Uncatalyzed hydrogen transfer during 100-250 °C conversion of asphaltenes

  • Author / Creator
    Naghizada, Nazim
  • Asphaltene is the most difficult fraction of bitumen and heavy oil to upgrade, as it has low H/C, high molecular weight and metal content. Asphaltenes are insoluble in paraffinic solvents, and undergo aggregation and coke formation during refining and cracking processes. No attention is usually paid to reaction chemistry of asphaltenes during low temperature processing, which assumes that asphaltenes remain unreactive upon exposure to mild heating. However, asphaltenes have ‘stable’ free radicals in their natural state. The free radical chemistry of asphaltenes might have relevance to its storage, processing, and transport in industry where exposure to heating is inevitable. The work employed industrially n-pentane precipitated asphaltenes from Athabasca oil sands bitumen. Free radical-radical reactions, hydrogen disproportionation and hydrogen donor/acceptor properties were investigated over the temperature range 100 to 250 °C. The objective was to observe any changes in terms of yield of each soluble or insoluble fraction, gas product, aromatic hydrogen content and free radical content. The potential effect of the metal wall of the reactor was tested on control reactions and subsequent work took this into account (Chapter 3). When heating asphaltenes on their own, it was found that the yield of the n-heptane insoluble fraction of the asphaltenes feed increased from 67 to 75 % as the temperature was increased to 150 °C (Chapter 4). The n-heptane insoluble fraction also gained more aromaticity upon heating. These observed changes were ascribed to hydrogen transfer and addition-combination reactions, which eventually lead to formation of more condensed and aromatic products. It appeared that addition products became less soluble in paraffinic solvents. It was also considered that mild heating might have enabled free-radical reactions by improving molecular mobility and disruption of aggregation, which provided “caging” effect role for free radicals, as suggested in the literature. Direct evidence for hydrogen transfer was found by using reactions of mixtures of asphaltenes with model compounds α-methylstyrene, cumene, 9,10-dihydroanthracene, and anthracene as probe molecules to facilitate analysis at species level (Chapter 5). The amount of the hydrogen transfer from asphaltenes to α-methylstyrene was evaluated and was of the order 1.8 mg H/ g asphaltenes in 1 h at 250 °C. Evidence for the reverse reaction was not found, indicating asphaltene free radicals are incapable of forming bonds which could be stronger than the weakest C–H bond in tertiary carbon atom of cumene (353 kJ/mol). Asphaltenes also donated hydrogen to anthracene at 250 °C, indicating the presence of transferrable hydrogens with bond strength less than 315 kJ/mol which is the bond strength of the C–H at 9- and 10-positions of 9,10-dihydroanthracene. The presence of free radicals in the asphaltenes feed was confirmed, and a minor decrease was observed in the free radical concentration due to heating alone, which indirectly supported the possibility of hydrogen transfer and free radical combination reactions. It was shown that some free radical fragments are sterically hindered with respect to each other, which were stabilized significantly by addition of 9,10-dihydroanthracene. The results suggested that asphaltenes can act both as hydrogen acceptor and hydrogen donor. Indirect evidence for the formation of combination or addition products was also shown with the model compounds through reactions that were induced by asphaltenes. In conclusion, asphaltenes were reactive over the temperature range studied.

  • Subjects / Keywords
  • Graduation date
    Fall 2017
  • Type of Item
  • Degree
    Master of Science
  • DOI
  • License
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