How to Choose a Thermal Hazard Test for Chemical Process Safety in Australia
Every chemical process that involves an exothermic reaction carries a question: what happens if the heat is not removed? Answering that question is the job of thermal hazard testing. It sits at the centre of chemical reaction hazard assessment, from early research through to plant scale-up and the storage and transport of finished products.
This practical guide to process safety testing in Australia explains which test to use at each stage, how the techniques compare and what the results mean.
Why thermal hazard testing matters
A runaway reaction starts when a reaction generates heat faster than the system can lose it. The temperature rises, the reaction speeds up, and the heat output rises again. In a closed vessel this usually comes with a pressure rise from vapour or gas generation, which can rupture equipment.
Runaway reaction prevention depends on knowing three things before you scale up:
At what temperature does an exothermic reaction or decomposition begin?
How much energy can it release, and how high could the temperature and pressure go?
How fast could it run away, and how much time would operators have to respond?
No single instrument answers all of these efficiently. That is why many laboratories use a testing ladder: quick screening first, detailed adiabatic testing where screening shows a hazard, and reaction calorimetry to understand the desired reaction itself.
Step 1: Screen first
Screening is about finding out quickly which materials and mixtures are energetic, so you can focus detailed testing where it is needed. A good screening test gives you an approximate onset temperature, an indication of the energy released, and, ideally, whether gas or pressure is generated.
How screening compares with DSC
Differential scanning calorimetry (DSC) is the most familiar screening tool. It uses milligram samples, is fast, and gives a good estimate of the heat of decomposition. Its limitations for process safety are well known:
The very small sample may not represent a heterogeneous mixture.
Standard DSC crucibles do not measure pressure, so gas-generating decompositions can look less hazardous than they are.
Scanning methods tend to detect onset at a higher temperature than will occur at plant scale, so DSC onset temperatures need a safety margin and should not be treated as safe operating limits.
A pressure-capable screening calorimeter addresses some of these gaps. The Rapid Screening Device for exothermic reaction screening that ProDigitek supplies from Thermal Hazard Technology (THT) measures temperature and pressure together on up to 6 samples at once, with sample sizes from 10 mg to 100 g and a typical test time of around 30 minutes. It covers -100 to 400 °C and 0 to 160 bar, using temperature ramps with isothermal soak options.
When to use screening
New raw materials, intermediates, products and waste streams
Comparing formulations or process conditions
Deciding which samples need adiabatic testing
If screening shows a significant exotherm or pressure rise, move up the ladder.
Step 2: Adiabatic runaway testing
Screening tells you a hazard exists. Adiabatic testing tells you how it behaves when the heat cannot escape, which is the worst case for a large vessel. Large reactors and storage containers lose heat slowly relative to their volume, so they behave much closer to adiabatic than a small laboratory sample does.
An accelerating rate calorimeter (ARC) is the established instrument for this. The sample is held in a small sealed bomb inside a calorimeter whose walls are controlled to follow the sample temperature, so no heat is lost. In the Heat-Wait-Seek method, the instrument heats in small steps, waits for equilibrium, then looks for self-heating. Once it detects an exotherm, it tracks the reaction adiabatically and records temperature and pressure against time.
The ES Accelerating Rate Calorimeter for adiabatic runaway testing covers ambient to 550 °C (down to -40 °C with a cryogenic option) and 0 to 200 bar as standard. It complies with ASTM E1981, the standard guide for assessing thermal stability of materials by accelerating rate calorimetry.
ARC vs DSC
The ARC vs DSC question comes up often. They are complementary rather than competing:
DSC | ARC | |
Purpose | Fast screening, heat of decomposition | Worst-case adiabatic runaway behaviour |
Sample | Milligrams | Grams, in a sealed bomb |
Conditions | Scanning, heat lost to surroundings | Adiabatic, no heat loss |
Pressure data | Usually none | Yes, throughout the test |
Main outputs | Onset estimate, energy | Onset, adiabatic temperature rise, TMR, pressure rate, kinetics |
Test time | Short | Longer, often many hours |
Use DSC or another screening tool to find what needs attention, then the ARC to quantify it.
