During his shift, an employee of a special chemical products plant noticed at 1:20 pm a release of white smoke through the vents on an intermediate storage tank containing acrylic acid (AA, C3H4O2, flammable and corrosive substance, boiling point: 141°C). The tank (70 m³, 75 mm stainless steel shell) had been filled in preparation for a distillation column filling test; moreover, it was heat insulated and inerted with nitrogen. The employee sounded the alarm; a crew of internal fire-fighters attempted to stop the exothermic polymerisation reaction by sprinkling with water cannons, since the addition of water and inhibitor was no longer feasible due to the smoke. Faced with the inefficacy of such measures, at 1:51 pm the plant operator called the municipal fire department, which dispatched a team of 25 men and 3 trucks who reached the site at 2:05. Around 2:30 pm, the highly exothermic reaction led the product to boil; leaking then occurred through cracks in the shell caused by a rise in tank pressure (2.6 bar). Upon arriving at the scene, fire-fighters were notified by company technicians of a tank explosion risk. Despite this warning, they were setting up additional water cannons to back up the internal crew when a BLEVE-type explosion burst the tank (P = 6 bar, 3 kg TNT equivalent): several pieces of debris and a large quantity of overheated polymers were projected within a 70 m radius. Next, 66 m³ of AA and 28 m³ of toluene, originating in 5 adjacent tanks damaged during the blast, poured into the retention basin fuelling a pool fire that ignited by a hotspot and raged until 10:30 pm. The explosion resulted in 37 victims: one fire-fighter burned to death, 5 were seriously injured, and another 31 responders sustained slight injuries. The survivors, who referred to “a rain of fire falling from the sky burning rescue workers’ clothing and protective masks”, escaped thanks to a quick getaway. Two of the three rescue vehicles were destroyed and over 500 employees in neighbouring companies evacuated. The fire was only brought under control at 3:30 pm the next day. The entire site was closed, in accordance with an administrative order, for a full month and the unit affected was closed for 9 months, causing a 10% drop in the world’s acrylic acid production capacity. Production losses amounted to €450 million (45 billion yen), while property damage was assessed at €15 million (1.5 billion yen).

Results of the official investigation commission indicated that:

• The liquid stemming from the bottom of a column had been heated to prevent it from solidifying during a transfer taking place at 100°C instead of the 60°C planned prior to storage;
• The system used to re-circulate liquid from the tank bottom, where it was cooled by upward movement in order to stabilise temperature, had not been activated during the 77 hours of filling preceding the accident, thus triggering a thermal runaway reaction of AA despite the high inhibitor content in the liquid;
• The technician was not equipped with any way to continuously monitor tank temperature, nor was he given any operating guidelines relative to temperature monitoring, which explains the delayed response;
• Since this scenario had not been anticipated, no efficient means of controlling the reaction were made available, as sprinkling the tank proved ineffectual given that it was heat insulated;
• No instructions for responding to this type of accident had been provided.

An analysis of these initial causes revealed that:

• The tank’s cooling capacity had not been taken into account as a safety factor, due to the concentration of inhibitor in the liquid and the presence of a steam heating regulation system; hence, the temperature of the liquid upstream of the tank inlet had not been checked either;
• This regulation system had been disassembled shortly prior due to repeated malfunctions, yet without performing any analysis of safety-related consequences;
• Technicians were not given any up-to-date operating procedures and start-up of the upward circulation system had only occurred on an exceptional basis during the previous 2 years (subsequent to a process modification), with just a single information panel placed 15 m from the technician’s station indicating the need to open the system for large storage volumes;
• The importance of tracking temperature in the tank had not been registered due to high inhibitor content in the liquid combined with low volumes during normal operations, which explains the absence of instruments and continuous control instructions;
• A lack of internal feedback, given that an incident of accidental AA polymerisation had occurred on the same type of tank in another unit in 1994, but only tanks receiving liquids from a column bottom at over 80°C had been fitted with continuous measurement devices;
• The previous incidents of polymerisation had been controlled by means of sprinkling water, leading to the belief that polymerisation could be controlled similarly.

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