Pollution
Humain
Environnement
Economique

A reactor overflow with the release of sulphur dioxide (SO2) occurred around 8 am inside a chemical plant during the 5th production run of an intermediate composite.

A double sulphite formation process was underway in the reactor, which had not been fitted with a very high level safety system. At the end of the reaction, the technician who had been trained and familiar with this synthesis process drew a sample for analysis: when opening the sampling orifice beneath the condenser, 200 l of reactive medium spilled along with a release of SO2. The technician, who was wearing individual protective gear (helmet, goggles, chemical gloves, suit and safety shoes) was overcome by the SO2 and slightly burned on the neck by a spattering of the reactive medium. He then closed the sampling orifice and sounded the alarm. His co-workers showered him and escorted him to the infirmary; he required a few hours of hospitalisation to perform additional tests.

The internal emergency plan was triggered at 8:15 am. The site’s first response team rinsed with water the overflowed product before discharging it to the building’s retention basin. After analysis of the 15 m³ of effluent, this quantity was submitted for biological treatment at the end of the day. The operator issued a press release for distribution.

From the operator’s standpoint, the accident was caused by a leak on a water valve located on the lower part of the reactor. This leak, along with the network pressure, would have been responsible for introducing water into the bottom of the reactor, generating a 1,300-litre excess volume in the reactor that went undetected due to the absence of a very high level gauge. The presence of foam inside the glass exchanger (gradual accumulation upon each batch run) was no better able to signal the reactor’s overfilling to the technician. According to the principle of communicating vessels, the reactor’s overfilling spilled into the exchanger and then via the sampling orifice upon its opening.

The operator introduced the following remedial measures: replacement of the reactor’s outlet valve by a sealed manual valve; verification of all line meters (for a reliable accounting of volumes); additional protection offered technicians for manual sampling tasks (gas mask); insertion of an intermediate washing step once every 3 batches in order to limit foam accumulation; and a sampling procedure modification (addition of a visual level inspection via the exchanger). Over the longer term, the operator would adopt a sampling protocol in isolation without preliminary opening of the reactor, along with a very high level detection system installed on the reactor.