The sulphuric acid production unit of a chemical plant was being restarted after a maintenance operation. The operation consisted of warming up equipment with natural gas and ambient air, which resulted in a plume of smoke. At 6:25 a.m., following a technical incident, the unit was shut down. Alerted by calls from local residents, the fire brigade and the gendarmerie went to the site. An SO3 concentration of 0.6 ppm was measured at the base of the building. Measurements at the site’s property limit were negative. The facility operator established real-time emergency communication with the administrations and town authorities. The unit completed its restart procedure after the incident was corrected.

Around 10:50 a.m. the following day, clouds of white vapours leaving the unit’s chimney were reported by a local resident to the plant’s dedicated telephone number. Other residents contacted the emergency services. Complaints were received regarding odours and eye irritation, while some people noticed acidification of the water in their swimming pool compared to the day before. The workshop’s production was slowed down. The release occurred during normal operations of the workshop, which was put into emergency shutdown.

A control failure of the acid titration of the unit’s scrubbing column caused incomplete absorption of the SO3 to H2O/SO4 and the release of 55,000 N m³/h of an H2O/SO3 mixture (odorous and irritant) for 50 minutes. The failure of the SO3 detection sensor delayed the alert. The unit was stopped at 11:50 a.m. The SO3 measurements taken around the site by the plant’s safety department and the fire brigade were negative. The Inspection authorities for classified facilities visited the site. The workshop was restarted at 7 p.m. During this phase, a smaller plume of smoke was noticeable, and precautionary measurements, taken by the safety department and the fire brigade, again turned out to be negative.

The facility operator put the following measures in place to prevent these accidents from reoccurring:

• improvement in the reliability of the unit’s boiler to avoid unscheduled shutdowns,
• improvement in the reliability of SO3 atmospheric sensors through the use of a 3rd redundant sensor,
• launch of a €1 million investment project to eliminate the plume of smoke during the installation’s start-up phases, with the installation of a gas-gas exchanger,
• modification of procedures for communication with neighbouring residents and factories to improve information delays and encourage anticipation of planned events.