Page 221 - Principles of Catalyst Development

210 CHAPTER 8
These are most likely edge-corner atoms. Additional poisoning destroys all
activity eventually. In cases such as this, selective poisoning of undesired
activity gives improved yields of desired products.

8.3.8. Coking

"Coke" is a term given to carbonaceous residues on a catalyst
surface. (27H) Varying in structure from hydrogen-deficient, aromatic-type
polymers to graphic carbon, these deposits are found in amounts up to
20 wt % whenever carbon-containing feeds are exposed to catalytic surfaces.
All catalysts experience this deactivation to some extent. In extreme cases,
the catalyst surface is covered with layers of coke deposit, thereby decreasing
the accessible surface areas, active components become encapsulated in
carbon, and pores are blocked with heavy buildup. When these effects
become significant, the coked catalysts must be replaced or regenerated.
Coke accumulation is the difference between coke deposition and
removal. Both occur, although at different rates. Well-designed catalysts
provide an economic balance between the two. Deposition from hydrocar-
bons occurs in two ways, on acid sites and on dehydrogenation sites:

Acid sites: (1) c"Hm -- (CH,),. (8.8 )

Dehydrogenation sites: (2) c" Hm -- yC (metals and oxides)

With carbon monoxide and dioxide, two other possibilities occur:

Dissociation sites: (3) 2CO ~ C + CO 2 ( 8.9)

Coke removal involves a number of reactions

(5) C + O 2 ~ CO 2

(8.10)

(8) C+C02~2CO

where C represents the carbon-containing residue.