Molecular nature of athabasca bitumen

Athabasca bitumen is a heavy hydrocarbon recovered from oil sands. During upgrading, bitumen is first distilled to remove lighter components which are processed in hydrotreaters. This distillable portion, heavy gas oil, accumulates nearly 80 w/w% of the saturates present in the original material. The aromatic character and heteroatoms content of the molecules in this fraction increase with the boiling point of the components. The residue from distillation, bitumen pitch, is subjected to thermal cracking followed by hydrotreating. The extractable front fractions from pitch show a trend for increasing aromatic content with a concomitant decrease in H/C atomic ratios. This is a reflection of greater numbers of aromatic rings with a higher degree of condensation and decreasing degree of substitution. The insoluble end-cut from pitch is characterised by the presence of "core" structures comprising condensed polyaromatic rings associated with heteroatoms (N) and trace metals (Ni, V). The heaviest sub-fractions from the end-cut contain more than 10 condensed aromatic rings and are enriched in heteroatoms (N) and metals (Ni, V). By comparison, the lighter end-cut material comprises relatively non-polar molecules with an average of only 7 aromatic ring structures. Because these "cores" are both coke precursors and strong chromophors, their light absorbing propensity, measured by K/C values, may be indicators of coke farming propensity.