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The Chemistry and Technology of Petroleum
agent is usually an olefin, although cyclopropane, alkyl halides, aliphatic alcohols, ethers, and esters
may also be used. The alkylation of aromatic hydrocarbons is presumed to occur through the agency
of the carbonium ion.
Thermal alkylation is also used in some plants, but like thermal cracking, it is presumed to
involve the transient formation of neutral free radicals and therefore tends to be less specific in
production distribution.
16.6 POLYMERIZATION
Polymerization is a process in which a substance of low molecular weight is transformed into one of
the same compositions but of higher molecular weight while maintaining the atomic arrangement
present in the basic molecules. It has also been described as the successive addition of one molecule
to another by means of a functional group, such as that present in an aliphatic olefin.
In the petroleum industry, polymerization is used to indicate the production of, say, gasoline
components that fall into a specific (and controlled) molecular weight range, hence the term polymer
gasoline. Furthermore, it is not essential that only one type of monomer be involved:
CH 3 CH=CH 2 + CH 2 =CH 2 → CH 3 CH 2 CH 2 CH=CH 2
This type of reaction is correctly called copolymerization, but polymerization in the true sense of
the word is usually prevented, and all attempts are made to terminate the reaction at the dimer or
trimer (three monomers joined together) stage. It is the 4- to 12-carbon compounds that are required
as the constituents of liquid fuels. However, in the petrochemical section of the refinery, polymerization, which results in the production of, say, polyethylene, is allowed to proceed until materials
of the required high molecular weight have been produced.
16.7 PROCESS CHEMISTRY
In a mixture as complex as petroleum, the reaction processes can only be generalized because of
difficulties in analyzing not only the products but also the feedstock as well as the intricate and
complex nature of the molecules that make up the feedstock. The formation of coke from the higher
molecular weight and polar constituents of a given feedstock is detrimental to process efficiency and
to catalyst performance (Speight, 1987; Dolbear, 1998).
Refining the constituents of heavy oil and bitumen has become a major issue in modern refinery
practice. The limitations of processing heavy oils and residua depend to a large extent on the amount
of higher molecular weight constituents (i.e., asphaltenes) present in the feedstock (Speight, 1984,
2000, 2004b; Schabron and Speight, 1997), which are responsible for high yields of thermal and
catalytic coke (Chapters 10 and 18).
16.7.1 tHermAl CHemIstry
When petroleum is heated to temperatures in excess of 350°C (660°F), the rate of thermal decomposition of the constituents increases significantly. The higher the temperature, the shorter the
time to achieve a given conversion, and the severity of the process conditions is a combination of
residence time of the crude oil constituents in the reactor and the temperature needed to achieve a
given conversion.
Thermal conversion does not require the addition of a catalyst. This approach is the oldest technology available for residue conversion, and the severity of thermal processing determines the conversion and the product characteristics. As the temperature and residence time are increased, the primary
products undergo further reaction to produce various secondary products, and so on, with the ultimate
products (coke and methane) being formed at extreme temperatures of approximately 1000°C (1830°F).
The Chemistry and Technology of Petroleum
agent is usually an olefin, although cyclopropane, alkyl halides, aliphatic alcohols, ethers, and esters
may also be used. The alkylation of aromatic hydrocarbons is presumed to occur through the agency
of the carbonium ion.
Thermal alkylation is also used in some plants, but like thermal cracking, it is presumed to
involve the transient formation of neutral free radicals and therefore tends to be less specific in
production distribution.
16.6 POLYMERIZATION
Polymerization is a process in which a substance of low molecular weight is transformed into one of
the same compositions but of higher molecular weight while maintaining the atomic arrangement
present in the basic molecules. It has also been described as the successive addition of one molecule
to another by means of a functional group, such as that present in an aliphatic olefin.
In the petroleum industry, polymerization is used to indicate the production of, say, gasoline
components that fall into a specific (and controlled) molecular weight range, hence the term polymer
gasoline. Furthermore, it is not essential that only one type of monomer be involved:
CH 3 CH=CH 2 + CH 2 =CH 2 → CH 3 CH 2 CH 2 CH=CH 2
This type of reaction is correctly called copolymerization, but polymerization in the true sense of
the word is usually prevented, and all attempts are made to terminate the reaction at the dimer or
trimer (three monomers joined together) stage. It is the 4- to 12-carbon compounds that are required
as the constituents of liquid fuels. However, in the petrochemical section of the refinery, polymerization, which results in the production of, say, polyethylene, is allowed to proceed until materials
of the required high molecular weight have been produced.
16.7 PROCESS CHEMISTRY
In a mixture as complex as petroleum, the reaction processes can only be generalized because of
difficulties in analyzing not only the products but also the feedstock as well as the intricate and
complex nature of the molecules that make up the feedstock. The formation of coke from the higher
molecular weight and polar constituents of a given feedstock is detrimental to process efficiency and
to catalyst performance (Speight, 1987; Dolbear, 1998).
Refining the constituents of heavy oil and bitumen has become a major issue in modern refinery
practice. The limitations of processing heavy oils and residua depend to a large extent on the amount
of higher molecular weight constituents (i.e., asphaltenes) present in the feedstock (Speight, 1984,
2000, 2004b; Schabron and Speight, 1997), which are responsible for high yields of thermal and
catalytic coke (Chapters 10 and 18).
16.7.1 tHermAl CHemIstry
When petroleum is heated to temperatures in excess of 350°C (660°F), the rate of thermal decomposition of the constituents increases significantly. The higher the temperature, the shorter the
time to achieve a given conversion, and the severity of the process conditions is a combination of
residence time of the crude oil constituents in the reactor and the temperature needed to achieve a
given conversion.
Thermal conversion does not require the addition of a catalyst. This approach is the oldest technology available for residue conversion, and the severity of thermal processing determines the conversion and the product characteristics. As the temperature and residence time are increased, the primary
products undergo further reaction to produce various secondary products, and so on, with the ultimate
products (coke and methane) being formed at extreme temperatures of approximately 1000°C (1830°F).