At around 9.30 am, a release of carbon monoxide (CO) and hydrogen sulphide (H2S) occurred and required the evacuation of 980 people present on the worksite of a unit under construction towards one of the refinery’s entrances. The refinery’s firefighters took measurements and detected H2S concentrations up to 7 ppm.

Eight people exposed were taken to the sick bay; 7 of them were hospitalised and stayed under observation for 24 hours. Traces of CO were detected in their blood but the symptoms were those caused by H2S.

None of the installations of this new unit were likely to emit H2S, and the operator identified the CR4/FCC unit (catalytic cracking unit) as causing the emission of H2S. Its restart after 5 days of shutdown, had generated the formation of CO and H2S discharged directly into the chimney in exceptional weather conditions (light wind).

During the FCC’s startup phase, CO was generated for approximately 30 min, during which the load passed through the catalyst at a temperature below 700°C. Similarly, the H2S originating from the smoke from the furnace receiving the unit’s stripping water, was discharged directly into the air. The amount of H2S discharged was estimated at 225 kg.

No anomaly in the startup procedure was noted and the consequences of this event were attributed to the combination of particular atmospheric conditions and massive human presence in the vicinity of the chimney.

To reduce the likelihood of this event happening again, meteorological data will be taken into account before each FCC restart to ensure good smoke dispersion conditions with delayed startup if necessary. The operator also undertook to strip the H2S in a high-pressure stripper and then treat it in acid gas treatment units after 1 January 2009.