New insights into secondary gas generation from the thermal cracking of oil: Methylated monoaromatics. A kinetic approach using 1,2,4-trimethylbenzene. Part III: An isotopic fractionation model

Abstract The scope of the present study was to develop an empirical kinetic model predicting, over the whole range of reactant conversions (0–100%), the yield of CH 4 generated from the thermal degradation of 1,2,4-trimethylbenzene, a model compound for methylated mono-aromatic hydrocarbons present in oil. Most of the chemical equations of the model were constrained by our previous mechanistic model ( Fusetti et al., 2010 ). The resulting reaction scheme was composed of four CH 4 generation pathways: ( P a ) dimerization of 1,2,4-trimethylbenzene, ( P b ) demethylation of 1,2,4-trimethylbenzene into xylenes, ( P c ) condensation reactions of dimers and C 18+ products into (prechar + char) components and ( P d ) dimerization of xylenes and their demethylation into toluene. Associated activation energies were in the range 52–61 kcalmol -1 and frequency factors were all in the neighborhood of 10 12  s −1 . Below 5% conversion, P b and P c governed CH 4 generation, followed by P a . Above 5% conversion, P c was the main source of CH 4 , followed by P b and P a , respectively. P d showed negligible CH 4 yields up to 95% conversion. Above 100% conversion, the degradation of (prechar + char) components seemed the most likely new source of gas which was not accounted for in the model. Using a unique chemical equation with a maximum CH 4 yield of 7.6 wt% per CH 3 group and an associated set of kinetic parameters E a  = 58.5 kcalmol -1 and A  = 10 11.96  s −1 , we demonstrated CH 4 generation kinetics from the thermal degradation of 1,2,4-trimethylbenzene to be similar to CH 4 generation kinetics previously reported from the thermal degradation of methylated polyaromatic hydrocarbons. Eventually, the four-equation empirical model was used to perform simulations under temperature conditions usually encountered in deeply buried reservoirs (DBR). Under these conditions, the simulations revealed the CH 4 prone character of methylated mono-aromatic hydrocarbons. Moreover, these simulations demonstrated that the thermal stability increased as follows: methylated polyaromatics