Styrene

Guy B. WoodleUOP LLC, Des Plaines, Illinois, U.S.A.

INTRODUCTION

Styrene is one of the most important aromaticmonomers used for the manufacture of plastics.Small-scale commercial production of styrene beganin the 1930s. Demand for styrene-based plastics hasgrown significantly, and in 2003 the worldwide annualproduction capacity was approximately 24.5 millionmetric tons.[1]

About 65% of styrene is used to produce polystyrene.Polystyrene is used in the manufacture of manycommonly used products such as toys, household andkitchen appliances, plastic drinking cups, housingsfor computers and electronics, foam packaging, andinsulation. Polystyrene finds such widespread use becauseit is relatively inexpensive to produce and is easy to poly-merize and copolymerize, resulting in plastics with abroad range of characteristics. In addition to poly-styrene, styrene is used to produce acrylonitrile–butadiene–styrene polymer, styrene–acrylonitrile polymer, andstyrene–butadiene synthetic rubber (SBR).

The development of styrene technologies wasmainly driven by demand for cheap synthetic rubberduring and immediately after World War II. Between5% and 10% of total styrene produced becomes acomponent of synthetic rubbers, which are copolymersof styrene and butadiene (SBR). Styrene copolymerscontaining acrylonitrile are specialty materials thatare used for specific applications. Demand for styrenefor the period 2004–2009 is estimated to grow at a rateof approximately 4% per year.[1]

PHYSICAL AND CHEMICAL PROPERTIES

Styrene is a colorless aromatic liquid. It is only veryslightly soluble in water, but infinitely soluble inalcohol and ether. Additional properties are listed inTable 1.

Styrene is chemically reactive with the mostimportant reaction being its polymerization to formpolystyrene. Styrene can also copolymerize withother monomers, such as butadiene and acrylo-nitrile, to produce a variety of industrially importantcopolymers.

In addition to polymerization, styrene can undergoother types of reactions due to the chemical nature of

its unsaturated side chain and aromatic ring. Forexample, styrene can be oxidized to form benzoic acid,benzaldehyde, styrene oxide, and other oxygenatedcompounds. Styrene oxide is used in the productionof various cosmetics, perfumes, agricultural andbiological chemicals.

REACTION KINETICS ANDTHERMODYNAMICS

Essentially all commercially produced styrene usesethylbenzene (EB) as a feedstock. Between 85% and90% of worldwide styrene production is based on EBdehydrogenation. The remaining 10–15% of styreneis obtained as a coproduct in a process to producepropylene oxide.

Ethylbenzene Dehydrogenation

Ethylbenzene is catalytically dehydrogenated in thepresence of steam according to the equation:

CH3 CH2

H2+

The reaction is highly endothermic and conversionis limited in extent by equilibrium. The reactionequilibrium constant is defined as:

Keq ¼ ðPsty � Ph2Þ=Peb

where Psty is the partial pressure of styrene, Ph2 is thepartial pressure of hydrogen; and Peb is the partialpressure of ethylbenzene.

High temperature, steam dilution, and low systempressure produce an equilibrium more favorable tostyrene. For endothermic vapor-phase reactions, theequilibrium constant increases with temperature and

Encyclopedia of Chemical Processing DOI: 10.1081/E-ECHP-120007970Copyright # 2006 by Taylor & Francis. All rights reserved. 2859

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can be determined according to the followingequation:[2]

ln Keq ¼ 16:12 � ð15; 350=TÞ

where Keq is the equilibrium constant in atmospheresand T is the temperature in K.

The equilibrium constant has the dimension ofpressure since two moles of products are formed foreach mole of EB converted. Therefore, a higher totalpressure will shift the reaction equilibrium to the leftand reduce EB conversion. Lower pressure results ingreater EB conversion without an accompanyingsignificant decrease in styrene selectivity.

Another method to create a positive shift inequilibrium is the use of steam dilution to reduce thepartial pressures of EB, styrene, and hydrogen. Steamdilution provides the same effect as a reduction in totalpressure.

