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Appendix - Case Studies

Flixborough, UK (June 1, 1974)

The accident at the Nypro Ltd caprolactam plant outside of Flixborough, is a typical example of a vapor cloud explosion (VCE). An intermediate in the production of caprolactam is cyclohexanol, which is produced by partial oxidation of cyclohexane (see Reaction). The conversion must be kept low (usually below 10%) in order to avoid complete oxidation of the feed and product.

A sudden failure of a temporary cyclohexane line bypassing Reactor No.5 that had been removed for repair (see Figure) led to the vaporization of an estimated 30 metric ton of cyclohexane. The vapor cloud dispersed throughout the plant and was ignited by an unknown source. The entire plant site was leveled and 28 people were killed.

The failure of the bypass pipe was attributed to inadequate support of the pipe and a poor design (consisting only of a full-scale sketch in chalk on the workshop floor) that failed to account for the movement of the pipe due to pressure inside it. When the pressure rose a little above the normal level, the bellows at each end of the pipe began to rotate and failed.

 

Bhopal, India (December 3, 1984)

The most notorious example of the hazards involved when dealing with highly toxic materials is the Bhopal accident. The Bhopal plant (partially owned by Union Carbide and partially owned locally) produced pesticides. An intermediate compound in this process is methyl isocyanate (MIC), which is extremely dangerous. It is reactive, toxic, volatile and flammable. The TLV for MIC is 0.02 ppm.

The disaster started with the contamination of a large MIC storage tank with a large amount of water (how is still unclear), leading to an exothermic chemical reaction. Insufficient cooling of the storage tank led to a runaway reaction with an accompanying temperature increase past the boiling point of MIC (312K). The MIC vapors traveled through a pressure relief system and into a scrubber and flare system installed to consume the MIC in case of a release. Unfortunately, the scrubbing and flare systems were not operating. An estimated 40 metric tons of MIC vapor was released, and the vapor stayed close to the ground due to the density of about twice that of air. The toxic cloud spread to the adjacent town, killing over 2,500 civilians and injuring an estimated 200,000 more.

The exact cause for the contamination of the MIC is not known. However, several design measures could have prevented the disaster:

The Bhopal event and other major chemical plant incidents have changed the chemical processing industry profoundly; the result has been new legislation with better enforcement, enhancement in process safety, development of inherently safer plants, harsher court judgments, management willing to invest in safety related equipment and training, etc.

 

Seveso, Italy (July 9, 1976)

The Seveso accident happened at a chemical plant manufacturing pesticides and herbicides. An exothermic runaway reaction occurred in a reactor used for the production of 2,4,5-trichlorophenol (TCP) from 1,2,4,5 tetrachlorobenzene (TCB) by hydrolysis with NaOH (see Reaction).

 

How could this runaway reaction have happened? The plant was being shut down for the weekend, leaing the reactor half full of unreacted material at elevated temperature. The runaway reaction in the unstirred mixture was triggered by the slight, unexpected, heat input from the hot, dry wall of the reactor to the upper layer of the reaction mixture. The temperature of this upper layer reached a level at which slow and weak exothermic reactions started. After 7 hours these reactions resulted in the start of other, more rapid, exothermic reactions.

The rupture disk on the reactor burst as a result of excessive pressure, and an aerosol cloud containing 2,3,7,8 tetrachlorodibenzo-p-dioxin (TCDD) was released into the air. TCDD was a by-product of the uncontrolled exothermic reaction (see Reaction):

 

The reactor had no automatic cooling system and only maintenance personnel were there at the time of the accident, so there was no one to start cooling manually and suppress the reaction. Fortunately, a worker noticed the cloud and stopped the release after only 20 minutes. TCDD is commonly known as dioxin, which is widely believed to be one of the most toxic man-made chemicals (LD50 = 0.001 mg/kg, compared to DDT with LD50 =150 mg/kg). Although no immediate fatalities were reported, between a few 100-g and a few kilograms of dioxin were widely dispersed, which resulted in an immediate contamination of some 30 km2 of land and vegetation. More than 600 people had to be evacuated and 2,000 were treated for dioxin poisoning. The best-known consequence of the Seveso disaster and previous serious chemical accidents (e.g. Flixborough) was the impulse that it gave to the creation of the European Community's Seveso Directive in 1982, a new system of industrial regulation for ensuring the safety of hazardous operations.

 

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