Incorrect equipment component installation is also a common cause. Similarly, in flow reactors, localized insufficient mixing causes hotspots to form, wherein thermal runaway conditions occur, which causes violent blowouts of reactor contents and catalysts. Failure of the mixer can result in localized heating, which initiates thermal runaway. Thermal runaway is most often caused by failure of the reactor vessel's cooling system. This scenario was behind the Seveso disaster, where thermal runaway heated a reaction to temperatures such that in addition to the intended 2,4,5-trichlorophenol, poisonous 2,3,7,8-tetrachlorodibenzo- p-dioxin was also produced, and was vented into the environment after the reactor's rupture disk burst. Thermal runaway may result from unwanted exothermic side reaction(s) that begin at higher temperatures, following an initial accidental overheating of the reaction mixture. For example, oxidation of cyclohexane into cyclohexanol and cyclohexanone and ortho-xylene into phthalic anhydride have led to catastrophic explosions when reaction control failed. These include hydrocracking, hydrogenation, alkylation (S N2), oxidation, metalation and nucleophilic aromatic substitution. Many reactions are highly exothermic, so many industrial-scale and oil refinery processes have some level of risk of thermal runaway. Chain branching is an additional positive feedback mechanism which may also cause temperature to skyrocket because of rapidly increasing reaction rate.Ĭhemical reactions are either endothermic or exothermic, as expressed by their change in enthalpy. Frank-Kamenetskii theory provides a simplified analytical model for thermal explosion. This has contributed to industrial chemical accidents, most notably the 1947 Texas City disaster from overheated ammonium nitrate in a ship's hold, and the 1976 explosion of zoalene, in a drier, at King's Lynn. It is a process by which an exothermic reaction goes out of control: the reaction rate increases due to an increase in temperature, causing a further increase in temperature and hence a further rapid increase in the reaction rate. Chemical engineering Ĭhemical reactions involving thermal runaway are also called thermal explosions in chemical engineering, or runaway reactions in organic chemistry. For example, releases of methane, a greenhouse gas more potent than CO 2, from wetlands, melting permafrost and continental margin seabed clathrate deposits could be subject to positive feedback. Some climate researchers have postulated that a global average temperature increase of 3–4 degrees Celsius above the preindustrial baseline could lead to a further unchecked increase in surface temperatures. In astrophysics, runaway nuclear fusion reactions in stars can lead to nova and several types of supernova explosions, and also occur as a less dramatic event in the normal evolution of solar-mass stars, the " helium flash". Thermal runaway can occur in civil engineering, notably when the heat released by large amounts of curing concrete is not controlled. In electrical engineering, thermal runaway is typically associated with increased current flow and power dissipation. In chemistry (and chemical engineering), thermal runaway is associated with strongly exothermic reactions that are accelerated by temperature rise. It is a kind of uncontrolled positive feedback. Thermal runaway occurs in situations where an increase in temperature changes the conditions in a way that causes a further increase in temperature, often leading to a destructive result. Thermal runaway describes a process that is accelerated by increased temperature, in turn releasing energy that further increases temperature.
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