Explosion in a refinery

During the polymerization of ethylene in a 750 litres pressurized reactor, an exothermic runaway decomposition occurred and ethylene and its decomposition products were released through the two reactor vent stacks and via the dump valves on the supply lines. The released gas (cloud) was ignited apparently by the hot carbon particles obtained by the decomposition of the ethylene and an aerial explosion occurred.

The following substances were involved in the accident: Ethylene (60 kg). Ethylene (together with the decomposition products about 60 kg). The decomposition products are normally carbon, hydrogen and methane, but no data are available about their amounts.

The explosion caused structural damages to the surrounding plants but no data are available about their cost. The released gas was ignited because the automatic water quenching system operated but failed to prevent the aerial ignition. This was due to a quenching system design not appropriate. The intervention of automatic isolation valves on the reactor were not fast enough to prevent more unused ethylene entering the reactor and therefore to feed the aerial explosion. Automatic dump valves on the feeding line operated 2 seconds to dump residual ethylene to atmosphere resulting also in feeding the aerial explosion.

The initiating (process-related) cause was an exothermic runaway decomposition which caused a reactor pressure increase with consequent release of ethylene and its decomposition products.
The cause of the ignition of the released gas was probably the hot carbon particles obtained by the decomposition of the ethylene).
The automatic water quenching system operated but failed to prevent the aerial ignition. The intervention of automatic isolation valves on the reactor were not fast enough to prevent more unused ethylene entering the reactor and therefore to feed the aerial explosion.
The root cause could be attributed to inadequate system design (quenching system and valve opening).

polymerization reactor, exhasut valves, quenching system

Polymerization of ethylene was carried out in a 750 litres pressurized 19 years old reactor in a polyethylene plant of a petrochemical factory.

MEASURES TO MITIGATE THE EFFECTS OF THE ACCIDENT: After the accident, the following measures were established: 1- to use new smart sensors to speed up the reactor isolation and, therefore, to reduce the amount of gas released; 2- to vent ethylene to a separate, contained system or increase considerably the delay before the automatic dumping of the lines contents.

Event sequence:
19:55. In a reactor with a height of 13 m, vacuum distillate containing hydrogen blew from a flange of the upper piping. It ignited and produced a 0.5-2 m high flame.
20:00. An emergency call to the fire department was made and the emergency shutdown was started. Pressure was reduced from operating pressure of 10 MPa(g).
20:08. Public fire brigades arrived. For fear of a secondary disaster with hydrogen sulphide, they stood by without extinguishing the fire with water. 20:44. The fire extinguishment was confirmed.
20:45. The pressure in the reactor lowered to 0.6 MPa(g). Nitrogen gas was introduced into the system.
21:20. Pressure lowered to 0.3 MPa(g) or less. Cooling by feeding nitrogen gas was continued.

After the accident, the following measures were established: 1- to use new smart sensors to speed up the reactor isolation and, therefore, to reduce the amount of gas released; 2- to vent ethylene to a separate, contained system or increase considerably the delay before the automatic dumping of the lines contents.