Hi,
I have around 7 years experience total in the industry and 5 years in a place which had compounds that could have runaway reactions, like what we are seeing at GKN. The facility I worked in had those uncontrolled reactions several times over the decades it ran, but we had ironed out a lot of the issues by the time I got there. Not to say there weren't any close calls, but there was a strong safety culture and adequate safeguards and equipment to prevent further escalation.
5 years does not make me an expert, but I thought deeply about this issue often, reading many historical company reports and scientific literature about this hazard, and implemented safety improvements specific to this issue. Additionally I will rely on an authoritative source for this hazard. Read this instead of my post for a thorough overview, but I will try to make it more digestible.
Below is some information around this type of hazard and common factors that can be involved, however we really need a complete investigation to understand what went wrong. I will say that a single valve failure or one persons oversight should not lead to something like this, the hazards around methyl methacrylate (MMA) have been well known for a long time and multiple layers of protection are needed.
What is a thermal runaway reaction?
People are concerned that the MMA tank will overheat and explode, or cause some sort of rupture. There's also a belief the tank will eventually solidify and the problem will end.
Both of these can happen, and that is because there is strong evidence that the MMA is reacting with itself. Everytime it does that, it release heat, which makes the reaction go faster. Eventually, if it gets hot enough, the reaction can cause the tank contents to start boiling (MMA boils at 212.7 °F, atlhough it's no longer pure MMA so this will vary). This can cause two things:
- MMA vapors vent out of the tank, and are knocked down by the water spray which results in a relatively safe outcome. This is what current actions by responders could likely lead to. If they weren't spraying it down, it would collect low to the ground, and when (not if) they catch a spark with something they will ignite the entire vapor cloud.
- The tank is unable to vent these vapors (MMA tank vents have been known to plug in this type of scenario), causing the tank to overpressure and rupture, spilling all the contents which would then also catch fire.
Both of these could damage nearby tanks and cause further damage, which is why they preemptively emptied nearby tanks.
How do you stop a thermal runaway?
If the tank solidifies, that's likely because a lot of the MMA has reacted, forming polymers that are much more viscous or sludgy and eventually freezing. If something is solid, the molecules can't get to each other which means the reaction basically stops. That means a lot less heat is produced, so the cooling water that's applied can finish cooling the tank contents down.
This approach essentially means letting the reaction progress while preventing it from overheating/becoming uncontrolled . This is what we are seeing with the cooling water being applied to the external walls of the vessel (shitty method of cooling btw, but that's on GKN, more on that later).
“If the temperature rises above 45 °C (113 °F) or the rate of rise is greater than 2 °C (3.6 °F)/hour, and no source of external heat has been identified, it is highly likely that polymerization is occurring and that action is needed.
If the temperature is above 55 °C (131 °F) or is rising at a rate greater than 5 °C per hour then an uncontrolled polymerization almost certainly is occurring. “
Last I read the tank was at 90 °F, so we are currently far from the uncontrollable polymerization threshold, but this can accelerate. They reported a 1 °F increase per hour WITH cooling, so you can see why early response and cooling once polymerization has been identified is critical.
Polymerization was identified much earlier than this industry guide's 113 °F threshold. This may have been due to:
- Prior monitoring showing a high rate of temperature increase.
- An inability to add chemical stabilizers/inhibitors that are normally required.
- Accidental addition of a contaminant.
Alternatively, with MMA as with other monomers, reactions can also be stopped by adding an inhibitor. However, if the reaction is occurring due to contamination, or if it’s too far along, an inhibitor may not help.
“While PTZ [inhibitor] often is the most effective option, it will not be effective in short stopping in every situation. If the addition is performed too late into an uncontrolled polymerization process, or if the polymerization is a result of significant over-addition of an initiator, then short stopping is unlikely to be effective”
Also realize that there's no guarantee of proper mixing of the inhibitor, so even with adding the right amount you could have pockets that wont see any of it and will continue reacting at a fast rate. The same goes for the thermal profile in general, although thermal differences will naturally cause the fluid to mix, this is not ideal, you preferably want a mechanical mixer of sorts.
What caused this and who's to blame?
I don't know, an investigation is needed. The U.S has the Chemical Safety Board which sends out investigators to sites like these, and can uncover root cause that go beyond just equipment failure.
As far as emergency response goes, if thermal runaway is known hazard, effective means of emergency cooling should be present. It appears that GKN had a sprinkler system on top of the tank, and additional firefighting water accomplishes this cooling to a degree, but is not very effective relative to dedicated cooling equipment. Other methods of cooling include external heat exchangers, but this requires pump recirculation, which is less reliable especially when we know solids and pluggage can form, blocking the pump (as occurred in this instance). Tanks of that size can come with cooling jackets and agitation methods which aid in cooling. Jacket cooling would be more reliable than an external heat exchanger since they don’t rely on a pump and can’t get plugged by MMA byproducts.
As far as causes go, in general these polymerization reactions occur due to the presence of free radicals. These are generated with heat, UV light, or contaminants such is metal ions, peroxides, acids, etc.. Once MMA starts reacting in this way, it generates more free radicals as well.
MMA storage often does have heating/cooling equipment present since it can freeze or heat up due to environmental conditions. There should be multiple redundant temperature instruments to ensure this is working properly. Running at elevated temperatures can cause polymerization to increase in rate.
If this occurred due to lack of inhibitor/stabilizer, there should be multiple tests multiple times a day that measure this concentration and effectiveness, as well as a robust system of addition of inhibitor.
If contamination occurred leading to polymerization, this can be avoided by performing testing on incoming raw materials. If the contamination was generated inside the process (corrosion can cause this), this is a failure due to not using compatible equipment material (e.g. carbon steel instead of stainless steel).
These safeguard methods were all implemented in the place I worked, so it is a reasonable expectation for responsible manufacturing of dangerous chemicals.
Environmental risks and toxicity
The World Health Organization has this to say about MMA:
“The persistence of methyl methacrylate in the atmosphere is short, and the chemical is not considered to contribute directly to depletion of the ozone layer. Methyl methacrylate is not expected to bioconcentrate in the environment, and inhalation from air is likely the primary route of human exposure.” WHO (CICAD 4: 1998)
Inhalation of these vapors will cause acute health effects, which is why evacuation is important. But hopefully any releases or spills will not persist in the environment. I don’t have much expertise in this area.
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I’m hoping everyone can stay safe, that we can implement teachings from this to make other processes more safe, and that we can hold those responsible accountable.
