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Catalytic Cracking
19.5 CATALYSTS
Cracking crude oil fractions occurs over many types of catalytic materials, but high yields of
desirable products are obtained with hydrated aluminum silicates. These may be either activated
(acid-treated natural clays of the bentonite type) or synthesized silica-alumina or silica-magnesia
preparations. Their activity to yield essentially the same products may be enhanced to some extent
by the incorporation of small amounts of other materials, such as the oxides of zirconium (zirconia,
ZrO 2 ), boron (boria, B 2 O 3 , which has a tendency to volatilize away on use), and thorium (thoria,
ThO 2 ). Both the natural and the synthetic catalysts can be used as pellets or beads, and also in the
form of powder; in either case, replacements are necessary because of attrition and gradual loss of
efficiency (DeCroocq, 1984; LePage et al., 1987, 1992; Speight and Ozum, 2002; Ancheyta and
Speight, 2007; Speight, 2011). The catalysts may also be dispersed throughout the feedstock to
assure higher conversion to the desired product (Panariti et al., 2000).
19.5.1 CAtAlyst ProPertIes
The catalysts must be stable to physical impact loading and thermal shocks and must withstand the
action of carbon dioxide, air, nitrogen compounds, and steam. They should also be resistant to sulfur
compounds; the synthetic catalysts and certain selected clays appear to be better in this regard than
average untreated natural catalysts. The silica-alumina catalysts are reported to give the highest
octane gasoline and silica-magnesia the largest yields, with the natural clays falling between them.
Neither silica (SiO 2 ) nor alumina (Al 2 O 3 ) alone is effective in promoting catalytic cracking
reactions. In fact, they (and also activated carbon) promote decomposition of hydrocarbon constituents that match the thermal decomposition patterns. Mixtures of anhydrous silica and alumina
(SiO 2 . Al 2 O 3 ) or anhydrous silica with hydrated alumina (2SiO 2 . 2Al 2 O 3 . 6H 2 O) are also essentially
not effective. A catalyst having appreciable cracking activity is obtained only when prepared from
hydrous oxides followed by partial dehydration (calcining). The small amount of water remaining
is necessary for proper functioning.
The catalysts are porous and highly adsorptive, and their performance is affected markedly by
the method of preparation. Two catalysts that are chemically identical but have pores of different
size and distribution may have different activity, selectivity, temperature coefficient of reaction rate,
and response to poisons. The intrinsic chemistry and catalytic action of a surface may be independent of pore size, but small pores appear to produce different effects because of the manner and time
in which hydrocarbon vapors are transported into and out of the interstices.
Commercial synthetic catalysts are amorphous and contain more silica than is called for by the preceding formulae; they are generally composed of 10%–15% alumina (Al 2 O 3 ) and 85%–90% silica (SiO 2 ).
The natural materials, montmorillonite, a non-swelling bentonite, and halloysite, are hydrosilicates of
aluminum, with a well-defined crystal structure and approximate composition of Al 2 O 3 . 4Si 2 O . xH 2 O.
Some of the newer catalysts contain up to 25% alumina and are reputed to have a longer active life.
Commercially used cracking catalysts are insulator catalysts possessing strong acidic properties. They function as catalysts by altering the cracking process mechanisms through an alternative
mechanism involving chemisorption by proton donation and desorption, resulting in cracked oil
and theoretically restored catalyst. Thus, it is not surprising that all cracking catalysts are poisoned
by proton-accepting vanadium.
The catalyst-oil volume ratios range from 5:1 to 30:1 for the different processes, although most
processes are operated to 10:1. However, for moving-bed processes, the catalyst-oil volume ratios
may be substantially lower than 10:1.
19.5.2 CAtAlyst treAtment
The latest technique developed by the refining industry to increase gasoline yield and quality is to
treat the catalysts from the cracking units to remove metal poisons that accumulate on the catalyst
Catalytic Cracking
19.5 CATALYSTS
Cracking crude oil fractions occurs over many types of catalytic materials, but high yields of
desirable products are obtained with hydrated aluminum silicates. These may be either activated
(acid-treated natural clays of the bentonite type) or synthesized silica-alumina or silica-magnesia
preparations. Their activity to yield essentially the same products may be enhanced to some extent
by the incorporation of small amounts of other materials, such as the oxides of zirconium (zirconia,
ZrO 2 ), boron (boria, B 2 O 3 , which has a tendency to volatilize away on use), and thorium (thoria,
ThO 2 ). Both the natural and the synthetic catalysts can be used as pellets or beads, and also in the
form of powder; in either case, replacements are necessary because of attrition and gradual loss of
efficiency (DeCroocq, 1984; LePage et al., 1987, 1992; Speight and Ozum, 2002; Ancheyta and
Speight, 2007; Speight, 2011). The catalysts may also be dispersed throughout the feedstock to
assure higher conversion to the desired product (Panariti et al., 2000).
19.5.1 CAtAlyst ProPertIes
The catalysts must be stable to physical impact loading and thermal shocks and must withstand the
action of carbon dioxide, air, nitrogen compounds, and steam. They should also be resistant to sulfur
compounds; the synthetic catalysts and certain selected clays appear to be better in this regard than
average untreated natural catalysts. The silica-alumina catalysts are reported to give the highest
octane gasoline and silica-magnesia the largest yields, with the natural clays falling between them.
Neither silica (SiO 2 ) nor alumina (Al 2 O 3 ) alone is effective in promoting catalytic cracking
reactions. In fact, they (and also activated carbon) promote decomposition of hydrocarbon constituents that match the thermal decomposition patterns. Mixtures of anhydrous silica and alumina
(SiO 2 . Al 2 O 3 ) or anhydrous silica with hydrated alumina (2SiO 2 . 2Al 2 O 3 . 6H 2 O) are also essentially
not effective. A catalyst having appreciable cracking activity is obtained only when prepared from
hydrous oxides followed by partial dehydration (calcining). The small amount of water remaining
is necessary for proper functioning.
The catalysts are porous and highly adsorptive, and their performance is affected markedly by
the method of preparation. Two catalysts that are chemically identical but have pores of different
size and distribution may have different activity, selectivity, temperature coefficient of reaction rate,
and response to poisons. The intrinsic chemistry and catalytic action of a surface may be independent of pore size, but small pores appear to produce different effects because of the manner and time
in which hydrocarbon vapors are transported into and out of the interstices.
Commercial synthetic catalysts are amorphous and contain more silica than is called for by the preceding formulae; they are generally composed of 10%–15% alumina (Al 2 O 3 ) and 85%–90% silica (SiO 2 ).
The natural materials, montmorillonite, a non-swelling bentonite, and halloysite, are hydrosilicates of
aluminum, with a well-defined crystal structure and approximate composition of Al 2 O 3 . 4Si 2 O . xH 2 O.
Some of the newer catalysts contain up to 25% alumina and are reputed to have a longer active life.
Commercially used cracking catalysts are insulator catalysts possessing strong acidic properties. They function as catalysts by altering the cracking process mechanisms through an alternative
mechanism involving chemisorption by proton donation and desorption, resulting in cracked oil
and theoretically restored catalyst. Thus, it is not surprising that all cracking catalysts are poisoned
by proton-accepting vanadium.
The catalyst-oil volume ratios range from 5:1 to 30:1 for the different processes, although most
processes are operated to 10:1. However, for moving-bed processes, the catalyst-oil volume ratios
may be substantially lower than 10:1.
19.5.2 CAtAlyst treAtment
The latest technique developed by the refining industry to increase gasoline yield and quality is to
treat the catalysts from the cracking units to remove metal poisons that accumulate on the catalyst