Why Are Early-Stage Design Decisions Critical for Process Safety?

If you answered “no” to any of the following questions, you may want to consider creating a chemical process evaluation testing strategy:

Early-stage design choices in chemical processes often determine whether a system will operate safely, or drift toward potential failure. At this stage, chemists and engineers select reaction routes, evaluate materials, and begin considering scale-up from the laboratory to pilot and production environments. 

What is often overlooked is that these same decisions can also create the conditions where process failure can begin.

Process failure occurs when a system slips outside its intended operating window and can no longer be safely controlled. In the lab, this risk feels remote as small batches shed heat quickly, glassware can vent pressure, and human intervention is easy. 

But scale changes everything. In an industrial reactor, heat can accumulate faster than it can be removed, pressure can rise abruptly, and small deviations can escalate into uncontrolled events.

Both reactive systems (where runaway reactions or decompositions may release uncontrolled energy) and non-reactive systems (where dusts, vapours, or gases can form explosive atmospheres) carry serious hazard potential. 

Recognising these hazards early and embedding safeguards into your process design from the start is responsible engineering.

Process failure often occurs when energy release exceeds system capacity, driving runaway reactions, pressure surges, or explosions.

Reactive systems can exhibit uncontrolled energy release from exothermic reactions, thermal decomposition, or runaway behaviour. Common examples include nitrations, oxidations, polymerisations and reactions that may run smoothly under controlled conditions but escalate to crisis if cooling capacity is lost, dosing is mismanaged, or impurities alter the pathway.

In fact, HSE guidance emphasises that scale-up to pilot plant is the critical stage for revealing thermal hazards that bench work can mask. This is when a thermal behaviour assessment (including calorimetry) becomes essential to identify runaway potential.

Non-reactive systems are equally as dangerous. Powders such as flour, sugar, or wood dust, which can be safe in bulk, may explode when dispersed in air. Solvent vapours or intermediates accumulating in poorly ventilated spaces may ignite with a spark. Even static electricity, if not controlled, can initiate combustion. 

In both cases, the mechanism of failure is the same: energy is released faster than the system can absorb or control it. As a result, safeguards will fail either because they were not designed for credible scenarios, or because the hazards were never identified.

In both cases, the mechanism of failure is the same: energy is released faster than the system can absorb or control it. As a result, safeguards will fail either because they were not designed for credible scenarios, or because the hazards were never identified.

Most runaway reactions begin when cooling capacity is exceeded, and the Maximum Temperature of the Synthesis Reaction (MTSR) surpasses decomposition limits.

When this happens, a secondary exothermic decomposition can begin. These secondary decompositions are often more energetic than the intended synthesis. Gases are then produced which, in turn, leads to rapid increases in both temperature and pressure. 

According to the CSB, approximately 60% of reactive system accidents are linked to poor or absent thermal hazard assessment. Too often, calorimetry is not performed at the design stage, leaving critical gaps in understanding runaway potential. Again, DSEAR and ATEX explicitly require thermal hazard assessments before scale-up.

A process cannot be considered safe if it has only been evaluated under ideal conditions. Instead, chemical engineers must analyse normal operation and any potentially credible misoperations.

Reactive Systems

For reactive systems, hazard evaluation must extend well beyond measuring the intended heat of synthesis. Processes need to be stress-tested against abnormal conditions that can occur during real-world operation:

• Loss of Cooling Capacity: If cooling is reduced or lost altogether, reaction heat may accumulate faster than it can be removed. This can push the system toward self-heating, where temperature rises uncontrollably and initiates runaway.
• Accelerated or Uncontrolled Dosing: Overfeeding a reactant, whether due to pump malfunction or operator error, can overwhelm the reaction’s thermal balance. Instead of steady heat release, the system can tip into rapid acceleration, raising the Maximum Temperature of the Synthesis Reaction (MTSR).
• Agitation Failure: Mixing is often assumed to be constant, but loss of agitation creates stagnant zones. In exothermic reactions, these localised regions can overheat, leading to decomposition or hot-spot-driven runaway.
• Thermal Stability of Reagents, Intermediates, and Products: Testing must also identify the decomposition onset of all materials involved, not just the target product. Unstable intermediates or impurities can decompose at lower temperatures, producing secondary exotherms and potentially large volumes of gas. Without this insight, relief systems may be severely under-designed.

