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The Chemistry and Technology of Petroleum
produce a less complex and much narrower chromatograph that may even approximate a single peak
which may prove much more difficult to separate by a detector.
These data provide strong indications of the ring-size distribution of the polycyclic aromatic
systems in petroleum asphaltenes. For example, from an examination of various functional subfractions, it was shown that amphoteric species and basic nitrogen species contain polycyclic aromatic systems having two to six rings per system. On the other hand, acid sub-fractions (phenolic/
carboxylic functions) and neutral polar sub-fractions (amides/imino functions) contain few, if any,
polycyclic aromatic systems having more than three rings per system (Chapter 12).
In all cases, the evidence favored the preponderance of the smaller (one to four) ring systems
(Speight, 1986). But, perhaps what is more important about these investigation is that the data show
that the asphaltene fraction is a complex mixture of compound types which confirms fractionation
studies and cannot be adequately represented by any particular formula that is construed to be
average. Therefore, the concept of a large polycyclic aromatic ring system as the central feature of
the asphaltene fraction must be abandoned for lack of evidence.
In summary, the premise is that petroleum is a natural product and that the aromatic systems are
based on identifiable structural systems that are derived from natural product precursors.
8.3.1.4 Unsaturated Hydrocarbons
The presence of olefins (R.CH=CH.R 1 ) in petroleum has been under dispute for many years because
there are investigators who claim that olefins are actually present; in fact, these claims usually refer
to distilled fractions, and it is very difficult to entirely avoid cracking during the distillation process.
Nevertheless, evidence for the presence of considerable proportions of olefins in Pennsylvanian
crude oils has been obtained; spectroscopic and chemical methods showed that the crude oils, as
well as all distillate fractions, contained up to 3% w/w olefins. Hence, although the opinion that
petroleum does not contain olefins requires some revision, it is perhaps reasonable to assume that
the Pennsylvania crude oils may hold an exceptional position and that olefins are present in crude oil
in only a few special cases. The presence of dienes (R.CH=CH–CH=R′) and acetylenes (RC≡CR′)
is considered to be extremely unlikely.
In summary, a variety of hydrocarbon compounds occur throughout petroleum. Although the
amount of any particular hydrocarbon varies from one crude oil to another, the family from which
that hydrocarbon arises is well represented.
8.3.2 nonHydroCArBon ComPonents
The previous sections present some indication of the types and nomenclature of the organic hydrocarbons that occur in various crude oils. Thus it is not surprising that petroleum which contains only
hydrocarbons is, in fact, an extremely complex mixture. The phenomenal increase in the number of
possible isomers for the higher hydrocarbons makes it very difficult, if not impossible in most cases,
to isolate individual members of any one series having more than, say, 12 carbon atoms.
Inclusion of organic compounds of nitrogen, oxygen, and sulfur serves only to present crude
oil as an even more complex mixture than was originally conceived. Nevertheless, considerable
progress has been made in the isolation and/or identification of the lower molecular weight hydrocarbons, as well as accurate estimations of the overall proportions of the hydrocarbon types present
in petroleum. Indeed, it has been established that, as the boiling point of the petroleum fraction
increases, not only the number of the constituents but the molecular complexity of the constituents
also increases (Figure 8.1) (Speight, 2000 and references cited therein).
Crude oils contain appreciable amounts of organic nonhydrocarbon constituents, mainly sulfur-,
nitrogen-, and oxygen-containing compounds and, in smaller amounts, organometallic compounds
in solution and inorganic salts in colloidal suspension. These constituents appear throughout the
entire boiling range of the crude oil but tend to concentrate mainly in the heavier fractions and in
the nonvolatile residues.
The Chemistry and Technology of Petroleum
produce a less complex and much narrower chromatograph that may even approximate a single peak
which may prove much more difficult to separate by a detector.
These data provide strong indications of the ring-size distribution of the polycyclic aromatic
systems in petroleum asphaltenes. For example, from an examination of various functional subfractions, it was shown that amphoteric species and basic nitrogen species contain polycyclic aromatic systems having two to six rings per system. On the other hand, acid sub-fractions (phenolic/
carboxylic functions) and neutral polar sub-fractions (amides/imino functions) contain few, if any,
polycyclic aromatic systems having more than three rings per system (Chapter 12).
In all cases, the evidence favored the preponderance of the smaller (one to four) ring systems
(Speight, 1986). But, perhaps what is more important about these investigation is that the data show
that the asphaltene fraction is a complex mixture of compound types which confirms fractionation
studies and cannot be adequately represented by any particular formula that is construed to be
average. Therefore, the concept of a large polycyclic aromatic ring system as the central feature of
the asphaltene fraction must be abandoned for lack of evidence.
In summary, the premise is that petroleum is a natural product and that the aromatic systems are
based on identifiable structural systems that are derived from natural product precursors.
8.3.1.4 Unsaturated Hydrocarbons
The presence of olefins (R.CH=CH.R 1 ) in petroleum has been under dispute for many years because
there are investigators who claim that olefins are actually present; in fact, these claims usually refer
to distilled fractions, and it is very difficult to entirely avoid cracking during the distillation process.
Nevertheless, evidence for the presence of considerable proportions of olefins in Pennsylvanian
crude oils has been obtained; spectroscopic and chemical methods showed that the crude oils, as
well as all distillate fractions, contained up to 3% w/w olefins. Hence, although the opinion that
petroleum does not contain olefins requires some revision, it is perhaps reasonable to assume that
the Pennsylvania crude oils may hold an exceptional position and that olefins are present in crude oil
in only a few special cases. The presence of dienes (R.CH=CH–CH=R′) and acetylenes (RC≡CR′)
is considered to be extremely unlikely.
In summary, a variety of hydrocarbon compounds occur throughout petroleum. Although the
amount of any particular hydrocarbon varies from one crude oil to another, the family from which
that hydrocarbon arises is well represented.
8.3.2 nonHydroCArBon ComPonents
The previous sections present some indication of the types and nomenclature of the organic hydrocarbons that occur in various crude oils. Thus it is not surprising that petroleum which contains only
hydrocarbons is, in fact, an extremely complex mixture. The phenomenal increase in the number of
possible isomers for the higher hydrocarbons makes it very difficult, if not impossible in most cases,
to isolate individual members of any one series having more than, say, 12 carbon atoms.
Inclusion of organic compounds of nitrogen, oxygen, and sulfur serves only to present crude
oil as an even more complex mixture than was originally conceived. Nevertheless, considerable
progress has been made in the isolation and/or identification of the lower molecular weight hydrocarbons, as well as accurate estimations of the overall proportions of the hydrocarbon types present
in petroleum. Indeed, it has been established that, as the boiling point of the petroleum fraction
increases, not only the number of the constituents but the molecular complexity of the constituents
also increases (Figure 8.1) (Speight, 2000 and references cited therein).
Crude oils contain appreciable amounts of organic nonhydrocarbon constituents, mainly sulfur-,
nitrogen-, and oxygen-containing compounds and, in smaller amounts, organometallic compounds
in solution and inorganic salts in colloidal suspension. These constituents appear throughout the
entire boiling range of the crude oil but tend to concentrate mainly in the heavier fractions and in
the nonvolatile residues.