Correcting for the phi factor
In any adiabatic test, part of the heat warms the sample container rather than the sample. The phi factor is the ratio of the total heat capacity of sample plus container to that of the sample alone. A phi factor of 1 would be a perfect adiabatic test. Real tests have a higher value, so measured temperature rises and rates are lower than at full scale. ARC analysis software corrects the data to phi = 1, and low phi test containers reduce the correction needed.
Step 3: Reaction calorimetry for heat of reaction and scale-up
The first two steps focus mostly on unwanted reactions and decompositions. The desired reaction matters too. If the main reaction releases a lot of heat and cooling fails, the temperature can rise high enough to trigger a secondary decomposition.
Reaction calorimetry measures the heat released by the intended reaction under process-like conditions, including how fast heat is released as reagents are added. That tells you whether your plant cooling can keep up, how dosing rate affects heat accumulation, and how high the temperature could climb if cooling stopped.
The Micro Reaction Calorimeter for heat of reaction measurement does this on a small scale. It uses 1.5 ml glass vials with µW sensitivity, controls temperature from -20 to 170 °C, and adds reagents from a computer-controlled syringe (1 to 250 µL) for titration studies. Options include pressure measurement, gas flow and solid addition.
When to use reaction calorimetry
Early process development, when material is scarce
Choosing dosing rates and process temperatures
Estimating heat accumulation in semi-batch processes
Providing heat of reaction and heat capacity data for scale-up calculations
What the results mean
Onset temperature
The temperature at which self-heating is first detected. It depends on the sensitivity of the instrument and the test conditions, so it is not a fixed property of the material. A more sensitive adiabatic test usually detects onset at a lower temperature than a scanning test.
Adiabatic temperature rise
The temperature increase if all the reaction energy stays in the material. It equals the heat of reaction divided by the specific heat capacity. A large adiabatic temperature rise means a runaway could reach temperatures where boiling, gas generation or secondary decomposition occur.
Time to maximum rate (TMR)
The time a runaway would take, under adiabatic conditions, to reach its maximum rate from a given starting temperature. It is a direct measure of how long operators would have to act. A common reference is the temperature at which TMR is 24 hours, often called ADT24, used to judge whether a process or storage temperature leaves enough margin.
SADT
The self-accelerating decomposition temperature is the lowest temperature at which a substance in its transport packaging can undergo self-accelerating decomposition. It is a key figure for classifying and transporting self-reactive substances and organic peroxides, and for setting control temperatures. Because SADT depends on package size and heat loss, it can be estimated from adiabatic data combined with kinetic modelling, as well as by the standard UN transport tests.
How to choose a calorimeter
A simple way to plan your testing:
Unknown material or new mixture: start with screening to find exotherms and pressure generation.
Exotherm found, or large scale or storage planned: run adiabatic tests to get onset, adiabatic temperature rise, TMR and pressure data.
Developing or scaling a reaction: use reaction calorimetry to measure heat of reaction, heat release rate and heat capacity.
Storage and transport: use adiabatic data and kinetics to assess TMR at storage temperature and support SADT assessment.
Local support in Australia
ProDigitek supplies, installs and supports the full range of Thermal Hazard Technology calorimeters in Australia, with sales and support Australia-wide and installation and training on the east coast. If you are planning process safety testing in Australia and want to talk through which instrument suits your materials, request a quote or get in touch with our team.
Frequently asked questions
What is the difference between ARC and DSC?
DSC is a fast scanning screen on milligram samples, usually without pressure data. An ARC tests gram-scale samples in a sealed bomb under adiabatic conditions, recording temperature and pressure to show worst-case runaway behaviour.
What does TMR mean in thermal hazard testing?
Time to maximum rate is how long a runaway would take under adiabatic conditions to reach its maximum rate from a given temperature. It indicates how much time is available to respond.
What is SADT testing?
It determines the self-accelerating decomposition temperature, the lowest temperature at which a packaged substance can undergo self-accelerating decomposition. It is used for transport classification and control temperatures.
Which test should I run first?
Start with screening to find exotherms and pressure generation, then use adiabatic testing where a hazard is found, and reaction calorimetry to understand the heat of the desired reaction.
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