What is process safety?
Key terms, the runaway problem, and the functional groups that warn you a molecule can release energy.
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.
Process safety is not personal safety
Personal (occupational) safety is about slips, falls, cuts and PPE: frequent, usually small events that affect one person. Process safety is about keeping hazardous materials and energy inside the equipment: rare events, but when they happen they can kill many people at once, destroy a plant and reach the neighbourhood. A plant can have an excellent lost-time injury record and still be one valve away from disaster. That was true at Bhopal, Flixborough and Buncefield.
In pharma and fine-chemical plants the largest source of energy is usually not a pressurised gas or a flammable inventory. It is the chemistry itself: an exothermic reaction, or a molecule that can decompose.
The runaway problem in one picture
An exothermic reaction stays under control only while heat is removed as fast as it is produced. Heat production rises exponentially with temperature (Arrhenius), while heat removal through the jacket rises only linearly (UA × ΔT). Above a certain temperature, production wins: the batch heats itself, which speeds the reaction, which heats it faster. That positive feedback is a thermal runaway. If the batch reaches a temperature where a decomposition starts, the energy and gas released can be many times larger than the main reaction.
Heat removal: qex = U · A · (T − Tc) (linear in T)
Terms you will meet in every lesson
| Term | Meaning |
|---|---|
| Exothermic reaction | Releases heat: the reaction enthalpy ΔHr is negative. |
| Adiabatic | No heat exchange with the surroundings. A large, well-insulated reactor with failed cooling is close to adiabatic, so it is the worst case. |
| Adiabatic temperature rise, ΔTad | How much the temperature would rise if all the remaining reaction heat stayed in the mass: ΔTad = Q′ / c′p. |
| Activation energy, Ea | Sets how strongly the rate rises with temperature. A higher Ea means a steeper rise once things get hot. |
| Runaway | A thermally unstable system in which temperature and rate keep accelerating, possibly to an explosion. |
| Induction time / TMRad | The time, under adiabatic conditions, from a given temperature until the reaction or decomposition reaches its maximum rate. It is the time you have to act. |
| Autocatalytic reaction | A reaction sped up by its own products or intermediates. It can sit quietly for a long time, then accelerate suddenly, so simple “onset temperatures” are misleading. |
| SADT | Self-Accelerating Decomposition Temperature: the lowest ambient temperature at which a packaged substance (e.g. an organic peroxide in its shipping container) runs away within 7 days. It applies only to that package size. |
Molecules that warn you
Some groups of atoms store a lot of energy or release gas when they break down. If one of them is in a starting material, intermediate, product or residue, treat the process as potentially hazardous until thermal testing says otherwise.
| Family | Examples of energetic groups |
|---|---|
| Nitrogen-rich | Azides (–N3), diazo and diazonium compounds, azo (–N=N–), triazenes, tetrazoles and triazoles, hydrazines, N-halo compounds |
| Oxygen-rich | Peroxides and hydroperoxides (–O–O–), peracids, ozonides, epoxides (strained ring) |
| Nitrogen + oxygen | Nitro (–NO2), nitrate esters (–ONO2), nitrites, nitroso, N-oxides, oximes, hydroxylamine and its salts |
| Halogen + oxygen | Chlorates, perchlorates, hypochlorites, perchloryl compounds |
| Unsaturated carbon | Acetylenes and metal acetylides, allenes, conjugated dienes (polymerisation) |
| Metal compounds | Organometallics and metal hydrides (pyrophoric), heavy-metal salts of energetic anions |
Oxygen balance is a quick indicator. It is the oxygen a molecule has, compared with what it would need to burn its own carbon to CO2 and hydrogen to H2O. The closer to zero, the more the molecule behaves like an explosive: it carries its own oxidant.
Key takeaways
- Process safety is about keeping energy and hazardous material inside the equipment. It is measured differently from personal safety.
- Runaway = exponential heat production beating linear heat removal.
- ΔTad, Ea and TMRad describe how big, how steep and how fast a runaway is.
- Energetic functional groups (azides, nitro, peroxides, diazo, N–O, acetylenes…) mean: test before you scale up.