517
Catalytic Cracking
Aromatic hydrocarbons exhibit wide variations in their susceptibility to catalytic cracking.
The benzene ring is relatively inert, and condensed-ring compounds, such as naphthalene, anthracene, and phenanthrene, crack very slowly. When these aromatics crack a substantial part of their
conversion is reflected in the amount of coke deposited on the catalyst. Alkylbenzenes with attached
groups of C 2 or larger primarily form benzene and the corresponding olefins, and heat sensitivity
increases as the size of the alkyl group increases.
19.2 EARLY PROCESSES
In general, catalytic cracking may be regarded as the modern method for converting high-boiling
petroleum fractions, such as gas oil, into high-quality gasoline and other added value products.
In the 1930s, thermal cracking units produced ∼50% of the total gasoline. The octane number
of this gasoline was about 70 compared to 60 for straight-run (distilled) gasoline. The thermal
reforming and polymerization processes that were developed during the 1930s could be expected
to further increase the octane number of gasoline to some extent, but an additional innovation was
needed to increase the octane number of gasoline to enhance the development of more powerful
automobile engines.
In 1936, a new cracking process opened the way to higher octane gasoline; this process was
catalytic cracking. The process is basically the same as thermal cracking but differs by the use of
a catalyst, which is not (in theory) consumed in the process and directs the course of the cracking
reactions to produce more of the desired higher octane hydrocarbon products.
19.3 COMMERCIAL PROCESSES
Catalytic cracking is another innovation that truly belongs to the twentieth century. It is the modern
method for converting high-boiling petroleum fractions, such as gas oil, into gasoline and other
low-boiling fractions. The several processes currently employed in catalytic cracking differ mainly
in the method of catalyst handling, although there is an overlap with regard to catalyst type and
the nature of the products. The catalyst, which may be an activated natural or synthetic material,
is employed in bead, pellet, or microspherical form and can be used as a fixed bed, moving-bed, or
fluid-bed configurations.
The fixed-bed process was the first to be used commercially and uses a static bed of catalyst in
several reactors, which allows a continuous flow of feedstock to be maintained. Thus, the cycle of
operations consists of (1) flow of feedstock through the catalyst bed, (2) discontinuance of feedstock
flow and removal of coke from the catalyst by burning, and (3) insertion of the reactor on-stream.
The moving-bed process uses a reaction vessel in which cracking takes place and a kiln in which
the spent catalyst is regenerated, catalyst movement between the vessels is provided by various
means.
The fluid-bed process differs from the fixed-bed and moving-bed processes insofar as the powdered catalyst is circulated essentially as a fluid with the feedstock (Sadeghbeigi, 1995). The several
fluid catalytic cracking processes in use differ primarily in mechanical design (e.g., Hemler, 1997;
Hunt, 1997; Johnson and Niccum, 1997; Ladwig, 1997). Side-by-side, reactor–regenerator configuration or the reactor either above or below the regenerator is the main mechanical variation. From
a flow standpoint, all fluid catalytic cracking processes contact the feedstock and any and recycle
streams with the finely divided catalyst in the reactor.
Feedstocks may range from naphtha to atmospheric residuum (reduced crude). Feed preparation
(to remove metallic constituents and high-molecular weight nonvolatile materials) is usually carried out through any one of the following ways: coking, propane deasphalting, furfural extraction,
vacuum distillation, viscosity breaking, thermal cracking, and HDS (e.g., Speight, 2000).
The major process variables are temperature, pressure, catalyst/feedstock ratio (ratio of the
weight of catalyst entering the reactor per hour to the weight of feedstock charged per hour), and
Catalytic Cracking
Aromatic hydrocarbons exhibit wide variations in their susceptibility to catalytic cracking.
The benzene ring is relatively inert, and condensed-ring compounds, such as naphthalene, anthracene, and phenanthrene, crack very slowly. When these aromatics crack a substantial part of their
conversion is reflected in the amount of coke deposited on the catalyst. Alkylbenzenes with attached
groups of C 2 or larger primarily form benzene and the corresponding olefins, and heat sensitivity
increases as the size of the alkyl group increases.
19.2 EARLY PROCESSES
In general, catalytic cracking may be regarded as the modern method for converting high-boiling
petroleum fractions, such as gas oil, into high-quality gasoline and other added value products.
In the 1930s, thermal cracking units produced ∼50% of the total gasoline. The octane number
of this gasoline was about 70 compared to 60 for straight-run (distilled) gasoline. The thermal
reforming and polymerization processes that were developed during the 1930s could be expected
to further increase the octane number of gasoline to some extent, but an additional innovation was
needed to increase the octane number of gasoline to enhance the development of more powerful
automobile engines.
In 1936, a new cracking process opened the way to higher octane gasoline; this process was
catalytic cracking. The process is basically the same as thermal cracking but differs by the use of
a catalyst, which is not (in theory) consumed in the process and directs the course of the cracking
reactions to produce more of the desired higher octane hydrocarbon products.
19.3 COMMERCIAL PROCESSES
Catalytic cracking is another innovation that truly belongs to the twentieth century. It is the modern
method for converting high-boiling petroleum fractions, such as gas oil, into gasoline and other
low-boiling fractions. The several processes currently employed in catalytic cracking differ mainly
in the method of catalyst handling, although there is an overlap with regard to catalyst type and
the nature of the products. The catalyst, which may be an activated natural or synthetic material,
is employed in bead, pellet, or microspherical form and can be used as a fixed bed, moving-bed, or
fluid-bed configurations.
The fixed-bed process was the first to be used commercially and uses a static bed of catalyst in
several reactors, which allows a continuous flow of feedstock to be maintained. Thus, the cycle of
operations consists of (1) flow of feedstock through the catalyst bed, (2) discontinuance of feedstock
flow and removal of coke from the catalyst by burning, and (3) insertion of the reactor on-stream.
The moving-bed process uses a reaction vessel in which cracking takes place and a kiln in which
the spent catalyst is regenerated, catalyst movement between the vessels is provided by various
means.
The fluid-bed process differs from the fixed-bed and moving-bed processes insofar as the powdered catalyst is circulated essentially as a fluid with the feedstock (Sadeghbeigi, 1995). The several
fluid catalytic cracking processes in use differ primarily in mechanical design (e.g., Hemler, 1997;
Hunt, 1997; Johnson and Niccum, 1997; Ladwig, 1997). Side-by-side, reactor–regenerator configuration or the reactor either above or below the regenerator is the main mechanical variation. From
a flow standpoint, all fluid catalytic cracking processes contact the feedstock and any and recycle
streams with the finely divided catalyst in the reactor.
Feedstocks may range from naphtha to atmospheric residuum (reduced crude). Feed preparation
(to remove metallic constituents and high-molecular weight nonvolatile materials) is usually carried out through any one of the following ways: coking, propane deasphalting, furfural extraction,
vacuum distillation, viscosity breaking, thermal cracking, and HDS (e.g., Speight, 2000).
The major process variables are temperature, pressure, catalyst/feedstock ratio (ratio of the
weight of catalyst entering the reactor per hour to the weight of feedstock charged per hour), and