Prevent Runaway Chemical Reactions

Analyzing reaction dynamics allows scientists to model runaway reaction scenarios and establish the ideal reaction procedure. Measuring, calculating, and understanding the parameters below are essential to assess and avoid risk in a chemical process. This allows scientists to make predictions about temperature and dosing profiles, concentrations, or maximum operating temperature. From the view point of thermochemistry, the key information is:

1. Accumulation
2. Adiabatic Temperature Rise
3. Heat Capacity
4. Reaction Enthalpy
5. Heat Production Rate/Heat of Reaction
6. MTSR (Maximum Temperature of the Synthesis Reaction)
7. Desired Reaction, Undesired Reaction, Decomposition, Thermal Runaway

Accumulation usually refers to either material being dosed during a semi-batch reaction, which does not react right away, or energy that is accumulated because the heat production is larger than the heat removal capacity.

If the concentration of unreacted raw material builds up or accumulates, it poses a hazard if cooling fails. This can be measured using a reaction calorimeter by determining the rate at which heat is generated, compared to the rate of the addition. It is also possible to use in situ FTIR spectroscopy to directly measure the concentration of the unreacted reagent.

Thermal accumulation describes the variation of the energy contents of a system and is caused by the heat production being larger than the heat removal capacity resulting in a change of temperature. Measuring the heat capacity and the temperature change by a reaction calorimeter allows the calculation of the thermal accumulation and derivation of the true heat of reaction.

The adiabatic temperature rise is a property that is calculated for any given reaction to indicate the potential severity of a cooling failure or other process upset. It depends on the total amount of energy that is released (reaction enthalpy), the total mass of the reaction mixture, and the specific heat capacity of the system. It is of particular importance to know if it could bring the reaction mixture to a temperature where a secondary reaction or decomposition reaction could occur. The adiabatic temperature rise is easily calculated using a reaction calorimeter to give a simple ‘worst-case’ indication of the temperature increase. More sophisticated calculations, such as those performed by iC Safety, can derive related parameters such as the MTSR (Maximum Temperature of the Synthesis Reaction) which is a more useful parameter for scale-up studies.

Heat capacity or specific heat capacity of a material describes the amount of energy required to increase the temperature of one kilogram of the material by one degree Celsius. The standard unit of the heat capacity is thus kJ/kg.K-1. This property plays a direct role in the calculation of the accumulated heat, the adiabatic temperature rise, and MTSR. Since it varies according to the nature of the reactor contents, it is necessary to experimentally determine the heat capacity of the mixture as part of the process safety study. This confers the additional benefit that such data is usually welcomed by engineers working on scale-up since it helps to calculate the overall energy balance of the process.

The reaction enthalpy of a chemical reaction is the enthalpy change that occurs when substances are transformed by a chemical reaction. It tells the scientist how much energy is evolved from a process in total and is measured using a reaction calorimeter. The reaction enthalpy of the reaction is calculated by integrating the heat of reaction trend over time.

The heat production rate or heat of reaction is related to the overall enthalpy of the reaction but considers how the energy is released in function of time. This is of fundamental importance, because the total enthalpy of the reaction may not be critical, but the rate at which that energy is released can be the difference between a safe process and a dangerous one.

The heat of reaction provides vital information regarding the start and end of the reaction, possible accumulation of reactants, the maximum heat evolution, the reaction kinetics, and the necessary reactor cooling capacity and kinetics. The rate at which heat is produced is influenced by several factors, including temperature, dosing rate, concentration, kinetics, and mixing.

The determination of the heat of reaction requires the knowledge of the overall heat flow balance, including the amount of energy that flows through the reactor wall (qflow), the amount that may be added/consumed during dosing (qdos), and the amount that is accumulated due to temperature increase (qaccu). Its integral value represents the enthalpy of the reaction.

The maximum temperature of the synthesis reaction (MTSR) is one of the cornerstones of chemical process safety for a semi-batch reactor. It describes the temperature evolution after a cooling failure due to the amount of unreacted material that was present at the time of cooling failure. Starting with the process temperature, the calculation of MTSR is performed dynamically to determine the highest temperature that the reaction mixture will reach assuming that all of the reaction energy remains in the reactor (i.e. the system is or becomes adiabatic). The reaction calorimeter is an essential tool for the measurement of the heat of reaction, the reagent accumulation, and MTSR. It is important to note that this property concerns the desired (synthesis) reaction only, and additional procedures (Differential Scanning Calorimetry, DSC) need to be executed to understand the effects of any subsequent secondary or decomposition reactions.

A thermal runaway or thermal explosion may be the consequence of an adiabatic process of a reaction or series of reactions. It occurs when heat produced by an exothermic reaction is accumulated leading to an increase in temperature of the reaction mixture. Subsequently, the reaction rate increases leading to an increased rate of heat generation.

The temperature increase may trigger secondary reactions, such as decomposition of the reaction mass, the intermediates, or the final product leading to a thermal runaway. By conducting proper process safety studies at laboratory scale, the conditions leading to a thermal runaway can be identified and avoided before the process is scaled-up.

Studying both, the desired as well as potentially undesired reactions are of utmost importance. While the undesired reactions are investigated using adiabatic calorimeters, Dewar flasks, or DSC, the desired reaction is typically studied with a reaction calorimeter.

The Reaction Calorimeter can be used to determine key factors such as the Heat of Reaction, the accumulation, the Adiabatic Temperature Rise and MTSR in order to understand the consequence of a cooling failure. In fact, it is used to determine whether the desired synthesis reaction could cause the process to become unstable leading to a secondary reaction.

In most cases, the secondary reaction liberates more energy and faster than the desired reaction, with the production facility not being capable of safely removing this energy. Therefore, it is necessary to design the reaction so that potentially dangerous conditions cannot occur, or to define appropriate measures to ensure that the process remains under control safely at all times.

Reaction calorimeters determine a range of safety-related parameters with high accuracy. The use of designed-for-purpose systems is critical for the delivery of precise, high-quality calorimetric data.

METTLER TOLEDO thermostats are equipped with unique instant cooling principles to provide fast response and accurate control to guarantee the best and safest working conditions.

The use of purpose-designed hardware, together with advanced, user-friendly software ensures that parameters, such as specific heat, heat transfer, the true heat of reaction, and the accumulation of product and heat are calculated accurately throughout the entire chemical reaction. This provides chemical engineers with essential data for process safety and scale-up.