Identification, conversion and reactivity of diolefins in thermally cracked naphtha

  • Author / Creator
    Páez Cárdenas, Nidia Y
  • Bitumen extracted from Canadian oil-sands has a high viscosity, which limits its transportation. Field upgrading has become an option to overcome that limitation, without having to dilute the bitumen. During the field upgrading of bitumen, solvent deasphalting in combination with thermal cracking (visbreaking) can be used to produce a material ready to be transported for refining. In the thermal cracking process, mono-olefins and diolefins are generated especially in the light fractions. In many cases the olefin content exceeds the Canadian pipeline specification of <1% as 1-decene equivalent, required to have a product suitable for pipeline transportation. The usual treatment is hydrogenation, however, that treatment is not an option for a field upgrader facility due to the cost of H2 production and the size of the facility. Diolefins and especially conjugated diolefins are very reactive at high temperatures. They are capable of undergoing addition reactions to form molecules of high molecular weight better known as gums, that later could form coke. In previous research a non-conventional aromatic alkylation process was proposed to treat mono-olefins, but the conversion of diolefins at high temperatures contributed to catalyst deactivation by fouling. Therefore a hydrogen-free and low temperature alternative was sought to eliminate the diolefins. Three objectives were set in order to solve that problem: (1) to identify diolefins thermally cracked naphtha, (2) to explore alternative low temperature treatments for diolefins conversion and (3) to stablish a reactivity sequence of representative olefinic and diolefinic species present in cracked naphtha. The approach taken to develop the research involved three steps that corresponded to each objective. The identification of diolefins in thermally cracked naphtha was done by means of the gas chromatography-mass spectrometry (GC-MS) coupled with two chemical reactions: hydrogenation and Diels-Alder cycloaddition. This work was done in collaboration with Alberta Innovates Future Technologies (AITF), which analyzed the same sample using gas chromatography with vacuum ultra violet detector (GC-VUV). There was matrix interference due to the variety of compounds present in the naphtha. Four compounds were identified as diolefins, all of them with a conjugated structure: trans- 1,3-pentadiene, cis-1,3-pentadiene, 2-methyl-1,3-pentadiene and a cyclic diolefin of 7 carbons, possibly 5,5-dimethyl-1,3-cyclopentadiene. Five diolefins were identified by GC-VUV, two conjugated, two isolated and one cumulated: 2,3-dimethyl-1,3-butadiene, 3-methyl-1,3-pentadiene, trans-1,4-hexadiene, 1,7-octadiene and tetramethylallene. The low temperature treatment reactions explored were hydration and Diels-Alder cycloaddition. Hydration was first attempted on a model compound at 110ᵒC, 3 MPa and using four acid catalysts: sulfuric acid in aqueous solution, solid phosphoric acid, Siral-5 and H-ZSM-5. The model compound used was 2,5-dimethyl-2,4-hexadiene. The expected alcohol products of the water addition were not seen, instead a mixture of cracking, oxygenate and addition products was formed. The compounds present in the mixture were: trans-1,3-pentadiene, 1,5-hexadiene, 1,3-hexadiene, 1,3-cycloheptadiene, 2,5-dimethyl-2,4-hexadiene and cyclopentene and benzene as impurities. Conjugated and isolated linear diolefins underwent double bond and cis-trans isomerization. The disubstituted conjugated diolefin 2,5-dimethyl-2,4-hexadiene was converted into a cyclic ether. In the case of the Diels-Alder cycloaddition, the anticipated cyclohexene derivatives, were formed at the conditions of 60ᵒC and using 10 and 15% of AlCl3 as catalyst. The reaction was done using model compounds and a mixture of diolefins with two dienophiles: 3-buten-2-ol and methyl vinyl ketone (MVK). In the case of the model compounds, stereoisomers of 2,4-hexadiene were used, the diolefin trans-trans-2,4-hexadiene was the most reactive towards the dienophile. The mixture of diolefins was formed by 2,3-dimetyl-1,3-butadiene, 1,3-hexadiene, cis-3-methyl- 1,3-pentadiene, trans-3-methyl-1,3-pentadiene, 1,3-cycloheptadiene and 2,5-dimethyl-2,4-hexadiene. The diolefins of open chain were more prone to the cycloaddition, showing higher conversions than the cyclic diolefin. Similar to the reaction using model compounds, when the model mixture was used, the compounds with the trans configuration were more reactive. To stablish a reactivity sequence, hydrogenation was used as test reaction using Pt/C as catalyst. Eight compounds with different structure, but olefinic nature were selected. The order in reactivity found in this work from the most reactive to the least was: 1,4-pentadiene > 1-hexene > trans-1,3-pentadiene > 1-methylcyclohexene > 3-methyl-1,3-pentadiene > cyclohexene > 1,3-cylclohexadiene > vinylcyclopentane. According to the results, compounds with linear structure were more reactive, and the presence of branches or a cycle in the molecules decreased their reactivity for hydrogenation.

  • Subjects / Keywords
  • Graduation date
  • Type of Item
  • Degree
    Master of Science
  • DOI
  • 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.
  • Language
  • Institution
    University of Alberta
  • Degree level
  • Department
    • Department of Chemical and Materials Engineering
  • Specialization
    • Chemical Engineering
  • Supervisor / co-supervisor and their department(s)
    • De Klerk, Arno (Chemical and Materials Engineering)
  • Examining committee members and their departments
    • Nazemifard, Neda (Chemical and Materials Engineering)
    • Semagina, Natalia (Chemical and Materials Engineering)