Regeneration - Spent Hydroprocessing Catalysts - page 130

Figure 6.6: Effect of time on stream on temperature programmed oxidation (TPO) profiles of
spent catalysts (Mo/Al 2 O 3 , atmospheric residue [AR], 653 K, 12MPa) [From ref. 240 . Reprinted
with permission].
within the first 120 h, whereas the “hard” coke that of THF insolubles coke formed after
6500 h. The chemical transformations occurring during the conversion of a “soft” coke to
“hard” coke are shown in Figs 4.20 and 4.21 [238,239] . As expected, the latter coke had very
low solubility. It is believed that the “soft” coke was formed predominantly on the uncovered
support. With time on stream, the “soft” coke was gradually converted to more refractory coke.
This was supported by the decrease of the low temperature and increase of the high
temperature CO 2 peak between 1 and 240 h shown in Fig. 6.6 [240] . After 120 h on stream,
most of the coke was refractory and was insoluble in THF. More nitrogen and sulfur were
concentrated in the refractory coke than in the “soft” coke. These observations agreed with the
structural mechanism of coke formation proposed by Hauser et al. [238,239] . The distinction
between the “soft” coke and “hard” coke in Fig. 6.6 may be affected by the involvement of
active metals during TPO. Thus, the transition metals catalyzed oxidation of coke, i.e., oxygen
transfer from active metals to carbon has been proposed by several authors [181,208,369] .
Then, the coke in the vicinity of active metals would exhibit higher reactivity (e.g., be “softer”)
than its actual reactivity at the end of hydroprocessing operation. With respect to the overall
mechanism of oxidative regeneration, the hydrogen removing reactions during the oxidation of
“soft” coke should be much more pronounced than those during the oxidation of “hard” coke.
The study of Callejas et al. [181] gives another dimension to the coke structure issue. During
the TPO, these authors observed the emergence of another coke peak with maximum at about
700 ◦ C. This peak appeared for the catalyst after 1100 h on stream, whereas there was no such