Steam dilution has several other important benefits.First, steam supplies heat to the reacting mixture.Consequently, the drop in temperature for a givenEB conversion is lower, allowing greater EB conver-sions to be obtained with the same inlet temperature.Second, a minimum amount of steam appears to keepthe catalyst in the required oxidation state for highactivity. The actual quantity of steam varies with thetype of catalyst used. Third, steam is believed tosuppress the deposition of carbonaceous material onthe catalyst. If the carbonaceous material is allowedto accumulate, the catalyst will become fouled and itsactivity will decline to unacceptable levels.

The reaction feed mixture undergoes certain otherreactions that are not equilibrium limited under typicaloperating conditions. Most important among theseare the dealkylation reactions that result in theformation of benzene and ethylene or toluene andmethane. Other reactions produce small amountsofa-methylstyrene and other high boiling components.

The key dealkylation reactions can be described bythe following equations:

CH3

+ H2C CH2

CH3 CH3

H2+ CH4+

Both methane and ethylene undergo steam reform-ing reactions according to the following equations:

CH4 þ H2O ! CO þ 3H2

C2H4 þ 2H2O ! 2CO þ 4H2

The water–gas shift reaction also occurs and isgenerally near equilibrium at the reaction temperature:

CO þ H2O $ CO2 þ H2

The combination of dealkylation, steam reforming,and water–gas shift side reactions should be avoided,if possible. In addition to losing valuable EB feed bydealkylation, the resultant net formation of carbondioxide and hydrogen by this combination of reactionsinhibits the primary dehydrogenation reaction. The nethydrogen formation gives an unfavorable shift in equi-librium, while the presence of carbon dioxide has anegative effect on dehydrogenation catalyst activity.[3]

Table 1 Physical properties of styrene

Molecular weight 104.152

Specific gravitya 0.903

Melting point, �C �30.628Boiling point, �C 145.2

Critical temperature, �C 373

Critical pressure, atm 46.1

Vapor pressure, mm�Hgat T �C

1 5 20 10 40 60 100 200 400 760

�7.0 18.0 30.8 44.6 59.8 69.5 82.0 101.3 122.5 145.2aDensity is at 20�C referred to water at 4�C.(From Perry, R.H., Green, D.W., Eds.; Perry’s Chemical Engineers Handbook, 6th Ed.; McGraw-Hill: New York, 1984; 3-60 and Miller, S.A.,

Ed.; Ethylene and Its Industrial Derivatives; 901 pp.)

2860 Styrene

Typically, there is less methane and ethylene presentin the effluent of a reactor than would be expected fromthe benzene and toluene formation. Carbon monoxideis generally about 10mol% of the total carbon oxides.

The critical operating and design parameters for EBdehydrogenation are discussed in the followingparagraphs.

Reaction temperature

Because the dehydrogenation reaction is endothermic,the reaction mixture temperature decreases as thereaction proceeds. The reaction rate slows because ofthe closer approach to equilibrium and the decreasein kinetic reaction rate with the decreasing tempera-ture. Furthermore, the equilibrium constant is lessfavorable at lower temperature. Therefore, in a normaldesign, about 80% of the temperature drop occurs inapproximately the first third of the catalyst bed.

As a result, a high inlet catalyst temperature isrequired. However, high temperature also increasesthe rates of nonselective thermal reactions and dealky-lation reactions, which form benzene and tolueneby-products. In particular, as temperature is increased,the rate of benzene formation increases significantlyrelative to the rate of styrene formation. This meansthere is an effective upper limit to the inlet temperatureif high styrene selectivity is a required criterion. Reac-tion temperature is generally adjusted by changingeither the steam temperature or the steam-to-oil ratio.

Catalyst quantity

The amount of catalyst relative to EB feed is animportant parameter for optimum reactor perfor-mance. Too little catalyst will prevent a close approachto equilibrium. If EB conversion is low, then distilla-tion costs associated with recovery and recycle of theunconverted EB can become significant. With toomuch catalyst, the EB conversion reaches equilibriumbefore the outlet of the catalyst bed, while the sidereactions continue leading to loss of selectivity.

