estireno

PEP Process Module 1 6/23/99SRI Consulting

PRODUCTION OF STYRENE BY ADIABATIC DEHYDROGENATION:A TWO-STAGE REACTOR WITH STEAM REHEAT

Aspen Model Documentation

Index

• Process Summary

• About This Process

• Process Definition

• Process Conditions

• Physical Property Models and Data

• Chemistry/Kinetics

• Key Parameters

• Selected Simulation Results:BlocksStreams

• References


Process Summary

This Aspen Plus simulation models the production of styrene from EB by adiabatic dehydrogenation: a

process using a two-stage reactor with steam reheat. It is intended to resemble the “CLASSIC” technology of

Lummus Crest/Monsanto/UOP. In the Aspen model, the plant (base case) is designed to produce 1 billion lb/yr

(454,000 t/yr) of styrene. The process consists of three sections: dehydrogenation, vent gas and condensate

treatment, and styrene recovery. The major differences between this process and the design that used two-reactor

process with interstage reheat (resembling the Fina/Badger process) are in the dehydrogenation section. In the

styrene recovery section, low-grade energy can be recovered by azeotropical vaporization of an EB/water mixture

in the condenser of the EB/styrene splitter. Results from the Aspen simulation showed that the purity of styrene

obtained is 99.9%. Other valuable by-products include benzene (purity 96.9%) and toluene (purity 94.8%) at a

rate of approximately 1572 and 2717 lb/hr, respectively.


About This Process

Styrene is produced from EB primarily by adiabatic dehydrogenation. Styrene is also produced by oxidation

and dehydration as a coproduct with propylene oxide (PO) from EB via ∝-methyl benzyl alcohol. Other oxidative

dehydrogenation processes that react EB with an oxygen-containing compound can also produce styrene. In

addition, styrene can be prepared by the following processes: stilbene disproportionation with ethylene; toluene

oxidative coupling with methane; toluene alkylation with acetone, formaldehyde, and methanol; benzene

alkylation with ethylene oxide; aromatization of ethylene or n-octane; and thermolytic formation or selective

adsorption of C8 aromatics. Table 1 provides a summary of the commercial processes for styrene and their

licensor.

TABLE 1. COMMERCIAL PROCESSES FOR PRODUCING STYRENE

Process Type / Technology Licensor / Developer

EB Dehydrogenation Adiabatic, steam reheat Adiabatic, steam reheat Adiabatic, steam reheat (CLASSIC) Adiabatic, oxidative reheat (Smart™) Adiabatic, oxidative rehear (Styro-Plus™) Isothermal, molten salt bath heating Isothermal, hot flue gas heating

DowFina/BadgerLummus Crest/Monsanto/UOPLummus Crest/Monsanto/UOPUOPMontedison/DWE/LurgiBASF

Coproduction with propylene oxide EB oxidation, propylene epoxidation with EB hydroperoxide and α-methyl benzyl alcohol dehydration EB oxidation, propylene epoxidation with EB hydroperoxide and α-methyl benzyl alcohol dehydration

Arco

Shell

This Aspen Plus simulation models the production of styrene from EB by adiabatic dehydrogenation: a

process using a two-stage reactor with steam reheat. It is intended to resemble the “CLASSIC” technology of

Lummus Crest/Monsanto/UOP. In the EB dehydrogenation process, the important variables are the reaction

temperature, the reaction pressure, and the steam/EB ratio. Because the process is endothermic, the conversion of

EB increases as the reaction temperature increases. For a given temperature and a fixed EB/steam ratio, the

conversion of EB also increases as the reaction pressure decreases. Under constant temperature and pressure, a

higher EB/steam ratio results in greater EB conversion because the dilute steam reduces the partial pressure of the

reacting components and gives an effect similar to that achieved by reducing the reaction pressure. However,

steam quantities are limited by the pressure drop allowed and by the energy cost increase that would be required to

improve performance. In addition to steam, nitrogen can also be used as diluent (517063, 517060, 517134).


To improve dehydrogenation, catalysts and increased heat economy are being emphasized. Low-temperature

heat from the reactor effluent (517022) or the overhead of the EB column (517020) is recovered azeotropically by

vaporizing the EB/water mixture; no compression equipment is required. Steam consumption can be reduced by

periodically changing the steam/hydrocarbon ratio (517199).

Unlike traditional steam-reheated processes in the Styrene Monomer Advanced Reheat Technology

(SMART) process of Lummus Crest/Monsanto/UOP and UOP’s Styro-Plus, reactants leaving the

dehydrogenattion catalyst bed are reheated by selectively oxidizing part of the hydrogen coproduced (517038,

517236, 517232). The reaction is fed by air or by oxygen–rich air and takes place over a proprietary UOP high

selectivity catalyst (501284, 553004). In a semi-commercial operation, selectivity is 92-96%, with conversion in

the range of 77-93% (517203). Advantages of the Styro-Plus process include high conversion, high styrene

selectivity, reduced steam requirements, and less high-temperature heat transfer equipment. However, some

hydrocarbon coproduct has to be consumed, and an appreciable amount of phenylacetylene may form at high

conversion levels.

