Ethylbenzene
E
Guy B. Woodle
UOP LLC, Des Plaines, Illinois, U.S.A.
INTRODUCTION
Benzene Alkylation with Ethylene
Ethylbenzene (EB) is a single-ring alkylaromatic compound that is used almost exclusively as an intermediate
in the production of styrene monomer. On a commercial
scale, essentially all EB is produced by alkylating
benzene with ethylene. As of April 2004 approximately
30% of the worldwide EB production is carried out by
liquid phase alkylation using a homogenous aluminum
chloride catalyst in a process that was first commercialized in the 1930s.[1] Ethylbenzene was first produced
commercially using a zeolite catalyst in a vapor phase
reactor in 1980, which was a significant improvement
over the aluminum chloride process. However, it was
the development and commercialization of liquid phase
and mixed liquid–vapor phase technologies in the
1990s that allowed for efficient production of high-purity
EB. In 2003 the annual world EB production capacity
was around 28.6 million metric tons, and EB production
capacity is forecast to grow at an annual rate of just
under 5% from 2004 to 2014.[2]
In the production of EB, alkylation is the reaction of
ethylene with benzene according to the equation:
Successive alkylation reactions occur to a limited
extent resulting in the formation of diethylbenzene
and other higher ethylated benzenes, commonly called
polyethylbenzene (PEB).
PHYSICAL AND CHEMICAL PROPERTIES
Ethylbenzene is a colorless aromatic liquid. It is only
slightly soluble in water, but infinitely soluble in alcohol
and ether. Additional properties are listed in Table 1.
Ethylbenzene is chemically reactive with the most important reaction being its dehydrogenation to form styrene.
Styrene is used to produce polystyrene, which is used in
the manufacture of many commonly used products such
as toys, household and kitchen appliances, plastic drinking cups, housings for computers and electronics, foam
packaging, and insulation. In addition to polystyrene,
styrene is used to produce acrylonitrile–butadiene–
styrene polymer (ABS), styrene–acrylonitrile polymer
(SAN), and styrene–butadiene synthetic rubber (SBR).
Ethylbenzene can also be oxidized to form ethylbenzene hydroperoxide, an intermediate in a process
to produce propylene oxide.
REACTION KINETICS AND THERMODYNAMICS
Commercially produced EB is based on alkylating
benzene with ethylene.
Encyclopedia of Chemical Processing DOI: 10.1081/E-ECHP-120016171
Copyright # 2006 by Taylor & Francis. All rights reserved.
Kinetic reaction rate constants increase with the
number of ethyl groups alkylated on the benzene ring.
For example, the relative rate constant for alkylation
of EB is roughly twice that for the alkylation of benzene. Reaction rate constants continue to increase with
each successive alkylation reaction until a limitation is
reached, such as steric hindrance. The formation of pentaEB and hexa-EB proceeds very slowly for this and other
reasons so that only trace quantities are formed.
Alkylation reactions are exothermic. The initial
alkylation of ethylene to benzene and each successive
alkylation reaction generate roughly the same amount
of heat (Table 2).
In addition to alkylation reactions, transalkylation
reactions play a significant role in EB production.
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Ethylbenzene
Table 1 Physical properties of EB
Molecular weight
106.169
Specific gravitya
0.867
Melting point ( C)
–94.975
Boiling point ( C)
136.19
Critical temperature ( C)
346.4
Critical pressure (atm)
b
37
Vapor pressure (mmHg at T C)
T ( C)
Vapor pressure (mmHg)
1
9.8
5
13.9
10
25.9
20
38.6
40
52.8
60
61.8
100
74.1
200
92.7
400
113.8
760
136.2
a
From Perry’s Chemical Engineers’ Handbook, 6th Ed.; p. 3–34. Density is at 20 C referred to water at 4 C.
b
From Perry’s Chemical Engineers’ Handbook, 6th Ed.; p. 3–56.
(From Ethylene and Its Industrial Derivatives; S.A. Miller, Ed.; p. 900.)
