PRODUCTS & MARKETS

STYRENE


MONOMERS



Styrene, also known as vinyl benzene, is an organic compound with the chemical formula C6H5CH=CH2. This cyclic hydrocarbon is a colorless oily liquid that evaporates easily and has a sweet smell, although high concentrations confer a less pleasant odor. Styrene is the precursor to polystyrene and several copolymers. Approximately 15 billion lt are produced annually.

Occurrence, history, and use
Styrene is named after the styrax trees from whose sap (benzoin resin) it can be extracted. Low levels of styrene occur naturally in plants as well as a variety of foods such as fruits, vegetables, nuts, beverages, and meats. The production of styrene in the United States increased dramatically during the 1940s, when it was popularized as a feedstock for synthetic rubber.
The presence of the vinyl group allows styrene to polymerize. Commercially significant products include polystyrene, ABS, styrene-butadiene (SBR) rubber, styrene-butadiene latex, SIS (styrene-isoprene-styrene), S-EB-S (styrene-ethylene/butylene-styrene), styrene-divinylbenzene (S-DVB), and unsaturated polyesters. These materials are used in rubber, plastic, insulation, fiberglass, pipes, automobile and boat parts, food containers, and carpet backing.

Production
Styrene is produced in industrial quantities from ethylbenzene, which is in turn prepared on a large scale by alkylation of benzene with ethylene.

Dehydrogenation of ethylbenzene
Styrene is most commonly produced by the catalytic dehydrogenation of ethylbenzene. Ethylbenzene is mixed in the gas phase with 1015 times its volume in high-temperature steam, and passed over a solid catalyst bed. Most ethylbenzene dehydrogenation catalysts are based on iron(III) oxide, promoted by several percent potassium oxide or potassium carbonate.   
Steam serves several roles in this reaction. It is the source of heat for powering the endothermic reaction, and it removes coke that tends to form on the iron oxide catalyst through the water gas shift reaction. The potassium promoter enhances this decoking reaction. The steam also dilutes the reactant and products, shifting the position of chemical equilibrium towards products. A typical styrene plant consists of two or three reactors in series, which operate under vacuum to enhance the conversion and selectivity. Typical per-pass conversions are ca. 65% for two reactors and 70-75% for three reactors. Selectivity to styrene is 93-97%. The main byproducts are benzene and toluene. Because styrene and ethylbenzene have similar boiling points (145 and 136 C, respectively), their separation requires tall distillation towers and high return/reflux ratios. At its distillation temperatures, styrene tends to polymerize. To minimize this problem, early styrene plants added elemental sulfur to inhibit the polymerization. During the 1970s, new free radical inhibitors consisting of nitrated phenol-based retarders were developed. More recently, a number of additives have been developed that exhibit superior inhibition against polymerization. However, the nitrated phenols are still widely used because of their relatively low cost. These reagents are added prior to the distillation.
Improving conversion and so reducing the amount of ethylbenzene that must be separated is the chief impetus for researching alternative routes to styrene. Other than the POSM process, none of these routes like obtaining styrene from butadiene have been commercially demonstrated.

Via ethylbenzenehydroperoxide
Commercially styrene is also co-produced with propylene oxide in a process known as POSM (Lyondell Chemical Company) or SM/PO (Shell) for styrene monomer / propylene oxide. In this process ethylbenzene is treated with oxygen to form the ethylbenzene hydroperoxide. This hydroperoxide is then used to oxidize propylene to propylene oxide. The resulting phenylethanol is dehydrated to give styrene:
C6H5CH2CH3 + O2 → C6H5CH2CH2O2H
C6H5CH2CH2O2H + CH3CH=CH2 → C6H5CH2CH2OH + CH3CHCH2O
C6H5CH2CH2OH → C6H5CH=CH2 + H2O

Laboratory synthesis
A laboratory synthesis of styrene entails the decarboxylation of cinnamic acid. Styrene was first prepared by this method.

Other methods
Styrene can be produced from toluene and methanol, which are cheaper raw materials than those in the conventional process. Historically, however, this process has suffered from low selectivity due to competing decomposition of methanol. Exelus Inc. claims to have developed this process with commercially viable selectivities, at 400-425 C and atmospheric pressure, by forcing these components through a proprietary zeolitic catalyst. It is reported that an approximately 9:1 mixture of styrene and ethylbenzene is obtained, with a total styrene yield of over 60%.
Another developing route to styrene is via benzene and ethane. This process is being developed by Snamprogetti S.p.A. and Dow. Ethane, along with ethylbenzene, is fed to a dehydrogenation reactor with a catalyst capable of simultaneously producing styrene and ethylene. The dehydrogenation effluent is cooled and separated and the ethylene stream is recycled to the alkylation unit. The process attempts to overcome previous shortcomings in earlier attempts to develop production of styrene from ethane and benzene, such as inefficient recovery of aromatics, production of high levels of heavies and tars, and inefficient separation of hydrogen and ethane. Development of the process is ongoing.

Health effects
Styrene is only weakly toxic, with an LD50 of 500-5000 mg/kg (rats). The US EPA has described styrene as "a suspected carcinogen" and "a suspected toxin to the gastrointestinal, kidney, and respiratory systems, among others."
The U.S. EPA does not have a cancer classification for styrene, but currently is evaluating styrene's cancer-causing potential through its Integrated Risk Information System (IRIS) program. The National Toxicology Program of the U.S. Department of Health and Human Services also currently is evaluating styrene's potential toxicity. To date, no regulatory body anywhere in the world has classified styrene as a known human carcinogen, although several refer to it in various contexts as a possible or potential human carcinogen. The International Agency for Research on Cancer considers styrene to be "possibly carcinogenic to humans." Chronic exposure to styrene leads to tiredness/lethargy, memory deficits, headaches and vertigo.[citation needed]
Polystyrene plastic, according to the Styrene Information and Research Center, neither contains, nor breaks down into bisphenol A (BPA), a chemical used in plastic compounds that leads to developmental and reproductive problems in both adults and children.

 



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