Dynamic modeling and bifurcation analysis for the methyl isocyanate hydrolysis reaction

The aim of this work was to characterize the methyl isocyanate hydrolysis reaction and to identify its operational criteria. The parametric sensitivity and dynamic stability methodologies were performed at the Bhopal disaster circumstances, over the relevant operating parameter space. Stable and unstable conditions, bifurcations points, turning points and oscillatory behavior were determined. The combined methodology give useful guidance on the operative conditions selection and the appropriate strategy to overcome hazardous situations. The obtained results demonstrated high sensitivity to small perturbations (thermal runaway) and prevalent oscillatory behavior. Moreover, the following critical parameters for the studied dynamic system were defined: the inverse residence time of 1.5700103 and the heat transfer coefficient of 752.394.     

Parametric sensitivity analysis for thermal runaway in semi-batch reactors: Application to cyclohexanone peroxide reactions

The semi-batch reactors (SBRs) system, which is widely used in industrial processes, possesses an intrinsic parametric sensitivity, in which infinitesimal disturbances of input parameters can result in large variations in output variables. In this work, local parametric sensitivity analysis (PSA) was used to understand parameter variations and global PSA was conducted to examine the interaction of input parameters. The effects of these parameters on the output of the system model were analyzed based on the Monte Carlo method with Latin hypercube sampling and the extended Fourier amplitude sensitivity test model. The results showed that the evolution of thermal behaviors in SBRs were observed: marginal ignition; thermal runaway; and the quick onset, fair conversion, and smooth temperature profile. The threshold point of transition from marginal ignition to thermal runaway was at the maximal value of local sensitivity, for which the slope with respect to cooling temperature equaled zero. Moreover, the sequence of the global sensitivity of six common input parameters was computed and evaluated. The reliability of the numerical models was verified by using our previous experimental results of cyclohexanone peroxide reaction. This comprehensive sensitivity analysis could provide valuable operating information to improve chemical process safety.     

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.     

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.     

连续搅拌釜式反应器放热反应失控事故预防

运用敏感度方法研究连续搅拌釜式反应器(CSTR)放热反应的参数敏感性行为及规律,探讨CSTR反应器的安全操作区域和敏感操作区域。研究发现:无因次活化能小于4的反应系统,其整个操作参数空间都是安全的;无因次活化能大于4的反应系统,只有选择无因次活化能小于其临界值,反应系统才能安全、稳定地运行;临界无因次反应热的值与无因次丹克莱尔数和无因次反应热有关;丹克莱尔数和无因次反应热越大,则临界无因次反应热越小,安全操作区域也越小。     

The critical runaway condition and stability criterion in the phenol–formaldehyde reaction

The procedure of phenol–formaldehyde polymerization is a rather important and complicated reaction in the chemical industry. This exothermic polymeric reaction releases a huge amount of heat. The high amount of energy accumulated and increasing temperature in this reaction process always lead to runaway reaction and a hazard situation owing to the high released heat and improper operation. In this investigation, we used sodium hydroxide as alkali–catalyst in the phenol–formaldehyde polymerization and estimated the reaction kinetics parameters to evaluate the thermal hazard conditions. The critical temperatures and stable criteria of the runaway reaction in this exothermic polymerization were evaluated. This technique is important and useful for safe operation in the phenol–formaldehyde polymerization process.     

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.     

Investigation of the decomposition reaction and dust explosion characteristics of crystalline dicumyl peroxide

The dicumyl peroxide (DCP) is widely used as a polymerization initiator, catalyst and vulcanizing agent in the chemical industry. A number of accidents have been caused by its thermal instability in storage or manufacturing process. Thus, its hazard characteristics have to be clearly identified. First of all, the differential scanning calorimeter (DSC) is used to measure the heat of decomposition reaction, which can contribute to understanding the reaction characteristics of DCP. The accelerating rate calorimeter (ARC) is used to measure the rates of temperature and pressure rises of decomposition reaction, and then the kinetics parameters are estimated. Furthermore, the MIKE 3 apparatus and the 20-l-Apparatus are used to measure and analyze the dust explosion characteristics of DCP at room temperature and atmospheric pressure. Finally, Semenov's thermal explosion theory is applied to investigate the critical runaway condition and the stability criterion of decomposition reaction, and to build the relationship of critical temperature, convective heat transfer coefficient, heat transfer surface area and ambient temperature. These results contribute to improving the safety in the reaction, transportation and storage processes of DCP.     

