Asphaltene Gasification : Soot Formation and Metal Distribution

• Author / Creator

  Kurian, Vinoj

• Athabasca oil sands contain 8 to 14% bitumen and are recovered using surface mining or steam assisted gravity drainage (SAGD). During primary upgrading of bitumen, the vacuum residuum from the distillation column is sent to the solvent de-asphalting unit where a paraffinic solvent is used to remove the high carbon content asphaltenes from the vacuum tower bottoms. These asphaltenes, about one-third of the vacuum residuum, are liquid at operating temperatures and are very problematic in bitumen upgrading facilities. Considering factors such as an increase in world energy demand, large reserves of Canadian oil sands, improved upgrading technologies and an alternative to natural gas for SAGD operation, asphaltene gasification could be a great option to maximize its usability and reduce the waste product. In this work, the soot formed during the pyrolysis, partial oxidation and steam gasification of Athabasca asphaltene in an electrically heated entrained-flow reactor is analyzed. The effects of feed particle size, temperature, and residence time on the properties of soot particles were investigated for pyrolysis. The experiments were conducted at temperatures between 800 °C and 1400 °C and the residence time was varied between 5 to 12 s by controlling the carrier gas nitrogen flow rate. The asphaltene feed particle size was varied from 53 to 212 µm. Morphological, structural, and elemental properties of collected soot were also investigated. The asphaltene devolatilizes to produce char, light gases and tar. The feed asphaltene particle size had a negligible effect on the properties of the soot formed. The yield of soot formed increased with pyrolysis temperature. It was observed that the average size of primary soot particles decreases with an increase in temperature. With increase in residence time the average size of the primary soot particles increases. Vanadium, nickel, and other trace metal distribution is investigated in char and soot formed during pyrolysis, partial oxidation, and steam gasification. The pyrolysis temperature plays a major role in the trace metal content of the char and soot. With an increase in the pyrolysis temperature, the V and Ni contents decrease for both char and soot. An increase in the stoichiometric oxygen content decreases the soot yield during partial oxidation of asphaltenes as a result of the increase in the oxygen/carbon ratio. It is found that the V and Ni contents in both the char ash and soot ash decrease with an increase in stoichiometric oxygen in the feed. During asphaltene gasification with steam, the V content increases with the increase in the steam/fuel weight ratio for both char and soot. The Ni content slightly decreases in char with an increase in the steam/fuel ratio, whereas the Ni content increases in soot with an increase in the steam/fuel ratio. The soot-water, soot-cake and cooler fouling samples collected from an industrial asphaltene gasifier are analyzed to compare with the soot from lab scale entrained-flow reactor. The metal species evolved during gasification are of different types and major part of it is captured or associated with soot formed and exit the gasification system as soot-water and soot-cake. But a minor quantity of metals which are not associated with soot are getting deposited when cooled in the syngas cooler. The high metallic content of the soot-water and soot-cake shows that collection of metallic species, the act for which soot blowing is intended, has been to some extent successful in preventing slag formation in the gasifier. The very low soot content in the fouling sample (2-3 % soot) collected from the syngas cooler concludes that the deposits are formed in the cooler wall with a different mechanism for which soot blowing has not had a very positive effect. A CFD model is developed using ANSYS FLUENT by incorporating the single rate devolatilization model, volumetric species transport reactions and Moss-Brooks-Hall soot model into the basic continuity, momentum and energy equations. The model acts as a simple tool to estimate the soot formation in a reactor where the reaction conditions are favorable for soot formation. The influence of different steps of soot formation like nucleation, coagulation, and surface growth on the soot formation can be analyzed using the model.

• Subjects / Keywords

  • Vanadium
  • Gasification
  • Fouling
  • Pyrolysis
  • Asphaltenes
  • Soot

• Graduation date

  Fall 2016

• Type of Item

  Thesis

• Degree

  Doctor of Philosophy

• DOI

  https://doi.org/10.7939/R3707WT5M

• License

  This thesis is made available by the University of Alberta Libraries with permission of the copyright owner solely for non-commercial purposes. This thesis, or any portion thereof, may not otherwise be copied or reproduced without the written consent of the copyright owner, except to the extent permitted by Canadian copyright law.