Environmental Engineering Reference
In-Depth Information
temperatures. In view of the results shown in
Fig. 6.3 [322]
, the kinetics of carbon removal
from catalyst are dominated by the formation of CO
2
as the contribution of CO appears to be
rather minor. This, of course, does not take into consideration the rate determining initial step,
which may involve a surface reaction. The kinetic data obtained by measuring weight loss
during the burn-off are less reliable. Thus, under such conditions, the contribution of coke and
mineral matter to the overall weight loss cannot be decoupled.
The opposite trends in the effect of temperature on hydrogen removal from coke compared
with carbon removal have been noted (
Fig. 6.4
). It is evident that at early stages of burn-off at
350
◦
C, the overall kinetics are dominated by hydrogen removal similarly as it was indicated
above for the overall mechanism of regeneration. However, at 450
◦
C and above, the
involvement of hydrogen removing reactions becomes unimportant.
Catalyst regeneration is usually influenced by diffusion phenomena. This may be seen in
Figs 6.3 and 6.11
(at 500
◦
C), which indicates the presence of two burn-off regions, i.e.,
chemically controlled, occurring during the early stages of burn-off and predominantly
diffusion controlled, occurring during the final stages of burn-off. The former involved the
coke deposited on the exterior of catalyst particles, whereas the diffusion controlled burn-off
involved the coke deposited in pores. During the transition stage, both chemical and diffusion
controlled kinetics influence the burn-off. The slow burn-off during the later stages resulted
from the diminished accessibility of O
2
in the pores. Thus, the diffusion of O
2
into the catalyst
interior was obstructed by the burn-off products (CO, CO
2
,SO
2
, and NO
X
) exiting from the
pores. Consequently, carbon-coke interface is developed in the interior of catalyst particles. At
low temperatures and/or at low O
2
concentrations in oxidizing gas, the burn-off is chemically
controlled. Under such conditions, carbon-coke interface may not be developed because the
slow reaction with coke ensures enough time for O
2
to diffuse into the catalyst particles
interior, rather than being consumed as it is the case at higher temperatures. Therefore, at
higher temperatures, the formation of two regions in the coke layer, differing markedly in the
H/C ratio, may be evident. In this case, the external region would have a lower
H/C ratio.
6.2.2.1 Chemically Controlled Kinetics
Assuming first-order kinetics with respect to carbon removal, the following general equation
describes the carbon burning process:
kC
(6.1)
At constant partial pressure of O
2
,
k
includes also partial pressure of O
2
. After integration for
initial conditions, i.e.,
C
d
C/
d
t
=−
−
C
o
at
t
=
0, the form of this equation is changed to:
=−
ln(
C/C
o
)
kt
(6.2)
