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The Chemistry and Technology of Petroleum
the deposition of asphaltenic material. In other circumstances, such as the use of well stimulation
fluids that contain acidic species, there is also the potential to create salts of the asphaltenic species
(i.e., asphaltenesH + X − or resinsH + X − ). Whether this be through the agency of the charges species
within the asphaltenic molecules or through the agency of the basic (pyridine-type) nitrogen that
is known to exist in the ring systems of the asphaltenic constituents is unknown. But it will be sufficient to change the natural order of the constituents within the crude oil leading to the deposition
of incompatible (insoluble) material.
In summary, there are several effects that can occur during recovery processes that will lead
to the deposition of asphaltenic material. When this happens, problems such as interruption or
even cessation of the recovery operations can occur. In fact, when the increased popularity of
asphaltene-containing crude oil is taken into consideration, it is surprising that problems of the
deposition of asphaltenic material is not more widespread, assuming that all such occurrences
are reported!
13.5.2 eFFeCts on reFInIng oPerAtIons
When petroleum is heated to temperatures in excess of 350°C (660°F), the rate of thermal decomposition of the constituents increases significantly. The higher the temperature, the shorter the time
to achieve a given conversion, and the severity of the process conditions is a combination of the
residence time of the crude oil constituents in the reactor and the temperature needed to achieve a
given conversion.
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.
Polynuclear aromatic systems that are denuded of the attendant hydrocarbon moieties are somewhat less soluble in the surrounding hydrocarbon medium than their parent systems (Bjorseth, 1983;
Dias, 1987, 1988). This is due to the solubilizing effect of the alkyl moieties and the enrichment of
the liquid medium in paraffinic constituents. Again, there is an analogy with the deasphalting process, 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.
Instability or incompatibility, resulting in the separation of solids during refining, can occur
during a variety of processes, either by intent (such as in the deasphalting process) or inadvertently
when the separation is detrimental to the process. Thus, separation of solids occurs whenever the
solvent characteristics of the liquid phase are no longer adequate to maintain polar and/or high
molecular weight material in solution. Examples of such occurrences are (1) asphaltene separation
when the paraffin content or character of the liquid medium increases; (2) sludge or sediment formation in a reactor, which occurs when the solvent characteristics of the liquid medium change so
that asphaltic or wax materials separate; and (3) coke formation that occurs at high temperatures
and commences when the solvent power of the liquid phase is not sufficient to maintain the coke
precursors in solution.
Thus, there is a need to define adequately the nature of a refinery feedstock before use as well
as the nature of the products in order to anticipate the onset of instability or incompatibility. The
occurrence of such phenomena during thermal processes will be discussed in this chapter as it
relates to the thermal chemistry of crude oil constituents.
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 changes in chemical composition
during the process. In fact, the prime mover in the formation of incompatible products during the
The Chemistry and Technology of Petroleum
the deposition of asphaltenic material. In other circumstances, such as the use of well stimulation
fluids that contain acidic species, there is also the potential to create salts of the asphaltenic species
(i.e., asphaltenesH + X − or resinsH + X − ). Whether this be through the agency of the charges species
within the asphaltenic molecules or through the agency of the basic (pyridine-type) nitrogen that
is known to exist in the ring systems of the asphaltenic constituents is unknown. But it will be sufficient to change the natural order of the constituents within the crude oil leading to the deposition
of incompatible (insoluble) material.
In summary, there are several effects that can occur during recovery processes that will lead
to the deposition of asphaltenic material. When this happens, problems such as interruption or
even cessation of the recovery operations can occur. In fact, when the increased popularity of
asphaltene-containing crude oil is taken into consideration, it is surprising that problems of the
deposition of asphaltenic material is not more widespread, assuming that all such occurrences
are reported!
13.5.2 eFFeCts on reFInIng oPerAtIons
When petroleum is heated to temperatures in excess of 350°C (660°F), the rate of thermal decomposition of the constituents increases significantly. The higher the temperature, the shorter the time
to achieve a given conversion, and the severity of the process conditions is a combination of the
residence time of the crude oil constituents in the reactor and the temperature needed to achieve a
given conversion.
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.
Polynuclear aromatic systems that are denuded of the attendant hydrocarbon moieties are somewhat less soluble in the surrounding hydrocarbon medium than their parent systems (Bjorseth, 1983;
Dias, 1987, 1988). This is due to the solubilizing effect of the alkyl moieties and the enrichment of
the liquid medium in paraffinic constituents. Again, there is an analogy with the deasphalting process, 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.
Instability or incompatibility, resulting in the separation of solids during refining, can occur
during a variety of processes, either by intent (such as in the deasphalting process) or inadvertently
when the separation is detrimental to the process. Thus, separation of solids occurs whenever the
solvent characteristics of the liquid phase are no longer adequate to maintain polar and/or high
molecular weight material in solution. Examples of such occurrences are (1) asphaltene separation
when the paraffin content or character of the liquid medium increases; (2) sludge or sediment formation in a reactor, which occurs when the solvent characteristics of the liquid medium change so
that asphaltic or wax materials separate; and (3) coke formation that occurs at high temperatures
and commences when the solvent power of the liquid phase is not sufficient to maintain the coke
precursors in solution.
Thus, there is a need to define adequately the nature of a refinery feedstock before use as well
as the nature of the products in order to anticipate the onset of instability or incompatibility. The
occurrence of such phenomena during thermal processes will be discussed in this chapter as it
relates to the thermal chemistry of crude oil constituents.
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 changes in chemical composition
during the process. In fact, the prime mover in the formation of incompatible products during the