Sizing of safety valves for multi-purpose plants according to ISO 4126-10

Multi-purpose plants are frequently protected with mechanical safety devices like safety valves or bursting disks. Due to many changes of recipes it must be checked regularly whether the safety devices are sufficiently sized. But the sizing procedure of individual safety devices can be very tedious. Therefore energy specific relief areas (effective relief area per kW of energy input) have been determined for approx. 60 typical solvents. They are indicated for reactors with safety devices which have a set pressure of 7 bar (abs) or 11 bar (abs). These values are independent of the size of the reactors for vaporizing systems and arbitrary safety valves. The energy specific relief areas allow the minimum required relief area quickly to recalculate if the energy input of the reactor is known. In addition, the application of solvents in multi-purpose plants can be evaluated from a safety point of few.The energy specific relief areas are calculated based on a relief of two-phase gas/liquid mixtures. The data have been determined with the non-equilibrium HNE-DS method, which takes into account the boiling delay of the liquid in the safety device and the slip between gases and liquids. The method is recommended in the international standard ISO 4126 part 10. In addition, practical advice and possible improvements are outlined. The method leads to significantly smaller relief areas than according to the API 520. For multi-purpose plants with available safety devices this method allows for a considerable expansion of the application range of reactors.     

Vent sizing for gas-generating runaway reactions

Exothermic runaway reactions that generate non-condensible gas as the temperature increases, as is typical of decompositions for example, can reach extremely high rates of pressure rise necessitating emergency relief of the process vessel containing the reactant. Sizing of a relief device using presently recommended methods (e.g. DIERS) frequently leads to extremely large and expensive vents. This paper presents a methodology that leads to a simple but much improved method for vent sizing, fully allowing for two-phase release of the gas—liquid mixture. A number of examples are presented which lead to interesting conclusions about the influence of plant variables.     

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。     

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.     

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.     

A hybrid approach to faults detection and diagnosis in batch and semi-batch reactors by using EKF and neural network classifier

This work deals with a new hybrid approach for the detection and diagnosis of faults in different parts of fed-batch and batch reactors. In this paper, the fault detection method is based on the using of Extended Kalman Filter (EKF) and statistical test. The EKF is used to estimate on-line in added to the state of reactor the overall heat transfer coefficient (U). The diagnosis method is based on a probabilistic neural network classifier. The Inputs of the probabilistic classifier are the input–output measurements of reactor and the parameter U estimated by EKF, while the outputs of the classifier are fault types in reactor. This new approach is illustrated for simulated as well as experimental data sets using two cases of reactions: the first is the oxidation of sodium thiosulfate by hydrogen peroxide and the second is alkaline hydrolyse of ethyl benzoate in homogeneous hydro-alcoholic. Finally, the combination of the estimated parameter U using EKF and probabilistic neural network classifier provided the best results. These results show the performance of the proposed approach to monitoring the semi-batch and batch reactors.     

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.     

Database-supported documentation and verification of pressure relief device design in chemical plants

In this paper the results of a multi-year analysis of the pressure relief devices located in several plants in major chemical sites are summarized. The analysis consisted of a systematic evaluation of existing safety valves and rupture discs including the identification of the service conditions and design cases as well as the sizing calculations of the individual device and associated piping. Furthermore, from the total amount and the hazardous potential of the effluents the necessity of retention systems is evaluated to ensure a safe disposal.Because the knowledge in the field of emergency pressure relief changed very rapidly in recent years, the design of some safety devices was not according to the state of the art. An essential part of the verification program was the recommendation of measures to find the most economical yet technically correct way to correct these deficiencies. Rather than by carrying out wholesale replacement of an incorrectly sized safety device or vent line, often a reduction of the mass flow rate to be discharged, for example, by an orifice in a supply line, is sufficient.Results of the analysis were recorded on a novel database to capture the sizing information and maintain correct pressure relief device sizing into the future. The systematic databased approach has been used for the evaluation of about 4000 safety devices so far. The procedure has been proven to enable an experienced design engineer to carry out the analysis of a great number of pressure relief devices in a time-saving, reliable and reviewable way.     

