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The Chemistry and Technology of Petroleum
typically phenols, naphthenic acids, and esters (Chapter 8). They increase in quantity and complexity as molecular weight increases, just as sulfur and nitrogen compounds do.
A variety of sulfur compounds occur in petroleum (Chapter 8) and include mercaptans (–SH),
sulfides (–S–), and disulfides (–S–S–). They are much less thermally stable than the thiophene
derivatives and often lose hydrogen sulfide on heating. They can also react thermally to form more
stable sulfur compounds. In the chromatographic separation, these sulfur compounds are found in
the polar aromatics fraction, even though they may not all be aromatic. Thiophenes tend to exhibit
aromatic behavior and are collected with the aromatics when adsorption separation is used on petroleum fractions.
9.4.2 FrACtIonAtIon metHods
9.4.2.1 General Methods
Separation of petroleum, heavy oil, tar sand bitumen, and residua by adsorption chromatography
essentially commences with the preparation of a porous bed of finely divided solid, the adsorbent
(Hoiberg, 1964). The adsorbent is usually contained in an open tube (column chromatography); the
sample is introduced at one end of the adsorbent bed and induced to flow through the bed by means of
a suitable solvent. As the sample moves through the bed the various components are held (adsorbed) to
a greater or lesser extent depending on the chemical nature of the component. Thus, those molecules
that are strongly adsorbed spend considerable time on the adsorbent surface rather than in the moving
(solvent) phase, but components that are slightly adsorbed move through the bed comparatively rapidly.
Numerous factors randomly affect the process of migration through a bed, and in fact, the
total distance traveled in a given time by different molecules of the same material is not constant.
Nevertheless, the suitable choice of a bed and a moving (solvent) phase allows adequate separation
of even multi-component mixtures to be achieved. Thus, the overall adsorption chromatographic
process may be viewed as migration of different compounds along the bed, which varies with compound structure as well as a range of migrations by different molecules of the same compound.
The fractionation of petroleum and residua by adsorption on such materials as fuller’s earth,
animal charcoal, and various types of clay dates back to the beginning of the twentieth century
(Hoiberg, 1964). These materials effect an arbitrary separation of the material into a number of
fractions that have variously been described as oil, resins, hard resins, and soft resins, to mention
only the more commonly used terms.
It is essential that, before application of the adsorption technique to the petroleum, the asphaltenes
first be completely removed, for example, by any of the methods outlined in the previous section.
The prior removal of the asphaltenes is essential insofar as they are usually difficult to remove from
the earth or clay and may actually be irreversibly adsorbed on the adsorbent.
Nevertheless, careful monitoring of the experimental procedures and the nature of the adsorbent
has been responsible for the successes achieved with this particular technique. Early procedures consisted of warming solutions of the petroleum fraction with the adsorbent and subsequent filtration.
This procedure has continued to the present day, and separation by adsorption is used commercially
in plant operations in the form of clay treatment of crude oil fractions and products (Chapter 24).
In the laboratory very little use is made of the technique of warming a solution of the sample
with the adsorbent. Rather, a chromatographic technique is employed in which the sample is washed
through a column of the adsorbent using various solvents.
Early investigations involved filtration of crude oils through a column of fuller’s earth and it
was observed that the gasoline components appeared in the initial part of the filtrate. Subsequent
investigations showed that if light oil is drawn upward by a pump through a column of earth, the
light aliphatic hydrocarbons accumulate in the top section. The aromatic constituents did not rise
as high and the nitrogen- and sulfur-containing compounds were largely retained on the adsorbent.
A later chromatographic method involved separation into four principal fractions—asphaltenes,
asphaltic resins, dark oils, and water white oils—and a fifth fraction constituting the balance of
The Chemistry and Technology of Petroleum
typically phenols, naphthenic acids, and esters (Chapter 8). They increase in quantity and complexity as molecular weight increases, just as sulfur and nitrogen compounds do.
A variety of sulfur compounds occur in petroleum (Chapter 8) and include mercaptans (–SH),
sulfides (–S–), and disulfides (–S–S–). They are much less thermally stable than the thiophene
derivatives and often lose hydrogen sulfide on heating. They can also react thermally to form more
stable sulfur compounds. In the chromatographic separation, these sulfur compounds are found in
the polar aromatics fraction, even though they may not all be aromatic. Thiophenes tend to exhibit
aromatic behavior and are collected with the aromatics when adsorption separation is used on petroleum fractions.
9.4.2 FrACtIonAtIon metHods
9.4.2.1 General Methods
Separation of petroleum, heavy oil, tar sand bitumen, and residua by adsorption chromatography
essentially commences with the preparation of a porous bed of finely divided solid, the adsorbent
(Hoiberg, 1964). The adsorbent is usually contained in an open tube (column chromatography); the
sample is introduced at one end of the adsorbent bed and induced to flow through the bed by means of
a suitable solvent. As the sample moves through the bed the various components are held (adsorbed) to
a greater or lesser extent depending on the chemical nature of the component. Thus, those molecules
that are strongly adsorbed spend considerable time on the adsorbent surface rather than in the moving
(solvent) phase, but components that are slightly adsorbed move through the bed comparatively rapidly.
Numerous factors randomly affect the process of migration through a bed, and in fact, the
total distance traveled in a given time by different molecules of the same material is not constant.
Nevertheless, the suitable choice of a bed and a moving (solvent) phase allows adequate separation
of even multi-component mixtures to be achieved. Thus, the overall adsorption chromatographic
process may be viewed as migration of different compounds along the bed, which varies with compound structure as well as a range of migrations by different molecules of the same compound.
The fractionation of petroleum and residua by adsorption on such materials as fuller’s earth,
animal charcoal, and various types of clay dates back to the beginning of the twentieth century
(Hoiberg, 1964). These materials effect an arbitrary separation of the material into a number of
fractions that have variously been described as oil, resins, hard resins, and soft resins, to mention
only the more commonly used terms.
It is essential that, before application of the adsorption technique to the petroleum, the asphaltenes
first be completely removed, for example, by any of the methods outlined in the previous section.
The prior removal of the asphaltenes is essential insofar as they are usually difficult to remove from
the earth or clay and may actually be irreversibly adsorbed on the adsorbent.
Nevertheless, careful monitoring of the experimental procedures and the nature of the adsorbent
has been responsible for the successes achieved with this particular technique. Early procedures consisted of warming solutions of the petroleum fraction with the adsorbent and subsequent filtration.
This procedure has continued to the present day, and separation by adsorption is used commercially
in plant operations in the form of clay treatment of crude oil fractions and products (Chapter 24).
In the laboratory very little use is made of the technique of warming a solution of the sample
with the adsorbent. Rather, a chromatographic technique is employed in which the sample is washed
through a column of the adsorbent using various solvents.
Early investigations involved filtration of crude oils through a column of fuller’s earth and it
was observed that the gasoline components appeared in the initial part of the filtrate. Subsequent
investigations showed that if light oil is drawn upward by a pump through a column of earth, the
light aliphatic hydrocarbons accumulate in the top section. The aromatic constituents did not rise
as high and the nitrogen- and sulfur-containing compounds were largely retained on the adsorbent.
A later chromatographic method involved separation into four principal fractions—asphaltenes,
asphaltic resins, dark oils, and water white oils—and a fifth fraction constituting the balance of