Explosion in a refinery

The event occurred at a reactor for the polymerisation of ethylene. An exothermic runaway reaction took place, which decomposed ethylene into carbon, hydrogen and methane.
The Automatic dump valves (blow-off valves) on the feeding line operated and released the ethylene and its decomposition products through the stacks to the atmosphere. The released gas cloud was probably ignited by the hot carbon particles obtained by the decomposition of the ethylene and exploded.

The explosion caused structural damages to the surrounding plants.

The INITIATING CAUSE was an exothermic runaway decomposition which caused a pressure increase in the reactor with consequent venting of ethylene and its decomposition products.

The reasons for the decomposition of ethylene are not explicitly provided. However, it is reported that the automatic isolation valves on the reactor were not fast enough to prevent more unused ethylene entering the reactor. 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 ROOT CAUSE could be attributed to inadequate system design, unable to control he reaction and to properly handle consequences.

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.

BASIC DESCRIPTIONF OT THE PROCESS
The process affected by the event was the production of polyethylene, a polymer of ethylene, the most common plastic.
Ethylene (IUPAC name ethene) is a gaseous hydrocarbon with formula C2H4. Polymerisation of ethylene to polyethylene occurs according to the following formula:
n CH2=CH2 (gas) → [−CH2−CH2−]n (solid).
A catalyst is used, for example titanium chloride or metal oxides.

Depending on the final product, there are two production processes:
High-Pressure Process at a Pressure of 100–300 MPa and a temperature of 200–300 °C.
Low-Pressure Process at a Pressure of 1–8 MPa and a temperature of 70–300 °C,
The plant involved in the event was probably based on a low-pressure process for the production of LDPE (low-density polyethylene).

Runaway reaction caused by the decomposition of ethylene are reported in the scientific literature as consequence of too high temperature and/or pressure, and in presence of too high oxygen concentrations.

60 kg of ethylene were released, unknown if all combusted.

The water quenching system automatically operated but it failed to prevent the aerial ignition of the released gas. 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 water quenching system automatically operated but it failed to prevent the aerial ignition of the released gas. 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.

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.