Page 154 - Fluid Catalytic Cracking Handbook

Chemistry of FCC Reactions 129

Table 4-2
Comparison of Products of Thermal and Catalytic Cracking

Hydrocarbon Type Thermal Cracking Catalytic Cracking

n-Paraffms C 2 is major product, with C 3 to C 6 is major product;
much C 3 and C 3, and C 4 to few n-olefins above C 4;
C 16 olefins; little branching much branching
Olefins Slow double-bond shifts Rapid double-bond shifts,
and little skeletal extensive skeletal isomeri-
isomerization; H-transfer zation, H-transfer is major
is minor and nonselective and selective for tertiary
for tertiary olefins; only olefins; large amounts of
small amounts of aromatics formed from
aromatics formed from aliphatics at 932°F (500°O
aliphatics at 932°F (500°C)
Naphthenes Crack at slower rate than If structural groups are
paraffins equivalent, crack at about
the same rate as paraffins
Alkyl-aromatics Cracked within side chain Crack next to ring

Source: Venuto [2]

15 percent alumina replaced the natural clay catalysts. The synthetic
silica-alumina catalysts were more stable and yielded superior products.
In the mid-1950s, alumina-silica catalysts, containing 25 percent
alumina, came into use because of their higher stability. These synthetic
catalysts were amorphous; their structure consisted of a random array
of silica and alumina, tetrahedrally connected. Some minor improve-
ments in yields and selectivity were achieved by switching to catalysts
such as magnesia-silica and alumina-zirconia-silica.

Impact of Zeolites

The breakthrough in FCC catalyst was the use of X and Y zeolites
during the early 1960s. The addition of these zeolites substantially
increased catalyst activity and selectivity. Product distribution with a
zeolite-containing catalyst is different from the distribution with an
amorphous silica-alumina catalyst (Table 4-3). In addition, zeolites are
1,000 times more active than the amorphous silica alumina catalysts.