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Structural Group Analysis
adsorptive percolation. Since these complex molecules cannot be described as aromatics, naphthenes, or paraffins, it is logical that their composition should be expressed in terms of structural
fragments.
Structural group analysis may be seen as an analytical method giving information somewhere
between that obtained by elemental analysis, on the one hand, and by analysis for individual hydrocarbons (molecular type analysis), on the other. Elemental analysis, as such, is unattractive since
it gives only limited information about the constitution of petroleum because of the remarkable
constancy of the elemental composition (Chapter 8). Thus, instead of molecules or atoms, certain
structural types are considered components of the oil.
Methods for structural group analysis usually involve the determination of physical constants
of the sample. However, since there is no simple relation between physical properties and chemical
composition, a reliable correlation can only be obtained by studying properties of a great variety of
oil fractions or pure compounds according to exact methods, laborious though that may be. The data
collected statistically in this way may form the basis for chemical analysis by physical constants.
The better the representation and the greater the number of these basic data, the more reliable the
resulting method for structural group analysis will be. However, it should be remembered that structural group analysis is not the ultimate answer when applied to the heavier petroleum fractions or
residua. Nevertheless, its importance lies in the straight correlation existing between the information derived from such analysis and physical properties, but it is often sufficient to get an overall
description of the material in terms of its average structural group composition.
High-boiling petroleum fractions, petroleum products, and residua can be analyzed in terms of
groups of hydrocarbons, and four classes are generally recognized: (1) aromatic, if the molecule
contains at least one aromatic ring; (2) olefinic, if the molecule contains at least one olefine bond;
(3) naphthenic, if the molecule contains at least one naphthene ring; and (4) paraffinic, if the molecule contains neither an aromatic nor a naphthenic ring nor an olefinic double bond.
Furthermore, aromatic hydrocarbons are also subdivided according to aromatic type, a term
that describes compounds having the same number and grouping of aromatic rings. If an aromatic
hydrocarbon contains two aromatic rings, three types may be distinguished: (1) the rings may be
condensed, that is, fused together, to form the naphthalene nucleus, (2) the rings may be joined
by an inter-ring bond as in the biphenyl nucleus, and (3) the rings are separated by one or more
nonaromatic carbon atoms, such as diphenylmethane. This nomenclature is open to extension to
polyaromatics with more than two aromatic rings. Alkyl chains and naphthene rings are generally
not considered in discussions of aromatic type.
If a hydrocarbon contains structural groups of various types, it can be placed in more than one
class. In such cases, in consequence of the preceding definitions, the total of aromatics + olefins +
naphthenes + paraffins in a petroleum fraction may be considerably over 100%. To avoid this complication, other classes of hydrocarbons may be defined, such as naphthenoaromatics. Owing to the
rapidly increasing number and complexity of compounds in the higher boiling ranges, that part of
the crude oil that might be broadly designated the lubricant portion is in practice hardly suitable for
this type of analysis.
There are two ways of reporting the results of a structural group analysis. One method is to
determine the number of rings or other structural groups relative to a hypothetical average molecule of the sample; that is, a molecule containing the structural groups in the proportions found
by structural group analysis. Such figures referring to the average number of aromatic rings (R A ),
naphthene rings (R N ) and the total number of rings (R T = R A + R N ) are designated ring content. The
other method is to determine the number of carbon atoms in aromatic (%C A ), naphthenic (%C N ),
and paraffinic structures (%C P ), all expressed per 100 carbon atoms in the sample. Such figures may
be designated carbon distribution because the analysis gives the distribution of carbon over various
structures, such as aromatic, naphthenic, and paraffinic structures. If the mean molecular weight of
the sample is known and if an assumption is made about the type of rings present, the ring content
can be recalculated as carbon distribution; the converse is also applicable.
Structural Group Analysis
adsorptive percolation. Since these complex molecules cannot be described as aromatics, naphthenes, or paraffins, it is logical that their composition should be expressed in terms of structural
fragments.
Structural group analysis may be seen as an analytical method giving information somewhere
between that obtained by elemental analysis, on the one hand, and by analysis for individual hydrocarbons (molecular type analysis), on the other. Elemental analysis, as such, is unattractive since
it gives only limited information about the constitution of petroleum because of the remarkable
constancy of the elemental composition (Chapter 8). Thus, instead of molecules or atoms, certain
structural types are considered components of the oil.
Methods for structural group analysis usually involve the determination of physical constants
of the sample. However, since there is no simple relation between physical properties and chemical
composition, a reliable correlation can only be obtained by studying properties of a great variety of
oil fractions or pure compounds according to exact methods, laborious though that may be. The data
collected statistically in this way may form the basis for chemical analysis by physical constants.
The better the representation and the greater the number of these basic data, the more reliable the
resulting method for structural group analysis will be. However, it should be remembered that structural group analysis is not the ultimate answer when applied to the heavier petroleum fractions or
residua. Nevertheless, its importance lies in the straight correlation existing between the information derived from such analysis and physical properties, but it is often sufficient to get an overall
description of the material in terms of its average structural group composition.
High-boiling petroleum fractions, petroleum products, and residua can be analyzed in terms of
groups of hydrocarbons, and four classes are generally recognized: (1) aromatic, if the molecule
contains at least one aromatic ring; (2) olefinic, if the molecule contains at least one olefine bond;
(3) naphthenic, if the molecule contains at least one naphthene ring; and (4) paraffinic, if the molecule contains neither an aromatic nor a naphthenic ring nor an olefinic double bond.
Furthermore, aromatic hydrocarbons are also subdivided according to aromatic type, a term
that describes compounds having the same number and grouping of aromatic rings. If an aromatic
hydrocarbon contains two aromatic rings, three types may be distinguished: (1) the rings may be
condensed, that is, fused together, to form the naphthalene nucleus, (2) the rings may be joined
by an inter-ring bond as in the biphenyl nucleus, and (3) the rings are separated by one or more
nonaromatic carbon atoms, such as diphenylmethane. This nomenclature is open to extension to
polyaromatics with more than two aromatic rings. Alkyl chains and naphthene rings are generally
not considered in discussions of aromatic type.
If a hydrocarbon contains structural groups of various types, it can be placed in more than one
class. In such cases, in consequence of the preceding definitions, the total of aromatics + olefins +
naphthenes + paraffins in a petroleum fraction may be considerably over 100%. To avoid this complication, other classes of hydrocarbons may be defined, such as naphthenoaromatics. Owing to the
rapidly increasing number and complexity of compounds in the higher boiling ranges, that part of
the crude oil that might be broadly designated the lubricant portion is in practice hardly suitable for
this type of analysis.
There are two ways of reporting the results of a structural group analysis. One method is to
determine the number of rings or other structural groups relative to a hypothetical average molecule of the sample; that is, a molecule containing the structural groups in the proportions found
by structural group analysis. Such figures referring to the average number of aromatic rings (R A ),
naphthene rings (R N ) and the total number of rings (R T = R A + R N ) are designated ring content. The
other method is to determine the number of carbon atoms in aromatic (%C A ), naphthenic (%C N ),
and paraffinic structures (%C P ), all expressed per 100 carbon atoms in the sample. Such figures may
be designated carbon distribution because the analysis gives the distribution of carbon over various
structures, such as aromatic, naphthenic, and paraffinic structures. If the mean molecular weight of
the sample is known and if an assumption is made about the type of rings present, the ring content
can be recalculated as carbon distribution; the converse is also applicable.