Other novel reaction systems have also been proposed. A transient reactor system is reported to provide

better styrene yield than does a conventional steady state reactor system (517229). The transient system consists

of 2 reactors, with one of them on stream while a favorable temperature profile is being regenerated in the other.

Quick Contact (QC), a process developed by Stone & Webster, is a fluid-solid downflow adiabatic reaction

system with a short residence time; QC is capable of plug flow operation in the 200-500 millisecond contact time

range (517235). Although this technology was developed for ethylene production by pyrolysis in the 1970s, it is

not process-specific and may be appropriate for EB dehydrogenation with a suitable abrasion-resistant catalyst.


Process Definition

The Aspen Plus model is developed to simulate the production of styrene from EB by adiabatic

dehydrogenation: a process using a two-stage reactor with steam reheat, resembling the “CLASSIC” technology

of Lummus Crest/Monsanto/UOP. Figure 1 in the Aspen Plus model file shows the process flow diagram, which

consists of three sections: dehydrogenation, vent gas and condensate treatment, and styrene recovery.

In the Aspen model, 1,1,2-triphenylethane is used to represent the heavies. Compound m-xylene is used to

represent the xylenes, and 1,4-diethylbenzene is used to represent diethylbenzene. Aspen Plus Radfrac models are

used to represent the distillation columns. Due to insufficient kinetics information, Aspen Plus RSTOIC reactor

models is used to represent R-101. The reactor is considered to have 2 valid phases; vapor and liquid phases.

Fresh EB and recycled EB are combined with water to form a feed stream with a water/EB weight ratio of

about 0.41. This stream is a minimum-temperature azeotrope at 87oC (189oF) and 14 psia. By recovering low-

temperature heat, the feed stream is vaporized azeotropically in condensers E-301A&B of the EB/styrene splitter.

The vaporized feed stream is then preheated to 538oC (1000oF) in feed/effluent exchangers E-103A through D and

E-101A through D by exchanging heat with the reactor effluent.

EB is dehydrogenated in two-stage reactor R-101, where one stage of the reaction takes place above the

other. A static mixer heat exchanger is positioned on the reactor top, and a perforated inner core pipe runs through

the center of both stages to supply steam. The feed stream from E-103A through D enters the bottom of R-101 and

is heated to 693oC by mixing with superheated low-pressure (25 psig) steam from the inner core pipe. The mixture

then passes radially through the catalyst bed of the first stage, and about 35% of the EB is converted. Because EB

dehydrogenation is endothermic, a temperature drop occurs across the catalyst bed.

From the annular space around the catalyst bed of the first stage, the effluent passes through apertures to the

outer annular space of the upper second stage, rises to the reactor top, and passes through the tube side of a static

mixer heat exchanger. In the exchanger, the stream is reheated by the superheated low-pressure steam from F-101

that passes through the shell. The reheated stream then becomes the feed to the second stage of R-101 and flows

downward to the inner annular space of the catalyst bed of the second stage. Part of the steam from the exchanger

diffuses through the inner core pipe to mix with the feed to the second stage; the remaining steam flows down to

the first stage. The total steam/EB weight ratio is 1.8 (or a molar ratio of about 10.6). Another 35% of the EB is

converted as the reaction mixture passes radially through the catalyst bed of the second stage. The effluent flows

downward through apertures to the outer annular space of the first stage and leaves the reactor at 577oC (1071oF).


The heat content in the reactor effluent is recovered sequentially to preheat the feed in feed/effluent

exchangers E-101A through D, to generate medium-pressure (150 psig) steam in E-102A&B, and to preheat the

feed in E-103A through D. The reactor effluent is further cooled and partially condensed in effluent air cooler E-

104 and condenser E-105; the effluent is subsequently separated into vapor, aqueous, and organic streams in V-

101. The vapor and the aqueous streams are sent to the vent gas and condensate treatment section, and the organic

stream is routed to the styrene recovery section. The vent gas and condensate treatment section is similar to that

for the oxidative reheat process; the vapor stream is cooled and flashed for aromatics recovery in two stages with

vent gas compressor K-201 in between the stages. The noncondensable gases, mainly hydrogen, are used as fuel.

The aqueous stream is combined with the water layer of the distillate from the EB/styrene splitter column in the

styrene recovery section. The combined stream is then fed to condensate stripper C-201 and condensate treaters C-

202A&B, which contain diatomaceous earth to remove organic impurities. The treated condensate is used as

boiler feedwater.