The optimum catalyst quantity is achieved bybalancing the EB conversion level and the styreneyield. Catalysts typically lose activity with time on-stream, which has the effect of decreasing the effectiveactive catalyst quantity for reaction. Compensation foraging catalyst is achieved by adjusting other operatingparameters, in particular, the reaction temperature.

Reaction pressure

Ethylbenzene dehydrogenation results in a significantincrease in the volume of reactants due to the reac-tion stoichiometry. Lower pressure favors higher

equilibrium conversion to styrene. Reaction pressureis established during the plant design at the lowestpractical level. Modern commercial reactors operatebelow atmospheric pressure. Pressures as low as300mmHg or lower are common. The key side reac-tions are largely independent of reaction pressure;hence, operating at lower pressures also provideshigher styrene yield.

Steam dilution or steam-to-oil ratio

The main functions of steam dilution are to act as adiluent to reduce the hydrocarbon partial pressures,providing heat for the endothermic dehydrogenationreaction, and maintaining the catalyst’s active surfacein a desirable state. Increasing the steam-to-oil ratiohas the net effect of improving the EB conversionand styrene yield. However, costs associated withgenerating and superheating the dilution steam alsoincrease and eventually offset the reaction advantages.

Catalyst type and properties

Ethylbenzene dehydrogenation is generally catalyzedby a potassium-promoted iron oxide catalyst. Themost widely used catalysts are composed of iron oxide,potassium carbonate, and various metal oxide promo-ters. Examples of metal oxide promoters includechromium oxide, cerium oxide, molybdenum oxide,and vanadium oxide.[4] The potassium componentsubstantially increases catalyst activity relative to anunpromoted iron oxide catalyst. Potassium has beenshown to provide other benefits. In particular, itreduces the formation of carbonaceous deposits onthe catalyst surface, which prolongs catalyst life.

Properties such as catalyst size and shape alsoimpact performance. In theory, smaller sized catalystwill increase reaction rates by providing more availablecatalyst surface area than larger sized catalyst. Smallcatalyst particles, however, have a disadvantage in thatthey result in greater pressure drop through a reactorand higher overall reaction pressures. To address this,catalyst developers have used specialized shapes, suchas ribbed extrudates, to gain the advantage ofincreased surface area without incurring the penaltyof increased pressure drop and reaction pressure.

The Sud-Chemie Group and Criterion Catalysts arethe major catalyst developers and manufacturers forthe styrene industry. Both companies offer a widerange of catalysts to suit individual processing needs.Ethylbenzene conversion, styrene selectivity, catalystactivity, and catalyst stability can be optimized byselecting the best catalyst or a combination of catalystsfor a particular application. Dow and BASF manufac-ture proprietary catalysts, which have been mainly foruse in their own respective technologies.

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Propylene Oxide with Styrene Co-production

In the late 1960s, a method was discovered to producepropylene oxide by the epoxidation of propylene usingorganic hydroperoxides as the epoxidizing agent.[5]

During the epoxidation reaction, the hydroperoxideis essentially converted to the corresponding alcohol,which in turn can be dehydrated to a more desirablecoproduct. Styrene is coproduced in the form of thisprocess that uses EB hydroperoxide as the epoxidizingagent. The chemistry of this process can be brokendown into three main reactions as shown in Fig. 1.

The first step is oxidation of EB to form EB hydro-peroxide. The oxidation is carried out in the liquidphase with a target EB conversion of approximately13%.[6] Although higher conversions are attractivefrom an EB recovery and recycle standpoint, there isa significant disadvantage because the EB hydroperox-ide selectivity declines sharply. The second step isepoxidation of propylene to form propylene oxideproduct and 1-phenylethanol. In the last step, the1-phenylethanol is dehydrated to styrene and water.The dehydrated reaction mixture is typically strippedof light components and rerun in a styrene column toremove heavy by-products, resulting in a purifiedstyrene product.

The design and operation of a propylene oxide=styrene process plant is complicated and includesnumerous pieces of equipment. As a result, the totalinvestment cost for a commercial-scale plant is aboutfour times that of an EB dehydrogenation plant toproduce the same quantity of styrene product.

