447
Refining Chemistry
paraffinic constituents. Again, there is an analogy with the deasphalting process (Chapters 9, 10,
and 20), except that the paraffinic material is a product of the thermal decomposition of the asphaltene molecules and is formed in situ rather than being added separately.
The coke has a lower hydrogen-to-carbon atomic ratio than the hydrogen-to-carbon ratio of any
of the constituents present in the original crude oil. The hydrocarbon products may have a higher
hydrogen-to-carbon atomic ratio than the hydrogen-to-carbon ratio of any of the constituents present in the original crude oil or hydrogen-to-carbon atomic ratios at least equal to those of many of
the original constituents. It must also be recognized that the production of coke and volatile hydrocarbon products is accompanied by a shift in the hydrogen distribution.
Mild-severity and high-severity processes are frequently used for the processing of residue
fractions, whereas conditions similar to those of ultrapyrolysis (high temperature and very short
residence time) are used commercially only for cracking ethane, propane, butane, and light distillate
feeds to produce ethylene and higher olefins.
The formation of solid sediments, or coke, during thermal processes is a major limitation on
processing. Furthermore, the presence of different types of solids shows that solubility controls the
formation of solids. And the tendency for solid formation changes in response to the relative amounts
of the light ends, middle distillates, and residues and to their changing chemical composition during
the process (Gray, 1994). In fact, the prime mover in the formation of incompatible products during
the processing of feedstocks containing asphaltenes is the nature of the primary thermal decomposition products, particularly those designated as carbenes and carboids (Chapter 1) (Speight, 1987,
1992; Wiehe, 1992, 1993).
Coke formation during the thermal treatment of petroleum residua is postulated to occur by a
mechanism that involves the liquid–liquid phase separation of reacted asphaltenes (which may be
carbenes) to form a phase that is lean in abstractable hydrogen. The unreacted asphaltenes were
found to be the fraction with the highest rate of thermal reaction but with the least extent of reaction. This not only described the appearance and disappearance of asphaltene constituents but also
quantitatively described the variation in molecular weight and hydrogen content of the asphaltenes
with reaction time. Thus, the main features coke formation are (1) an induction period prior to
coke formation, (2) a maximum concentration of asphaltene constituents in the reacting liquid,
(3) a decrease in the asphaltene concentration that parallels the decrease in heptane-soluble material, and (4) high reactivity of the unconverted asphaltene constituents.
The induction period has been observed experimentally by many previous investigators (Levinter
et al., 1966, 1967; Magaril and Aksenova, 1970; Valyavin et al., 1979; Takatsuka et al., 1989a) and
makes visbreaking and the Eureka processes possible. The postulation that that coke formation
is triggered by the phase separation of asphaltenes (Magaril et al., 1971) led to the use of linear
variations of the concentration of each fraction with reaction time, resulting in the assumption of
zero-order kinetics rather than first-order kinetics. More recently (Yan, 1987), coke formation in
visbreaking was described as resulting from a phase separation step, but the phase-separation step
was not included in the resulting kinetic model for coke formation.
This model represents the conversion of asphaltenes over the entire temperature range and of
heptane-soluble materials in the coke induction period as first-order reactions. The data also show
that the four reactions give simultaneously lower aromatic and higher aromatic products, on the
basis of other evidence (Wiehe, 1992). Also, the previous work showed that residua fractions can be
converted without completely changing solubility classes (Magaril et al., 1971) and that coke formation is triggered by the phase separation of converted asphaltenes.
The maximum solubility of these product asphaltenes is proportional to the total heptanesoluble materials, as suggested by the observation that the decrease in asphaltenes parallels the
decrease of heptane-soluble materials. Finally, the conversion of the insoluble product asphaltenes
into toluene-insoluble coke is pictured as producing a heptane-soluble by-product, which provides
a mechanism for the heptane-soluble conversion to deviate from first-order behavior once coke
begins to form.
