Olefin Hydrotreating and Characterization of Olefins in Thermally Cracked Naphtha

• Author / Creator

  Budnar Subramanya, Annapoorna Shruthi

• Bitumen partial upgrading technologies aim to produce a product that meets viscosity and density specifications for pipeline transport with little to no addition of diluents. Thermal cracking processes are attractive for partial upgrading since they result in an increase in the light fractions. Thermal cracking, however, results in the formation of olefins that are concentrated in the lower boiling cuts of the cracked product. The olefin content must be reduced sufficiently to improve the storage stability of the cracked product and to meet the regulatory specifications for transportation by pipelines. The BituMaxTM partial upgrading process makes use of an olefin-aromatic alkylation process to reduce the olefin content in the product. Determination of the nature and abundance of olefins provides knowledge that is needed to support the development of the BituMaxTM olefin treatment process.
  One objective of this study was to characterize olefins in thermally cracked naphtha (boiling point 15 – 240 °C) obtained from a bitumen upgrader facility located in Alberta, Canada. The cracked naphtha was sub-fractionated into smaller boiling cuts in a lab-scale atmospheric distillation column. The sub-fractions were analyzed using gas chromatography with mass spectrometer (GC-MS) and flame ionization detector (GC-FID). Identification of olefinic species was confirmed by subjecting the cracked naphtha to mild hydrotreatment and comparison of chromatograms before and after hydrotreatment. This was necessary because assignment of compound identity based solely on the suggestions of the Mass Spectral Library was unreliable for distinguishing between olefins and cycloparaffins with the same molecular formula.
  A reaction study was conducted with model compounds to evaluate the hydrogenation activity of Ni/Al2O3 catalytic systems, with an aim to better understand olefin hydrotreating in the presence of aromatics, which also had some relevance to the naphtha characterization.
  Hydrogenation reactions with model naphtha were conducted in the temperature range 60 to 280 °C, 1 MPa H2 pressure at 1000 mL H2/mL liquid ratio and weight hourly space velocity of 0.5 h-1 based on the liquid feed. Model naphtha was made of 1-hexene (10 wt%), toluene (5 wt%) and n-octane (85 wt%). The reactions were studied over both reduced and sulfided Ni/Al2O3 catalysts. For the study of sulfided Ni/Al2O3, the catalyst was subjected to in situ sulfiding, and the feed was spiked with dimethyl disulfide (0.5 wt%) to keep the catalyst in a sulfided state. Hydrogenation of the model naphtha over reduced Ni/Al2O3 catalyst resulted in near complete conversion of 1-hexene to n-hexane at temperatures as low as 80 °C and near complete conversion of toluene to methylcyclohexane at 160 °C. Hydrogenation over sulfided Ni/Al2O3 catalyst resulted in selective conversion of 1-hexene compared to toluene, but the products formed below 240 °C were n-hexane and internal isomers cis-3-hexene, trans-3-hexene, cis-2-hexene and trans-2-hexene. Conversion to n-hexane was higher than 20% only at temperatures above 240 °C. Trace concentrations of a skeletal isomerization product of 1-hexene was also detected at temperatures above 240 °C.
  A first-pass identification of compounds in the cracked naphtha by GC-MS was verified using model compounds and by spiking sub-fractions. The identification of olefinic species was checked by comparison of chromatograms before and after mild hydrotreatment coupled with comparison of non-isothermal retention indices with the values reported in literature, when such data was available. The concentration of the species was calculated by evaluating average FID response factors for various compound classes identified in the cracked naphtha sample. Approximately 74 wt% of compounds that made up the cracked naphtha were identified by the characterization study.
  The identified species consisted of 40 wt% paraffins, 13 wt% olefins, 12 wt% cycloparaffins, 4 wt% aromatic compounds and 5 wt% sulfur compounds. The majority of the olefinic species found were in the C5-C7 range. The nature of the olefinic species was determined – straight chained (~6 wt%), branched olefins (~4 wt%), cyclic olefins (~3 wt%) and diolefins (~0.2 wt%).

• Subjects / Keywords

  • partial upgrading
  • cracked naphtha
  • thermal cracking
  • olefin hydrotreating
  • olefin characterization

• Graduation date

  Spring 2020

• Type of Item

  Thesis

• Degree

  Master of Science

• DOI

  https://doi.org/10.7939/r3-mg1m-d443

• License

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