The impact of cut point on the processability of Athabasca bitumen

The processability of four extra-heavy Athabasca bitumen vacuum bottoms hav ing different cut points was investigated. The characteristics of these hea vy bitumen fractions changed significantly with the boiling point. The mole cular weight, asphaltenes and metals content and microcarbon residue all in creased with increasing boiling point. High temperature/high pressure micro scopy data indicated that the coke induction period of the four fractions u nder either a hydrogen or nitrogen atmosphere was not affected by the incre ase in boiling point or asphaltenes content. Although the mesophase size an d growth rates were higher under a hydrogen atmosphere, large isotropic are as (no coke) were observed in the presence of hydrogen after a long reactio n 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 th e resid fractions was also studied using a micro-autoclave. At a relatively moderate severity (440 degreesC, 32 min), the coke yield based on feed, in creased with boiling point. However, when coke yield based on the original bitumen was calculated, it decreased as the cut point of the fraction incre ased. These findings indicate that processing higher boiling point feedstoc ks can be achieved without generating larger amounts of coke. A reasonable correlation was obtained between coke yield and asphaltenes content. The co ke yield also correlated with the molecular weight of the fractions. Compar ed with the feeds, the thermal hydrocracking products from all four fractio ns were characterized by higher concentrations of saturates and mono-diarom atics at the expense of polyaromatics and polars. The characteristics of th e products point to side chain fragmentation and C-C bond cleavage as the r esult of thermal treatment. Crown Copyright (C) 2001 Published by Elsevier Science Ltd. All rights reserved.