Identifying and understanding decomposition and secondary thermal runaway (the undesired reactions)
The use of calorimetry can help us characterize our target chemical reactions: power required to maintain isothermal conditions, runway reactions when cooling is not supplied, and what maximum temperature the reactor can reach.
However, additional reactions may occur in many cases when some of the reagents or products can undergo subsequent chemical reactions. The velocity of these side reactions can exponentially increase when the temperature rises due to the progression of the desired reactions. Among these side reactions, decomposition reactions are of special relevance. Decomposition reactions are those chemical reactions in which one reactant breaks down into two or more products.
A secondary thermal runaway is likely to occur if MTSR is greater than the onset temperature of a component in the reaction mix (Td). The onset temperature can be defined as the temperature at which the decomposition reaction starts to be detected. This scenario is illustrated in Figure 6.
As previously described, MTSR is estimated from reaction calorimetry, while the onset temperatures of materials are determined using thermal screening tools, such as the Thermal Screening Unit (TSu).
A more accurate value of the onset temperature can be determined using adiabatic calorimetry. Still, it is often considered sufficient to use a more straightforward thermal screening tool at this stage. The TSu enables representative samples of the reaction mix to be screened and reliable onset temperatures determined for the reaction intermediates and products. Thermal stability studies also need to take place on the reaction waste stream, which the TSu also facilitates with isothermal stability screening. Just as it is important to identify hazards during the reaction itself, it is also important to understand if there are any longer-term stability issues in the waste products formed so that these issues can also be mitigated.
Suppose it becomes apparent that there is a risk of a secondary thermal runaway occurring. In that case, it is necessary to determine the adiabatic temperature rise of the decomposition/secondary reaction. This allows an assessment to be made as to whether there will be a sufficient emergency cooling capacity to deal with the increase in temperature.
As this reaction involves a secondary thermal runaway, scaling up is generally a hazardous process. Therefore, although these parameters can be estimated from micro-calorimetry, it is advisable to use adiabatic calorimetry to get an accurate assessment.
The data collected enables the criticality of the reaction to be classified, as described by Stoessel , as a possible assessment of hazards and depicted in Figure 7.
The schematic references the key reaction parameters introduced in Figure 5:
- the isothermal temperature of the reaction (Tp)
- the decomposition temperature of a component in the reaction mix (Td)
- the Maximum Temperature of the Synthesis Reaction (MTSR)
It also introduces an additional parameter, the boiling point (BP) of the solvent or active chemicals used, as this is another factor that needs to be taken into account when considering the overall safety of a reaction. The relative order of these parameters in terms of temperature determines the criticality of the reaction.
- Scenario (A): the thermal order of the parameters is Tp<MTSR<BP<Td. This represents the least hazardous reaction scenario, and no special measures are required as the MTSR does not exceed that of the BP of the solvent nor the Td.
- Scenario (B): as with Scenario (A), the MTSR does not exceed Td, although in this case, the BP of the solvent is higher than Td (Tp<MTSR< Td< BP). This means that, unlike Scenario (A), the BP will not act as a limit on the maximum temperature the reaction will attain, and heat accumulation conditions should be avoided.
- Scenario (C): the MTSR is higher than the BP but less than Td (Tp< BP< MTSR<Td). This means that risk reduction measures are required but that the BP of the solvent could be used as a non-ideal safety mechanism for the reaction.
- Scenario (D): the MTSR is higher than Td, which means there is a risk that the decomposition could be triggered should the system fail. However, the BP of the solvent could be used as a limiting mechanism to prevent triggering the decomposition reaction, as BP is lower than Td (Tp<BP<Td<MTSR). In this case, risk reduction measures are required.
- Scenario (E): as with Scenario (D), the MTSR is greater than Td, which means there is a risk that the decomposition could be triggered. However, the BP of the solvent is also above that of Td, so it can’t be used as a safety barrier (Tp<Td<MTSR<BP). In this case, emergency control measures are also required, which may include redesigning the process.
Although the Stoessel Diagram can help assess how hazardous a reaction is, caution needs to be exercised . For example, the solvent boiling point temperature assumes atmospheric pressure. Therefore, if the operating pressure becomes higher than this after a cooling failure, then the solvent boiling temperature also increases, effectively removing the safety mechanism in Scenario C and Scenario D. Similarly, if the solvent is rapidly vaporized by a fast energy release from decomposition, the reactor could become over-pressurized before the plant pressure relief mechanisms kick in. Therefore, while the Stoessel Diagram is useful to facilitate the development of intrinsically safer processes, it is important to recognize that a wider range of potential hazards must also be considered, pressure chief among them.
Suppose the MTSR is greater than a component’s onset temperature (Td) within the reaction mix. In that case, an undesired side reaction or decomposition may be triggered, leading to a secondary thermal runaway.
To better understand the risks of thermal runaway in a reaction, the TSu (Thermal Screening Unit) can be used. The TSu supports large-volume measurements, enabling representative samples of the reaction mix to be screened and reliable onset temperatures to be determined for the reactants, intermediates, and products in the reaction mix. It also enables the vital assessment of pressure events in the reaction mix and can find use in studying the waste streams from reactions.