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The Chemistry and Technology of Petroleum
The hydrocarbons with the least thermal stability are the paraffins, and the olefins produced by
the cracking of paraffins are also reactive. Cycloparaffins (naphthenes) are less easily cracked, their
stability depending mainly on any side chains present, but ring splitting may occur, and dehydrogenation can lead to the formation of unsaturated naphthenes and aromatics. Aromatics are the most
stable (refractory) hydrocarbons, the stability depending on the length and stability of side chains.
Very severe thermal cracking of high-boiling feedstocks can result in condensation reactions of ring
compounds, yielding a high proportion of coke (Speight, 1986).
The higher boiling oils produced by cracking are light and heavy gas oils as well as a residual oil,
which in the case of thermal cracking is usually (erroneously) called tar and in the case of catalytic
cracking is called cracked fractionator bottoms. The residual oil may be used as heavy fuel oil,
and gas oils from catalytic cracking are suitable as domestic fuel oil and industrial fuel oil or as
diesel fuel if blended with straight-run gas oils. Gas oils from thermal cracking must be mixed with
straight-run (distilled) gas oils before they become suitable for domestic fuel oils and diesel fuels.
The gas oil produced by cracking is, in fact, a further important source of gasoline. In a oncethrough cracking operation, all the cracked material is separated into products and may be used as
such. However, cracked gas oil is more resistant to cracking (more refractory) than straight-run gas
oil but can still be cracked to produce gasoline. This is done in a recycling operation in which the
cracked gas oil is combined with fresh feed for another trip through the cracking unit. The operation
may be repeated until the cracked gas oil is almost completely decomposed (cracking to extinction)
by recycling (recycling to extinction) the higher boiling product, but it is more usual to withdraw
part of the cracked gas oil from the system according to the need for fuel oils. The extent to which
recycling is carried out affects the amount or yield of cracked gasoline resulting from the process.
The gases formed by cracking are particularly important because of their chemical properties
and their quantity. Only relatively small amounts of paraffinic gases are obtained from crude oil,
and these are chemically inactive. Cracking produces both paraffinic gases (e.g., propane, C 3 H 8 ) and
olefin gases (e.g., propene, C 3 H 6 ); the latter are used in the refinery as the feed for polymerization
plants where high-octane polymer gasoline is made. In some refineries, the gases are used to make
alkylate, a high-octane component for aviation gasoline and for motor gasoline. In particular, the
cracked gases are the starting points for many petrochemicals (Chapter 27).
In summary, the cracking of petroleum constituents can be visualized as a series of thermal
conversions (Chapter 16). The reactions involve the formation of transient free-radical species that
may react further in several ways to produce the observed product slate. Because of this, the slate of
products from thermal cracking is considered difficult to predict (Germain, 1969).
The available data suggest that thermal conversion (leading to coke formation) is a complex process involving both chemical reactions and thermodynamic behavior (Speight, 1994; Speight and
Ozum, 2002) (Chapters 12 and 16) and can be summarized as follows: (1) thermal reactions of crude
oil constituents result in the formation volatile products, (2) thermal reactions of crude oil constituents also result in the formation of high molecular weight and high-polarity aromatic components,
and (3) once the concentration of the high molecular weight high-polarity material reaches a critical
concentration, phase separation occurs giving a denser, aromatic liquid phase.
Reactions that contribute to this process are cracking of side chains from aromatic groups,
dehydrogenation of naphthenes to form aromatics, condensation of aliphatic structures to form
aromatic, condensation of aromatics of form higher fused-ring aromatics, and dimerization or
oligomerization reactions. Loss of side chains always accompanies thermal cracking, while
dehydrogenation and condensation reactions are favored by hydrogen deficient conditions.
Formation of oligomers is enhanced by the presence of olefins or diolefins, which themselves
are products of cracking. The condensation and oligomerization reactions are also enhanced by
the presence of Lewis acids, for example, aluminum chloride (AlCl 3 ).
The importance of solvents to mitigate coke formation has been recognized for many years,
but their effects have often been ascribed to hydrogen-donor reactions rather than phase behavior.
