The testing toolkit: DSC, TSU, ARC, RC1
What each instrument measures, when to use it, and how to read a DSC thermogram.
Educational overview. The principles here are general and simplified, and the figures and rules of thumb are typical values, not limits for your plant. Decisions need process-specific data, the applicable codes and standards, and a qualified assessment.
The questions testing must answer
- Is every starting material, intermediate, product, mixture and residue stable at the temperature it will see in the plant, including upsets?
- At what temperature do heat and gas generation start on plant scale, and how much energy is involved?
- How is the reaction heat released: instantly with the dosing, or with accumulation?
- Can the plant cooling keep up with the maximum heat release rate?
The instruments
| Instrument | What you get | Strengths | Limits |
|---|---|---|---|
| DSC Differential scanning calorimetry, a few mg in a sealed, high-pressure (often gold-plated) crucible | Exotherms and endotherms, their energy (J/g), approximate onset | Fast, tiny sample, quantitative heat | Least sensitive, so onsets appear too high; no pressure / gas data; poor for heterogeneous mixtures |
| TSU Thermal screening unit, ~8 mL test cell with pressure transducer | Onset of exotherm, pressure and gas generation | Gas data, short tests | No quantitative heat; still a screening test |
| ARC Accelerating rate calorimeter, “heat-wait-search” adiabatic test | Adiabatic self-heating rate, pressure, TMRad, kinetics | Sensitive, close to plant conditions, good for reaction mixtures and all-in batches | Slow (about 1 day per test), expensive; needs a correction for the thermal inertia of the cell (φ-factor) |
| Reaction calorimeter (RC1) Lab reactor run like the plant process | Heat release rate vs time, total reaction heat, cp, accumulation, gas flow | Measures the desired reaction under real dosing | Does not measure decompositions; needs the actual process |
Each instrument in motion
Drag the slider, or press Play, to watch what each test measures.
The usual flow: DSC screening of all materials and mixtures → if an exotherm is close to process conditions, ARC / TSU for the decomposition → reaction calorimetry for the main reaction → combine the results into the cooling-failure scenario (lesson 4) and the Stoessel class (lesson 5). Powders get their own tests (lesson 7).
Reading a DSC thermogram
- Endotherms (down, or up, depending on the plot convention) are melting, evaporation, solvent loss. Look for the arrow marked exo.
- An exotherm at moderate temperature in a mixture of the reagents is usually the desired reaction. Its area gives Q′rx.
- An exotherm at higher temperature is a decomposition. Its area gives Q′dc and its severity: ΔTad,dc = Q′dc / c′p.
- A sharp, symmetrical peak after a flat baseline in an isothermal test is the fingerprint of an autocatalytic decomposition. Do not extrapolate it like a normal reaction.
- Compare thermograms of the right mixtures: final reaction mass, mixture of reagents (worst case of accumulation), and “what if one component is forgotten?”
Scanning DSC onsets depend on scan rate and sensitivity. A decomposition that shows an onset at 220 °C in DSC can run away from 140 °C or lower in a large insulated vessel. Old rules of thumb (“stay 100 K below the onset”) have failed. Use kinetics: TMRad and TD24, measured or modelled (isothermal DSC series, ARC).
Key takeaways
- DSC screens everything; TSU adds pressure; ARC gives adiabatic kinetics and TMRad; reaction calorimetry measures the desired reaction and accumulation.
- Severity from energy: ΔTad = Q′ / c′p.
- DSC onset temperatures are not safe limits. Use TMRad / TD24.
- Watch for autocatalytic signatures and test the mixtures that matter, including mistakes.