Commercially it is found to be economically attractive
to transalkylate all the PEB formed as a result of successive alkylation reactions with benzene in a separate
transalkylation reactor to produce additional EB.
Transalkylation reaction rates are relatively slow
and conversion is generally limited by equilibrium.
For transalkylation reaction, the heat of reaction is
essentially zero, which leads to a reactor that operates
nearly isothermally.
The occurrence of both alkylation and transalkylation reactions results in a reaction chemistry that is
affected by equilibrium. The equilibrium has been
studied and is illustrated in Fig. 1. The horizontal
axis is the ratio of ethyl groups to benzene rings
and is often referred to as the ethyl-to-phenyl ratio.
In the case of an alkylation reaction, the ethyl-tophenyl ratio is the moles of ethylene to moles of
benzene. Similarly, for a transalkylation reaction,
the ethyl-to-phenyl ratio is equivalent to the moles
of ethyl groups contributed by PEB to the moles of
phenyl groups contributed by PEB plus benzene. At
ethyl-to-phenyl ratios above about 0.6, the equilibrium
EB concentration is relatively constant at about
48 wt% whereas the PEB concentration continues to
increase as the ratio approaches 1.0. Most commercial
reactors operate with ethyl-to-phenyl ratios less than
0.6. The equilibrium composition varies only slightly
across the temperature range of commercial interest.
Table 2 Benzene ethylation thermodynamics
Alkylation reaction
DH (500 K)
(kcal/mol)
Ethylene þ benzene ! ethylbenzene (EB)
36.7
Ethylene þ EB ! di-ethylbenzene (DEB)
28.3
Ethylene þ DEB ! tri-ethylbenzene (TEB)
25.9
Ethylene þ TEB ! tetra-ethylbenzene
25.5
Ethylbenzene
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E
Fig. 1 Ethylbenzene
composition.
Other reactions, such as oligomerization, cracking,
and isomerization, can also occur resulting in the formation of compounds such as cumene, butylbenzene,
xylene, diphenylethane, and other high-boiling compounds. The formation of these by-products is impacted
by the alkylation reaction conditions, in particular
whether the reactions occur in the vapor phase or the
liquid phase. For example, EB isomerization to xylene
typically only occurs under vapor phase reaction conditions where reaction temperatures are relatively high.
Isomerization does not occur to a great extent in the
liquid phase because of the lower operating temperatures. The formation of by-products is also affected by
the type of catalyst.
Acid catalysts are used to promote the alkylation of
ethylene to benzene. Acid catalysts suitable for benzene
alkylation include protonic acids (i.e., H2SO4, HF, and
H3PO4), Friedel–Crafts catalysts (i.e., AlCl3 and BF3),
and more recently, solid acid catalysts. Solid acid
catalysts used for the commercial manufacture of EB
are typically zeolitic molecular sieves and materials
such as ZSM-5, faujasite, MCM-22, and zeolite beta.[3]
Zeolites’ physical and chemical properties can be
modified to optimize the activity, selectivity, and stability of the catalysts. This flexibility of zeolites has made
them the preferred catalyst of choice.
Many zeolites occur naturally as minerals. Some of
these are natrolite, chabazite, sodalite, faujasite, and
mordenite. Several of these naturally occurring zeolites
can be produced synthetically, which makes them
suitable for commercial application. In particular,
equilibrium
faujasite-type structures zeolite X and Y have been
broadly used in the petrochemical and chemical industries. A large number of new zeolite materials have
been discovered and developed that cannot be found
in nature. These specialty synthetic materials include
MCM-22 and zeolite beta. The use of these synthetic
zeolites has enabled the production of EB to become
the highly efficient process it is today.[4]
Because the liquid phase process is predominantly
used for new EB plants, the critical operating and
design parameters for liquid phase benzene alkylation
are discussed below.
Alkylation Benzene-to-Ethylene Ratio
The benzene-to-ethylene molar ratio (B=E) for the
alkylation reaction section is the most important
parameter for design and operation of an EB plant.
