Biomedical Engineering Reference
In-Depth Information
To prevent sintering, catalysts may be doped with stabilizers (high melting temperature and
preventing agglomeration of small crystals) such as chromia, alumina, and magnesia. For
platinum catalysts, adding trace amount of chlorinated compounds in the process stream.
Chlorine increases the activation energy for the sintering process of Pt and thus reduces
the sintering rate for platinum catalysts.
16.4.4. Catalyst Activity Decay
Deactivation by sintering (aging) can usually be modeled as a second-order decay in cata-
lyst activity:
1þ k
cd
t
& k
cd
¼ k
cd0
exp
d
a
d
t
0
1
E
cd
RT
r
cd
¼ k
cd
a
2
¼
a ¼
(16.35)
Deactivation by coking is usually modeled with:
1
1þa
2
C
Coke
C
Coke
¼ At
n
a ¼
or
a ¼ expða
1
C
Coke
Þ
(16.36)
And deactivation by poisoning is commonly modeled with a power-law kinetics,
d
a
d
t
r
cd
¼ k
cd
C
P
a
n
¼
(16.37)
where P is the poison compound to the catalyst. It can either be a reactant or just an inert
substance in the reaction mixture.
16.4.5. Spent Catalyst Regeneration
Sintered catalysts are usually not regenerated on site. Catalyst structural changes are not
easy to recover and will required remaking.
Fouled catalysts can normally be regenerated by “burning.” However, temperature must
be controlled to minimize sintering.
Poisoned catalysts can be regenerated by 1) Desorption (with poison free stream) and 2)
Steam treatment at high temperature (however, sintering can be a problem). For example:
Ni
S
þ
H
2
O
/
NiO
þ
H
2
S
H
2
S
þ2
H
2
O
/
SO
2
þ
3H
2
16.5. GENETIC INSTABILITY
Genetic instability is part of the evolution process. The change of microorganisms
subject to hostile environment has been exploited for bioprocesses as well, for example
adaptation microorganisms to environments that they are not commonly in and produc-
tive. However, in this section, we focus more on the negative effects of genetic instability.
Microorganism instability is an issue in particular for genetically altered or genetically
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