I’ve turned my attention to reaction calorimetry recently. A reaction calorimeter (i.e., Mettler-Toledo RC1) is an apparatus so constructed as to allow the reaction of chemical substances with the benefit of measuring the heat flux evolved. Reaction masses may absorb heat energy from the surroundings (endothermic) or may evolve heat energy into the surroundings (exothermic).
Calorimetry has been around for a very long time. What is relatively recent is the development of instrumentation, sensor, and automation packages that are sufficiently user friendly that RC can be plausibly used by people like me: chemists who are assigned to implement a technique new to the organization. What I mean by “user friendly” is not this: an instrument that requires the full time attention of a specialist to operate and maintain it.
A user friendly instrument is one engineered and automated to the extent that as many adjustments as possible are performed by the automation and that the resulting sysem is robust enough that operational errors and conflicting settings are flagged prior to commencing a run. A dandy graphic user interface is nice too. Click and drag has become a normal expectation of users.
An instrument that can be operated on demand by existing staff is an instrument that nullifies the need for specialists. Not good for the employment of chemists, but normal in the eternal march of progress. My impression is that RC is largely performed by dedicated staff in safety departments. What the MT RC1 facilitates is the possibility for R&D groups to absorb this function and bring the chemists closer to the thermal reality of their processes. Administratively, it might make more sense for an outside group to do focus on process safety, however.
In industrial chemical manufacture the imperative is the same as for other capitalistic ventures- manufacture the goods with minimal cost inputs to provide acceptable quality. Reactions that are highly exothermic or are prone to initiation difficulties are reactions that may pose operational hazards stemming from the release of hazardous energy. A highly exothermic reaction that initiates with difficulty- or at temperatures that shrink the margin of safe control- is a reaction that should be closely studied by RC, ARC, and DSC.
It is generally desirable for a reaction to initiate and propagate under positive administrative and engineeering controls. Obviously, it is desirable for a reaction to be halted by the application of such controls. Halting or slowing a reaction by adjustment of feed rate or temperature is a common approach. For second order reactions, the careful metering of one reactant to the other (semi-batch) is the most common approach to control of heat evolution.
For first order reactions, control of heat evolution is had by control of the concentration of unreacted compound or by brute force management of heating and cooling.
Safe operation of chemical processing is about controlling the accumulated energy in the reactor. The accumulated energy is the result of accumulated unreacted compounds. Some reactions can be safely conducted in batch form, meaning that all of the reactants are charged to the reactor at once. At t=0, the accumulation of energy is 100 %. A reliable and properly designed heat exchange system is required for safe operation (see CSB report on T2). In light of T2, a backup cooling system or properly designed venting is advised.
The issue I take with the designers of the process performed at T2 is this: They chose to concentrate the accumulated energy by running the reaction as a batch process. This is a philosphical choice. The reaction could have been run as a semibatch process by feeding the MeCp to the Na with a condenser on the vessel. Control of the exotherm could have been had by control of the feed rate and clever use of the evaporative endotherm. A properly sized vent with rupture disc should always be used. These are three layers of protection.
Instead, they chose on a batchwise process relying on a now obviously inadequate pressure relief system, and the proper functioning of water to the jacket.
No doubt the operators of the facility were under price and schedule pressure. The MeCp manganese carbonyl compound they were making is an anti-knock additive for automotive fuels and therefore a commodity product. I have no doubt at all that their margins may have been thin and that resources may not have been there to properly engineer the process. This process has “expedient” written all over it in my view.
Reactions that have a latent period prior to noticeable reaction are especially tricky. Often such reactions can be rendered more reliable by operation at higher temperatures. Running exothermic reactions at elevated temperatures is somewhat counter-intuitive, but the issue of accumulation may be solved.
Disclaimer: The opinions expressed by Th’ Gaussling are his own and do not necessarily represent those of employers past or present (or future).