A high B=E is beneficial from an equilibrium and catalyst selectivity standpoint, but the large molar excess
of benzene relative to ethylene requires substantial
energy to recover and recycle. The first liquid phase
plants were designed and operated with B=E equal
to 6 or greater. Over time, as improved catalysts were
developed, the B=E has steadily decreased. In 2004,
units are typically designed with B=E in the range
of approximately 3.0–3.5. This substantial decrease
in B=E has resulted in significantly lower capital
costs, because nearly three-fourths of a plant’s equipment cost is associated with recovery and recycle of
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the excess benzene used in the reactors. Generally, a
commercial plant is operated at or very close to the
design B=E and is not frequently adjusted. With
further improvements in the near future, commercial
units will operate at even lower B=E ratios with B=E
ratios possibly approaching 1.5 being made possible.
Alkylation Reaction Temperature
For liquid phase alkylation, the reaction temperature is
normally in the range of 170–270 C to achieve an
acceptable reaction rate using typical commercial catalysts. Excessive temperatures can increase by-product
formation, so reactor design temperatures are often
set with this in mind. Temperature also affects the reactor operating pressure and hence the equipment cost.
Varying temperature within a relatively small range
has little impact on the alkylation reaction overall, so
generally, a commercial scale plant is maintained at a
constant temperature near the design value throughout
its operation cycle.
Reactor design plays a significant role in temperature
control. Multiple ethylene injection points and heat
removal stages are incorporated into the reactor section
design to allow reaction temperature to be maintained
in the desired range. A common design for reactor
sections contains four ethylene injections with one
heat removal stage. Another design option uses six
ethylene injections with two heat removal stages. Other
configurations have been used commercially.
Alkylation Reaction Pressure
Reaction pressure is sufficiently high so as to prevent
any components from vaporizing in the alkylation
reactor section. Alkylation reactors are typically
operated at about 35–40 bar to maintain the reactor
catalyst outlet streams in the liquid phase even at
the maximum operating temperature. The ethylene
injected into the reactor dissolves into the liquid hydrocarbon mixture such that the alkylation catalyst beds
are always in the liquid phase. Reaction pressure is
not normally varied during operation.
Transalkylation Benzene-toPolyethylbenzene Ratio
To obtain an economically viable PEB conversion in
the transalkylation reactor, a molar excess of benzene
relative to PEB is needed. A high benzene-to-PEB ratio
(Bz=PEB) results in high-equilibrium PEB conversion,
but at the expense of increased capital cost and
Ethylbenzene
operating cost associated with the recovery and recycle
of the excess benzene. As the Bz=PEB is decreased,
these capital and operating costs decrease, but the
PEB conversion level declines and formation of heavy
by-products increases.
Similar to the B=E in the alkylation reactor section,
the first liquid phase plants were designed and operated
with Bz=PEB close to 10 or higher. Over time, transalkylation catalyst system stability has been improved
and the Bz=PEB has steadily decreased and in 2004
plants are typically designed with Bz=PEB in the range
of 2.0–4.0. Generally, a commercial plant is operated at
or very close to its design Bz=PEB and the operating
Bz=PEB is not frequently adjusted.
Transalkylation Reaction Temperature
The reaction temperature is the key variable for controlling the operation and performance of the transalkylation reactor. Transalkylation reactors are designed
to operate across a relatively wide temperature range.
During initial operation when catalyst activity is high,
relatively low reaction temperatures are sufficient
to obtain the desired conversion of polyalkylated compounds. As the catalyst ages and loses activity, the
temperature is increased to maintain PEB conversion
at or near the desired level. Liquid phase transalkylation reactors typically operate between 170 C
and 270 C.
Catalyst Poisons
Because of the acidic nature of the zeolite catalysts
used in the production of EB, a number of materials
can interact with the zeolite and negatively impact its
performance. These compounds are referred to as
catalyst poisons and mostly impact the catalyst activity,
although selectivity can also be affected.
Any basic or alkaline material can react with a
zeolite to effectively neutralize the acidic active sites,
which generally results in irreversible loss of catalyst
activity. Basic compounds found in the ethylene or
benzene feedstocks can include amines, amides, nitriles,
and trace metal cations such as sodium and potassium.