COMMERCIAL PRODUCTION

Reactor Design

One important aspect of modern day EB dehydro-genation reactor design is managing the operatingconditions to minimize thermal reactions. The majorby-product from the thermal reaction of EB to styreneis benzene with significant subsequent conversions to acomplex mixture of higher aromatics, such as anthra-cene and=or pyrene, as well as coke. Thermal reactionsdo not occur at a significant level below about 600�C,but become a considerable factor affecting overall yieldwhen temperatures rise above 655�C.

One technique to reduce thermal reactions is todelay heating the EB to the reaction inlet temperatureuntil the last possible moment before being exposed tothe catalyst. The method involves superheating EBvapor, along with a portion of the dilution steam, toa temperature below approximately 580�C. The EB isvaporized with a certain amount of steam—commonlycalled primary steam—to suppress coking. The EBprimary steam is combined with the major part ofthe dilution steam immediately prior to entering thedehydrogenation catalyst bed. The major portion ofthe dilution steam is generally referred to as mainsteam. The main steam is superheated to a temperaturesuch that, when it is mixed with the EB and theprimary steam, the total combined feed mixturereaches the desired catalyst inlet temperature.

Reactor design and catalyst bed configurationare key factors for controlling thermal reactions.

Fig. 1 Propylene oxide–styrene process

chemistry.

2862 Styrene

Commercial adiabatic reactors are typically of radialflow construction with the flow path moving from into out. This radial outflow geometry requires a muchlower inlet volume to obtain proper distribution ofthe feed vapor through the catalyst bed than eitheran axial flow or a radial inflow reactor configuration.The radial flow reactor design also provides the advan-tage of low pressure drop since the flow path throughthe catalyst is much shorter relative to an axial flowreactor. To minimize thermal reactions, the reactorcenterpipe diameter should be as small as possible tominimize residence time at the highest temperaturethroughout the reactor. However, too small a diameterwill produce a high pressure drop through the center-pipe, potentially causing flow maldistribution andcausing the feed vapor to enter the catalyst bed witha velocity that can result in erosion and attrition of cat-alyst particles.

A single-stage reactor with practical limits oftemperature, pressure, and steam dilution is limitedto 40–50% per pass conversion of EB. If the single-stage reactor effluent is reheated, the reaction mixturemoves away from equilibrium allowing for higher EBconversion. When the reheated reaction mixture isfed to a second stage of catalyst, then total EB conver-sions of 60–75% per pass can be achieved. This processof reheating and adding catalyst stages can be repeatedas frequently as economically feasible. With each addi-tional reaction stage, however, a progressively smallerincremental EB conversion is achieved, generally witha corresponding decrease in styrene selectivity.

To obtain high EB conversions, typically two orthree reactors are used in series with some type ofreheating between the reactors to raise the temperatureof the reaction mixture. Modern day commercial

reactors are highly engineered. Designers use specia-lized computational fluid dynamics programs to studyflow characteristics throughout a reactor.

Commercial Adiabatic DehydrogenationProcesses

Most commercial styrene plants are based on either theLummus=UOP technology or the Fina=Badger technol-ogy. Dow Chemical is a major styrene producer and usesits own technology. These technologies are generallysimilar, but there are key differences in the details.

Lummus/UOP Classic SMTM Process

The first commercial plant based on the Lummus=Monsanto technology, which later became the Lummus=UOP technology, was commissioned in 1972. Since thattime, more than 50 projects have been licensed with morethan 40 plants in commercial operation as of 2004.

A typical Lummus=UOP Classic SM process flowdiagram is shown in Fig. 2. Fresh and recycled EBare combined with steam and fed to the dehydrogena-tion reaction section of the plant. The reactor effluentis condensed and separated into off-gas, processcondensate, and a dehydrogenated mixture. The hydro-gen rich off-gas stream is recovered through an off-gascompressor for use as a fuel gas. The process conden-sate is stripped of organics and either recycled for usewithin the styrene plant or exported. The dehydro-genated mixture, consisting mainly of unconvertedEB, styrene product, benzene, and toluene, is fed tothe distillation section of the plant.

