Basics of Polystyrene Production.docx

8/13/2019 Basics of Polystyrene Production.docx

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Basics of Polystyrene Production

Polystyrene is a widely used polymer. After production of the monomer, from one of a few

processes, the monomer proceeds to further processing to form polystyrene.

Styrene Monomer Production:

The energy needed for the reaction is supplied by superheated steam (at about 720 C) that is

injected into a vertically mounted fixed bed catalytic reactor with vaporized ethylbenzene. Thecatalyst is iron oxide based and contains Cr2O3and a potassium compound (KOH or K2CO3)

which act as reaction promoters.

Typically, 2.5-3 kg steam are required for each kilogram of ethylbenzene to ensure sufficiently

high temperatures throughout the reactor. The superheated steam supplies the necessary

reaction temperature of 550-620C throughout the reactor. Ethylbenzene conversion is typically

60-65%. Styrene selectivity is greater than 90%. The three significant byproducts are toluene,benzene, and hydrogen.

After the reaction, the products are cooled rapidly (perhaps even quenched) to preventpolymerization. The product stream (containing styrene, toulene, benzene, and unreacted

ethylbenzene) is fractionally condensed after the hydrogen is flashed from the stream. Thehydrogen from the reaction is used as fuel to heat the steam (boiler fuel). After adding a

polymerization inhibitor (usually a phenol), the styrene is vacuum distilled in a series of four

columns (often times packed columns) to reach the required 99.8% purity. The separation is

difficult due to the similar boiling points of styrene and ethylbenzene. Typical capacity perplant ranges from 70,000 to 100,000 metric tonnes per year in each reactor and most plants

contain multiple reactors or units.


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Polystyrene Production:

In 1996, world production capacity for styrene was near 19.2 million metric tonnes per year.

Dow Chemical is the world's largest producer with a total capacity of 1.8 million metric tonnesin the USA, Canada, and Europe (1996 figures). The main manufacturing route to styrene is the

direct catalytic dehydrogenation of ethylbenzene (above).

The reaction shown above has a heat of reaction of -121 KJ/mol (endothermic). Nearly 65% of

all styrene is used to produce polystyrene.

The overall reaction describing the styrene polymerization is:

This reaction is carried out in an inert organic solvent environment which provides the reaction

medium for this cationic polymerization reaction. The most common solvent used for this

reaction is 1,2-dichloroethane (EDC). Other suitable solvents may include carbon tetrachloride,

ethyl chloride, methylene dichloride, benzene, toluene, ethylbenzene, or chlorobenzene. The

preferred initiator is a mixture of boron trifluoride and water.

The initiator solution is prepared by incorporating 1.5% by weight boron trifluoride gas into theorganic solvent (EDC) containing 280 ppm water. This solution is continuously prepared in a

holding vessel and will then be injected into the reactor system.


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Figure 1: Block Diagrame for Polystyrene Process

Typical feed to the first reactor would consist of 50 weight percent styrene monomer, 100 ppm

water (based on styrene weight), 2000 ppm boron trifluoride (based on styrene weight), with thebalance being organic solvent. The polymerization reaction gives off heat that is carried away

from the reactors by jacketing them with a heat transfer fluid. The temperature of the reactants

should not vary by more than 15C throughout the reactor series. Temperature control is very

important in this reaction because as the reaction temperature increases, the average molecular

weight of the polystyrene decreases. The reaction temperature range is 40-70C. Temperature

can also be controlled by intermediate shell and tube heat exchangers.

The reaction vessels are typically elongated vessels made of stainless steel. The initiator is

introduced as shown below:

Figure 2: Typical Reactor Overview

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Basics of Polystyrene Production.docx