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A kinetic study of the hydrogenation and dimerization of styrene and α-methylstyrene on Ni-Mo-S catalyst Zhao, Xu

Abstract

Oilsands bitumen upgrading to produce naphtha, among other products, is a feasible approach to increasing the supply of refined oil products. However, naphtha derived from bitumen is relatively unstable as it contains unsaturated hydrocarbons (5 vol% diolefins, 15 wt% aromatics). The unsaturated hydrocarbons tend to polymerize and form carbonaceous deposits on the catalyst during mild hydrotreating (~200 °C), resulting in pressure build-up in the reactor that causes the early shut down of the hydrotreating unit. This dissertation addresses diolefin hydrogenation and dimerization kinetics and gum formation over a commercial Ni-Mo-S hydrotreating catalyst. Two model compounds, styrene and α-methylstyrene (AMS), were selected to represent the diolefin present in the naphtha feed. Styrene reactions were based on orthogonal analysis with temperature (200, 225, 250 °C), diolefin concentration (3.7-7.4 wt%), and catalyst amount (0.5, 1, 2 g) varied. For the AMS reactions, a single variable test was applied by changing the AMS content (4.2-6.3 wt%) or temperature (200, 225 and 250 °C). The styrene and AMS hydrogenation kinetics were developed as 1st-order in reactant and 0-order in H₂, based on a simplified Langmuir-Hinshelwood (L-H) model. Pseudo 1st-order in model compound kinetics was employed for the dimerization reaction. The results revealed that the rate of hydrogenating or dimerizing styrene was faster than AMS due to steric hindrance effects. The activation energy for styrene and AMS hydrogenation was 45.3 and 87.7 kJ/mol, respectively. The activation energy for styrene dimerization was 99.6 kJ/mol. Additionally, the relationship between dimer content and gum formation at the end of the reaction indicated that higher dimer concentration increased gum content in styrene reactions. However, this relationship was not observed in AMS reactions because of steric hindrance effects. Finally, competitive reactions between olefins and diolefins were also examined. Cyclohexene hydrogenation to cyclohexane was initially suppressed by AMS hydrogenation to cumene. With longer reaction time (510 mins), the cyclohexane concentration exceeded cumene, suggesting that competitive hydrogenation occurred between the cyclohexene and AMS. Adding cyclohexene to the AMS significantly reduced the dimer content in the product possibly due to competitive adsorption on the acidic sites.

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Attribution-NonCommercial-NoDerivs 2.5 Canada

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