Elaboration of a kinetic model in order to predict the molecular and isotopic composition of natural gas generated during the thermal cracking of hydrocarbons

Abstract

The scope of the present study was to validate an approach that could be used to elaborate a model that would predict the δ[superscript]13C of the gases generated during thermal cracking of oil. The attention was focused on C[subscript]14- methylaromatics and alkylaromatics but the entire methodology was demonstrated on one component i.e. 1,2,4-trimethylbenzene.Pyrolysis experiments at temperatures of 395, 425, 450, and 475 °C and at a pressure of 100 bar were performed in order to study the whole range of conversions. All pyrolysis fractions were recovered and quantified. All identified products were also quantified individually. A free-radical mechanism until 70% conversion of 1,2,4-trimethylbenzene was achieved. This mechanism was then used to characterize some CH[subscript]4 generation pathways at 425 and 200 °C. In both cases the identified pathways included: (i) demethylation of 1,2,4- trimethylbenzene into xylenes (and to a lesser extent demethylation of xylenes into toluene), (ii) dimerizarion of monoaromatics, (iii) intramolecular ring closure reaction of dimers into triaromatics.In a second step, the free-radical mechanism was used to constrain the chemistry of a simpler lumped kinetic model predicting CH[subscript]4 generation under laboratory and geological conditions for the whole range of conversions. The resulting scheme was composed of four pathways P[subscript]i for methane generation: Reactant --> Dimers (P[subscript]a), Reactant --> Xylenes (P[subscript]b), Dimers --> {Prechar + Char} (P[subscript]c), and Xylenes --> Dimers + Toluene (P[subscript]d). Optimization yielded activation energies in the range of 50-60 kcal/mol, and frequency factors in the neighbourhood of 10[superscript]12 s[superscript]-1. Simulations revealed that P[subscript]b and P[subscript]c led to the greatest amounts of CH[subscript]4 below 5% conversion, followed by P[subscript]a. Above 5% conversion, CH[subscript]4 generated via P[subscript]c became dominant but P[subscript]a and P[subscript]b were also found to be of importance. Contribution of P[subscript]d was found to be negligible, except for when 100% conversion was almost reached. Simulations under geological heating rates revealed that significant amounts of CH[subscript]4 were generated by methylated monoaromatics in deeply buried reservoirs and that methylated monoaromatics thus had a higher thermal stability than their polyaromatic counterparts but lower than the saturated hydrocarbons.CH[subscript]4 yield was also modelled using a unique stoichiometric equation (CH[subscript]4max = 7.6 wt% per methyl group) associated with Ea = 58.5 kcal/mol and A = 10[superscript]11.96 s[superscript]-1, showing relative similarities to other reported values for methylated polyaromatics. In the final stage, P[subscript]a, P[subscript]b, and P[subscript]c were selected as relevant contributions to δ[superscript]13C[subscript]CH4 until 100% conversion. Kinetics for the generation of [superscript]12CH[subscript]4 and [superscript]13CH[subscript]4 were then expressed separately and implemented into the lumped model. Optimization yielded a ratio of frequency factors Ω = 1.028, variations of activation energy ΔE[subscript]i in the range of 36-79 cal/mol (kinetic effect); and a δ[superscript]13C[subscript]p of CH[subscript]4 precursor groups equal to -32.7‰ (precursor effect). Simulations performed under geological heating rates illustrated the greater isotopic fractionation of CH[subscript]4 generated under geological conditions compared with laboratory conditions. The comparison at high maturity with δ[superscript]13C[subscript]CH4 during thermal cracking of 1-methylpyrene and mature kerogen under the same simulation conditions emphasized the need to determine the magnitude of the precursor effect for natural compounds.