574
The Chemistry and Technology of Petroleum
The expansion and turbulence of gas and oil passing upward through the expanded catalyst bed
are sufficient to cause almost complete random motion in the bed (particulate fluidized). This effect
produces the isothermal operation. It also causes almost complete back-mixing. Consequently, in
order to effect near complete sulfur removal (over 75%), it is necessary to operate with two or more
reactors in series. The ability to operate at a single temperature throughout the reactor or reactors,
and to operate at a selected optimum temperature rather than an increasing temperature from the
start to the end of the run, results in more effective use of the reactor and catalyst contents. When
all these factors are put together, that is, use of a smaller catalyst particle size, isothermal, fixed
temperature throughout run, back-mixing, daily catalyst addition, and constant product quality,
the reactor size required for an expanded bed is often smaller than that required for a fixed bed to
achieve the same product goals. This is generally true when the feeds have high initial boiling points
and/or the hydrogen consumption is very high.
21.2.6.3 Ebullating Bed Reactor
In ebullated bed hydroprocessing, the catalyst within the reactor bed is not fixed (Eccles, 1993).
In such a process, the hydrocarbon feed stream enters the bottom of the reactor and flows
upward through the catalyst—the catalyst is kept in suspension by the pressure of the fluid
feed. Ebullating bed reactors are capable of converting the most problematic feeds, such as
atmospheric residua, vacuum residua, heavy oil, and tar sand bitumen (all of which have a high
content of asphaltene constituents as well as metal constituents, sulfur constituents, and constituents ready to form sediment) to lower-boiling, more valuable products while simultaneously
removing the contaminants.
In the unit, the hydrogen-rich recycle gas is bubbled up through a mixture of oil and catalyst
particles which provides three-phase turbulent mixing, which is needed to ensure a uniform temperature distribution. At the top of the reactor, the catalyst is disengaged from the process fluids,
which are separated in downstream flash drums. Some of the catalyst is withdrawn and replaced
with fresh catalyst while the majority of the catalyst is returned to the reactor.
The function of the catalyst is to remove contaminants such as sulfur and nitrogen heteroatoms,
which accelerate the deactivation of the catalyst, while cracking (converting) the feed to lighter
products. Because ebullating bed reactors perform both hydrotreating and hydrocracking functions,
they are considered to be dual purpose reactors. Ebullating bed catalysts are made of pellets that are
<1 mm in size to facilitate suspension by the liquid phase in the reactor.
In contrast to fixed-bed hydrocracking units for vacuum gas oil, ebullating bed units are suitable
for processing residua and other heavy feedstocks, such as tar sand bitumen. The main advantages
are: (1) high conversion of atmospheric residue, up to 90% v/v, (2) better product quality than many
other residue conversion processes, especially delayed coking, and (3) long run length—catalyst life
does not limit these units since fresh catalyst is added and spent catalyst is removed continuously
and (subject only to mechanical issues) the units can typically run for a much longer time than fixed
bed units for the same heavy feedstock.
Examples of processes which use ebullated bed reactors are (1) H-Oil process, (2) LC-Fining
process, and (3) T-Star process.
21.2.6.4 Demetallization Reactor (Guard Bed Reactor)
The demetallization reactor (guard reactor, guard bed reactor) is a reactor that is placed in front of
hydrocracking reactors to remove contaminants, particularly metals, prior to hydrocracking. Such
reactors may employ an inexpensive catalyst to remove metals from expanded bed feed. Spent
demetallization catalyst can be loaded to more than 30% vanadium. A catalyst support having large
pores preferentially demetallized with a low degree of desulfurization.
Feedstocks that have relatively high metals contents (>300 ppm) substantially increase catalyst
consumption because the metals poison the catalyst, thereby requiring frequent catalyst replacement. The usual desulfurization catalysts are relatively expensive for these consumption rates but
The Chemistry and Technology of Petroleum
The expansion and turbulence of gas and oil passing upward through the expanded catalyst bed
are sufficient to cause almost complete random motion in the bed (particulate fluidized). This effect
produces the isothermal operation. It also causes almost complete back-mixing. Consequently, in
order to effect near complete sulfur removal (over 75%), it is necessary to operate with two or more
reactors in series. The ability to operate at a single temperature throughout the reactor or reactors,
and to operate at a selected optimum temperature rather than an increasing temperature from the
start to the end of the run, results in more effective use of the reactor and catalyst contents. When
all these factors are put together, that is, use of a smaller catalyst particle size, isothermal, fixed
temperature throughout run, back-mixing, daily catalyst addition, and constant product quality,
the reactor size required for an expanded bed is often smaller than that required for a fixed bed to
achieve the same product goals. This is generally true when the feeds have high initial boiling points
and/or the hydrogen consumption is very high.
21.2.6.3 Ebullating Bed Reactor
In ebullated bed hydroprocessing, the catalyst within the reactor bed is not fixed (Eccles, 1993).
In such a process, the hydrocarbon feed stream enters the bottom of the reactor and flows
upward through the catalyst—the catalyst is kept in suspension by the pressure of the fluid
feed. Ebullating bed reactors are capable of converting the most problematic feeds, such as
atmospheric residua, vacuum residua, heavy oil, and tar sand bitumen (all of which have a high
content of asphaltene constituents as well as metal constituents, sulfur constituents, and constituents ready to form sediment) to lower-boiling, more valuable products while simultaneously
removing the contaminants.
In the unit, the hydrogen-rich recycle gas is bubbled up through a mixture of oil and catalyst
particles which provides three-phase turbulent mixing, which is needed to ensure a uniform temperature distribution. At the top of the reactor, the catalyst is disengaged from the process fluids,
which are separated in downstream flash drums. Some of the catalyst is withdrawn and replaced
with fresh catalyst while the majority of the catalyst is returned to the reactor.
The function of the catalyst is to remove contaminants such as sulfur and nitrogen heteroatoms,
which accelerate the deactivation of the catalyst, while cracking (converting) the feed to lighter
products. Because ebullating bed reactors perform both hydrotreating and hydrocracking functions,
they are considered to be dual purpose reactors. Ebullating bed catalysts are made of pellets that are
<1 mm in size to facilitate suspension by the liquid phase in the reactor.
In contrast to fixed-bed hydrocracking units for vacuum gas oil, ebullating bed units are suitable
for processing residua and other heavy feedstocks, such as tar sand bitumen. The main advantages
are: (1) high conversion of atmospheric residue, up to 90% v/v, (2) better product quality than many
other residue conversion processes, especially delayed coking, and (3) long run length—catalyst life
does not limit these units since fresh catalyst is added and spent catalyst is removed continuously
and (subject only to mechanical issues) the units can typically run for a much longer time than fixed
bed units for the same heavy feedstock.
Examples of processes which use ebullated bed reactors are (1) H-Oil process, (2) LC-Fining
process, and (3) T-Star process.
21.2.6.4 Demetallization Reactor (Guard Bed Reactor)
The demetallization reactor (guard reactor, guard bed reactor) is a reactor that is placed in front of
hydrocracking reactors to remove contaminants, particularly metals, prior to hydrocracking. Such
reactors may employ an inexpensive catalyst to remove metals from expanded bed feed. Spent
demetallization catalyst can be loaded to more than 30% vanadium. A catalyst support having large
pores preferentially demetallized with a low degree of desulfurization.
Feedstocks that have relatively high metals contents (>300 ppm) substantially increase catalyst
consumption because the metals poison the catalyst, thereby requiring frequent catalyst replacement. The usual desulfurization catalysts are relatively expensive for these consumption rates but