In the styrene recovery section, a non-sulfur process polymerization inhibitor is added to the organic stream

from V-101 then is fed to EB/styrene splitter column C-301. C-301 contains structured packing and operates

under vacuum with a mixed benzene/toluene/ethylbenzene cut recovered overhead. The styrene recovery section

differs only in the additional energy recovery scheme for azeotropical vaporization of EB/water feed. The

equipment in the styrene recovery section is also larger because of conversion and selectivity that are lower than

in the oxidative reheat process. Benzene recovered from the overhead of C-303 is credited as a by-product or, in

the case of integrated EB and styrene production, recycled to the EB plant. Toluene is taken from the bottom of C-

303 and is credited as a by-product.

The bottoms from C-302 are recycled to the reaction section. Styrene with 99.9% purity is recovered in the

overhead of C-304. The bottom contains heavies, styrene, and the inhibitor. The product styrene is then inhibited

with p-tert-butyl catechol (TBC), a product polymerization inhibitor, and cooled to 160C (600F) by brine before

being sent to rundown tanks and product storage. Residue from the styrene column is used as fuel. Medium-

pressure steam (150 psig) is used in the reboiler of the benzene column, whereas low-pressure steam (25 psig) is

used in other reboilers.


Process Conditions

Table 2 provides the list of important blocks, design bases and assumptions for the process.TABLE 2. STYRENE FROM ETHYLBENZENE BY ADIABATIC DEHYDROGENATION:

A TWO-STAGE REACTOR WITH STEAM REHEAT, DESIGN BASES AND ASSUMPTIONSCapacity 1 billion lb/yr (454,000 t/yr), Styrene at 0.90 stream factor

Process resemblance CLASSICLummus Crest/Monsanto/UOP

References 330696, 517020, 517202Reactor A two-stage reactor

with steam reheatCatalysts G84Ca or C025b

Catalyst cycle life (years) 2Reactor temperature (oC) 550-640Reactor pressure (psia) 8-14WHSV (lb of EB/lb of catalyst/hr) 0.42Steam/EB (wt ratio) 1.8Conversion (%), 70.0Selectivity (mol%)

To styrene 95.5To benzene 2.1To toluene 2.3To others 0.1

Plant yields (mol%)To styrene 95.5To benzene 0.2To toluene 2.3To others 2.0

Net consumption/production (lb/lb of styrene)EB 1.076Benzene 0.01242Toluene 0.02147Vent gas 0.05153Residue 0.00284

ColumnsBenzene/toluene column

Design SpecValve traysMass recovery of toluene in the distillate is 97.9%

Benzene column Design Specs

Valve traysMass recovery of benzene in the distillate is 99%Mass recovery of toluene in the bottoms is 99%

EB/styrene splitter Structured packingStyrene column

Design SpecSieve traysMass recovery of styrene in the distillate is 99.87%

Components Carbon monoxide By-product Carbon dioxide By-product Hydrogen By-product Methane By-product Ethylene By-product Benzene By-product / Constituent of raw material Toluene By-product Styrene Product 126,564 lb/hr Xylenes By-product EB Raw material 136,147 lb/hr D-EB Constituent of raw material Heavies By-product Water Raw material Polymerization inhibitor Inhibitor p-Tert-butyl catechol Inhibitor a By United Catalysts Inc.b by Criterion Catalyst Co.


Physical Property Methods and Data

The physical property method used in the Aspen Plus simulation is UNIQUAC. The UNIQUAC model

calculates liquid activity coefficients for these property methods: UNIQUAC, UNIQ-2, UNIQ-HOC, UNIQ-NTH,

and UNIQ-RK. It is recommended for highly non-ideal chemical systems, and can be used for VLE and LLE

applications. This model can also be used in the advanced equations of state mixing rules, such as Wong-Sandler

and MHV2. The UNIQUAC model can handle any combination of polar and non-polar compounds, up to very

strong nonideality. Parameters should be fitted in the temperature, pressure, and composition range of operation.

No component should be close to its critical temperature

The UNIQUAC model can describe strongly nonideal liquid solutions and liquid-liquid equilibria. The model

requires binary parameters. Many binary parameters for VLE and LLE, from literature and from regression of

experimental data, are included in the ASPEN PLUS databanks. The property method has a vapor phase model

that can be used up to moderate pressures, have the Poynting correction included in the liquid fugacity coefficient

calculation. Heats of mixing are calculated using the UNIQUAC model.


