Runaway Reactions and Vapor Cloud Explosions: the Synthron Case Study

Abstract

Thermal runaway in chemical batch and semibatch reactors is one of the major plagues in fine chemical, pharmaceutical and plastic industries. Quite often the root cause of such events is a poor knowledge of the process kinetics and thermodynamics. For this reason, accidents continue to occur with a high frequency in both European and American countries. The present work is focused on the accident occurred in 2006 at Synthron Inc. (Morganton, NC), an American company that manufactured a great variety of powder coatings and paint additives. The accident has been originated by a runaway reaction and subsequent vapor cloud explosion (that killed one worker and injured 14 others) due to a wrong scaling-up of the original process recipe. Particularly, the standard synthesis was carried out in a 1,500 gallon semibatch reactor by polymerizing liquid acrylic monomers in a high flammable solvent blend. On the day of the accident, in order to produce slightly more of the desired product, plant managers decided to scale-up the process recipe in a single larger batch. In order to perform the synthesis in the same time of the standard recipe, they also decided to load almost all of the additional monomer required into the initial reactor charge: that is, using a quasi-batch operating mode. Unfortunately, adding all the monomer in one shot, more than doubled the rate of energy release in the reactor, exceeding the cooling capacity of the equipped condenser and causing a runaway reaction. The reactor pressure increased rapidly. Solvent vapors, vented from the reactor’s rupture disc, formed a flammable cloud inside the building, found an ignition source and, finally, resulted in a violent explosion. This work presents a detailed reconstruction of the dynamics of the accident occurred at Synthron Inc. using all data collected by the U.S. Chemical Safety Board (CSB) and implementing a detailed mathematical model capable of describing monomer conversion, temperature and pressure evolution inside the reactor together with the condenser efficiency. Particularly, all constitutive equations of material and energy balance, dosing policies and mixing rules have been considered. Results arising from simulations have been found to be in good agreement with both CSB collected data and successive reconstructions.