Monday, August 15, 2022

Styrene Rail Shipping Problems

There is an interesting (and well written) article over on about an ongoing incident with a styrene railcar in Southern California. The car started heating up, presumably from a self-polymerization reaction, and vented styrene fumes into the neighborhood around the railyard. Styrene is a central nervous system depressant (a ‘huffing’ problem in the 70’s) and a known carcinogen so there are potential long-term health effects. More importantly the material is flammable and the outgassing from the safety venting on the railcar poses a danger of fire and or explosion if the gas reaches an ignition source.

Styrene is an important monomer, that is a chemical that reacts with itself to form long chains of chemicals. While some monomers require the addition of a special chemical to initiate the polymerization reaction, styrene is one of a class of monomer that self-reacts so readily that the chemical has to have a chemical added to it to inhibit the reaction from taking place. Even with that chemical in place, reactions between styrene molecules continue to take place, normally at a very low rate.

The problem with this polymerization reaction is that it is exothermic, it produces heat. As the styrene temperature starts to increase, the rate of reaction begins to accelerate. The faster reaction produces more heat more quickly, reinforcing that acceleration. In a chemical manufacturing process a number of things can be done to control that reaction rate, cooling can be applied, diluents can be added, air can be introduced, and other, slower reacting monomers can be added. Unfortunately, none of this is much use in a railcar.

To protect the public from the affects of a runaway reaction in a railcar during shipment, DOT requires that there is a minimum level of inhibitor present in the styrene, that the styrene is loaded below a certain temperature, that the rail car be insulated from outside heat addition, and only allows the car to be in transit for a limited period of time. That way, there is only a very minor chance of the polymerization reaction to proceed to the point where it becomes a hazardous self-accelerating reaction.

According to the article, this railcar had been in transit for 50 days, the DOT limit is 45. There is a safety margin in the DOT rules so that should not be the sole issue driving this incident. While the tank is insulated to protect against outside heating, unusually high temperatures while in transit, could have increased the temperature in the product enough to raise the reaction rate. That needs further study as we expect to see wider spread higher temperatures in the future, DOT might need to reduce their transit time standards. Additional things that need to be looked at include the inhibitor level in the loaded styrene, the loading temperature, and the oxygenation of the material prior to loading. Similar railcars loaded at the same time and place should be checked for evidence of heating.

It will be interesting to see if the longer transit time for this shipment is the result of the railroad service issues that the Surface Transportation Safety Board has been investigating.

Another interesting issue here, who investigates the incident. It will not be the Chemical Safety Board, they only investigate incidents at stationary facilities. This falls under the purview of the National Transportation Safety Board (NTSB). I am not sure what triggers an investigation by that body, but there could be potentially systemic issues at play here, so this deserves their attention.

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