Highlighting the “structure–reactivity” relationship and the impact on the kinetics for the autoxidation reaction of hydrocarbons

Abstract

The interactions between heteroatomic species and the hydrocarbons present in Jet A-1 make the determination of degradation mechanisms difficult to elucidate and predict. This study aims to investigate the reaction mechanisms and kinetics of hydrocarbon molecules constituting jet fuel, excluding molecules with polar species. Five model hydrocarbons were selected (n-dodecane and its isomeric mixture, n-butylcyclohexane, 1,2,4-trimethylbenzene and 1-methylnaphthalene) and individually subjected to oxidation using the PetroOXY apparatus, focusing on the onset of autoxidation (ΔP/Pmax = 2 to 10 %). For the first time, an experimental protocol has been developed for the monitoring of gas and liquid phase reagent consumption and the identification and quantification of oxidation products formed in these phases. The results showed that oxygen consumption was always higher than that of the initial hydrocarbons, suggesting competitive reactions, with alkanes consuming more oxygen than aromatic molecules, as confirmed by induction periods (IP), demonstrating the different reactivity depending on the structure of the oxidized hydrocarbon molecule. A relationship between initial hydrocarbon structure and the type and number of carbon atoms in oxidation products was also demonstrated. A gel, probably a precursor deposit, was obtained after oxidation of the molecules, indicating a potential precipitation of polar species due to the difference in polarity with the hydrocarbon molecules. In addition, the overall kinetic and hydroperoxide dissociation constants of the reaction could be calculated from tests carried out at different temperatures (140, 150 and 160 °C). The Arrhenius parameter values were higher for alkanes than for aromatics, confirming the structure–reactivity relationship of hydrocarbons. A strong correlation between the Arrhenius parameters and the nature of the oxidized molecules was also demonstrated. This experimental study will be useful for the calibration of future kinetic models according to the chemical structure of the oxidized hydrocarbons and for the assessment of thermal stability behavior of 100 % Synthetic Alternative Fuels (SAF).