Fig. 2 Lummus=UOP classic SM

process.

Styrene 2863

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The main equipment in the dehydrogenation reac-tion section of a Lummus=UOP Classic SM plantincludes a steam superheater, two dehydrogenationreactors, a series of waste heat exchangers, and anoff-gas compressor (Fig. 3). The equipment is designedto minimize pressure drop from the dehydrogenationreactors inlet to the off-gas compressor.

The main steam is superheated and used to reheatthe reaction mixture for the second stage dehydrogena-tor. The reaction mixture is reheated in a speciallydesigned interchanger located inside the second stagedehydrogenator vessel shell. The cooled steam exitingthe interchanger is reheated in the steam superheaterprior to being fed to the first stage dehydrogenator.The superheated steam can range from 700�C to ashigh as approximately 850�C to achieve the desiredinlet temperature for the first stage dehydrogenator.

Superheated main steam is mixed with the EB and theprimary steam immediately before entering the first stagedehydrogenator. The reactor is designed to provide a uni-form reaction mixture while minimizing residence time inthe centerpipe to avoid thermal reactions. The reactoreffluent is cooled in a series of three waste heat exchan-gers before final cooling and condensing.

The first stage of waste heat recovery is used to super-heat the EB and the primary steam. Subsequent stagesare used to generate steam at different pressures. Typi-cally intermediate pressure steam and low pressuresteam are generated, which are directed for use else-where in the styrene plant or larger EB–styrene complex.

Hydrogen and light hydrocarbons removed fromthe condensed reactor effluent are compressed andused as fuel gas in the steam superheater. The processsteam from the reactor effluent stream is condensedand separated by gravity from the liquid hydrocarboncomponents. The condensate is stripped of hydrocar-bons and revaporized for use as process steam.

The distillation section of a Lummus=UOP ClassicSM plant consists of four distillation columns. The first

column in the sequence splits the EB and the lightercomponents from styrene. The EB=styrene monomer(EB=SM) splitter is operated under vacuum and usesstructured packing, such as Sulzer Mellapak Pluspacking, to minimize temperature and polymer forma-tion.[7] Polymerization inhibitors are injected into thesplitter to restrict polymer formation, in particular intothe bottom section of the column.

The overhead product from the EB=SM splitter isfed to an EB recovery column. The EB recoverycolumn net bottoms’ stream is recycled to the dehydro-genation section. Benzene and toluene by-products inthe recovery column overhead stream are separatedin a benzene=toluene splitter. Oftentimes, the benzenerecovered in this scheme is recycled as feed to theupstream EB plant.

The EB=SM splitter bottoms’ stream is fed to theSM column where the styrene is purified by removalof any heavy residual tars. Tertiary-butyl catechol(TBC) is injected into the overhead of the SM column,and the column is operated under vacuum to minimizepolymer formation.

A unique feature of the Lummus=UOP Classic SMprocess is the noncompressive azeotropic heat recoveryoption.[8] In this option, the EB=SM splitter overheadvapor is used to boil an EB–water azeotrope mixture,which is then fed to the dehydrogenation reactors.The condensation of the splitter overhead vaporproduces approximately 500 kcal=kg styrene. Thisenergy savings potential makes the azeotropic heatrecovery option economically attractive, in particular,in regions with moderate to high steam costs.

Lummus/UOP Smart SMTM Process

The Lummus=UOP Smart SM process is based on anoxidative reheat technology invented by UOP.[9]

Although this technology can be used in the design of

Fig. 3 Lummus=UOP classic SM processdehydrogenation section.

2864 Styrene

a grassroots plant, it is most commonly used in a revampof an existing plant to increase styrene production by asmuch as 60% with minimal capital investment cost.

