Catalytic Dehydrogenation of Ethylbenzene to Styrene

LSU ChE 4205 Industrial Catalysis paper. By: Byron McCaughey

Dehydrogenation Reaction

Introduction Top

The majority of industrial production of styrene follows from the dehydrogenation of ethylbenzene. This dehydrogenation process involves the catalytic reaction of ethylbenzene. Fresh ethylbenzene is mixed with a recycle stream and vaporized. Steam is then added before feeding the effluent into a train of 2 to 4 reactors. 1 This process involves a highly endothermic reaction carried out in the vapor phase over a solid catalyst. Steam is used to provide heat of this reaction, to prevent excessive coking or carbon formation, to shift equilibrium of the reversible reaction towards the products, and to clean the catalyst of any carbon that does form. 45 The reactors are run adiabatically in multiple reactors with steam added before each stage with typical yields of 88-94%. 4

Crude styrene from the reactors is then fed into a distillation train. Because of the possibility of polymerization of the styrene during distillation, small residence time, avoidance of high temperature, and addition of inhibitor are necessary. 1

Some companies use the oxidation of ethylbenzene as an alternative to dehydrogenation. This reaction path produces water as a byproduct as opposed to hydrogen and is therefore an exothermic reaction. 5% of styrene is produced by oxidation. 4

Important reactions

Dehydrogenation of ethylbenzene to styrene:

C6H5CH2CH3 ---> C6H5CH=CH2 + H2     Enthalpy (600 C) = 124.9 kJ/mol 1

Main side reactions:

C6H5CH2CH3 ---> C6H6 + C2H4

C6H5CH2CH3 ---> 8 C + 5 H2

C6H5CH=CH2 + 2 H2 ---> C6H5CH3 + CH4

Uses of styrene

65% of world production of styrene is polymerized to make polystyrene and other plastics. These plastics are commonly used in toys, television shells, and insulation foam. The remainder of styrene produced is used for elastomers, thermoset plastics, and other products. 1

Worldwide demand for the product

World-wide capacity of styrene was reported to be 17*106 ton/annum in 1993.1 The price for styrene monomer on the market is 0.39 - 0.42 $/lb for 99.6% purity. Most of the styrene produced is used in Western European and North American markets although the Asian market is growing rapidly. The following companies dominate the world markets: 1

  • Badger
  • ABB Lummus Crest
  • PO/Styrene
  • Dow Chemical
  • BASF

Operation conditions in the Process 

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Present day reactions are usually run at 600 to 650 C with a contact time of about 1 second between the feedstock and the catalyst. Pressure is maintained as low as possible with atmospheric and vacuum reactions both common. 4 Feed ratios of steam to ethylbenzene are typically between 10 and 15. A yield of 90% is typical with 65% conversion on a per pass basis.7 The distillation train is run at low pressures in order to maintain low temperatures and avoid polymerization.

The temperature of the reaction train can be lowered when newer catalysts are utilized. For example, zeolite catalysts such as ZSM-5 can have reaction conditions of 1.4 to 2.7 MPa and 370 C.7

Schematic 

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Catalysts 

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A large proportion of modern day styrene is produced with an unsupported iron(III) oxide catalyst promoted with a potassium compound.27 Potassium carbonate or hydroxide and chromium oxides are added to the catalyst to improve reaction selectivity.4 The Shell 105 catalyst which has dominated the market for many years has the following composition: 84.3% Fe2O3 , 2.4% Cr2O3 , 13.3% K2CO3.4 Improvement on this basic catalyst design is carried out with addition of metal oxides such as vanadium, cerium, molybdenum, and also manganese compounds.2 New developments that are just recently coming onto the market include Shell 015 ,205, Dow Chemical, and United Catalysts Inc. (G64 series).4

History of development

Styrene was first isolated in 1786 from liquidambar with steam distillation on the laboratory scale. It was not until 1925 that commercial production of styrene commenced. In 1938 Dow chemical began using the cracking of ethylbenzene with zinc oxide as the catalyst. 5 Although the catalysts used today are different, the basic reaction pathway is so efficient that it is still used today. 4

Preparation, Activation, Utilization, and Regeneration

The preparation of modern catalysts involves the addition of a small amount of water to a potassium and iron oxide mixture. The iron oxide mixture is then extruded to form 4 to 6 mm diameter pellets.7 These pellets are dried and heated to a high temperature to increase catalyst strength.

