Environmental Engineering Reference
In-Depth Information
of ammonia strongly adsorbed on catalyst surface, whereas the second peak from the oxidation
of nitrogen in coke. It was reported that much more of N
2
O was formed when nitrogen was
bound in the five-membered heterorings than that in the six-membered heteroring
[375,376]
.
Because of its impact on environment, the occurrence of N
2
O in the burn-off gas from
oxidative regeneration of spent catalysts needs to be addressed.
A part of the nitrogen is removed simultaneously with the inorganic sulfur, as it is indicated by
an overlap of the SO
2
and low temperature NO regions
[12,240]
. Most likely, this part of the
NO originated from the ammonia and other N-containing compounds adsorbed on and/or in
the proximity of catalytically active phase, rather than on the bare support. It was suggested
that the last part of carbon and nitrogen may be gradually converted to metal carbides and
nitrides
[377,378]
. These phases are catalytically active for hydroprocessing reactions.
Therefore, the last amount of carbon and nitrogen left in catalyst after regeneration may not be
detrimental, although their removal requires prolong exposure to the oxidation medium. This
was supported by the results published by Noguchi et al.
[379]
who reported that the activity
of the spent catalyst was almost completely restored as soon as the amount of carbon on the
catalyst was less than 3 wt.%.
Some primary reactions, which are part of the mechanism of conversion of nitrogen in coke on
catalyst, are similar as those occurring during the oxidation of other carbonaceous solids
[380]
. Thus, at low O
2
concentrations (e.g.,
2 vol.%), NH
3
and HCN are among important
products. This is supported by the product distribution obtained during the isothermal
oxidation of spent catalyst from an industrial hydroprocessing operation, shown in
Table 6.2
[322]
. The evolution of NH
3
and HCN coincided with that of CO
2
and CO, therefore, it was
associated with the oxidation of coke. Increasing the O
2
concentration from 2 to 4 vol.%
resulted in the formation of N
2
O in addition to NH
3
and HCN. At 4 vol.% O
2
concentration,
temperature increase from 350 to 450
◦
C resulted in the significant decrease in the yield of
∼
Table 6.2: Distribution of N-containing compounds during burn-off [From ref.
322
. Reprinted
with permission].
2% O
2
4% O
2
350
◦
C
450
◦
C
350
◦
C
450
◦
C
N total
a
20
.
7
13
.
0
23
.
9
8
.
8
HCN
10
.
8
0
.
6
11
.
3
0
.
9
NH
3
8
.
3
0
.
2
5
.
1
0
.
4
NO
1
.
5
5
.
2
1
.
3
5
.
1
N
2
O
0
7
.
0
6
.
2
1
.
6
C total
a
63
.
9
35
.
0
98
.
3
0
.
8
a
N total and C total in wt.% of N and C in catalyst, respectively.
