Recycle Hydrogen System
After the reactor effluent’s gas and liquid phases are separated, the gas flows to the
recycle gas compressor. In some cases, the recycle gas will be sent first to an amine
scrubber to remove most of the hydrogen sulfide. Most often, the recycle gas
compressor is a separate centrifugal machine, but it could also be a part of the
makeup gas compressors, as additional cylinders in a reciprocating compressor.
The recycle gas compressor is designed to pump a large volume of gas at a
relatively low compression ratio.
Recycle Gas Scrubbing
The recycle gas stream will typically contain hydrogen sulfide. The hydrogen
sulfide reduces the reactor hydrogen partial pressure and thus suppresses the
catalyst activity. This effect is more pronounced with a high-sulfur feed stream,
and for the same feedstock, the heavier the cut, the higher the sulfur content.
Recycle gas scrubbing is typically included in the design of selective FCC naphtha
hydrotreaters to minimize the temperature severity of hydrotreating, which directly
impacts monoolefin hydrogenation and octane retention.
Reactor(s)
Once the feed and recycle gas have been heated to the desired temperature, the
reactants enter the reactor inlet. As the reactants flow downward through the catalyst
bed, various exothermic reactions generate heat, and the temperature increases.
Multiple catalyst beds with interbed quenching may be required, depending mostly
upon the feedstock quality and the product specifications. Specific reactor designs
will depend upon several variables. Reactor diameter is typically set by the crosssectional liquid flux. As the unit capacity increases, the reactor diameter increases to
the point where two parallel trains would be considered. Reactor height is a function
of the amount of catalyst and number of beds required. Depending on the expected
heat of reaction, cold recycle gas or cold product separator liquid is brought into the
reactor at the interbed quench points in order to cool the reactants and thus control the
reaction rate. Good distribution of reactants at the reactor inlet and at the top of each
subsequent catalyst bed is essential for optimum catalyst performance. There are
many companies that design proprietary internals: reactor inlet diffuser, top liquid
distribution tray, quench section which includes quench inlet assembly, quench and
reactant mixing device and redistribution tray, as well as the reactor outlet device, or
collector. Not all reactors are specified with all the internals described above.
Reactor Effluent Water Wash
Cooling of the reactor effluent is accomplished in the feed/effluent exchangers,
which are typically the shell-and-tube design. Final cooling of the reactor effluent is
obtained in air fin coolers and/or water trim coolers. Water is injected directly into
the stream before it enters the coolers to prevent the deposition of salts. The salts
would tend to corrode and foul the coolers. The sulfur and nitrogen contained in the
feed are converted to hydrogen sulfide and ammonia in the reactor. These two
reaction products combine to form ammonium salts that can solidify and precipitate
368
P. Kokayeff et al.
After the reactor effluent’s gas and liquid phases are separated, the gas flows to the
recycle gas compressor. In some cases, the recycle gas will be sent first to an amine
scrubber to remove most of the hydrogen sulfide. Most often, the recycle gas
compressor is a separate centrifugal machine, but it could also be a part of the
makeup gas compressors, as additional cylinders in a reciprocating compressor.
The recycle gas compressor is designed to pump a large volume of gas at a
relatively low compression ratio.
Recycle Gas Scrubbing
The recycle gas stream will typically contain hydrogen sulfide. The hydrogen
sulfide reduces the reactor hydrogen partial pressure and thus suppresses the
catalyst activity. This effect is more pronounced with a high-sulfur feed stream,
and for the same feedstock, the heavier the cut, the higher the sulfur content.
Recycle gas scrubbing is typically included in the design of selective FCC naphtha
hydrotreaters to minimize the temperature severity of hydrotreating, which directly
impacts monoolefin hydrogenation and octane retention.
Reactor(s)
Once the feed and recycle gas have been heated to the desired temperature, the
reactants enter the reactor inlet. As the reactants flow downward through the catalyst
bed, various exothermic reactions generate heat, and the temperature increases.
Multiple catalyst beds with interbed quenching may be required, depending mostly
upon the feedstock quality and the product specifications. Specific reactor designs
will depend upon several variables. Reactor diameter is typically set by the crosssectional liquid flux. As the unit capacity increases, the reactor diameter increases to
the point where two parallel trains would be considered. Reactor height is a function
of the amount of catalyst and number of beds required. Depending on the expected
heat of reaction, cold recycle gas or cold product separator liquid is brought into the
reactor at the interbed quench points in order to cool the reactants and thus control the
reaction rate. Good distribution of reactants at the reactor inlet and at the top of each
subsequent catalyst bed is essential for optimum catalyst performance. There are
many companies that design proprietary internals: reactor inlet diffuser, top liquid
distribution tray, quench section which includes quench inlet assembly, quench and
reactant mixing device and redistribution tray, as well as the reactor outlet device, or
collector. Not all reactors are specified with all the internals described above.
Reactor Effluent Water Wash
Cooling of the reactor effluent is accomplished in the feed/effluent exchangers,
which are typically the shell-and-tube design. Final cooling of the reactor effluent is
obtained in air fin coolers and/or water trim coolers. Water is injected directly into
the stream before it enters the coolers to prevent the deposition of salts. The salts
would tend to corrode and foul the coolers. The sulfur and nitrogen contained in the
feed are converted to hydrogen sulfide and ammonia in the reactor. These two
reaction products combine to form ammonium salts that can solidify and precipitate
368
P. Kokayeff et al.