Assessment of Thermal Risks

2-Chloro-5-nitrobenzene sulfonic acid is synthesized by addition of p-chloro-nitrobenzene as a melt to 20% Oleum (20% S03 in H2S04) at 100 °C [1], This is added over 20 minutes to Oleum heated at 50 °C. The temperature then rises to 120-125 °C due to the heat of reaction. The conversion is achieved by maintaining this temperature during several hours with 2 bar of steam. 

Before the incident, neither the reaction and decomposition energy potentials nor the triggering conditions of the decomposition were known. Thus, a potentially severe process was entirely under manual control, without provision for an alarm system and emergency measures. A correct assessment of the energies and triggering conditions of the decomposition predicts such an incident, giving the opportunity to design a process that will avoid such incidents. 

In this chapter, after introducing some definitions, a systematic assessment procedure, based on the cooling failure scenario, is outlined. This scenario formulates six key questions that comprise the database for the assessment. Relying on the characteristic temperature levels arising from the scenario, criticality classes are defined. They provide a selection of the required risk-reducing 

Thermal Safety of Chemical Processes Risk Assessment and Process Design. Francis Stoessel Copyright 2008 WILEY-VCH Verlag GmbH Co. KGaA, Weinheim ISBN 978-3-527-31712-7 

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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 ... 

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. 

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). 

Firstly it is an essential tool for the assessment of thermal risks related to the performance of the reacting system at industrial scale (i.e. the capability of a system to enter into a runaway reaction). This type of safety data is particularly important for reactions like hydrogen generation by hydrolysis of borohydrides, where the rapid increase in temperature may result in a sharp pressure increase. Secondly the thermodynamic data are of primary interest for the determination of the reaction/diffusion and reaction/absorption mechanisms. 

Gygax, R., "Explicit and Implicit Use of Scale-Up Principles for the Assessment of Thermal Runaway Risks in Chemical Production," in Proceedings of the International Symposium on Runaway Reactions, Center for Chemical Process Safety/AIChE, New York, NY (1989). 

For a study of methods of assessment of thermal runaway risk from laboratory to industrial scales [2], A more detailed but eminently clear treatment of this and other needful safety considerations on scaling reactions up to production has since been published [3], So slight a scale-up as replacing two charcoal filters by one bigger one may cause a fire because heat loss was reduced [4], A journal largely devoted to scale-up of organic chemical processes has been launched [5]. 

Most of the chemical reactions performed in the fine chemicals industry are exothermal, meaning that thermal energy is released during the reaction. It is obvious that the amount of energy released is directly linked to the potential damage in the case of an incident. For this reason, the heat of reaction is one of the key data, which allow assessment of the risks linked to a chemical reaction at the industrial scale. 

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 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. 

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 ... 

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). 

Assess the thermal risks linked to the industrial performance of the process. 

As in every safety study, the solution of heat accumulation problems can be undertaken by applying the two commonly used principles of simplification and worst-case approach, as described in Section 3.4.1. A typical procedure following these principles is illustrated with the example of a decision tree for the assessment of the risks linked with thermal confinement (Figure 13.7). 

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. 

Gygax, R., 1989, Explicit and implicit use of scale-up principles for the assessment of thermal runaway risks in the chemical production. Int Symp on Runaway Reactions, 52-73 (CCPS, AIChE. USA). 

Atmospheric releases of flammable gases such as hydrogen may lead to major fires with extensive effects on the surroundings. In activities where hazards are associated with cloud fires, there is the need of societal risk assessment that involves the estimation of hazardous zones due to the resulting thermal radiation. However, till now only limited work has been done on modeling the effects of flash fires, in a way that available techniques may be judged insufficient [47],... 

A designer, as part of his facility design analysis, should perform a hazards analysis or risk assessment of the various processes which will be conducted within the facility in order to determine what potential thermal dangers or threats exist to personnel and equipment. A hazards analysis or risk assessment will provide for the identification of potential hazards and of the necessary corrective actions/measures to prevent or control the hazard. Early in the design of a facility, the processes and equipment may be conceptual and at this stage, a preliminary hazards analysis can be performed. It is early in the design that a preliminary hazards analysis can be most helpful because its... 

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