321
Asphaltene Constituents
and Mouton, 1977; Speight, 1986a). Indeed, the asphaltene fraction can be subdivided by a variety
of techniques (Bestougeff and Darmois, 1947, 1948; Bestougeff and Mouton, 1977; Francisco and
Speight, 1984). Of specific interest is the observation that when asphaltenes are fractionated on the
basis of aromaticity and polarity, it appears that the more aromatic species contain higher amounts
of nitrogen (Speight, 1984), suggesting that the nitrogen species are located predominantly in aromatic systems (Speight, 1994).
The fractionation of the asphaltene component of petroleum showed that it is possible to obtain
asphaltene fractions characterized by different degrees of aromaticity or heteroatom content by
using benzene/pentane (toluene/pentane) or benzene/methanol (toluene/methanol) mixtures in variable ratios (Speight, 1979; Andersen et al., 1997; Miller et al., 1998). The use of mixtures of a polar
and a nonpolar solvent in order to fractionate an asphaltene sample will tend to direct the fractionation by introducing polar forces and hydrogen bonding, as well as dispersion forces, as factors
determining which components of the asphaltene sample are soluble in the mixture.
In contrast, the exclusive use of saturated hydrocarbons whose solvent power derives entirely
from the dispersion forces should, in principle, permit the fractionation to be carried out with
variation of only a single component of the solubility parameter. It is known that the quantity of
asphaltenes precipitated by linear paraffins decreases with increasing carbon number until a limiting value is reached above n-heptane (n-C 7 H 16 ) to n-nonane (n-C 9 H 20 ) (Girdler, 1965; Mitchell and
Speight, 1973).
The cycloparaffins have better solvating power toward the asphaltenes and often fail to induce
asphaltene separation from crude oil. It is possible to fractionate asphaltenes using mixtures of
solvents, such as cyclohexane and n-alkanes, and any differences in the precipitation threshold
underscores the point that the solubility properties of the asphaltene fractions of different crude oils
can vary markedly (Acevedo et al., 1995).
It appears, therefore, that the tendency of asphaltenes to precipitate from a given crude oil
because of a variation in the solubility parameter of the precipitating solvent mixture is more closely
related to the aromaticity and polarity of the asphaltenes than to their dimensions. However, it is
possible to conclude (Acevedo et al., 1995) that molecular weight, aromaticity, and polarity combine
to determine asphaltene solubility in hydrocarbon media.
The fractionation of asphaltenes into a variety of functional (and polar) types (Figure 12.1;
Francisco and Speight, 1984) has confirmed the complexity of the asphaltene fraction. Other studies
(Schmitter et al., 1984) have brought to light the occurrence of four-ring aromatic nitrogen species
in petroleum. These findings are of particular interest since they correspond to the ring systems that
have been tentatively identified by application of the HPLC technique to the basic nitrogen fraction
of asphaltenes (Speight, 1986b, 1994).
The molecular weight/polarity concept in which asphaltene fractions are defined on the basis
of molecular weight and polarity (Figure 12.3) can be used to indicate the relative nature of carbenes and carboids (thermal decomposition products of asphaltenes), which are of lower molecular weight and more polar than the asphaltene constituents from which they are formed (Speight,
1994). Furthermore, the slope of the line as illustrated is purely arbitrary and will vary with the
source and composition of different fractions (Figure 12.4). However, it should not be construed
that this two-dimensional diagram is truly representative of the nature of asphaltenes and a threedimensional diagram involving functionality or molecular weight might be more appropriate for
showing the chemical and physical differences between the various asphaltenes, carbenes, and carboids. Nevertheless, the concept does emphasize the lower molecular weight and increased polarity
of the various asphaltene constituents.
Unfortunately, in many studies, too little emphasis has been placed on determining the nature
and location of the nitrogen, oxygen, and sulfur atoms in the asphaltene structure.
