At a chemical plant, a confined explosion occurred at 5:30 a.m. on an empty 700 m³ tank containing methyl esters (flash point: 110 °C). These esters are produced by transesterification of oils with methanol. The tank in question was being used for storage between the single transesterification and hydrogenation operations and separation of glycerol by decantation. The tank was heated by a steam system. The overpressure deformed the tank, without cracking it, while it was empty for cleaning. The anchors were ripped out of the ground. Alerted by the sound of the explosion, technicians noticed that the tank was deformed and that smoke was coming from its vents. A water curtain was set up and a steam inerting process was used inside the tank to extinguish a possible fire on the inside. Inerting operations continued for more than 24 hours.

Production was not affected as other tanks were used for the activity. The property damage was estimated at 450 k€.

Causes

The explosion was caused by the self-ignition of residues in the tank as a result of the heat coming from the heating system, and the excessively low water level in the empty tank which had exposed the heating coil. The reactor was opened following the accident and a solid deposit was noted inside the tank. The deposit essentially contained esters, glycerol and traces of acrolein. The facility operator calculated that the deposit self-ignited at 144 °C, which was the temperature of the heating system at the time of the accident. The deposit’s porosity contributed to this phenomenon. The operator employed the same process on two other production sites and this accident had never occurred there.

Measures taken

After the accident, the facility operator implemented regular cleaning of all the tanks containing methyl esters. The operating instructions were modified to integrate a minimum drainage level so as to avoid venting the coil and possible formation of deposits. For cleaning operations that require complete emptying, the heating system is switched off and the tanks are inerted with nitrogen. The operator is considering modifying the tank heating system to incorporate a heater control on the fill level system, as well as a systematic inerting system. The possibility of using hot water instead of superheated steam is being studied to reduce the temperature of the surfaces inside the tank.