Chemistry/KineticsReactors

This Aspen simulation models the production of styrene from EB by adiabatic dehydrogenation: a process

using a two-stage reactor with steam reheat, which resembled the “CLASSIC” technology of Lummus

Crest/Monsanto/UOP. The main reaction in the adiabatic EB dehydrogenation process is:

In the presence of steam, the equilibrium constant can be expressed as:

Where P is the total pressure, x the equilibrium conversion, and S/EB the steam EB molar ratio. The side

reactions are thermal cracking, hydrocracking, and steam cracking:

And the water-gas shift reaction is:

At high steam concentrations, the following reactions may occur:

The reactions (with their conversions) used to model RSTOIC reactor R-101 in the Aspen Plus model are as

follows:

22563256 HCHCHHCCHCHHC +=→

2246623256

2235623256

H2COCHHCOH2CHCHHCH3COCHHCOH2CHCHHC+++→+

++→+

222 HCOOHCO +→+

S/EB)xx)(1(1PxK

2P ++−=

2242

224

222

626623256

435623256

4663256

4HCO2OH2HC3HCOOHCH

CO2HO2HCHCHCHCHCHHC

CHCHHCHCHCHHCCHHCCHCHHC

+→+

+→+

+→+

+→+

+→+

+→

EB) 0.055% conversion l(Fractiona 22HEthyleneHeavies2EBBenzene (8)

lbmol/hr) 24.2021extent (Molar O2HCO2H2CO (7)

lbmol/hr) 12.0675extent (Molar O22HEthylene24H2CO (6)

O)2H 0.0192% conversion l(Fractiona 23HCOO2H4CH (5)

EB) 1.376% conversion l(Fractiona 22H 2CO4CHBenzeneO22HEB (4)

EB) %609.1 conversion l(Fractiona 23HToluene2CO O22HEB (3)

EB) 0.1096% conversion l(Fractiona Ethylene BenzeneEB (2)

EB) 66.86% conversion l(Fractiona 2HStyrene EB (1)

:101-R

=++→+

+→+

+→+

=+→+

=+++→+

=++→+

=+→

=+→


References

-- Wang, S.-H., “ Styrene, Supplement C,” Process Economics Program Report No. 33C (March 1993)

517063 Zietz, A., et al. (to Standard Oil, Illinois), “ Doped Aluminum Borate,” US 4,645,753 (Feb 24, 1987)

517060 Satek, L. C. (to Amoco), “ Dehydrogenation of Alkylaromatics,” US 4,590,324 (May 20, 1986)

517134 Ueno, A., et al. (to Nissan Girdler Catalyst), “ Manufacture of Nonferrous Metal Oxide Catalysts for EBDehydrogenation,” Japan Kokai 2-78435 (March 19, 1990)

517202 Sundaram, K.M., et.al., “ Styrene Plant Simulation and Optimization,” Hydrocarbon Processing, 70, 1(January 1991), 93-7

517037 Sardina, H., et al., “ New Styrene Plant Operation Optimizer,” Paper No. 65B presented at the 1988AlChE Spring National Meeting, New Orleans, Louisiana, March 6-10, 1988

517020 Sardina, H. (to Lummus Crest), “ Dehydrogenation Process for Production of Styrene from EBComprising Low Temperature Heat Recovery and Modification of the EB-Steam Feed Therewith,” US4,628,136 (Dec 9, 1986)

517022 Whittle, L. F. (to Badger), “ Method of Recovering Heat from Low Temperature Effluent,” US4,695,664 (Sept 22, 1987)

517199 Choi, J., et al., “ Saving of Steam by Periodic Catalyst Reactivation in Styrene Synthesis,” Chem.-Ing.-Tech., 61, 8, 641-4

517038 Imai, T., et al., “ The Principle of Styro-Plus,” Paper No. 64A presented at the 1988 AlChE SpringNational Meeting, New Orleans, Lousiana, March 6-10, 1988

517236 Romatier, J., et al., “ The Smart SM Process. The Lowest Investment Route to a Styrene UnitRevamp,” paper presented at 1992 DeWitt Petrochemical Review, Houston, Texas, March 25-27, 1992

517232 Egawa, K., et al., “ New Sytrene Monomer Process,” Aromatics, 43, 5 & 6 (1991), 17-25

501284 Imai, T. (to UOP), “ Dehydrogenation of Dehydrogenatable Hydrocarbons,” US 4,435,607 (March 6,1984)

553004 Herber, R., et al. (to UOP), “ Dehydrogenation of Dehydrogenatable Hydrocarbons,” US 4,827,066(May 2, 1989)

517203 Ward, D.J., et al., “How New Styrene Unit is Working,” Hydrocarbon Processing, 66, 3 (March 1987),47-8

517229 Heggs, P.J., et al., “ Experimental Investigation of a Transient Catalytic Reactor for theDehydrogenation of EB to Styrene,” Ind.Eng.Chem.Res., 27, 12 (1988), 2241-46

517235 “ QC Reaction System,” Licensing Information (nonconfidential disclosure), Stone & Webster (1990)

Report by: Noni LimJune 23, 1999

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