At 10:30 pm in a fine chemistry plant, a reactor underwent a pressure rise during chlorination of alcohol using thionyl chloride (SOCl2). This slightly exothermic reaction took place in a solvent medium (1,2 dichloroethane, or DCE) and under a slight pressure drop (10 to 20 cm of water). A vapour injection served to maintain the temperature at around 70°C. At the outset, the reactor contained SOCl2 in the DCE solution, with alcohol subsequently added during a 30-hour controlled process. At the time of the accident, the reactor was being fed for 3 hours by successive 200-litre loads of alcohol, and the 1st casting had not yet been completed. The monitoring by 2 technicians, one of whom was in training, included an hourly verification of both the temperature and pressure drop: no anomaly had yet to be observed.
Around 9:30 pm, the technician turned off the control box selector for the powder loading chute used during the previous operation carried out in the same reactor. At that time, he was surprised by a noise, accompanied by the break of a protection disc on the glass column connected to the reactor, and noticed smoke in the vicinity of the disc joints. He proceeded by closing the alcohol inlet valve and was moving towards the vapour injection closing valve upon noticing that the leak was expanding on the column; he immediately left the unit, in requesting that his co-worker follow him out, when the explosion occurred.
A rupture disc calibrated at 0.3 bar and the glass enclosure on top of the device burst. Either the explosion or the toxic gases released following the shattering of equipment caused the death of the trainee technician, who did not immediately leave the premises. No environmental consequences ensued, as the toxic gaseous mix released (HCl / SO2) stayed confined inside the building.
A judicial investigation was conducted; analyses confirmed that all materials used onsite indeed complied with specifications. The anhydrous fluid feeding the condenser had not reacted with the medium. Moreover, no trace of sodium was detected inside either the reactor or the trap drum, thereby excluding the hypothesis of a return of the hydro-alkaline removal system used to neutralise the SO2 generated by the reaction. The plant’s equipment was also compliant with specifications; 1,500 litres of solution were found in the damaged reactor. The powder loading chute had been fitted with 2 valves; the upper one (on the loading booth side) was found closed, while the lower one (on the reactor side) was open with its pneumatic control hoses placed in the reverse position. These observations favoured the hypothesis of an accidental addition of water into the reaction medium via the powder loading chute. The laboratory simulation of such an addition indicated that SOCl2 hydrolysis with SO2 and HCl formation could lead to a sudden pressure rise that the safety device was unable to suppress.
This accident was due to a combination of several factors: reversal of control hoses on the lower chute valve, creating a position opposite the logic programmed into the local controller; failure to process the chute upon changing synthesis procedures, thus leading to an accidental introduction of water; and the inappropriately-timed action carried out on the chute control box.
The operator adopted a series of measures, namely:
- Regarding powder loading chutes: replacement of the fast pneumatic control hose connections on valves by non-interchangeable screw-on connectors; display of powder loading system valve position synoptic diagrams based on limit switching; and a mechanical relay system.
- Regarding production machinery: replacement of glass devices by glazed steel equipment for gas-generating reactors; plus the systematic recording of process parameters.
- Regarding the production building: enhanced signalling systems for emergency resources, intervention and exits; installation of directional lighting.
A number of general measures were also adopted or improved: the inspection, certification or reconfiguration (if necessary) of equipment prior to every new batch start-up; introduction of a monitoring logbook for anomalies in each unit; systematic and periodic audits of units / processes coupled with an assessment of installation layout and premises, in conjunction with production documents; and mandatory wearing of leak protection masks.