Of particular concern are nitrogen-containing organic
compounds typically present in the benzene feed.
There are different types of nitrogen compounds
that have been identified as benzene feed contaminants.
The most common ones are N-formyl morpholine
(NFM), N-methyl pyrolidone (NMP), morpholine,
monoethanol amine (MEA), diethanol amine (DEA),
and acrylonitrile (ACN). Both NFM and NMP are
common aromatic extraction solvents that are used to
Ethylbenzene
purify benzene. They are used commonly in Europe and
less frequently in the rest of the world. Morpholine is a
decomposition product of NFM and has also
been identified in various benzene feeds. Corrosion
inhibitors, including MEA and DEA and other similar
compounds, are used in benzene recovery columns and
can be found in the feed to an EB plant. Transoceanic
shipment containers sometimes alternate loads between
benzene and ACN, which can lead to nitrile contamination. In addition to these most common compounds,
other basic nitrogen compounds can be present in
the feed depending on its origin, processing, and
handling.
Feed Treatment to Remove Catalyst Poisons
Even trace quantities of catalyst poisons can lead to
significant catalyst deactivation and have a large
impact on commercial production. There are varieties
of adsorbent materials that are capable of removing
basic nitrogen compounds from the feed streams. The
type of nitrogen species in the feed is an issue that
affects the choice of material as well as the guard bed
design. Additionally, it is necessary to consider both
the equilibrium capacity of these materials and their
mass transfer properties when selecting a guard bed
material and design.
The most common guard bed materials are acidified
resin, clay, and zeolite. The choice of optimum guard
bed material is generally a function of several variables, including nitrogen adsorption capacity, mass
transfer properties, disposal methods, regenerability,
and cost. These variables also influence the design of
the guard bed.
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COMMERCIAL PRODUCTION
Liquid Phase Aluminum Chloride
Catalyst Process
The primary means of producing EB from the 1930s to
about 1980 was the liquid phase aluminum chloride
catalyst process. Although the aluminum chloride
catalyst process is still in use at many plants, its share
of worldwide EB production is diminishing as all new
plants use a solid acid catalyst.
A flow diagram of a typical aluminum chloride
catalyzed EB plant, based on the widely used
Monsanto=Lummus technology, is shown in Fig. 2.
The Monsanto=Lummus technology is used in the
majority of aluminum chloride catalyst plants because
it significantly reduces aluminum chloride catalyst consumption by operating at higher reaction temperatures
than competing processes.
In the latest version of aluminum chloride plant
designs, the alkylation reactions occur in a homogenous liquid phase at 160–180 C. The conditions of
the alkylation reactor prohibit the recycle of PEB to the
reactor. As a result, these plants have a separate transalkylation reaction zone. The recycle PEB stream is
mixed with the alkylation reactor effluent and fed to
the transalkylation reaction zone. The aluminum
chloride present in the alkylation reactor effluent catalyzes the transalkylation reactions.
The effluent stream from the alkylation–transalkylation reaction section is cooled, washed, and neutralized to remove and recover the AlCl3 catalyst. The
washed hydrocarbon stream contains unconverted
benzene, EB, PEB, and other minor reaction byproducts. It is separated into product and recycle
Fig. 2 Monsanto=Lummus aluminum
chloride catalyzed EB process.
E
934
streams by fractionation in a series of three distillation
columns. The first column recovers unconverted benzene in the overhead stream, which is dried in a drying
column before being recycled to the alkylation reactor
section. The second column separates the product EB
as the overhead stream. The last column recovers
PEB from the high-boiling, heavy by-product tar components. The PEB is recycled to the transalkylation
reaction section.
The handling and disposal of the aluminum chloride
catalyst and waste has become increasingly more costly
and complicated because of environmental considerations. Equipment and piping corrosion and fouling
along with the related environmental issues led to the
development of EB processes based on solid acid, heterogenous catalysts. These are the main reasons why
new plants are not based on the Friedel–Craft-type
catalysts. Major equipment pieces needed to be replaced
on a regular schedule because of corrosion. This
resulted in extensive turnarounds, poor plant onstream
efficiency, and thus, are primary contributors to the
high operating costs associated with the aluminum
chloride process.
