Nitrous vapours (NOx) were released during the emergency shutdown of an adipic acid production unit protected by a programmable controller and comprised of 4 reactors in a low-pressure state. The NOx by-product suctioned by a compressor was transformed into nitric acid in an oxidation reactor under 2 bar of pressure. Placing the unit in safe operating mode shut down the compressor and opened an automatic valve. The reactor returned to normal pressure; the NOx was discharged via a chimney 15 m aboveground and for a small fraction above the adipic reactors. Given the absence of wind, internal fire-fighters were able to disperse the highly visible reddish cloud using fire nozzles. One of the responders died from a heart attack suffered during the intervention. Cables had been improperly connected when unplugging the controller to perform tests after intervention. A short-circuit, responsible for burning the controller’s incoming fuse during a simulation signal, actually triggered the shutdown of this unit.

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