In a chemical plant, sulphur trioxide (SO3) was released into the atmosphere via the chimney of the sulphuric acid (H2SO4) production unit and a greyish cloud was blown over the city. Sulphur dioxide (SO2) is produced by the combustion of sulphur in the presence of air, then the SO2 formed is converted into SO3 by catalytic reaction. The SO3 is directed towards an absorption column where it is hydrolysed into sulphuric acid by spraying H2SO4 (recirculation of the H2SO4 produced). On the day of the accident, a sudden drop of the absorption tower’s acid spraying rate was noted at 11:38 am. At 11:45 am, the control room technician stopped the production unit: the injection of sulphur was stopped. The establishment’s internal emergency plan was activated at 11:48 am. The installation was completely shut down at 11:51 am. With no spraying, SO3 was released directly into the atmosphere via the chimney of the final absorption column.

The operator estimated that 123 kg of SO2 were released in 7 minutes, as the concentration of the discharge was 139 ppm and the concentration in the environment never exceeded 15 ppm (VLE). The failure of the immersed H2SO4 recirculation pump caused the accident: the hub had detached from the body of the spray pump turbine, but its motor continued to operate, and the pump’s rotational detection alarm did not detect the failure. This failure resulted from repair work carried out 9 days earlier on one of the unit’s heat exchangers: when looking for the leak on the heat exchanger, water in all likelihood passed through its shell and into the acid circuit, thus reducing the acid concentration. This drop in the acid content conveyed by the pump led to its corrosion, amplified by the high velocity of the fluid at the surface of the turbine. Corrective measures are being implemented: deployment of an instrumental safety system on the acid spraying flour rates, intensity measurement on the acid spray pumps controlling the sulphur feed pump, integration of a zero spray flow rate in the absorption columns into the emergency procedure and staff training. A discussion is also held on how to prevent the possibility of water entering the acid recirculation tank.