Non-Reactive Systems

Non-reactive systems demand equal scrutiny. Engineers, during the design phase, must evaluate:

• How easily the material disperses,
• Whether credible ignition sources exist nearby, and
• How much dust could be mobilised during maintenance, cleaning, or equipment upset.

A material that appears stable in bulk may present fire and explosion risks once conditions deviate.

Testing is the most reliable way to replace assumptions with actionable data in chemical process design. Without it, engineers are forced to extrapolate from small-scale observations or literature values, which can mask hidden hazards and create unsafe designs. We’ve listed a few of our testing methods and how they can help below. 

Thermal Screening
Thermal screening is often the first step in evaluating a new chemical process. By heating reagents, intermediates, and products under controlled conditions, we can detect low-temperature decomposition events, unstable exotherms, or unexpected reactions that could pose serious hazards at scale. 

Early identification of these risks allows unsuitable synthetic routes to be eliminated before significant investment in equipment, materials, or pilot studies.

Reaction Calorimetry
Once a reaction route is selected, reaction calorimetry provides quantitative data for scale-up. This technique allows us to measure the rate and magnitude of heat release under actual process conditions, allowing us to calculate adiabatic temperature rise, identify peak heat generation, and determine safe cooling requirements. 

The insights gained include the sizing of cooling systems, the design of dosing strategies, and reactor configuration. Test data can be used to ensure that energy generated by the reaction can be safely managed.

Adiabatic Calorimetry
Adiabatic calorimetry goes a step further by simulating worst-case scenarios, such as cooling failure, dosing surges, or agitation loss. These tests reveal whether a system can self-heat to decomposition, whether gases are generated during runaway reactions, and whether relief systems are adequate to prevent overpressure. 

Many historical runaway reactions could have been prevented if adiabatic testing had been performed during the design stage. This data is crucial for engineering safe processes and demonstrating due diligence under regulatory frameworks.

Together, the above tests can provide a comprehensive understanding of how your process behaves under both normal and abnormal conditions. 

Process failures rarely happen by chance. In most cases, they can be traced back to assumptions, oversights, or gaps in knowledge made at the very start of process design. By the time a design reaches pilot or production scale, those early decisions may already have created the conditions for failure.

• Scale-Up Assumptions: Lab behaviour is often assumed to translate directly to pilot scale. But heat dissipation, mixing, and manual oversight differ radically.
• Over-Reliance on Normal Operation: Cooling, venting, and dosing are often sized for intended synthesis, not deviations. In one nitration case, adequate cooling for the primary exotherm failed instantly when decomposition began.
• Equipment Reliability: Designs assume agitators, pumps, and cooling systems will not fail. Fouling and breakdowns occur. A stalled agitator in an exothermic system can create hot spots that trigger runaway.
• Human Factors: Many failures arise from operator missteps such as wrong valve sequence, bypassed alarms, or dosing errors. Design must tolerate human error through clear instrumentation, interlocks, and relief sizing.
• Incomplete Hazard Data: Without calorimetry, explosibility tests, or electrostatic hazard characterisation, risks remain invisible. Literature values and “similar compound” data are insufficient, and small variations can drastically alter hazard behaviour.

Process safety and responsible engineering is built in the design phase. The earliest choices in reaction route, scale-up strategy and data collection set the limits of what a process can withstand under stress. 

Most failures trace back to assumptions made during this stage. These failures are usually related to scaling laboratory behaviour directly, designing only for normal operation, or relying on incomplete hazard data.

By replacing assumptions with testing, anticipating both normal and abnormal scenarios, and embedding safeguards into every phase of operation, engineers can design systems that are resilient rather than fragile.

Early-stage design decisions determine whether processes run smoothly or escalate toward failure. Early awareness, rigorous testing, and conscious design are the engineer’s strongest defences against accidents that are otherwise preventable.

Learn more about process safety testing in our FREE on-demand webinar.