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The Chemistry and Technology of Petroleum
to a designated degree of hydrocracking) also cause an overall increase in hydrogen consumption
when middle distillates are employed as feedstocks in place of the naphtha.
High-boiling distillates, such as the atmospheric and vacuum gas oils, are not usually produced
as a refinery product but merely serve as feedstocks to other processes for conversion to lowerboiling materials. For example, gas oils can be desulfurized to remove more than 80% of the sulfur
originally in the gas oil with some conversion of the gas oil to lower-boiling materials. The treated
gas oil (which has a reduced carbon residue as well as lower sulfur and nitrogen contents relative to
the untreated material) can then be converted to lower-boiling products in, say, a catalytic cracker
where an improved catalyst life and volumetric yield may be noted.
The conditions used for the hydrodesulfurization of gas oil may be somewhat more severe than
the conditions employed for the hydrodesulfurization of middle distillates with, of course, the feedstock in the liquid phase.
In summary, the hydrodesulfurization of the low-, middle-, and high-boiling distillates can be
achieved quite conveniently using a variety of processes. One major advantage of this type of feedstock is that the catalyst does not become poisoned by metal contaminants in the feedstock since
only negligible amounts of these contaminants will be present. Thus, the catalyst may be regenerated several times and on-stream times between catalyst regeneration (while varying with the process conditions and application) may be of the order of 3–4 years.
21.2.6 reACtors
Reactor configurations within petroleum hydroprocessing units may include catalysts beds that are
fixed or moving (Speight, 2000; Ancheyta, 2007). Most hydroprocessing reactors are fixed-bed
reactors. Hydroprocessing units with fixed-bed reactors must be shut down to remove the spent
catalyst when catalyst activity declines below an acceptable level (due to the accumulation of coke,
metals, and other contaminants). There are also hydroprocessing reactors with moving or ebullating
catalyst beds.
21.2.6.1 Downflow Fixed-Bed Reactor
The reactor design commonly used in hydrodesulfurization of distillates is the fixed-bed reactor
design in which the feedstock enters at the top of the reactor and the product leaves at the bottom
of the reactor (Figure 21.5). The catalyst remains in a stationary position (fixed-bed) with hydrogen
and petroleum feedstock passing in a downflow direction through the catalyst bed. The hydrodesulfurization reaction is exothermic and the temperature rises from the inlet to the outlet of each
catalyst bed. With a high hydrogen consumption and subsequent large temperature rise, the reaction
mixture can be quenched with cold recycled gas at intermediate points in the reactor system. This
is achieved by dividing the catalyst charge into a series of catalyst beds and the effluent from each
catalyst bed is quenched to the inlet temperature of the next catalyst bed.
The extent of desulfurization is controlled by raising the inlet temperature to each catalyst bed
to maintain constant catalyst activity over the course of the process. Fixed-bed reactors are mathematically modeled as plug-flow reactors with very little back mixing in the catalyst beds. The
first catalyst bed is poisoned with vanadium and nickel at the inlet to the bed and may be a cheaper
catalyst (guard bed). As the catalyst is poisoned in front of the bed, the temperature exotherm
moves down the bed and the activity of the entire catalyst charge declines, thus requiring a raise
in the reactor temperature over the course of the process sequence. After catalyst regeneration, the
reactors are opened and inspected, and the high metal content catalyst layer at the inlet to the first
bed may be discarded and replaced with fresh catalyst. The catalyst loses activity after a series of
regenerations and, consequently, after a series of regenerations it is necessary to replace the complete catalyst charge. In the case of very high metal content feedstocks (such as residua), it is often
necessary to replace the entire catalyst charge rather than to regenerate it. This is due to the fact
that the metal contaminants cannot be removed by economical means during rapid regeneration
The Chemistry and Technology of Petroleum
to a designated degree of hydrocracking) also cause an overall increase in hydrogen consumption
when middle distillates are employed as feedstocks in place of the naphtha.
High-boiling distillates, such as the atmospheric and vacuum gas oils, are not usually produced
as a refinery product but merely serve as feedstocks to other processes for conversion to lowerboiling materials. For example, gas oils can be desulfurized to remove more than 80% of the sulfur
originally in the gas oil with some conversion of the gas oil to lower-boiling materials. The treated
gas oil (which has a reduced carbon residue as well as lower sulfur and nitrogen contents relative to
the untreated material) can then be converted to lower-boiling products in, say, a catalytic cracker
where an improved catalyst life and volumetric yield may be noted.
The conditions used for the hydrodesulfurization of gas oil may be somewhat more severe than
the conditions employed for the hydrodesulfurization of middle distillates with, of course, the feedstock in the liquid phase.
In summary, the hydrodesulfurization of the low-, middle-, and high-boiling distillates can be
achieved quite conveniently using a variety of processes. One major advantage of this type of feedstock is that the catalyst does not become poisoned by metal contaminants in the feedstock since
only negligible amounts of these contaminants will be present. Thus, the catalyst may be regenerated several times and on-stream times between catalyst regeneration (while varying with the process conditions and application) may be of the order of 3–4 years.
21.2.6 reACtors
Reactor configurations within petroleum hydroprocessing units may include catalysts beds that are
fixed or moving (Speight, 2000; Ancheyta, 2007). Most hydroprocessing reactors are fixed-bed
reactors. Hydroprocessing units with fixed-bed reactors must be shut down to remove the spent
catalyst when catalyst activity declines below an acceptable level (due to the accumulation of coke,
metals, and other contaminants). There are also hydroprocessing reactors with moving or ebullating
catalyst beds.
21.2.6.1 Downflow Fixed-Bed Reactor
The reactor design commonly used in hydrodesulfurization of distillates is the fixed-bed reactor
design in which the feedstock enters at the top of the reactor and the product leaves at the bottom
of the reactor (Figure 21.5). The catalyst remains in a stationary position (fixed-bed) with hydrogen
and petroleum feedstock passing in a downflow direction through the catalyst bed. The hydrodesulfurization reaction is exothermic and the temperature rises from the inlet to the outlet of each
catalyst bed. With a high hydrogen consumption and subsequent large temperature rise, the reaction
mixture can be quenched with cold recycled gas at intermediate points in the reactor system. This
is achieved by dividing the catalyst charge into a series of catalyst beds and the effluent from each
catalyst bed is quenched to the inlet temperature of the next catalyst bed.
The extent of desulfurization is controlled by raising the inlet temperature to each catalyst bed
to maintain constant catalyst activity over the course of the process. Fixed-bed reactors are mathematically modeled as plug-flow reactors with very little back mixing in the catalyst beds. The
first catalyst bed is poisoned with vanadium and nickel at the inlet to the bed and may be a cheaper
catalyst (guard bed). As the catalyst is poisoned in front of the bed, the temperature exotherm
moves down the bed and the activity of the entire catalyst charge declines, thus requiring a raise
in the reactor temperature over the course of the process sequence. After catalyst regeneration, the
reactors are opened and inspected, and the high metal content catalyst layer at the inlet to the first
bed may be discarded and replaced with fresh catalyst. The catalyst loses activity after a series of
regenerations and, consequently, after a series of regenerations it is necessary to replace the complete catalyst charge. In the case of very high metal content feedstocks (such as residua), it is often
necessary to replace the entire catalyst charge rather than to regenerate it. This is due to the fact
that the metal contaminants cannot be removed by economical means during rapid regeneration