Consequence Analysis
How Thermal Hazard Testing Works for API Synthesis Routes
Vinit Pandey · Published 4 August 2026
In short: Thermal hazard testing for API synthesis uses calorimetric techniques to characterise the exothermic behaviour of a reaction before it is scaled up from laboratory to plant scale, where reduced surface-area-to-volume ratio makes heat removal proportionally harder.
Key takeaways
- Reduced surface-area-to-volume ratio at scale makes heat removal proportionally harder than at lab scale
- DSC is a rapid first-pass screen; adiabatic calorimetry follows where a hazard is flagged
- Adiabatic calorimetry gives plant-relevant safe holding time data after a simulated cooling failure
- Route or reagent changes generally require re-testing rather than reusing prior thermal hazard data
A reaction that appears well-controlled at laboratory scale can behave very differently at plant scale, because the surface-area-to-volume ratio of a reactor decreases as scale increases, reducing the relative rate of heat removal available to counter reaction heat generation — a key reason thermal hazard testing precedes scale-up decisions.
Differential Scanning Calorimetry (DSC) is typically used first as a rapid, small-sample screening test to detect whether a reaction mixture or intermediate has any exothermic or decomposition event within the relevant process temperature range, flagging candidates for more detailed testing.
Adiabatic calorimetry follows DSC screening where a potential hazard is flagged, measuring the adiabatic temperature rise and time to maximum rate under conditions that closely simulate a real cooling-failure scenario, giving plant-relevant safe holding time data.
For pharmaceutical route changes — a common occurrence given frequent formulation and synthesis optimisation — thermal hazard testing needs repeating for any step where reagents, solvents, concentrations, or addition rates materially change, since prior testing data does not necessarily remain valid.