New insights into secondary gas generation from the thermal cracking of oil: Methylated mono-aromatics. A kinetic approach using 1,2,4-trimethylbenzene. Part II: An empirical kinetic model

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 CH4 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 < methylated mono-aromatics < saturates. The risk for the decrease in the porosity of reservoirs was also quantified via the prediction of the yield of (prechar + char) components. (C) 2009 Elsevier Ltd. All rights reserved.