage, is anymore necessary in MR proposed solution owing to the higher CO conversion achieved thank to the hydrogen removal through the membrane. Passing from a feed molar ratio of 5 to 3, MR volume corresponds to 33 and 21%, respectively, of the one required by the whole TR process. A further reduction of the feed molar ratio, towards the stoichiometric value of 1, leads to an extra reduction of this parameter. This also has consequence on the reduction of the plant footprint.

In addition, around 90% of the H₂ fed and produced by reaction in the Pd-Ag MR was recovered as permeate at 15 bar and ca. 450°C (outlet temperature). This stream, completely pure in H₂, did not require any separation/purification, contrarily to the one exiting from the traditional process. Furthermore, other hydrogen was also contained in retentate stream. MR retentate concentrated (ca. 60-70%) and compressed in CO₂ can be easily captured.

 

 

[1] Barbieri G., Brunetti A., Caravella A., Drioli E., “Pd-based Membrane Reactors for one-stage Process of Water Gas Shift”, RSC Adv., 2011, 1 (4), 651-661 http://dx.doi.org/10.1039/C1RA00375E A

Acknolewdgements

The research under this project is co-funded by the European Union Seventh Framework Programme (FP7/2007 - 2013) under DEMCAMER project (NMP3-LA-2011-262840)