565
Hydrotreating and Desulfurization
streams will contain a range of aromatic compounds that been to be removed to enable many of
the product streams to meet product specifications.
Of the aromatic constituents, the polycyclic aromatics are first partially hydrogenated before
cracking of the aromatic nucleus takes place. The sulfur and nitrogen atoms are converted to hydrogen sulfide and ammonia, but a more important role of the hydrogenation is probably to hydrogenate
the coke precursors rapidly and prevent their conversion to coke.
Hydrotreating catalysts are usually cobalt plus molybdenum or nickel plus molybdenum (in the
sulfide) forms, impregnated on an alumina base. The hydrotreating operating conditions (1000–
2000 psi hydrogen and about 370°C, 700°F) are such that appreciable hydrogenation of aromatics
does not occur. The desulfurization reactions are invariably accompanied by small amounts of
hydrogenation and hydrocracking, the extent of which depends on the nature of the feedstock and
the severity of desulfurization.
One of the problems in the processing of high-sulfur and high-nitrogen feeds is the large quantity of hydrogen sulfide (H 2 S) and ammonia (NH 3 ) that are produced. Substantial removal of both
compounds from the recycle gas can be achieved by the injection of water in which, under the highpressure conditions employed, both hydrogen sulfide and ammonia are very soluble compared with
hydrogen and hydrocarbon gases. The solution is processed in a separate unit for the recovery of
anhydrous ammonia and hydrogen sulfide.
Hydrotreating is carried out by charging the feed to the reactor, together with a portion of all the
hydrogen produced in the catalytic reformer. Suitable catalysts are tungsten-nickel sulfide, cobalt–
molybdenum–alumina, nickel oxide–silica–alumina, and platinum–alumina. Most processes
employ cobalt–molybdenum catalysts, which generally contain about 10% by weight molybdenum
oxide and <1% by weight cobalt oxide supported on alumina. The temperatures employed are in the
range of 300°C–345°C (570°F–850°F), and the hydrogen pressures are about 500–1000 psi.
The reaction generally takes place in the vapor phase but, depending on the application, may
be a mixed-phase reaction. The reaction products are cooled in a heat exchanger and led to a
high-pressure separator where hydrogen gas is separated for recycling. Liquid products from
the high-pressure separator flow to a low-pressure separator (stabilizer) where dissolved light
gases are removed. The product may then be fed to a reforming or cracking unit if desired.
Generally, it is more economical to hydrotreat high-sulfur feedstocks before catalytic cracking
than to hydrotreat the products from catalytic cracking. The advantages are: (1) the products require
less finishing, (2) sulfur is removed from the catalytic cracking feedstock, and corrosion is reduced
in the cracking unit, and (3) coke formation during cracking is reduced and higher conversions
result, and the catalytic cracking quality of the gas oil fraction is improved.
Although hydrocracking will occur during hydrotreating, attempts are made to minimize such
effects but the degree of cracking is dependent on the nature of the feedstock. For example, decalin (decahydronaphthalene) cracks more readily than the corresponding paraffin analog, n-decane,
[CH 3 (CH 2 ) 8 CH 3 ] to give higher iso-paraffin to n-paraffin product ratios than those obtained from the
paraffin. A large yield of single-ring naphthenes is also produced, and these are resistant to further
hydrocracking and contain a higher than equilibrium ratio of methylcyclopentane to cyclohexane.
When applied to residua the hydrotreating processes can be used for processes such as: (1) fuel
oil desulfurization and (2) residuum hydrogenation, which is accompanied by hydrodesulfurization,
hydrodenitrogenation, as well as partial conversion to produce products suitable as feedstocks for
other processes, such as catalytic cracking.
One of the chief problems with hydroprocessing residua is the deposition of metals, in particular vanadium, on the catalyst. It is not possible to remove vanadium from the catalyst, which
must therefore be replaced when deactivated, and the time taken for catalyst replacement can
significantly reduce the unit time efficiency. Fixed-bed catalysts tend to plug owing to solids in
the feed or carbon deposits when processing residual feeds. As mentioned previously, the highly
exothermic reaction at high conversion gives difficult reactor design problems in heat removal and
temperature control.
