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Asphaltene Constituents
The resins are dark, semisolid or solid, very adhesive materials of high molecular weight. Their
composition can vary depending on the kind of precipitating liquid and on the temperature of the
liquid system. They become quite fluid on heating but often show pronounced brittleness when cold.
The oils fraction (comprising the saturates fraction plus the aromatics fraction) comprises
the lowest molecular weight fraction of petroleum and is the dispersion medium for the peptized
asphaltenes. Although the oils fraction is sometimes colored, fractions obtained by chromatography
are colorless and are similar to white medicinal oils or lubricating oils of high purity. The oils fraction may be quite viscous because of the presence of paraffin waxes that vary over a wide range for
crude oil and other feedstocks. The oils fraction is miscible with low-molecular-weight liquid hydrocarbons (such as n-pentane and n-heptane), as well as most organic solvents, and may be separated
into various hydrocarbon subfractions by suitable solvent or adsorbent methods.
12.3 COMPOSITION
Asphaltene fractions isolated from different sources are remarkably constant in terms of ultimate
composition, although there can be variations in terms of heteroatom content because of local and
regional variations in the plant precursors and mineralogical composition of the nearby geological
formations. Even though the nature of the source material and subtle regional variations in maturation conditions serve to differentiate one crude oil (and hence one asphaltene) from another, it might
be suggested that the maturation process tends to be a molecular equalizer over the time periods
involved. Thus, differences between the asphaltene constituents reflect differences in the relative
amounts of the functional molecular types present and the way in which the structural types are
combined in the various asphaltenes. This is reflected, in part, by changes in the functional group
composition of asphaltene fractions from different crude oils (see, e.g., Koots and Speight, 1975;
Huc et al., 1984).
The elemental composition of asphaltenes isolated by use of excess (greater than 40) volumes of
n-pentane as the precipitating medium shows that the amounts of carbon and hydrogen usually vary
over only a narrow range (Speight, 1994). These values correspond to a hydrogen-to-carbon atomic
ratio of 1.15% ± 0.5%, although values outside this range are sometimes found. The near constancy of the H/C ratio is surprising when the number of possible molecular permutations involving
the hetero elements is considered. In fact, this property, more than any other, is the cause for the
general belief that unaltered asphaltene constituents from crude oil have a definite composition.
Furthermore, it is still believed that asphaltenes are precipitated from petroleum by hydrocarbon
solvents because of this composition, not only because of solubility properties.
In contrast to the carbon and hydrogen contents of asphaltenes, notable variations occur in the
proportions of the hetero elements, in particular in the proportions of oxygen and sulfur. Oxygen
contents vary from 0.3% to 4.9%, and sulfur contents from 0.3% to 10.3%. On the other hand, the
nitrogen content of the asphaltenes has a somewhat lesser degree of variation (0.6%–3.3% at the
extremes). However, exposing asphaltenes to atmosphere oxygen can substantially alter the oxygen
content and exposing a crude oil to elemental sulfur or even to sulfur-containing minerals can result
in excessive sulfur uptake. Perhaps, oxygen and sulfur contents vary more markedly than do nitrogen contents because of these conditions.
The use of n-heptane as the precipitating medium yields a product that is substantially different
from the n-pentane-insoluble material. For example, the hydrogen-to-carbon atomic ratio of the
n-heptane precipitate is lower than that of the n-pentane precipitate (Speight, 1994, 2001). This
indicates a higher degree of aromaticity in the n-heptane precipitate. Nitrogen-to-carbon, oxygento-carbon, and sulfur-to-carbon ratios are usually higher in the n-heptane precipitate, indicating
higher proportions of the hetero elements in this material (Speight and Moschopedis, 1981a).
