Pollution
Humain
Environnement
Economique

At 10.40 pm, during a styrene/acrylonitrile copolymerisation reaction in a plastics manufacturing plant, the operator in the control room detected an abnormal temperature increase to 125 °C on one of the 75-m³ reactors. The visual display screen in the control room confirmed the request for cooling. An operator then went to the cooling tower to check the water level in the pool, and noted that the water was at the “very low” level: the industrial make-up water supply was no longer operating.

The operator was unable to reprime the cooling pumps. The control room operator initiated the emergency procedure in case of reactor runaway: 3 loads of cold water, 2 m³ each, were introduced into the reactor in an attempt to bring the temperature to 121 °C maximum. The procedure proved to be inefficient, each load lowering the temperature only 0.7 °C. The reactor’s volume prevented any additional water from being added. As stipulated in the emergency procedure, a reaction inhibitor was then introduced to prevent the product from solidifying before the reactor was completely emptied into the “dump” tank placed be low the reactor, i.e. 65 t of styrene-acrylonitrile mixture. At the time of emptying, the limits of the process had been reached (a temperature of 140°C, pressure of 5.2 bar). The thermal runaway of the reaction was due to a lack of water in the reactor’s jacket circuit connected to a low level in the water-receiving tank associated with the atmospheric cooling tower. The operator inspected the tank and noted that 2 vibrating blade sensors were fouled. The failure of the “low” level sensor did not allow the tank’s water makeup valve to open automatically. As regards the “very low” level sensor, its fouling was such that the control room alarm was not triggered. Authorisation for the starting of the manufacturing cycle was therefore not blocked during the preliminary tests carried out on the reactor. Initially, the reactor in question was locked out while the operator had the level sensors cleaned. A maintenance procedure was established: the condition of the sensors will be checked during each reactor cleaning operation, performed every 15 days. A series of tests of the “low” and “very low” level sensor alarms will also be initiated before each production cycle. The operator also intends to install level sensors with different technologies. The installation in question was re-started on July 26th, in late evening, after being subjected to the checking programmes and tests defined above.

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