Studies on the disposition of nitrogen in petroleum asphaltenes indicated the existence of nitrogen as various heterocyclic types (Clerc and O’Neal, 1961; Nicksic and Jeffries-Harris, 1968;
Moschopedis and Speight, 1976b, 1979; Jacobson and Gray, 1987; Mullins, 1995). The more
Asphaltene Constituents
and Mouton, 1977; Speight, 1986a). Indeed, the asphaltene fraction can be subdivided by a variety
of techniques (Bestougeff and Darmois, 1947, 1948; Bestougeff and Mouton, 1977; Francisco and
Speight, 1984). Of specific interest is the observation that when asphaltenes are fractionated on the
basis of aromaticity and polarity, it appears that the more aromatic species contain higher amounts
of nitrogen (Speight, 1984), suggesting that the nitrogen species are located predominantly in aromatic systems (Speight, 1994).
The fractionation of the asphaltene component of petroleum showed that it is possible to obtain
asphaltene fractions characterized by different degrees of aromaticity or heteroatom content by
using benzene/pentane (toluene/pentane) or benzene/methanol (toluene/methanol) mixtures in variable ratios (Speight, 1979; Andersen et al., 1997; Miller et al., 1998). The use of mixtures of a polar
and a nonpolar solvent in order to fractionate an asphaltene sample will tend to direct the fractionation by introducing polar forces and hydrogen bonding, as well as dispersion forces, as factors
determining which components of the asphaltene sample are soluble in the mixture.
In contrast, the exclusive use of saturated hydrocarbons whose solvent power derives entirely
from the dispersion forces should, in principle, permit the fractionation to be carried out with
variation of only a single component of the solubility parameter. It is known that the quantity of
asphaltenes precipitated by linear paraffins decreases with increasing carbon number until a limiting value is reached above n-heptane (n-C 7 H 16 ) to n-nonane (n-C 9 H 20 ) (Girdler, 1965; Mitchell and
Speight, 1973).
The cycloparaffins have better solvating power toward the asphaltenes and often fail to induce
asphaltene separation from crude oil. It is possible to fractionate asphaltenes using mixtures of
solvents, such as cyclohexane and n-alkanes, and any differences in the precipitation threshold
underscores the point that the solubility properties of the asphaltene fractions of different crude oils
can vary markedly (Acevedo et al., 1995).
It appears, therefore, that the tendency of asphaltenes to precipitate from a given crude oil
because of a variation in the solubility parameter of the precipitating solvent mixture is more closely
related to the aromaticity and polarity of the asphaltenes than to their dimensions. However, it is
possible to conclude (Acevedo et al., 1995) that molecular weight, aromaticity, and polarity combine
to determine asphaltene solubility in hydrocarbon media.
The fractionation of asphaltenes into a variety of functional (and polar) types (Figure 12.1;
Francisco and Speight, 1984) has confirmed the complexity of the asphaltene fraction. Other studies
(Schmitter et al., 1984) have brought to light the occurrence of four-ring aromatic nitrogen species
in petroleum. These findings are of particular interest since they correspond to the ring systems that
have been tentatively identified by application of the HPLC technique to the basic nitrogen fraction
of asphaltenes (Speight, 1986b, 1994).
The molecular weight/polarity concept in which asphaltene fractions are defined on the basis
of molecular weight and polarity (Figure 12.3) can be used to indicate the relative nature of carbenes and carboids (thermal decomposition products of asphaltenes), which are of lower molecular weight and more polar than the asphaltene constituents from which they are formed (Speight,
1994). Furthermore, the slope of the line as illustrated is purely arbitrary and will vary with the
source and composition of different fractions (Figure 12.4). However, it should not be construed
that this two-dimensional diagram is truly representative of the nature of asphaltenes and a threedimensional diagram involving functionality or molecular weight might be more appropriate for
showing the chemical and physical differences between the various asphaltenes, carbenes, and carboids. Nevertheless, the concept does emphasize the lower molecular weight and increased polarity
of the various asphaltene constituents.
Unfortunately, in many studies, too little emphasis has been placed on determining the nature
and location of the nitrogen, oxygen, and sulfur atoms in the asphaltene structure.
Studies on the disposition of nitrogen in petroleum asphaltenes indicated the existence of nitrogen as various heterocyclic types (Clerc and O’Neal, 1961; Nicksic and Jeffries-Harris, 1968;
Moschopedis and Speight, 1976b, 1979; Jacobson and Gray, 1987; Mullins, 1995). The more