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Hydrocracking
In the first type, the undesirable compounds are removed from the unconverted hydrocarbons
before the latter are charged to the cracking reactor. This type is called the two-stage process.
The other variety is often referred to as series-flow hydrocracker. This type uses a catalyst with an
increased tolerance toward nitrogen, both as ammonia and in organic form.
In the two-stage configuration, fresh feed is preheated by heat exchange with effluent from the
first reactor. It is combined with part of a not fresh gas–recycle gas mixture and passes through a
first reactor for desulfurnation/denitrogenation step. These reactions, as well as those of hydrocracking, which occurs to a limited extent in the first reactor, are exothermic. The catalyst inventory is
therefore divided among a number of fixed beds. Reaction temperatures are controlled by introducing part of the recycle gas as a quench medium between beds. The ensuing liquid is fractionated to
remove the product made in the first reactor. Unconverted material, with low nitrogen content and
free of ammonia, is taken as a bottom stream from the fractionation section. After heat exchange
with reactor effluent and mixing with heated recycle gas, it is sent to the second reactor. Here, most
of the hydrocracking reactions occur. Strongly acidic catalyst with a relatively low hydrogenation
activity (metal sulfides on, e.g., amorphous silica–alumina) is usually applied. As in the first reactor,
the exothermic nature of the process is controlled by recycle gas as the quench medium the catalyst
beds. Effluent from the second reactor is cooled and joins first-stage effluent for separation from
recycle gas and fractionation. The part of the second reactor feed that has remained unconverted is
recycled to the reactor. Feedstock is thereby totally converted to the product boiling range.
In the series-flow configuration, the principal difference is the elimination of first stage cooling
and gas–liquid separation and the ammonia removal step. The effluent from the first stage is mixed
with more recycle gas and routed direct to the inlet of the second reactor. In contrast with the amorphous catalyst of the two-stage process, the second reactor in series flow generally has a zeolite
catalyst, based on crystalline silica–alumina. As in the two-stage process, material not converted to
the product boiling range is recycled from the fractionation section.
Both two stage and series flow hydrocracking are flexible process: they may yield, in one mode of
operation, only naphtha and lighter products and, in a different mode, only gas oil and lighter products. In the naphtha mode, both configurations have comparable yield patterns. In modes for heavier
products, kerosene and gas oil, the two-stage process is more selective, because product made in the
first reactor is removed from the second reactor feed. In series flow operation, this product is partly
cracked into lighter products in the second reactor.
Hydrocracking reactor stages usually have similar process flow schemes. The oil feed is combined with a preheated mixture of makeup hydrogen and hydrogen-rich recycle gas and heated to
reactor inlet temperature via a feed-effluent exchanger and a reactor charge heater. The reactor
charge heater design philosophy is based on many years of safe operation with such two-phase
furnaces. The feed-effluent exchangers take advantage of special high-pressure exchanger design
features developed by Chevron engineers to give leak-free end closures. From the charge heater, the
partially vaporized feed enters to the top of the reactor. The catalyst is loaded in separate beds in
the reactor with facilities between the beds for quenching the reaction mix and ensuring good flow
distribution through the catalyst.
The reactor effluent is cooled through a variety of heat exchangers including the feed-effluent
exchanger and one or more air coolers. De-aerated condensate is injected into the first-stage reactor
effluent before the final air cooler in order to remove ammonia and some of the hydrogen sulfide.
This prevents solid ammonium bisulfide from depositing in the system. A body of expertise in the
field of materials selection for hydrocracker cooling trains is quite important for proper design.
The reactor effluent leaving the air cooler is separated into hydrogen-rich recycle gas, a sour
water stream, and a hydrocarbon liquid stream in the high-pressure separator. The sour water effluent
stream is often then sent to a plant for ammonia recovery and for purification, so water can be recycled back to the hydrocracker. The hydrocarbon-rich stream is pressure reduced and fed to the distillation section after light products are flashed off in a low-pressure separator. The hydrogen-rich
gas stream from the high-pressure separator is recycled back to the reactor feed by using a recycle
Hydrocracking
In the first type, the undesirable compounds are removed from the unconverted hydrocarbons
before the latter are charged to the cracking reactor. This type is called the two-stage process.
