Explosion from flammable gases from a de-hydrogenation reactor

An explosion occurred in a water skim tank associated with a reactor for dehydrogenation of the ethyl benzene. The reaction products from this dehydrogenation reaction consisted in a mixture of styrene, ethylbenzene, benzene, toluene, hydrogen, C02 and steam. Following a treatment in a "separator", the water still entrained hydrocarbons and styrene polymers and passed into the skim tank where a further separation of the two phases took place. This process occurred in overpressure in respect to atmospheric pressure, to allow for venting of the product of the separation process.
At this point, an emergency plant shutdown was carried out, due to a partial electric power failure. An explosion occurred one hour later. It was probably caused by the entrapment of air in the skim tank, which then formed a flammable mixture with the hydrocarbons and hydrogen. The accidental air injection was due to the sinking of the water level following the power cut, what caused the pressure in the tank to fell below the external atmospheric pressure, creating the condition for air to be sucked in via the atmospheric vent. The ignition of the explosive mixture occurred probably by friction over heating of the neck bush on the tank's submerged pump shaft.

The INITIATING CAUSE was the fall of the water level in the skim tank due to the shut down. This created under-pressure in the tank and the suction of the air forming a flammable mixture with hydrogen and the hydrocarbon.

This could have happened at any time, since it was not foreseen to purge the tank with nitrogen.

The ROOT CAUSE is related to a process design mistake, rather than to a improper operative procedure.

tank

The explosion occurred one hour after a plant shut down due to a power supply failure.

DESCRIPTION OF THE INDUSTRIAL PROCESS
The process regards the production of by styrene (C6H5CH=CH2) by dehydrogenation of ethylbenzene (C6H5CH2CH3). The reaction is achieved by using superheated steam (up to 600 °C) and a solid catalyst:

ethylbenzene + water vapour (inert) => styrene + hydrogen.

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The main recommendations were of technical nature, to prevent the creation of flammable atmospheres:
1. Blanketing with insert gas
2. Preventing air ingress
3. Replacing of the submerged pump by an external pump
4. Installation of a panel monitoring the water level in the tank and issuing a low-level alarm
5. Automatic closure of the tank level control valve on power failure.