446
The Chemistry and Technology of Petroleum
An additional corollary to this work is that conventional models of petroleum asphaltenes (which,
despite evidence to the contrary, invoked the concept of a large polynuclear aromatic system) offer
little, if any, explanation of the intimate events involved in the chemistry of coking. Models that
invoke the concept of the asphaltene fractions as a complex solubility class with molecular entities
composed of smaller polynuclear aromatic systems (Chapter 12) are more in keeping with the present data. But the concept of an average structure is not in keeping with the complexity of the fraction
and the chemical or thermal reactions of the constituents (Chapter 12).
Little has been acknowledged here of the role of low-molecular-weight polar species (resins) in
coke formation. However, it is worthy of note that the resins are presumed to be lower molecular
weight analogs of the asphaltenes. This being the case, similar reaction pathways may apply.
Thus, it is now considered more likely that molecular species within the asphaltene fraction,
which contains nitrogen and other heteroatoms (and have lower volatility than the pure hydrocarbons), are the prime movers in the production of coke (Speight, 1987). Such species, containing various polynuclear aromatic systems, can be denuded of the attendant hydrocarbon moieties
and are undoubtedly insoluble (Bjorseth, 1983; Dias, 1987, 1988) in the surrounding hydrocarbon
medium. The next step is gradual carbonization of such entities to form coke (Cooper and Ballard,
1962; Magaril and Ramazaeva, 1969; Magaril et al., 1970).
Thermal processes (such as visbreaking and coking) are the oldest methods for crude oil conversion and are still used in modern refineries. The thermal chemistry of petroleum constituents has
been investigated for more than five decades, and the precise chemistry of the lower-molecularweight constituents has been well defined because of the bountiful supply of pure compounds. The
major issue in determining the thermal chemistry of the nonvolatile constituents is, of course, their
largely unknown chemical nature and, therefore the inability to define their thermal chemistry with
any degree of accuracy. Indeed, it is only recently that some light has been cast on the thermal
chemistry of the nonvolatile constituents.
Thus, the challenges facing process chemistry and physics are determining (1) the means by
which petroleum constituents thermally decompose, (2) the nature of the products of thermal
decomposition, (3) the subsequent decomposition of the primary thermal products, (4) the interaction of the products with each other, (5) the interaction of the products with the original constituents,
and (6) the influence of the products on the composition of the liquids.
When petroleum is heated to temperatures over approximately 410°C (770°F), the thermal or
free-radical reactions start to crack the mixture at significant rates. Thermal conversion does not
require the addition of a catalyst; therefore, this approach is the oldest technology available for
residue conversion. The severity of thermal processing determines the conversion and the product
characteristics.
Asphaltene constituents are major components of residua and heavy oils, and their thermal
decomposition has been the focus of much attention (Wiehe, 1993; Gray, 1994; Speight, 1994). The
thermal decomposition not only produces high yields (40 wt.%) of coke but also, optimistically and
realistically, produces equally high yields of volatile products (Speight, 1970). Thus, the challenge
in studying the thermal decomposition of asphaltenes is to decrease the yields of coke and increase
the yields of volatile products.
Several chemical models describe the thermal decomposition of asphaltene constituents (Wiehe,
1993; Gray, 1994; Speight, 1994). Using these available asphaltene models as a guide, the prevalent
thinking is that the asphaltene nuclear fragments become progressively more polar as the paraffinic
fragments are stripped from the ring systems by scission of the bonds (preferentially) between the
carbon atoms alpha and beta to the aromatic rings.
