A comparative analysis between temperature and pressure measurements for early detection of runaway initiation

In this work, we have analysed the use of pressure instead of temperature measurements for the early warning detection of runaway initiation. This is possible due to the fact that our runaway criterion, i.e. div>0, does not depend specifically on which state space variable we are using for divergence calculation. A series of runaway experiments, carried out in a 250 l pilot-scale reactor, has been used to compare the results. In accordance with previous analysis, we show that by using temperature, early detection of runaway initiation is achieved. Analogously to temperature, pressure may be also used for runaway detection. By comparing the different types of reactive systems analysed (vapour and gassy), it can be observed that temperature works better, in terms of earlier detection, than pressure but the differences are more pronounced for vapour than for gassy systems.     

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.     

On the divergence criterion for runaway detection: Application to complex controlled systems

It is well-known that, for certain values of the operative parameters influencing the dynamic behavior of a chemical reactor, a phenomenon known as thermal runaway (that is, a loss of the reactor temperature control) may arise. Such a situation can be really dangerous because above a certain threshold temperature value unwanted side reactions or, worse, decompositions of the reacting mixture may be triggered evolving high amounts of flammable or toxic gases that can cause reactor pressurization and, eventually, its explosion. For this reason, since the beginning of the previous century a number of studies concerning the prediction of the so called runaway boundaries has been carried out. In this work, a modified version of the divergence criterion for runaway detection, originally developed by Zaldívar and co-workers, is presented. Such a modified divergence criterion is capable of treating whatever type of complex controlled reacting system (taking into account not only temperature control but also dosing strategies) and its reliability has been demonstrated for isoperibolic semibatch reactors using literature experimental data concerning the nitration of 4-Chlorobenzotrifluoride in mixed acids and the nitric acid oxidation of 2-octanol to 2-octanone and further carboxylic acids.     

Setting optimal operating conditions for a catalytic reactor for butane oxidation using parametric sensitivity analysis and failure probability indices

Safe operation of a catalytic reactor remains a sensitive issue when highly exothermic reactions are conducted and hazardous side reactions may occur. Derivation of the optimal operating conditions must include economic but also safety criteria, technological constraints, beside controllability and stability aspects. The present work introduces a criterion based on a joint failure probability index related to uncertainty in the runaway boundaries and the random disturbances of the operating parameters. The use of such a safety criterion is even more important when setting the optimal operating policy of the reactor in the vicinity of the runaway boundaries that often correspond to a high productivity. The paper indicates how an economically efficient but more prudent operating policy can be selected, by simultaneously considering the economic and safety objectives. An example is provided for the case of an industrial fixed-bed tubular reactor, of high thermal sensitivity, used for the catalytic oxidation of butane to maleic anhydride in vapour phase. The multi-objective optimization can lead to a prudent trade-off operating solution (corresponding to a failure probability of maximum 3–4%) that limits the reactor productivity. Being based on a local and global sensitivity analysis of the reactor, the proposed rule is generally applicable by minimizing the probability with which control variables violate their safety limits.     

Variability of operating safety limits with catalyst within a fixed-bed catalytic reactor for vapour-phase nitrobenzene hydrogenation

The safety in operation of a fixed-bed catalytic reactor remains a sensitive issue when a highly exothermic reaction is conducted and various process development elements such as controllability, stability, risk, and economic aspects are considered. Several model-based methods are used to estimate the safe operating region limits. Nominal conditions are set to limit the hot spot in the tubular reactor and avoid excessive thermal sensitivity to variations in the process parameters. When the catalyst or its characteristics are changed, the operating conditions have to be adjusted accordingly. The safety problem becomes more important when the production optimization requires setting the nominal operating point in the vicinity of the safety limits. This paper investigates the advantages as well as the precautions that need to be taken when using a more sophisticated model-based global sensitivity criterion (MV of Morbidelli & Varma) to routinely update the runaway critical conditions when changes in the investigated system frequently occur. A concrete example is provided for the case of an industrial fixed-bed catalytic reactor for nitrobenzene hydrogenation in vapour-phase. The analysis points out the discrepancies in predicting the runaway boundaries for complex processes between precise sensitivity-based MV-method and shortcut methods, and the importance of accounting for parameter uncertainty for both evaluation of the confidence region around the runaway boundaries and for the optimal set-point location. The close connection between the operating risk limits and the process kinetics is also highlighted even if the reactor geometry and the main flow conditions are kept unchanged.     

