Selectivity Boudouard reactions

Rostrup-Nielsen and Pedersen (209) recently studied sulfur poisoning of supported nickel catalysts in both methanation and Boudouard reactions by means of gravimetric and differential packed-bed reactor experiments. In their gravimetric experiments a synthesis mixture (H2/CO/He = 5/7/3) containing 1-2 ppm H2S was passed over a catalyst pellet of 13% Ni/Al203-MgO at 673 K and 1 atm. The rates of Boudouard and methanation reactions were determined from weight increases and exit methane concentrations respectively. In the presence of 2 ppm H2S a factor of 20 decrease was observed in both methanation and Boudouard rates over a period of 30-60 min. However, the selectivity or ratio of the rates for Boudouard and methanation reactions was constant with time. From these results the authors concluded that the methanation and Boudouard reactions involve the same intermediate, carbon, and that sulfur blocks the sites for the formation of this intermediate. 

The maximum absolute methane conversion of 21% was obtained at the temperature of 500 °C, while the maximum carbon dioxide conversion of 23% was achieved at 400—450 °C. Moreover, carbon monoxide selectivity was always higher than hydrogen selectivity, and this was read as an empirical proof of the effect of reverse water gas shift reaction and Boudouard reaction (Paturzo et al., 2003). 

A second point is that selective catalysts can promote desirable reactions at the expense of undesirable ones (e.g. carbon formation according to Boudouard). Finally, while reaction rates increase with increasing temperature, in many cases the overall thermodynamic efficiency decreases with temperature (due to entropy production). 

For the equilibrium model to accurately estimate the yields of the compounds forming the synthesis gas, it is necessary to select the most representative reactions. In this sense it is appropriate to focus all attention on the sections of oxidation and reduction of the gasifier. Because the oxygen fed to the gasifier is completely consumed in the oxidation section, while the carbon of the vacuum residue is oxidized and passes into the gas phase, this phase is really critical to the process and it is here where the conversion of carbon into the gas phase is defined. Thus, the methanation (Equation 4.4) and Boudouard (Equation 4.6) reactions are the ones representing the equilibrium of the carbon oxidation step toward the formation of compounds of synthesis gas. The main compounds produced in the oxidation step reach equilibrium in the reduction section for reactions (4.7) and (4.8). Reactions (4.9) and (4.10) are not considered important because of the lack of oxygen in the system and the low methane production. Also, the reactions (4.11) through (4.16) are neither taken into account because they only define the equilibrium for trace compounds in the synthesis gas. 

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