Railcar filled with styrene begins to polymerize in Cincinnati - 8/28/05

The end of August of 2005 saw news in Cincinnati covering a story of a railcar spontaneously polymerizing styrene. The railcar was venting styrene and getting hotter thanks to the exothermic polymerization process.

You can catch a glimpse of the railcar being doused with water from 0:30-0:38 in this video...


The report from isitech's website says this...

The escape of gaseous styrene was observed from a safety valve on a stationary railway tank vehicle on the 28th August 2005 at approx. 5 pm, near Cincinnati at the regional airport Lunken in the state of Ohio. According to media reports, the tank vehicle contained approx. 24'000 gallons, that is about 90'000 liters of styrene. The tank vehicle belongs to a company that has been admitted to official quotation on the stock exchange with an ISO 9000 certification and a safety award "OSHA star site".

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Because the opening of a safety valve requires an increase in internal pressure, one can assume that an exothermic (heat generating) reaction had taken place inside the tank. In the case of styrene, a well known reaction is the polymerization of styrene to polystyrene.
A stabiliser like 4-tertiary-butyl-catechol (TBC), which prevents polymerization, is usually added to styrene for transport and storage. In order for TBC to be effective, it is necessary that a certain concentration of oxygen is dissolved in the styrene solution besides TBC. Should no stabiliser be present or it has been used up, styrene can polymerize with oxygen to form a styrene-oxygen copolymer, benzaldehyde or formaldehyde.

Between 10-15 ppm TBC is added to styrene. Under ideal conditions, 10-15 ppm TBC stabilises styrene for approximately 3 months. The TBC can be used up faster according to oxygen concentration, temperature, humidity, rust or other impurities in the tank. In addition, a minimal oxygen concentration of 10 ppm. and preferentially of 15-20 ppm. is necessary.

The higher the temperature is, the faster the TBC concentration falls.

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According to media reports, the tank wagon had been stationary at the site of the accident for 9 months. Due to this lengthy stationary period, the polymerisation would be likely to account for the rise in pressure.

Aristatek's website uses the incident as an example of a training document...

What caused the rail car to vent styrene monomer? The venting occurred because of an increase in pressure inside the tank. The website listed above explained that the increase in pressure was due to heat generated within the tank due to polymerization of the styrene monomer within the tank. Normally, a chemical inhibitor such as 15 parts per million of 4-tertiary-butyl-catechol (TBC) is added to the tank during transport to prevent polymerization. This inhibitor scavenges rust and other impurities within the tank that can act to initiate polymerization. Oxygen (about 10 ppm) is also required to be dissolved in the styrene monomer for the TBC to do its job. The TBC concentration decreases with time as it scavenges impurities; 15 ppm concentration would probably be mostly used up in possibly 3 months (even less time if ambient temperatures are warmer). The website mentioned that the rail car had been sitting there for 9 months. Without the inhibitor, the styrene monomer can polymerize with oxygen to form a styrene-oxygen copolymer or benzaldehyde and/or formaldehyde and polymerize with the release of heat. The heat further accelerates the polymerization releasing more heat. Fortunately, no explosion occurred, the chemical was not released all at once, and people were evacuated to safe distances. The safety valve did t he job it was designed to do, to release excessive pressure buildup slowly avoiding a catastrophic explosion. The error was that the rail car was allowed to sit there for nine months, during which time the inhibitor became depleted.

The Cincinnati Enquirer's website provides a day-by-day recounting of the entire event...

Westlake Chemical Company, the responsible party, did remove the railcar once the polymerization was complete and set up a claims process for local residents. One nearby resident died a few weeks later, and his death was attributed to inhalation of the styrene gas. Settlements ended up topping over $2 million (plus $400,000 for the city of Cincinnati, itself.)

This might've been bigger national news if it weren't for a little storm that hit New Orleans at about the same time...