Page 233 - Introduction to chemical reaction engineering and kinetics

8.6 Catalyst Deactivation and Regeneration 215

carbonaceous compound. Very little coke forms on silica or carbon supports, but acidic
supports or catalysts are especially prone to coking.
To minimize coking, the reactor may be operated at short residence times, or hydro-
gen may be added to the process stream to convert gas-phase carbon into methane. It
is also advantageous to minimize the temperature upstream of the catalyst bed, since
gas-phase carbon is less readily formed at low temperatures.

8.6.2 Poisoning

Poisoning is caused by chemisorption of compounds in the process stream; these com-
pounds block or modify active sites on the catalyst. The poison may cause changes in
the surface morphology of the catalyst, either by surface reconstruction or surface relax-
ation, or may modify the bond between the metal catalyst and the support. The toxicity
of a poison (P) depends upon the enthalpy of adsorption for the poison, and the free en-
ergy for the adsorption process, which controls the equilibrium constant for chemisorp-
tion of the poison (Kp). The fraction of sites blocked by a reversibly adsorbed poison
(0,) can be calculated using a Langmuir isotherm (equation 8.4-23a):

8, = KPPP (8.6-1)
1 + KAPA + KPPP
where KA and Kp are the adsorption constants for the reactant (A) and the poi-
son, respectively, and PA and pp are the partial pressures of the reactant and poi-
son. The catalyst activity remaining is proportional to the fraction of unblocked sites,
1 - 8,.
The compound responsible for poisoning is usually an impurity in the feed stream;
however, occasionally, the products of the desired reaction may act as poisons. There
are three main types of poisons:
(1) Molecules with reactive heteroatoms (e.g., sulfur);
(2) Molecules with multiple bonds between atoms (e.g., unsaturated hydrocarbons);
(3) Metallic compounds or metal ions (e.g., Hg, Pd, Bi, Sn, Cu, Fe).

The strength of the bond between the poison and the catalyst (or support) may be
relatively weak, or exceptionally strong. In the latter case, poisoning leads to an ir-
reversible loss of activity. However, if the chemisorption bond is very weak, the ob-
served loss of activity can be reversed by eliminating the impurity (poison) from the
feed stream. Poisons may be eliminated by physical separation, or in the case of a
type (1) or type (2) poison, the poison may be converted to a nontoxic compound
by chemical treatment (oxidation for type (l), and hydrogenation for type (2)). If a
product is responsible for poisoning, it may be helpful to operate the reactor at low
conversion, and/or selectively remove product at intermediate stages of a multistage
reactor.

8.6.3 Sintering
Sintering is caused by growth or agglomeration of small crystals which make up the
catalyst or its support. The structural rearrangement observed during sintering leads
to a decrease in surface area of the catalyst, and, consequently, an irreversible reduc-
tion in catalyst sites. Sintering generally occurs if the local temperature of the catalyst
exceeds approximately one-third to one-half of its melting temperature (T,). The up-
per limit (i.e., (1/2)T,,,) applies under “dry” conditions, whereas the lower temperature
limit (i.e., (1/3)T,) applies if steam is present, since steam facilitates reorganization of