Reference

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".

	USGS satellite photo. Position of the source (red circle) with evacuation zone

Order to evacuate

The population was evacuated up to a distance of 0.5 miles around the tank vehicle and a curfew with a radius of 1 mile was imposed. The regional airport Lunken was temporarily closed.

Two policemen were taken to hospital after they had inhaled gas, but they were released again. No further persons were reported to have been injured as a result of the accident.

Possible Reason for the Increase in Pressure

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. The decrease in the TBC concentration from 15 ppm to 10 ppm. at the corresponding temperature over a period of days has been specified in the following table (stored in air Reference):

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.

Chemical hazard symbols

The first hazard symbol to the left is the ‘NFPA hazard diamond’ (according to NFPA 704, cf. also to University of Oregon):  

	Health hazard = 2 (blue square): Hazardous!  Only to be entered in full protective clothing and breathing apparatus during the duration of exposure in the hazard area.
	Fire hazard = 3 (red square) : Danger of ignition at normal temperatures
	Reactivity = 2 (yellow square):  Capable of violent chemical reaction. Increased safety measures. Fire fighting method only to be undertaken from a safe distance.

The illustration in the middle displays the UN number for identifying hazards:

	The Hazard identification No. 39 stands for 3 = ignitable liquid substance, 9= danger of a violent reaction which results from autodecomposition or polymerisation.
	The UN-No. 2055 is equivalent to styrene.

The illustration on the right shows that styrene belongs to UN Hazard Class 3:

Ignitable liquid substances

Physical Properties

Styrene (also referred to as vinyl benzene, phenyl ethylene) is a colourless, benzene-like smelling, strongly refractive, ignitable liquid. 

	Styrene has a boiling point of 145 degrees Celsius and exists as a liquid under standard conditions. The vapour pressure is small at  5 hPa = 5 mbar at standard conditions.
	The flash point is at 31 degrees Celsius and a mixture with air is ignitable within 1 to 9 Vol %.
	Styrene vapour with a vapour density ratio of 3.6 is heavier than air. Following a release there is therefore a danger of the vapour subsiding and settling within the canalization or e.g. accumulating within underground car parks and cellars and forming an ignitable mixture there.
	Styrene has an aqueous solubility of 0.24 g per 1 kg Water and therefore has a low solubility in water. Due to the density of styrene at 910 kg/m3  being smaller than that of water, fluid styrene forms an overlying phase on a body of water.
	Forms explosive, oxidising peroxide compounds.

Toxic Properties

Styrene vapour irritates the eyes and skin. Blistering is possible after more prolonged exposure.

Styrene is assessed as being less toxic e.g. to acetic acid (the ERPG-1 value of styrene is 50 ppm, the ERPG-1 value of acetic acid is 5 ppm.).  However, the EPA categorises styrene as being possibly carcinogenic for humans (see  EPA Air Toxics Website Styrene).

Hazard Assessment using the MET

To assess hazards, we assume two escalating cases: 

We will assume the following weather data:  

Wind velocity: 3 m/s, temperature: 25°C, relative air humidity:  70%, sky less than 50% overcast, daytime, roughness of terrain: village.  

Using standard settings, we obtain the following using the MET:

For case 1 (refer to illustration on the left): 

Toxic hazard outdoors: 1150 m and within the protective shelter of a building: 120 m. 

Explosion hazard: (1% chance of damage to the eardrum): 1430 m.

For case 2 (refer to illustration on the right):

Toxic hazard outdoors:  250 m and within the protective shelter of a building: 20m. 

Explosion hazard: (1% chance of damage to the eardrum): 660 m.

Tank explosion: Fragments: 630 m, overpressure hazard: 120 m.

Fireball (during a BLEVE):  550 m 1st degree burns on exposed skin 1.

When a scenario like this occurs, prior to its release, a certain amount of styrene will be converted into a polymer. This is regarded as being far less dangerous. This fraction of converted styrene can be neglected. However, the fraction of styrene that will be converted prior to release cannot be easily predicted and it is therefore not taken into account. 

Click to enlarge the program mask (German version)

Interpretation

Case where no fire has broken out:

A hazard analysis using the MET depicts an endangerment caused by the explosion of a gas cloud for the unfavourable case in the absence of a fire at approx. 1430 m. This distance is comprised of the dispersal of the styrene after a tank bursts and the detonation of styrene at a maximum distance at which an air-styrene mixture is still just capable of exploding. This distance is therefore a conservative measurement, because a fraction possibly only fulminates without generating a dangerous overpressure. One should not forget that styrene fumes are heavier than air: there is therefore the possibility of a build-up of fumes in cellars, car parks or the canalisation. The critical toxic distance ranges from 1150 m for persons outdoors and 120 m for those who are protected indoors.

In the case of a fire:

In the case of a fire, one has to reckon with hazards due to a gas cloud explosion up until a distance of 660m. For a BLEVE, a fireball would have to be calculated for with a critical distance up to 550 m (first-degree burns).

Hazard potential outdoors

Due to the critical toxic distance of 1150 m and the danger of a gas cloud explosion the critical distance value outdoors should be fixed at 1500 m.

Hazard potential within buildings

The hazard within buildings ranges from a minimum of 150 m (the ignition of wood due to a fireball at 150 m, the critical toxic distance within a building is 120m). One should not neglect the fact that parts of the tank can be flung off up to 630 m (90% of all parts) and that buildings can be heavily damaged. The critical distance within buildings should therefore be extended to 630 m.

Measures taken in reality

The actual evacuation zone set up by the catastrophe defence personnel for this accident comprised 0.5 miles (approx. 800 meters).  A curfew was set up within a 1-mile limit (approx. 1.6 km).

Other accidents involving styrene

We recently reported on a release of styrene in France (refer to it here).

Acknowledgements

We would like to thank the news broadcasting station WCPO-TV (http://www.wcpo.com) for providing us with graphical material.