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The essence of reaction thermal runaway is the vicious cycle between uncontrolled reaction rate and uncontrolled reaction temperature. The structure of a batch reactor is shown in Fig. 2, which is the most widely used in the pharmaceutical industry, pesticide industry, and other fine chemical industries. It can be seen that the basic construction of a batch reactor includes the reactor shell, jacket for temperature control, dosing hole, emergency relief system, agitators, bottom valve, and reactants. The reactor shell is the container of reactants and used to bear the pressure generated by chemical reactions. The jacket for temperature control of the reactor is used to provide energy to maintain the continuous progress of the reaction. The dosing hole on the top cover of the reactor is used to add reaction required raw materials and observe the state of the reactants. The pressure relief pipeline is used to balance the pressure between the reactor and the atmosphere in normal production process. However, the pressure relief pipeline is used to reduce overpressure in the reactor in the case of an emergency. The reaction stirring or agitator is used for mixing and mass transfer of different reactants, and promoting heat exchange between reactants and cooling jackets. The bottom valve is used to discharge the reaction product. In case of emergency, it is used to discharge the active substance to the containment device for diluting, quenching, or inertion. Chemical reactions are carried out between the molecules of reactants. The capacity of reactants should comply with the specified dosage. With the continuous progress of the reaction, the chemical composition and physical property of reactants are constantly changing.
The common causes of reaction thermal runaway can be classified into three categories. The first one is the hot spot in the reactor caused by insufficient mixing of reactants or agitator failure. The second is weak cooling capacity of the cooling jacket failure, resulting in reaction heat accumulation in the reactor. The third is excessive feeding caused by human error or metering device failure. Regardless of whether the reaction thermal runaway occurs in batch reactors, tubular reactors, continuous-flow stirred tank reactors, or fixed bed catalytic reactors, the emergency response and loss prevention methods for reaction thermal runaway follow three basic principles: relieving overpressure in the reactor, controlling the reaction temperature, and retarding the reaction rate.
Relief of overpressure
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For liquid phase reactions, overpressure in the reactor is usually caused by the vapor pressure of the superheated liquid reactants. For gas phase reactions, the pressure in the reactor is generally positively correlated with the reaction rate. When thermal runaway happens, the reaction rate and reaction temperature are out of control, which may accelerate the vaporization of the reactants, resulting in a sharp increase in the vapor space pressure of the reactor. When the inner pressure exceeds the maximum limit stress of the shell material, the catastrophic explosion of reaction vessel will happen. The shock wave formed by overpressure released during the explosion is the main cause of casualties and property loss around the plant area.
Therefore, releasing the overpressure is one of the most basic operations of the emergency disposal for reaction thermal runaway. Releasing the overpressure so that the pressure difference between the inside and outside of the reactor shell is within the limit of yielding, and ensures the equipment integrity. The vapor of the reactants is largely combustible or toxic. Therefore, the released vapor from the relief system also needs follow-up treatment procedures, such as collection by storage tanks, combustion by flare systems, and absorption or degradation by special devices.
Control of reaction temperature
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The uncontrolled increase of reaction temperature is the direct cause of the acceleration of the reaction rate and overpressure in the reactor. The reaction temperature is adjusted according to the heat exchange between the cooling jacket and the reaction material. Therefore, insufficient cooling capacity or cooling failure will lead to heat accumulation in the reactor. The temperature uniformity of the reaction mixture is realized by stable stirring. Hence, insufficient stirring or stirring failure will lead to uneven mixing of reactants and hot spots in reactors. This phenomenon is more likely to occur in polymerization processes, especially in bulk polymerization and solution polymerization. Due to the high monomer content in these polymerization processes, the viscosity of the reactants changes greatly during the polymerization process. Generally, the viscosity of reactants rises with the increase of the polymerization degree. The increased viscosity requires greater stirrer torque with larger agitator power. At the same time, the increase of viscosity results in poor heat exchange effect between the reactants and the reactor. It is an effective method for the inhibition of reaction thermal runaway.
Quenching the runaway reaction
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The above-mentioned two principles, relief of overpressure and control of the reaction temperature, can be classified as passive emergency disposal strategies, which are committed to mitigating the catastrophic consequences of reaction thermal runaway. However, starting from the root of reaction thermal runaway, quenching the active substance and inhibiting the progress of the chemical reaction could extinguish the 'flame' of runaway reaction.
According to Arrhenius equation, as expressed in Eq. (1), the reaction rate constant is affected by the pre-exponential factor, apparent activation energy, and thermodynamic temperature. With the exception of the control of reaction temperature mentioned above, the reaction rate constant can be reduced by diluting the concentration of reactants, which may decrease the probability of collision between different reactive molecules. In addition, quenching the reactive intermediates could reduce the reaction rate by increasing the activation energy of the reaction.
$ k=A{e}^{-\frac{{E}_{a}}{RT}} $ (1) where k is the reaction rate constant, A is the pre-exponential factor, Ea is the apparent activation energy, R is the molar gas constant, T is the thermodynamic temperature.
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Reaction thermal runaway has been a common hazard leading to process safety-related accidents. In this work, the principles of the emergency response technologies for reaction thermal runaway were summarized. According to the published literature, several loss prevention methods of reaction thermal runaway were developed, but there are still many aspects worthy of further investigation. Suggestions for future work can be summarized into the following aspects:
(1) The calculation model of safety venting mostly obtained by theoretical derivation and verified by lab-scale experiments. With high integration, large scale and complex process of the chemical industry, experiential equipment and test methods for large scale safety venting testing is necessary.
(2) With the development of multiphase fluid model and CFD simulation technology at different scales, the mixing process of coolant and reactant in complex reaction processes could be further studied, such as the heat and mass transfer in microreactors.
(3) The combination of thermal storage and material functionalization may provide an intensive and practical approach for thermal management and runaway inhibition in complex chemical processes.
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About this article
Cite this article
Chen Q, Wang J, Gao M, Liu L, Tao J. 2022. Review on loss prevention of chemical reaction thermal runaway: Principles and application. Emergency Management Science and Technology 2:10 doi: 10.48130/EMST-2022-0010
Review on loss prevention of chemical reaction thermal runaway: Principles and application
- Received: 28 August 2022
- Accepted: 27 September 2022
- Published online: 14 October 2022
Abstract: Reaction thermal runaway has been extensively characterized as a major hazard for fine chemical industry. It is necessary to develop safety technologies for the control of reaction thermal runaway in emergencies and mitigating the subsequent hazards. To date, literature review on the loss prevention methods of chemical reaction thermal runaway is insufficient. In this paper, a comprehensive review is delivered to outline the progress of emergency response technologies for reaction thermal runaway in recent years, major principles and potential applications of those loss prevention methods. It is expected that this review article has certain reference value for the further understanding of thermal runaway, the design of mitigation systems, and the formulation of emergency response strategy for runaway reactions.
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Key words:
- Loss prevention /
- Thermal runaway /
- Chemical reaction /
- Emergency response /
- Reaction inhibition