When Cooling Methods Were Changed, a Tragedy Occurred

When Cooling Methods Were Changed, a Tragedy Occurred 

Operators conducted the chemical reaction in an aqueous solution at about 32° F (0° C) to minimize side reactions and maintain product quality. The operating team keep the reactor temperature controlled by adding an excess of crushed ice. As the exothermic reaction melted the ice, the operators replenished the crushed ice by adding it into the system via the manway. 

Unfortunately, by focussing on the potential cooling aspects the operators did not understand that dry ice in water reacts to become carbonic acid. NaSH, a reaction product of H2S and sodium hydroxide, is stable at high pH, but liberates toxic H2S when acidified. As operators added pieces of dry ice, the pieces instantly created acidic pockets that reacted with the sodium hyrosulfide, and immediately liberated poisonous hydrogen sulfide gas. Events happened so quickly that the process operators were overcome by the toxic fumes before they were able to add all of the dry ice. They perished from acute hydrogen sulfide inhalation—while trying to be helpful. 

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When cooling methods were changed, a tragedy occurred... 

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