The other variety is often referred to as series-flow hydrocracker. This type uses a catalyst with an
increased tolerance toward nitrogen, both as ammonia and in organic form.
In the two-stage configuration, fresh feed is preheated by heat exchange with effluent from the
first reactor. It is combined with part of a not fresh gas–recycle gas mixture and passes through a
first reactor for desulfurnation/denitrogenation step. These reactions, as well as those of hydrocracking, which occurs to a limited extent in the first reactor, are exothermic. The catalyst inventory is
therefore divided among a number of fixed beds. Reaction temperatures are controlled by introducing part of the recycle gas as a quench medium between beds. The ensuing liquid is fractionated to
remove the product made in the first reactor. Unconverted material, with low nitrogen content and
free of ammonia, is taken as a bottom stream from the fractionation section. After heat exchange
with reactor effluent and mixing with heated recycle gas, it is sent to the second reactor. Here, most
of the hydrocracking reactions occur. Strongly acidic catalyst with a relatively low hydrogenation
activity (metal sulfides on, e.g., amorphous silica–alumina) is usually applied. As in the first reactor,
the exothermic nature of the process is controlled by recycle gas as the quench medium the catalyst
beds. Effluent from the second reactor is cooled and joins first-stage effluent for separation from
recycle gas and fractionation. The part of the second reactor feed that has remained unconverted is
recycled to the reactor. Feedstock is thereby totally converted to the product boiling range.
In the series-flow configuration, the principal difference is the elimination of first stage cooling
and gas–liquid separation and the ammonia removal step. The effluent from the first stage is mixed
with more recycle gas and routed direct to the inlet of the second reactor. In contrast with the amorphous catalyst of the two-stage process, the second reactor in series flow generally has a zeolite
catalyst, based on crystalline silica–alumina. As in the two-stage process, material not converted to
the product boiling range is recycled from the fractionation section.
Both two stage and series flow hydrocracking are flexible process: they may yield, in one mode of
operation, only naphtha and lighter products and, in a different mode, only gas oil and lighter products. In the naphtha mode, both configurations have comparable yield patterns. In modes for heavier
products, kerosene and gas oil, the two-stage process is more selective, because product made in the
first reactor is removed from the second reactor feed. In series flow operation, this product is partly
cracked into lighter products in the second reactor.
Hydrocracking reactor stages usually have similar process flow schemes. The oil feed is combined with a preheated mixture of makeup hydrogen and hydrogen-rich recycle gas and heated to
reactor inlet temperature via a feed-effluent exchanger and a reactor charge heater. The reactor
charge heater design philosophy is based on many years of safe operation with such two-phase
furnaces. The feed-effluent exchangers take advantage of special high-pressure exchanger design
features developed by Chevron engineers to give leak-free end closures. From the charge heater, the
partially vaporized feed enters to the top of the reactor. The catalyst is loaded in separate beds in
the reactor with facilities between the beds for quenching the reaction mix and ensuring good flow
distribution through the catalyst.
The reactor effluent is cooled through a variety of heat exchangers including the feed-effluent
exchanger and one or more air coolers. De-aerated condensate is injected into the first-stage reactor
effluent before the final air cooler in order to remove ammonia and some of the hydrogen sulfide.
This prevents solid ammonium bisulfide from depositing in the system. A body of expertise in the
field of materials selection for hydrocracker cooling trains is quite important for proper design.
The reactor effluent leaving the air cooler is separated into hydrogen-rich recycle gas, a sour
water stream, and a hydrocarbon liquid stream in the high-pressure separator. The sour water effluent
stream is often then sent to a plant for ammonia recovery and for purification, so water can be recycled back to the hydrocracker. The hydrocarbon-rich stream is pressure reduced and fed to the distillation section after light products are flashed off in a low-pressure separator. The hydrogen-rich
gas stream from the high-pressure separator is recycled back to the reactor feed by using a recycle