During cleaning operations at a chemical site that had ceased its activities, hydrogen chloride was released for a few minutes when a tank containing 11.7 tonnes of benzoyl chloride was being drained. The substance was no longer used and had been stocked in the tank for 3 years. Draining tests conducted a few days earlier proved unsuccessful owing to the presence of a solid decomposition product: benzoic acid. Laboratory dissolution tests with methanol showed no abnormal reaction, and tank drainage operations were planned using 2 m³ of methanol injected into the tank by a pump. At around 9.30 am, an employee began the operation with a fireman from an outside company. Slightly thereafter, 200 l of methanol had already been introduced into the tank when the pump malfunctioned. The operation was suspended and spraying equipment was placed into operation. At 10 am, a weak explosion was heard, most likely associated with overpressure, followed by a gas cloud released from between the flanges of a manhole due to a faulty seal. A water curtain was set up and a fire nozzle was used to dissipate the cloud that was drifting north of the site toward a residential area just 150 m from the tank.
A few local residents were effected by the cloud. An elderly woman was hospitalised for testing and remained under observation for 24 hours.
Hydrochloric acid was involved and its highly exothermic hydrolysis leads to the formation of benzoic acid and HCl. A chemical reaction with alcohols leads to the formation of esters and HCl; the accident was a result of this reaction, 90 kg of HCl (55 m³) were likely released with high heat release for the 100 l of methanol added.
Several alarms were triggered in the guard’s shack who then alerted the internal firemen. The neighbours informed the public emergency services. Only 2 plant employees were present besides the subcontractors. No production manager or plant representative was at the site at the time; the site’s internal contingency plan was initiated only at 3.15 pm. The Classified Installations Inspectorate recorded several anomalies: the accident was not declared, a misunderstanding of the dangers associated with the substances stocked and certain installations, disregard for labelling rules, poor internal organisation and security not in place after business hours… This accident highlights the importance of removing unused chemical products within a reasonable period of time to prevent new hazards (degradation of products, equipment condition, loss of information).
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