Hydrokrackning av FT vaxer på ädelmetall / kiseloxid - aluminiumoxidkatalysatorer: en experimentell och modelleringsstudie
Independent thesis Advanced level (degree of Master (Two Years)), 20 credits / 30 HE creditsStudent thesisAlternative title
Hydroconversion of Fischer - Tropsch Waxes over Noble Metal/Silica - Alumina Catalysts: an Experimental and Modelling Study (English)
Synthetic fuel production employing the Fischer-Tropsch (FT) technology will, most likely, play an important role in the future energy system. FT waxes can be upgraded to middle distillates through a hydroprocessing step that combines isomerization and cracking reactions.
Different catalysts can be employed in the hydroconversion of FT waxes. It is usually carried out on bifunctional catalysts that possess a metal and an acidic function. The former provides the hydrogenation/dehydrogenation activity while the latter catalyzes isomerization and cracking reactions. Due to the absence of poisoning compounds, noble metals (e.g. Pt, Pd) can be employed together with a mesoporous silica-alumina support. The aim of this study was to prepare and test noble metal/silica-alumina catalysts for the hydroconversion of Fischer-Tropsch waxes. A Pd- and a Pt-based catalyst were prepared and fully characterized according to commonly used techniques. From these analyses, it was found that a higher metal dispersion and a lower total acidity characterize the Pt-based catalyst. The experimental testing demonstrated that the Pt-based catalyst is the most suitable for the hydroconversion of FT waxes since it possesses higher activity (evaluated as C22+ conversion) and higher selectivity towards the middle-distillate fraction (i.e. the desired product). Furthermore, both catalysts were tested under about 200 hours on stream showing a good stability.
The last part of this study was devoted to the modelling of the hydroconversion reaction. Firstly, a deep literature review of the already published models was carried out. Afterwards, the considered most advanced model was implemented in a commercial simulator in order to test its prediction power with respect to the main features of the reacting system. In addition, an improved version of this model was proposed considering the variability of a stoichiometric factor, previously kept constant. The former model and its improved version were tested and compared according to a statistical study. Thanks to this, it was possible to state that the improved hydroconversion model has a slightly better prediction power.
Finally, a comparison of the two catalysts was carried out by evaluating their kinetic constants derived from the improved model. From this point of view, the Pd-based catalyst presents stronger Langmuir adsorption constants. This could explain its lower reactivity since the involved species have a higher energy barrier prior to desorb. Moreover, this catalytic system presents a higher probability of producing isomerized fragments after a cracking after (86% on Pd, 78% on Pt on average). This suggests that more isomerization steps occur in the case of the Pd-based catalyst prior to undergo cracking reactions. This aspect could add some further explanations to its lower activity since the conversion of high boiling point fractions is slower.
Place, publisher, year, edition, pages
2015. , 100 p.
Fischer-Tropsch wax, hydrocracking modelling, bifunctional catalyst, noble metal, silica-alumina
Other Chemical Engineering
IdentifiersURN: urn:nbn:se:kth:diva-172440OAI: oai:DiVA.org:kth-172440DiVA: diva2:848223