In a chemical plant, a flash occurred during the cleaning of a 700 l reactor in the new synthesis development workshop (stage preceding the industrial start of manufacture and carried out following semi-industrial departmental and laboratory tests). The 2 technicians present nearby were treated by the emergency services. One of them received serious burns to his face and was hospitalised.

The accident occurred after the reactor’s last methanol cleaning phase following the 3rd manufacturing campaign of a product using a palladium acetate catalyst. According to the guidelines, the reactor had undergone the 4 specified cleaning stages (solvent then with acid solution, sodium and finally acetone) under nitrogen inerting. Noting the presence of palladium residues, the technicians decided to clean it with methanol brought to boiling point. After draining the methanol and cooling (30 °C), the operator opened the reactor’s manhole. He wiped the edge of the reactor using a paper towel to check that the wall was clean, then went back to the document-holder console located 2 m away without resealing the manhole. While he was observing the persistent traces of black product presumed to be catalyst residues, which are difficult to clean, the flash occurred.

Ignition of the residual methanol vapours from the manhole, initiated either by a palladium/methanol/oxygen (highly likely) reaction, or by an electrostatic discharge from the console (not very likely) would have caused the accident.

The use of methanol was not specified in the cleaning instructions and it had not been used for the previous 2 synthesis campaigns. Nevertheless, its use was recommended by the semi-industrial department according to the transfer meeting report.

For feedback purposes, the staff were made aware of the risks related to not rapidly closing the manhole; for reactions using similar compounds, the manual check of the cleanliness using a paper towel (swabbing) in the presence of solvent vapours in the reactor was prohibited: before opening the manhole, this involved depressurising the reactor then breaking the vacuum with nitrogen or filling the tank with water.