On May 24, 2007 an explosion and subsequent fire destroyed a tank farm located at a gasoline purification plant in Sløvåg, Norway. In addition to the tank farm, an office building and some neighboring tank trucks were destroyed. There were no casualties in the accident, but at least two people received medical treatment for injuries sustained during the incident. GexCon performed an investigation to determine the probable cause of the accident. The results of the investigation are presented in the paper.
The purpose of the purification process performed was to reduce the content of sulphur containing organic components, especially mercaptans (thiols). The essential chemical principle behind the cleaning process was the higher solubility of mercaptans in an alkaline aqueous solution (caustic soda) compared to gasoline (less polar solvent). To this end the gasoline was mixed with caustic soda in a storage tank, followed by a gravity based separation process: the denser NaOH solution with dissolved mercaptans and precipitated sludge settled in the bottom of the tank, while the top layer of treated gasoline could be pumped out of the tank. The cleaning process resulted in precipitation of solid waste that accumulated in the bottom of the tanks, and the amount of waste eventually reached a level where it was no longer possible to treat gasoline. To remove the waste from the tank, a solution of hydrochloric acid was added to dissolve the solid deposits. During this process, the explosion occurred.
GexCon's investigation included multiple onsite inspections, various experiments such as determination of the release rate of mercaptans from the sludge in the storage tank, and determination of the electrostatic properties of the liquids in the storage tank. The CFD-tool FLACS was used to study the accumulation of flammable vapors in the storage tank and their dispersion around the tank, to determine whether the mixture could have been ignited outside of the tank. The investigation concluded that addition of hydrochloric acid to the alkaline solution resulted in reduced solubility of mercaptans (reduction of the pH value), causing a flammable mixture that eventually filled the entire tank. Self-heating of activated carbon in the filter, installed on top of the storage tank, due to adsorption of mercaptans caused ignition of the flammable mixture in the tank.
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