The International Biosynthetics (IBIS) plant was, at the time of the accident, a wholly owned subsidiary of Shell UK Ltd. and employed some 250 people in the manufacture of fine chemicals.
The release occurred on the phosgene plant at 11:27 hours on 7 December 1991. The batch reaction involved the phosgenation of dimethyl aniline (DMA) in a toluene solution. The process involved the addition of 1 tonne of recycled toluene to the reactor, then as no more recycled toluene was available fresh toluene was to be added. Attempts were made to fill the reactor with 2 tonnes of fresh toluene. The flow indicator showed that the required amount of toluene had been added to the reactor, however a control valve between the pump and the vessel was closed and none of the toluene had been added. A level measurement was available for the vessel but as the process appeared to be proceeding normally this was not checked.
The next stage was to add 20 kg of phosgene to check if any water was present in the reactor. This would have resulted in a temperature rise of the solution. Because there was insufficient toluene in the vessel, the temperature indicator was not in the solution and therefore showed no temperature rise. As there appeared to be no water in the reactor, 0.8 tonnes of phosgene were then added to the vessel. After a shift changeover the next steps in the process were carried out. These were to add 1.6 tonnes of DMA and heat to 65°C. The operating temperature was reached but the temperature continued to rise to well above 100°C. As the pressure increased the pressure control valve, pressure relief valve and bursting disc all operated as designed and relieved the vessel to a scrubbing column. The reaction was more violent than had been predicted and the relief system had insufficient capacity to deal with the pressure rise. This resulted in a connection on the condenser line failing and releasing the contents of the vessel to atmosphere. Fortunately the phosgene had been consumed in the reaction. However, the vapour cloud drifted for 4 km affecting about 60 people and staining some property blue.
Health and Safety Executive, ‘Release of Chemicals From International Biosynthetics Limited. A report of the investigation by the Health and Safety Executive into the chemical emission from International Biosynthetics Ltd. on 7 December 1991’, ISBN 0 11 882 154 7, 1993.