Explosion and fire in a chemical plant producing aromatic hydrocarbon

The incident occurred at the hydro-dealkylation (HDA) plant of an aromatics manufacturer. The affected component was a heat exchanger part of the hydrogen purification system. This part failed releasing a gaseous mixture of hydrogen (H2), methane (CH4) and ethane (CH3-CH3) at 54 bar, causing an explosion followed by a fire.
The heater used to regenerate the adsorbent beds of the purification unit had been repaired a few hours earlier, but the automatic heating mode had not been activated upon restart. Consequently, two regeneration cycles had been carried out cold, causing an overload of the adsorber and the fouling of the heat exchanger. The operators detected a pressure problem in the exchanger and shut down the installation. The explosions occurred 40 minutes later.
The investigation showed that the seals and fins of the heat exchanger had been corroded by condensing aqueous components in the hydrogen purification gas flow. The facility had been in operation for 20 years.

The INITIATING CAUSE was the loss of containment caused by the failure of a component of a heat exchanger.
A material analysis revealed that the only cause for containment failure was the corrosion on the fins and the soldering points, caused by small amounts of aqueous components and traces of acidic and basic components.
As a result, during the 20 years of operation, weakened fins gradually broke off and eventually the self-supporting structure ruptured, tearing open the heat exchanger. Inspections of neighbouring parts of the system did not reveal any comparable corrosion damage.
CONTRIBUTING CAUSE was the erroneous execution of two purification cycles after a repair.
The ROOT CAUSE was the lack of a procedure for inspection, maintenance and repair.

Aluminium plate nd fins, heat exchanger, hydrogen purification unit

The plant was restarted after repairing the catalytic absorbed bed. Its heater however was not switched on as required.

DESCRIPTION OF THE PROCESS
The hydrogen purification system of this plant had the goal to separate the methane and ethane produced in the reaction part by hydrodealkylation of toluene from the hydrogen. To this purpose, the methane, ethane and hydrogen mixture was first air-cooled to low temperatures in a so-called cold box before being separated by means of an adsorber.
Upstream of the cold box, a heat exchanger had the function of cooling the gas mixture to 5-9° C. The heat exchange element consisted in an aluminium plate equipped with fins.
As part of the process, small amounts of water (400 ppm) condense together with toluene (700 vol ppm) from a gas flow of 18,000 Nm3/h.
This process had been running for around 20 years.

Only materials damage (2 MEuro).
The damaged parts were repaired or replaced. Clean-up work was necessary and the disposal of the water used during fire-fighting.
1,320 kg of hydrogen, 792 kg of methane and 88 kg of ethane were involved in the accident.

The facility was shut down and sealed off the operational part of the plant. An alarm was sent to the plant and the local fire brigades, which extinguished the fire after approximately 2 and half hours.
TIMELINE
00:53 The shift crew on hand decided to shut down the system and turn off the cold box to thaw.
01:40 There was a dull double bang in quick succession. This was probably the heater rupturing and a subsequent deflagration with subsequent fire.
01:42 Report to control centre
01.50 Block valves around the cold box were closed.
03:30 The fire was extinguished.

The failing of switching on the heater of the catalytic bed would probably require a review of the operative and repair/maintenance procedures and their implementation.
On top of that, It will be necessary:
(1) To control of corrosion and erosion by installing samples.
(2) To continuously control the pH value of the condensed water
(3) To put in place a regular inspection/maintenance and replacement procedure

The plant operator reviewed the corrosion monitoring system and considered regular replacement of all or part of the equipment.