Thermal risk in batch reactors: Theoretical framework for runaway and accident

Thermal safety and risk of accidents are still challenging topics in the case of batch reactors carrying exothermic reactions. In the present paper, the authors develop an integrated framework focusing on defining the governing parameters for the thermal runaway and evaluating the subsequent risk of accident. A relevant set of criteria are identified in order to find the prior conditions for a thermal runaway: failure of the cooling system, critical temperature threshold, successive derivatives of the temperature (first and second namely) vs. time and no detection in due time (reaction time) of the runaway initiation. For illustrative purposes, the synthesis of peracetic acid (PAA) with hydrogen peroxide (HP) and acetic acid (AA) is considered as case study. The critical and threshold values for the runaway accident are identified for selected sets of input data. Under the conditional probability of prior cooling system failure, Monte Carlo simulations are performed in order to estimate the risk of thermal runaway accident in batch reactors. It becomes then possible to predict the ratio of reactors, within an industrial plant, potentially subject to thermal runaway accident.     

Inherent thermal runaway hazard evaluation method of chemical process based on fire and explosion index

Thermal runaway hazard assessment provides the basis for comparing the hazard levels of different chemical processes. To make an overall evaluation, hazard of materials and reactions should be considered. However, most existing methods didn't take the both into account simultaneously, which may lead the assessment to a deviation from the actual hazard. Therefore, an integrated approach called Inherent Thermal-runaway Hazard Index (ITHI) was developed in this paper. Similar to Dow Fire and Explosion Index(F&EI) function, thermal runaway hazard of chemical process in ITHI was the product of material factor (MF) and risk index (RI) of reaction. MF was an indicator of material thermal hazards, which can be determined by initial reaction temperature and maximum power density. RI, which was the product of probability and severity, indicated the risk of thermal runaway during the reaction stage. Time to maximum rate under adiabatic conditions and criticality classes of scenario were used to indicate the runaway probability of the chemical process. Adiabatic temperature rise and heat of the desired reaction and secondary reaction were used to determine the severity of runaway reaction. Finally, predefined hazard classification criteria was used to classify and interpret the results obtained by this method. Moreover, the method was validated by case studies.     

苯和甲苯硝化及磺化反应热危险性分级研究

首先介绍了化工工艺热安全性的内涵,并从反应过程热危险性分析的方法学出发,介绍间隙、半间歇化学反应工艺热危险性分级研究的总体思路及方法。然后,围绕甲苯和苯的硝化、磺化反应,用全自动反应量热仪(RC1e)和加速度量热仪(ARC)测定其反应过程的绝热温升(△Tad)、目标反应所能达到的最高温度(TM)、分解反应最大速率到达时间(θD)等参数。运用风险评价指数矩阵法(方法1)和基于失控过程温度参数的热危险评估法(方法2)分别对其硝化和磺化反应过程的热危险性进行了分级评估。结果表明,这两种方法具有良好的一致性;给定工艺条件下甲苯和苯的一段硝化反应过程的热危险度等级较低;而磺化反应的热危险较高。尽管这两种方法还有一定的局限性,但对于间歇、半间歇合成工艺的本质安全化设计、工艺热危险性的评估具有重要的参考价值和实用意义。     

Thermal hazards analysis of styrene in contact with impurities

Styrene is a reactive monomer commonly used to produce polystyrene and other copolymers. Unintended thermal runaway polymerization reactions of styrene keep reoccurring and have led to catastrophic consequences. One of the possible causes of these runaway incidents involves the contamination of the styrene monomer by incompatible species, which was not adequately investigated and documented. This study focuses on the quantification of thermal runaway hazards of styrene in contact with a series of contamination substances by adopting calorimetric analysis. Both Differential Scanning Calorimeter (DSC) and Advanced Reactive System Screening Tool (ARSST) were employed to examine the exothermic characteristics of styrene mixed with contaminating substances at different concentration levels and mixing conditions. Key safety parameters of the exothermic reaction, such as the onset temperature, the overall heat release, the maximum self-heating rate, as well as the activation energy, were obtained. The results indicated that the thermal runaway polymerization of purified styrene was significantly altered by the presence of contaminant species. Water effectively retarded and quenched the runaway polymerization at a higher temperature range. Alkaline had no substantial effect on the thermal runaway characteristics. The presence of acid solution under both static contact and vigorous mixing condition significantly promoted the thermal polymerization of styrene. A trace amount of concentrated acid initiated violent exothermic activity even at room temperature; and the severity of the reaction was profoundly impacted by the mass-transfer. Our study demonstrates significant implications in the prevention of runaway incidents during transportation and storage of styrene.     

