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Impact of conjugated olefins on nickel-molybdenum-sulphide supported on gamma-alumina catalyst deactivation and fouling of naphtha hydrotreaters Alzaid, Ali H.


This dissertation investigates the reactions of conjugated olefins that lead to catalyst deactivation and fouling in naphtha hydrotreater reactors using a commercial Ni-Mo-S/γ-Al₂O₃ catalyst. The reactions were performed in a micro-scale fixed bed reactor system operated at 150-250°C, 3-4 MPa H₂, LHSV of 1-8 hr-¹ and a H₂/feed ratio of 392-1200 standard mL/mL. During isoprene hydrogenation, an increase in dimerization activity with temperature was attributed to a higher activation energy of dimerization compared to hydrogenation. Conjugated olefin content was also shown to impact oligomerization as an increase in the conjugated olefin content resulted in a decrease in hydrogenation product yield while the oligomerization activity and gum content increased. By investigating different olefin structures, conjugation was shown to enhance dimerization/oligomerization while steric hindrance limited dimer/oligomer formation by limiting access and reactivity of the double bonds. The addition of cyclohexene to 4-methylstyrene resulted in a significant loss in catalyst hydrogenation activity while the dimerization activity remained almost the same for a period of up to 30 days time-on-stream. The loss in catalyst activity can be attributed to a higher concentration of 4-methylstyrene when the overall conversion was lower, resulting in higher dimerization and gum formation. This in turn resulted in increased catalyst deactivation compared to the case of no cyclohexene in the feed. Reactor fouling was shown to be linked to dimer and gum formation, as the pressure drop across the reactor increased with higher dimerization yield and gum formation. The increase in pressure drop was well described by a decreasing average reactor bed voidage caused by cumulative gum deposition within the catalyst bed. An overall trend of increasing gum yield with increasing dimer yield is reported, suggesting that the dimers are precursors for gum formation. In addition, catalyst deactivation was linked to carbon deposition on the catalyst caused by dimer and gum formation; increased dimer and gum formation were accompanied by an increased carbon deposition and decreased BET surface area of the catalyst. A kinetic model of the hydrogenation and dimerization of 4-methylstyrene over spent commercial Ni-Mo-S/γ-Al₂O₃ showed that hydrogenation has much lower activation energy (24.8 kJ/mol) than dimerization (68.2 kJ/mol).

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