At 3:15 p.m., a burning leak occurred on a defluoridation tank containing butane, propane, traces of organic fluoride, and traces of hydrogen fluoride (HF) at a refinery’s alkylation unit. The operator implemented the refinery’s internal emergency plan. The unit was placed in a safe state. The HF used in the process was transferred to its storage tank. The onsite firefighters extinguished the fire at 3:28 p.m. One fell ill from the smoke and was taken to hospital. He was released the same day. The facilities were cooled and depressurised.

A 50 cm opening was found on the defluoridation tank (V=2770 l). A super-concentration of HF caused a reaction peak with the alumina present in the defluoridation tank, resulting in an uncontrolled increase in temperature (+220 °C in 30 s) and causing the equipment to lose its mechanical characteristics, rupture at its operating pressure and release 1.5 t of gas that, upon contact with a hot spot, formed a 7 m flame.

Undetected loss of process control

The defluoridation tank is part of the alkylation unit’s fractionation section. Located downstream of a column, it is used to separate HF from hydrocarbons that are supplied from a settling tank. Multiple technical failures occurred on the day of the accident:

• the sensor used to measure pressure in the separator column, and thus control the column’s operation, was fouled;
• the sensors used to measure the flows entering and exiting the column, and which are compared to allow technicians to check the stability of the operations, were also fouled;
• the liquid level sensor in the settling tank was also defective.

The unit was being restarted following a shutdown. The incorrect pressure reading caused the separator column to malfunction and a super-concentration of HF formed at the bottom. The technicians, trusting the instrumentation, suspected that HF had been drawn into the settling tank. They asked an instrumentation specialist to check its level. As the same time, they stopped the transfer of the bottoms fraction to the alkylation unit. However, the sensor used to measure the column’s outlet temperature (which, if low, could indicate poor separation) is located on this circuit. When the instrumentation specialist confirmed that there was no HF in the settling tank and that its level was correct, the technicians opened the defluoridation tank’s feed valve. It ruptured within seconds of this.

Supplemented restart protocol

The operator took the following corrective measures:

• it moved the temperature measurement sensor to the column’s outlet to keep it operational;
• it modified the restart procedure to include the sensor’s location;
• it added alarms to the defluoridation tank’s temperature (high and differential alarms);
• it modified the restart procedure to include a check of the separator column’s instrumentation (pressure and flow) and the settling tank’s instrumentation.