The simple explanation of what is happening…
Styrene (Stabilized) is shipped with an “inhibitor.” This inhibitor is TIME and TEMPERATURE dependent. Should the inhibitor expire and/or the solution exceed its safe temperature, the inhibitor weakens, and the Styrene will begin to polymerize (chemical reaction), which is an EXOTHERMIC reaction with “run-away” potential. This means the reaction can generate a lot of heat and ultimately cause the catastrophic failure of the container.
Imagine a snowball the size of a baseball being rolled down the side of a mountain. As it rolls down the hill, it gets bigger and bigger, which causes it to go faster and faster. At some point, we can no longer stop this snowball; however, if we can get in front of this snowball early, we have a chance of preventing it from reaching the bottom of the mountain. When and where that point is has many factors, some of which are in the control of the responders and some which are NOT.
As the Styrene solution heats up, it increases its volatility, which increases the vapor pressure within the container. The pressure relief valve on the container will lift to relieve this increased pressure, releasing the Styrene vapor (which is almost 1.5 times heavier than air).This is what we are seeing in the video below.
The “catastrophic risk” in this scenario is the catastrophic failure of the container (a railcar in this event), as the polymerization reaction can generate temperatures that exceed the container’s Maximum Allowable Working Temperature (MAWT) due to the heat generated within the container.
NOTE:
If the temperature of the solution rises 1°C/day, it may be an early indication of spontaneous polymerization.
A 2-3°C/day temperature increase is a typical indication of the onset of a runaway polymerization.
The critical temperature of the Styrene solution is 52ºC (125°F). At this temperature, the stabilizing effect of the inhibitor 4-tert-butylcatechol (TBC) will be lost. There is then a severe risk that the liquid will auto-polymerize, generating considerable heat.
Source: TotalEnergies Petrochemicals & Refining USA, Inc.
Video Source: Aaron Vetter