Molecular nature of athabasca bitumen

Athabasca bitumen is a heavy hydrocarbon recovered from oil sands. During u pgrading, bitumen is first distilled to remove lighter components which are processed in hydrotreaters. This distillable portion, heavy gas oil, accum ulates nearly 80 w/w% of the saturates present in the original material. Th e aromatic character and heteroatoms content of the molecules in this fract ion increase with the boiling point of the components. The residue from dis tillation, bitumen pitch, is subjected to thermal cracking followed by hydr otreating. The extractable front fractions from pitch show a trend for increasing arom atic content with a concomitant decrease in H/C atomic ratios. This is a re flection of greater numbers of aromatic rings with a higher degree of conde nsation and decreasing degree of substitution. The insoluble end-cut from p itch is characterised by the presence of "core" structures comprising conde nsed polyaromatic rings associated with heteroatoms (N) and trace metals (N i, V). The heaviest sub-fractions from the end-cut contain more than 10 con densed aromatic rings and are enriched in heteroatoms (N) and metals (Ni, V ). By comparison, the lighter end-cut material comprises relatively non-pol ar molecules with an average of only 7 aromatic ring structures. Because th ese "cores" are both coke precursors and strong chromophors, their light ab sorbing propensity, measured by K/C values, may be indicators of coke farmi ng propensity.