327
Asphaltene Constituents
In summary, asphaltene molecular weights are variable (Yen, 1974; Speight et al., 1985; Speight,
1994). There is a tendency to associate even in dilute solution in nonpolar solvents. However, data
obtained using highly polar solvents indicate that the molecular weights, in solvents that prevent
association, usually fall in the range 2000 ± 500.
12.5 REACTIONS
The relative reactivity of petroleum constituents can be assessed on the basis of bond energies and
the asphaltene constituents are no exception to this generalization. While the use of bond energy
data is a method for predicting the reactivity or the stability of specific bonds under designed conditions, it must be remembered that the reactivity of a particular bond is also subject to its environment. Thus, it is not only the reactivity of the constituents of petroleum that is important in
processing behavior, but the stereochemistry of the constituents as they relate to one another is also
of some importance (Chapter 13). It must be appreciated that the stereochemistry of organic compounds is often a major factor in determining reactivity and properties.
Thus, it is not surprising that the chemical reactions of the constituents of the asphaltene fraction have received some attention and provide valuable information about the potential chemical
conversion that can be performed with a view to the chemical functions within the asphaltenes.
However, these reactions also provide valuable information about the structural types in these constituents and also what effects changes in the hydrocarbon structure or functionality will have on
the properties of the asphaltene constituents. This information is valuable when perturbations to the
petroleum system are given consideration vis-à-vis the potential effects that can cause deposition of
sediment after the delicate balance of the system has been changed or even destroyed (Leontaritis
and Mansoori, 1988; Speight, 1994; Mansoori, 1996; Chapters 13 and 14).
The desirability of removing the asphaltene fraction from a crude oil has been advocated many
times, the principal reasons being the production of a cracking stock low in metal impurities and
heteroatom compounds and a low carbon residue. As a result of the structural studies just discussed,
it is evident that petroleum asphaltenes are agglomerations of compounds of a particular type. Thus,
it is not surprising that asphaltenes undergo a wide range of interactions of a chemical and physical nature based not only on their condensed aromatic structure but also on the attending alkyl and
naphthenic moieties.
Asphaltenes can be thermally decomposed under conditions similar to those employed for visbreaking (viscosity breaking; ∼470°C, 880°F) to afford, on the one hand, light oils that contain
higher (to >C 40 ) paraffins and, on the other hand, coke:
Asphaltene fraction → H 2 , CO, CO 2 , H 2 S, SO 2 , H 2 O + CH 2 =CH 2 , CH 4 , CH 3 CH 3 , CH 3 (CH 2 ) n CH 3
The thermal decomposition of asphaltenes provides an excellent example of inconsistencies in
the derivation of structural types from spectroscopic materials (i.e., magnetic resonance) in which
alkyl side chains are deduced to contain approximately four carbon atoms (Speight, 1970a, 1971).
Asphaltene pyrolysis (350°C–800°C, 660°F–1470°F) produces substantial amounts of alkanes
(having up to 40 carbon atoms in the molecule) in the distillate that can only be presumed to reflect
the presence of such chains in the original asphaltene. Transalkylation studies (Farcasiu et al., 1983)
also provide evidence for longer alkyl chains. Obviously, recognition of the inconsistencies of the
spectroscopic method with respect to the paraffinic moieties must lead to the recognition of similar
inconsistencies when considering the aromatic nucleus.
The application of thermal techniques to study the nature of the volatile thermal fragments
from petroleum asphaltenes has produced some interesting data relating to the polynuclear aromatic systems (Speight, 1971; Ritchie et al., 1979; Schucker and Keweshan, 1980; Gallegos, 1981).
These thermal techniques have produced strong evidence for the presence of small (one-ring to
four-ring) polynuclear aromatic systems (Speight and Pancirov, 1984) and now, application of the
Asphaltene Constituents
In summary, asphaltene molecular weights are variable (Yen, 1974; Speight et al., 1985; Speight,
1994). There is a tendency to associate even in dilute solution in nonpolar solvents. However, data
obtained using highly polar solvents indicate that the molecular weights, in solvents that prevent
association, usually fall in the range 2000 ± 500.
12.5 REACTIONS
The relative reactivity of petroleum constituents can be assessed on the basis of bond energies and
the asphaltene constituents are no exception to this generalization. While the use of bond energy
data is a method for predicting the reactivity or the stability of specific bonds under designed conditions, it must be remembered that the reactivity of a particular bond is also subject to its environment. Thus, it is not only the reactivity of the constituents of petroleum that is important in
processing behavior, but the stereochemistry of the constituents as they relate to one another is also
of some importance (Chapter 13). It must be appreciated that the stereochemistry of organic compounds is often a major factor in determining reactivity and properties.
Thus, it is not surprising that the chemical reactions of the constituents of the asphaltene fraction have received some attention and provide valuable information about the potential chemical
conversion that can be performed with a view to the chemical functions within the asphaltenes.
However, these reactions also provide valuable information about the structural types in these constituents and also what effects changes in the hydrocarbon structure or functionality will have on
the properties of the asphaltene constituents. This information is valuable when perturbations to the
petroleum system are given consideration vis-à-vis the potential effects that can cause deposition of
sediment after the delicate balance of the system has been changed or even destroyed (Leontaritis
and Mansoori, 1988; Speight, 1994; Mansoori, 1996; Chapters 13 and 14).
The desirability of removing the asphaltene fraction from a crude oil has been advocated many
times, the principal reasons being the production of a cracking stock low in metal impurities and
heteroatom compounds and a low carbon residue. As a result of the structural studies just discussed,
it is evident that petroleum asphaltenes are agglomerations of compounds of a particular type. Thus,
it is not surprising that asphaltenes undergo a wide range of interactions of a chemical and physical nature based not only on their condensed aromatic structure but also on the attending alkyl and
naphthenic moieties.
Asphaltenes can be thermally decomposed under conditions similar to those employed for visbreaking (viscosity breaking; ∼470°C, 880°F) to afford, on the one hand, light oils that contain
higher (to >C 40 ) paraffins and, on the other hand, coke:
Asphaltene fraction → H 2 , CO, CO 2 , H 2 S, SO 2 , H 2 O + CH 2 =CH 2 , CH 4 , CH 3 CH 3 , CH 3 (CH 2 ) n CH 3
The thermal decomposition of asphaltenes provides an excellent example of inconsistencies in
the derivation of structural types from spectroscopic materials (i.e., magnetic resonance) in which
alkyl side chains are deduced to contain approximately four carbon atoms (Speight, 1970a, 1971).
Asphaltene pyrolysis (350°C–800°C, 660°F–1470°F) produces substantial amounts of alkanes
(having up to 40 carbon atoms in the molecule) in the distillate that can only be presumed to reflect
the presence of such chains in the original asphaltene. Transalkylation studies (Farcasiu et al., 1983)
also provide evidence for longer alkyl chains. Obviously, recognition of the inconsistencies of the
spectroscopic method with respect to the paraffinic moieties must lead to the recognition of similar
inconsistencies when considering the aromatic nucleus.
The application of thermal techniques to study the nature of the volatile thermal fragments
from petroleum asphaltenes has produced some interesting data relating to the polynuclear aromatic systems (Speight, 1971; Ritchie et al., 1979; Schucker and Keweshan, 1980; Gallegos, 1981).
These thermal techniques have produced strong evidence for the presence of small (one-ring to
four-ring) polynuclear aromatic systems (Speight and Pancirov, 1984) and now, application of the