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Adsorption of long chain alkanes on Fisher-Tropsch wax hydroconversion catalysts: an experimental and modeling study

Monday, 4 July, 2011 - 14:00
Campus: Brussels Humanities, Sciences & Engineering campus
Ben Li
phd defence

Nowadays, hydroconversion of Fischer-Tropsch linear paraffins plays an important
role in petroleum refinery. Conversion of synthesis gas through the Fischer-Tropsch
process leads to the formation of products essentially made up of n-paraffins whose
distribution covers a wide range of molecular weights and is free of aromatic, sulfur
and nitrogen compounds. However, long chain n-paraffins or waxes have poor cold
flow properties, hampering their direct use as transportation fuels.

An attractive way to improve the quality of the waxes produced via the
Fischer-Tropsch consists of the isomerisation of long linear paraffin chains into
branched paraffins. This process is not yet fully understood, given its very large
complexity, involving chemical reactions of many hundreds of components. The
efficiency and selectivity of the catalysts used in this hydroconversion process is
strongly affected by the adsorption mechanisms of the paraffin molecules in the
micro- and mesopores of the catalyst. In the present work, we studied for the first time
the adsorption mechanisms of long chain paraffins in these hydroconversion catalysts.
The experiments reveal completely different mechanisms in liquid or vapor phase
conditions; pressure and temperature of the process strongly affect the competitive
adsorption equilibrium. To rationalize these effects, theoretical models were
developed allowing to predict the adsorption behavior in real process conditions. As
such, this work contributes to a better understanding of the hydroconversion reaction,
and could eventually lead to an improvement of the global process.

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