A flashover followed by a fire occurred at a fine chemicals plant whilst a technician was emptying a flexible intermediate bulk container (FIBC) of crude loxapine (a wet intermediate of heptane) into a dryer. The worker, who was slightly stunned, noticed that his suit had melted and that the FIBC was charred. He pressed the emergency circuit breaker button. He tried to put the fire out with a dry chemical extinguisher, but it failed to operate. He then grabbed a second one located at the end of a corridor and was able to extinguish the fire. Managers secured the plant. They isolated the dryer by closing its lid and injecting nitrogen into it for 10 minutes to evacuate the remaining oxygen and solvent vapours.

Loxapine, which is used in the manufacture of an active pharmaceutical ingredient, is generally obtained after being placed in a spin-dryer. However, as the plant’s four spin-dryers were out of commission, it was passed through a filter and then emptied into two FIBCs for drying. This change in procedure resulted in a higher residual solvent content.

The operator identified multiple causes:

• The flashover was ignited by an electrostatic discharge caused by the simultaneous presence of a wet powder of flammable solvent, oxygen in the dryer and a loading chute made of non-conducting material (which did not dissipate the static charges built up by friction between the powder particles and the equipment);
• The flow of oxygen, bringing together the ignition/combustion conditions required for the reaction medium, led to a loss of inertness. This loss was due to the absence of a stream of nitrogen inside the cavity and the dryer’s loss of inertness. The flow of oxygen that was drawn in when the dryer was opened up for the unloading of the FIBCs, and which was not offset by a flow of nitrogen, created a non-inert zone with sufficient oxygen content to create an explosive atmosphere in the presence of heptane vapours and powder particles.
• The drying procedure does not require the maintenance of inert conditions inside the dryers during FIBC loading. This is to prevent the risk of asphyxiation in the event of a nitrogen leak (the flow of nitrogen used for inerting is located at the loading station, i.e. adjacent to the technicians’ work area).
• The humidity level in the building was abnormally low. Electrostatic discharges are more likely to occur in low-humidity conditions.

To prevent the same type of accident recurring, the operator:

• is looking into the possibility of replacing the silicone loading chute with an antistatic one;
• is going to repair the building’s humidifier;
• has incorporated a dryer-inerting stage and the maintenance of a nitrogen flow during FIBC loading into the procedure;
• raised technicians’ awareness of the importance of systematically carrying out inerting procedures;
• equipped the dryers with a nitrogen flow meter;
• added a nitrogen or breathing-air system of pipework to eliminate the potential risk of asphyxiation.