Environmental Engineering Reference
In-Depth Information
6.2 Oxidative Regeneration
The most established regeneration method involves the oxidative removal (by burn-off) of
carbonaceous deposits, generally referred to as coke, which deposited on catalyst surface
during the operation. To ensure safety, the diluted air has been used as the oxidation medium
during the early stages of regeneration. The last stages of regeneration process, i.e., after the
most reactive components of coke were removed, may be completed using air.
Figure 6.1
[367,368]
shows that under properly controlled regenerating conditions, almost complete
removal of carbon deposited on catalyst can be achieved. The oxidative regeneration using air
is also possible provided that the regenerator temperature is carefully monitored. In this case,
either the design of regenerator or an installation of cooling system may be necessary.
However, the additional cost of these changes must be compensated by improved efficiency of
regeneration.
6.2.1 Mechanism of Oxidative Regeneration
A significant complexity of the regeneration mechanism is indicated by the simultaneous
involvement of organic coke, contaminant metals associated with coke and those on catalyst
surface as well as the inorganic portion of the spent catalyst. Because of catalyst porosity, two
main regions, i.e., chemically controlled and diffusion controlled, are always part of the
overall regeneration mechanism. In addition, variable conditions (e.g., O
2
concentration,
temperature, particle size of spent catalyst, method of spent catalyst pretreatment, etc.) can
have a pronounced effect during regeneration.
6.2.1.1 Oxidation of Coke
The distribution of gaseous products provides essential information on the mechanism of
oxidative regeneration. Various experimental techniques used for this purpose were compared
elsewhere
[13]
. In the presence of O
2
under typical regeneration conditions, both inorganic
part and organic part of spent catalyst are oxidized. The involvement of the latter becomes
more evident with the gradual removal of coke. Typical analyses of the coke on spent catalysts
are shown in
Table 6.1 [13]
. They indicate that the oxidation of coke will involve the following
general reaction in which the formation of all major gaseous products is shown:
+
+
+
+
=
+
+
+
+
H
m
C
n
S
x
N
y
Coke
(0.5
m
n
x
0
.
5
y
)O
2
H
2
O
CO
SO
2
NO
CO
2
Products
Depending on the experimental conditions, the formation of N
2
O, NO
2
, and SO
3
can be
anticipated as well. The difference between these reactions and the oxidation of coke during
storage discussed in the previous section should be noted. Thus, under regeneration conditions,
the spent catalyst is contacted with the oxidation medium at much higher temperatures
