Reactor runaway temperature excursion

[jeem]

In a scenario of reactor runaway temperature excursion, what control & safety layers should respond?

[sys1]

That is one of the highest-consequence scenarios in the process industries because temperature rise accelerates reaction rate, which generates more heat, creating a positive feedback loop. According to the LOPA philosophy in Center for Chemical Process Safety guidance and the IEC 61511 functional safety lifecycle, protection should be provided by multiple independent layers—not a single control loop. The basic line of defense is the temperature control loop coming under basic process control system.
Typical actions should include:
Increase cooling water or thermal oil circulation.
Reduce steam or heating medium.
Reduce catalyst addition rate.
Reduce reactant feed.
Adjust recycle ratio.
Slow agitator only if process-specific analysis supports it.

[ivani1]

A reactor runaway is first controlled by the Basic Process Control System through cooling and feed control. If temperature continues to rise, alarms prompt operator intervention. Should the operator fail or the reaction accelerate beyond control, the Safety Instrumented System initiates an automatic shutdown by isolating reactant feeds, stopping heating, activating emergency cooling or quenching, and initiating depressurization if required. If pressure still exceeds the design limit, the PSV or rupture disc protects the reactor by relieving to the flare. Finally, passive protection and emergency response systems minimize the consequences of any loss of containment.

[tuan]

In a reactor runaway temperature excursion, protection is based on the Layers of Protection Analysis concept.
Ideally, multiple independent layers should respond in sequence before the situation escalates into vessel overpressure, loss of containment, or explosion.
I have attached a typical sequence.

Reactor_Runaway.jpeg