Steam is an integral part of the activation and regeneration of the catalyst. After superheated steam is initially passed over the bed to activate it, the process steam provides the endothermic heat of reaction and serves to clean the catalyst surface of carbon. The steam reacts with surface carbon to produce carbon dioxide and hydrogen.4

Typical Problems

Chromia was used in the past as a promoter for the dehydrogenation but its utility was limited by poisoning in the presence of steam. The catalyst is self-cleaning when potassium is used with steam.7 However, potassium promoted catalyst life is on the order of 1 to 2 years. Deactivation occurs when the potassium promoter migrates to the center of the pellet because of a temperature gradient.7 The center of the catalyst is cooler because of the endothermic dehydrogenation reaction and slow rate of heat transfer. During the dehydrogenation reaction, toluene is produced as a byproduct.7 The selectivity for styrene over toluene is improved with a smaller size pellet but this causes an increase in pressure drop over the bed. 7 This shows that the reaction is substantially diffusion limited. 7

Alternative catalysts 

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Because of the large worldwide demand for styrene, a number of alternative catalysts are being actively investigated. Corporations such as Mobil and UOP have a modern process development that focuses on the use of ZSM-5.7 Unocal is investigating the possibility of milder reaction temperatures through the use of its H-Y type catalyst.7

Scientific endeavors include, J. Wu, D. Liu, and A. Ko, who have shown that TiO2-Fe2O3, ZrO2-Fe2O3, and TiO2-Fe2O3-ZrO2 have higher activities than the conventional K-promoted iron catalysts.8 In another study, higher activities were also obtained with V-MgO mixed catalysts promoted by Cr, Co, and Mo doping5.6

The mechanics of the selectivity of the dehydrogenation of ethylbenzene was looked at by Jebarathinam et. al. who studied spinel oxides containing Ni, Cr, Zn, Cu, Fe, and Al. Their findings include the fact that basic sites and acidic sites on the catalyst surface influence ethylbenzene conversion.3

Investigation is also underway about the possibility of replacing the dehydrogenation of ethylbenzene to styrene with oxidative dehydrogenation. This produces water as a byproduct as is therefore an exothermic reaction.

C6H5CH2CH3 + 1/2 O2 -----> C6H5CH=CH2 + H2O

Ikenaga et. al. studied the catalytic activities of various metal oxides and phosphates for the oxidative dehydrogenation of ethylbenzene to styrene. 2

Physical Properties of Styrene1 Top

Styrene safety


 

Molecular weight

104.153

Boiling point

145.15 C

Freezing point

-30.6 C

Critical density

0.297 g/mL

Critical pressure

3.83 MPa

Critical Temperature

362.1 C

Critical Volume

3.37mL/g

Flammable limits in air

1.1 – 6.1 vol%

Flash point

31.1 C

Autoignition point

490 C

Heat of combustion at constant P

-4.263 MJ/mol

Links to Vendors Top

Other Relevant Links

 

References Top

  1. Elvers, Barbara, Stephen Hawkins, William Russey. Ullman’s Encyclopedia of Industrial Chemistry. Fifth edition. VCH Weinheim: 1994. Volume A25 pgs.329-339.
  2. Ikenaga, Na-oki, Tadatoshi Tsuruda, Kazuhiro Senma. "Dehydrogenation of Ethylbenzene with Carbon Dioxide Using Activated Carbon-Supported Catalysts." Industrial Engineering Chemical Research. American Chemical Society. March 28, 2000.
  3. Jebarathinam, N. J.; Eswaramoorthy, M.; Krishnasamy, V. "Dehydrogenation of Ethylbenzene over Spinel Oxides." Bull. Chem. Soc. Jpn. 1994, 67, 3334. [CAS]
  4. Mark, Herman F. Encyclopedia of Polymer Science and Engineering. John Wiley and Sons. New York: 1989. Volume 16, pgs. 6-12.
  5. Mitchell, Ernest. "The Dow Process for Styrene Production." Transactions of the American Institute of Chemical Engineers. American Institute of Chemical Engineers, New York:1946. pgs. 299-308.
  6. Oganowski, W.; Hanuza, J.; Drulis, H.; Mista, W.; Macalik, L. "Promotional Effect of Molybdenum, Chromium and Cobalt on a V-Mg-O Catalyst in Oxidative Dehydrogenation of Ethylbenzene to Styrene." Appl. Catal. 1996, 136, 143.[CAS]
  7. Satterfield, Charles N. Heterogeneous Catalysis in Industrial Practice. Second Edition. Krieger Publishing. Malabar: 1996. pgs. 403 – 405.
  8. Wu, J.-C.; Liu, D.-S.; Ko, A.-N. "Dehydrogenation of Ethylbenzene over TiO2-Fe2O3 and ZrO2-Fe2O3 Mixed Oxide Catalyst." Catal. Lett. 1993, 20, 191.[CAS]