Vapor Phase Zeolite Catalyst Process
The first commercial plant based on the Mobil=Badger
vapor phase technology was commissioned in 1980.[5]
From 1980 until the early 1990s, use of the vapor phase
process gained in popularity because it offered several
advantages over the aluminum chloride process. A
major benefit of the vapor phase process was the use
of a zeolite catalyst that eliminated the issues associated with corrosion and waste disposal of aluminum
chloride.
The alkylation of benzene is performed in a vapor
phase, fixed-bed reactor using a ZSM-5 based catalyst.
ZSM-5 is an aluminosilicate zeolite with a high silica
and low aluminum content. ZSM-5, a highly porous
material, is considered a medium-pore zeolite with
two types of pores, both formed by 10-membered oxygen rings. The first type of pore is straight and elliptical
in cross section and the second type of pore is circular
in cross section and intersects the straight pores at
right angles in a zigzag pattern. Therefore, throughout
its crystalline structure, ZSM-5 has an intersecting
two-dimensional pore structure.
The original vapor phase design accomplished the
alkylation and transalkylation reactions in a single
reactor. Subsequent designs performed the transalkylation reactions in a vapor phase, secondary reactor
that was separate from the alkylation reactor. Fig. 3
shows a flow diagram for the latest publicly disclosed
version of the process, sometimes referred to as
the third-generation EB process. The alkylation and
Ethylbenzene
transalkylation reaction section consists of a fired
reactor feed heater, a multibed alkylation reactor, a
transalkylation reactor, and various heat exchange
equipment. Both the alkylation and the transalkylation
reactors are vapor phase, operating at temperatures in
the range of 370–420 C and pressure in the range of
0.69–2.76 MPa.
The third-generation process is capable of achieving
an EB yield greater than 99%. However, the hightemperature vapor phase operation of the reactors is
not trouble free. The significant extent of the isomerization reactions and the catalyst deactivation by
deposition of carbonaceous material are the most
important problems associated with the high temperature. Any xylene formed because of isomerization cannot be separated and therefore ends up contaminating
the EB product. While the xylene impurity is not a
significant problem in the vapor phase EB plant, it is
not desired because it results in higher operating cost
in the downstream styrene plant. Catalyst deactivation
occurs at a rate that requires periodic catalyst
regeneration. The length of time between regeneration
can vary from as little as 2 mo to slightly more than
1 yr depending on the specific plant design and
operating conditions. Because the reactors must be
taken off-line for regeneration, the onstream efficiency
can be low, resulting in high operating costs for a
vapor phase plant. Additional equipment may be
required for the regeneration procedure, depending
on the specific plant design, which adds capital cost
to the plant.
The Mobil=Badger vapor phase process includes
four distillation columns. The first major separation
is in a benzene recovery column where unconverted
benzene is recovered as an overhead product for
recycle to the alkylation and transalkylation reactors.
The bottom stream is fed to an EB recovery column
where EB product is separated from cumene, the
PEB, and other heavy components. The cumene,
PEB, and other heavy by-products are further separated in the PEB recovery column. The heavy residue
is typically used as fuel in the reactor feed heater.
The PEB fraction is recovered in the overhead stream
and recycled to the transalkylation reactor where it
reacts to form additional EB. A fourth column is used
as a stabilizer column to vent any light components
and to remove water from the system.
Liquid Phase Zeolite Catalyst Processes
EBOneTM process offered by Lummus=UOP
One of the shortcomings of the vapor phase zeolitic EB
process is the occurrence of side reactions that can
lead to high levels of contaminants in the EB product.
Ethylbenzene
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E
Fig. 3 Mobil=Badger
EB process.
