286
The Chemistry and Technology of Petroleum
As a result, as composition studies in petroleum advance to the higher molecular-weight ranges,
it is impractical to consider the determination of the individual compounds and very difficult, if not
impossible, to segregate and identify fractions of the required simplicity.
It should be noted that, however, a problem occasionally encountered in petroleum technology is
that of establishing the structure of a single compound. Correlation between spectra and structure is
useful for this purpose and, although it may never be possible to write the structural formula from
spectral data alone, in combination with other forms of data, a choice between several alternate
structures is often possible.
For this reason, investigators must be content with information on molecular types of hydrocarbons and on structural groups and it is perhaps in this field that spectroscopy has its most valuable
application.
Although gas–liquid chromatography and other techniques have been applied successfully to the
identification of a considerable number of petroleum constituents, they are, however, mainly limited
to the so-called front end (i.e., the volatile portion) of petroleum. Since the majority of crude oils
contain significant proportions of a nonvolatile residuum, it is not possible to identify individual
components in this part of the petroleum by a technique that requires some degree of volatility of
the constituents. It is because of this that other methods of identification have been pursued.
However, despite many attempts, the separation of higher boiling fractions and residua into
individual components appears to be an altogether hopeless enterprise. Even the preparation and
identification of uniform fractions containing exclusively molecules of the same size and type are
extremely complicated and success is not guaranteed. Pursuit of the composition of both virgin
petroleum fractions and refinery products in terms of paraffinic, naphthenic, and aromatics content
has given rise to structural group analysis.
Structural group analysis is, in fact, the determination of the statistical distribution of these
structural elements in the oil fraction, irrespective of the way in which the elements are combined in
molecules. Thus, structural group analysis occupies a position midway between ultimate analysis, in
which atoms are the components, and molecular analysis, in which molecules are the components.
A method for structural group analysis seems to be complete only if structural elements are chosen
in such a way that the sum of all equals 100% (or unity), for instance, by considering the distribution
of carbon in aromatic, naphthenic, and paraffinic locations in petroleum, its fractions, and products.
Structural group analysis has been widely applied to the analysis of petroleum and its fractions.
Comparative data have been collected pertaining to the character of crude oils and variations in composition of fractions within the same crude, as well as the composition of intermediate and finished
products. These data are often helpful in identifying products of unknown origin or in giving valuable indications concerning their manufacture. Much benefit has also been derived from structural
group analysis in physical separation processes, such as solvent extraction, solvent dewaxing, and
percolation, and in elucidating overall reaction schemes in conversion processes, such as cracking.
When more thorough knowledge of hydrocarbon types is required than can be derived from
the average proportion of structural elements of petroleum, structural group analysis may also
render good service. However, statistical analysis often requires assumptions that may be without
true scientific foundation. The failure to recognize the distinction between matters of statistical
significance and practical importance leads to interpretation of the data on the basis of statistical significance alone and without consideration of proper experimental design of the test for the
detection of systemic errors of a magnitude that has serious consequences for the end result.
One of the basic ideas underlying structural group analysis is the conception of a high molecular weight petroleum constituent as complex in structure and hence in properties. High molecular
weight petroleum constituents may contain (and very frequently do contain) aromatic as well as
naphthenic rings and paraffinic side chains, which is reflected in their physical and chemical properties. For instance, the viscosity and viscosity index (Chapter 10) of such hydrocarbons will to a
large extent be determined by the relative quantity of each structural element; the same is true for
the behavior of these hydrocarbons in physical separation processes, such as solvent extraction and
The Chemistry and Technology of Petroleum
As a result, as composition studies in petroleum advance to the higher molecular-weight ranges,
it is impractical to consider the determination of the individual compounds and very difficult, if not
impossible, to segregate and identify fractions of the required simplicity.
It should be noted that, however, a problem occasionally encountered in petroleum technology is
that of establishing the structure of a single compound. Correlation between spectra and structure is
useful for this purpose and, although it may never be possible to write the structural formula from
spectral data alone, in combination with other forms of data, a choice between several alternate
structures is often possible.
For this reason, investigators must be content with information on molecular types of hydrocarbons and on structural groups and it is perhaps in this field that spectroscopy has its most valuable
application.
Although gas–liquid chromatography and other techniques have been applied successfully to the
identification of a considerable number of petroleum constituents, they are, however, mainly limited
to the so-called front end (i.e., the volatile portion) of petroleum. Since the majority of crude oils
contain significant proportions of a nonvolatile residuum, it is not possible to identify individual
components in this part of the petroleum by a technique that requires some degree of volatility of
the constituents. It is because of this that other methods of identification have been pursued.
However, despite many attempts, the separation of higher boiling fractions and residua into
individual components appears to be an altogether hopeless enterprise. Even the preparation and
identification of uniform fractions containing exclusively molecules of the same size and type are
extremely complicated and success is not guaranteed. Pursuit of the composition of both virgin
petroleum fractions and refinery products in terms of paraffinic, naphthenic, and aromatics content
has given rise to structural group analysis.
Structural group analysis is, in fact, the determination of the statistical distribution of these
structural elements in the oil fraction, irrespective of the way in which the elements are combined in
molecules. Thus, structural group analysis occupies a position midway between ultimate analysis, in
which atoms are the components, and molecular analysis, in which molecules are the components.
A method for structural group analysis seems to be complete only if structural elements are chosen
in such a way that the sum of all equals 100% (or unity), for instance, by considering the distribution
of carbon in aromatic, naphthenic, and paraffinic locations in petroleum, its fractions, and products.
Structural group analysis has been widely applied to the analysis of petroleum and its fractions.
Comparative data have been collected pertaining to the character of crude oils and variations in composition of fractions within the same crude, as well as the composition of intermediate and finished
products. These data are often helpful in identifying products of unknown origin or in giving valuable indications concerning their manufacture. Much benefit has also been derived from structural
group analysis in physical separation processes, such as solvent extraction, solvent dewaxing, and
percolation, and in elucidating overall reaction schemes in conversion processes, such as cracking.
When more thorough knowledge of hydrocarbon types is required than can be derived from
the average proportion of structural elements of petroleum, structural group analysis may also
render good service. However, statistical analysis often requires assumptions that may be without
true scientific foundation. The failure to recognize the distinction between matters of statistical
significance and practical importance leads to interpretation of the data on the basis of statistical significance alone and without consideration of proper experimental design of the test for the
detection of systemic errors of a magnitude that has serious consequences for the end result.
One of the basic ideas underlying structural group analysis is the conception of a high molecular weight petroleum constituent as complex in structure and hence in properties. High molecular
weight petroleum constituents may contain (and very frequently do contain) aromatic as well as
naphthenic rings and paraffinic side chains, which is reflected in their physical and chemical properties. For instance, the viscosity and viscosity index (Chapter 10) of such hydrocarbons will to a
large extent be determined by the relative quantity of each structural element; the same is true for
the behavior of these hydrocarbons in physical separation processes, such as solvent extraction and