573
Hydrotreating and Desulfurization
and the metals have been reported to interfere with the combustion of carbon and sulfur, catalyzing the conversion of sulfur dioxide (SO 2 ) to sulfate (SO 4
2− ) that has a permanent poisoning effect
on the catalyst.
Fixed-bed hydrodesulfurization units are generally used for distillate hydrodesulfurization and
may also be used for residuum hydrodesulfurization but require special precautions in processing.
The residuum must undergo two-stage electrostatic desalting so that salt deposits do not plug the
inlet to the first catalyst bed and the residuum must be low in vanadium and nickel content to avoid
plugging the beds with metal deposits. Hence the need for a guard bed in residuum hydrodesulfurization reactors.
During the operation of a fixed-bed reactor, contaminants entering with fresh feed are filtered
out and fill the voids between catalyst particles in the bed. The buildup of contaminants in the bed
can result in the channeling of reactants through the bed and reducing the hydrodesulfurization efficiency. As the flow pattern becomes distorted or restricted the pressure drop throughout the catalyst
bed increases. If the pressure drop becomes high enough physical damage to the reactor internals
can result. When high-pressure drops are observed throughout any portion of the reactor, the unit is
shut down and the catalyst bed is skimmed and refilled.
With fixed-bed reactors, a balance must be reached between reaction rate and pressure drop
across the catalyst bed. As catalyst particle size is decreased, the desulfurization reaction rate
increases but so does the pressure drop across the catalyst bed. Expanded-bed reactors do not have
this limitation and small 1/32 in. (0.8 mm) extrudate catalysts or fine catalysts may be used without
increasing the pressure drop.
21.2.6.2 Upflow Expanded-Bed Reactor
Expanded-bed reactors are applicable to distillates, but are commercially used for very heavy,
high metals, and/or dirty feedstocks having extraneous fine solids material. They operate in
such a way that the catalyst is in an expanded state so that the extraneous solids pass through
the catalyst bed without plugging. They are isothermal, which conveniently handles the hightemperature exotherms associated with high hydrogen consumptions. Since the catalyst is in an
expanded state of motion, it is possible to treat the catalyst as a fluid and to withdraw and add
catalyst during operation.
Expanded beds of catalyst are referred to as particulate fluidized insofar as the feedstock and
hydrogen flow upward through an expanded bed of catalyst with each catalyst particle in independent motion. Thus, the catalyst migrates throughout the entire reactor bed. Expanded-bed reactors are mathematically modeled as back-mix reactors with the entire catalyst bed at one uniform
temperature. Spent catalyst may be withdrawn and replaced with fresh catalyst on a daily basis.
Daily catalyst addition and withdrawal eliminate the need for costly shutdowns to change out
catalyst and also result in a constant equilibrium catalyst activity and product quality. The catalyst is withdrawn daily and has a vanadium, nickel, and carbon content that is representative on a
macro-scale of what is found throughout the entire reactor. On a micro-scale, individual catalyst
particles have ages from that of fresh catalyst to as old as the initial catalyst charge to the unit
but the catalyst particles of each age group are so well dispersed in the reactor that the reactor
contents appear uniform.
In the unit, the feedstock and hydrogen recycle gas enter the bottom of the reactor, pass up
through the expanded catalyst bed, and leave from the top of the reactor. Commercial expanded-bed
reactors normally operate with 1/32 in. (0.8 mm) extrudate catalysts that provide a higher rate of
desulfurization than the larger catalyst particles used in fixed-bed reactors. With extrudate catalysts
of this size, the upward liquid velocity based on fresh feedstock is not sufficient to keep the catalyst
particles in an expanded state. Therefore, for each part of the fresh feed, several parts of product oil
are taken from the top of the reactor, recycled internally through a large vertical pipe to the bottom
of the reactor, and pumped back up through the expanded catalyst bed. The amount of catalyst bed
expansion is controlled by the recycle of product oil back up through the catalyst bed.
