Criticality classes

Assessment of Severity and Probability for the Different Criticality Classes 

Obviously not all the parameters described above need to be evaluated for each scenario. In this context, the criticality classes are a useful tool in that they help in selecting the required data for the assessment of severity and probability (see Section 3.3.6). The criticality classes also give backbone to the systematic design procedure (Table 10.3). The procedure to follow for this assessment is presented below for each criticality class. 

For semi-batch reactions, it is important to check if the rating as class 2 is due to the control of the accumulation by the feed rate. Often reactions belong to class 

5 if the feed is not immediately halted when a failure occurs. It is important to recognize these shifting class reactions, since they require a reliable interlock to stop the feed, in case of deviation from the desired temperature towards higher and lower temperature, in order to avoid any undesired accumulation of reactant. 

This situation is similar to class 3, except that the MTSR is above Tm4, meaning that if the temperature cannot be stabilized at MTT, a secondary reactions could be triggered. Thus, the potential of the secondary reactions cannot be neglected and must be included in evaluation of the severity. The calculation of the potential produced gas volume must also take into account the secondary reaction. The final temperature is given by 

---

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... 

Figu re 3.5 Criticality Classes of Scenario, obtained by combining the four temperature levels TP, MTSR, TD24 and MTT. 

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

Hence, the succession of characteristic temperatures is Tp < MTT < Td24 < MTSR, which corresponds to a criticality class 4, which requires technical measures. The high latent heat of evaporation of water and ammonia would allow stopping the runaway at 240 °C by depressurizing the reactor in order to use evaporation cooling. This possibility will be analysed in Chapter 10. 

Describe the required experiments, allowing for the determination of the criticality class. 

Hence the intended process belongs to the criticality class 5 ... 

The choice and design of technical protection measures against runaway is in accordance with the risk level. This means that the consequences and controllability of the commencing runaway must be assessed. The criticality classes, based on four characteristic temperatures, are at the root of this assessment and serve in the design of protection measures. 

In criticality classes 1 to 3, the energy to be considered is the reaction energy (QL) only, whereas in classes 4 and 5, the energy to be considered is the total energy, that is, the sum of the reaction and decomposition energies ( il rx + Qj,). The temperature increase may represent a threat in itself, but in most cases, it will result in a potential pressure increase. 

The volume of gas potentially released by a reaction (including secondary reactions in criticality classes 4 and 5) can be known from the chemistry or measured experimentally by appropriate calorimetric methods, as for example, Calvet calorimetry, mini-autoclave, Radex, or Reaction Calorimetry (as V at T k. and /Jmes). It must be corrected for the temperature to be considered, MTSR (class 2), MTT (class 3 or 4), or Tf (class 5). Where the gas stems from the main reaction, only the accumulated fraction (X) will be released ... 

Table 10.3 Required data set for the different criticality classes.

The situation is similar to class 1, except that the MTT is above Tm4. This means that under heat accumulation conditions, the activity of secondary reactions cannot be neglected, leading to a slow but significant pressure increase, or gas or vapor release. Nevertheless, the situation may become critical only if the reaction mass is left for a longer time at the level MTT. The assessment can be made using the same procedure as for criticality class 1, represented in Figure 10.8. The gas or vapor flow rate is an important parameter for the design of the required protection measures such as condenser, scrubber, or other treatment units. 

The criticality class is 4 (Tp < MTT < TD24 < MTSR). Thus, the secondary reaction could theoretically be triggered and the total energy release is... 

Since the system is in criticality class 4, the main contribution to the activity at MTT stems from the synthesis reaction. Nevertheless, the contribution of the decomposition reaction should also be checked, since MTT and TD24 are close together. 

Class 100 conditions require that air in the immediate proximity of exposed product be of an acceptable quality with a particle count of no more than 100 0.5-pm particles per cubic foot of air. This is obtauned by utilizing HEPA filters and laminar flow conditions with the room HVAC. Room pressurization is also critical. Class 100 rooms are required to maintain a positive pressure to surrounding rooms of at least... 

---