Cooling failure scenario

A common practice in the assessment of risks due to runaway reactions is to use the cooling failure scenario as developed by Gygax [1, 6]. This is a worst-case as- 

What temperature can be attained after runaway of the desired reaction The 

What temperature can be attained after runaway by decomposition The thermal data of the secondary decomposition reactions allow calculation of the adiabatic temperature rise and determination of the final temperature, starting from the level of the MTSR. This temperature gives a direct indication of the possible consequences of a runaway. In polymerization reactions, the temperature reached after loss of control of the reaction itself will often determine the consequences. Polymerization reaction masses are often thermally stable, so this question is often not relevant. Exceptions are polycondensation reactions, especially hot melts or reactive resins. 

At which moment does the cooling failure have the worst consequences Since the amount of unconverted reactants and the thermal stability of the reaction mass may vary with time, it is important to know at which instant the accumulation, and therefore the thermal potential, is highest. This will be the worst case, and obviously the safety measures have to account for it. 

How fast is the runaway of the deshed reaction Generally, industrial reactors are operated at temperatures where the deshed reaction is fast. Hence, a temperature increase above the normal process temperature will cause a significant acceleration of the reaction therefore, in most cases, this period of time is short. For polymerization reactions, where decomposition of the reaction mass is not critical, this time will determine the choice of technical risk reduction measures. The concept of time to maximum rate under adiabatic conditions (TMRad) as used for decomposition reactions can be applied to the polymerization itself, starting from the process temperature. It allows estimation of the probability of entering a runaway situation, as explained below for decomposition reactions. 

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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. 

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...

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 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. 

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

Build a cooling failure scenario As worst case the temperature would increase to boiling point and the solvent evaporate. What would the required energy be to evaporate the water from the reaction mass (Latent heat of evaporation of water AH = 2200 kjkg-1). 

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 ... 

Thus, the temperature can be controlled using cold water as a coolant, but the reaction requires practically the full available cooling capacity of the reactor (Question 1 in the cooling failure scenario Section 3.3.1). 

The MTSR (Question 2 in the cooling failure scenario) can be directly determined using Equation 7.22, by reading the data from the thermogram (Figure 7.7). The accumulation of 25% is reached at the stoichiometric point, that is, after 3.2 hours of feed (Question 4 in the cooling failure scenario) ... 

At 127 °C, the decomposition reaction is critical, that is, the time to maximum rate (Question 6 in the cooling failure scenario) is shorter than 8 hours (see Table 5.4). 

Fig. 2-4. Schematic presentation of a cooling failure scenario according...

This very plausible and easy to follow approach to an evaluation of a cooling failure scenario for the batch reactor can in numerous ways be transferred to other reactor types as well as the assessment of the consequences of other maloperation scenarios. 

All further assumptions for the design are based on a cooling failure scenario without specifying any of the causes leading to it. Thus it is assumed that at least two independent failures occur simultaneously ... 

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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