At around 12.00 noon, the emergency services were on the ground in Saint-Louis in response to complaints received from several dozen residents about pungent odours. A patrol carrying out visual and olfactory checks traced the source of the odour to inside a chemical plant. The operator was contacted and reported that, between 11:00 and 11:30 a.m., a tank containing a preparation of ethyl acrylate based lubricant (inflammable, highly reactive and highly odorous: odour threshold of 0.001 ppm) had accidentally risen in temperature (118 °C instead of 58 °C) and pressure (1 bar instead of vacuum conditions). The tank was rinsed out and degassed. The gases emitted were diverted towards the unit’s scrubber where they were filtered, condensed and scrubbed. The wash water was collected in a retention pit pending pumping and treatment by a wastewater treatment plant in Switzerland.

The incoming shift took up their posts in the manufacturing workshop at 5.00 a.m. They were not informed (or were incorrectly informed) by the outgoing shift that the tank containing ethyl acrylate was full and awaiting continuation of the manufacturing process. Thinking that it was empty, the new shift did not think to check its status. In the control room, the shift foreman saw on the screens that the tank’s vacuum pump, allowing a reaction control pressure to be maintained, was in operation. He decided to shut it down because he thought that the previous shift had forgotten to do so. Due to the increase in pressure in the tank as a result of the pump being shut down, the manufacturing reaction restarted and went out of control. As it was not possible to regulate the flow of gases in the tank’s relief conduit (DN 500 and on-off valve), the gases from the reaction were released too quickly to allow the unit’s purification systems to neutralise them (insufficient size).

The tank was rinsed out and degassed. The gases emitted were diverted towards the unit’s scrubber where they were filtered, condensed and scrubbed. The wash-water was collected in a retention pit pending pumping and treatment by a wastewater treatment plant in Switzerland.

The owner reiterated the requirement for the incoming shift to systematically check the production line tanks, regardless of the information given by the outgoing shift. Awareness was raised among staff of the need to follow instructions precisely and a technical change was made to the system for releasing gas from the tank to the unit’s condensers and scrubbers (fitting the relief conduit with a low-diameter bypass equipped with a valve).