Refining Chemistry
paraffinic constituents. Again, there is an analogy with the deasphalting process (Chapters 9, 10,
and 20), except that the paraffinic material is a product of the thermal decomposition of the asphaltene molecules and is formed in situ rather than being added separately.
The coke has a lower hydrogen-to-carbon atomic ratio than the hydrogen-to-carbon ratio of any
of the constituents present in the original crude oil. The hydrocarbon products may have a higher
hydrogen-to-carbon atomic ratio than the hydrogen-to-carbon ratio of any of the constituents present in the original crude oil or hydrogen-to-carbon atomic ratios at least equal to those of many of
the original constituents. It must also be recognized that the production of coke and volatile hydrocarbon products is accompanied by a shift in the hydrogen distribution.
Mild-severity and high-severity processes are frequently used for the processing of residue
fractions, whereas conditions similar to those of ultrapyrolysis (high temperature and very short
residence time) are used commercially only for cracking ethane, propane, butane, and light distillate
feeds to produce ethylene and higher olefins.
The formation of solid sediments, or coke, during thermal processes is a major limitation on
processing. Furthermore, the presence of different types of solids shows that solubility controls the
formation of solids. And the tendency for solid formation changes in response to the relative amounts
of the light ends, middle distillates, and residues and to their changing chemical composition during
the process (Gray, 1994). In fact, the prime mover in the formation of incompatible products during
the processing of feedstocks containing asphaltenes is the nature of the primary thermal decomposition products, particularly those designated as carbenes and carboids (Chapter 1) (Speight, 1987,
1992; Wiehe, 1992, 1993).
Coke formation during the thermal treatment of petroleum residua is postulated to occur by a
mechanism that involves the liquid–liquid phase separation of reacted asphaltenes (which may be
carbenes) to form a phase that is lean in abstractable hydrogen. The unreacted asphaltenes were
found to be the fraction with the highest rate of thermal reaction but with the least extent of reaction. This not only described the appearance and disappearance of asphaltene constituents but also
quantitatively described the variation in molecular weight and hydrogen content of the asphaltenes
with reaction time. Thus, the main features coke formation are (1) an induction period prior to
coke formation, (2) a maximum concentration of asphaltene constituents in the reacting liquid,
(3) a decrease in the asphaltene concentration that parallels the decrease in heptane-soluble material, and (4) high reactivity of the unconverted asphaltene constituents.
The induction period has been observed experimentally by many previous investigators (Levinter
et al., 1966, 1967; Magaril and Aksenova, 1970; Valyavin et al., 1979; Takatsuka et al., 1989a) and
makes visbreaking and the Eureka processes possible. The postulation that that coke formation
is triggered by the phase separation of asphaltenes (Magaril et al., 1971) led to the use of linear
variations of the concentration of each fraction with reaction time, resulting in the assumption of
zero-order kinetics rather than first-order kinetics. More recently (Yan, 1987), coke formation in
visbreaking was described as resulting from a phase separation step, but the phase-separation step
was not included in the resulting kinetic model for coke formation.
This model represents the conversion of asphaltenes over the entire temperature range and of
heptane-soluble materials in the coke induction period as first-order reactions. The data also show
that the four reactions give simultaneously lower aromatic and higher aromatic products, on the
basis of other evidence (Wiehe, 1992). Also, the previous work showed that residua fractions can be
converted without completely changing solubility classes (Magaril et al., 1971) and that coke formation is triggered by the phase separation of converted asphaltenes.
The maximum solubility of these product asphaltenes is proportional to the total heptanesoluble materials, as suggested by the observation that the decrease in asphaltenes parallels the
decrease of heptane-soluble materials. Finally, the conversion of the insoluble product asphaltenes
into toluene-insoluble coke is pictured as producing a heptane-soluble by-product, which provides
a mechanism for the heptane-soluble conversion to deviate from first-order behavior once coke
begins to form.