On September 5, 2023, at 11:50 a.m., a distillation tower (“Tower”) collapsed (Figure 1) at a Petrochemical in Texas. The collapse was the result of a fire inside the distillation tower. The incident resulted in approximately $194 million in property damage.
A few weeks before the incident, the company shut down its Pyrolysis Gasoline unit, including its extractive Tower. The company followed its procedure to remove solvent and hydrocarbons from within the Tower to prepare for maintenance work. On the day of the incident, a maintenance crew began opening manways to prepare the Tower for entry. At 9:30 a.m., temperatures with the Tower’s stainless steel structured packing beds began to rise. The company sounded an emergency alarm at 10:44 a.m. after smoke and flames showed a fire within the Tower. Personnel were evacuated after adding nitrogen and trying to stop air from entering the Tower. At 11:53 a.m., the Tower folded over (collapsed).
The company investigation found that the Tower fire resulted from a chain of undesired reactions. About a month before the unit shut down, equipment leaks in another area of the process allowed water to enter the extractive distillation unit, leading to internal corrosion that removed iron from the metal walls. This iron formed iron sulfide (a pyrophoric material) from hydrogen sulfide that is usually present in the process equipment. When the Tower’s manways were opened, oxygen (air) entered the Tower, starting an exothermic iron sulfide oxidation reaction. The company concluded that the heat from this iron sulfide oxidation reaction resulted in localized heating (1,300 to 1,800 degrees Fahrenheit) within the structured packing that weakened the Tower’s walls and ultimately caused the Tower to collapse.
In addition, The company’s investigation found that a 2016 incident had revealed the potential for iron sulfide to accumulate within the Tower. The investigation team determined that this knowledge had not been effectively transferred to the existing site personnel. Had the site’s operations team known about the potential for iron sulfide within the Tower, changes to the procedure to remove solvent and hydrocarbon could have been made to mitigate the heating from the iron sulfide oxidation.
During the incident, approximately 160 pounds of sulfur dioxide and an unknown amount of other reaction and combustion products were accidentally released.
Probable Cause
Based on The company’s investigation, the CSB determined that the probable cause of the fire was smoldering iron sulfide oxidation within the structured packing of the Tower. Contributing to the incident was that the company did not effectively maintain its knowledge from its 2016 incident finding that iron sulfide could accumulate within the Tower.
Source: CSB.gov
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