Parametric study of the explosivity of graphite-metals mixtures

The explosivity of dust clouds is greatly influenced by several parameters which depend on the operating conditions, such as the initial turbulence, temperature or ignition energy, but obviously also on the materials composition. In the peculiar case of a mixture of two combustible powders, the physical and chemical properties of both dusts have an impact on the cloud flammability and on its explosivity. Nevertheless, no satisfactory ‘mixing laws’ predicting the mixture behavior are currently available and the composition variable to be considered for such models greatly depend on the safety parameters which have to be determined: from volume ratios for some thermal exchanges and ignition phenomena, to surface proportions for some heterogeneous reactions and molar contents for chemical reactions. This study is mainly focused on graphite/magnesium mixtures as they are encountered during the decommissioning activities of UNGG reactors (Natural Uranium Graphite Gas). Due to the different nature and reactivity of both powders, these mixtures offer a wide range of interests. Firstly, the rate-limiting steps for the combustion of graphite are distinct from those of metals (oxygen diffusion or metal vaporization). Secondly, the flame can be thickened by the presence of radiation during metal combustion, whereas this phenomenon is negligible for pure graphite. Finally, the turbulence of the initial dust cloud is modified by the addition of a second powder. In order to assess the explosivity of graphite/magnesium clouds, a parametric study of the effects of storage humidity, particle size distribution, ignition energy, and initial turbulence has been carried out. In particular, it was clearly demonstrated that the turbulence significantly influences the explosion severity by speeding up the rate of heat release on the one hand and the oxygen diffusion through the boundary layer surrounding particles on the other hand. Moreover, it modifies the mean particle size and the spatial dust distribution in the test vessel, impacting the uniformity of the dust cloud. Thus, the present work demonstrates that the procedures developed for standard tests are not sufficient to assess the dust explosivity in industrial conditions and that an extensive parametric study is relevant to figure out the explosive behavior of solid/solid mixtures subjected to variations of operating conditions.     

某恒温间歇反应的热失控研究

为得到某恒温间歇反应冷却系统临界温度Tc并评估该反应体系的热危险性,基于间歇反应体系热参数的敏感性,探讨数值计算方法和选用4种反应热失控临界判据得到的不同Tc值。结果表明,数值计算方法得到了反应体系的Tc为31.7℃,Semenov判据、Sliding判据、Da/St判据得到的Tc值分别为10℃,17℃和27℃;无量纲绝热温升B给出的结论是,在允许的工艺温度范围内,该反应对任意的冷却系统温度都处于难以控制状态。数值计算方法及各判据得到的Tc的较大差异性说明各方法均有一定的局限性。在运用参数敏感性分析的基础上,结合风险矩阵图方法对系统冷却失效的热危险性进行评估,得到不同Tc下风险可接受、有条件接受及不可接受对应的参数范围的物料累积度Xac与最大反应速率到达时间θ间的关系。     

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.     

无机酸对硝酸铵热稳定性影响的研究

为了研究硫酸、盐酸两种无机酸对含能材料硝酸铵热稳定性的影响,使用绝热加速量热仪ARC和微量量热仪C80,对纯硝酸铵及硝酸铵和硫酸、盐酸的混合物进行了热分析实验并研究各种样品在恒温以及升温条件下的吸、放热特性。根据化学反应动力学和热力学理论,确定了硝酸铵及其与无机酸的混合物发生放热分解反应的反应动力学参数和热力学参数。基于Semenov热爆炸模型,计算并比较了各样品标准包装的自加速分解反应温度。     

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.     

