In a Seveso classified factory manufacturing intermediary compounds for organic chemistry, a chemical runaway followed by an explosion occurred at 9:10 p.m. in the glass column of a 3000 l reactor.The light cladding of the building housing the reactor is washed away. testimonies report to have seen and fireball rising. The explosion ignites a fire in the unit. A thick cloud containing 110 kg of hydrochloric acid (HCl) forms above the site and is dispersed in a few minutes towards the ocean due to the wind.
Employees trigger the emergency shutdown and begin to fight the fire with the local resources. On site at 9:20 p.m, firefighters mobilize significant resources (50 men, 9 trucks) and find upon arrival that the fire is almost contained. They baste the damaged reactor with four nozzles to cool it while the site is evacuated and the nearby higway access is closed by the police. With the help op someemployees, they inspect the neighboring units to detect any toxic leaks caused by domino effect. Access to the highway is reopened at 11 p.m while the intrenal emergency plan is lifted. Firefighters leave the site at 0:30 a.m while a reduced team is left on site for monitoring during the night. The 180 m³ of water used for extinction is collected in the retention basin of the plant. 8 employees are given medical attention, 5 of which remain under observation overnight. Other units / reactors of the plant are not damaged. The operator cleans the accidented area to prevent injury by falling pieces of siding still clinging to the building structure. The site activities resume 48 h after the accident. Damages are assessed at 700 k of Euros.
The day of the accident, a manufacturing batch had been ongoing since 3 p.m by adding at low temperature 1000 kg of liquid ethylene compound with 750 kg of a highly flammable and volatile liquid silylated compound (hydrosilane). The homogeneous mixture was then poured into a second reactor at 100 ° C in the presence of catalyst (chloroplatinic acid hydrate) to form the final product. Control of the hydrosilation reaction is ensured by the progressive introduction of the mixture. At 9:10 p.m, a sharp rise in the temperature of the mixture caused overpressure and the bursting of the column. The hydrosilane was then hydrolyzed in HCL on contact with moist air and broken down into H2 as the source of the collateral fire.
The investigation showed that, given the loss of catalyst activity (7th consecutive batch) leading to a lengthening of the duration of the batch, a technician decided to introduce tens of gramme of fresh catalyst in the first reactor along with the raw materials. Bibliographical studies and laboratory tests showed that the reaction can not start into this reactor at low temperature ( 5-20°C) , the difference with the temperature required for the synthesis (90 ° C) in the second reactor appearing as a guarantee for the safety of this modification judged as minor. However, tests conducted by the operator after the accident revealed that an hydrosilylation exothermic reaction can occur at these temperatures, after an induction period of several hours, in the presence of traces of alcohol. Given that the catalyst has been dissolved with a ketone, a small amount (about 0.01% ) was present in the mixture inside the reactor and was reduced by hydrosilane, this reduction being favored by the mass effect. Despite a rigorous process of reactionnal risks analysis and 36 batch manufactured over 6 years without any accident, this accident occurred on the very batch for which the process has been slightly modified. The operator reminded its employees that 1) according to the safety criteria of the plant, this change should have been classified as significant and lead to a collective in-depth analysis before implementation 2 ) any unilateral change of a process must be substantiated and be subject to compensatory safety measures.