Hydrotreating and Desulfurization
streams will contain a range of aromatic compounds that been to be removed to enable many of
the product streams to meet product specifications.
Of the aromatic constituents, the polycyclic aromatics are first partially hydrogenated before
cracking of the aromatic nucleus takes place. The sulfur and nitrogen atoms are converted to hydrogen sulfide and ammonia, but a more important role of the hydrogenation is probably to hydrogenate
the coke precursors rapidly and prevent their conversion to coke.
Hydrotreating catalysts are usually cobalt plus molybdenum or nickel plus molybdenum (in the
sulfide) forms, impregnated on an alumina base. The hydrotreating operating conditions (1000–
2000 psi hydrogen and about 370°C, 700°F) are such that appreciable hydrogenation of aromatics
does not occur. The desulfurization reactions are invariably accompanied by small amounts of
hydrogenation and hydrocracking, the extent of which depends on the nature of the feedstock and
the severity of desulfurization.
One of the problems in the processing of high-sulfur and high-nitrogen feeds is the large quantity of hydrogen sulfide (H 2 S) and ammonia (NH 3 ) that are produced. Substantial removal of both
compounds from the recycle gas can be achieved by the injection of water in which, under the highpressure conditions employed, both hydrogen sulfide and ammonia are very soluble compared with
hydrogen and hydrocarbon gases. The solution is processed in a separate unit for the recovery of
anhydrous ammonia and hydrogen sulfide.
Hydrotreating is carried out by charging the feed to the reactor, together with a portion of all the
hydrogen produced in the catalytic reformer. Suitable catalysts are tungsten-nickel sulfide, cobalt–
molybdenum–alumina, nickel oxide–silica–alumina, and platinum–alumina. Most processes
employ cobalt–molybdenum catalysts, which generally contain about 10% by weight molybdenum
oxide and <1% by weight cobalt oxide supported on alumina. The temperatures employed are in the
range of 300°C–345°C (570°F–850°F), and the hydrogen pressures are about 500–1000 psi.
The reaction generally takes place in the vapor phase but, depending on the application, may
be a mixed-phase reaction. The reaction products are cooled in a heat exchanger and led to a
high-pressure separator where hydrogen gas is separated for recycling. Liquid products from
the high-pressure separator flow to a low-pressure separator (stabilizer) where dissolved light
gases are removed. The product may then be fed to a reforming or cracking unit if desired.
Generally, it is more economical to hydrotreat high-sulfur feedstocks before catalytic cracking
than to hydrotreat the products from catalytic cracking. The advantages are: (1) the products require
less finishing, (2) sulfur is removed from the catalytic cracking feedstock, and corrosion is reduced
in the cracking unit, and (3) coke formation during cracking is reduced and higher conversions
result, and the catalytic cracking quality of the gas oil fraction is improved.
Although hydrocracking will occur during hydrotreating, attempts are made to minimize such
effects but the degree of cracking is dependent on the nature of the feedstock. For example, decalin (decahydronaphthalene) cracks more readily than the corresponding paraffin analog, n-decane,
[CH 3 (CH 2 ) 8 CH 3 ] to give higher iso-paraffin to n-paraffin product ratios than those obtained from the
paraffin. A large yield of single-ring naphthenes is also produced, and these are resistant to further
hydrocracking and contain a higher than equilibrium ratio of methylcyclopentane to cyclohexane.
When applied to residua the hydrotreating processes can be used for processes such as: (1) fuel
oil desulfurization and (2) residuum hydrogenation, which is accompanied by hydrodesulfurization,
hydrodenitrogenation, as well as partial conversion to produce products suitable as feedstocks for
other processes, such as catalytic cracking.
One of the chief problems with hydroprocessing residua is the deposition of metals, in particular vanadium, on the catalyst. It is not possible to remove vanadium from the catalyst, which
must therefore be replaced when deactivated, and the time taken for catalyst replacement can
significantly reduce the unit time efficiency. Fixed-bed catalysts tend to plug owing to solids in
the feed or carbon deposits when processing residual feeds. As mentioned previously, the highly
exothermic reaction at high conversion gives difficult reactor design problems in heat removal and
temperature control.