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.     

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.     

Gas-phase thermal explosions in catalytic direct oxidation of alkenes

The metal-based catalytic oxidation of alkenes to the corresponding epoxides is playing a significant role in the modern chemical industry. Nevertheless, these key processes are still lacking proper understanding with respect to the gas-phase runaway behaviour (thermal explosion) and to the hot spot formation on the catalytic surface, under the typical process conditions.This work aims to enlighten these aspects by considering either the catalytic or the gas-phase chemistry for the development of reactor operative diagrams, in order to define the best-operating conditions with respect to the selectivity, the productivity, and the process safety aspects.The proposed methodology has been applied to the oxidation of ethylene and propylene for the direct oxidation process by pure oxygen, considering a detailed kinetic model accounting for the homogeneous reactions, coupled with the heterogeneous catalytic mechanisms.Sensitivity and reaction path analyses were performed to individuate the ruling species and reactions determining the transition to runaway conditions.     

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.     

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 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.     

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.     

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.     

Model-based zero emission safety concept for reactors with exothermal reactions for chemical plants

To avoid major accidents caused by runaway reactions, a new safety concept called SmartHIP¹ is introduced, with the objective to prevent runaway reactions and avoid emissions. The basic principle is based on an online model-based approach by using safety instrumented systems. Different criteria exist for calculating the hazard potential of a runaway reaction and its early detection. The approaches of the literature from the last years are analysed and divided into three different groups: Divergence, Accumulation and Adiabatic Criterion. Each approach is compared to the other by applying it to an esterification reaction of acetic anhydride with methanol. The approaches are evaluated according to different requirements, like accuracy, wide range of application or reliability.     

Development of a risk-based maintenance strategy using FMEA for a continuous catalytic reforming plant

Petrochemical facilities and plants require essential ongoing maintenance to ensure high levels of reliability and safety. A risk-based maintenance (RBM) strategy is a useful tool to design a cost-effective maintenance schedule; its objective is to reduce overall risk in the operating facility. In risk assessment of a failure scenario, consequences often have three key features: personnel safety effect, environmental threat and economic loss. In this paper, to quantify the severity of personnel injury and environmental pollution, a failure modes and effects analysis (FMEA) method is developed using subjective information derived from domain experts. On the basis of failure probability and consequence analysis, the risk is calculated and compared against the known acceptable risk criteria. To facilitate the comparison, a risk index is introduced, and weight factors are determined by an analytic hierarchy process. Finally, the appropriate maintenance tasks are scheduled under the risk constraints. A case study of a continuous catalytic reforming plant is used to illustrate the proposed approach. The results indicate that FMEA is helpful to identify critical facilities; the RBM strategy can increase the reliability of high-risk facilities, and corrective maintenance is the preferred approach for low-risk facilities to reduce maintenance expenditure.     

Planning protection measures against runaway reactions using criticality classes

A systematic approach to the assessment of thermal risks linked with the performance of exothermal reactions at industrial scale was proposed a long time ago. The approach consisted of a runaway scenario starting from a cooling failure and a classification of these scenarios into criticality classes. In the mean time these tools became quite popular and many chemical companies use them. Recently, the international standard IEC 61511 required the use of protection systems with reliability depending on the risk level. Since the criticality classes were developed as a tool for the choice of risk reducing measures as a function of the criticality, it seems obvious that the criticality classes may be used in the context of the standard IEC 61511, which provides a relation between the risk level and the reliability of protection systems.Firstly, the runaway scenario and the criticality classes will be shortly described. Secondly, the assessment criteria for severity and probability of occurrence of a runaway scenario will be described together with the required data and their interpretation in terms of risk. Thirdly, the assessment procedure is exemplified for the different criticality classes. Finally, the design of protection measures against runaway and the required IPL and SIL are based on the risk assessment obtained from the criticality classes. This approach allows minimising the required data set for the safety assessment and for the definition of the protection system designed in order to avoid the development of the runaway.     

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.     

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.     

Uncertainty analysis framework for tubular connection sealability of underground gas storage wells

The seal failure of tubing and casing connections compromises underground gas storage well safety. This work proposes a systematic uncertainty analysis framework for connection sealability assessment. The framework covers reliability analysis and reliability sensitivity analysis and attempts to provide more effective support for the reliability design of connection seals. The reliability analysis introduces an adaptive Kriging with stopping criterion P-Monte Carlo simulation (AKP-MCS) method, which can provide a satisfactory estimate of failure probability with a small number of performance function evaluations. This metamodeling technology can effectively reduce the numerical efforts required for the reliability assessment of connection sealability. In the reliability sensitivity analysis, the refined metamodel obtained from the reliability analysis is coupled into a single-loop Monte Carlo simulation (MCS) method. The classifier attribute of this metamodel can meet the requirement of the single-loop MCS method to classify the signs of sampling points. This attribute enables sample matrices to be evaluated on this metamodel instead of the performance function, making the reliability sensitivity assessment more feasible. The proposed method is first demonstrated with four academic examples with promising results. Next, an illustrative tubular connection case is provided. The proposed scheme gives estimates of the failure probability and reliability sensitivity close to the classical model but requires less computational cost. The results of the analysis can provide useful information for the scheme decision-making and reliability optimization of connection seal design.     

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.     

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.     

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。     

数据挖掘与现场调查结合的电动汽车事故分析

为了解电池热失控引起的电动汽车自燃事故起因,降低事故发生率,统计梳理2020年新能源汽车起火事故概况,并基于动力电池失效机制和车辆运行数据,提出一种融合事故阶段安全参数关联分析、事故现场调查和全生命周期数据一致性分析的事故多维分析方法,采用该方法深入剖析一起电动汽车起火事故.研究结果表明:事故多维分析方法可通过探究事故...