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 18.104.22.168. 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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