448
The Chemistry and Technology of Petroleum
In support of this assumption, it is known (Langer et al., 1961) that partially hydrogenated refinery process streams provide abstractable hydrogen and as a result, inhibit coke formation during
residuum thermal conversion. Thus, the heptane-soluble fraction of a residuum that contains naturally occurring partially hydrogenated aromatics can provide abstractable hydrogen during thermal
reactions.
As the conversion proceeds, the concentration of asphaltene cores continues to increase and
the heptane-soluble fraction continues to decrease until the solubility limit, S L is reached. Beyond the
solubility limit, the excess asphaltene cores, A * ex , phase separate to form a second liquid phase
that is lean in abstractable hydrogen. In this new phase, asphaltene radical—asphaltene radical
recombination is quite frequent, causing a rapid reaction to form solid coke and a by-product of a
heptane-soluble core.
The asphaltene concentration varies little in the coke induction period (Wiehe, 1993) but then
decreases once coke begins to form. Observing this, it might be concluded that asphaltenes are
unreactive, but it is the high reactivity of the asphaltenes down to the asphaltene core that offsets
the generation of asphaltene cores from the heptane-soluble materials to keep the overall asphaltene
concentration nearly constant.
Previously, it was demonstrated (Schucker and Keweshan, 1980; Savage et al., 1988) that the
hydrogen-to-carbon atomic ratio of the asphaltenes decreases rapidly with reaction time for asphaltene thermolysis and then approaches an asymptotic limit at long reaction times, which provides
qualitative evidence for asphaltene cracking down to a core.
The measurement of the molecular weight of petroleum asphaltenes is known to give different
values depending on the technique, the solvent, and the temperature (Chapters 10 and 12) (Dickie
and Yen, 1967; Moschopedis et al., 1976; Speight et al., 1985). As shown by small-angle x-ray (Kim
and Long, 1979) and neutron (Overfield et al., 1989) scattering, this is because asphaltenes tend to
self-associate and form aggregates.
Thus, coke formation is a complex process involving both chemical reactions and thermodynamic behavior. Reactions that contribute to this process are cracking of side chains from aromatic
groups, dehydrogenation of naphthenes to form aromatics, condensation of aliphatic structures to
form aromatics, condensation of aromatics to form higher fused-ring aromatics, and dimerization or
oligomerization reactions. Loss of side chains always accompanies thermal cracking, and dehydrogenation and condensation reactions are favored by hydrogen-deficient conditions.
The importance of solvents in coking has been recognized for many years (e.g., Langer et al.,
1961), but their effects have often been ascribed to hydrogen donor reactions rather than phase
behavior. The separation of the phases depends on the solvent characteristics of the liquid.
Addition of aromatic solvents suppresses phase separation, whereas paraffins enhance separation.
Microscopic examination of coke particles often shows evidence for the presence of mesophase,
spherical domains that exhibit the anisotropic optical characteristics of liquid crystals.
This phenomenon is consistent with the formation of a second liquid phase; the mesophase liquid
is denser than the rest of the hydrocarbon, has a higher surface tension, and probably wets metal
surfaces better than the rest of the liquid phase. The mesophase characteristic of coke diminishes as
the liquid phase becomes more compatible with the aromatic material.
The phase separation phenomenon that is the prelude to coke formation can also be explained by
use of the solubility parameter, δ, for petroleum fractions and for the solvents (Yen, 1984; Speight,
1994) (Chapter 12). As an extension of this concept, there is sufficient data to draw a correlation
between the atomic hydrogen/carbon ratio and the solubility parameter for hydrocarbons and the
constituents of the lower-boiling fractions of petroleum (Speight, 1994). Recognition that hydrocarbon liquids can dissolve polynuclear hydrocarbons, a case in which there is usually less than a
three-point difference between the lower solubility parameter of the solvent and the higher solubility parameter of the solute. Thus, a parallel, or near-parallel, line can be assumed that allows the
solubility parameter of the asphaltenes and resins to be estimated.