The Chemistry and Technology of Petroleum
The hydrocarbons with the least thermal stability are the paraffins, and the olefins produced by
the cracking of paraffins are also reactive. Cycloparaffins (naphthenes) are less easily cracked, their
stability depending mainly on any side chains present, but ring splitting may occur, and dehydrogenation can lead to the formation of unsaturated naphthenes and aromatics. Aromatics are the most
stable (refractory) hydrocarbons, the stability depending on the length and stability of side chains.
Very severe thermal cracking of high-boiling feedstocks can result in condensation reactions of ring
compounds, yielding a high proportion of coke (Speight, 1986).
The higher boiling oils produced by cracking are light and heavy gas oils as well as a residual oil,
which in the case of thermal cracking is usually (erroneously) called tar and in the case of catalytic
cracking is called cracked fractionator bottoms. The residual oil may be used as heavy fuel oil,
and gas oils from catalytic cracking are suitable as domestic fuel oil and industrial fuel oil or as
diesel fuel if blended with straight-run gas oils. Gas oils from thermal cracking must be mixed with
straight-run (distilled) gas oils before they become suitable for domestic fuel oils and diesel fuels.
The gas oil produced by cracking is, in fact, a further important source of gasoline. In a oncethrough cracking operation, all the cracked material is separated into products and may be used as
such. However, cracked gas oil is more resistant to cracking (more refractory) than straight-run gas
oil but can still be cracked to produce gasoline. This is done in a recycling operation in which the
cracked gas oil is combined with fresh feed for another trip through the cracking unit. The operation
may be repeated until the cracked gas oil is almost completely decomposed (cracking to extinction)
by recycling (recycling to extinction) the higher boiling product, but it is more usual to withdraw
part of the cracked gas oil from the system according to the need for fuel oils. The extent to which
recycling is carried out affects the amount or yield of cracked gasoline resulting from the process.
The gases formed by cracking are particularly important because of their chemical properties
and their quantity. Only relatively small amounts of paraffinic gases are obtained from crude oil,
and these are chemically inactive. Cracking produces both paraffinic gases (e.g., propane, C 3 H 8 ) and
olefin gases (e.g., propene, C 3 H 6 ); the latter are used in the refinery as the feed for polymerization
plants where high-octane polymer gasoline is made. In some refineries, the gases are used to make
alkylate, a high-octane component for aviation gasoline and for motor gasoline. In particular, the
cracked gases are the starting points for many petrochemicals (Chapter 27).
In summary, the cracking of petroleum constituents can be visualized as a series of thermal
conversions (Chapter 16). The reactions involve the formation of transient free-radical species that
may react further in several ways to produce the observed product slate. Because of this, the slate of
products from thermal cracking is considered difficult to predict (Germain, 1969).
The available data suggest that thermal conversion (leading to coke formation) is a complex process involving both chemical reactions and thermodynamic behavior (Speight, 1994; Speight and
Ozum, 2002) (Chapters 12 and 16) and can be summarized as follows: (1) thermal reactions of crude
oil constituents result in the formation volatile products, (2) thermal reactions of crude oil constituents also result in the formation of high molecular weight and high-polarity aromatic components,
and (3) once the concentration of the high molecular weight high-polarity material reaches a critical
concentration, phase separation occurs giving a denser, aromatic liquid phase.
Reactions that contribute to this process are cracking of side chains from aromatic groups,
dehydrogenation of naphthenes to form aromatics, condensation of aliphatic structures to form
aromatic, condensation of aromatics of form higher fused-ring aromatics, and dimerization or
oligomerization reactions. Loss of side chains always accompanies thermal cracking, while
dehydrogenation and condensation reactions are favored by hydrogen deficient conditions.
Formation of oligomers is enhanced by the presence of olefins or diolefins, which themselves
are products of cracking. The condensation and oligomerization reactions are also enhanced by
the presence of Lewis acids, for example, aluminum chloride (AlCl 3 ).
The importance of solvents to mitigate coke formation has been recognized for many years,
but their effects have often been ascribed to hydrogen-donor reactions rather than phase behavior.