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.     

Calorimetric study of the inhibition of runaway reactions during methylmethacrylate polymerization processes

Reaction inhibition was adopted as a method to halt runaway phenomena during polymerization experiments. Use of reaction calorimetry coupled with a particular system for early detection of the onset of runaway (early warning detection system) has allowed to investigate the behaviour of two substances that can influence the reaction rate: hydroquinone and 1,4-benzoquinone. The process studied was the free-radical polymerization of methylmethacrylate carried out under batch conditions in bulk or in emulsion. The results show that hydroquinone and 1,4-benzoquinone behave differently: the first is an inhibitor because it completely stops the process; the second behaves as a retarder and could be used industrially to control the process and keep the reactor temperature within safe limits.     

Safer management of process changes in chemical reactors

Nowadays many chemical industries are SMEs where multi-purpose batch or semi-batch reactors are commonly used. Vent sizing for realistic runaway scenario is not an easy task for such enterprises since they have usually few resources and use multi-purpose reactors with fast process turnovers. As a consequence these batch and semi-batch reactors are usually equipped with emergency relief systems sized once forever when the reactor is designed. This can lead to a large underestimation of the vent area in case of runaway reactions occurring when processes different from the ones considered for originally sizing the vent are carried out.The approach proposed in this work aims to identify the maximum reactor load leading to safe conditions even in case of runaway phenomena to be handled with the emergency relief system already installed (or even with a smaller vent area). This approach allows avoiding the change of the emergency relief system with a larger vent area (as required every time a new more hazardous process has to be carried out on existing reactors) at the price of lower plant productivity.     

基于VSP2的引发剂质量分数对醋酸乙烯聚合反应失控特性及安全泄放设计影响研究

为了解决醋酸乙烯聚合反应失控所引起的超压问题,通过VSP2绝热量热仪研究了醋酸乙烯聚合反应的失控特性,并通过Leung's法对某醋酸乙烯聚合反应器的安全泄放面积进行了计算;然后,在其他条件不变的情况下,研究引发剂质量分数对失控特性和泄放面积的影响,结果表明,引发剂质量分数对反应总放热量的影响不大,体系绝热温升为105~115℃;但引发剂质量分数越大,失控反应的最大温升速率和最大压升速率越大。这是因为引发剂质量分数越大,在相同泄放压力和最大累积压力下,单位质量反应物的放热速率就越大,也就需要更大的泄放面积;最后,引入无量纲数W~*、G~*和A~*,拟合出它们与引发剂质量分数X*的关系式,结果表明,在研究范围内所需安全泄放面积随引发剂质量分数线性增大。     

Thermal risk in batch reactors: Case of peracetic acid synthesis

The present paper deals with accidents risk in batch reactors. It identifies the conditions for the occurrence of a thermal runaway and develops a probabilistic approach to assess the relevant risk. It investigates also the conditions for optimal synthesis of peracetic acid (PAA) with hydrogen peroxide (HP) and acetic acid (AA). The kinetic model of reversible reaction and side reaction of PAA synthesis is used to predict reactor temperature and molar ratio of PAA by ASPEN PLUS software. A sensitivity analysis is performed under different conditions such as constant temperature or adiabatic process with different concentrations of sulfuric acid. Assuming a prior cooling system failure, the conditions for reaction runaway triggering a thermal accident are identified in the case of PAA synthesis. Monte Carlo simulations are used in order to calculate the conditional probability of accident and optimize the synthesis of PAA.     

Thermal runaway criterion for chemical reaction systems: A modified divergence method

The chemical reaction in certain range of operating conditions may exhibit parametric sensitivity where small changes in one or more of the input parameters lead to changes in the output variables (eg. reaction temperature). The sharp rise of the reaction temperature is a critical behavior that may lead to runaway conditions. Thus, it is of vital importance to determine the critical operating parameters consisting of the parametric sensitivity region under the consideration of intrinsic safety. In this paper, a modified divergence criterion is proposed based on the trace of Jacobian matrix at the maximum temperature. The nonlinear differential equations describing the dynamic behavior of the chemical reaction is linearized locally in the vicinity of the equilibrium point by the small perturbation analysis. The relationship between the perturbation equation and parametric sensitivity of the reaction system is investigated. The critical values computed by the modified divergence criterion are compared with Morbidelli and Varma criterion (MV criterion), Adler and Enig criterion (AE criterion) and divergence criterion (Div criterion). The comparison demonstrates the validity of the new criterion. In addition, the critical explosion pressures of two kinds of hazardous chemicals are computed by the various critical criteria and compared with published experimental data. The results show that the modified divergence criterion could give smaller computational error compared with the previous criteria.     