The Chemistry and Technology of Petroleum
In support of this assumption, it is known (Langer et al., 1961) that partially hydrogenated refinery process streams provide abstractable hydrogen and as a result, inhibit coke formation during
residuum thermal conversion. Thus, the heptane-soluble fraction of a residuum that contains naturally occurring partially hydrogenated aromatics can provide abstractable hydrogen during thermal
reactions.
As the conversion proceeds, the concentration of asphaltene cores continues to increase and
the heptane-soluble fraction continues to decrease until the solubility limit, S L is reached. Beyond the
solubility limit, the excess asphaltene cores, A * ex , phase separate to form a second liquid phase
that is lean in abstractable hydrogen. In this new phase, asphaltene radical—asphaltene radical
recombination is quite frequent, causing a rapid reaction to form solid coke and a by-product of a
heptane-soluble core.
The asphaltene concentration varies little in the coke induction period (Wiehe, 1993) but then
decreases once coke begins to form. Observing this, it might be concluded that asphaltenes are
unreactive, but it is the high reactivity of the asphaltenes down to the asphaltene core that offsets
the generation of asphaltene cores from the heptane-soluble materials to keep the overall asphaltene
concentration nearly constant.
Previously, it was demonstrated (Schucker and Keweshan, 1980; Savage et al., 1988) that the
hydrogen-to-carbon atomic ratio of the asphaltenes decreases rapidly with reaction time for asphaltene thermolysis and then approaches an asymptotic limit at long reaction times, which provides
qualitative evidence for asphaltene cracking down to a core.
The measurement of the molecular weight of petroleum asphaltenes is known to give different
values depending on the technique, the solvent, and the temperature (Chapters 10 and 12) (Dickie
and Yen, 1967; Moschopedis et al., 1976; Speight et al., 1985). As shown by small-angle x-ray (Kim
and Long, 1979) and neutron (Overfield et al., 1989) scattering, this is because asphaltenes tend to
self-associate and form aggregates.
Thus, coke formation is a complex process involving both chemical reactions and thermodynamic behavior. Reactions that contribute to this process are cracking of side chains from aromatic
groups, dehydrogenation of naphthenes to form aromatics, condensation of aliphatic structures to
form aromatics, condensation of aromatics to form higher fused-ring aromatics, and dimerization or
oligomerization reactions. Loss of side chains always accompanies thermal cracking, and dehydrogenation and condensation reactions are favored by hydrogen-deficient conditions.
The importance of solvents in coking has been recognized for many years (e.g., Langer et al.,
1961), but their effects have often been ascribed to hydrogen donor reactions rather than phase
behavior. The separation of the phases depends on the solvent characteristics of the liquid.
Addition of aromatic solvents suppresses phase separation, whereas paraffins enhance separation.
Microscopic examination of coke particles often shows evidence for the presence of mesophase,
spherical domains that exhibit the anisotropic optical characteristics of liquid crystals.
This phenomenon is consistent with the formation of a second liquid phase; the mesophase liquid
is denser than the rest of the hydrocarbon, has a higher surface tension, and probably wets metal
surfaces better than the rest of the liquid phase. The mesophase characteristic of coke diminishes as
the liquid phase becomes more compatible with the aromatic material.
The phase separation phenomenon that is the prelude to coke formation can also be explained by
use of the solubility parameter, δ, for petroleum fractions and for the solvents (Yen, 1984; Speight,
1994) (Chapter 12). As an extension of this concept, there is sufficient data to draw a correlation
between the atomic hydrogen/carbon ratio and the solubility parameter for hydrocarbons and the
constituents of the lower-boiling fractions of petroleum (Speight, 1994). Recognition that hydrocarbon liquids can dissolve polynuclear hydrocarbons, a case in which there is usually less than a
three-point difference between the lower solubility parameter of the solvent and the higher solubility parameter of the solute. Thus, a parallel, or near-parallel, line can be assumed that allows the
solubility parameter of the asphaltenes and resins to be estimated.