Calculation of the stability criterion in the reaction of energetic materials

The stability in the energetic materials' thermal decomposition reaction is developed from both transient mass and heat transfer equations simultaneously. The derived equation is the sufficient and necessary condition for the reaction system's stability, which was expressed as:

In this stability equation, a modified Semenov number φ is the expressed function of three kinetic parameters, i.e., n, and β and two variables, i.e., x and θ, respectively. A reaction system is stable if its evaluated modified Semenov number satisfies this stability criterion. In the case of the minimum modified Semenov number φ_m, we can deduce the stability equation at critical condition as

The criteria of stability for the explosives TNT, RDX, PETN and HMX were evaluated using this formula, which was expressed θ_c as function of x at given n, η and various β as parameters.     

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。     

Influence of parameter uncertainty on modeling of industrial ammonia reactor for safety and operability analysis

Chemical reactors represent probably the most hazardous units of chemical industry. Safety analysis of a chemical reactor requires basic knowledge of all particular processes which can be described by mathematical models. Most of the model parameters involved in the prediction of reactor behavior are uncertain. These uncertainties can cause discrepancies mainly in the prediction by models with nonlinear behavior and they can be the source of confusion in the design of chemical reactors and consequently also in the safety and operability analysis.The main aim of this work was to analyze the influence of uncertainties in the model parameters on the prediction of operating quantities by mathematical models with nonlinear behavior. Such analysis can be used for safety and operability analysis of an industrial catalytic ammonia reactor. The industrial fixed-bed reactor was used by a mathematical model with nine parameters. Analyses of the influence of uncertainty in a single model parameter and their combination were carried out by the Monte Carlo approach. It is shown that even a small uncertainty in one of the key parameters or in a combination of these key parameters can result in several steady states results of the operating quantities and can be the source of confusion in the design and consequently also in the safety and operability analysis.     

CFD simulation of shortstopping runaway reactions in vessels agitated with impellers and jets

Runaway reactions are continuing to be a problem in the chemical industry. A recent study showed that 26% of our major chemical plant accidents are due to runaways. The consequences of runaway reactions are usually mitigated with (a) reliefs and containment systems or (b) shortstopping (reaction inhibition). This study covers the concept of shortstopping.

One of the major reasons for runaways is power failure. In the advent of a power failure, mixing an inhibiting agent with the reactor contents is challenging. However, jets or impellers driven by a small generator can be used for mixing. This study compares shortstopping results in vessels agitated with jets and impellers using computational fluid dynamics (CFD). A commercial CFD code, Fluent is used.

For shortstopping systems relying on jet mixing, angle and diameter of jet nozzle and jet velocity are the key design/operating parameters. For the systems with impellers, type, size and RPM of impeller are the key parameters. In this work, mixing with a jet mixer is first investigated for three nozzle diameters and two angles of injection. The best jet mixer configuration on the basis of mixing time is used for shortstopping studies. The simulated shortstopping results with the jet mixer are then compared with those obtained with impeller (Rushton and pitched blade turbine) stirred vessels. Our results identify the conditions for effective shortstopping; i.e., agitation requirements, locations for adding the inhibitor, and the quantity of inhibitor.

The distribution of excess inhibitor is shown to be an important and essential design criterion for effective shortstopping when using impeller stirred vessels. The comparative study with a single jet shows that jet mixer is ineffective when used for shortstopping. Efforts such as adding excess inhibitor and inhibition with higher reaction rates at the same power, proved to be ineffective when using jet mixer compared to the results with impellers.     

简单放热化学反应体系热安全性研究判据

许多危险化学物质在发生热化学反应的同时通常会放出热量,如果能量不能有效释放就可能引起火灾和爆炸事故,考虑到化学物质对热的响应方式非常复杂,从分析绝热条件下化学物质的热化学反应动力学入手,利用化学物质的物理化学特性参数计算化学反应体系在绝热条件下发生热化学反应的温升速率dT/dt,进而获得有关的动力学因子A,E等。结果表明,尽管在近似绝热条件下化学反应体系的热化学反应与其本身的特性和反应容器有关,但温升速率dT/dt只与物质的物理化学特性参数有关。含氯酸钾的几种简单放热反应体系的ARC实验结果进一步验证了这一结论。因此,同一种化学物质与不同物质构成的多元混合反应体系在相同近似绝热条件下的热化学反应特征参数,可以作为判据用来比较并评价体系的相对安全性,该判据对表征热化学反应的难易程度以及物质的相对安全性起到一定的指导意义。     

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.     

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.