反应性液体紧急泄放面积设计进展

为防止反应失控造成爆炸事故,减少事故损失,在介绍模拟反应失控的实验装置ARC、VSP以及RSST等的基础上,针对不同的紧急泄放类型,如气相系统、蒸气系统和混合系统的紧急泄放研究进展,进行了分析论述,旨在发现解决反应失控紧急泄放问题的更好方法,从而为进一步研究反应失控的紧急泄放问题打下基础.     

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 instability and runaway criteria: The dangers of disregarding dynamics

Two exemplary exothermic processes, synthesis of nitroglycerine in a continuous stirred tank reactor (CSTR) and synthesis of the explosive RDX in a CSTR, are used to demonstrate the dangers of ignoring the system dynamics when defining criteria for thermal criticality or runaway. Stability analyses are necessary to prescribe such criteria, and for these systems prove the presence of dangerous oscillatory thermal instability which cannot be detected using the steady state thermal balances.     

Safety assessment of potential supercritical solvents for Fischer–Tropsch Synthesis

Fischer–Tropsch Synthesis (FTS) is a primary pathway for gas-to-liquid (GTL) technology. In order to overcome commercial problems associated with reaction and transport phenomena, the use of supercritical solvents has been proposed to enhance conversion, catalyst stability and improve temperature control in fixed-bed reactors. One of the major challenges in designing the supercritical FTS reactor unit is selecting appropriate solvents of critical properties within the required reaction operating conditions. Numerous alternatives exist and should be screened based on relevant criteria. The main aim of this paper is to develop a screening methodology to identify an optimum supercritical solvent or a mixture of solvents that meet the aforementioned criteria while minimizing the cost and more importantly satisfying the safety constraints. A safety metric system was developed in order to compare the risk issues associated with using different solvents. In addition, an economic analysis of using the different solvents was performed. Finally, a case study was solved to illustrate the use of the proposed metrics and the selection of solvents based on safety and techno-economic criteria.     

Catalytic decomposition of hydrogen peroxide in the presence of alkylpyridines: Runaway scenarios studies

A number of runaway scenarios of the excess of hydrogen peroxide used during the N-oxidation of alkylpyridines, under closed and open conditions, were examined. It was found that, in most cases, if the volume of the liquid hydrogen peroxide solution occupies more than 10% of the total volume of a closed system (e.g. reactor and vent line between reactor and blockage), the production of gases raises the pressure so quickly that evaporation is completely suppressed. Higher than 70% filling levels result in complete expansion of the liquid. The MTSR(t) of the system falls rapidly if the normal process temperature is high, but if a runaway occurs exactly at the end of dosing, MTST will be very high and secondary decompositions will rapidly develop. The results of this study are currently being used to critically assess the current approaches and further the study of inherently safer designs.     

过氧丙酸分解反应的失控泄放特性研究

为研究过氧丙酸分解反应的失控泄放特性,利用泄放模式实验装置对过氧丙酸在不同泄放口径和泄放压力下的顶部和底部的泄放过程进行了试验模拟,得到了过氧丙酸的失控特性参数和不同条件下的泄放特征。结果表明:过氧丙酸失控反应泄放易出现二次峰值现象,初次峰值为气相泄放,二次峰值为气液两相泄放;二次峰值的出现取决于泄放口径及泄放时的物料温度,与泄放压力无关;恒压泄放容易出现非平衡泄放,导致较高最大累积压力和较高的釜内物料温度;底部泄放能够使釜内物料快速排空。     

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.     

Influence of calorimetric approximations on the design of emergency relief systems

The design of an emergency relief system (that is, a pressure safety valve or a rupture disk) for vessels, which may involve runaway reactions, requires knowledge of the chemical kinetics of the reactions involved. When safety-related problems are considered this is usually achieved using calorimetric tests, coupled with some suitable approximations on the kinetics of the reacting system. In this work we have analysed the extent to which the precise knowledge of the chemical kinetics influences the size of the relief system device for different reaction conditions. Decision criteria are proposed to identify situations where approximations in the kinetic mechanism lead to underestimation in the venting area.