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On April 8, 2021, an explosion and accident occurred at the Yenkin-Majestic OPC polymer resin plant in Columbus, Ohio. The accident resulted in 1 death and 8 employees injured. What happened? According to the investigation report by the U.S. Chemical Safety and Hazard Investigation Board (CSB), the accident occurred toward the end of a chemical reaction taking place in a reactor. Before the accident, the agitator of this reactor stopped, and the operator was not present next to the reactor at that time. A few minutes later, the operator began to cool the material in the reactor, yet the mixer, which should have been in operation, remained stopped. When the operator added the solvent from the top of the reactor, the temperature of the material in the reactor for that batch was approximately 430°F (221°C), while the temperature of the solvent being added was around 70°F (21°C). The operator noticed that the temperature of the material in the tank was not dropping; by looking through the sight glass on the manhole, he found that the mixer had stopped. He knew that the agitator was supposed to run during the cooling process, so he restarted it. The restarted mixer stirred the materials in the stationary layer of hot resin and the liquid solvent layer. A large amount of solvent evaporated, causing the pressure inside the reactor to rise rapidly, which triggered the high-pressure alarm in the reactor. Within a few seconds, liquid resin and flammable solvent vapors sprayed out from the manhole, quickly filling the enclosed room with white vapor. The operator tried to turn off the mixer but failed, as he could not see clearly what was happening, and he was also splashed with hot resin; he then evacuated the area. About 2 minutes after the leak started, the vapor cloud was ignited and exploded, resulting in the death of 1 employee, 8 others were taken to the hospital for treatment, and the resin production facility was damaged. Cause analysis: The mixer may stop due to mechanical, power supply, or control failures; mixer faults can be checked through the control system or visually. When a mechanical failure occurs, the mixer motor may still be running, but the mixer may not be rotating. Certain process steps, such as sampling, may require the mixer to be temporarily stopped. The operating procedures should specify when to stop and when to restart the mixer. When adding volatile materials or solvents to a process where the temperature is higher than their boiling point, rapid boiling and a rise in pressure occur. The stirring action moves the material to the cooling surface. When stirring stops, the cooling effect also decreases. When the mixer is restarted, volatile materials may evaporate and increase the pressure inside the reactor. Restarting the mixer depends on many factors, such as how long it has been stopped, the materials in the process, and other conditions. When conducting a Process Hazard Analysis (PHA), mixer failures and restart operations should be among the topics reviewed and discussed. Corrective and preventive measures: When operating chemical processes, it is necessary to closely monitor process parameters such as temperature, pressure, and the status of agitators. When the procedure requires you to stop the agitator and perform other operations, please read all the steps to determine whether to restart the agitator after completing those operations. If the agitator stops, or if you notice that it does not restart, please contact your supervisor to determine the proper course of action. During the process hazard analysis, it is necessary to carefully assess the potential hazards associated with mixer failures. Many parameters need to be considered to determine the hazard, and appropriate corrective actions must be taken.