17-09-2012, 03:31 PM
Water Gas Shift Reaction
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Introduction:
To obtain higher conversions overall, Water Gas Shift(WGS) is normally conducted in multiple adiabatic stages with inter-stage cooling. the first stage is High temperature shift (HTS) with an operating temperature range of 350°C to 600°C, while the second stage is low temperature shift (LTS) with anoperating temperature range of 150°C to 300°C. Copper based catalysts are typically used industrially for LTS operations. The exact composition of these catalysts may vary according to their specific applications and their accompanying supports. The lower limit of the operating temperature in the LTS reactor is 190-2000 C and 30 bar which happens to be the dew point of water indicating the effect of steam on the catalytic activity of the Cu-based, LTS catalysts.
Commercial hydrogen is usually produced from Water Gas Shift reaction which is one of the main industrial reactions. The interest in this grew because of its application in fuel cell, wherein the simultaneous production of hydrogen while decreasing the carbon monoxide is dual benefit, CO being the catalyst poison.
Reaction Mechanisms:
The regenerative mechanism is believed to be the dominant pathway for the HTWGS reaction over Fe-Cr catalysts. The importance of the redox mechanism in these high temperature catalysts has been confirmed by Boreskov1 who showed that a Fe2+/Fe3+ coupling existed in Fe3O4-Cr2O3 catalysts, with Fe2+ being oxidised to Fe3+by water and Fe3+ being reduced by carbon monoxide.
The Low Temperature Water Gas Shift
Three groups of catalysts which are used in The LTS are (1) Cu-ZnO-Al2O3, which is a standard catalysts currently used in the industries (2) the Pt group metals supported on partially reducible oxides, like ceria, Titania, zirconia or their mixtures, and © Au supported on oxides, like ceria, titania, zirconia or their mixtures. The reaction mechanism on oxide-supported Au catalysts is different from that on supported platinum metal catalysts for the following reasons (1) the lower adsorption energy of CO on the Au nanoparticles; (2) the inactivity of Au (unlike the Pt- group of metals) for H2O dissociation.
The LTS reaction over Cu-ZnO catalysts are distinguished from the HTS reaction over iron oxide – chromia because of the following: (1)the dissociation of H2O to H2 and O over copper metal or ZnO, at these low temperatures, is less documented compared to that on the iron oxide, Fe3O4, at the higher temperatures. (2)In the low operating temperature of the LTS the amount of CO adsorbed on metallic copper, is lower compared to the platinum group of metals at similar temperatures (3) the WGS rate is proportional to the CO partial pressure to the first order over the Cu-ZnO compared to the zero order observed over the Pt-based catalysts.