Rule of 100: An inherent limitation or performance measure in oxidative coupling of methane?

The oxidative coupling of methane over La2O3/CaO type-catalyst in a fixed-bed reactor is studied under a wide range of operating conditions (973 < T< 1103 K, 55< p(total) < 220 kPa, and 3.7 < m(cat)/V-STP <50 kg.s/m(3)). A ten-step kinetic model incorporating all main products was used to predict the behavior of the system. Methane conversions and C-2 selectivities were calculated by varying methane to oxygen ratios in the feed under different operating conditions which were also compared with the rule of 100. The results show that deviation from this rule depends on the operating conditions. Within the range studied, an increase in pressure, temperature and contact time results in smaller deviation from the rule. This rule is best approximated when the catalyst operates near its optimal performance. For negative deviations, common to the most catalysts, it is found that the optimal performance should occur at methane conversion levels lower than 50%. A plot of selectivity versus conversion for high-yield reported performance data of a large variety of catalysts shows that data points concentrated roughly in 20%-50% methane conversion region, confirming the generality and prediction of modeling.