Some interesting compositional relationships can also be derived from the data. These data confirm the consistency of asphaltene composition but show that when composition varies, this is usually due to heteroatom content. It is also possible to show from the data how the asphaltenes compare
Asphaltene Constituents
The resins are dark, semisolid or solid, very adhesive materials of high molecular weight. Their
composition can vary depending on the kind of precipitating liquid and on the temperature of the
liquid system. They become quite fluid on heating but often show pronounced brittleness when cold.
The oils fraction (comprising the saturates fraction plus the aromatics fraction) comprises
the lowest molecular weight fraction of petroleum and is the dispersion medium for the peptized
asphaltenes. Although the oils fraction is sometimes colored, fractions obtained by chromatography
are colorless and are similar to white medicinal oils or lubricating oils of high purity. The oils fraction may be quite viscous because of the presence of paraffin waxes that vary over a wide range for
crude oil and other feedstocks. The oils fraction is miscible with low-molecular-weight liquid hydrocarbons (such as n-pentane and n-heptane), as well as most organic solvents, and may be separated
into various hydrocarbon subfractions by suitable solvent or adsorbent methods.
12.3 COMPOSITION
Asphaltene fractions isolated from different sources are remarkably constant in terms of ultimate
composition, although there can be variations in terms of heteroatom content because of local and
regional variations in the plant precursors and mineralogical composition of the nearby geological
formations. Even though the nature of the source material and subtle regional variations in maturation conditions serve to differentiate one crude oil (and hence one asphaltene) from another, it might
be suggested that the maturation process tends to be a molecular equalizer over the time periods
involved. Thus, differences between the asphaltene constituents reflect differences in the relative
amounts of the functional molecular types present and the way in which the structural types are
combined in the various asphaltenes. This is reflected, in part, by changes in the functional group
composition of asphaltene fractions from different crude oils (see, e.g., Koots and Speight, 1975;
Huc et al., 1984).
The elemental composition of asphaltenes isolated by use of excess (greater than 40) volumes of
n-pentane as the precipitating medium shows that the amounts of carbon and hydrogen usually vary
over only a narrow range (Speight, 1994). These values correspond to a hydrogen-to-carbon atomic
ratio of 1.15% ± 0.5%, although values outside this range are sometimes found. The near constancy of the H/C ratio is surprising when the number of possible molecular permutations involving
the hetero elements is considered. In fact, this property, more than any other, is the cause for the
general belief that unaltered asphaltene constituents from crude oil have a definite composition.
Furthermore, it is still believed that asphaltenes are precipitated from petroleum by hydrocarbon
solvents because of this composition, not only because of solubility properties.
In contrast to the carbon and hydrogen contents of asphaltenes, notable variations occur in the
proportions of the hetero elements, in particular in the proportions of oxygen and sulfur. Oxygen
contents vary from 0.3% to 4.9%, and sulfur contents from 0.3% to 10.3%. On the other hand, the
nitrogen content of the asphaltenes has a somewhat lesser degree of variation (0.6%–3.3% at the
extremes). However, exposing asphaltenes to atmosphere oxygen can substantially alter the oxygen
content and exposing a crude oil to elemental sulfur or even to sulfur-containing minerals can result
in excessive sulfur uptake. Perhaps, oxygen and sulfur contents vary more markedly than do nitrogen contents because of these conditions.
The use of n-heptane as the precipitating medium yields a product that is substantially different
from the n-pentane-insoluble material. For example, the hydrogen-to-carbon atomic ratio of the
n-heptane precipitate is lower than that of the n-pentane precipitate (Speight, 1994, 2001). This
indicates a higher degree of aromaticity in the n-heptane precipitate. Nitrogen-to-carbon, oxygento-carbon, and sulfur-to-carbon ratios are usually higher in the n-heptane precipitate, indicating
higher proportions of the hetero elements in this material (Speight and Moschopedis, 1981a).
Some interesting compositional relationships can also be derived from the data. These data confirm the consistency of asphaltene composition but show that when composition varies, this is usually due to heteroatom content. It is also possible to show from the data how the asphaltenes compare