The higher polarity polynuclear aromatic systems that have been denuded of the attendant
hydrocarbon moieties are somewhat less soluble in the surrounding hydrocarbon medium than their
parent systems (Bjorseth, 1983; Dias, 1987, 1988). Two factors are operative in determining the
solubility of the polynuclear aromatic systems in the liquid product. The alkyl moieties that have
a solubilizing effect have been removed, and there is also enrichment of the liquid medium in
The Chemistry and Technology of Petroleum
An additional corollary to this work is that conventional models of petroleum asphaltenes (which,
despite evidence to the contrary, invoked the concept of a large polynuclear aromatic system) offer
little, if any, explanation of the intimate events involved in the chemistry of coking. Models that
invoke the concept of the asphaltene fractions as a complex solubility class with molecular entities
composed of smaller polynuclear aromatic systems (Chapter 12) are more in keeping with the present data. But the concept of an average structure is not in keeping with the complexity of the fraction
and the chemical or thermal reactions of the constituents (Chapter 12).
Little has been acknowledged here of the role of low-molecular-weight polar species (resins) in
coke formation. However, it is worthy of note that the resins are presumed to be lower molecular
weight analogs of the asphaltenes. This being the case, similar reaction pathways may apply.
Thus, it is now considered more likely that molecular species within the asphaltene fraction,
which contains nitrogen and other heteroatoms (and have lower volatility than the pure hydrocarbons), are the prime movers in the production of coke (Speight, 1987). Such species, containing various polynuclear aromatic systems, can be denuded of the attendant hydrocarbon moieties
and are undoubtedly insoluble (Bjorseth, 1983; Dias, 1987, 1988) in the surrounding hydrocarbon
medium. The next step is gradual carbonization of such entities to form coke (Cooper and Ballard,
1962; Magaril and Ramazaeva, 1969; Magaril et al., 1970).
Thermal processes (such as visbreaking and coking) are the oldest methods for crude oil conversion and are still used in modern refineries. The thermal chemistry of petroleum constituents has
been investigated for more than five decades, and the precise chemistry of the lower-molecularweight constituents has been well defined because of the bountiful supply of pure compounds. The
major issue in determining the thermal chemistry of the nonvolatile constituents is, of course, their
largely unknown chemical nature and, therefore the inability to define their thermal chemistry with
any degree of accuracy. Indeed, it is only recently that some light has been cast on the thermal
chemistry of the nonvolatile constituents.
Thus, the challenges facing process chemistry and physics are determining (1) the means by
which petroleum constituents thermally decompose, (2) the nature of the products of thermal
decomposition, (3) the subsequent decomposition of the primary thermal products, (4) the interaction of the products with each other, (5) the interaction of the products with the original constituents,
and (6) the influence of the products on the composition of the liquids.
When petroleum is heated to temperatures over approximately 410°C (770°F), the thermal or
free-radical reactions start to crack the mixture at significant rates. Thermal conversion does not
require the addition of a catalyst; therefore, this approach is the oldest technology available for
residue conversion. The severity of thermal processing determines the conversion and the product
characteristics.
Asphaltene constituents are major components of residua and heavy oils, and their thermal
decomposition has been the focus of much attention (Wiehe, 1993; Gray, 1994; Speight, 1994). The
thermal decomposition not only produces high yields (40 wt.%) of coke but also, optimistically and
realistically, produces equally high yields of volatile products (Speight, 1970). Thus, the challenge
in studying the thermal decomposition of asphaltenes is to decrease the yields of coke and increase
the yields of volatile products.
Several chemical models describe the thermal decomposition of asphaltene constituents (Wiehe,
1993; Gray, 1994; Speight, 1994). Using these available asphaltene models as a guide, the prevalent
thinking is that the asphaltene nuclear fragments become progressively more polar as the paraffinic
fragments are stripped from the ring systems by scission of the bonds (preferentially) between the
carbon atoms alpha and beta to the aromatic rings.
The higher polarity polynuclear aromatic systems that have been denuded of the attendant
hydrocarbon moieties are somewhat less soluble in the surrounding hydrocarbon medium than their
parent systems (Bjorseth, 1983; Dias, 1987, 1988). Two factors are operative in determining the
solubility of the polynuclear aromatic systems in the liquid product. The alkyl moieties that have
a solubilizing effect have been removed, and there is also enrichment of the liquid medium in