Environmental Engineering Reference
In-Depth Information
Figure 6.4: Effect of temperature on evolution of H
2
O during isothermal burn-off of spent
CoMo/Al
2
O
3
catalyst.
The oxidation of S and N heteroatoms, which are part of the heteroring structures of coke,
precedes the formation of SO
2
and NO. This is depicted in
Fig. 6.2 [332]
by the formation of
the sulfoxide and nitroxide entities. These results in the weakening of the C S and C N bonds
in the oxidized structures compared with the corresponding bond in heterorings. Such situation
favors decomposition of the transition complexes to produce SO and NO. Very low stability of
the former species favors rapid oxidation to SO
2
even under a limited availability of O
2
. The
higher oxidation state oxides, such as SO
3
and NO
2
can also be present, particularly after a
diluted air is replaced with air. In fact, a gas phase oxidation of SO
2
to SO
3
and NO to NO
2
can proceed while burn-off gases are exiting regeneration zone.
All carbon and nitrogen oxides produced during the oxidative regeneration are of an organic
origin. At the same time, the structure of spent catalysts suggests that both organic coke and
catalyst contribute to the SO
2
formation. Two different sources of SO
2
can be clearly
distinguished in
Fig. 6.5 [369]
. In this case, the oxidic form of CoMo/Al
2
O
3
(catalyst A) was
used for hydroprocessing the mixture of hexadecane + phenol. The same reaction was repeated
with the sulfidic form of the CoMo/Al
2
O
3
(catalyst B). Then, the sulfided CoMo/Al
2
O
3
catalyst was used for hydroprocessing VGO (catalyst C). The TPO profiles of the spent
catalyst C indicate two regions of the SO
2
formation. The lower region coincides with that of
catalyst B and is attributed to the oxidation of metal sulfides. On the other hand, the higher
