(28b) Heavy Oil Tutorial: Polynuclear Aromatics, the Barrier to Conversion

The basic problem of the conversion of petroleum resids and heavy oils are the presence of polynuclear aromatics (PNS's) greater than four rings while the only aromatics in transportation fuels that are desirable are alkylated single rings in gasoline. It is these polynuclear aromatics that provide a thermal reaction limit and insolubility, resulting in fouling and coking. Although analytical tools cannot determine the full PNA distribution, HPLC provides data on the heptane soluble fraction for four rings and less and even hydrogen content provides insight. It is the objective of primary upgrading to produce the maximum feed for secondary upgrading that is relatively free of catalyst poisons, including PNA's greater than four rings, while rejecting a low valued byproduct. Fortunately, even with the limited understanding of the chemical structure of petroleum macromolecules, the path is clear on how to greatly improve such primary upgrading processes as coking and hydroconversion.