231
Fractional Composition
involves the fractionation of Athabasca tar sand bitumen into four gross fractions and sub-fractionation of these four fractions, which allows further study of the distribution of the functional types
within the feedstock.
A combination chromatography using alumina and silica gel is also suitable for deasphaltened
oils. A more complex scheme, also involving the use of silica, resulted in the successful separation
of hexane-deasphalted crude oil (Seifert, 1975). There is also a report of the direct fractionation of
hydrocarbon and hetero compounds from deasphaltened residua on a dual alumina–silica column
with subsequent treatment of the polar fraction with cation- and anion-exchange resins into basic,
acid, and neutral materials. The method also includes chromatography of the asphaltenes, but only
with highly polar asphaltene samples are the basic and acid compounds first removed with ionexchange resins. The remainder of the feedstock is separated into saturates, aromatics, and hetero
compounds using alumina–silica adsorption. Separation of the saturates fraction into n-alkanes
and iso-alkanes plus cycloalkanes is achieved by the use of urea (H 2 N . C=O . NH 2 ) and thiourea
(H 2 N . C=S . NH 2 ).
One of the problems of such a fractionation scheme is the initial separation of the feedstocks
into two ill-defined fractions (colloids and dispersant) without first removing the asphaltenes. As
already noted, asphaltenes are specifically defined by the method of separation. They are less well
defined using such liquids as ethyl acetate in place of the more often used hydrocarbons, such as
pentane and heptane (Speight, 1979). The use of ethyl acetate undoubtedly leads to asphaltene material in the dispersoids and non-asphaltene material in the colloids. Application of a more standard
deasphalting technique would undoubtedly improve this method and provide an excellent insight
into feedstock composition.
One of the common issues related to the use of any adsorption-based fractionation scheme is
the nature of the adsorbent. In the early reports of petroleum fractionation (Pfeiffer, 1950), clays
often appeared as an adsorbent to effect the separation of the feedstock into various constituent
fractions. However, clay (fuller’s earth, attapulgus clay, and the like) is often difficult to define with
any degree of precision from one batch to another. Variations in the nature and properties of the
clay can, and will, cause differences not only in the yields of composite fractions but also in the distribution of the compound types in those fractions. In addition, irreversible adsorption of the more
polar constituent to the clay can be a serious problem when further investigations of the constituent
fractions are planned.
One option for resolving this problem has been the use of more standard adsorbents, such as
alumina and silica. These materials are easier to define and are often accompanied by guarantees
of composition and type by various manufacturers. They also tend to irreversibly adsorb less of
the feedstock than clay. Once the nature of the adsorbent is guaranteed, reproducibility becomes a
reality. Without reproducibility the analytic method does not have credibility.
9.4.2.2 ASTM Methods
There are three ASTM methods that provide for the separation of a feedstock into four or five
constituent fractions and it is interesting to note that as the methods have evolved there has been a
change from the use of pentane (ASTM D2006 and ASTM D2007) (Figures 9.7 and 9.8) to heptane
(ASTM D4124) (Figure 9.9) to separate asphaltenes. This is, in fact, in keeping with the production
of a more consistent fraction that represents these higher molecular weight, more complex constituents of petroleum (Girdler, 1965; Speight et al., 1984).
Two of the methods (ASTM D2007 and ASTM D4124) use adsorbents to fractionate the deasphaltened oil, but the third method (ASTM D2006) advocates the use of various grades of sulfuric
acid to separate the material into compound types. Caution is advised in the application of this
method since the method does not work well with all feedstocks. For example, when the sulfuric
acid method (ASTM D2006) is applied to the separation of heavy feedstocks, complex emulsions
can be produced.
Fractional Composition
involves the fractionation of Athabasca tar sand bitumen into four gross fractions and sub-fractionation of these four fractions, which allows further study of the distribution of the functional types
within the feedstock.
A combination chromatography using alumina and silica gel is also suitable for deasphaltened
oils. A more complex scheme, also involving the use of silica, resulted in the successful separation
of hexane-deasphalted crude oil (Seifert, 1975). There is also a report of the direct fractionation of
hydrocarbon and hetero compounds from deasphaltened residua on a dual alumina–silica column
with subsequent treatment of the polar fraction with cation- and anion-exchange resins into basic,
acid, and neutral materials. The method also includes chromatography of the asphaltenes, but only
with highly polar asphaltene samples are the basic and acid compounds first removed with ionexchange resins. The remainder of the feedstock is separated into saturates, aromatics, and hetero
compounds using alumina–silica adsorption. Separation of the saturates fraction into n-alkanes
and iso-alkanes plus cycloalkanes is achieved by the use of urea (H 2 N . C=O . NH 2 ) and thiourea
(H 2 N . C=S . NH 2 ).
One of the problems of such a fractionation scheme is the initial separation of the feedstocks
into two ill-defined fractions (colloids and dispersant) without first removing the asphaltenes. As
already noted, asphaltenes are specifically defined by the method of separation. They are less well
defined using such liquids as ethyl acetate in place of the more often used hydrocarbons, such as
pentane and heptane (Speight, 1979). The use of ethyl acetate undoubtedly leads to asphaltene material in the dispersoids and non-asphaltene material in the colloids. Application of a more standard
deasphalting technique would undoubtedly improve this method and provide an excellent insight
into feedstock composition.
One of the common issues related to the use of any adsorption-based fractionation scheme is
the nature of the adsorbent. In the early reports of petroleum fractionation (Pfeiffer, 1950), clays
often appeared as an adsorbent to effect the separation of the feedstock into various constituent
fractions. However, clay (fuller’s earth, attapulgus clay, and the like) is often difficult to define with
any degree of precision from one batch to another. Variations in the nature and properties of the
clay can, and will, cause differences not only in the yields of composite fractions but also in the distribution of the compound types in those fractions. In addition, irreversible adsorption of the more
polar constituent to the clay can be a serious problem when further investigations of the constituent
fractions are planned.
One option for resolving this problem has been the use of more standard adsorbents, such as
alumina and silica. These materials are easier to define and are often accompanied by guarantees
of composition and type by various manufacturers. They also tend to irreversibly adsorb less of
the feedstock than clay. Once the nature of the adsorbent is guaranteed, reproducibility becomes a
reality. Without reproducibility the analytic method does not have credibility.
9.4.2.2 ASTM Methods
There are three ASTM methods that provide for the separation of a feedstock into four or five
constituent fractions and it is interesting to note that as the methods have evolved there has been a
change from the use of pentane (ASTM D2006 and ASTM D2007) (Figures 9.7 and 9.8) to heptane
(ASTM D4124) (Figure 9.9) to separate asphaltenes. This is, in fact, in keeping with the production
of a more consistent fraction that represents these higher molecular weight, more complex constituents of petroleum (Girdler, 1965; Speight et al., 1984).
Two of the methods (ASTM D2007 and ASTM D4124) use adsorbents to fractionate the deasphaltened oil, but the third method (ASTM D2006) advocates the use of various grades of sulfuric
acid to separate the material into compound types. Caution is advised in the application of this
method since the method does not work well with all feedstocks. For example, when the sulfuric
acid method (ASTM D2006) is applied to the separation of heavy feedstocks, complex emulsions
can be produced.