Community Research and Development Information Service - CORDIS

Coke deactivation of catalysts

Catalysts used for the hydrotreating of heavy petroleum fractions suffer very rapid initial coke build up as a result of strong initial adsorption of the polyaromatic fractions including the nitrogen containing aromatic compounds on the surface.
Some sulphur may be located in the coke, but contrary to nitrogen compounds, the sulphur containing molecules are not adsorbed on the surface.
The fact that the product sulphur concentration stays constant through this initial period shows that the hydrodesulphurization (HDS) activity is not affected by initial coking within the first hour of vaccum gas oil (VGO) hydrotreating.
The hydrodenitrification (HDN) activity may appear to decrease, but more likely this is a result of nitrogen removal by initial adsorption of nitrogen species combined with a constant HDN conversion.
On the basis of studies by temperature programmed techniques it can be concluded that the majority of the nitrogen appears to be present as strongly adsorbed organic nitrogen species, which are oxidized at a relatively high temperature. Some coke may cover these species. Evidence of a small amount of adsorbed nitrogen hydrogen species is also found. The majority of these species is to be found in the active phase of the catalyst.
Nuclear microprobe methods have been developed that allow the measurement of the distribution of carbon, hydrogen, nitrogen and heavier elements in coked catalyst pellets.
Applications have included studies of the variability of profiles in batches of used pellets, investigation of interrelationships between coke components and limited kinetic studies. Many of these applications have proven to be successful and nuclear microprobe methods will continue to be used in the study of catalyst coking.
Solid state carbon-13 cross polarization-magic angle spinning nuclear magnetic resonance (CP-MAS NMR) has in priciple been found to be a powerful tool for the characterization of coke samples.
The application has included calculati ons of aromaticity and hydrogen to carbon ratios of the coke deposits. A careful interpretation of the results is very important since this kind of study involves a lot of assumptions. Correlations, however, begin to emerge and a continued research as concerns methods and application will secure a better understanding of the coke formation at the molecular level.

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