Using thermal analysis with kinetic calculation method to assess the thermal stability of azo compound on construction materials

The application of construction polymers in engineering and alternative materials has always occupied a place in the market. In the production process of polymer resins, initiators can be used to lower the polymerization reaction energy threshold, which can improve reaction efficiency and reduce energy loss. However, as a commonly used energetic substance in the polymerization process, azos have caused related process hazards due to their exothermic characteristics. Because of this, it is essential to examine and analyze the thermal hazard characteristics of emerging azo substances, such as 2-cyanopropan-2-imemicarbazide (CABN). Although previous literature performs the calculation on related thermal hazard parameters of CABN, there is still exists a void for discussion in estimating the reaction model to avoid analogous hazards and enhance the existing thermal analysis. Based on the past literature, the reaction model is improved with thermogravimetric analysis as evidence. The revised thermal hazard parameters are calculated as the basis of control and mitigation measures, the kinetic model is used to estimate the modified safety parameters, and in the judgment of the runaway reaction, the critical temperature of the runaway is found by analyzing the influence of slight changes in ambient temperature on the reaction temperature. The results show that the critical temperature that causes CABN to enter the runaway reaction is delayed, and the hazard is lower than in the storage situation. Therefore, the thermal hazard to CABN mainly focuses on the safety environment and measures during storage.     

Alternative method to prevent thermal runaway in case of error on operating conditions continuous reactor

Thermal runaway was studied in a continuous tubular pilot reactor under steady-state regime. Different accident scenarii were conducted by making some errors on reactant concentrations and/or temperature feed. To prevent thermal runaway, control by direct contact by solvent injection was used at different reactor locations. This injection allowed controlling the maximum reaction temperature. A simplified analytical method to estimate the maximum reaction temperature along the reactor was used.Benefit of this control method was the diminution of computational time. Furthermore, by injecting solvent to control maximum reaction temperature, there is no need to shut down the unit. The control method was validated experimentally.     

Thermal hazard assessment for synthesis of 3-methylpyridine-N-oxide

The exothermic oxidation of 3-methylpyridine with hydrogen peroxide was analyzed by Reaction Calorimeter (RC1e) in semi-batch operation. Heat releasing rate and heat conversion were studied at different operating conditions, such as reaction temperature, feeding rate, the amount of catalyst and so on. The thermal hazard assessment of the oxidation was derived from the calorimetric data, such as adiabatic temperature rise (ΔT_ad) and the maximum temperature of synthesis reaction (MTSR) in out of control conditions. Along with thermal decomposition of the product, the possibility of secondary decomposition under runaway conditions was analyzed by time to maximum rate (TMR_ad). Also, risk matrix was used to assess the risk of the reaction. Results indicated that with the increase of the reaction temperature, the reaction heat release rate increased, while reaction time and exotherm decreased. With the increase of feeding time, heat releasing rate decreased, but reaction time and exotherm increased. With the amount of the catalyst increased, heat releasing rate increased, reaction time decreased and exothermic heat increased. The risk matrix showed that when the reaction temperature was 70 °C, feeding time was 1 h, and the amount of catalyst was 10 g and 15 g, respectively, the reaction risk was high and must be reduced.     

Comparison of criteria for prediction of runaway reactions in the sulphuric acid catalyzed esterification of acetic anhydride and methanol

Loss of temperature control is one of the major reasons that can lead to runaway reaction. This occurrence is commonly named thermal runway. The aim of this paper is the application of thermal runaway criteria in order to predict the onset of runaway phenomena and define the range of stability related to operating conditions in the reactor, with specific reference to the esterification of acetic anhydride and methanol catalysed by sulphuric acid tested in isoperibolic conditions. The isoperibolic calorimeter has also been used to obtain thermodynamic, kinetic and physical chemistry data necessary to develop a model for the reaction. Some runaway criteria applied in this work require a model for the process, so a model for the analyzed system been developed.Because of the modest reaction enthalpy and low activation energy this reacting system provide a severe test to the runaway criteria.In this work, various runaway criteria have been applied to the experimental and simulated data and the results obtained have been compared.     

