At a chemical plant producing polystyrene, styrene was released following an uncontrolled pressure rise. Around 6:30 pm, the transfer pump from reactor 1 to reactor 2 on the DC2 polystyrene production unit stopped subsequent to excessive pressure on the transfer line. This shutdown in turn caused the pressure to increase inside reactor 1. At 7 pm, it was decided to stop operations on the entire unit. The reactor was cooled and the monomer supply cut. Since the reaction was heat releasing, temperature gradually rose until reaching 190°C with a pressure at 2.2 bar. The transfer pump was restarted, but at a slower speed to avoid interruptions due to high pressure in the transfer line. At 8:10 pm, in accordance with the emergency procedure, the vent was opened in order to reduce pressure inside the reactor, which at 8:05 pm equalled 3.35 bar. Heat was no longer being released, and the reaction had consumed a good proportion of the monomer. The premises were vented for 9 minutes. The quantity of styrene/polystyrene mix sent into the atmosphere was estimated at 1 tonne. This vent was positioned at a height of 17 m.
The cloud moved towards the site’s administrative building and towards the nearby factory and a residential zone in a neighbouring city. Fire-fighters were called to the scene after notification from neighbours disturbed by the strong odours (olfactory threshold for styrene is very low). The styrene concentration at breast height was evaluated at 1 to 2 ppm for a 500-m distance from the release site. With respect to styrene exposure limit values, the operator concluded that this incident led to a temporary environmental nuisance, without any consequence for human health.
Subsequent to this incident, production lines were shut down for maintenance. According to the site operator, it was possible that a piece of polymer actually broke loose in the transfer line between the 2 reactors, thereby partially blocking this line and causing the pressure rise in the line. A Prefect’s emergency measure order was enacted on 3rd May, 2005.
