Being a reactive hazards person, I try to keep up on the reports posted by the US Chemical Safety Board (CSB). In my view, the CSB does exemplary work in root cause analysis of what are often very complex events leading to disaster. I wholeheartedly recommend that people in the process side of chemistry peruse the many reports and videos posted on the CSB website.

The development of any technology in the real world involves what I refer to as

“the discovery of new failure modes”.

While it is possible to anticipate many kinds of failure modes, it often happens that plant operations will present the opportunity to line up the planets in a particular way that was left out of the failure analysis.

A recent account from the CSB is the report on the T2 Laboratories accident in late 2007 in Jacksonville, FL.  This accident killed 4 employees and injured 32 in many of the adjacent businesses. The explosive yield was estimated by the CSB investigators to be equivalent to 1450 lbs of TNT.

What is most instructive about this incident is the extent to which the thermokinetic behaviour was unknown to the owner/operators. This accident illustrates that thermal decomposition modes leading to runaway can happen despite a large number of successful runs.

I won’t go into too much detail since the report itself should be read by those interested in such things. But the upshot is that the reactor contents (MeCp dimer, Na, and diglyme) accelerated to a temperature that lead to the exothermic reaction of sodium metal and solvent diglyme. The reaction contents accelerated, raising the temperature and pressure to the rupture disk yield pressure of 400 psi. However, the acceleration was far too energetic for this safety device. The vessel exploded, hurling fire and fragments off the site. Just prior to the explosion, the owner/engineer directed the operators to leave the control room, saying prophetically, “there is going to be a fire”.

While the owners did perform some process development and did have the used vessel professionally inspected, what was left out was a study of the aptitude of the reaction to self-heat into a runaway condition. The company rightly anticipated the exothermicity of the sodium reaction with MeCp monomer and in fact, relied on the exotherm to raise the rxn temperature to a level where the economics would be more favorable. But what nobody at T2 anticipated was the runaway potential of the reaction of the sodium with the diglyme. No doubt they thought that the cooling jacket would prevent temperature excursions leading to a runaway.

The various glymes are often chosen as reaction solvents owing to their diether character as well as their high boiling points. Troublesome compounds or reactions requiring a polar solvent can be dissolved at high temperature and reacted in this high boiler. In certain cases, reactions can be run in a glyme and the product conveniently distilled out of the reaction mixture. Perhaps this is what they were doing in the MCMT process, I don’t know. This level of detail was not provided in the report.

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