The Lummus=UOP Smart SM technology usesa specially designed reactor that contains twoconcentric catalyst zones. A cross-sectional view ofthe concentric oxidation and dehydrogenationcatalyst beds is also shown in Fig. 4. In the first zone,hydrogen is selectively oxidized across a noble metal-containing catalyst. The direct combustion of hydro-gen reheats the reaction mixture, which is directly fedinto the second zone where the standard EB dehydro-genation reaction occurs. In addition to providing thefull reheating requirement, another benefit of thistechnology is it shifts the reaction equilibrium in afavorable direction by removing the hydrogen by-product. This shift in equilibrium allows for higherEB conversion without a corresponding decrease instyrene yield.

The Lummus=UOP Smart SM technology was firstcommercialized in 1995 at Mitsubishi Chemical inKashima, Japan. The Mitsubishi Chemical plant wasdesigned with a dehydrogenation section containingtwo combination oxidation–dehydrogenation reactorsas shown in Fig. 4.

The temperature rise in the oxidation zone isproportional to the amount of oxygen reacted acrossthe catalyst bed. The oxygen is diluted in steam andthe oxygen=steam mixture is well mixed to ensure thereaction mixture remains outside the flammabilityenvelope at all times.

Fina/Badger Styrene Process

The Fina=Badger styrene process has evolved throughmany generations. The most recent design uses a flowdiagram as shown in Fig. 5. Recycled and fresh EB

Fig. 4 Lummus=UOP smart SM processdehydrogenation section.

Fig. 5 Fina=Badger styrene pro-

cess.

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are mixed with steam and fed to the primary and thesecondary dehydrogenation reactors. The reactor efflu-ent is condensed and separated into vent gas, conden-sate, and hydrocarbon. The vent gas, the majority ofwhich is hydrogen, is used as fuel gas. The condensateis stripped and used as feed water for steam generation.The hydrocarbon portion of the reactor effluent is fedto the distillation section of the plant, which consists ofthree distillation columns.

The main types of equipment in the dehydrogena-tion section of the plant are the steam superheater,the primary and secondary dehydrogenation reactors,and a series of feed=effluent exchangers (Fig. 6). Highpressure steam is also generated by the recovery of heatfrom the reactor effluent stream.

The major portion of steam is superheated andused to reheat the reaction mixture for the secondarydehydrogenation reactor. As the cooled steam exitsthe reheater it is superheated again in the steamsuperheater, prior to being fed to the primarydehydrogenation reactor. The dehydrogenation reac-tors are designed to provide low pressure drop anduniform flow distribution. The reactor effluent iscooled in a series of three heat exchangers that heatthe EB and steam feed to the reactors and generatesteam.

The Fina=Badger distillation section consists ofthree distillation columns. All the columns aredesigned to operate under vacuum to minimize tem-perature and polymer formation. The first columnin the sequence splits the benzene and toluene by-products from the unconverted EB and styrene pro-duct. The benzene and toluene mixture is typically sentto an integrated EB plant where it is further fractio-nated. In this case, the benzene by-product is ulti-mately consumed in the EB unit and the toluenebecomes a by-product stream from the EB plant.

The EB recycle column separates the unconverted EBfor recycle to the dehydrogenation reactors. Recent EBrecovery columns use high efficiency packing to obtainminimum pressure drop through the column. Thisallows the column bottoms’ temperature to be main-tained below 100�C. This is an important aspect of thedesign as styrene polymerization becomes significant attemperatures higher than approximately 100�C.

The EB recovery column bottoms’ stream is fed to afinishing column where the styrene is purified by theremoval of any heavy residue. Tertiary-butyl catecholis injected into the overhead of the finishing columnto prevent polymerization. Tertiary-butyl catechol iswidely used to prevent styrene polymerization duringstorage.

In 1997, Fina=Badger joined with Shell TechnologyVentures, a subsidiary of Shell Oil Company, to developa reheating technology called Flameless DistributedCombustion (FDC) for application in EB dehydrogena-tion.[10] Flameless Distributed Combustion technology ispatented by Shell Oil Company and was originally usedas a heat injector for enhanced recovery of hydrocarbonsfrom subterranean formations.

