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The Chemistry and Technology of Petroleum
types with the accompanying variation in polarity. Removal of alkyl side chains from the asphaltenes
will decrease the H/C ratio and increase the solubility parameter thereby bringing about a concurrent decrease of the asphaltene product in the hydrocarbon solvent. In fact, on the molecular weight
polarity diagram for asphaltenes, carbenes, and carboids can be shown as lower molecular weight,
highly polar entities in keeping with molecular fragmentation models.
12.7 STRUCTURAL ASPECTS
The molecular nature of the nonvolatile fractions of petroleum has been the subject of numerous
investigations (Speight, 1972, 1994; Yen, 1974), but determining the actual structures of the constituents of the asphaltene fraction has proved to be difficult. It is the complexity of the asphaltene
fraction that has hindered the formulation of the individual molecular structures. Nevertheless,
various investigations have brought to light some significant facts about asphaltene structure. There
are indications that asphaltenes consist of condensed aromatic nuclei that carry alkyl and alicyclic
systems with hetero elements (i.e., nitrogen, oxygen, and sulfur) scattered throughout in various,
including heterocyclic, locations.
Other basic generalizations have also been noted: with increasing molecular weight of the
asphaltene fraction, both aromaticity and the proportion of hetero elements increase (Koots and
Speight, 1975). In addition, the proportion of asphaltenes in petroleum varies with source, depth of
burial, the specific (or API) gravity of the crude oil, and the sulfur content of the crude oil as well
as nonasphaltene sulfur (Koots and Speight, 1975). However, many facets of asphaltene structure
remain unknown, and it is the purpose of this chapter to bring together the pertinent information on
asphaltene structure as well as the part played by asphaltenes in the physical structure of petroleum.
In fact, it is this overall lack of structural detail that indicates that current, and future, chemical research should focus on several items. These are (1) the skeletal structure, (2) the size of the
polynuclear aromatic systems, (3) the internuclear bonds that need to be thermally or catalytically
cleaved to produce more identifiable lower molecular weight fragments (Chapters 7 and 15), (4) the
heteroatom types and subsequent ease or difficulty of removal (Chapter 7), (5) the metallic constituents and their occurrence in porphyrinic or nonporphyrinic locations (Chapter 7), and (6) the physicochemical relationships that exist between the asphaltenes and the other constituents of the oil
(Speight, 1994; Merdrignac and Espinat, 2007) (Chapters 12 and 13).
Despite numerous investigations, determination of the actual molecular structures that exist in
petroleum has proved difficult. It is no doubt the great complexity of these materials, which are
actually isolated by physical phenomena rather than by virtue of their chemical structures, that has
hindered the formulation of individual molecular structures. Nevertheless, the various investigations have brought to light some significant facts about the structures that exist in petroleum.
There are indications that the asphaltenes consist of condensed aromatic nuclei that carry alkyl
and cylcoalkyl (naphthenic) substituents; heteroatoms (i.e., sulfur, nitrogen, and oxygen) are scattered throughout the hydrocarbon systems. In addition, it appears that with increasing molecular
weight both aromaticity and the proportion of the heteroatoms increase. There is also fragmentary
evidence that, with increasing molecular weight, the heteroatoms become incorporated into the
more stable heterocyclic systems. This is especially true for sulfur constituents in which dibenzothiophene systems are believed to predominate in the higher molecular weight asphalts; it is presumed that similar arrangements occur for the nitrogen atoms, and oxygen may also occur in cyclic
systems or in various functional groups.
If the residuum is produced by a process in which thermal decomposition has occurred, there
may be traces of thiols (mercaptans, R–SH) in the mix that would, along with any hydrogen sulfide
generated during the process, impart an objectionable odor to the residuum. The occurrence of thiols in heavy oils that have not been subjected to thermal treatment is also possible but is generally
not considered likely.
The Chemistry and Technology of Petroleum
types with the accompanying variation in polarity. Removal of alkyl side chains from the asphaltenes
will decrease the H/C ratio and increase the solubility parameter thereby bringing about a concurrent decrease of the asphaltene product in the hydrocarbon solvent. In fact, on the molecular weight
polarity diagram for asphaltenes, carbenes, and carboids can be shown as lower molecular weight,
highly polar entities in keeping with molecular fragmentation models.
12.7 STRUCTURAL ASPECTS
The molecular nature of the nonvolatile fractions of petroleum has been the subject of numerous
investigations (Speight, 1972, 1994; Yen, 1974), but determining the actual structures of the constituents of the asphaltene fraction has proved to be difficult. It is the complexity of the asphaltene
fraction that has hindered the formulation of the individual molecular structures. Nevertheless,
various investigations have brought to light some significant facts about asphaltene structure. There
are indications that asphaltenes consist of condensed aromatic nuclei that carry alkyl and alicyclic
systems with hetero elements (i.e., nitrogen, oxygen, and sulfur) scattered throughout in various,
including heterocyclic, locations.
Other basic generalizations have also been noted: with increasing molecular weight of the
asphaltene fraction, both aromaticity and the proportion of hetero elements increase (Koots and
Speight, 1975). In addition, the proportion of asphaltenes in petroleum varies with source, depth of
burial, the specific (or API) gravity of the crude oil, and the sulfur content of the crude oil as well
as nonasphaltene sulfur (Koots and Speight, 1975). However, many facets of asphaltene structure
remain unknown, and it is the purpose of this chapter to bring together the pertinent information on
asphaltene structure as well as the part played by asphaltenes in the physical structure of petroleum.
In fact, it is this overall lack of structural detail that indicates that current, and future, chemical research should focus on several items. These are (1) the skeletal structure, (2) the size of the
polynuclear aromatic systems, (3) the internuclear bonds that need to be thermally or catalytically
cleaved to produce more identifiable lower molecular weight fragments (Chapters 7 and 15), (4) the
heteroatom types and subsequent ease or difficulty of removal (Chapter 7), (5) the metallic constituents and their occurrence in porphyrinic or nonporphyrinic locations (Chapter 7), and (6) the physicochemical relationships that exist between the asphaltenes and the other constituents of the oil
(Speight, 1994; Merdrignac and Espinat, 2007) (Chapters 12 and 13).
Despite numerous investigations, determination of the actual molecular structures that exist in
petroleum has proved difficult. It is no doubt the great complexity of these materials, which are
actually isolated by physical phenomena rather than by virtue of their chemical structures, that has
hindered the formulation of individual molecular structures. Nevertheless, the various investigations have brought to light some significant facts about the structures that exist in petroleum.
There are indications that the asphaltenes consist of condensed aromatic nuclei that carry alkyl
and cylcoalkyl (naphthenic) substituents; heteroatoms (i.e., sulfur, nitrogen, and oxygen) are scattered throughout the hydrocarbon systems. In addition, it appears that with increasing molecular
weight both aromaticity and the proportion of the heteroatoms increase. There is also fragmentary
evidence that, with increasing molecular weight, the heteroatoms become incorporated into the
more stable heterocyclic systems. This is especially true for sulfur constituents in which dibenzothiophene systems are believed to predominate in the higher molecular weight asphalts; it is presumed that similar arrangements occur for the nitrogen atoms, and oxygen may also occur in cyclic
systems or in various functional groups.
If the residuum is produced by a process in which thermal decomposition has occurred, there
may be traces of thiols (mercaptans, R–SH) in the mix that would, along with any hydrogen sulfide
generated during the process, impart an objectionable odor to the residuum. The occurrence of thiols in heavy oils that have not been subjected to thermal treatment is also possible but is generally
not considered likely.