Avoid Explosion Risks & Hazards of Chemical Reactions

An explosion is a rapid increase in volume and release of energy in an extreme manner, usually with the generation of high temperatures and the release of gases. Explosions in  a chemical plant need to be avoided at all costs. There are two primary  causes of explosions during a chemical process:

• The material is energetically unstable, and has explosive properties. A comprehensive safety study of the process will identify whether materials are sensitive to shock, friction, heat, etc., and may dictate special procedures for safe handling.
• The process could lead to uncontrolled heat or gas generation, resulting in an escape of flammable vapor, or an over-pressurization of the reactor leading to rupture and loss of contents, which may be flammable.

In order to avoid the risk of uncontrolled heat generation, it is imperative that the process is studied using a Reaction Calorimeter in order to determine the Heat of Reaction (enthalpy) and the rate of heat release, so that a process can be designed that minimizes risk of loss of control.

Accumulation typically refers to material being dosed during a semi-batch reaction which does not react right away.  The concentration of this material builds up/accumulates and poses a hazard if the reaction then begins to run at a faster rate, or if there is a cooling failure. Accumulation can be measured using a Reaction Calorimeter to determine the rate at which heat is produced, and how that compares to the rate of addition. It is also possible to use in situ analysis to directly measure the concentration of the unreacted reagent.

When a process is being studied for safety, it is essential to confirm whether gas is being produced, and under what circumstances. In some cases, such as decarboxylation releasing CO₂, this is an obvious and expected part of the process.  In other cases,  such as at the onset of runaway during nitration, gas production may be due to an unexpected side-reaction or decomposition. Unexpected gas evolution can quickly lead to over-pressurization of the equipment, with risk of loss of containment or even rupture the plant, with catastrophic consequences.

Measurement of gas evolution is a crucial part of process safety investigations at the laboratory scale, and the Reaction Calorimeter can easily be equipped with a meter or sensor system to quantitatively detect the time and rate of gas evolution so that suitable provision may be made at scale.

When a liquid evaporates, it requires a significant amount of energy to change from the liquid phase to the vapor phase. This is known as the latent heat of evaporation. This phenomenon is often used during large-scale chemical manufacturing in order to help remove heat from the system, and thereby reduce the costs and increase the safety of the process. It is thus common practice to operate at least part of a batch process under conditions of reflux, in order to maximize the rate of reaction while maintaining a safe and efficient process. Operation of a Reaction Calorimeter under reflux conditions requires special attention to capture the heat that is taken away in the reflux condenser, and to ensure that the complete energy balance of the process is maintained even under reflux conditions.

This paper assesses the following questions about thermal accumulation through practical case studies from both the lab and plant:

• Why and when does thermal accumulation occur?
• Is thermal accumulation important to consider, and how big is it?
• What is the impact of an incorrect calculation of thermal accumulation?

Autocatalytic reactions have a tendency to begin very slowly, and then suddenly increase in rate due to the formation of catalytic compounds or intermediates. This can be extremely dangerous at large scale, since it may mean the rapid production of large amounts of heat that the cooling system is unable to remove. It is critically important to know in advance if a system exhibits autocatalytic behavior, so that the process can be properly designed. The Reaction Calorimeter allows direct measurement of the rate of heat production, and in situ analysis can give further information about the rate at which the reaction takes place.

Initiation is the point at which the chemical reaction begins, and it is not necessarily when the reagents are mixed. In some cases, initiation only occurs after a certain time period has elapsed (during autocatalysis), when an inhibiting material has been consumed, or when a certain temperature has been reached (during a non-isothermal reaction or a decomposition). The existence of an initiation period does not necessarily mean that the reaction is dangerous, but it is vitally important that the parameters of the event (time, duration, etc.) are well-understood in order to avoid hazardous conditions. For example, Grignard reactions typically undergo a delayed initiation, and it is important not to add too much halide to the reactor before initiation has been confirmed. Otherwise, there is a high risk of accumulation and runaway.

Learn how reaction calorimetry helps scientists to quickly screen conditions and safely scale-up - without the need for large testing volumes. Examples presented include:

• Detecting Heat Release While Changing Temperature
• Understanding the Effect of Heat Accumulation
• Screening For Scalability Issues at Small Scale
• Eliminating Risk of Runaway Reactions 

Deflagration occurs when a solid material, such as a reactor vessel or storage container, undergoes a self-sustaining decomposition reaction that liberates significant quantities of energy (heat).  Deflagration is essentially burning, but can occur in the absence of oxygen from the atmosphere. Deflagration differs from detonation, primarily in respect of the speed of the reaction, with a detonation occurring at supersonic velocity, and deflagration at sub-sonic velocity. During a chemical process, deflagration needs to be avoided at all costs. Chemical process safety studies should always include an assessment of the susceptibility of the material to deflagration (and explosion), as well as an understanding of what temperatures could be reached during the process, especially in cases of process upset or cooling failure.

The heating and cooling system of the RC1 Reaction Calorimeter is based on circulating heat transfer fluid with a high velocity and a large amount of pre-cooled oil. While the high velocity ensures fast response to temperature changes, the reservoir with the pre-cooled oil guarantees instantaneous cooling in case of a large exotherm or emergency. Along with the algorithms that ensure the highest precision of calculation, the thermostat is designed to quickly respond to changing conditions in the reactor in order to maintain the desired temperature accurately.

The temperature control system of the EasyMax and OptiMax Reaction Calorimeter uses the concept of electrical heating and Peltier-based solid-state cooling technology. Both heating and cooling are extremely fast due to the compact thermostat design, and enable the system to achieve temperatures well below 0 °C without the use of a cryostat. As a result of the design, the space requirements are minimal.