Thermal hydrocracking of Cold Lake vacuum bottoms asphaltenes and their subcomponents

Abstract

The thermal chemistry of bitumen and heavy oils is extremely complicated, mostly because of the complex nature and the unknown molecular structure of different components that are present in these materials. In order to understand the molecular transformation of bitumen during upgrading better, the following approach was adopted: separate the bitumen into different components (asphaltenes and maltenes), characterize them in detail and react each fraction separately. In the present work, the results of thermal reaction of asphaltenes and its subfractions (A1–A4) separated by column chromatography are presented. Thermal hydrocracking of asphaltenes derived from Cold Lake vacuum bottoms (CLVB) was investigated, neat and in a hexadecane solvent at 440 °C (30 min and 13.8 MPa H 2). The amount of coke measured as methylene chloride insolubles remained relatively constant at about 25 wt.%, whether the asphaltenes reacted neat or in hexadecane. Increasing asphaltenes concentration in hexadecane had little effect on coke formation. Results from the present study were compared with those reported previously from our laboratory for the same CLVB asphaltenes, which were thermal hydrocracked in CLVB maltenes and in decalin, and produced significantly less coke than in hexadecane. Unlike the reaction in hexadecane, changing the concentration of asphaltenes in maltenes had a significant effect on coke formation. The results from the previous study were rationalized in terms of competing reactions between maltenes and asphaltene-derived radicals for hydrogen, solvating effects of the media on coke formation, and viscosity effects on the radical–radical combination to form coke. Hexadecane as a solvent was relatively inert when compared to maltenes under the same severity conditions and did not stabilize the reactive radicals or prevent retrogressive reactions. The thermal hydrocracking of CLVB asphaltenes subcomponents (A1–A4) was also investigated in hexadecane. A better correlation was obtained between the coke yield and the molecular weight than either microcarbon residue or the aromaticity of the subcomponents. Based on the lack of additivity in coke yield, it was concluded that a synergistic effect among the subcomponents was most likely responsible for the reduced coke yield obtained from processing the asphaltenes feed.