The Chemical Composition of Solubility Classes from Athabasca Bitumen Pitch Fractions

Abstract

As the Canadian supply of light crudes diminishes, refineries have necessarily been required to deal with more difficult to process oilsands bitumen and heavy oils. Nearly 50% of bitumen consists of a residuum that cannot be easily upgraded. A better understanding of the chemistry of this material is the key to achieving overall optimal yields of commercially useful products. In recent years, the application of supercritical fluid extraction (SCFE) with pentane has been successful in separating the bitumen residuum into a number of narrow fractions based on solubility parameters and molecular weight. In combination with advanced characterization techniques this approach has proved to be useful in elucidating the complexity of bitumen chemistry. To provide further insights into the chemistry of bitumen pitch we now extend this established approach to the solubility classes (saturate, aromatics and resins) derived from the bitumen fractions, or front-cuts, separated during the SCFE process with pentane. The amount of the saturate class present in the front-cuts decreases rapidly with cumulative yield. In the same range, resin content increases continuously while aromatic content passes through a maximum. No asphaltene is present in the front-cuts; this component is separated in the pentane insoluble end-cut fraction during SCFE. Both aromatics and resins from the front-cuts show a trend for increasing aromatic content and decrease in H/C atomic ratios at higher cumulative yields. This is a reflection of increasing number of aromatic rings, with a higher degree of condensation and decreasing degree of substitution, for larger and more complex molecules. In terms of structural parameters and molecular sizes we find relatively minor differences between the corresponding aromatic and resin fractions in each front-cut. The most noteworthy difference between these SARA classes lies in the significantly higher nitrogen contents of the resin class.