Catalyst Deactivation - Spent Hydroprocessing Catalysts - page 56

sites. This slows down the activation of hydrogen, the availability of which is crucial for
hydroprocessing reactions to occur as well as for slowing down coke formation [55] .
Moreover, the prolonged adsorption of N-compounds diminishes the access of other reactant
molecules to catalytic sites. Therefore, at least part of the coke is formed as a consequence of
the catalyst poisoning by N-compounds. The extensive information on catalyst deactivation by
coke and N-compounds was reviewed in details and published elsewhere [49] . It should be
noted that a deep or even ultradeep HDS could not be accomplished without minimizing
poisoning effect of N-compounds. There may be a difference between the poisoning effects of
the N-compounds present in VGO compared with that in HGO. This is supported by the
presence of the fractions boiling below 350 ◦ C in the latter, whereas such fraction is not
present in VGO. Thus, depending on the preparation of HGO, this may represent as much as
30% of the HGO fraction. It was indicated earlier that the poisoning effect of N-compounds
increased with decreasing boiling range of the fractions [10,49,165] . For example, the rate of
the HDN of quinoline, which was added to the 616-666 K, 706-756 K and 797+K fractions,
increased in the same order [166] . In practical situation, e.g., between the inlet and outlet of
the fixed-bed reactor, the inhibiting effect of N-compounds may exhibit a maximum before
most of the N-compounds were converted to hydrocarbons [115] . Similarly, in a multistage
system, the inhibiting effect of N-compounds will increase from the first stage and reach a
maximum in one of the downstream reactors.
The poisoning effect of N-compounds on catalyst activity was clearly demonstrated in the
study published by Kaernbach et al. [167] on HDS of the distillate feed derived from the
Russian crude. In this case, N-compounds were separated from the feed by ion exchange
chromatography prior to the experiments performed at 633 K and 7MPa in the continuous
fixed-bed reactor. As expected, the HDS conversion was much greater in the absence of
N-compounds. Similarly, the HDS activities increased by almost 60% after the N-compounds
were removed from the feed by adsorption with silica-alumina [168] . The poisoning by
N-compounds decreased with increasing temperature because of their diminished adsorption
on catalytic sites. The adverse effect of N-compounds in the feed on catalyst activity was also
confirmed by Massoth et al. [169,170] .
The catalyst samples taken after 12 months on stream from the different depths of the single
fixed-bed used for hydroprocessing of a VGO (633 to 673 K; 8MPa) had different coke
deposition patterns [171] . The amount of coke increased with the increasing depth of the bed.
The graphitic nature of coke increased towards the end of the bed as well. The predominantly
amorphous structure of coke on the inlet and graphitic structure on the outlet of catalyst bed
observed by Koizumi et al. [172] is in agreement with the results of Anemia et al. [171] .Itwas
proposed that the increasing temperature towards the end of fixed-bed (because of the
increased rate of exothermic reactions) was the main contributor to the difference in coke
structure. Almost certainly, the increased rate of poisoning by N-compounds was an important
contributor as well. Thus, the HYD of N-heterorings, occurring near the front of fixed-bed,