低压环境对锂电池热失控释放温度影响

为研究锂电池在民航飞行低压特殊环境的安全性及发生热失控灾害后的高温危险性,通过可模拟飞行变动条件的动压变温实验舱开展系列实验,研究锂电池在不同低压环境下的（101,60,30 kPa）多节18650型锂离子电池热失控温度特性,采集电池池体温度及热失控喷射释放温度等参数。研究结果表明:随环境压力降低,圆柱锂电池间的热失控传播并不能被阻断,但锂电池热失控灾害所释放产生的高温区域减少,且高温持续时间变短,释放所产生温度的高温危险性随环境压力的降低而有所降低。     

过氧乙酸溶液的热爆炸分析

为有效预防生产、储运和使用中过氧乙酸引发的火灾爆炸事故,采用绝热加速量热仪模拟了15%和10%浓度的过氧乙酸溶液的热爆炸过程,得到了两种浓度的PAA溶液的热分解温度、压力、温升速率随时间变化的关系曲线,并用速率常数法分别计算了反应级数n、表观活化能Ea和指前因子A。经过绝热修正,得到最危险状态下的温度和压力等相关热危险参数,并基于Semenov热爆炸理论推算了三种包装条件下两种样品的不可逆温度和自加速分解温度。结果表明,15%PAA和10%PAA溶液热分解反应级数均为一级,表观活化能分别为1044kJ·mol-1和1032kJ·mol-1;绝热条件下初始放热温度分别为429℃和293℃;自加速分解温度受反应系统到达最大反应速率的时间、物料存储规模及散热条件的影响,建议PAA应储存在通风背阴处且单个包装容积应控制在25L以下。     

过氧化氢热爆炸研究进展

过氧化氢作为绿色环保的氧化剂,广泛应用于工业的各个领域,同时也因其热分解爆炸危险性导致了一系列严重的火灾爆炸事故。过氧化氢在高温或与一些不兼容化学物质作用下,将会激发其热危险性,进而引发热失控反应,最终导致爆炸事故的发生。结合近年来国内发生的过氧化氢热爆炸事故,简要概述了其热爆炸事故历程,并从理论研究和实验研究两个方面综述了过氧化氢热爆炸的研究进展。理论研究方面,主要介绍了化学反应失控模型和基于热动力学的研究方法,尤其对基于热失控模型的热风险评估进行了详细的阐述。实验研究方面,分析了高温条件下与杂质催化作用下过氧化氢的热危险性,包括无机杂质和有机杂质。最后就过氧化氢热爆炸的研究提出了进一步的研究方向。     

化学放热系统热爆炸潜在危险性评价

讨论化学放热系统的热稳定性和临界条件,用化学反应物无消耗的假设推导化学放热系统热失控(热爆炸)时的动力学参数临界值,得到热失控的判据、临界点火温度和熄火温度。提出用系统安全指数概念来评价放热反应系统发生热爆炸的潜在危险性,分析化学放热系统的平衡域。用硝酸甲酯分解爆炸实例,说明如何利用安全指数对具有热爆炸可能性的系统的潜在危险性进行定量评价,其预测结果与实验结果一致。     

Thermal risk analysis of cumene hydroperoxide in the presence of alkaline catalysts

Many studies have been performed to clarify the basic thermal runaway hazards and kinetics of cumene hydroperoxide (CHP) decomposition. However, materials that are incompatible with CHP have not been clearly identified. Alkaline solutions have been used as a catalyst to form dimethylphenyl carbinol (DMPC) and dicumyl peroxide (DCPO); however, these solutions also affect the reaction and storage temperature of CHP. In this study, thermal calorimeters, differential scanning calorimetry (DSC) and vent sizing package 2 (VSP2), were used to compare the effects of various bases on the decomposition of CHP in cumene. Specifically, the exothermic onset temperature, change in pressure over time, self-heating rate and heat of decomposition were evaluated. Moreover, to appraise the degree of hazard associated with the use of CHP, the compatibility of CHP with various substances was analyzed, and a risk matrix for thermal runaway reactions was obtained. The results of the present study could be used to design safety procedures for the production of CHP and its derivatives.     

二叔丁基过氧化物的热失控动力学研究

为研究二叔丁基过氧化物(DTBP)热失控危险性,利用C600微量量热仪对DTBP热分解动力学进行试验研究,测定DTBP在不同升温速率下的起始放热温度和分解热,分别用非等转化率法和等转化率法得到DTBP热分解反应的动力学参数。用非等转化率法确定反应的最佳反应级数为1,相应的活化能分别为137.75、132.60、128.61和122.93 kJ/mol,指前因子分别为8.82×1012、6.69×1012、2.06×1012和3.89×10111/s。用等转化率法确定的活化能范围为102～138 kJ/mol,并拟合出活化能与转化率的关系曲线。结合计算出的动力学参数,通过对DTBP分解机理的分析,可以推断其具有热失控危险性。     

交联剂TAIC反应风险评估分析

利用全自动反应量热仪和绝热加速量热仪等相关实验仪器检测出TAIC(三烯丙基异氰尿酸酯)合成反应的反应热、比热容及热稳定性等数据,依据绝热温升、工艺温度、技术最高温度、最大反应速率到达时间及失控体系可能达到的最高温度这5个温度参数按照评估标准从分解热、严重度、可能性、矩阵、工艺危险度这5个方面分别进行评估。通过对热参数及实验过程进行分析提出降低工艺危险等级的工艺优化方法。根据最终评估结果对TAIC生产装置的安全性进行评价,提出相应的整改措施及建议。     

