315
12
Asphaltene Constituents
12.1 INTRODUCTION
As the boiling points of petroleum fractions increase, the complexity of the constituents in these
fractions also increases, and the differences between the main classes of hydrocarbons become
less well pronounced. The high-boiling fractions of petroleum (and this may commence with the
saturates fraction and the aromatics fractions depending on the boiling profile of the crude oil) may
be considered as having an extremely complicated composition. In fact, any attempted isolation of
individual compounds is a frustrating and an impossible task.
Nevertheless, the properties of any material are dictated by its chemical composition, and once
the chemical composition of a material has been established, it becomes possible to correlate composition and properties. Petroleum is no exception to this generality; it is recognized that the constituents of petroleum contribute to its physical state as well as to its behavior during processing.
It is therefore desirable to investigate the general composition of crude oil, and it is not surprising
that the separation of the constituents of crude oil by virtue of their physical properties has received
considerable attention.
It is generally recognized that crude oils are composed of four major fractions (saturates, aromatics, resins, and asphaltenes) that differ from one another sufficiently in solubility and adsorptive character that the separation can be achieved by application of relevant methods (Chapter 10).
Indeed, although these four fractions are chemically complex, the methods of separation have
undergone several modifications to such an extent that the evolution of the separation techniques is
a study in itself (Chapter 10). And because the fractions are in a balanced relationship in crude oil,
the chemical and physical characters of these constituents need reference in this chapter.
It will become immediately apparent from the contents of this chapter that the majority of the
work carried out on the higher boiling constituents of petroleum concentrated on the asphaltene
fraction. Parallel work on the resin and oil fractions is extremely sketchy and has not been considered worthwhile, insofar as it is generally believed that the constituents of the asphaltene fraction
have markedly adverse affects on the processability of crude oils. Furthermore, it is the asphaltene
constituents that also play a role in the rheological properties of asphalt (Nellensteyn, 1924, 1938;
Katz and Beu, 1945; Speight, 1995, 1999; Ancheyta et al., 2002; Sheu, 2002).
For the purposes of this chapter, the resin constituents are regarded as those materials soluble
in n-pentane or n-heptane (i.e., whichever hydrocarbon is used for the separation of asphaltenes)
but insoluble in liquid propane. In addition, resin constituents are also those materials soluble in
n-pentane or n-heptane but that cannot be extracted from an earth by n-pentane or n-heptane. The
oils fraction is, therefore, that fraction of crude oil soluble in n-pentane or in n-heptane that is
extractable from an earth by n-pentane or by n-heptane. The oils fraction is composed of saturates
and aromatics fractions (Chapter 1).
Nevertheless, attempts have been made to collate the results of work carried out on the resin and
oil fractions, as it is now evident that petroleum is a delicately balanced physical system (Chapter 13).
Each fraction depends on the other fractions for complete mobility and maintaining the stability of
the petroleum.
Questions may also be raised regarding the usefulness of attempting to determine the exact structure of each complex material, such as asphaltenes, resins, and nonvolatile oils. Perhaps the answers lie
not only in determining the locations of the heteroatoms (nitrogen, oxygen, and sulfur). It is the hope
12
Asphaltene Constituents
12.1 INTRODUCTION
As the boiling points of petroleum fractions increase, the complexity of the constituents in these
fractions also increases, and the differences between the main classes of hydrocarbons become
less well pronounced. The high-boiling fractions of petroleum (and this may commence with the
saturates fraction and the aromatics fractions depending on the boiling profile of the crude oil) may
be considered as having an extremely complicated composition. In fact, any attempted isolation of
individual compounds is a frustrating and an impossible task.
Nevertheless, the properties of any material are dictated by its chemical composition, and once
the chemical composition of a material has been established, it becomes possible to correlate composition and properties. Petroleum is no exception to this generality; it is recognized that the constituents of petroleum contribute to its physical state as well as to its behavior during processing.
It is therefore desirable to investigate the general composition of crude oil, and it is not surprising
that the separation of the constituents of crude oil by virtue of their physical properties has received
considerable attention.
It is generally recognized that crude oils are composed of four major fractions (saturates, aromatics, resins, and asphaltenes) that differ from one another sufficiently in solubility and adsorptive character that the separation can be achieved by application of relevant methods (Chapter 10).
Indeed, although these four fractions are chemically complex, the methods of separation have
undergone several modifications to such an extent that the evolution of the separation techniques is
a study in itself (Chapter 10). And because the fractions are in a balanced relationship in crude oil,
the chemical and physical characters of these constituents need reference in this chapter.
It will become immediately apparent from the contents of this chapter that the majority of the
work carried out on the higher boiling constituents of petroleum concentrated on the asphaltene
fraction. Parallel work on the resin and oil fractions is extremely sketchy and has not been considered worthwhile, insofar as it is generally believed that the constituents of the asphaltene fraction
have markedly adverse affects on the processability of crude oils. Furthermore, it is the asphaltene
constituents that also play a role in the rheological properties of asphalt (Nellensteyn, 1924, 1938;
Katz and Beu, 1945; Speight, 1995, 1999; Ancheyta et al., 2002; Sheu, 2002).
For the purposes of this chapter, the resin constituents are regarded as those materials soluble
in n-pentane or n-heptane (i.e., whichever hydrocarbon is used for the separation of asphaltenes)
but insoluble in liquid propane. In addition, resin constituents are also those materials soluble in
n-pentane or n-heptane but that cannot be extracted from an earth by n-pentane or n-heptane. The
oils fraction is, therefore, that fraction of crude oil soluble in n-pentane or in n-heptane that is
extractable from an earth by n-pentane or by n-heptane. The oils fraction is composed of saturates
and aromatics fractions (Chapter 1).
Nevertheless, attempts have been made to collate the results of work carried out on the resin and
oil fractions, as it is now evident that petroleum is a delicately balanced physical system (Chapter 13).
Each fraction depends on the other fractions for complete mobility and maintaining the stability of
the petroleum.
Questions may also be raised regarding the usefulness of attempting to determine the exact structure of each complex material, such as asphaltenes, resins, and nonvolatile oils. Perhaps the answers lie
not only in determining the locations of the heteroatoms (nitrogen, oxygen, and sulfur). It is the hope