A fixed roof acrylic acid tank exploded in which there was one fatality and 36 people were injured. The tank acted as an intermediate storage between two distillation columns for purifying acrylic acid. Originally the tank had been used at full capacity. The contents were cooled and mixed by pumping from the bottom of the tank to the top. Later, the normal operating level was reduced to a level below the cooling coil.
After routine shutdown maintenance, the recirculation circuit was not commissioned. Thus there was no recirculation of the liquid to the top of the tank.
At the time of explosion, the plant was conducting a test of the downstream distillation column, which required stopping feed from this tank. The level of the tank gradually increased to its original operating level.
Without recycling of the liquid to the top of the tank, the acrylic acid above the cooling coils was not mixed and cooled. The temperature of the incoming acrylic acid was believed to be below the onset temperature for polymerization, and the acrylic acid contained a polymerization inhibitor.
However, the temperature in the tank built up causing the tank to get over-pressurized, leading to the explosion and consecutive fire.
Findings
Originally the pipe feeding the tank was hot water jacketed to provide freeze protection, but this was changed to steam.
Removal of a steam trap made temperature control unreliable.
The top layer was no longer mixed with cooler acrylic acid , and stayed warm from incoming acrylic acid .
There are two exothermic acrylic acid self-reactions: dimerization and polymerization. Polymerization inhibitors do not stop the dimerization reaction. Experiments showed that heat from dimerization raised the temperature sufficiently to start a runaway polymerization reaction.
The hazard of heat from dimerization was not recognized, so recirculation to the top of the tank was not resumed.
The tank had no temperature indicator. The first indication of a problem was observation of acrylic acid vapors escaping from the top vent on the tank.
Direct Cause
Insufficient Cooling and onset of Exothermic Reaction.
Root Cause
Recirculation system was not commissioned after maintenance, which suggests that the criticality of the recirculation system was not established in the system.
There were no temperature sensors in the system to warn the operator of a potential runaway reaction.
MSDS of the material was not frequently reviewed
Corrective Actions
List of safety critical equipment which are required for the smooth functioning of the plant shall be identified and communicated.
There shall be suitable interlocks in place which shall prevent system operation with the safety critical equipment under bypass condition.
Any and all changes shall be routed through Management of Change Process.
MSDS of the materials shall be reviewed at set frequency or a similar system shall be in place to track and update the changes in the MSDS.
Evacuation zone for the worst case scenarios to be established with explosion proof shelter.
Process critical parameters such as flow, temperature, pressure and flow shall be properly identified with effective ways to monitor them.
Ref: Nippon Shokubai Co., Ltd. Himeji Plant Explosion and Fire at Acrylic Acid Production Facility Investigation Report March 2013.