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9
Fractional Composition
9.1 INTRODUCTION
Refining petroleum involves subjecting the feedstock to a series of physical and chemical processes
(Chapter 15) as a result of which a variety of products are generated. In some of the processes, for
example, distillation, the constituents of the feedstock are isolated and unchanged, whereas in other
processes, for example, cracking, considerable changes are brought about to the constituents.
Recognition that refinery behavior is related to the composition of the feedstock has led to a multiplicity of attempts to establish petroleum and its fractions as compositions of matter. As a result,
various analytical techniques have been developed for the identification and quantification of every
molecule in the lower boiling fractions of petroleum (ASTM, 2012). It is now generally recognized
that the name petroleum does not describe a composition of matter but rather a mixture of various
organic compounds that includes a wide range of molecular weights and molecular types that exist
in balance with each other (Speight, 1994; Subramanian et al., 1996; McLean and Kilpatrick, 1997;
Long and Speight, 1998). There must also be some questions of the advisability (perhaps futility is
a better word) of attempting to describe every molecule in petroleum. The true focus should be to
what ends these molecules can be used.
Thus, investigations of the character of petroleum need to be focused on the influence of its
character on refining operations and the nature of the products that will be produced. Furthermore,
one means by which the character of petroleum has been studied is through its fractional composition. However, the fractional composition of petroleum varies markedly with the method of
isolation or separation, thereby leading to potential complications (especially in the case of the
heavier feedstocks) in the choice of suitable processing schemes for these feedstocks. Crude oil
can be fractionated into three or four general fractions: (1) asphaltene fraction, (2) resin fraction,
(3) aromatics fraction, and (4) saturates fraction (Figure 9.1). Thus, it is possible to compare interlaboratory investigations and thence to apply the concept of predictability to refining sequences
and potential products.
Investigations of the character of petroleum through fractionation studies have been practiced for
more than 170 years (Boussingault, 1837), although modern fractionation techniques are essentially
a twentieth-century approach to examining petroleum composition. In fact, the fractionation of
petroleum has evolved to such an extent that it is now possible to determine with a high degree of
accuracy the types of compounds present in a crude oil.
The fractionation methods available to the petroleum industry allow a reasonably effective
degree of separation of hydrocarbon mixtures. However, the problems are separating the petroleum
constituents without alteration of their molecular structure and obtaining these constituents in a
substantially pure state. Thus, the general procedure is to employ techniques that segregate the
constituents according to molecular size and molecular type.
It is more generally true, however, that the success of any attempted fractionation procedure
involves not only the application of one particular technique but also the utilization of several integrated techniques, especially those techniques involving the use of chemical and physical properties
to differentiate among the various constituents. For example, the standard processes of physical
fractionation used in the petroleum industry are those of distillation and solvent treatment, as
well as adsorption by surface-active materials. Chemical procedures depend on specific reactions,
such as the interaction of olefins with sulfuric acid or the various classes of adduct formation.
9
Fractional Composition
9.1 INTRODUCTION
Refining petroleum involves subjecting the feedstock to a series of physical and chemical processes
(Chapter 15) as a result of which a variety of products are generated. In some of the processes, for
example, distillation, the constituents of the feedstock are isolated and unchanged, whereas in other
processes, for example, cracking, considerable changes are brought about to the constituents.
Recognition that refinery behavior is related to the composition of the feedstock has led to a multiplicity of attempts to establish petroleum and its fractions as compositions of matter. As a result,
various analytical techniques have been developed for the identification and quantification of every
molecule in the lower boiling fractions of petroleum (ASTM, 2012). It is now generally recognized
that the name petroleum does not describe a composition of matter but rather a mixture of various
organic compounds that includes a wide range of molecular weights and molecular types that exist
in balance with each other (Speight, 1994; Subramanian et al., 1996; McLean and Kilpatrick, 1997;
Long and Speight, 1998). There must also be some questions of the advisability (perhaps futility is
a better word) of attempting to describe every molecule in petroleum. The true focus should be to
what ends these molecules can be used.
Thus, investigations of the character of petroleum need to be focused on the influence of its
character on refining operations and the nature of the products that will be produced. Furthermore,
one means by which the character of petroleum has been studied is through its fractional composition. However, the fractional composition of petroleum varies markedly with the method of
isolation or separation, thereby leading to potential complications (especially in the case of the
heavier feedstocks) in the choice of suitable processing schemes for these feedstocks. Crude oil
can be fractionated into three or four general fractions: (1) asphaltene fraction, (2) resin fraction,
(3) aromatics fraction, and (4) saturates fraction (Figure 9.1). Thus, it is possible to compare interlaboratory investigations and thence to apply the concept of predictability to refining sequences
and potential products.
Investigations of the character of petroleum through fractionation studies have been practiced for
more than 170 years (Boussingault, 1837), although modern fractionation techniques are essentially
a twentieth-century approach to examining petroleum composition. In fact, the fractionation of
petroleum has evolved to such an extent that it is now possible to determine with a high degree of
accuracy the types of compounds present in a crude oil.
The fractionation methods available to the petroleum industry allow a reasonably effective
degree of separation of hydrocarbon mixtures. However, the problems are separating the petroleum
constituents without alteration of their molecular structure and obtaining these constituents in a
substantially pure state. Thus, the general procedure is to employ techniques that segregate the
constituents according to molecular size and molecular type.
It is more generally true, however, that the success of any attempted fractionation procedure
involves not only the application of one particular technique but also the utilization of several integrated techniques, especially those techniques involving the use of chemical and physical properties
to differentiate among the various constituents. For example, the standard processes of physical
fractionation used in the petroleum industry are those of distillation and solvent treatment, as
well as adsorption by surface-active materials. Chemical procedures depend on specific reactions,
such as the interaction of olefins with sulfuric acid or the various classes of adduct formation.