A fire occurred during the start-up phase of a pyrolysis installation which had been shut down the day before due to a propane compressor failure. After nitrogen flushing of the piping and heating up of the reaction tubes, feeding of the pyrolysis furnace with EDC (1,2-dichloroethane) was started at 10:50 hrs. The feeding control was set on “manual operation” which according to experience creates the feeding conditions foreseen by the operating instructions (manual).
Between 11:00 and 11:30 hrs, 3 heating gas breakdowns (e.g. failures of the supply) occurred due to an exceeding heating gas pressure. The heating gas supply was restarted each time. At 12:33 hrs. alarms signalled increasing temperatures of the reaction-tube walls. The technician attempted in vain to reduce the temperature by increasing the EDC (1,2-dichloroethane) feed. The heating gas of the furnace was shut-off completely 12h58.
Although the tube-wall temperatures were decreasing, the pyrolysis-gas transfer pipe between the furnace and the cooler burst and the released pyrolysis gas ignited immediately. EDC feed-valves were shut-off. Back-flowing EDC from a quencher fed the fire.
The fire brigade cooled adjacent equipment with water and abated most of the forming hydrogen chloride with a water curtain. In order to avoid a flash-fire or an explosion, the released gas (1,5 t EDC / 700 kg MVC) was intentionally allowed to burn off. The fire stopped by itself in 30 min. Material damage to steel supports, piping, valves, cables etc was estimated at 1 M euros.
Exothermic reactions in the pyrolysis tubes are thought to be the cause of the pipe rupture. During the start-up phase, insufficient EDC quantities (with respect to the start-up procedure) were fed to the furnace, thus allowing unusual temperatures in the reacting tubes.
To secure start-ups, each pyrolysis gas tube will be equipped with a pyrolysis gas temperature measurement system for cut-off according to a 2 of 3 redundant logic (class A safeguard). The reaching of certain temperatures will shut off the heating gas and EDC-feed and launch a nitrogen washing of pyrolysis tubes. The frequence of inspections is increased for the early recognition of corrosion- or erosion-damages.