594
The Chemistry and Technology of Petroleum
off to yield iso-butane and benzene. Overall, coke formation is very low in hydrocracking, since
the secondary reactions and the formation of the precursors to coke are suppressed as the hydrogen
pressure is increased.
When applied to residua, the hydrocracking process can be used for processes such as
1. Fuel oil desulfurization
2. Residuum conversion to lower boiling distillates
The products from hydrocracking are composed of either saturated or aromatic compounds; no
olefins are found. In making gasoline, the lower paraffins formed have high octane numbers; for
example, the five- and six-carbon number fractions have leaded research octane numbers of 99–100.
The remaining gasoline has excellent properties as a feed to catalytic reforming, producing a highly
aromatic gasoline that is capable of a high octane number. Both types of gasoline are suitable for
premium-grade motor gasoline. Another attractive feature of hydrocracking is the low yield of gaseous components, such as methane, ethane, and propane, which are less desirable than gasoline.
When making jet fuel, more hydrogenation activity of the catalysts is used, since jet fuel contains
more saturates than gasoline.
Like many refinery processes, the problems encountered in hydrocracking heavy feedstocks can
be directly equated to the amount of complex, higher boiling constituents that may require pretreatment (Speight and Moschopedis, 1979; Reynolds and Beret, 1989; Gray, 1994; Speight, 2000;
Speight and Ozum, 2002). Processing these feedstocks is not merely a matter of applying know-how
derived from refining conventional crude oils but requires knowledge of composition and properties (Chapters 8 through 10). The materials are not only complex in terms of the carbon number
and boiling point ranges but also because a large part of this envelope falls into a range of model
compounds where very little is known about the properties. It is also established that the majority
of the higher molecular weight materials produce coke (with some liquids), but the majority of the
lower molecular weight constituents produce liquids (with some coke). It is to both of these trends
that hydrocracking is aimed.
It is the physical and chemical composition of a feedstock that plays a large part not only in
determining the nature of the products that arise from refining operations but also in determining the precise manner by which a particular feedstock should be processed (Speight, 1986).
Furthermore, it is apparent that the conversion of heavy oils and residua requires new lines of
thought to develop suitable processing scenarios. Indeed, the use of thermal (carbon rejection)
processes and of hydrothermal (hydrogen addition) processes, which were inherent in the refineries designed to process lighter feedstocks, has been a particular cause for concern. This has
brought about the evolution of processing schemes that accommodate the heavier feedstocks
(Khan and Patmore, 1998; Speight, 2011). As a point of reference, an example of the former
option is the delayed coking process in which the feedstock is converted to overhead with the
concurrent deposition of coke, for example that used by Suncor, Inc., at their oil sands plant
(Speight, 1990).
The hydrogen addition concept is illustrated by the hydrocracking process in which hydrogen is
used in an attempt to stabilize the reactive fragments produced during the cracking, thereby decreasing their potential for recombination to heavier products and ultimately to coke. The choice of
processing schemes for a given hydrocracking application depends upon the nature of the feedstock
as well as the product requirements (Suchanek and Moore, 1986). The process can be simply illustrated as a single-stage or as a two-stage operation (Figure 22.3).
The single-stage process can be used to produce gasoline but is more often used to produce
middle distillate from heavy vacuum gas oils. The two-stage process was developed primarily
to produce high yields of gasoline from straight-run gas oil, and the first stage may actually
be a purification step to remove sulfur-containing (as well as nitrogen-containing) organic
materials.
The Chemistry and Technology of Petroleum
off to yield iso-butane and benzene. Overall, coke formation is very low in hydrocracking, since
the secondary reactions and the formation of the precursors to coke are suppressed as the hydrogen
pressure is increased.
When applied to residua, the hydrocracking process can be used for processes such as
1. Fuel oil desulfurization
2. Residuum conversion to lower boiling distillates
The products from hydrocracking are composed of either saturated or aromatic compounds; no
olefins are found. In making gasoline, the lower paraffins formed have high octane numbers; for
example, the five- and six-carbon number fractions have leaded research octane numbers of 99–100.
The remaining gasoline has excellent properties as a feed to catalytic reforming, producing a highly
aromatic gasoline that is capable of a high octane number. Both types of gasoline are suitable for
premium-grade motor gasoline. Another attractive feature of hydrocracking is the low yield of gaseous components, such as methane, ethane, and propane, which are less desirable than gasoline.
When making jet fuel, more hydrogenation activity of the catalysts is used, since jet fuel contains
more saturates than gasoline.
Like many refinery processes, the problems encountered in hydrocracking heavy feedstocks can
be directly equated to the amount of complex, higher boiling constituents that may require pretreatment (Speight and Moschopedis, 1979; Reynolds and Beret, 1989; Gray, 1994; Speight, 2000;
Speight and Ozum, 2002). Processing these feedstocks is not merely a matter of applying know-how
derived from refining conventional crude oils but requires knowledge of composition and properties (Chapters 8 through 10). The materials are not only complex in terms of the carbon number
and boiling point ranges but also because a large part of this envelope falls into a range of model
compounds where very little is known about the properties. It is also established that the majority
of the higher molecular weight materials produce coke (with some liquids), but the majority of the
lower molecular weight constituents produce liquids (with some coke). It is to both of these trends
that hydrocracking is aimed.
It is the physical and chemical composition of a feedstock that plays a large part not only in
determining the nature of the products that arise from refining operations but also in determining the precise manner by which a particular feedstock should be processed (Speight, 1986).
Furthermore, it is apparent that the conversion of heavy oils and residua requires new lines of
thought to develop suitable processing scenarios. Indeed, the use of thermal (carbon rejection)
processes and of hydrothermal (hydrogen addition) processes, which were inherent in the refineries designed to process lighter feedstocks, has been a particular cause for concern. This has
brought about the evolution of processing schemes that accommodate the heavier feedstocks
(Khan and Patmore, 1998; Speight, 2011). As a point of reference, an example of the former
option is the delayed coking process in which the feedstock is converted to overhead with the
concurrent deposition of coke, for example that used by Suncor, Inc., at their oil sands plant
(Speight, 1990).
The hydrogen addition concept is illustrated by the hydrocracking process in which hydrogen is
used in an attempt to stabilize the reactive fragments produced during the cracking, thereby decreasing their potential for recombination to heavier products and ultimately to coke. The choice of
processing schemes for a given hydrocracking application depends upon the nature of the feedstock
as well as the product requirements (Suchanek and Moore, 1986). The process can be simply illustrated as a single-stage or as a two-stage operation (Figure 22.3).
The single-stage process can be used to produce gasoline but is more often used to produce
middle distillate from heavy vacuum gas oils. The two-stage process was developed primarily
to produce high yields of gasoline from straight-run gas oil, and the first stage may actually
be a purification step to remove sulfur-containing (as well as nitrogen-containing) organic
materials.