A cloud of ammonia emanated from a reactor at a chemical manufacturing plant five minutes after 100 kg of 2-Cyanoguanidine had been loaded through the manhole. During the manufacturing process, once the reactor cools to 145 °C, a technician must add 600 g of defoamer followed by 794 kg of 2-Cyanoguanidine powder (C2H4N4) packaged in 25 kg bags. The cloud formed after part of the powder was added.

A faulty temperature sensor caused the technician to shut off the cooling too early and start manufacturing outside the expected ranges

Seeing the cloud of ammonia, the technician measured the temperature with the sensor of a nearby reactor to compare it against the value registered on the sensor of the reactor being used, which was 142 °C. At 160 °C, the sensor was at the maximum of its measurement range (actual temperature estimated between 180 and 200 °C). The technician reclosed the manhole (which was not sealed), started degassing the reactor to the ammonia trap, turn on the reactor cooling, and sprayed it with a hose to cool it down and knock down the ammonia vapours. The cloud of ammonia drifted south-west, towards an uninhabited area and dissipated at around 5:45 p.m.

Poor risk analysis

The absence of redundant temperature measurement, an essential monitoring parameter to avoid reaction runaways, is one of the causes identified afterwards by the operator. Newly installed unshielded mixers had generated electromagnetic interference that affected several sensors that were found to be faulty on several reactors. The operator replaced them without reconducting a risk analysis.

After the event, the operator decided to install an ammonia scrubber to minimise the risk of any accidental release.