The impact of cut point on the processability of Athabasca bitumen

The processability of four extra-heavy Athabasca bitumen vacuum bottoms having different cut points was investigated. The characteristics of these heavy bitumen fractions changed significantly with the boiling point. The molecular weight, asphaltenes and metals content and microcarbon residue all increased with increasing boiling point. High temperature/high pressure microscopy data indicated that the coke induction period of the four fractions under either a hydrogen or nitrogen atmosphere was not affected by the increase in boiling point or asphaltenes content. Although the mesophase size and growth rates were higher under a hydrogen atmosphere, large isotropic areas (no coke) were observed in the presence of hydrogen after a long reaction time. The similarity in the induction period indicates that, even in the higher boiling point fractions, sufficient maltenes were present to peptize the high content of asphaltenes, thus preventing a phase separation and a reduction of the coke induction period from occurring. The reactivity of the resid fractions was also studied using a micro-autoclave. At a relatively moderate severity (440°C, 32 min), the coke yield based on feed, increased with boiling point. However, when coke yield based on the original bitumen was calculated, it decreased as the cut point of the fraction increased. These findings indicate that processing higher boiling point feedstocks can be achieved without generating larger amounts of coke. A reasonable correlation was obtained between coke yield and asphaltenes content. The coke yield also correlated with the molecular weight of the fractions. Compared with the feeds, the thermal hydrocracking products from all four fractions were characterized by higher concentrations of saturates and mono–diaromatics at the expense of polyaromatics and polars. The characteristics of the products point to side chain fragmentation and C–C bond cleavage as the result of thermal treatment.