Heavy products formation in free radical systems during thermal conversion of heavy oil

• Author / Creator

  Tannous, Joy Hikmat

• Thermal cracking of heavy oils is important industrially in processes such as visbreaking, coking and residue hydroconversion. In these processes, residue conversion is limited by formation of heavy products. Heavy product formation also contributes to industrially relevant problems such as fouling and gum formation. In this study, heavy product formation in free radical systems was investigated.
  The study of free radical systems requires preparatory work for the quantification of free radical content. Electron spin resonance spectroscopy (ESR) was employed to perform quantitative analysis of the free radical content of oilsands bitumen, asphaltenes, deasphalted oil, vacuum residue and vacuum gas oil fractions, as well as thermally converted product fractions. The main contribution from this work was to show that for bitumen and bitumen derived materials the bulk liquid properties affected the measured free radical concentration, even after compensating for effects that could affect the spectroscopy. These differences were explained in terms of the “equilibrium” composition that resulted from dimerization and decomposition of free radical pairs. It suggested that the free radical concentration and availability of reactive free radicals could be independently manipulated through temperature and the bulk liquid properties in thermal conversion of bitumen. This was a noteworthy contribution, because it suggested a new way to approach the control of free radical availability during thermal conversion.
  Naphtheno-aromatics were suspected to be one of the compound classes responsible for heavy products formation. The reactivity of indan, indene, thianaphthene, indole and benzofuran was investigated at thermal cracking conditions of 400 °C and 2 MPa under nitrogen environment and provided relevant information about the reactivity of such compound classes for the rest of the study. Several of the test compounds were capable of self-reacting and forming heavy products.
  Heavy product formation caused by free radical addition reactions was evaluated. The importance of hydrogen transfer reactions were highlighted, and how the bulk liquid properties affected the probability of free radical chain transfer, two aspects that are seldom captured in models describing thermal cracking for visbreaking processes.
  However, it was speculated that hydrogen transfer was not the only type of transfer that could take place and that methyl transfer could also take place. There was a large body of literature on intramolecular methyl migration, but little mention of intermolecular methyl transfer as a potentially important reaction type in free radical systems. In a series of reactions, the potential transfer of a methyl group in free radical systems was tested using indene, 2-methylindene and α-methylstyrene. Besides the formation of products typical of hydrogen transfer, evidence of methyl transfer was provided. When methyl transfer was favored by the presence of methylated compounds in the feed, there was less heavy products formed in the vacuum residue range, the products were of lower density, and there was less free radical content in the products. This was a noteworthy and new contribution, because it not only showed the importance of intermolecular methyl transfer in free radical systems, but also indicated that methyl transfer as opposed to hydrogen transfer affected the properties of the reaction product.
  Tracking the effect of methyl and hydrogen transfer as well as addition reactions in free radical systems was suspected to be possible using traceable compounds. A new way of synthesis of 13C-labelled indene was evaluated. The synthesis of 13C-labelled indene provided a method to use labelled compounds to track addition reactions in bitumen and deasphalted oil. This work can serve as the starting point for future investigations related to heavy products formation in free radical systems.

• Subjects / Keywords

  • visbreaking
  • asphaltenes
  • free radical chemistry
  • heavy oil
  • thermal cracking
  • upgrading
  • hydrogen transfer

• Graduation date

  Fall 2019

• Type of Item

  Thesis

• Degree

  Doctor of Philosophy

• DOI

  https://doi.org/10.7939/r3-d7g8-2c69

• License

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