584
The Chemistry and Technology of Petroleum
low to moderate porosity (from 12 D pore diameter with crystalline acidic components to 50 D 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 >80 D 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.
Aromatic hydrogenation in petroleum refining may be carried out over supported metal or
metal sulfide catalysts depending on the sulfur and nitrogen levels in the feedstock. For hydrorefining of feedstocks that contain appreciable concentrations of sulfur and nitrogen, sulfided nickel–
molybdenum (Ni–Mo), nickel–tungsten (Ni–W), or cobalt–molybdenum (Co–Mo) on alumina
(γ-Al 2 O 3 ) catalysts are generally used, whereas supported noble metal catalysts have been used
for sulfur- and nitrogen-free feedstocks. Catalysts containing noble metals on Y-zeolites have been
reported to be more sulfur tolerant than those on other supports (Jacobs, 1986). Within the series of
cobalt-promoted or nickel-promoted group VI metal (Mo or W) sulfides supported on γ-Al 2 O 3 , the
ranking for hydrogenation is
Ni W Ni Mo Co Mo Co W
–
–
–
–
>
>
>
Nickel–tungsten (Ni–W) and nickel–molybdenum (Ni–Mo) on Al2O3 catalysts are widely used to
reduce sulfur, nitrogen, and aromatics levels in petroleum fractions by hydrotreating.
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 to the base metal sulfides is palladium sulfide (PdS). Although it is expensive, palladium sulfide forms the basis for several very
active catalysts.
The life of a catalyst used to hydrotreat petroleum residua is dependent on the rate of carbon
deposition and the rate at which organometallic compounds decompose and form metal sulfides on
the surface. Several different metal complexes exist in the asphaltene fraction of the residuum and
an explicit reaction mechanism of decomposition that would be a perfect fit for all of the compounds
is not possible. However, in general, terms, the reaction can be described as hydrogen (A) dissolved
in the feedstock contacting an organometallic compound (B) at the surface of the hydrotreating
catalyst and producing a metal sulfide (C) and a hydrocarbon (D):
A B C D
+ Æ +
Different rates of reaction may occur with various types and concentrations of metallic compounds. For example, a medium metal-content feedstock will generally have a lower rate of demetallization compared to high-metal content feedstock. And, although individual organometallic
compounds decompose according to both first- and second-order rate expressions, for reactor
design, a second-order rate expression is applicable to the decomposition of residuum as a whole.
Obviously, choice of hydrogenation catalyst depends on what the catalyst designer wishes to
accomplish. In catalysts to make gasoline, for instance, vigorous cracking is needed to convert a
large fraction of the feed to the kinds of molecules that will make a good gasoline blending stock.
For this vigorous cracking, a vigorous hydrogenation component is needed. Since palladium is the
The Chemistry and Technology of Petroleum
low to moderate porosity (from 12 D pore diameter with crystalline acidic components to 50 D 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 >80 D 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.
Aromatic hydrogenation in petroleum refining may be carried out over supported metal or
metal sulfide catalysts depending on the sulfur and nitrogen levels in the feedstock. For hydrorefining of feedstocks that contain appreciable concentrations of sulfur and nitrogen, sulfided nickel–
molybdenum (Ni–Mo), nickel–tungsten (Ni–W), or cobalt–molybdenum (Co–Mo) on alumina
(γ-Al 2 O 3 ) catalysts are generally used, whereas supported noble metal catalysts have been used
for sulfur- and nitrogen-free feedstocks. Catalysts containing noble metals on Y-zeolites have been
reported to be more sulfur tolerant than those on other supports (Jacobs, 1986). Within the series of
cobalt-promoted or nickel-promoted group VI metal (Mo or W) sulfides supported on γ-Al 2 O 3 , the
ranking for hydrogenation is
Ni W Ni Mo Co Mo Co W
–
–
–
–
>
>
>
Nickel–tungsten (Ni–W) and nickel–molybdenum (Ni–Mo) on Al2O3 catalysts are widely used to
reduce sulfur, nitrogen, and aromatics levels in petroleum fractions by hydrotreating.
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 to the base metal sulfides is palladium sulfide (PdS). Although it is expensive, palladium sulfide forms the basis for several very
active catalysts.
The life of a catalyst used to hydrotreat petroleum residua is dependent on the rate of carbon
deposition and the rate at which organometallic compounds decompose and form metal sulfides on
the surface. Several different metal complexes exist in the asphaltene fraction of the residuum and
an explicit reaction mechanism of decomposition that would be a perfect fit for all of the compounds
is not possible. However, in general, terms, the reaction can be described as hydrogen (A) dissolved
in the feedstock contacting an organometallic compound (B) at the surface of the hydrotreating
catalyst and producing a metal sulfide (C) and a hydrocarbon (D):
A B C D
+ Æ +
Different rates of reaction may occur with various types and concentrations of metallic compounds. For example, a medium metal-content feedstock will generally have a lower rate of demetallization compared to high-metal content feedstock. And, although individual organometallic
compounds decompose according to both first- and second-order rate expressions, for reactor
design, a second-order rate expression is applicable to the decomposition of residuum as a whole.
Obviously, choice of hydrogenation catalyst depends on what the catalyst designer wishes to
accomplish. In catalysts to make gasoline, for instance, vigorous cracking is needed to convert a
large fraction of the feed to the kinds of molecules that will make a good gasoline blending stock.
For this vigorous cracking, a vigorous hydrogenation component is needed. Since palladium is the