The commercialization of liquid phase processes, which
operate at substantially lower temperatures, decreased
the side reactions dramatically, resulting in ultrahigh-purity EB product. This improvement alleviated
problems previously encountered in the downstream
styrene plant. The first liquid phase commercial plant
based on the Lummus=UOP process was commissioned in 1990. Since then, more than 25 projects have
been licensed with more than 17 plants in commercial
operation as of 2004. The liquid phase plants typically
achieve high onstream efficiency, often greater than
99%, which results in low turnaround and maintenance
costs. This technology is now licensed by UOP LLC
and ABB Lummus Global.
The first liquid phase plants used a zeolite Y based
catalyst for both the alkylation and the transalkylation
reactions. A significant improvement in the process
occurred in the mid-1990s when EBZ-500TM catalyst
was developed and put into commercial operation.[6]
EBZ-500 catalyst is based on zeolite beta, which has
unique characteristics that make it highly suitable for
benzene alkylation. Zeolite beta has a tetragonal crystal structure with straight 12-membered ring channels
with crossed 10-membered ring channels. This crystal
arrangement gives zeolite beta a unique threedimensional structure that results in high catalyst
activity, an important feature in the relatively lowtemperature liquid phase process. Furthermore, benzene alkylation in the liquid phase is typically limited
by diffusion, so zeolite beta with its relatively large
pore dimensions is well suited for the application.
vapor
phase
A typical EBOne plant flow diagram is shown in
Fig. 4. The alkylation reactor is maintained in the
liquid phase and uses multiple catalyst beds and ethylene injections to improve the reaction selectivity.
Dividing the ethylene into multiple feed streams keeps
the alkylation catalyst deactivation rate very low. In
some plants using EBZ-500 catalyst, operating lengths
of more than 8 yr have been obtained without catalyst
regeneration. The ethylene conversion is essentially
100% in the alkylation reactors, and the reactors
operate nearly adiabatically. The exothermic heat of
reaction is recovered and used within the process to
heat internal process streams or to generate steam.
In the few instances when EBZ-500 catalyst has
been regenerated, it has been restored to essentially
the same activity and selectivity as fresh catalyst. The
regeneration is a mild carbon burn procedure that is
relatively inexpensive. If required, in situ regeneration
equipment can be incorporated into the design. This
is not common and is usually considered only for locations were ex situ regeneration facilities are not readily
accessible.
The transalkylation reactor is also maintained in the
liquid phase but uses EBZ-100TM catalyst, which is
made using zeolite Y. Transalkylation reaction is
nearly thermo-neutral, so it operates essentially isothermally. The reactor temperature is generally adjusted to
provide the desired level of PEB conversion. While a
high temperature results in high PEB conversion that
closely approaches equilibrium composition, these
conditions can result in undesired side reactions.
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Ethylbenzene
Fig. 4 Lummus=UOP’s EBOneTM
process.
Deactivation of EBZ-100 catalyst is rare, usually
only occurring because of unusual upsets or operation
of the transalkylation reactor. Plants have operated for
approximately 10 yr without regenerating the transalkylation catalyst. If EBZ-100 catalyst requires regeneration, an inexpensive, mild carbon burn procedure is used.
The alkylation and transalkylation reactor effluent
streams are sent to the distillation section, which
consists primarily of three fractionation columns. The
first column is the benzene column. It separates unconverted benzene into the overhead stream for recycle to
the reactors. The benzene column bottom stream is the
feed to the EB column. The EB column recovers the
EB product in an overhead stream at purities as high
as 99.98 wt%. The bottom stream of the EB column
feeds a relatively small PEB column where PEB is fractionated overhead and recycled to the transalkylation
reactor. The bottom stream of the PEB column,
referred to as flux oil, is generally used as fuel in an
integrated styrene complex.
Ethylbenzene yields greater than 99.5% can be
achieved by the Lummus=UOP technology.
EBMaxTM process offered by Mobil=Badger
The EBMax process offered by Mobil=Badger is a
liquid phase alkylation reaction using a catalyst based
Fig. 5 Mobil=Badger’s EBMaxTM
process.
