229
Fractional Composition
the specimen. The method consists of precipitation of asphaltenes with 40 volumes of n-pentane
(or n-heptane) and elution of the n-pentane (or n-heptane) soluble fraction from a chromatographic
column of fuller’s earth. The elution technique gave a series of fractions:
1. Oils eluted with n-pentane
2. Dark oils eluted with methylene chloride
3. Resins with methyl ethyl ketone
4. Hard resins that were desorbed with an acetone–chloroform mixture
The proportions of each fraction are subject to the ratio of fuller’s earth to n-pentane soluble materials. For example, a change in the ratio from 10:1 to 25:1 causes a decrease in the percentage of water
white oils by a factor of 3:2, and the percentage of dark oils and asphaltic resins increases by about
the same factor. The method, like all chromatographic procedures proposed for the fractionation
of crude oils, is dominated by equilibrium conditions, and does not give fractions that are different
components but only blends of the same components in different proportions.
Other method of fractionation by the use of adsorbents include separation of the maltene fraction by elution with n-heptane from silica gel into two fractions named aromatics and nonaromatics
and is in fact a separation into the two broad groups called resins and oils in other methods. The
silica gel method may also be modified to produce three fractions: (1) nonaromatics eluted with
n-heptane, (2) aromatics eluted with benzene, and (3) compounds that contain oxygen as well as
sulfur and nitrogen, eluted with pyridine. Prior separation of the asphaltenes renders the procedure
especially suitable and convenient for use with heavy oil and bitumen. Other modifications include
successive elution with n-pentane, benzene, carbon tetrachloride, and ethanol.
Alumina has also been used as an adsorbent and involves (1) precipitation of asphaltenes with
normal pentane, (2) elution of oils from alumina with pentane, and (3) elution of resins from alumina with a methanol–benzene mixture. In fact, the choice of the adsorbent appears to be arbitrary,
as does the choice of the various solvents or solvent blend. The use of ill-defined adsorbents, such as
earths or clays, is a disadvantage in that certain components of the petroleum may undergo changes
(e.g., polymerization) caused by the catalytic nature of the adsorbent and can no longer be extracted
quantitatively. Furthermore, extraction of the adsorbed components may require the use of solvents
of comparatively high solvent power, such as chloroform or pyridine, which may be difficult to
remove from the product fractions.
It is also advisable, once a procedure using an earth or clay has been established, that the same
type of adsorbent be employed for future fractionation since the ratio of the product fractions varies
from adsorbent to adsorbent. It is also very necessary that the procedure be used with caution and
that the method not only be reproducible but quantitative recoveries be guaranteed; reproducibility
with only, say, 85% of the material recoverable is not a criterion of success.
There are two procedures that have received considerable attention over the years and these
are (1) the United States Bureau of Mines–American Petroleum Institute (USBM-API) method
(Figure 9.6) and (2) the saturates–aromatics–resins–asphaltenes (SARA) method. This latter
method is often also called the saturates–aromatics–polars–asphaltenes (SAPA) method. These
two methods are used as representing the standard methods of petroleum fractionation. Other
methods are also noted, especially when the method has added further meaningful knowledge to
compositional studies.
The USBM-API method employs ion-exchange chromatography and coordination chromatography with adsorption chromatography to separate heavy oils and residua into seven broad fractions:
acids, bases, neutral nitrogen compounds, saturates, and mono-, di-, and polyaromatic compounds.
The acid and base fractions are isolated by ion-exchange chromatography, the neutral nitrogen
compounds by complexation chromatography using ferric chloride, and the saturates and aromatics
by adsorption chromatography on activated alumina (Jewell et al., 1972) or on a combined alumina–
silica column.
Fractional Composition
the specimen. The method consists of precipitation of asphaltenes with 40 volumes of n-pentane
(or n-heptane) and elution of the n-pentane (or n-heptane) soluble fraction from a chromatographic
column of fuller’s earth. The elution technique gave a series of fractions:
1. Oils eluted with n-pentane
2. Dark oils eluted with methylene chloride
3. Resins with methyl ethyl ketone
4. Hard resins that were desorbed with an acetone–chloroform mixture
The proportions of each fraction are subject to the ratio of fuller’s earth to n-pentane soluble materials. For example, a change in the ratio from 10:1 to 25:1 causes a decrease in the percentage of water
white oils by a factor of 3:2, and the percentage of dark oils and asphaltic resins increases by about
the same factor. The method, like all chromatographic procedures proposed for the fractionation
of crude oils, is dominated by equilibrium conditions, and does not give fractions that are different
components but only blends of the same components in different proportions.
Other method of fractionation by the use of adsorbents include separation of the maltene fraction by elution with n-heptane from silica gel into two fractions named aromatics and nonaromatics
and is in fact a separation into the two broad groups called resins and oils in other methods. The
silica gel method may also be modified to produce three fractions: (1) nonaromatics eluted with
n-heptane, (2) aromatics eluted with benzene, and (3) compounds that contain oxygen as well as
sulfur and nitrogen, eluted with pyridine. Prior separation of the asphaltenes renders the procedure
especially suitable and convenient for use with heavy oil and bitumen. Other modifications include
successive elution with n-pentane, benzene, carbon tetrachloride, and ethanol.
Alumina has also been used as an adsorbent and involves (1) precipitation of asphaltenes with
normal pentane, (2) elution of oils from alumina with pentane, and (3) elution of resins from alumina with a methanol–benzene mixture. In fact, the choice of the adsorbent appears to be arbitrary,
as does the choice of the various solvents or solvent blend. The use of ill-defined adsorbents, such as
earths or clays, is a disadvantage in that certain components of the petroleum may undergo changes
(e.g., polymerization) caused by the catalytic nature of the adsorbent and can no longer be extracted
quantitatively. Furthermore, extraction of the adsorbed components may require the use of solvents
of comparatively high solvent power, such as chloroform or pyridine, which may be difficult to
remove from the product fractions.
It is also advisable, once a procedure using an earth or clay has been established, that the same
type of adsorbent be employed for future fractionation since the ratio of the product fractions varies
from adsorbent to adsorbent. It is also very necessary that the procedure be used with caution and
that the method not only be reproducible but quantitative recoveries be guaranteed; reproducibility
with only, say, 85% of the material recoverable is not a criterion of success.
There are two procedures that have received considerable attention over the years and these
are (1) the United States Bureau of Mines–American Petroleum Institute (USBM-API) method
(Figure 9.6) and (2) the saturates–aromatics–resins–asphaltenes (SARA) method. This latter
method is often also called the saturates–aromatics–polars–asphaltenes (SAPA) method. These
two methods are used as representing the standard methods of petroleum fractionation. Other
methods are also noted, especially when the method has added further meaningful knowledge to
compositional studies.
The USBM-API method employs ion-exchange chromatography and coordination chromatography with adsorption chromatography to separate heavy oils and residua into seven broad fractions:
acids, bases, neutral nitrogen compounds, saturates, and mono-, di-, and polyaromatic compounds.
The acid and base fractions are isolated by ion-exchange chromatography, the neutral nitrogen
compounds by complexation chromatography using ferric chloride, and the saturates and aromatics
by adsorption chromatography on activated alumina (Jewell et al., 1972) or on a combined alumina–
silica column.