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

Abstract

The scope of the present study was to characterize the pathways leading to CH 4 generation during the thermal degradation of a model compound representative of methylated monoaromatic hydrocarbons present in many crude oils. 1,2,4-trimethylbenzene was selected and subjected to pyrolysis experiments from 395 – 450 °C (at 100 bar). The whole range of reactant conversions was studied. All pyrolysis fractions were recovered and quantified. All products that could be quantified individually were used to develop a mechanistic kinetic model of 122 reversible reactions involving 47 species up to C 18. The model was validated on our experimental results for conversions below 70%. The model was then used to assess the relative contributions of specified CH 4 generation pathways at high (425 °C) and low (200 °C) temperatures. For the present case study, it was demonstrated that the pathways accounting for CH 4 generation were the same at both temperatures. At low conversions, CH 4 was mainly generated by the ‘polyaromatic route’, i.e. the dimerization of monoaromatics (step 1) followed by the intramolecular ring closure of these dimers (step 2). The contribution of the ‘monoaromatic route’, i.e. the successive demethylation reactions of methylated monoaromatics, was lower but not negligible. At higher conversions the contribution of the ‘monoaromatic route’ increased, eventually accounting for around 50% of the overall yield of CH 4 at 70% conversion. Within the ‘polyaromatic route’, step 1 and step 2 quickly exhibited similar contributions, eventually sharing the remaining 50% of the overall CH 4 yield at 70% conversion.