Relationship between aromatic hydrogen and aromatic carbon in straight run and converted oils

[Display omitted] •Aromatic hydrogen content, HAr, and aromatic carbon content, CAr, are only broadly related.•Boiling range of a fraction was an inadequate descriptor for the relationship of HAr to CAr.•An increase in heterocyclic aromatic content decreased the (H/C)Ar.•The HAr and CAr changed in different ways with respect to thermal conversion severity. In the conversion of high boiling oil fractions, it is difficult to track compositional changes that affect the chemical nature of species. For reaction engineering, monitoring changes related to hydrogen transfer reactions and formation of aromatic species as thermal conversion progresses to coking is useful. Proton and carbon-13 nuclear magnetic resonance (1H and 13C NMR) spectrometry can measure the aromatic hydrogen and aromatic carbon content in oils of diverse origin and distillation range. 13C NMR analyses are more time consuming than 1H NMR analyses. The ability to calculate the aromatic carbon content of oils directly from measured aromatic hydrogen content is alluring and is reportedly feasible (Cookson, et al. Fuel 1986; 65:1247–1253). In this work previously reported 1H and 13C NMR data for different petroleum and coal derived oils were collated and supplemented with experimental measurements that included narrow cuts of straight run and refined oils, as well as bitumen-derived mixtures that were thermally converted at different levels of severity. It was found that aromatic carbon content, CAr, was only broadly related to aromatic hydrogen content, HAr. A wide range of values for CAr was possible for a specific value of HAr. The medium atmospheric equivalent boiling point temperature, T50, was a poor descriptor for indicating a directional change in the relationship between CAr and HAr, both for wide and narrow boiling range distillation fractions. An increase in heterocyclic aromatic content caused a decrease in the (H/C)Ar ratio, since heteroatoms in aromatics are mainly found in ring-edge and not in ring-junction positions. When the changes in HAr and CAr of different bitumen-derived materials with respect to thermal conversion severity were evaluated, it was found that changes in HAr and CAr were not in a fixed relation to each other.