Regeneration - Spent Hydroprocessing Catalysts - page 127

Figure 6.3: Concentrations and yields of CO and CO 2 during temperature programmed oxidation
(TPO) of spent catalyst in 2% O 2 at 350 C [From ref. 322 . Reprinted with permission].
On the basis of the results shown in Figs 6.3 and 6.4 , two distinct regions and/or mechanisms
of oxidative regeneration may be applicable, i.e., the one dominated by removal of hydrogen
and the other dominated by removal of carbon. The latter involves much less reactive coke left
behind because most of the reactive moieties rich in hydrogen were already removed.
Mechanistically, the results in Fig. 6.4 can be interpreted in terms of the removal of the most of
aliphatic carbon and all hydrogen associated with it. Some aliphatic structures, i.e., alkyl
substituents and methylene bridges, which are part of coke molecules, can be identified in coke
model shown in Figs 4.20 and 4.21 [238,239] . This would leave behind the highly aromatic
and non-reactive residue. Figures 4.20 and 4.21 show a continuous change in coke structure
during various stages of the operation. This suggests that the burning profiles (e.g., Figs 6.3
and 6.4 ) of the spent catalysts taken at different stages on stream of hydroprocessing operation
will be different.