Thermal decomposition behaviour and structural characteristics of asphaltenes

Abstract

The thermal behavior of 7 characterized n-C7 asphaltenes has been investigated by thermogravimetry and pyrolysis/gas chromatography/mass spectrometry (Py/GC/MS). Analytical data and results of structural analysis reveal substantial differences among the n-C7 asphaltene samples. The aromatic carbon fraction ranged between 0.69 and 0.48 and the sulphur content was between 2.9 and 10.8%. A substantial fraction of peripheral carbon in hydroaromatic/aromatic sheets (0.2 – 0.3) was substituted with aliphatic chains and a significant fraction of the aliphatic moieties (15–48%) were found to be present as naphthenic structures. Results of TG pyrolysis and 13CNMR data showed that final yields of pyrolysis depended linearly on the aliphatic carbon fraction of the asphaltene. A linear relationship was also found between the fraction of sulphur evolved and final weight losses. Analysis of volatile matter by FT-ir and the Py/GC/MS technique indicated that its composition was predominantly aliphatic (∼ 80%) while the remainder was formed by alkyl-substituted aromatic compounds. The main classes of compounds identified were n-alkanes, l- n alkanes, isoalkanes, cycloalkane and alkyl-substituted benzene. The amount of methane formed during pyrolysis ranged between 5.8 and 8.7% of carbon present in the initial sample. Its formation occurs mainly at high degrees of conversion, apparently through two reaction pathways. A non isothermal technique using different heating rates was applied to determine the kinetic parameters of asphaltene pyrolysis. In each case, the data showed an increase of Eact during reaction, particularly at high conversion degrees.