Modeling Of The Venting Of A Gassy System Under Runaway Conditions

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Exothermic reactions that undergo uncontrolled self-heating as a result of loss of cooling lead to a thermal runaway. Such reactions can present a serious hazard in the petrochemical (e.g. polymerisation processes) and chemical industries. They can reach excessively high rates of temperature increase, either due to production of gaseous reaction products, or the boiling of reactor contents. This temperature rise in effect leads to a pressure increase higher than the process equipment is made to withstand. Since the temperature rate rapidly increases (several hundred degrees per minute), a thermal explosion may occur followed by the release of toxic and flammable gases, if there is no venting mechanism to relieve the system of the excess pressure. The heat produced by a reaction is proportional to the volume of reaction mixture, while the cooling capacity however is a function of surface area. This has larger implications for industrial scaling, and is said to be the cause of accidents involving thermal runaways as in the well-known cases of Seveso in 1976, Bhopal in 1984 and more recently the T2 Laboratories in 2007. The prediction of the consequences of a runaway reaction in term of temperature and pressure evolution in a reactor vessel requires the knowledge of the reaction kinetics, thermodynamics and fluid dynamics inside the vessel during venting. Such phenomena and their interaction are complex and still to be fully understood, especially for those reactions in which the pressure generation is totally or partially due to the production of permanent gases (gassy or hybrid systems). Moreover, they cannot be easily determined by laboratory scale experiments. Computer modeling is a growing field of research necessary to develop methods capable of predicting the onset of a runaway reaction. Also, adequate vent sizing calculation methods are widely investigated for relief vent sizing emergency actions. The work described in this poster presents a dynamic model that simulates the behavior of a gassy system, specifically the decomposition of an 80% Cumene Hydro-Peroxide solution in aryl hydrocarbon during venting. The model provides the temperature, pressure, mass inventory and conversion profiles throughout the reaction. A sensitivity study of the model was performed in order to study the effect of various parameters on the resulting behaviour of the system before and after venting. The outcomes of this model provide a deeper insight into the improvement of emergency relief systems design for hybrid and gassy systems, where significant progress is still to be made both in the experimental and modeling areas.