347
Structure of Petroleum
of the hydrocarbons. In addition to the naphthene rings, alkylated aromatic hydrocarbons are also
believed to be present. However, it is generally believed that this picture of oil structure is oversimplified and that the higher boiling, or nonvolatile oil fractions contain more complex structures of the
type illustrated for the resins, in which paraffinic chains and saturated ring systems coexist in a variety of ways. It is also considered likely that the saturated ring structures can be replaced by aromatic
ring structures in the resin formulae.
Of the three fractions that constitute the asphaltic portion of petroleum, the oils have received least
attention. Nevertheless, some aspects of their chemistry have been studied and are outlined here.
This fraction is thermally stable at ordinary temperatures and is quite resistant to attack by many
chemicals. At elevated temperatures, fission of side chains takes place. In the presence of oxygen
(or air), carbon dioxide, water, and products containing carbonyl >C=O and hydroxyl –O–H groups
are found. If the temperature is sufficiently high, the products containing oxygen functions are
unstable and dehydrogenation is the overall result.
Just as the asphaltene constituents and resins react with oxygen at low temperatures (<100°C,
<212°F), so do the oils. Thus, oxidation in benzene solution by air, in either the presence or absence
of metal salts, proceeds smoothly to yield asphaltene and resin products (Moschopedis and Speight,
1978b). In general, the majority of the oil recovered after oxidation has lower atomic N/C and S/C
ratios than the untreated oil; this indicates that it is the more polar components of the oil fraction that
are susceptible to oxidation. It is also evident that the presence of catalytic amounts of metal salts
causes acceleration in the oxidation rate of the oil fraction.
Halogens react with the oils relatively slowly, especially if there is no olefin bond (>C=C<) or
aromatic unsaturation; essentially substitution reactions occur. At elevated temperatures, dehydrohalogenation occurs, giving rise to resin and asphaltene products.
Dilute acids have little, if any, effect on the oils; concentrated nitric acid brings about oxidation; concentrated sulfuric acid (oleum: H 2 SO 4 + SO 3 ) produces predominantly oil-soluble sulfonate
derivatives.
13.2.2.3 Molecular Weight
Molecular weights of oil fractions usually vary over the range 250–600, and it is only rarely, if ever,
that molecular weights in excess of 600 are recorded.
The nature of the solvent has no effect on the molecular weight of the oils; this is also true of
the temperature at which the determination is carried out. It is therefore presumed that recorded
molecular weights of oils are true molecular weights and are not the result of molecular association
caused by intermolecular forces.
13.2.3 resIn ConstItuents
The separation of the resin constituents from the deasphaltened oil is a different problem, and it is
in this part of the separation scheme that opinions regarding isolation of the resin constituents from
deasphaltened oil most frequently differ.
It must always be remembered that the definition of the nonvolatile constituents of petroleum
(i.e., the asphaltene constituents, the resin constituents, and, to some extent, part of the oils fraction
insofar as nonvolatile oils occur in residua and other heavy feedstocks) is an operational aid. It is
difficult to base such separations on chemical or structural features (Girdler, 1965; Speight et al.,
1982, 1984; Speight, 1994). This is particularly true for the asphaltene fraction, for which the separation procedure not only dictates the yield but can also dictate the quality of the fraction insofar as
the yield and character of this is dependent upon the hydrocarbon used for the separation (Girdler,
1965; Mitchell and Speight, 1973). The technique employed also dictates whether or not the asphaltene contains coprecipitated resins (Ali et al., 1985; Speight, 1994; Andersen and Speight, 2001).
This is based on the general definition that asphaltenes are insoluble in n-pentane (or in n-heptane)
but resins are soluble in n-pentane (or in n-heptane).
Structure of Petroleum
of the hydrocarbons. In addition to the naphthene rings, alkylated aromatic hydrocarbons are also
believed to be present. However, it is generally believed that this picture of oil structure is oversimplified and that the higher boiling, or nonvolatile oil fractions contain more complex structures of the
type illustrated for the resins, in which paraffinic chains and saturated ring systems coexist in a variety of ways. It is also considered likely that the saturated ring structures can be replaced by aromatic
ring structures in the resin formulae.
Of the three fractions that constitute the asphaltic portion of petroleum, the oils have received least
attention. Nevertheless, some aspects of their chemistry have been studied and are outlined here.
This fraction is thermally stable at ordinary temperatures and is quite resistant to attack by many
chemicals. At elevated temperatures, fission of side chains takes place. In the presence of oxygen
(or air), carbon dioxide, water, and products containing carbonyl >C=O and hydroxyl –O–H groups
are found. If the temperature is sufficiently high, the products containing oxygen functions are
unstable and dehydrogenation is the overall result.
Just as the asphaltene constituents and resins react with oxygen at low temperatures (<100°C,
<212°F), so do the oils. Thus, oxidation in benzene solution by air, in either the presence or absence
of metal salts, proceeds smoothly to yield asphaltene and resin products (Moschopedis and Speight,
1978b). In general, the majority of the oil recovered after oxidation has lower atomic N/C and S/C
ratios than the untreated oil; this indicates that it is the more polar components of the oil fraction that
are susceptible to oxidation. It is also evident that the presence of catalytic amounts of metal salts
causes acceleration in the oxidation rate of the oil fraction.
Halogens react with the oils relatively slowly, especially if there is no olefin bond (>C=C<) or
aromatic unsaturation; essentially substitution reactions occur. At elevated temperatures, dehydrohalogenation occurs, giving rise to resin and asphaltene products.
Dilute acids have little, if any, effect on the oils; concentrated nitric acid brings about oxidation; concentrated sulfuric acid (oleum: H 2 SO 4 + SO 3 ) produces predominantly oil-soluble sulfonate
derivatives.
13.2.2.3 Molecular Weight
Molecular weights of oil fractions usually vary over the range 250–600, and it is only rarely, if ever,
that molecular weights in excess of 600 are recorded.
The nature of the solvent has no effect on the molecular weight of the oils; this is also true of
the temperature at which the determination is carried out. It is therefore presumed that recorded
molecular weights of oils are true molecular weights and are not the result of molecular association
caused by intermolecular forces.
13.2.3 resIn ConstItuents
The separation of the resin constituents from the deasphaltened oil is a different problem, and it is
in this part of the separation scheme that opinions regarding isolation of the resin constituents from
deasphaltened oil most frequently differ.
It must always be remembered that the definition of the nonvolatile constituents of petroleum
(i.e., the asphaltene constituents, the resin constituents, and, to some extent, part of the oils fraction
insofar as nonvolatile oils occur in residua and other heavy feedstocks) is an operational aid. It is
difficult to base such separations on chemical or structural features (Girdler, 1965; Speight et al.,
1982, 1984; Speight, 1994). This is particularly true for the asphaltene fraction, for which the separation procedure not only dictates the yield but can also dictate the quality of the fraction insofar as
the yield and character of this is dependent upon the hydrocarbon used for the separation (Girdler,
1965; Mitchell and Speight, 1973). The technique employed also dictates whether or not the asphaltene contains coprecipitated resins (Ali et al., 1985; Speight, 1994; Andersen and Speight, 2001).
This is based on the general definition that asphaltenes are insoluble in n-pentane (or in n-heptane)
but resins are soluble in n-pentane (or in n-heptane).