Chemical reactions are at the core of many processes in the field of chemical manufacturing. These reactions, while designed to yield specific products, often come with inherent hazards in the form of undesired by-products or heat energy.
These by-products can manifest as both exothermic and endothermic energy, as well as undesired materials. This will present challenges in maintaining a safe and efficient manufacturing environment.
Among the various hazards associated with chemical reactions, exothermic energy and the generation of permanent gases pose the highest risk potential to any manufacturing process.
The need for a thorough understanding of the process at scale is necessary for both the safety of the personnel involved, as well as the prevention of material and financial loss.
Pressure Emergence: Industrial Incident Case Studies
Runaway reactions have been implicated in several industrial incidents worldwide. To address this, testing on a small-scale is a key element of process development. The knowledge gained from small-scale testing can then be applied to ensure the reliability and safety of reactions conducted on a larger scale.
Pressure emerges as a hazard in chemical reactions, particularly for vessels. Vessel over-pressurization can then lead to either controlled venting or, in extreme cases, an explosion.
Like exothermic energy or temperature evolution, pressure fluctuations require consideration due to their impact on the overall safety of your process.
Overpressure in a chemical reaction can result from three potential sources:
- Gas Generation: During the normal course of the reaction, the production of gases, either as a product or a by-product, can contribute to pressure build-up.
- Vapour Pressure Effects: Heat generated or input during the reaction can cause changes in vapour pressure, affecting the overall pressure within the system.
- Heat from the Normal Process: The heat produced during the reaction may trigger secondary reactions, such as decomposition or thermal runaway, leading to the generation of additional gases or an increase in vapour pressure.
A comprehensive approach to chemical reaction hazards will therefore involve an understanding and management of the desired reactions and the potential risks associated with energy release, by-product formation, and pressure fluctuations.
The Seveso Disaster
Vapour Pressure Dynamics
Imagine a scenario involving a mass of water. As the temperature increases, more vapour is generated above it. This is a phenomenon tied to the natural logarithmic relationship of vapour pressure. Typically, the relationship between temperature and vapour pressure follows a linear scale, providing a predictable framework for understanding the behaviour of substances under varying thermal conditions.
Linear Trends and the Onset of Problems
Using the below graphical representation, you can see a clear linear correlation between temperature and vapour pressure. The graph, denoted by the green line moving from right to left, illustrates the expected linear increase in temperature. However, the plot takes a significant turn when vapour and gas pressures are combined.
The onset line, (the juncture in the graph), signifies the commencement of permanent gas generation, possibly from an exothermic reaction. At this point, the trajectory deviates from linearity, spiking upwards. The deviation from the expected linear slope is where challenges emerge, introducing unpredictability and potential risks into the chemical process.
Learning from Seveso
In the Seveso disaster, a thermal runaway event occurred during a standard weekend shutdown. The reactor vessel, employing steam for heating reactions, experienced a lack of stirring due to the shutdown. Localised heating ensued, triggering a thermal runaway reaction whereby more energy is released. This ultimately led to the release of six tons of chemicals into the surrounding area. Despite the activation of the emergency venting system, the escalating pressure, coupled with the absence of stirring, thwarted containment efforts.
While no direct human fatalities were reported, the incident resulted in numerous animal deaths and necessitated a massive clean-up operation.
By understanding the dynamics of vapour pressure and recognising the potential deviations from linearity, process safety professionals can implement targeted measures to mitigate risks and create a safer chemical manufacturing processes.
Understanding Chemical Reaction Hazard Analysis
Early Stage Identification: New Products and Potential Routes
At the outset of a manufacturing lifecycle, the identification of a new product will mark the commencement of the journey. The decision-making process involves exploring potential routes for synthesis, and here, the importance of Chemical Reaction Hazards analysis is important.
Engaging in early desktop studies is important, but equally valuable is the initiation of thermal screening. Thermal screening involves the scrutinisation of the thermal properties of raw materials and/or the final product. These insights will inform later decisions in the nascent stages of production.
Optimising Synthetic Routes
As the synthetic route is honed and optimized, the next logical step involves delving into reaction calorimetry. This entails a meticulous examination of the energy released alongside rate of reactions. Information gleaned from reaction calorimetry aids in refining the manufacturing process, guiding decisions on factors such as agitation methods and optimal heat points for specific reactions.
Scale-Up Challenges and Major Hazard Assessments
The transition from laboratory-scale experimentation to plant-scale production introduces a paradigm shift in the assessment of chemical hazards. It is important to recognise the limitations of lab assessments, particularly in the face of scale differences. This is where adiabatic calorimetry plays a pivotal role. It forms the foundation of safety considerations as processes move towards larger scales where greater energy can be absorbed.
