208
The Chemistry and Technology of Petroleum
8.4.3 vACuum resIduA (1050°F + )
This fraction, the vacuum bottoms, is the most complex of petroleum. Vacuum residua contain the
majority of the heteroatoms originally in the petroleum and molecular weight of the constituents
range, as near as can be determined subject to method dependence, up to several thousand. The
fraction is so complex that the characterization of individual species is virtually impossible, no
matter what claims have been made or will be made. Separation of vacuum residua by group type
becomes difficult and confused because of the multi-substitution of aromatic and naphthenic species as well as by the presence of multiple functionalities in single molecules.
Classically, n-pentane or n-heptane precipitation is used as the initial step for the characterization
of vacuum residuum. The insoluble fraction, the pentane- or heptane-asphaltenes, may be as much
as 50% by weight of a vacuum residuum. The pentane- or heptane-soluble portion (maltenes) of
the residuum is then fractionated chromatographically into several solubility or adsorption classes
for characterization. However, in spite of claims to the contrary, the method is not a separation by
chemical type. Kit is a separation by solubility and adsorption.
The separation of the asphaltene constituents does, however, provide a simple way to remove
some of the highest molecular weight and most polar components but the asphaltene fraction is so
complex that compositional detail based on average parameters is of questionable value.
The use of ion exchange chromatography (McKay et al., 1976; Green et al., 1989) has offered
some indications of chemical types within the complex high molecular weight fractions.
For the 565°C + (1050°F + ) fractions of petroleum, the levels of nitrogen and oxygen may begin
to approach the concentration of sulfur. These elements consistently concentrate in the most polar
fractions to the extent that every molecule contains more than one heteroatom. At this point, structural identification is somewhat fruitless and characterization techniques are used to confirm the
presence of the functionalities found in lower boiling fractions such as, for example, acids, phenols,
nonbasic (carbazole-type) nitrogen, and basic (quinoline-type) nitrogen.
Several models have been proposed based on the observed functionalities, apparent molecular
weight, and elemental analysis of the fraction but whether or not these models offer insights into
the nature and behavior of the asphaltene constituents remains open to speculation and question
(Speight, 1994b).
The nickel and vanadium that are concentrated into the vacuum residuum appear to occur in
two forms: (1) porphyrins and (2) nonporphyrins (Reynolds, 1998). Because the metalloporphyrins can provide insights into petroleum maturation processes, they have been studied extensively and several families of related structures have been identified. On the other hand, the
nonporphyrin metals remain not clearly identified although some studies suggest that some of
the metals in these compounds still exist in a tetrapyrrole (porphyrin-type) environment (Pearson
and Green, 1993).
It is more than likely that, in a specific residuum molecule, the heteroatoms are arranged in different functionalities, making an incredibly complex molecule. Considering how many different
combinations are possible, the chances of determining every structure in a residuum are very low.
Because of this seemingly insurmountable task, it may be better to determine ways of utilizing the
residuum rather attempting to determine (at best questionable) molecular structures.
REFERENCES
Aksenov, V.S. and Kanayanov, V.F. 1980. Regularities in composition and structures of native sulfur compounds from petroleum, Proceedings, 9th International Symposium on Organic Sulfur Chemistry, Riga,
USSR, June 9–14.
Altgelt, K.H. and Boduszynski, M.M. 1994. Compositional Analysis of Heavy Petroleum Fractions. Marcel
Dekker Inc., New York.
Baker, E.W. and Palmer, S.E. 1978. The porphyrins. In Structure and Synthesis, Volume I, Part A, D. Dolphin
(Ed.). Academic Press, New York.
The Chemistry and Technology of Petroleum
8.4.3 vACuum resIduA (1050°F + )
This fraction, the vacuum bottoms, is the most complex of petroleum. Vacuum residua contain the
majority of the heteroatoms originally in the petroleum and molecular weight of the constituents
range, as near as can be determined subject to method dependence, up to several thousand. The
fraction is so complex that the characterization of individual species is virtually impossible, no
matter what claims have been made or will be made. Separation of vacuum residua by group type
becomes difficult and confused because of the multi-substitution of aromatic and naphthenic species as well as by the presence of multiple functionalities in single molecules.
Classically, n-pentane or n-heptane precipitation is used as the initial step for the characterization
of vacuum residuum. The insoluble fraction, the pentane- or heptane-asphaltenes, may be as much
as 50% by weight of a vacuum residuum. The pentane- or heptane-soluble portion (maltenes) of
the residuum is then fractionated chromatographically into several solubility or adsorption classes
for characterization. However, in spite of claims to the contrary, the method is not a separation by
chemical type. Kit is a separation by solubility and adsorption.
The separation of the asphaltene constituents does, however, provide a simple way to remove
some of the highest molecular weight and most polar components but the asphaltene fraction is so
complex that compositional detail based on average parameters is of questionable value.
The use of ion exchange chromatography (McKay et al., 1976; Green et al., 1989) has offered
some indications of chemical types within the complex high molecular weight fractions.
For the 565°C + (1050°F + ) fractions of petroleum, the levels of nitrogen and oxygen may begin
to approach the concentration of sulfur. These elements consistently concentrate in the most polar
fractions to the extent that every molecule contains more than one heteroatom. At this point, structural identification is somewhat fruitless and characterization techniques are used to confirm the
presence of the functionalities found in lower boiling fractions such as, for example, acids, phenols,
nonbasic (carbazole-type) nitrogen, and basic (quinoline-type) nitrogen.
Several models have been proposed based on the observed functionalities, apparent molecular
weight, and elemental analysis of the fraction but whether or not these models offer insights into
the nature and behavior of the asphaltene constituents remains open to speculation and question
(Speight, 1994b).
The nickel and vanadium that are concentrated into the vacuum residuum appear to occur in
two forms: (1) porphyrins and (2) nonporphyrins (Reynolds, 1998). Because the metalloporphyrins can provide insights into petroleum maturation processes, they have been studied extensively and several families of related structures have been identified. On the other hand, the
nonporphyrin metals remain not clearly identified although some studies suggest that some of
the metals in these compounds still exist in a tetrapyrrole (porphyrin-type) environment (Pearson
and Green, 1993).
It is more than likely that, in a specific residuum molecule, the heteroatoms are arranged in different functionalities, making an incredibly complex molecule. Considering how many different
combinations are possible, the chances of determining every structure in a residuum are very low.
Because of this seemingly insurmountable task, it may be better to determine ways of utilizing the
residuum rather attempting to determine (at best questionable) molecular structures.
REFERENCES
Aksenov, V.S. and Kanayanov, V.F. 1980. Regularities in composition and structures of native sulfur compounds from petroleum, Proceedings, 9th International Symposium on Organic Sulfur Chemistry, Riga,
USSR, June 9–14.
Altgelt, K.H. and Boduszynski, M.M. 1994. Compositional Analysis of Heavy Petroleum Fractions. Marcel
Dekker Inc., New York.
Baker, E.W. and Palmer, S.E. 1978. The porphyrins. In Structure and Synthesis, Volume I, Part A, D. Dolphin
(Ed.). Academic Press, New York.