Kinetic effect of alkylaromatics on the thermal stability of hydrocarbons under geological conditions

A compound representative of the alkyl aromatic fraction in oils (decylbenzene) was artificially matured in the presence of an n-alkane, n-hexadecane ( nC 16) using confined pyrolysis (pressurized gold cells). A significant inhibition of the cracking of nC 16 was observed in the presence of decylbenzene. Identification and quantitation of all compounds has led to the writing of a reactive network model. The model is based on the cracking reactions of the pure compounds and the cross-reactions between the reactants. Conversion was held sufficiently low that secondary reactions were not too numerous to handle. The accuracy of the derived computer model was checked by comparing the predicted conversion and product yield curves with the experimental results. The kinetic model allows direct extrapolation of the behaviour of the hydrocarbon mixture to geological temperatures ( T=200 °C) without any user operated adjustment. The model predicts that decylbenzene has a strong inhibition effect on nC 16 cracking. The inhibition effect can be explained by the increased relevance of the retroene reaction (a molecular reaction) at low temperatures which directly forms toluene, a very strong inhibitor of alkane cracking. These considerations lead to the definition of a new geochemical parameter, the Inhibition Factor (I.F.) which measures the kinetic effect (inhibition or acceleration) of one hydrocarbon family on the other in complex mixtures. The Inhibition Factor brings a rational explanation for the so called “mixture effect” invoked by other authors when studying the thermal stability of alkanes in oils [Energy and Fuels 9 (1995) 1990; Geochim. Cosmochim. Acta 61C (1997) 3725; Org. Geochem. 29 (1998) 119; Energy and Fuels 16 (2002) 831]. One major conclusion of this paper is that kinetic behaviour of a single compound differs strongly from that of the same compound in mixtures. Thus, the kinetic study of the thermal decomposition of pure compounds cannot lead to the prediction of complex mixtures. Our model predicts that the thermal stability of n-alkanes in hydrocarbon mixtures is greatly enhanced by long-chain alkyl aromatics at geological temperatures.