Ethylbenzene
on MCM-22. A commercial plant based on the EBMax
technology was commissioned in 1995 at Chiba
Styrene Monomer Company.[7]
MCM-22 is classified as a medium-pore zeolite
consisting of two independent, nonintersecting, 10-membered ring channels. One of the channels contains ‘‘super
cages’’ that have a diameter defined by 12-membered
rings. The MCM-22 crystal surface is covered with
12-membered ring pockets with each pocket being half
of a ‘‘super cage.’’ It is within these surface pockets that
the alkylation reactions are thought to occur.
A typical EBMax plant flow diagram is shown in
Fig. 5. The alkylation reactor is maintained in the
liquid phase and uses multiple catalyst beds and ethylene injections. The ethylene conversion is essentially
100% in the alkylation reactors, and the reactors
operate nearly adiabatically. The exothermic heat of
reaction is recovered and used to generate steam, heat
reactor feed streams, or as heat duty in the distillation
columns.
The transalkylation reactor in an EBMax plant can
be either vapor phase or liquid phase. More recently,
the transalkylation reactor has been designed as liquid
phase because of its improved energy efficiency. The
transalkylation reaction is conducted in the liquid
phase using Mobil TRANS-4TM catalyst.
The alkylation and transalkylation reactor effluent
streams are sent to the distillation section, which
consists primarily of three fractionation columns. The
first column is a benzene column and it separates
unconverted benzene into the overhead stream for
recycle to the reactors. The benzene column bottom
stream feeds the EB column. The EB column recovers
937
the EB product in the overhead stream, and the bottom stream of the EB column feeds the PEB column
where PEB is fractionated overhead and recycled to
the transalkylation reactor. The bottom stream of the
PEB column is removed as a residue stream and is
generally used as fuel in an integrated styrene complex.
Mixed Liquid–Vapor Phase
Zeolite Catalyst Process
The CDTECH EBTM process is based on a mixed
liquid–vapor phase alkylation reactor section. The
design of a commercial plant is similar to the liquid
phase technologies except for the design of the alkylation reactor, which combines catalytic reaction with
distillation into a single operation.[8]
Theoretically, catalytic distillation can overcome
limitations in a typical two-step process consisting of
reaction followed by distillation or separation. Often,
a two-step process is limited by chemical equilibrium,
heat transfer, mass transfer, or some combination of
these. Catalytic distillation can overcome many of
these constraints by simultaneously separating products from reactants, maintaining nearly isothermal
operation and lowering the external ratio of reaction
diluents.
The CDTECH alkylation reactor consists of two
main sections—a catalytic distillation section and a
standard distillation section—as shown in Fig. 6.
Benzene is fed to the top of the alkylation reactor
and ethylene is fed as a vapor below the catalytic
distillation section, creating a countercurrent flow of
Fig. 6 CDTECH EBTM process.
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938
Ethylbenzene
the alkylation reactants through the catalytic distillation section. Throughout the catalytic distillation
section, a vapor–liquid equilibrium is established with
ethylene largely concentrated in the vapor phase. The
ethylene that dissolves into the liquid phase rapidly
alkylates benzene on the catalyst active sites to produce
EB. The rapid reaction of ethylene in the liquid phase
creates a driving force for additional ethylene to dissolve into the liquid phase where the alkylation reaction
occurs on the catalyst active sites. The exothermic heat
of reaction creates the vaporization necessary to effect
the distillation. The alkylation reaction products,
mainly EB, diethylbenzene, and smaller amounts of
other by-products, are continuously fractionated and
removed from the catalytic distillation section. In the
lower section of the alkylation reactor, standard distillation occurs and the reactor bottom stream contains
primarily EB, PEB, and other high-boiling by-products.
The catalytic distillation section uses a zeolite catalyst that is packaged into specially engineered bales.
The catalyst bales function similarly as typical column
structured packing and are designed to optimize both
the distillation and the chemical reaction processes that
occur in this portion of the alkylator.[9]
The alkylator typically does not achieve 100%
conversion of the ethylene, so the overhead stream
from the alkylation reactor contains some unconverted
ethylene and benzene. This overhead stream is fed to a
finishing reactor were the unconverted ethylene is fully
reacted. The finishing reactor is a fixed-bed reactor that
operates in the liquid phase.
