450
The Chemistry and Technology of Petroleum
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, which occurs when the paraffin content or character of the liquid
medium increases (Chapters 10 and 12)
2. Wax separation, which occurs when there is a drop in temperature or the aromatic content
or character of the liquid medium increases
3. Sludge or sediment formation in a reactor, which occurs when the solvent characteristics of
the liquid medium change so that asphalt or wax materials separate
4. Coke formation, which occurs at high temperatures and commences when the solvent
power of the liquid phase is not sufficient to maintain the coke precursors in solution
(Chapters 10 and 12)
5. Sludge or sediment formation in fuel products and occurs because of the interplay of
several chemical and physical factors
This mechanism also appears to be operable during residua hydroconversion, which has included
a phase-separation step (the formation of dry sludge) in a kinetic model but it was not included as a
preliminary step to coke formation in a thermal cracking model (Takatsuka et al., 1989a,b; Speight,
2004a,b).
16.7.2 HydroConversIon CHemIstry
There have also been many attempts to focus attention on the asphaltenes during hydrocracking
studies. The focus has been on the macromolecular changes that occur by investigation of the
changes to the generic fractions, that is, the asphaltenes, the resins, and the other fractions that make
up such a feedstock (Ancheyta and Speight, 2007). In terms of hydroprocessing, the means by which
asphaltene constituents are desulfurized, as one step of a hydrocracking operation, is also suggested
as part of the process. This concept can then be taken one step further to show the dealkylation of
the aromatic systems as a definitive step in the hydrocracking process (Speight, 1987).
When catalytic processes are employed, complex molecules (such as those that may be found
in the original asphaltene fraction or those formed during the process) are not sufficiently mobile
(or are too strongly adsorbed by the catalyst) to be saturated by the hydrogenation components.
Hence, these molecular species continue to condense and eventually degrade to coke. These deposits deactivate the catalyst sites and eventually interfere with the process.
Several noteworthy attempts have been made to focus attention on the asphaltene constituents during
hydroprocessing studies. The focus has been on the macromolecular changes that occur by investigation of the changes in the generic fractions, that is, the asphaltene constituents, the resin constituents,
and the other fractions that make up such a feedstock. This option suggests that the overall pathway by
which hydrotreating and hydrocracking of heavy oils and residua occur involves a stepwise mechanism:
Asphaltene constituents → resin-type constituents (polar aromatics)
Resin-type constituents → aromatics
Aromatics → saturates
A direct step from either the asphaltene constituents or the resin constituents to the saturates is not
considered a predominant pathway for hydroprocessing.
The means by which asphaltenes are desulfurized, as one step of a hydrocracking operation, is
also suggested as part of this process. This concept can then be taken one step further to show the
dealkylation of the aromatic systems as a definitive step in the hydrocracking process (Speight,
1987). It is also likely that molecular species (within the asphaltene fraction) that contain nitrogen
The Chemistry and Technology of Petroleum
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, which occurs when the paraffin content or character of the liquid
medium increases (Chapters 10 and 12)
2. Wax separation, which occurs when there is a drop in temperature or the aromatic content
or character of the liquid medium increases
3. Sludge or sediment formation in a reactor, which occurs when the solvent characteristics of
the liquid medium change so that asphalt or wax materials separate
4. Coke formation, which occurs at high temperatures and commences when the solvent
power of the liquid phase is not sufficient to maintain the coke precursors in solution
(Chapters 10 and 12)
5. Sludge or sediment formation in fuel products and occurs because of the interplay of
several chemical and physical factors
This mechanism also appears to be operable during residua hydroconversion, which has included
a phase-separation step (the formation of dry sludge) in a kinetic model but it was not included as a
preliminary step to coke formation in a thermal cracking model (Takatsuka et al., 1989a,b; Speight,
2004a,b).
16.7.2 HydroConversIon CHemIstry
There have also been many attempts to focus attention on the asphaltenes during hydrocracking
studies. The focus has been on the macromolecular changes that occur by investigation of the
changes to the generic fractions, that is, the asphaltenes, the resins, and the other fractions that make
up such a feedstock (Ancheyta and Speight, 2007). In terms of hydroprocessing, the means by which
asphaltene constituents are desulfurized, as one step of a hydrocracking operation, is also suggested
as part of the process. This concept can then be taken one step further to show the dealkylation of
the aromatic systems as a definitive step in the hydrocracking process (Speight, 1987).
When catalytic processes are employed, complex molecules (such as those that may be found
in the original asphaltene fraction or those formed during the process) are not sufficiently mobile
(or are too strongly adsorbed by the catalyst) to be saturated by the hydrogenation components.
Hence, these molecular species continue to condense and eventually degrade to coke. These deposits deactivate the catalyst sites and eventually interfere with the process.
Several noteworthy attempts have been made to focus attention on the asphaltene constituents during
hydroprocessing studies. The focus has been on the macromolecular changes that occur by investigation of the changes in the generic fractions, that is, the asphaltene constituents, the resin constituents,
and the other fractions that make up such a feedstock. This option suggests that the overall pathway by
which hydrotreating and hydrocracking of heavy oils and residua occur involves a stepwise mechanism:
Asphaltene constituents → resin-type constituents (polar aromatics)
Resin-type constituents → aromatics
Aromatics → saturates
A direct step from either the asphaltene constituents or the resin constituents to the saturates is not
considered a predominant pathway for hydroprocessing.
The means by which asphaltenes are desulfurized, as one step of a hydrocracking operation, is
also suggested as part of this process. This concept can then be taken one step further to show the
dealkylation of the aromatic systems as a definitive step in the hydrocracking process (Speight,
1987). It is also likely that molecular species (within the asphaltene fraction) that contain nitrogen