Structure Representation of Asphaltene GPC Fractions Derived from Kuwaiti Residual Oils

Abstract

Two asphaltenes obtained from a residual oil before and after hydrotreatment were subjected to preparative GPC. Selected GPC fractions were analyzed by NMR and XRD to derive structural parameters to investigate whether there is a relationship between the molecular size of the asphaltene fraction and its structural features. Under the GPC experimental conditions (∼5 wt % THF solution), some eluting species are aggregates rather than monomeric molecules. Nevertheless, a correlation between the molecular size of the GPC fraction and its structure was found. The higher the molecular weight (MW), the more aromatic and condensed is the fraction. Moreover, the asphaltene fractions with the highest MW of both residues, before and after hydrotreating, show very similar structural features. This fact suggests that during hydrotreatment the low MW asphaltenes are converted to distillates, whereas the higher MW fractions (>6000 Da) stay unaltered and can cause severe coking in the downstream processes because of the highly refractory asphaltenes, which are still present. Structure presentations derived from NMR and XRD data demonstrate that the higher MW fractions (≈5250 Da) of both residues consist of clusters of 4−5 aromatic rings, which are interlinked by aliphatic chains of 9−11 carbon atoms.