581
Hydrotreating and Desulfurization
and ebullating catalyst systems with continuous catalyst removal capability, and the Shell Process
(a conversion process) that may involve the use of a bunker flow reactor ahead of the main reactors
to allow periodic changeover of catalyst.
The primary objective in most of the residue desulfurization processes is to remove sulfur with
minimum consumption of hydrogen. Substantial percentages of nitrogen, oxygen, and metals are
also removed from the feedstock. However, complete elimination of other reactions is not feasible
and, in addition, hydrocracking, thermal cracking, and aromatic saturation reactions occur to some
extent. Certain processes, such as the H-Oil process (Chapter 22), which uses a single-stage or a
two-stage reactor, and the LC-Fining process (Chapter 22), which uses an expanded bed reactor, can
be designed to accomplish high yields of lower-boiling distillates at the expense of desulfurization.
Removal of nitrogen is much more difficult than removal of sulfur. For example, nitrogen removal
may be only about 25%–30% when sulfur removal is at a 75%–80% level. Metals are removed from
the feedstock in substantial quantities and are mainly deposited on the catalyst surface and exist as
metal sulfides at processing conditions. As these deposits accumulate, the catalyst pores eventually
become blocked and inaccessible, thus catalyst activity is lost.
Desulfurization of residua is considerably more difficult than desulfurization of distillates
(including vacuum gas oil) because many more contaminants are present and very large, complex
molecules are involved. The most difficult portion of feed in residue desulfurization is the asphaltene fraction that forms coke readily and it is essential that these large molecules be prevented from
condensing with each other to form coke, which deactivates the catalyst. This is accomplished
by selection of proper catalysts, use of adequate hydrogen partial pressure, and assuring intimate
contact of the hydrogen-rich gases and oil molecules in the process design.
21.5 CATALYSTS
Catalysts used in hydrotreating reactors include cobalt and molybdenum oxides on alumina, nickel oxide,
nickel thiomolybdate, tungsten and nickel sulfides, and vanadium oxide. Cobalt– molybdenum and nickel–
molybdenum are the most commonly used catalysts for hydrotreating. Both types of catalyst remove
sulfur, nitrogen and other contaminants from petroleum feed. Cobalt–molybdenum catalysts, however,
are selective for sulfur removal, while nickel–molybdenum catalysts are selective for nitrogen removal.
Typically, hydrodesulfurization catalysts consist of metals impregnated on a porous alumina
support. Almost all of the surface area is found in the pores of the alumina (200–300 m 2 /g) and the
metals are dispersed in a thin layer over the entire alumina surface within the pores (Table 21.3).
This type of catalyst does display a huge catalytic surface for a small weight of catalyst. Cobalt
(Co), molybdenum (Mo), and nickel (Ni) are the most commonly used metals for desulfurization
catalysts. The catalysts are manufactured with the metals in an oxide state. In the active form they
are iii the sulfide state, which is obtained by sulfiding the catalyst either prior to use or with the feed
during actual use. Any catalyst that exhibits hydrogenation activity will catalyze hydrodesulfurization to some extent. However, the Group VIB metals (chromium, molybdenum, and tungsten) are
particularly active for desulfurization, especially when promoted with metals from the iron group
(iron, cobalt, nickel).
Furthermore, the increasing importance of hydrodesulfurization (HDS) and hydrodenitrogenation (HDN) in petroleum processing in order to produce clean-burning fuels has led to a surge of
research on the chemistry and engineering of heteroatom removal, with sulfur removal being the
most prominent area of research. Most of the earlier works are focused on (1) catalyst characterization by physical methods, (2) on low-pressure reaction studies of model compounds having relatively high reactivity, (3) on process development, or (4) on cobalt–molybdenum (Co–Mo) catalysts,
nickel–molybdenum catalysts (Ni–Mo), or nickel–tungsten (Ni–W) catalysts supported on alumina,
often doped by fluorine or phosphorus.