One particular advantage of the CDTECH process
is the ability of the alkylation reactor to accept a dilute
ethylene feed. Because the alkylator operates in a
mixed vapor–liquid phase, it is capable of utilizing
dilute ethylene feeds, for example, offgas from a fluid
catalytic cracking plant or dilute ethylene from a steam
cracker plant. In general, ethylene feed streams containing significant amounts of hydrogen, methane, or
ethane do require some pretreatment and cannot be
used directly in the straight liquid phase technologies.
ECONOMICS
Although there are several different commercial technologies in use; the economic information described below
relates only to the liquid phase technology. The cost of
EB production consists of three main components: raw
materials, utilities, and the fixed cost associated with
the plant. The cost of utilities includes fuel, electricity,
steam, cooling water, catalyst, and chemical costs
required to operate the plant. Ethylbenzene plants typically have a small net negative utilities cost because the
credit value of steam generated usually exceeds the cost
of other utilities used throughout the plant.
The major cost components for EB production
using the liquid phase process are listed in Table 3.
The major cost of production is the cost of ethylene
and benzene raw materials, which accounts for more
than 95% of the total cost of production because of
the extremely high product yield of the commercial
processes. As seen in Table 3, more than 95% of the
total cost of production comes from the raw material
costs of the ethylene and benzene feedstocks. The
remaining cost, less than 5%, comes from fixed and
utilities costs. The utilities costs are zero or slightly
negative because all heat input and the heat of the
alkylation reaction is recovered as low-pressure steam,
which is valuable for a downstream styrene plant. The
efficiency of this liquid phase process delivers extremely high commercial yields. The benzene cost is
the largest cost component, so the economics of EB
production is highly dependent on benzene price.
Table 3 Typical economics for an EB liquid phase processa
Unit
Quantity
Unit/MT
Price
$/Unit
Cost
$/MTb
Product
Ethylbenzene
MT
1.0000
530
530.0
Raw materials
Ethylene
Benzene
MT
MT
0.2653
0.7387
629
453
166.9
334.6
By-product credits
Flux Oil
MT
0.0030
125
Net feedstock cost
(0.4)
501.1
Utilities cost
(0.6)
Fixed cost
12
Total cost of production
a
North America, 2003.
b
MT, metric ton.
512.5
Ethylbenzene
939
production of polystyrene, has become an important
feedstock for products that are used in everyday life.
Most people come in contact with numerous products
produced from styrene throughout the course of a
normal day. Because of its close link with styrene
production, the demand for EB is expected to continue
growing at a rate comparable to the demand growth
rate of styrene, which is nearly equal to the gross
domestic product (GDP) growth rate.
The chemical processing technologies that have
been developed are sophisticated and produce EB to
meet that demand at the lowest possible cost. Research
and development aimed at discovering further
improvements in existing technologies and identifying
new technologies for EB production remains an area
of great focus with strong potential for application in
the marketplace.
REFERENCES
Fig. 7 Distribution of EB cost components.
The raw material cost has two components—one
dictated by the stoichiometry and one caused by yield
losses occurring as a result of the process technology. If
the unalterable stoichiometric raw material consumption is removed from the cost of production, the
resultant distribution of cost components appears very
different, as illustrated in Fig. 7. From this perspective,
the raw material cost is only about 10% of the
incremental cost of production and the fixed costs
become dominant. Recent catalyst and process design
improvements have reduced the variable costs of EB
production, while ever-increasing plant complexity
and more stringent environmental regulations have
greatly increased the fixed costs. Other recent trends,
such as globalization of the EB–styrene market, have
also resulted in higher fixed costs.
The result of the shift in focus from variable costs to
fixed costs is that plants are being designed for larger
capacities. For example, in 2003, typical new EB plants
in the Asia Pacific Region produced an average of
368 KMTA EB per year, nearly double the capacity
of typical plants started up just 5 yr earlier.
CONCLUSIONS
Since its first commercial production in the 1930s, EB,
mainly through its role as an intermediate in the
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E