Partial oxidation of hydrocarbons: an experimental and kinetic modeling study

Partial oxidation offers an attractive alternative route to form olefins from alkanes in simple and low cost reactors. Using Pt catalyst on alpha-alumina support in a reactor with a contact time of a few milliseconds, olefin selectivities of up to 70% have been achieved at large alkane and O-2 conversion without catalyst fouling and deactivation. Under suitable conditions, cyclohexane oxidative dehydrogenation to form cyclohexene prevails over the decomposition reactions toward butadiene and smaller olefins. A general and extensive pyrolysis and oxidation reaction scheme, involving about 150 species and 3000 reactions, has been used to verify the importance of homogeneous reaction paths in explaining the experimental results of oxidative pyrolysis of alkanes from propane up to hexanes. For higher alkanes, relevant cracking occurs even at short contact times and considerable ethylene and propylene yields are obtained. Comparisons between model predictions and experiments at different temperatures (1000-1300 K), residence times (1-5 ms), and alkane to oxygen ratios (0.5-1.2) suggest that surface reaction steps play a significant role in this process especially for the smaller alkanes and at the lower temperatures (higher alkane to oxygen ratios). In these conditions, catalyst activity promotes the reactivity of the system and improves CO2 formation as well as oxidative dehydrogenation. Few simplified catalytic steps coupled with the homogeneous reaction scheme allow us to explain the experimental data. This model is a useful tool for investigating and optimizing operating conditions and reactor configurations.