Fire in a chemicals production plant

The fire occurred in a unit to produce hexamethylenediamine, an organic compound precursor of polymers (HMD). The process is based on the hydrogenation of a precursor in presence of a catalyst.

A jet fire of hydrogen was generated during shutdown of the hydrogenation reactor. The shutdown was part of the operative cycle, aiming at cleaning (regenerating) the nickel catalyst. The event initiated in the reactor exit tank, which was collecting the reaction products: HMD and water. The liquid level in this tank was monitored by two independent instruments: a floating gauge and a gamma-radiography technique.
A sudden increase of the signal indicating the level of liquid in in the exit tank caused the automatic full opening of the regulation valve and the emptying of the tank. This caused a back-flow of the hydrogen contained in the dome of the reactor into the tank through the degassing line. This line, under normal conditions, had the purpose to bring back to the reactor the residue of hydrogen dissolved in the reaction product.
The hydrogen leak (5000 Nm³/h in 10 min) was detected by an over consumption of hydrogen, which induced the operator to warn the operative room.
When closing the regulation valve, the workers saw a jet fire at the fire trap of the reactor placed outside the building. After closure of the valve, the fire extinguished automatically due to the absence of hydrogen.

The INITIATING cause was a sequences of events triggered by partially wrong readings of the monitoring system (see below).
The root case could be identified in operative shortcoming, which allowed functioning of a not properly adjusted instrument.

The two level measuring instruments were indicating different values. The gauge indicates 100%, and this led to discharging the reaction product tank and thus to hydrogen entry through relief pipe.
The emergency shutdown was based however on the reading of the radiography, which should have activated the shutdown when coming below the 10% level signal. Unfortunately, the equipment was not (well) calibrated and was indicating 12% (instead of 0). The emergency shut down was not triggered.
These two wrong measures lead to a continuous leak of hydrogen through discharging pipe. The leak was detected by an over consumption of hydrogen.

Hydrogenation reactor, collecting tank, liquid level detectors

The hydrogenation reactor was shut down for maintenance (regeneration of the catalyst)

DESCRIPTION OF THE CHEMCIAL PROCESS
Hexamethylenediamine or hexane-1,6-diamine, is the organic compound with the formula H2N(CH2)6NH2.
It is used almost exclusively for the production of polymers, mostly for the production of nylon 66.

It is produced by the hydrogenation of adiponitrile, in presence of a catalyst, according to the reaction:
NC(CH2)4CN + 4 H2 → H2N(CH2)6NH2

The source of this event does not provide information on the process parameters such as temperature, pressure and reactant masses. From the technical literature, the pressure ranges from 120 C to 170 C, and the pressure is in the 200 to 350 bar.

The increase in pressure was handled by performing a manual venting which brought back the pressure to a value lower than the nominal one.

The post-incident investigation found out that since at least three days the difference between the two measurement techniques for the level of liquid in the tank was more than 10 %. The system safeguard was based on the signal coming from the gamma-radiography technique. It is activated when its measurement show a level below 10%. The zero-shift on this instrument, however, was higher than this value. Therefore, for all this period the unit has operated without a safeguard in place.

In the past, in the case of a similar fire on the fire-trap of another site, the operator had replaced the open tank with a closed one. This solution was not adopted for the unit affected by this accident, because the operator in this case preferred the adoption of preventive measures in the form of safe barriers placed before the possible place of the accident.

One of the lessons drawn by the inspection was that measures limiting the occurrence of an accidents (preventive) and measures for limiting their consequences (mitigating) are not incompatible, but complementary.
Safety barriers and mitigation measures are complementary and should both be used.