Comparison study of the gas-phase oxidation of alkylbenzenes and alkylcyclohexanes

Abstract

The goal of this paper is to present new experimental results obtained during the study of the gas phase oxidation of ethyl-benzene, n-hexyl-benzene, ethyl-cyclohexane and n-butyl-cyclohexane which belongs to two molecule families present in diesel fuels: alkylbenzenes and alkylcyclohexanes. Experiments were carried out in a jet-stirred reactor over the temperature range 550–1100K. The new results have been compared with existing literature data obtained for alkylbenzenes and alkylcyclohexanes with alkyl chains of different size to highlight the influence of the chain size on the reactivity. The comparison showed that both alkylcyclohexanes exhibit reactivity at both low- and high-temperatures such as cyclohexane and that the reactivity was similar whatever the size of the alkyl chain. For the three compared alkylbenzenes, important differences were observed in the reactivity at low-temperature: ethylbenzene started to react only above 750K, while other compounds reacted from 550K. The comparison also showed that alkylbenzenes were less reactive than their alkylcyclohexane homologs and that the production of aromatic compounds known to promote soot formation was also significantly larger for alkylbenzenes. This paper also presents the effect of the equivalence ratio and pressure on the reaction kinetics. In a general manner, a decrease of the fuel/air ratio significantly increased the reactivity and the carbon monoxide selectivity below 800K, but decreased the selectivity of heavy oxygenated products, the atmospheric degradation of which can be a source of toxic oxygenated products. This decrease had a more limited effect on the reactivity at higher temperatures but disfavored the production of unburned species (oxygenated species like acetaldehyde and unsaturated hydrocarbons which are known to be soot precursors). A pressure increase from 1 to 10bar enhanced the reactivity of all these hydrocarbons over the full studied temperature range, with a start of the reaction at lower temperatures. A larger production of toxic oxygenated products was observed with increasing pressures, while low pressures promoted the formation of soot precursors. Alkylbenzene results were generally well reproduced by simulations using literature models.