Effect of Thermal and Hydro-Catalytic Treatment on the Molecular Chemistry of Narrow Fractions of Athabasca Bitumen Pitch

Abstract

As much as 50 w/w % of bitumen derived from Athabasca oil sands consists of a residuum that cannot be easily upgraded. A better understanding of the chemistry of this material is the key to achieving optimal yields of commercially useful products. In recent years, the application of supercritical fluid extraction (SCFE) with pentane has been successful in separating the residuum into a number of narrow fractions based on their solubility parameters. An important corollary to this separation is that the molecular weight of each fraction increases with the severity of conditions needed to extract the progressively heavier components in each residua. In combination with advanced characterization techniques this approach has proved to be useful in elucidating the complexity of bitumen chemistry. For the work described here, this approach has been applied to residua from commercial operations at the Syncrude Canada Ltd. plant in Northern Alberta. Base samples included virgin bitumen pitch (P) together with residua from the catalytic hydro-cracking (HC) unit and a once-through fluid-coking (CK) test.The SCFE insoluble, heavy sub-fractions in the pitch contain “core” structures, comprising condensed polyaromatic rings, associated with heteroatoms and trace metals. In the catalytic hydro-cracking process the cleavage of C−S side chains and the breakage of inter-unit bonds are the prevailing reactions. A comparison between the lighter, SCFE-soluble fractions from catalytic hydro-cracking and thermal cracking shows the latter to have lower unit molecular weights, higher sulfide contents, and shorter alkyl substituents. The highly aromatic, SCFE-insoluble molecules from the CK process are likely formed from condensation reactions involving various fragments of toluene-soluble and -insoluble material. Condensation reactions between heavy “core” molecules from the SCFE insolubles in pitch lead to formation of coke. In the catalytic hydro-cracking process single unit, highly aromatic, polar “cores” from pitch SCFE insolubles appear to remain toluene-soluble and thus constitute most of the HC residue. These coke precursors will be converted to coke, or pseudo-solid species, if subjected to the thermal treatment applied in coking or severe hydro-cracking.