Hydroisomerization of Alkanes over Metal-loaded Zeolite Catalysts

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Abstract

Zeolite catalysis plays an important role in many industrial applications due to their unique properties and has become widely used in the area of oil refining. Of particular interest is Zeolite Y, which can be hydrothermally treated into its ultrastable form, USY. USY offers a superior practicality, especially when dealuminated and metal-loaded.The importance of alkanes hydroisomerization arises from the continuingly stricter regulations imposed on the utilization of gasoline as an automotive fuel. The requirements to reduce the aromatics content in gasoline present a need to find an alternative way to maintain its research octane number (RON). An alternative to gasoline‟s high-octane aromatic content is to increase the RON for the paraffinic content of gasoline, which can be accomplished through hydroisomerization. Commercially, bifunctional metal-loaded zeolites are used to hydroisomerize the light naphtha stream produced at overheads of atmospheric distillation towers. However, no such process exists for the low-value heavy naphtha cut. This targeted process would, if successful, greatly improve refiner‟s profitability.In this work, bifunctional USY zeolite catalysts are studied in the hydroisomerization of a normal alkane (nC7, RON = 0). This nC7, found in heavy naphtha, has been used as the “model” compound. The impact of different reaction conditions and catalyst properties on catalyst activity and stability, in addition to the catalyst selectivity to high octane isomers is one step towards determining optimum conditions and preferential catalyst formulations that favour octane maximization. Six platinum-loaded USY zeolite catalysts, four in-house and two commercial, were tested in an atmospheric glass fixed-bed reactor and a stainless steel reactor purpose-built during the course of this thesis. Reaction temperatures ranged from 170 to 250oC at pressures between 1 and 15 bar. The hydrogen to hydrocarbon molar ratio was fixed at 9, with feed space time ranging from 35.14 to 140.6 kg.s/mol. In-house catalysts were hydrothermally treated at different severities, while commercial ones were originally dealuminated through acid-leaching treatments.Results have shown commercial catalyst CBV-712 gave the best performance and highest octane values for product isomers (>30). In addition, there was no coke generation. The next best catalyst was the most severely steamed in-house catalyst (USY-D) that has shown a remarkable performance at high pressures, almost eclipsing the performance of CBV-712, yet produced higher levels of coke. Other USY catalysts tested were less robust during reactions, probably due to imbalance in their acidic to metallic functions, or diffusion limitations arising from their pore structures. The best catalysts were, nonetheless, highly sensitive to sulfur presence in the feed, which severely impacted their activity, especially their metallic functions, and thus require sulfur-free feeds in order to demonstrate their full capacities.Simple kinetic modelling of experimental data was performed using the initial rates method and estimation of kinetic parameters, whose values were in good agreement with previous literature.

Keyword(s)

Zeolite, Alkane, Heptane, Hydroisomerization, Isomerization, Bifunctional, Catalysis

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