Gaseous chlorine was detected at a chlorine chemical plant at around 8:00 a.m. near the pit used to neutralise effluent from the towers used to scrub toxic gases produced by the production of propylene from hydrocarbon pyrolysis. Fire-fighters arrived to measure toxicity levels within the site and the immediate surroundings. These measures were found to be negative. Although the release was stopped by shutting down the facility, local residents complained of smells between 9:00 and 10:00 a.m. due to wind conditions. The plant operator issued a press release. The unit was started back up at 9:45 a.m. following work on a chiller.

The gases released by pyrolysis are made up of chlorine and hydrogen chloride. The hydrogen chloride is usually dried, compressed and purified in a fractionating tower so that it may be re-used in the unit. When the tower is started up, the purity of the hydrogen chloride at its top is insufficient. The hydrogen chloride is therefore sent to two scrubbing towers and water is injected to neutralise it as hydrogen chloride in solution. The chlorine is neutralised with sodium-bearing water (20% NaOH) in a third tower. The alkaline effluent from this tower is made up of sodium-bearing water and sodium hypochlorite resulting from neutralisation of the chlorine.

On the day of the accident, a problem with the supply of coolant (NH3) to the chiller used to cool the fractionating tower prevented the hydrochloric acid being re-used. The temperature at the top of the tower was 0 °C instead of less than –15 °C. The gas was then sent to the scrubbing towers to be neutralised. The high flow of hydrogen chloride through the scrubbing towers caused an unusual amount of acidic solution to flow into the neutralisation pit. As it mixed with the effluent from the chlorine scrubbing tower, the pH in the pit changed from alkaline to slightly acidic (pH 6.5). There was not enough residual sodium-bearing water to neutralise all the acids in solution in the pit. This slight acidity was sufficient to allow the chlorine trapped as sodium hypochlorite to be released as gas.

The operator changed the operating instructions so that technicians stop the pyrolysis furnace if a significant problem occurs on the chiller of the hydrochloric acid distillation tower (temperature greater than –15 °C at the top of the column). The operator is even considering fitting an automated control on the temperature measurement.