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The Chemistry and Technology of Petroleum
olefins formed in the cracking, and protect the catalysts from poisoning by coke. Zeolites or amorphous silica–alumina provide the cracking functions. The zeolites are usually type Y (faujasite),
ion exchanged to replace sodium with hydrogen and make up 25%–50% of the catalysts. Pentasils
(silicalite or ZSM-5) may be included in dewaxing catalysts.
Hydrocracking catalysts, such as nickel (5% by weight) on silica–alumina, work best on feedstocks that have been hydrofined to low nitrogen and sulfur levels. The nickel catalyst then operates
well at 350°C–370°C (660°F–700°F) and a pressure of about 1500 psi to give good conversion of
feed to lower boiling liquid fractions with minimum saturation of single-ring aromatics and a high
iso-paraffin to n-paraffin ratio in the lower molecular weight paraffins.
The poisoning effect of nitrogen can be offset to a certain degree by operating at a higher temperature. However, the higher temperature tends to increase the production of material in the methane
(CH 4 ) to butane (C 4 H 10 ) range and decrease the operating stability of the catalyst so that it requires
more frequent regeneration. Catalysts containing platinum or palladium (approximately 0.5% wet)
on a zeolite base appear to be somewhat less sensitive to nitrogen than are nickel catalysts, and successful operation has been achieved with feedstocks containing 40 ppm nitrogen. This catalyst is
also more tolerant of sulfur in the feed, which acts as a temporary poison, the catalyst recovering its
activity when the sulfur content of the feed is reduced.
On such catalysts as nickel or tungsten sulfide on silica–alumina, isomerization does not
appear to play any part in the reaction, as uncracked normal paraffins from the feedstock tend
to retain their normal structure. Extensive splitting produces large amounts of low-molecularweight (C 3 –C 6 ) paraffins, and it appears that a primary reaction of paraffins is catalytic cracking
followed by hydrogenation to form iso-paraffins. With catalysts of higher hydrogenation activity, such as platinum on silica–alumina, direct isomerization occurs. The product distribution is
also different, and the ratio of low- to intermediate-molecular-weight paraffins in the breakdown
product is reduced.
In addition to the chemical nature of the catalyst, the physical structure of the catalyst is also
important in determining the hydrogenation and cracking capabilities, particularly for heavy feedstocks (Fischer and Angevine, 1986; Kobayashi et al., 1987a,b). When gas oils and residua are used,
the feedstock is present as liquids under the conditions of the reaction. Additional feedstock and the
hydrogen must diffuse through this liquid before reaction can take place at the interior surfaces of
the catalyst particle.
At high temperatures, reaction rates can be much higher than diffusion rates and concentration gradients can develop within the catalyst particle. Therefore, the choice of catalyst porosity is
an important parameter. When feedstocks are to be hydrocracked to liquefied petroleum gas and
gasoline, pore diffusion effects are usually absent. High surface area (about 300 m 2 /g) and low to
moderate porosity (from 12 pore diameter with crystalline acidic components to 50 or more with
amorphous materials) catalysts are used. With reactions involving high-molecular-weight feedstocks,
pore diffusion can exert a large influence, and catalysts with pore diameters greater than 80 Å are
necessary for more efficient conversion.
Catalyst operating temperature can influence reaction selectivity, since the activation energy for
hydrotreating reactions is much lower than for hydrocracking reaction. Therefore, raising the temperature in a residuum hydrotreater increases the extent of hydrocracking relative to hydrotreating,
which also increases the hydrogen consumption.
Molybdenum sulfide (MoS 2 ), usually supported on alumina, is widely used in petroleum
processes for hydrogenation reactions. It is a layered structure that can be made much more
active by addition of cobalt or nickel. When promoted with cobalt sulfide (CoS), making what
is called cobalt-moly catalysts, it is widely used in hydrodesulfurization (HDS) processes.
The nickel sulfide (NiS)-promoted version is used for hydrodenitrogenation (HDN) as well as
hydrodesulfurization (HDS). The closely related tungsten compound (WS 2 ) is used in commercial hydrocracking catalysts. Other sulfides (iron sulfide, FeS; chromium sulfide, Cr 2 S 3 ; and
vanadium sulfide, V 2 S 5 ) are also effective and used in some catalysts. A valuable alternative
The Chemistry and Technology of Petroleum
olefins formed in the cracking, and protect the catalysts from poisoning by coke. Zeolites or amorphous silica–alumina provide the cracking functions. The zeolites are usually type Y (faujasite),
ion exchanged to replace sodium with hydrogen and make up 25%–50% of the catalysts. Pentasils
(silicalite or ZSM-5) may be included in dewaxing catalysts.
