At 12:34 hrs in a chemical factory, an explosion followed by a fire occurred in a thermo-blower during the drying process of a granulate (m-nitrobenzenesulfonic acid sodium salt). Brown smoke (nitric gas) was released by the roof exhaust of the dryer and 200 empty 200 litre polyethylene drums caught fire.

The spray nozzles in the closed loop dryer were switched over from nitrogen gas to ambient air 90 minutes before the explosion. Residues of another chemical (a cross-linked Polyvinylpyrrolidone – PVP) previously dried in the dryer started oxidising slowly due to the atmospheric air feed (sulphur dioxide could be smelled short before the explosion) , then ignited in the dryer, thus causing the decomposition of about 90 kg of dust of the granulate being processed. The fast accelerating uncontrolled decomposition reaction increased the pressure, despite the gas release through the rupture disk, causing the dryer shell to burst at a internal pressure of approximately 1.5 bar.

The dryer was destroyed, as well as neighbouring equipments; the damage costs were evaluated at 5.5 M euros. As a consequence of the extinguishing operations, a white precipitate (extinguishing foam with traces of chemicals) deposited in the neighbouring residential area. The population was warned and told to carefully wash their garden products.

The plant operator modified the procedures : Different substances can be dried in a closed loop gas dryer only if:

• possible product residues / deposits cannot cause hazardous conditions
• the dryer has to be built in such a manner that complete cleaning can be guaranteed.