The insights garnered through adiabatic calorimetry contribute towards building your foundation of safety measures, ensuring that the hazards identified at the laboratory level are effectively addressed and mitigated during large-scale production using exothermic and endothermic reactions.
Energetic Functional Groups
An aspect that demands early attention in your chemical process evaluation journey is the assessment of energetic functional groups. These groups, embedded within certain classes of chemicals, have a propensity to exhibit heightened reactivity and enthalpy change, particularly under elevated temperatures.
Initial Desktop Work
When embarking on a new chemical process, whether it involves reagents or the desired products, thorough desktop work will always be imperative. A facet of this preliminary exploration is the identification of certain classes of chemicals known to harbour energetic functional groups. These groups are characterised by their potential for increased reactivity, especially when exposed to higher temperatures.
Nitrogen Groups
A prominent category that frequently raises concerns in terms of stability is nitrogen-containing groups. Nitrogen, while an ubiquitous element, can contribute to the instability of chemicals, particularly when arranged in specific configurations.
Mitigating Risks with Informed Decision-Making
Armed with the knowledge of energetic functional groups, process safety chemists and analysts can make informed decisions during the early stages of process development. Recognising the instability associated with certain groups empowers professionals to choose alternative routes or implement precautionary measures to mitigate potential risks. This proactive approach not only contributes to the safety of the working environment but also aids in devising more robust and efficient chemical processes.
Scale Differences in Chemical Reactions: Heat Dynamics
The concept of scale differences influences how processes unfold at various volumes. From millilitres, to litres, to tonnes, the shift in scale introduces significant changes in the relationship between surface area, volume, and heat dynamics.
Surface Area and Volume
As the scale of a reactor changes, the relative surface area undergoes a transformation. Illustrated by a typical graph for a cylinder, the decrease in radius from right to left results in a steep increase in the surface area-to-volume ratio at smaller vessel sizes.
As the reactor volume expands, the relative surface area diminishes. This is particularly pertinent to reactor design, given that cooling capacity is intricately linked to the surface area of the reactor.
Cooling Capacity vs. Heat Generation
The differences of scale introduces an intriguing dichotomy between cooling capacity and heat generation. While the cooling capacity, represented by the dashed green line in the plot (graph below), tends to increase linearly, the heat generation, especially in exothermic processes, follows an exponential trajectory, represented by the red line (graph below).
The Arrhenius relationship dictates that reaction rates increase with temperature. This creates a scenario where the heat generated outpaces the linear increase in cooling capacity.
The Risk of Thermal Runaway
The below plot depicts a juncture where the red line of heat generation intersects with the green line of cooling capacity. This intersection signifies a point where the heat produced exceeds the system’s capacity to absorb energy and remove it through normal cooling processes.
The consequence is an accumulation of excess heat within the system, leading to a scenario known as thermal runaway.
In this situation, the reaction spirals out of control, posing not only safety risks but also potential damage to equipment and the integrity of the overall manufacturing process.
The intricate relationship between surface area, volume, heat generation, and cooling capacity underscores the need for meticulous planning and design considerations at each scale.
Safeguarding Chemical Processes
The first line of defence against process deviations lies in a comprehensive understanding of the chemistry, energy change, plant infrastructure, and available assessment methods. HAZOP (Hazard and Operability Study), CHAZOP (Computer HAZOP), what-if analysis, Failure Modes and Effects Analysis (FMEA), checklist analysis, and fault tree analysis are among the arsenal of tools that can be used to scrutinise potential deviations.
The Operating Envelope Concept
Central to mitigating process deviations is the concept of designing processes within a safe operating envelope. This involves creating a controlled environment where the operating temperature aligns with the heat generated by potential exothermic reactions.
This operating envelope must also consider the thermal decomposition point of substances within the vessel. Striking this delicate balance allows for the widest possible operating range, enhancing the system’s resilience against unexpected variations.
Small Changes, Minimal Impact
The ultimate goal in designing chemical processes is to achieve resilience in the face of small changes. Catastrophic outcomes can be averted by meticulously crafting a process that can absorb and adapt to deviations without spiralling out of control.
Chemical reaction hazards are unpredictable, whilst being both essential and challenging to assess.
Chemical Reaction Hazard Analysis (CRH) offers a roadmap for manufacturers to navigate new products, synthetic routes, and the inevitable challenges of scaling up. From early desktop studies to the optimization of synthetic routes, CRH provides a guiding light, allowing process safety professionals to make informed decisions that balance efficiency with safety.
The exploration of Chemical Reaction Hazards must be undertaken with vigilance and there must be a commitment to continuous improvement. Understanding, analysis, and proactive design, will create a safer, more efficient, and more resilient future in the chemical manufacturing industry.
