Detailed Chemical Kinetic Modeling of Cyclohexane Oxidation†

Abstract

A detailed chemical kinetic mechanism has been developed and used to study the oxidation of cyclohexane at both low and high temperatures. Reaction rate constant rules are developed for the low temperature combustion of cyclohexane. These rules can be used for in chemical kinetic mechanisms for other cycloalkanes. Since cyclohexane produces only one type of cyclohexyl radical, much of the low temperature chemistry of cyclohexane is described in terms of one potential energy diagram showing the reaction of cyclohexyl radical + O 2 through 2 five, six and seven membered ring transition states. The direct elimination of cyclohexene and HO 2 from RO 2 is included in the treatment using a modified rate constant of Cavallotti et al.Published and unpublished data from the Lille rapid compression machine, as well as jet-stirred reactor data are used to validate the mechanism. The effect of heat loss is included in the simulations, an improvement on previous studies on cyclohexane. Calculations indicated that the production of 1,2-epoxycyclohexane observed in the experiments can not be simulated based on the current understanding of low temperature chemistry. Possible 'alternative' H-atom isomerisations leading to different products from the parent O 2 QOOH radical were included in the low temperature chemical kinetic mechanism and were found to play a significant role.

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“…The chemistry involved in the oxidation of cycloalkanes being close to that proposed for alkanes, the groups of Livermore [224][225], Milano [227] and Nancy [228] have mainly considered the same types of reactions with close kinetic parameters to what is described in paragraph 2.4. We shall then only review here the differences which had to be taken into account due to the presence of the cycle, mainly in the case of isomerizations and the formations of bicyclic ethers.…”

“…Up-dating the classes of reactions and the kinetic data proposed for alkanes [70], the team of Westbrook and Pitz has proposed models for the oxidation of cyclohexane (5859 reactions) [224] and methylcyclohexane (7026 reactions) [225]. The team of Ranzi and Faravelli has first proposed a globalized mechanism to model the oxidation of cyclohexane [226], with 2 globalized steps to reproduce the high temperature decomposition and 6 reactions to take into account the low temperature behaviour, but a second more detailed mechanism, with 12 sensitive rate constants estimated through quantum mechanics, has been more recently proposed [227].…”

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“…Results of this are compared in Figure 7 shows that exclusion of formally direct pathways results in the removal of the peak at the short timescale for both mechanisms which confirms that prompt OH-formation at low temperatures is a signature for formally direct pathways. The differences in the concentrations between the Silke et al [30] and our model is due to the different level at which secondary chemistry is represented as well as different rate coefficients adopted. Therefore, the observed discrepancy between the two models on the long timescales is not unexpected.…”

“…In order to understand these pressure effects, we studied the OH formation in the cyclohexyl + O 2 reaction at higher pressures (6-20 bar) in the temperature range of 586-828 K. The ME based kinetic model of 44 reactions from Knepp et al [8] was adapted to calculate the OH time profiles for the experimentally investigated pressure ranges. The comprehensive mechanism from Silke et al [30] was also used to predict the OH concentration time profiles under our conditions. Fig.…”

“…rate coefficients connecting nonadjacent wells, reactants or bimolecular products. In the second case a detailed mechanism of Silke et al [30] was used in an unmodified manner, while in the fourth case a hybrid model was constructed by adding our ME rate coefficients to their model. Results of this are compared in Figure 7 shows that exclusion of formally direct pathways results in the removal of the peak at the short timescale for both mechanisms which confirms that prompt OH-formation at low temperatures is a signature for formally direct pathways.…”