A ‘flash’ type explosion happened while an esterification reactor was being filled inside a chemical plant. Two technicians were burned, one seriously. The accident occurred as a technician was pouring a bag of finely divided succinic anhydride (C4H4O3) into a reactor filled with recycled toluene through a manhole. The manufacturing procedure called for: filling the reactor with 1000 l of toluene (capable of containing traces of ethanol due to suction); starting the stirring operation; inerting the reactor in a nitrogen (N2) atmosphere by continuous sweeping; installing suction adjacent to the loading opening in the form of a mobile hose; and lastly opening the manhole and pouring 1000 kg (i.e. 40 bags) of C4H4O3.

According to initial observations, nitrogen sweeping had not been activated and no finding indicated an operable suction hose. Moreover, it seemed that no special precaution had been taken to prevent static electricity from forming. One of the hypotheses forwarded as a cause of this explosion suggested that exceeding the lower flammable limit of toluene generated electric potential, due to the absence of nitrogen inerting and activation of a toluene cloud when opening the polyethylene bags with a cutter.

The explosion did not lead to any plant destruction and had no environmental impact. In compliance with the emergency Prefectural order, an investigation was conducted. Restarting the plant was made contingent on completing a detailed analysis of accident causes and implementing both remedial and preventive actions.