A deflagration occurred inside a 200-gal (0.757 m 3 ) atmospheric mixing vessel during the course of apparently normal operations. The vessel had previously been used to dilute 70% aqueous tert-butyl hydroperoxide (t-BHP) with deionized water. Ten minutes earlier the mixed batch had been transferred out and the vessel had been given a brief rinse with deionized water about 4 minutes after the transfer. The deflagration occurred soon after starting the second of two deionized water rinses. Although the hinged vessel lid was forced open and damaged there were no injuries or chemical exposures since no personnel were in the area at the time. The incident was caused by a faulty metering system that overcharged sodium hypochlorite (bleach) solution to the deionized water supply. The roughly 100-fold overcharge of bleach resulted in a vigorous, gassy reaction with t-BHP in the mixing vessel. The reaction generated a gas-phase mixture comprising mostly acetone plus about 90% oxygen. The ignition energy of a replicated mixture was 0.015 mJ, similar to that of optimum hydrogen-air, but with poor upward mixing of acetone the ignition energy could have been as small as 2 μJ. The source of ignition could not be identified with certainty; the "least improbable" scenario was considered to be a spark from an electrically charged slug of rinse water falling through an electrically charged mist that had formed during the gassy reaction.
INCIDENTIn the batch process, 70% aqueous tert-butyl hydroperoxide (t-BHP) is diluted with deionized water in a stirred mixing vessel (mixer) then pumped to another vessel for reaction. Following liquid transfer, two separate additions of deionized water are made to rinse out the mixer. In each case the rinse water is sent to the reactor.On the day of the incident the dilute t-BHP mixture was made up as usual and pumped to the reactor. About 1 minute into the second rinse step a loud noise was heard by plant staff. Flammable gas detectors did not indicate flammable gas in any area. The control system was still operating normally with no alarms or signs of a process upset. Plant staff reviewed all the ongoing processes to help determine the source of the upset. The process was then put into hold. When the conditions in the plant were found to be safe, operators walked through the unit for a visual check of the process. The mixing vessel was found to have a damaged hinged lid, but no other signs of other damage, smoke, or soot.
BLEACH METERING SYSTEMVarious processes at the plant share a common supply of deionized water that has been treated with dilute sodium hypochlorite (bleach) to limit biological growth. The bleach metering system is designed to dilute 12.5% bleach concentrate to 30 ppm in a deionized water supply tank.
MIXING VESSELFigure 1 shows a sketch of the mixing vessel. The agitator is mounted near the side of the roof and the shaft angles down to the middle of the tank floor. The agitator shaft and deionized water nozzles are on opposite sides of the fixed half of the hinged lid. The o...