Explosion in a chemical plant manufacturing fuel additives

The accident occurred during the first phase of the batch synthesis of tricarbonyl compound (methylcyclopentadienyl) manganese (TMCM), while the reaction mixture in the reactor consisted of more than 0.5 t of sodium and organic compounds. Reaction runaway with hydrogen production caused a large increase in pressure and temperature leading to a rupture of the reactor (above 100 bar).
The ignition of the released flammable cloud led to the formation of a fireball and generated a large heat flux. A fragment of more than 100 kg from the upper part of the reactor were found 400 m from the site of the explosion.
The fire mobilized about 70 firefighters; a safety perimeter of 800 m was set up; a cloud of mushroom-shaped black smoke rose more than 600 m above the site.
The plant was employing 12 people. 4 employees died and 33 were injured, most of them outside the site, by projectiles from buildings impacted by the blast of the explosion. The damages were important, observed up to 300 m from the place of the explosion (breakage of windows, damages of the structures ...).

An investigation was conducted by a federal agency to determine the exact causes of the accident. The runaway of the reaction was due to a lack of cooling of the reactor. The reactor cooling system was sensitive to point failures due to the lack of redundancy of safety equipment during reactor design. In addition, the safety system was not able to evacuate the pressure generated by the runaway reaction.

The INITIATING CAUSE of the runaway reaction sustained by a temperature increase due to a lack of cooling.

The ROOT CAUSE was that fact that the plant operator failed to recognise the hazards related to the possibility of a chemical run-away and to act on time. This could be associated to a lack of knowledge (lack of training) or to inadequate diagnostic capacity of the installation.
Another component was related to deficiencies in the safety design. The cooling system was susceptible to single-point failures due to a lack of design redundancy. The MCMT reactor vessel relief system was incapable of relieving the pressure from the runaway reaction. Finally, the operation was highly manual and missing instructions in case of lack of cooling.

chemical reactor for TMCM synthesis

In this plant, the production process started with heating the reactor to melt sodium at 210°F (98.9°C), after which a mixer (agitator) had to be manually started. Mixing the raw materials increased the reaction rate, creating heat. Heat from the reaction and from the heating system continued raising the temperature.
At a temperature of about 300°F (148.9°C), the process operator likely turned off the heating system as specified in the procedure, but heat from the reaction continued increasing.
At a temperature of about 360°F (182.2°C), the process operator likely started cooling, using the process control system, as specified in the procedure. However, the mixture temperature in the reactor continued to increase.

6 hours after the start of the process, the operator reported a cooling problem and operators realised the existence of an emergency, also because various alarms were activated. Short before the explosion, the reactor was venting, but not at the speed required to counteract the increase of the internal pressure.

DESCRIPTION OF THE PROCESS
The tricarbonyl (methylcyclopentadienyl) manganese (TMCM) is an organomanganese compound with formula (C5H4CH3)Mn(CO)3. It is used as fuel supplement, in gasoline, specifically in unleaded gasoline to increase the octane rating.
TMCM is a combustible liquid and is very toxic by inhalation or skin contact.

The accident occurred during the first phase of the synthesis of the TMCM, while the reaction mixture in the reactor consisted of more than 0.5 t of sodium and organic compounds.

According to the normal operation procedure, hydrogen is among the reaction products and is vented to the atmosphere via a pressure relief valve. The reaction becomes self-sustaining and need to be cooled.

4 employees were dead and 33 injured, most of them, outside the site, by projectiles from buildings impacted by the blast of the explosion. The material damages were important, they were observed until 300 m of the place of the explosion (breakage of windows, damages of the structures ...).

The fire mobilized about 70 firefighters; a safety perimeter of 800 m was set up; a cloud of mushroom-shaped black smoke rose more than 600 m above the site.

In a previous analysis of the accidents related to reactive hazard, the CBS had issued the following general recommendations (HAZARD INVESTIGATION Report No. 2001-01-H Issue Date: October 2002 IMPROVING REACTIVE HAZARD MANAGEMENT):
1. Identify and thoroughly evaluate reactive hazards in their processes
2. Implement appropriate emergency pressure relief devices and other design safeguards
3. Develop effective operating procedures and training programs
4. Carefully manage changes to existing processes
5. Plan for possible accidents, including evacuation drills and emergency response exercises

These recommendations can be applied also to this specific accident. Moreover, one of the findings of CSB report dedicated to this case is the lack of reactive chemistry experience.

The CBS issued the following additional recommendations to the American Institute of Chemical Engineers:
6. To add reactive hazard awareness to baccalaureate chemical engineering curricula requirements.
7. To inform all student members about the Process Safety Certificate Program and encourage program participation.