Flameless Distributed Combustion technologyenables specific constraints in the conventional dehy-drogenation system to be overcome, in particulardesigning for low steam-to-oil ratios. A low steam-to-oil ratio is desirable because of the substantial energysavings associated with superheating less steam. How-ever, a practical lower steam-to-oil ratio limit existsdue to the metallurgy of the steam superheater, steamtransfer lines, and interstage reheater. FlamelessDistributed Combustion allows for operation at molarsteam-to-oil ratios less than 7 : 1 without a costlymetallurgy upgrade. This is accomplished by heatingthe reaction mixture more directly through a combus-tion and convective heat transfer process.

Fig. 6 Fina=Badger styrene process dehydro-

genation section.

2866 Styrene

Flameless Distributed Combustion technology,unlike the Lummus=UOP Smart SM technology, doesnot directly combust hydrogen from the reactionmixture; hence it does not obtain the benefit of a favor-able shift in equilibrium.

Other Processes

Propylene oxide/styrene process

Aside from EB dehydrogenation, the only other commer-cial-scale productionof styrene is throughapropyleneoxi-de=styrene process that produces roughly 15% ofworldwide styrene. This technology was developed as analternative to the chlorohydrin method for producingpropylene oxide.

Styrene from Butadiene

Because the conventional EB dehydrogenationtechnologies are relatively mature, there is little roomfor significant additional reduction in productioncosts. This situation has motivated a lot of researchtoward using alternative, lower cost feedstocks for styreneproduction. One area that has been examined involvesa two-step process to convert butadiene to styrene.

The first step of the process involves the cyclo-dimerization of butadiene to 4-vinylcyclohexene. Thereaction is exothermic and can be catalyzed by either acopper-containing zeolite catalyst or an iron dinitrosylchloride catalyst complex. Although both vapor-phaseand liquid-phase processes have been studied, itappears that liquid-phase reactions are preferredbecause they achieve higher butadiene conversionlevels. The second step is oxidative dehydroge-nation of the 4-vinylcyclohexene to produce styrene.Dow has led the research effort in this area and has

identified catalyst formulations that provide morethan 90% conversion of 4-vinylcyclohexene withapproximately 92% selectivity to styrene.[11]

Storage

Preventing polymerization is the key to successful styr-ene storage. Special handling and storage proceduresare required to maintain the styrene product qualityand to avoid a potentially dangerous situation invol-ving uncontrolled polymerization.

During storage, styrene polymerization is preventedby maintaining low temperature and using an appropri-ate polymerization inhibitor. The industry standardstyrene storage inhibitor is TBC and is typically usedat concentrations between 10ppm and 15ppm. To beeffective, TBC requires dissolved oxygen to be present inconcentrations roughly equal to the TBC concentration.

In addition to adding TBC inhibitor, maintainingthe styrene at the lowest practical temperature is criti-cal to preserving product quality. Styrene storage facil-ities are generally maintained at temperatures belowabout 20�C, which allows for storage times of around10 weeks. Even a 5�C increase in the storage tempera-ture to 25�C can reduce the storage time to less than 4weeks.[12] Tertiary-butyl catechol is added occasionallyduring storage to maintain the concentration in thedesired range.

ECONOMICS

The cost of styrene production can be broken downinto three main components: raw materials, utilities,and the fixed cost associated with the plant.The utilities cost includes fuel, electricity, steam,cooling water, catalyst, and chemical costs required to

Table 2 Styrene economics for conventional EB dehydrogenation process

UNIT

Quantity

UNIT/MT

Price

$/UNIT

Cost

$/MT

Produce

Styrene MT 1.0000 751 751.0

Raw materialsEthylene MT 0.2912 629 183.2Benzene MT 0.7898 453 357.8

By-product credits

Toluene MT 0.0401 378 (15.1)Light ends MT 0.0401 289 (11.6)

Net feedstock costs 514.2

Utilities 95.0

Fixed cost 35

Total cost of production 644.2

Basis: North America, 2003

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operate the plant. The major cost components forstyrene production using conventional adiabaticdehydrogenation process are listed in Table 2. The majorcost of production is for the ethylene and benzene rawmaterials, which account for approximately 80% of the

total cost of production. The benzene cost is the largestcost component; hence, the economics of styreneproduction are highly dependent on benzene price.

