How to prevent runaway chemical reactions - EPA

" PROBLEM: Many industrial chemical processes involve exothermic (heat generating) reactions. Uncontrolled, or runaway, reactions can occur as a result of various situations, such as mischarged raw materials, failure of a reactor's cooling system or the presence of contaminants. If the heat generation exceeds the reactor's ability to remove it, the reaction can accelerate - or run away - and cause the temperature and pressure to increase. A sudden energy release from such an uncontrolled reaction has the potential to harm workers, the public, and the environment. The following Case Study aims to increase awareness of possible hazards connected with exothermic reactions."

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Runaway reactions

"A typical runaway scenario involves reactants being charged into a reactor at room temperature and heated with stirring until the reaction temperature is reached. Temperature is held constant to optimise cycle time and yield. On completion, the reactor is cooled and emptied. However, if no provision is made in the process to account for cooling failure at reaction temperature e.g. due to power failure or operator error (forgot to start the stirrer), etc. then unconverted material still present in the reactor may react at an uncontrollable rate proportional to the amount of unreacted material. This may lead to over-pressure in the vessel and subsequent rupture by virtue of the normal reaction exotherm. Alternatively, a secondary decomposition reaction may be initiated and the heat so produced may lead to yet a further increase in temperature and eventual runaway conditions1 . The prime causes of runaways are associated2,3 with – process chemistry – inadequate design – substandard operational procedures – lack of training – raw-material quality control – temperature control – agitation – mischarging of reactants – maintenance – human factors (which may impact all of the foregoing)".

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