Assessment thermal risk

This example shows that with only sparse thermal data it is sometimes possible to assess thermal risks. This is possible due to the low concentration used in this hydrogenation. 

Gygax, R. W., Scaleup Principles for Assessing Thermal Runaway Risks, Chem. Eng. Prog, V. 86, No. 2, 1990. 

Traditionally, risk is defined as the product of the severity of a potential incident by its probability of occurrence. Hence, risk assessment requires the evaluation of both the severity and the probability. Obviously, the results of such an analysis aid in designing measures for the reduction of the risk (Figure 3.1). The question that arises now is What do severity and probability mean in the case of thermal risks inherent to a particular chemical reaction or process ... 

The thermal risk linked to a chemical reaction is the risk of loss of control of the reaction and associated consequences (e.g. triggering a runaway reaction). Therefore, it is necessary to understand how a reaction can switch from its normal course to a runaway condition. In order to make this assessment, the theory of thermal explosion (see Chapter 2) needs to be understood, along with the concepts of risk assessment. This implies that an incident scenario was identified and described, with its triggering conditions and the resulting consequences, in order to assess the severity and probability of occurrence. For thermal risks, the worst case will be to lose the cooling of a reactor or in general to consider that the reaction mass or the substance to be assessed is submitted to adiabatic conditions. Hence, we consider a cooling failure scenario. 

The six key questions presented above ensure that the essential knowledge about the thermal safety of a process is addressed. In this sense, they represent a systematic way of analysing the thermal safety of a process and building the cooling failure scenario. Once the scenario is defined, the next step is the actual assessment of the thermal risks, which requires assessment criteria. The criteria used for the assessment of severity and probability are presented below. 

At first glance, the data and concepts used for the assessment of thermal risks may appear complex and difficult to overlook. In practice, however, two rules simplify the procedure and reduce the amount of work to the required minimum ... 

The six key questions described in the cooling failure scenario allow us to identify and assess the thermal risks of a chemical process. The first steps allow building a failure scenario, which is easy to understand and serves as a base for the assessment The proposed procedure (Figure 3.6) is based on the separation of severity and probability, taking into account the economic aspects of data determination in a safety laboratory. In a second step, based on the scenario, the criticality index can be determined to help in the choice and design of risk-reducing measures. 

Assess the thermal risk linked to the performance of this process, and determine the criticality class. 

Assess the thermal risks linked to this hydrogenation reaction. 

The thermal risks of a synthesis step with an exothermal bimolecular reaction (A + B —> P) must be assessed. For this, the required thermal data have to be determined in a safety laboratory equipped with a reaction calorimeter and a DSC. 

Ubrich, O. and Lerena, P. (1996) Methodology for the assessment of the thermal risks applied to a nitration reaction, in Nitratzioni sicure in lahoratorio e in impianto industriale, Stazione sperimentalo combustibili, Milano. 

Assessing the thermal risks of the process means answering the six questions in the cooling failure scenario (see Section 3.3.1). The overall energy potential of the reaction is calculated from the molar reaction enthalpy of 200 kj moT1. The concentration to be used is that of the final reaction mass (2molkg 1), since the reactant B must be added to allow the reaction ... 

Besides the temperature, other possible consequences of decomposition reactions are flammable or toxic gas release, solidification, swelling, foaming, carbon-ation, that may cause the loss of a batch, but also damage leading to the loss of a plant unit and impinging on the production of the desired product. These consequences should also be considered in the assessment. Therefore, the determination of the decomposition energies is a preliminary to any assessment of thermal risks. 

For a first assessment of the thermal risks linked with the performance of a batch or semi-batch process, the thermal stability of a reacting mixture is of primary... 

For semi-batch reactions, it is wise to analyze a sample of the mixture present in the reactor before feed of the reactant is started. In fact, this mixture is often preheated to the process temperature before feeding, hence it is exposed to the process temperature and it could be useful to interrupt the process at this stage, in case of necessity. Such a thermogram assesses the thermal risks linked with such a process interruption (see case history at the beginning of this chapter). 

Part I gives a general introduction and presents the theoretical, methodological and experimental aspects of thermal risk assessment. The first chapter gives a general introduction on the risks linked to the industrial practice of chemical reactions. The second chapter reviews the theoretical background required for a fundamental understanding of mnaway reactions and reviews the thermodynamic and kinetic aspects of chemical reactions. An important part of Chapter 2 is dedicated to the heat balance of reactors. In Chapter 3, a systematic evaluation procedure developed for the evaluation of thermal risks is presented. Since such evaluations are based on data, Chapter 4 is devoted to the most common calorimetric methods used in safety laboratories. 

Adiabatic temperature rise values were obtained in this study as a index of thermal hazard prediction of MEKPO. Feasible reactions at every MEKPO decomposition steps were identified from the possible reaction clusters by obtaining Gibbs free energy of reaction. And for each feasible reaction, enthalpy of reaction, heat capacity values and adiabatic temperature rise were assessed. Thermal inertia and MEKPO mixture composition ratio were considered. Adiabatic temperature rise values for each reaction condition were easily obtained, and by this, it is shown that this approach in this study can be a good methodology to get both qualitative and quantitative risk assessment result for hazardous undesirable reaction. The results were compared with the experimental and simulation data from the reference, and the errors were less than reasonable range. 

The only test which can be used to assess the risk of thermomechani-cally induced failure is arguably the most arbitrary of all those to which plastic encapsulated ICs are subjected. This test involves thermal cycling the components several hundred times between -55 and 150°C with a dwell of 10 min at each temperature. At the end of this stress, outright failures and any thermal intermittent failures are detected by monitoring the operation of the components whilst the temperature is slowly ramped from 0 to 70°C. 

Fig. n.2. Cooling failure scenario, presenting the consequences to the desired reaction of loss of cooling and triggering of a secondary decomposition reaction. The numbers represent the key questions used in the assessment of thermal risks (see text). 

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