A reading above the alert level relative to sulphur dioxide concentration (500 µg/m³ of SO2) was detected by the Rhone Department air quality monitoring network between 4 am and 9 am. Two concentration peaks were measured at 7 and 9 am, equal respectively to 947 and 1,031 µg/m³ (average hourly concentration). An increase in hydrogen sulphide (H2S) content of the fuel oil-gas burned inside the furnaces and boilers on a neighbouring petrochemical platform, along with unfavourable weather conditions, was the cause of this SO2 emission. Once alerted, the operator adopted a series of measures (activation of the unit’s hydro-desulfurization (HDS) recirculation system, shutdown of the sulphur plant, change in combustible on the boilers and atmospheric distillation unit) to bring the situation back to normal.

An analysis of incident causes revealed that poor calibration of the amine section (extraction of acid gas) from the HDS unit led to deficient fuel oil-gas washing: instability on a regeneration column in this section, produced by a change in the unit’s charge, caused unpriming of a reboiler that the technicians were unable to reboot. Control of this type of reboiler (a thermosiphon) was known to be highly sensitive, as instability could stem from a temperature gradient between the vapour and the process that was either too low or too high, from problems in regulating the condensate drum level in case process equipment or waste debris would accumulate in the lower part of the column, thereby creating a loss of charge, etc.

Moreover, various factors contributed to exacerbating this situation: the HDS unit had been operating in ‘distillate’ mode, sulphur content of the unit’s incoming products was thus higher (1.5% to 2%), the installation’s H2S analysers were inoperable and not fitted with default alarms. Subsequent to this incident, various measures were adopted: connection to an alarm reporting system of SO2 analysers on the flue ducts, temperature sensors on the reboiler and air quality monitoring sensors, drafting of instructions associated with these alarms, reboiler priming training offered to technicians, personnel awareness of the environmental impacts from such incidents.