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However the reaction scheme of the cpom over ni

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Unformatted text preview: hydrocarbons on an industrial scale as they are active and inexpensive. However the reaction scheme of the CPOM over Ni catalysts is still under discussion. Generally two extremes and combinations of them are considered (Fig. 4). In the one extreme CO and H2 are the primary products of the oxidation of CH4. Reactions leading to the thermodynamically most stable reaction mixture containing next to CO and H2 also H2O and CO2 are secondary reactions. If this scheme was valid it would be possible to achieve CO and H2 yields higher than predicted by the overall process thermodynamics by suppressing the secondary reactions leading to the equilibration. In the other possible extreme first all O2 is consumed in the total oxidation of part of the CH4, yielding H2O and CO2. H2O and CO2 then react in the second step with the remaining CH4 yielding CO and H2. The overall thermodynamics present the maximum CO and H2 yields achievable. 35 1. Introduction 4 CH4 + 2 O2 - 36 kJ/mol - 803 kJ/mol 4 CO + 8 H2 + 767 kJ/mol CO2 + 2 H2O + 3 CH4 Fig. 4 Scheme of the two extreme reaction pathways for the partial oxidation of CH4 Direct syngas formation Choudhary et al. proposed that CO and H2 are the primary products over Ni catalysts [10, 104]. They studied CPOM over Ni catalysts at high space velocities and undiluted feed and found that the selectivities towards CO and H2 were higher than those predicted by the thermodynamics of the overall process including side products. They concluded that this was due to the fact that CO and H2 are primary products and that at short enough contact times the equilibration of the gas mixture is not possible. Other authors showed that under the applied conditions huge temperature gradients exist and that the local temperature in the catalyst can be more than 300°C higher than the measured one [9, 69, 109]. Therefore it is difficult to draw any comparison of these results with thermodynamic data. Qin et al. also observed a rise in the temperature due to the partial oxida...
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