Hydrocracking catalysts, such as nickel (5% by weight) on silica–alumina, work best on feedstocks that have been hydrofined to low nitrogen and sulfur levels. The nickel catalyst then operates
well at 350°C–370°C (660°F–700°F) and a pressure of about 1500 psi to give good conversion of
feed to lower boiling liquid fractions with minimum saturation of single-ring aromatics and a high
iso-paraffin to n-paraffin ratio in the lower molecular weight paraffins.
The poisoning effect of nitrogen can be offset to a certain degree by operating at a higher temperature. However, the higher temperature tends to increase the production of material in the methane
(CH 4 ) to butane (C 4 H 10 ) range and decrease the operating stability of the catalyst so that it requires
more frequent regeneration. Catalysts containing platinum or palladium (approximately 0.5% wet)
on a zeolite base appear to be somewhat less sensitive to nitrogen than are nickel catalysts, and successful operation has been achieved with feedstocks containing 40 ppm nitrogen. This catalyst is
also more tolerant of sulfur in the feed, which acts as a temporary poison, the catalyst recovering its
activity when the sulfur content of the feed is reduced.
On such catalysts as nickel or tungsten sulfide on silica–alumina, isomerization does not
appear to play any part in the reaction, as uncracked normal paraffins from the feedstock tend
to retain their normal structure. Extensive splitting produces large amounts of low-molecularweight (C 3 –C 6 ) paraffins, and it appears that a primary reaction of paraffins is catalytic cracking
followed by hydrogenation to form iso-paraffins. With catalysts of higher hydrogenation activity, such as platinum on silica–alumina, direct isomerization occurs. The product distribution is
also different, and the ratio of low- to intermediate-molecular-weight paraffins in the breakdown
product is reduced.
In addition to the chemical nature of the catalyst, the physical structure of the catalyst is also
important in determining the hydrogenation and cracking capabilities, particularly for heavy feedstocks (Fischer and Angevine, 1986; Kobayashi et al., 1987a,b). When gas oils and residua are used,
the feedstock is present as liquids under the conditions of the reaction. Additional feedstock and the
hydrogen must diffuse through this liquid before reaction can take place at the interior surfaces of
the catalyst particle.
At high temperatures, reaction rates can be much higher than diffusion rates and concentration gradients can develop within the catalyst particle. Therefore, the choice of catalyst porosity is
an important parameter. When feedstocks are to be hydrocracked to liquefied petroleum gas and
gasoline, pore diffusion effects are usually absent. High surface area (about 300 m 2 /g) and low to
moderate porosity (from 12 pore diameter with crystalline acidic components to 50 or more with
amorphous materials) catalysts are used. With reactions involving high-molecular-weight feedstocks,
pore diffusion can exert a large influence, and catalysts with pore diameters greater than 80 Å are
necessary for more efficient conversion.
Catalyst operating temperature can influence reaction selectivity, since the activation energy for
hydrotreating reactions is much lower than for hydrocracking reaction. Therefore, raising the temperature in a residuum hydrotreater increases the extent of hydrocracking relative to hydrotreating,
which also increases the hydrogen consumption.
Molybdenum sulfide (MoS 2 ), usually supported on alumina, is widely used in petroleum
processes for hydrogenation reactions. It is a layered structure that can be made much more
active by addition of cobalt or nickel. When promoted with cobalt sulfide (CoS), making what
is called cobalt-moly catalysts, it is widely used in hydrodesulfurization (HDS) processes.
The nickel sulfide (NiS)-promoted version is used for hydrodenitrogenation (HDN) as well as
hydrodesulfurization (HDS). The closely related tungsten compound (WS 2 ) is used in commercial hydrocracking catalysts. Other sulfides (iron sulfide, FeS; chromium sulfide, Cr 2 S 3 ; and
vanadium sulfide, V 2 S 5 ) are also effective and used in some catalysts. A valuable alternative