Imagine taking a snowball and rolling it down the side of Mount Everest. As the ball rolls down the mountain, it gets bigger and faster. If we can stop it early in its travel down the mountain, BEFORE it gets to BIG and FAST, we can save the town at the base of the moutain. But there comes a time and place in this snowball’s journey down the side of the mountain where it has gotten TOO BIG and TOO FAST that we can no longer stop it. The only option is to evacuate the town at the bottom of the mountain BEFORE the massive snowball reaches it. This is how I explan the “P” Hazard in my HAZMAT and Process Safety Training and PHAs.
But in a well designed process, we have an “emergency system” called a “quenching system” that can STOP the exothermic reaction and avoid the catastrophic event. An emergency quench system is an automated or manual safety system designed to “quench” runaway polymerization. Because the polymerization generates intense heat, runaway reactions can trigger catastrophic over-pressurization. Quench systems deploy chemical inhibitors and/or cooling diluents to safely terminate the “chain reaction”.
NOTE: We call the old manual systems the “short straw” design, as someone has to go to the reactor and load the “kill agent” and depress it into the reactor BEFORE the process reaches it critical point. It can be done safely “sometimes”, but sometimes the rate of rise in temp and pressure makes approaching the vessel risky; hence why we dont see many manual systems these days.
Quenching can either be used to
1) directly control a chemical reaction or
2) in the treatment of an emergency vent stream.