Page 234 - Introduction to chemical reaction engineering and kinetics

216 Chapter 8: Catalysis and Catalytic Reactions

Table 8.4 Sintering temperatures for common
metals

Metal Sintering temperature/‘C;[(1/3)T,1

many metals, aluminas, and silicas. Table 8.4 lists some common metal catalysts and the
temperature at which the onset of sintering is expected to occur.
To prevent sintering, catalysts may be doped with stabilizers which may have a high
melting point and/or prevent agglomeration of small crystals. For example, chromia,
alumina, and magnesia, which have high melting points, are often added as stabilizers
of finely divided metal catalysts. Furthermore, there is evidence that sintering of plat-
inum can be prevented by adding trace quantities of chlorinated compounds to the gas
stream. In this case, chlorine increases the activation energy for the sintering process,
and, thus, reduces the sintering rate.

8.6.4 How Deactivation Affects Performance

Catalyst deactivation may affect the performance of a reactor in several ways. A reduc-
tion in the number of catalyst sites can reduce catalytic activity and decrease fractional
conversion. However, some reactions depend solely on the presence of metal, while
others depend strongly on the configuration of the metal. Thus, the extent to which
performance is affected depends upon the chemical reaction to be catalyzed, and the
way in which the catalyst has been deactivated. For example, deposition/chemisorption
of sulfur, nitrogen, or carbon on the catalyst generally affects hydrogenation reactions
more than exchange reactions. Consequently, if parallel reactions are to be catalyzed,
deactivation may cause a shift in selectivity to favor nonhydrogenated products. Sim-
ilarly, heavy metals (e.g., Ni, Fe) present in the feed stream of catalytic crackers can
deposit on the catalyst, and subsequently catalyze dehydrogenation reactions. In this
case, the yield of gasoline is reduced, and more light hydrocarbons and hydrogen pro-
duced.
Another way in which catalyst deactivation may affect performance is by blocking
catalyst pores. This is particularly prevalent during fouling, when large aggregates of
materials may be deposited upon the catalyst surface. The resulting increase in diffu-
sional resistance may dramatically increase the Thiele modulus, and reduce the effec-
tiveness factor for the reaction. In extreme cases, the pressure drop through a catalyst
bed may also increase dramatically.

8.6.5 Methods for Catalyst Regeneration

In some cases, it is possible to restore partially or completely the activity of a catalyst
through chemical treatment. The regeneration process may be slow, either because of
thermodynamic limitations or diffusional limitations arising from blockage of catalyst
pores. Although the rate of desorption generally increases at high temperatures, pro-
longed exposure of the catalyst to a high-temperature gas stream can lead to sintering,
and irreversible loss of activity. If the bound or deposited species cannot be gasified at
temperatures lower than the sintering temperature (see Table 8.4) then the poisoning
or fouling is considered to be irreversible.