Pollution
Humain
Environnement
Economique

A benzoyl peroxide (BPO) explosion occurred in a paint and coatings manufacturing company. In a dedicated building, 98 % granular BPO is produced by drying batches of 75 % granular BPO using -since 2001- a spherical rotating vacuum dryer (jacketed and glass-lined). On Friday the 27/12/2002, the employees began normal procedures to prepare a batch of 98 % BPO. The vacuum dryer was loaded with 90 kg of granular 75 % BPO and started. As per practice, hot water to the dryer was shut off at 2 pm when the inner temperature reached 42°C to allow the material to cool. Since the plant did not operate over the weekend, the drying system remained off and sealed until Monday 30/12/2002, when operators followed normal procedure to restart the drying system. Again, due to the new years’ bank holidays, the drying system was not operated, and the dryer remained sealed until plant personnel returned to work on January 2, 2003. Because the production typically took 2.5 days, operators anticipated that the batch would be ready after completing one last drying cycle in the morning. At 8:50 am, the temperature inside the dryer had reached 42°C : as usual, the heating was stopped and the dryer continued to rotate under vacuum to allow the material to cool.

At 11:30 am, the operators took their lunch break at a table 10 m away from the dryer. One of the operators noted an unusual noise coming from the vacuum pump, which he planned to check after lunch. At 11:55 am, the vacuum dryer suddenly exploded with –according to the employees that quickly left the premises- “thick black smoke with rolling flames and a loud boom”. One of them was slightly injured by flying debris.

The automatic building sprinkler system activated and the Fire fighters arrived quickly on scene to extinguish a residual fire. Runoff water leaving the property was tested at several locations and determined to be non-hazardous. The dryer had been propelled through the corrugated steel dividing wall and through several pallets of filled fibber drums and landed 10 m away. The building was badly damaged.

The US CSB investigated the case and showed that the likely cause of the explosion was thermal decomposition of the peroxide, since the drying system was running very close to the thermal decomposition temperature. Indeed, the heating was stopped, but since the jacket was not designed to be drained, the hot water remained in the jacket at 82°C until it was cooled by heat losses to the surrounding environment.

The experts underlined that the hazards of BPO are well documented and that the operator should have reviewed consensus standards and guidance documents on the handling, storage, and manufacture of BPO, and implemented their recommended practices (such as gathering relevant hazard information, reviewing reactive hazards, developing a preventive maintenance program and developing and conducting training on operating procedures for normal and abnormal situations).