Failure cooling

Two types of initiators are internal and external. Internal initiators result from failures within a plant or the plant s support utilities. Thus, vessel rupture, human error, cooling failure, and loss of offsite power are internal events. All others are external events earthquakes, tornados, fires (external or internal), and floods (external or internal). Event trees can be used to analyze either type of initiator. 

In order to develop the safest process the worst runaway scenario must be worked out. This scenario is a sequence of events that can cause the temperature runaway with the worst possible consequences. Typically, the runaway starts with failure that results in an adiabatic course of exothermic reaction, inducing secondary reactions that proceed with a high thermal effect. Such a. sequence of typical events is shown in Fig. 5.4-55 (after Gygax, 1988-1990 Stoessel, 1993). It starts with, for instance, a cooling failure at time tx when the temperature is at the set level, Tv ,- Then the temperature rises up to the Maximum Temperature for Synthetic Reaction (MTSR) within the time Atn. Assuming adiabatic conditions MTSR = + ATad,R... 

Using the value of Fc found in Exercise 1, study the effect of differing valve discharge capacities (KV) following cooling failure at different stages of reaction time (FC = 0). 

Fig. 3.18 Scenario of cooling failure with thermal runaway. ATa(j, is the adiabatic temperature rise by desired reaction. ATaa2 is the adiabatic temperature increase by the decomposition reaction. The time required for this increase is TMRad-...

Fdisk = IF (P> = PBURST) THEN 10 ELSE 0. 0 Cooling failure ... 

P) solvent quantity D too small and cooling failure at the same time at the beginning of the reaction [failures I c) + II b)], TeXo may also be exceeded ... 

After a cooling failure, when boiling point is reached, a fraction of the energy released is used to heat the reaction mass to the boiling point and the remaining fraction of the energy results in evaporation. The amount of evaporated solvent can be calculated from the distance to the boiling point ... 

A chemical reaction is performed at 10 °C in cyclohexane. A secondary decomposition reaction becomes dominant above 30 °C. In case of cooling failure, the reaction mass will reach boiling point and cyclohexane will evaporate. Thermal data and physical properties of cyclohexane are summarized in Table 2.9. 

Assess the severity linked to this operation in case of a cooling failure. 

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... 

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 scenario presented here was developed by R. Gygax [1, 2]. Let us assume that while the reactor is at the reaction temperature (TP), a cooling failure occurs (point 4 in Figure 3.2). The scenario consists of the description of the temperature evolution after the cooling failure. If, at the instant of failure, unconverted material is still present in the reactor, the temperature increases due to the completion of the reaction. This temperature increase depends on the amount of non-reacted material, thus on the process conditions. It reaches a level called the Maximum Temperature of the Synthesis Reaction (MTSR). At this temperature, a secondary decomposition reaction may be initiated. The heat produced by this reaction may... 

Figure 3.2 Cooling Failure Scenario After a cooling failure, the temperature rises from process temperature to the maximum temperature of synthesis reaction. At this temperature, a secondary decomposition reaction may be triggered. The left-hand part of the scheme is devoted to the desired...

If after the cooling failure unconverted reactants are still present in the reaction mixture, they will react in an uncontrolled way and lead to an adiabatic temperature increase. The remaining unconverted reactants are referred to as accumulated reactants. The available energy is proportional to the accumulated fraction. Thus, the answer to this question necessitates the study of the reactant conversion as a function of time, in order to determine the degree of accumulation of unconverted reactants (Xac). The concept of Maximal Temperature of the Synthesis Reaction (MTSR) was developed for this purpose ... 

Question 4 At which moment does the cooling failure have the worst consequences ... 

Since the time of the cooling failure is unknown, it must be assumed that it occurs at the worse instant, that is, at the time where the accumulation is at a maximum and/or the thermal stability of the reaction mixture is critical. The amount of unconverted reactants and the thermal stability of the reaction mass may vary with time. Thus, it is important to know at which instant the accumulation, and therefore the thermal potential, is highest The thermal stability of the reaction mass may also vary with time. This is often the case when a reaction proceeds over intermediate steps. Hence, both the synthesis reaction and secondary reactions must be known in order to answer this question. The combination of a maximum accumulation with the minimum thermal stability defines the worst case. Obviously, the safety measures have to account for it. 

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. 

The probability can be evaluated using the time-scale If, after the cooling failure (Question 4), there is enough time left (Questions 5 and 6) to take emergency measures before the runaway becomes too fast, the probability of the runaway will remain low. 

The cooling failure scenario presented above uses the temperature scale for the assessment of severity and the time-scale for the probability assessment. Starting from the process temperature (TP), in the case of a failure, the temperature first increases to the maximum temperature of the synthesis reaction (MTSR). At this point, a check must be made to see if a further increase due to secondary reactions could occur. For this purpose, the concept of TMRad is very useful. Since TMRad is a function of temperature (see Section 2.5.5) it may also be represented on the temperature scale. For this, we can consider the variation of TMRad with temperature and look for the temperature at which TMRad reaches a certain value (Figure 3.4), for example, 24 hours or 8 hours, which are the levels in the assessment criteria presented in Sections 3.3.2 and 3.3.3. 

The process temperature (TP) the initial temperature in the cooling failure scenario. In case of non-isothermal processes, the initial temperature will be taken at the instant when the cooling failure has the heaviest consequences (worst case). 

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. 

In the present chapter, some important aspects of reactor stability and the corresponding assessment criteria for normal operating conditions will be presented. In a second section, the assessment criteria for deviating conditions, such as cooling failure, are introduced. 

Temperature in Cases of Cooling Failure The Concept of MTSR... 

Since the process temperature, as well as the degree of accumulation, may vary during the reaction, the temperature after cooling failure (Tf) depends strongly on the strategy of control of the reaction. The temperature is a function of time. 

Thus, for the prediction of the behavior of a reactor when there is a cooling failure, the knowledge of the instant at which it is maximum, is an important datum. The assessment of the process safety and the design of safety measures will be based on the MTSR corresponding to the maximum of Tcf ... 

Note When working at low temperatures (below ambient), the MTSR may be taken as the ambient temperature in cooling failure, even if the adiabatic temperature rise would not allow it to reach this point. This is because a reactor left at a subambient temperature will equilibrate with its surroundings. 

Figure 6.6 Cooling failure scenario for the substitution reaction performed in an isothermal batch reactor.

Hence the knowledge of the adiabatic temperature rise is sufficient to calculate the MTSR. The data required for the safety assessment are the maximum heat release rate of the reaction at the desired temperature (q ) and the reaction energy (Qpt). The first datum is needed to calculate the required cooling capacity of the industrial reactor. The second calculates the adiabatic temperature rise necessary to assess the behavior of the reactor in case of cooling failure. The calorimetric techniques used for batch reactors are presented in Section 6.9.1. 

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