Key points
As the market needs for chemical products increase, manufacturers are required to produce goods on larger and larger scales. Improved capabilities, including production at a larger scale, result in lower costs per batch, thus increasing the profitability of the process. Oftentimes, in order to achieve profitability, safety hazard evaluation is overlooked.
The majority of accidents in chemical processes occur during reagent dosing, maintenance, or the operation of the process itself. Exothermic reactions pose particular risks, especially when the temperature increases abruptly, resulting in heat generation rates higher than the removal capacity of the plant.
If this situation is not corrected, the chemical reaction can enter in a positive feedback loop, when the reaction heat further increases the rate at which processes occur. This phenomenon is called thermal runaway, and the result can be catastrophic, potentially causing significant damage to the production facility in terms of manpower, investment, product loss, and public image. Events that lead to thermal runaway include reagent accumulation or cooling failure.
Exotherm characterisation
In many cases, accidents can be avoided when safety risk assessments have been performed and, as a result, efficient mitigation strategies have been put in place. Calorimetry is incredibly powerful in understanding the safety of the process, and its evaluation should start even before the process.
Reagents can be prone to chemical decomposition, a process in which molecules spontaneously break down into smaller components. If this process is exothermic, thermal runaway events can occur ( www.aiche.onlinelibrary.wiley.com).
Solutions such as H.E.L’s TSu (Thermal Screening Unit) can be used to predict the behavior of chemicals, both reagents and products, during storage. The temperature at which the decomposition reaction accelerates, entering into a positive feedback loop, is called exotherm. The characterisation of the conditions at which this exotherm is triggered allows for the optimisation of storage.
Understanding potential side effects during the chemical reaction is fundamental. Exotherms can be dangerous when not properly managed, so the characterisation of such processes remains central in the design of safer processes.
However, this type of analysis is normally performed at bench scale. One of the main differences between small and large-volume reactions is the amount of energy absorbed by the vessel and dispersed to the surroundings. This is where phi-factor and adiabatic calorimetry shine through.
Phi-factor can be roughly described as the percentage of energy that needs to be transferred to the vessel in order to heat up the reaction. In large vessels, this value is very low due to the sheer mass of reagents inside in comparison to the mass of the vessel itself, whereas, at small scale, this situation is completely different.
The consequence of a high undetermined phi-factor is that the thermodynamic values determined during the analysis can be inaccurate, rendering them unreliable for large-volume simulations. On the other hand, at high volumes, the amount of energy dispersed to the surroundings is negligible compared to the energy produced, behaving de facto adiabatically. At small scale, the heat dissipation is a very efficient process.
To have in-depth knowledge of the process at a large scale, it is important to have phi-factor specific adiabatic calorimeters, such as Phi-TEC I and Phi-TEC II. A thoroughly characterised chemical reaction from a calorimetry perspective can help to predict fundamental values such as Time to Maximum Rate or the Adiabatic Temperature Rise. These parameters permit the design of safety windows in case the reaction gets into thermal runaway, ensuring minimal damage to people and goods.
Process optimisation
The last step in order to improve the safety of chemical processes is the optimisation of the reaction. The objective of this step is to increase the product yield, keeping resources (such as energy) utilisation as low possible but also ensuring that there is little risk and that mitigation strategies are present. Adiabatic calorimetry reigns supreme in this phase of development. The energy released during a reaction can be potentially dangerous, resulting in temperature and pressure increases.
Temperature can increase the yield of the reaction but also can lead to thermal runaways and the consequent risk of fires, scolds, and burns. The system can become over pressurised as a consequence of decomposition reactions, when the products are gases, or if the substances involved in the reaction are susceptible to evaporation.
Mitigation strategies, such as changing the feed regime from batch to semi-batch can help to soften the temperature increase, making it more manageable. However, sometimes, it is impossible to avoid dangerous scenarios, and calorimetry can provide a great deal of information that can mitigate the worst part.
Parameters such as the Maximum Temperature of Synthesis Reaction (MTSR) and Time to Maximum Rate (TMR) are useful to determine the inherent risk of the process or the time allowance to evacuate facilities, respectively. Adiabatic reaction calorimeters, such as H.E.L’s Simular are designed to accurately characterise the thermal behavior of processes, allowing for safer designs and mitigation strategies.
Early evaluations for safer chemical production
The trend to increase chemical production on larger scales brings both opportunities and challenges to the industry. While increased scale can lead to high production yields and profitability, it also exacerbates the risks associated with safety hazards. Neglecting safety evaluations can result in catastrophic events, damaging production facilities, reputation, and causing substantial losses.
It is important to emphasise that accidents can be averted, and there are tools, such as calorimetry, that can help to manage the safety of chemical processes. Starting with early assessments, like the prediction of decomposition reactions, plays a crucial role in optimising storage conditions and preventing otherwise avoidable accidents. When dealing with larger scales, tools such as adiabatic calorimetry become indispensable for accurate analysis and the design of safer processes.
Nevertheless, these predictions will only be reliable when they are equipped with precise phi factors. Thorough calorimetric characterisations enable users to calculate critical safety parameters, such as Time to Maximum Rate and Adiabatic temperature, minimising the harm in the event of thermal runaways. The integration of advanced calorimetry techniques promises to be the cornerstone of safer and more prosperous chemical manufacturing.
For further information contact victoriaordsmith@helgroup.com.