Investigation of a drum explosion of 30% aqueous KOCN

A 30% aqueous solution of KOCN placed in a 55 gallon HDPE drum at 50 °C began venting gas almost immediately. Although a vent was kept open the drum exploded within 1–2 h of being filled. This report reviews the steps taken after the accident to find its cause and to recommend safe operating conditions. The DIERS vent sizing package (VSP), used as a closed system adiabatic reactor, was able to simulate the incident under controlled laboratory conditions. Data were thereby collected for the first time on the runaway kinetics of the KOCN hydrolysis. Isothermal data were obtained in a highly sensitive microwatt heat flow calorimeter in an open system. It was demonstrated that even under isothermal conditions, the hydrolysis rates accelerated once underway, reaching maxima in 30 h at 25 °C and 6.7 h at 40 °C. There is satisfactory agreement of these results with other work on 0.5% KOCN solutions reported in earlier studies.     

Safety relief analysis for methanol-acetic anhydride system by stationary methods and dynamic simulation

The Leung method proposed by the Design Institute of Emergency Relief Systems (DIERS) is widely used in the design of relief systems involving two-phase flow. However, this method is not always suitable for all the situations. The calculating results may be unacceptably large, especially under high overpressure. To aid selection of appropriate vent sizing methods, a typical vapor system experiment (esterification of methanol-acetic and anhydride) was conducted by the vent sizing unit (VSU) of accelerating rate calorimeter (ARC). Seven different stationary methods were used to calculate the venting size under overpressure of 10%, 20%, 50%, 100% and 200%. Through the systematic comparison of different methods, a conservatism order of stationary methods was summarized as well as the selecting principles for these methods were discussed. Process simulation was also applied to investigate the relationship between reactor temperature/pressure and its relief size, which could be used in the prediction of vent size in vapor system conveniently without complex calculating procedure.     

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.     

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.     

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.     

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.     

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。     

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.     

Dynamics of explosion venting in a compartment with methane-air mixtures

A series of experiments on explosion venting of methane-air mixtures are performed to scrutinize the pressure evolution as well as the flame dynamics and morphology at various vent conditions. Specifically, a premixed flame is ignited at the center of a polycarbonate cylindrical compartment, with three various vent areas considered (with negligible vent relief pressure). As expected, the highest maximum pressure is observed in the case of the smallest vent area. For all three cases, the pressure evolution experiences two major peaks, associated with the instants (i) when the maximum flame front surface area in the chamber is reached and (ii) when an external explosion occurs due to venting of unburned gases, respectively. For the fuel-rich mixtures, a flashback is observed subsequent to the external explosion, constituting the key outcome of the present work. The flame tip velocities show two general trends, namely, exponential acceleration towards the vent, while a flame propagates towards the blocked side of the compartment with no acceleration, which is important to know in the fire/explosion safety applications.     

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.     

Vent Sizing Criteria for Partial Volume Deflagration

The accidental spill of volatile solvents or the release of flammable gases within equipment and buildings is likely to form fuel concentration gradients unless efficient mixing is provided. As a consequence, even small amounts of fuel can form flammable clouds, and partial volume deflagrations may occur. Nevertheless, few indications are given in international guidelines for vent sizing and only over-conservative well-mixed stoichiometric assumptions are used. In this paper, we propose a predictive methodology for the evaluation of the dynamics of partial volume deflagration, aiming at defining useful correlations for the design of vent devices, starting from the fundamental equation for the rate of pressure rise and flame propagation in closed vessel. We define a ‘stratified gas deflagration index’ K_G(m), where m is the filling ratio, and use it with the most common design equations for vent sizing. The approach has been validated by means of a CFD code for the simulation of stratified laminar methane–air explosion by varying both filling ratio and volume.