Biomedical Engineering Reference
In-Depth Information
60
50
f
2
= M
G
X
-
MR
X
40
30
2
4
6
8
10
S
, g/L
f
1
=
MS
S
-
MC
S
FIGURE E16-4.3
The trajectory from (S
¼
2.1922 g/L, X
¼
36.997 g-X/L) to the new steady state (S
¼
2.1922 g/L,
X
¼
62.597 g-X/L) on (S, X) plane (with f
1
and f
2
indicated). The conditions are reversed from those of
Fig. E16-4.2
.
by fouling, poisoning, and sintering due to heat and pressure. The catalyst activity is defined
based on the effective reaction rate as
r
eff
¼ ar (16.32)
where r is the rate of reaction with fresh catalyst (zero service time), r
eff
is the observed rate of
reaction after the catalyst has been put into service, and a is the catalyst activity. In general,
the catalyst activity loss (or decay) is a function of catalyst activity itself, concentration of
various components in the reaction mixture, and temperature. That is
d
a
d
t
¼ fða
r
cd
¼
;
C
;
TÞ
(16.33)
The exact form of the catalyst activity decay rate is dependent on the mechanism of catalyst
deactivation. Like the kinetic analysis, we commonly apply power-law relationships to
describe the catalyst deactivation.
16.4.1. Fouling
Catalyst fouling can occur in a number of ways. Materials (reactants, products, or interme-
diates) can deposit on catalyst to block active sites. Most common form of fouling: deposition
of carbonaceous species on catalyst, also known as coking. Coke can be laminar graphite,
high-molecular-weight polycyclic aromatics (tars), polymer aggregates, or metal carbides.
To minimize coking, one can operate at short catalyst residence time; apply hydrogen gas
to eliminate has phase carbon; and minimize temperature upstream (to prevent gas phase
carbon from forming).
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