At 7.35 am., a fire broke out in a dyes synthesis plant during the cleaning of a reactor used for the diazotation of p-nitroaniline (p-NA). The reaction occurs in an iced hydrous solution with sodium nitrite and hydrochloric acid (34 %) and liberates nitrous gases that are sent through a piping system to the alkaline absorption column.

The 18,5 m3 batch reactor had not been used for the past 2-3 days. In order to clean it before the next batch, it was filled to 2/3 of its volume with water with 200 l of sodium hydroxide. The technician then introduced low pressure vapour through a pipe directly into the cleaning solution.

But p-NA and NO2 had built-up in the upper part of the reactor, particularly in correspondence with the steam injection pipe as well as in the exhaust system. The dust deposits decomposed /ignited upon heating with the vapour (which heated not only the liquid phase but also the pipe-walls that were not in the liquid up to 100 °C).

The technician saw the flames and sounded the alarm. The unit is shut down, small explosions could be heard in the exhaust system. On-site fire brigades protected flammable material nearby to the fire, switched off the electric supply and shut off the channel system conveying water to the Rhine. The situation was back to normal at 9 am.

The plant operator introduced following measures :

• use of moist anilins, moistening/washing using spraying nozzles to prevent the formation of dust,
• strengthening og the temperature detection system with alarms (automatic shut-off of the ventilators, start of the flooding system…)
• modification of the exhaust system (installation of automatically closing fire dampers as well as spray nozzles in vertical pipes, exhaust air ducts that should not pass through firewalls…)
• modification od the cleaning procedure (insert the steam pipe horizontally under the surface of the liquid; use a double walled pipe as steam pipe…) and increasing of verifications / checks.