Hydrotreating and Desulfurization
and the metals have been reported to interfere with the combustion of carbon and sulfur, catalyzing the conversion of sulfur dioxide (SO 2 ) to sulfate (SO 4
2− ) that has a permanent poisoning effect
on the catalyst.
Fixed-bed hydrodesulfurization units are generally used for distillate hydrodesulfurization and
may also be used for residuum hydrodesulfurization but require special precautions in processing.
The residuum must undergo two-stage electrostatic desalting so that salt deposits do not plug the
inlet to the first catalyst bed and the residuum must be low in vanadium and nickel content to avoid
plugging the beds with metal deposits. Hence the need for a guard bed in residuum hydrodesulfurization reactors.
During the operation of a fixed-bed reactor, contaminants entering with fresh feed are filtered
out and fill the voids between catalyst particles in the bed. The buildup of contaminants in the bed
can result in the channeling of reactants through the bed and reducing the hydrodesulfurization efficiency. As the flow pattern becomes distorted or restricted the pressure drop throughout the catalyst
bed increases. If the pressure drop becomes high enough physical damage to the reactor internals
can result. When high-pressure drops are observed throughout any portion of the reactor, the unit is
shut down and the catalyst bed is skimmed and refilled.
With fixed-bed reactors, a balance must be reached between reaction rate and pressure drop
across the catalyst bed. As catalyst particle size is decreased, the desulfurization reaction rate
increases but so does the pressure drop across the catalyst bed. Expanded-bed reactors do not have
this limitation and small 1/32 in. (0.8 mm) extrudate catalysts or fine catalysts may be used without
increasing the pressure drop.
21.2.6.2 Upflow Expanded-Bed Reactor
Expanded-bed reactors are applicable to distillates, but are commercially used for very heavy,
high metals, and/or dirty feedstocks having extraneous fine solids material. They operate in
such a way that the catalyst is in an expanded state so that the extraneous solids pass through
the catalyst bed without plugging. They are isothermal, which conveniently handles the hightemperature exotherms associated with high hydrogen consumptions. Since the catalyst is in an
expanded state of motion, it is possible to treat the catalyst as a fluid and to withdraw and add
catalyst during operation.
Expanded beds of catalyst are referred to as particulate fluidized insofar as the feedstock and
hydrogen flow upward through an expanded bed of catalyst with each catalyst particle in independent motion. Thus, the catalyst migrates throughout the entire reactor bed. Expanded-bed reactors are mathematically modeled as back-mix reactors with the entire catalyst bed at one uniform
temperature. Spent catalyst may be withdrawn and replaced with fresh catalyst on a daily basis.
Daily catalyst addition and withdrawal eliminate the need for costly shutdowns to change out
catalyst and also result in a constant equilibrium catalyst activity and product quality. The catalyst is withdrawn daily and has a vanadium, nickel, and carbon content that is representative on a
macro-scale of what is found throughout the entire reactor. On a micro-scale, individual catalyst
particles have ages from that of fresh catalyst to as old as the initial catalyst charge to the unit
but the catalyst particles of each age group are so well dispersed in the reactor that the reactor
contents appear uniform.
In the unit, the feedstock and hydrogen recycle gas enter the bottom of the reactor, pass up
through the expanded catalyst bed, and leave from the top of the reactor. Commercial expanded-bed
reactors normally operate with 1/32 in. (0.8 mm) extrudate catalysts that provide a higher rate of
desulfurization than the larger catalyst particles used in fixed-bed reactors. With extrudate catalysts
of this size, the upward liquid velocity based on fresh feedstock is not sufficient to keep the catalyst
particles in an expanded state. Therefore, for each part of the fresh feed, several parts of product oil
are taken from the top of the reactor, recycled internally through a large vertical pipe to the bottom
of the reactor, and pumped back up through the expanded catalyst bed. The amount of catalyst bed
expansion is controlled by the recycle of product oil back up through the catalyst bed.