Runaway scenario

Unwanted reaction Clean and inspect equipment after each use Design with compatible materials contaminants. Maintain integrity of the system Design emergency relief system (ERS) for runaway scenario CCPS G-13 CCPS G-22 CCPS G-23 CCPS G-29... 

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... 

Figure 5.4-55. A typical runaway scenario (adapted from Gygax, 1988-1990).

Another list of approximate criteria for criticality in runaway scenarios was formulated by Stoessel (1993), see Table 5.4-36. 

The typical runaway scenario as illustrated in Fig. 5.4-55 shows that the following data are needed to determine what would happen if this scenario materialized (reference is made by numbers in circles in the figure) ... 

If a reliable kinetic model and data on cooling capacity are at hand, runaway scenarios can be examined by computer simulations and only final findings have to be tested experimentally. Such an approach has been presented, e.g. by Zaldivar et al. (1992). However, the detailed reaction mechanism and reaction kinetics are rarely known. Therefore, thermokinetic methods with gross (macro-)kinetics dominate among methods for data... 

At this stage, a number of credible maloperations will have been defined that can lead to vessel over-pressurisation. In order to cope with all the credible runaway scenarios, the relief system will need to be sized for the "worst case runaway" reaction that can occur, and this is normally the maloperation that will give rise to the highest rate of temperature and/or pressure rise over the relief range. 

It is also important to correct the raw vapour pressure data for any pad gas which was present in the test cell. This can be done by subtracting the partial pressure of any non-condensible pad gas which was present in the test cell, to obtain the vapour pressure (see A2.7.1). Because pressure transducers may not be very accurate at the bottom end of their range, it is advisable to vent the test cell to atmosphere, once it is filledand before sealing it and heating to the initial runaway temperature, so that a reliable initial pad gas pressure is known. (This may not always be compatible with the desire to simulate the runaway scenario within the test.) An alternative is to evacuate both test cell and containment vessel before the reactants are added so that there is no pad gas and no correction is needed,... 

The following questions represent six key points that help to develop the runaway scenario and provide guidance for the determination of data required for the risk assessment ... 

This chapter describes a runaway scenario. The first section presents a general review of the decomposition reaction characteristics. The second section is devoted to the energy release that defines the consequences of a runaway. The third section deals with triggering conditions of undesired reactions, based on the concept of TMRld. The next section reviews some important aspects for the experimental characterization of decomposition reactions. Finally, the last section gives some examples stemming from industrial practice. 

Figure 4-86 shows the most important runaway scenarios. As a consequence of the assumed cooling failure, an adiabatic temperature rise will be observed diich is identical to the reacting accumulation potential of the desired reaction. The maximum temperature reached corresponds to the MTSR, which has been discussed in the previous section. 

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