Environmental Engineering Reference
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NH 3 , HCN, and N 2 O. At the same time, NO became the major N-containing product. Based
on these observations, the initial stage of the mechanism of nitrogen conversion during the
oxidative regeneration of spent catalysts may involve thermal decomposition of
coke, i.e.:
There is a significant database to support the O 2 aided elimination of NH 3 and HCN from
carbonaceous solids [374-376] . There is the evidence supporting the preferential formation
of N 2 O from NH 3 and NO from HCN. These reactions occur in the gas phase, although the
formation of these products via surface reactions cannot be ruled out. Then, the mechanism
of nitrogen oxides formation in gas phase during catalyst regeneration can be refined as:
2NH 3 +
2O 2 =
N 2 O
+
3H 2 O
2HCN
+
2.5O 2 =
2NO
+
2CO
+
H 2 O
N 2 O
+
0.5O 2 =
2NO
NO
+
0.5O 2
=
NO 2
This mechanism suggests that during oxidative regeneration, there are two sources of N 2 O and
NO, i.e., one involving the oxidation of the NH 3 strongly held on catalyst surface and the other
originating from the decomposition of N-containing moieties in coke yielding precursors to
the formation of NH 3 and HCN, which are subsequently oxidized in gas phase to nitrogen
oxides.
6.2.1.2 Involvement of Metals
The coke in spent catalysts after hydroprocessing of residues contains metals, such as V and
Ni. Depending on the feed, other metals, i.e., Fe, Ti, Ca, As, etc., can also be present. It was
suggested that V in coke may be either in a partially oxidized or in a sulfided form, whereas
most of the Ni was in a sulfided form [27] . Under conditions of oxidative regeneration, these
metals will be converted to the corresponding oxides, i.e.:
VO x S y +
1.5 y O 2 =
VO( x +
0.5 y )
+ y SO 2
NiS x +
1.5 x O 2 =
NiO0.5 x
+
x SO 2
Obviously, all other metals that deposited on catalyst surface during the operation should be
oxidized in similar manners giving the corresponding oxides of Fe, Ti, Ca, etc.
 
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