In a chlorochemical plant, the chlorine detection network (Cl2, toxic gas) was activated near the chlorine gas compression unit at about 11:45 p.m. A quick investigation revealed a leak on the seal of a heat exchanger. This exchanger heats the carbon tetrachloride (CCl4) used to absorb any chlorine remaining in the inert gases through a column at the outlet of the electrolysis unit.
The unit’s pipe operating technicians switched over to an emergency exchanger. This switchover caused the pump to lose its prime, supplying the absorption tower with CCl4. Several unsuccessful attempts to re-prime led the technicians to divert the incoming inert gases to the unit’s safety column at 11:55 p.m. (soda neutralisation). The inert gases trapped in the absorption tower were directed towards the stack to be incinerated. At approximately 00:08 a.m., a failure of the hydrogen sensor (H2) in the stack triggered the closure of this circuit. Inert gases trapped in the absorption tower could no longer be destroyed.
To avoid an explosion at the top of the tower, the technicians diluted the H2 present in the trapped inert gases by maintaining the injection of air at the bottom of the column. This injection slightly increased the pressure in the column. Its discharge valves to the stack opened at 0:09 a.m. From 00:11 to 00:14 a.m., chlorine sensors from neighbouring workshops detected peaks in the atmosphere from 0.5 to 2.5 ppm. A technician, slightly intoxicated by the fumes while conducting his rounds, was examined in the infirmary. A chlorine smell spread through the control room of the neighbouring unit, even though it was equipped with an air detection and purification system. The facility operator initiated its internal contingency plan; toxicity measurements in the air proved to be negative. Production returned to normal levels in the late morning after the safety conditions had been checked. The chemical platform housing the plant issued a press release.
The CCl4 pump’s loss of prime was caused by the influx of air during the switchover to the emergency exchanger whose circuit, looped on the column, was empty. The accumulation of hot inert gases in the absorption tower at the beginning of the accident created back pressure in the CCl4 supply system, preventing the prime from being re-established in the pump. The penetration of a small amount of chlorine into the adjacent control room was caused by a failure of the chlorine analyser on its air intake vent. The sampling line on this detector was full of water. The water had absorbed the chlorine present in the air drawn in, preventing the intake from being switched over to the activated carbon filters.
The operator initiated the following corrective actions:
- improvement of heat exchanger reliability,
- switching of the air intake from the control room over to the active carbon filters pending a solution at the water inlet on the detector’s sampling line,
- integration of this accident into the unit’s hazard study scenarios.