A fire occurred in a continuous-feed autoclave system (fixed-catalyst-bed tubular reactor) when the rupture disc released, discharging hot oil, oil distillates, and hydrogen gas out a vent pipe into the autoclave cell. The flammable mixture was discharged directly into the cell because there was no system in place to catch or remotely exhaust the autoclave contents. The oil and gas ignited in a fireball that, in turn, ignited nearby combustibles (cardboard and paper), causing a sustained fire. The hydrogen gas and autoclave system were shutoff immediately. However, a lecture bottle containing hydrogen sulfide was heated by the surrounding fire and ultimately ruptured with enough force to cause facility structural damage. (Lecture bottles do not have a pressure-relief device.) The explosion occurred about five minutes after the rupture disc released.
One employee received minor burns on his arm from the initial fireball. No other injuries occurred. Fire damage to the laboratory and equipment was minimal, but the cost of the structural damage caused by the explosion was estimated at about $55,000.
The ignition of the fireball could have been caused by any of the following mechanisms:
- The inverse Joule-Thompson effect of hydrogen (i.e., heating upon expansion)
- Some of the oil and light ends were above their auto-ignition temperatures
- Contact with hot surfaces of the autoclave
- Static electricity
- Electrostatic discharge of the mixture.
The possibility that the explosion may have been caused by the hydrogen discharged from the autoclave was thoroughly investigated. However, there were no signs of combustion in the upper part of the cell. Also, the explosion occurred approximately five minutes after the rupture disc release, long after the hydrogen source had been shut off and more than one air exchange had occurred in the cell.
The following were identified as lessons learned from the incident:
- The reactor outlet line was plugged with viscous oil, which resulted in over-pressurization of the system. If the plug had been in the feed line, the rupture disc on the reactor would never have been over-pressurized. If the staff had added a second rupture disc near the pump outlet to relieve pressure before the reactor rupture disc, with a check valve in between the two, the release might have been avoided. This design change was incorporated into the system rebuild.
- The fixed-catalyst-bed tubular reactor system should have been reviewed and approved by health and safety, facilities engineering, or building management staff.
- The inadequate hazard analysis of the installation resulted in failure to anticipate the need for containment or remote discharge of the rupture disc effluent. A thorough hazard analysis should have been done before operating the system.
- The operators were not adequately trained on the hazards of the system. - Combustible materials and gas lecture bottles should not have been stored in the autoclave cell (i.e., poor housekeeping).
- The fire failed to activate the sprinkler system because the exhaust fan was turned on within seconds of the rupture disc release, so the air change volume minimized the temperature rise within the cell.
- The roof pressure vent panel failed to adequately relieve the explosion pressure.