Hydrodesulfurization and demetallization occur simultaneously on the active sites within the
catalyst pore structure. Sulfur and nitrogen occurring in residua are converted to hydrogen sulfide
Hydrotreating and Desulfurization
and ebullating catalyst systems with continuous catalyst removal capability, and the Shell Process
(a conversion process) that may involve the use of a bunker flow reactor ahead of the main reactors
to allow periodic changeover of catalyst.
The primary objective in most of the residue desulfurization processes is to remove sulfur with
minimum consumption of hydrogen. Substantial percentages of nitrogen, oxygen, and metals are
also removed from the feedstock. However, complete elimination of other reactions is not feasible
and, in addition, hydrocracking, thermal cracking, and aromatic saturation reactions occur to some
extent. Certain processes, such as the H-Oil process (Chapter 22), which uses a single-stage or a
two-stage reactor, and the LC-Fining process (Chapter 22), which uses an expanded bed reactor, can
be designed to accomplish high yields of lower-boiling distillates at the expense of desulfurization.
Removal of nitrogen is much more difficult than removal of sulfur. For example, nitrogen removal
may be only about 25%–30% when sulfur removal is at a 75%–80% level. Metals are removed from
the feedstock in substantial quantities and are mainly deposited on the catalyst surface and exist as
metal sulfides at processing conditions. As these deposits accumulate, the catalyst pores eventually
become blocked and inaccessible, thus catalyst activity is lost.
Desulfurization of residua is considerably more difficult than desulfurization of distillates
(including vacuum gas oil) because many more contaminants are present and very large, complex
molecules are involved. The most difficult portion of feed in residue desulfurization is the asphaltene fraction that forms coke readily and it is essential that these large molecules be prevented from
condensing with each other to form coke, which deactivates the catalyst. This is accomplished
by selection of proper catalysts, use of adequate hydrogen partial pressure, and assuring intimate
contact of the hydrogen-rich gases and oil molecules in the process design.
21.5 CATALYSTS
Catalysts used in hydrotreating reactors include cobalt and molybdenum oxides on alumina, nickel oxide,
nickel thiomolybdate, tungsten and nickel sulfides, and vanadium oxide. Cobalt– molybdenum and nickel–
molybdenum are the most commonly used catalysts for hydrotreating. Both types of catalyst remove
sulfur, nitrogen and other contaminants from petroleum feed. Cobalt–molybdenum catalysts, however,
are selective for sulfur removal, while nickel–molybdenum catalysts are selective for nitrogen removal.
Typically, hydrodesulfurization catalysts consist of metals impregnated on a porous alumina
support. Almost all of the surface area is found in the pores of the alumina (200–300 m 2 /g) and the
metals are dispersed in a thin layer over the entire alumina surface within the pores (Table 21.3).
This type of catalyst does display a huge catalytic surface for a small weight of catalyst. Cobalt
(Co), molybdenum (Mo), and nickel (Ni) are the most commonly used metals for desulfurization
catalysts. The catalysts are manufactured with the metals in an oxide state. In the active form they
are iii the sulfide state, which is obtained by sulfiding the catalyst either prior to use or with the feed
during actual use. Any catalyst that exhibits hydrogenation activity will catalyze hydrodesulfurization to some extent. However, the Group VIB metals (chromium, molybdenum, and tungsten) are
particularly active for desulfurization, especially when promoted with metals from the iron group
(iron, cobalt, nickel).
Furthermore, the increasing importance of hydrodesulfurization (HDS) and hydrodenitrogenation (HDN) in petroleum processing in order to produce clean-burning fuels has led to a surge of
research on the chemistry and engineering of heteroatom removal, with sulfur removal being the
most prominent area of research. Most of the earlier works are focused on (1) catalyst characterization by physical methods, (2) on low-pressure reaction studies of model compounds having relatively high reactivity, (3) on process development, or (4) on cobalt–molybdenum (Co–Mo) catalysts,
nickel–molybdenum catalysts (Ni–Mo), or nickel–tungsten (Ni–W) catalysts supported on alumina,
often doped by fluorine or phosphorus.
Hydrodesulfurization and demetallization occur simultaneously on the active sites within the
catalyst pore structure. Sulfur and nitrogen occurring in residua are converted to hydrogen sulfide