The raw materials cost has two components—onedictated by the stoichiometry and the other caused by

Table 3 Styrene economics for propylene oxide-styrene process

UNIT

Quantity

UNIT/MT

Price

$/UNIT

Cost

$/MT

Product

Styrene MT 1.0000 751 751.0

Raw materialsEthylene MT 0.3135 629 197.2Benzene MT 0.8194 453 371.2

Propylene MT 0.3541 465 164.6Oxygen MT 0.2529 43 10.9

By-product creditsLight ends MT 0.1000 289 (28.9)Propylene oxide MT 0.4500 1227 (552.1)

Tars MT 0.0400 257 (10.3)

Net feedstock costs 152.6

Utilities 65.0

Fixed cost 95

Total cost of production 312.6

Basis: North America, 2003

Fig. 7 Distribution of styrene productioncost components.

2868 Styrene

yield losses occurring as a result of the process technol-ogy. If the unalterable stoichiometric raw material con-sumption is removed from the cost of production, theresultant distribution of cost components appears verydifferent, as shown in Fig. 7. From this perspective, theraw materials’ cost is only about 15% of the incremen-tal cost of production and the utilities and fixed costsbecome dominant. Recent catalyst and process designimprovements have reduced the variable costs of styr-ene production, while ever-increasing complexity andmore stringent regulations have greatly increased thefixed costs. Other recent trends, such as larger plantcapacities and globalization of the styrene market, havealso resulted in higher fixed costs.[13]

The result of the shift of focus from variable to fixedcosts is that plants are being designed for larger capa-cities. For example in 2003, typical new styrene plantsin the Asia Pacific Region produced an average of 350KMTA styrene per year, nearly double the capacity oftypical plants started up just 5 years earlier. The driveto reduce fixed costs has led to numerous revamps ofexisting plants to substantially increase capacity. Inmany cases, capacity expansions on the order of 50%are being implemented.

The propylene oxide=styrene process, the only othercommercial process for production of styrene, is a grow-ing influence on the overall styrene market economics.When viewed from the perspective that styrene is theprimary product and propylene oxide is a by-product,the economics of this process appear encouraging (Table3). Depending on the credit value assigned to the propy-lene oxide coproduct, the total cost of styrene produc-tion can be approximately 50% of conventional EBdehydrogenation technology. Approximately 33% ofthe styrene capacity added between 1998 and 2003 wasproduced using propylene oxide=styrene technology.More recently, the trend appears to be reversing andpropylene oxide=styrene processes are accounting forless of newly installed capacity. Although propyleneoxide=styrene plants are built to produce propyleneoxide, there is a profound impact on the styrene marketsupply=demand balance.

CONCLUSIONS

Since the first commercial-scale production in the1930s, styrene, mainly through its derivatives, hasbecome an integral part of life. Most people come incontact with numerous styrene-based productsthroughout the course of a normal day. Demand forstyrene is expected to continue growing at a rate com-parable to the gross domestic product growth rate.

The chemical processing technologies that have beendeveloped are sophisticated, producing styrene to meetthe demand at low cost. Research and development

efforts are aimed at further improvements in existingtechnologies and identification of new technologiesfor styrene production opportunities.

REFERENCES

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2. Carra, S.; Forni, L. Kinetics of catalytic dehydro-genation of ethylbenzene to styrene. Ind EngChem Process Des. Dev. 1965, 4 (3), 281–285.

3. Matsui, J.; Sodesawa, T.; Nozaki, F. Influence ofcarbon dioxide addition upon decay of activity ofa potassium-promoted iron oxide catalyst fordehydrogenation of ethylbenzene. Appl. Catal.1991, 67, 179–188.

4. Hirano, T. Roles of potassium in potassium-pro-moted iron oxide catalyst for dehydrogenation ofethylbenzene. Appl. Catal. 1986, 26, 65–79.

5. Kollar, J. Epoxidation Process. US Patent3,351,635, November 7, 1967.

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