Chemical Reaction Hazards and Overpressure Protection

The most common hazards associated with chemical reactions are those which cause elevated pressures inside reaction vessels (or other inadequately vented vessels). Exothermic reactions generate heat and, in the presence of a volatile liquid (eg. solvent) can generate very high pressures associated with the volatile liquid. This normally happens when the reaction temperature rises above the atmospheric boiling point of the solvent. It is wise to note that a reaction may be exothermic even if you have to heat the reaction mass initially to get the reaction started. As the temperature of a reaction increases so this can lead to a thermal runaway created by a linear loss of temperature (due to accepted heat loss conditions) but an exponential production of temperature due to the exothermic reaction. This is a situation where control of the vessel is lost and there is little time for correcting the situation.

Therefore, the reaction vessel may be at risk from over-pressurisation due to violent boiling or rapid gas generation. The elevated temperatures may initiate secondary, more hazardous runaways or decompositions. If either of these scenarios generates sufficient pressure and the vessel relief systems are inadequately sized to contend with the rapidity of the pressure rise, there is a risk of vessel rupture or uncontrolled release of flammable or toxic gas.

Some of the largest incidents have been caused by runaway chemical reactions such as Seveso in Italy and Bhopal in India. Ananalysis of thermal runaways in the UK has indicated that most incidents occur because of:

inadequate understanding of the process chemistry and thermochemistry;
inadequate design for heat removal;
inadequate control systems and safety systems; and
inadequate operational procedures, including training.