Thermal hazard evaluation for γ-valerolactone production by using formic acid as hydrogen donor

Aiming at the green and sustainable energy substitution and supply, biomass valorization has become a potential strategy to face the energy crisis and increasing demand all over the world from long-term perspectives. Among the bio-based chemicals, γ-valerolactone (GVL) production from hydrogenation of levulinic acid (LA) and its esters has attracted great interests due to its wide applications, such as fuel, solvent, and additives. However, the safety evaluation for this hydrogenation reaction has received few attentions. To fill this gap, thermal hazard evaluation for GVL production from LA hydrogenation by using formic acid (FA) as hydrogen donor was first performed. The process conditions were optimized by using orthogonal experimental method for further calorimetry study. Thermal stability of chemicals and thermal risk of reaction process under adiabatic conditions were investigated by applying differential scanning calorimetry and accelerating rate calorimeter Phi-Tec II, respectively. The results revealed that the chemicals were stable in temperature range from 30 to 250 °C except FA due to its evaporation and decomposition with endothermic behaviors. The reaction process under isothermal and adiabatic conditions demonstrates that the decomposition of FA was rapid and followed by the hydrogenation of LA to GVL. Based on kinetic model under adiabatic conditions and risk matrix, the thermal runaway risk was found to be medium, indicating that certain safety measures should be properly designed and taken for loss prevention. This work could benefit the safety design and thermal risk prevention for GVL production by using FA as hydrogen donor.     

锂离子电池组热失控蔓延热传递机制的影响研究

分析通风和电池组数量对电池组热失控发展蔓延热传递机制的影响。选择荷电状态(SOC)为100%的镍钴锰(NCM523)三元锂离子动力电池组作为研究对象,改变电池组底部外加热源的热流量和加热时间,利用多物理场仿真软件COMSOL,进行热滥用导致不同风速通风环境和不同电池数量电池组热失控过程的模拟。结果表明:随着风速不断增大,电池组和周围环境的对流换热损失增强,电池组热失控蔓延进程受到了有效抑制。受热传导模式的影响,电池组数量和排列方式不同,电池组热失控蔓延的路线不同。越靠近外部热源的电池,触发热失控越早,触发热失控的起始温度越高;电池组所含电池数量越多,触发电池组热失控所需的热流量越大,但第一块电池热失控以后,后续电池触发热失控的时间间隔急剧缩短,电池组热失控后果的严重度增加。     

不同外热功率下18650锂离子电池热失控特性*

为探究不同外热功率（220,170,120,70 W）下锂离子电池的热失控特性,采用动压变温实验舱作为燃爆实验舱,并利用量热仪和ISO-9705烟气分析仪监测特征参数,对荷电状态（SOC）为100%的18650型锂离子电池进行高温热失控实验。结果表明:在不同的外热功率条件下,锂离子电池进入热失控的过程呈现出相似的趋势,但是各阶段的特性却存在差异。池体表面中心温度、HRR,THR和耗氧量均随外热功率的降低而降低。高外热功率下燃爆响应时间点明显提前,池体温度更高,220 W外热功率下,燃爆响时间点为176 s,池体温度为720.6 ℃,比70 W时提前366 s,高210.03 ℃,可见高外热功率时,电池热危害性更高。热解烟气CO的峰值体积百分比浓度随着外热功率的降低而升高,而CxHy的峰值质量百分比浓度降低,,CO2的峰值体积百分比浓度降低。在70 W外热功率时,CO峰值体积百分比浓度高达0.322%,220 W时CO峰值体积百分比浓度仅为0.165%,说明低外热功率时,电池毒危害性更高。     

A general criterion to define runaway limits in chemical reactors

A general runaway criterion valid for single as well as for multiple reaction types, i.e. consecutive, parallel, equilibrium, and mixed kinetics reactions, and for several types of reactors, i.e. batch reactor (BR), semibatch reactor (SBR) and continuous stirred tank reactor (CSTR) has been developed. Furthermore, different types of operating conditions, i.e. isoperibolic and isothermal (control system), have been analysed. The criterion says that we are in a runaway situation when the divergence of the system becomes positive (div>0) on a segment of the reaction path. The results show that this is a general runaway criterion than can be used to calculate the runaway limits for chemical reactors. The runaway limits have been compared with previous criteria. A considerable advantage, over existing criteria, is that it can be calculated on-line using only temperature measurements and, hence, it constitutes the core of an early warning runaway detection system we are developing.