Characterization of molecular change of heavy oil under mild thermal processing using FT-IR spectroscopy

In order to quickly and economically obtain the information of molecular changes occurring in heavy oils during thermal processing, simulated mixtures of heavy oils and their pyrolyzed products were prepared from four model compounds, i.e., naphthalene, tetralin, decalin, and n-heptane. FT-IR absorption characteristics and average molecular parameters of the mixtures were correlated; the molecular parameters included the number ratio of methylene to methyl (NCH2/NCH3), hydrogen aromaticity (faH), atomic ratio of hydrogen to carbon of the aromatic sheet (NHar/NCar), etc. Considering heavy coker gas oil (HCGO) as a heavy oil feedstock, HCGO was thermally processed in an autoclave and its gas oil fraction (GOF) was separated into group fractions and analyzed by FT-IR. The results show that the simulated mixtures show good linearity between faH and the absorption area ratio of the bands at 3100 - 2750 cm−1 to 3100 - 3000 cm−1 (S3000-3100/S2750-3100), and so does (NCH2/NCH3) and absorbance ratio of the bands at 2920 - 2960 cm−1 (A2920/A2960). These correlations could be useful tools for illustrating molecular changes occurring in heavy oil during thermal processing. As the processing proceeds, the NCH2/NCH3 of the saturate molecules increases first and then declines, in sharp contrast to that of the aromatic molecules, which shows a consistent decline. As far as faH or NHar/NCar is concerned, however, the aromatic molecules show a rising trend. The chemistry behind these phenomena is also discussed in some detail.