At around 5:00 p.m., an odour of methyl acrylate was detected near the floor where drums of methyl acrylate were being loaded at a research and production plant for pharmaceutical active ingredients. The operator had the building evacuated as a precautionary measure.

Initially assuming the odour was from the loading operation underway, he sent the reactor exhaust, a single vent until then, to the gas scrubbing tower. Loading was continued and the odour dissipated.

At around 9:00 p.m., residents 800 m away detected odours and called the emergency services. Firefighters conducted reconnaissance and made the connection with plant. The operator implemented the internal emergency plan at 10:10 p.m. He stopped the reaction underway by lowering the temperature from 80 to 30 °C. A family of eight was temporarily evacuated. Measurements taken at around 1:00 a.m. by a mobile chemical response unit (CMIC) showed levels of 0 ppm outside the site and 55 ppm inside the tower. The operator lifted the internal emergency plan at 12:45 a.m. He issued a press release.

Analysis of the causes

The odour did not come from the production floor where methyl acrylate was being unloaded or from the scrubbing tower; it in fact came from a waste storage tank. Before loading the reactor, the operator had drained the distillation residues containing methyl acrylate to the waste treatment area. Due to a valve failure, this waste was sent to a storage tank not typically used and whose set pressure was lower than that of the valve on the usual tank (50 mbar instead of 100 mbar). While the waste was being emptied, methyl acrylate gas formed in the headspace above the liquid in the tank. When the second container of residue was being emptied, the displaced volume and the saturated methyl acrylate vapour pressure (92 mbar at 20 °C) caused the valve to open and release the odour. The weather, temperature inversion, absence of wind followed by a change in direction of the wind, exacerbated the stagnation and development of the odour on and off the site.

The possibility of the formation of an incompatible mixture in the tank, which would have caused an exothermic reaction, could not be borne out because there were no temperature sensors.

As the waste disposal sheets did not contain the list of substances in the batch of liquid waste, it was impossible to prevent the risk of incompatible mixtures. In addition, the waste management procedure did not specify in which tank each type of liquid waste should be placed.

Actions implemented:

The inspection authorities for classified facilities went to the plant on the day of the accident and suggested that the prefect issue an emergency measures order stipulating that the operator must implement corrective actions before the facilities may be restarted. The operator put forward the following immediate actions:

• repairing the waste diverter valve;
• improving the off-gas treatment system;
• raising his technicians’ awareness to improve waste management;
• updating the procedure for identifying waste disposal routes based on type.

In the longer term, the operator plans to:

• add an ‘incompatible effluent mixture’ scenario to the waste-stock safety report;
• update and standardise the waste disposal sheets to be able to assess the risk of incompatible mixtures;
• review the tank design standards and then revise them based on the findings.

The inspection authorities also required the operator to facilitate decision-making so that the internal emergency plan may be implemented quickly and the facilities shut down in the event of an abnormal situation.