On the Merry Path of Calorimetry
I enjoy working with our RC1 reaction calorimeter. As we get more experience with thermal profiles of reactions, the utility of this instrument is made more evident. The Mettler-Toledo RC1 can be used to follow the heat evolution of a reaction for safety purposes, and/or it can be used to narrow in on optimum feed rates of reactants.
What is next on the agenda is to determine the heat transfer coefficient(s) and wetted heat transfer surface areas in selected reactors in order to gauge the upper heat load boundary that can be managed safely. There are many variables to contend with. Inevitably, one has to pick a finite range of operating parameters to evaluate. Agitation rate, fill level, and heat transfer medium are variables to take into account.
So, down the merry path we go, learning more and more about applied thermodynamics and chemical engineering. I can dig it.
In my experience with people in different organizations in the context of training and expertise, I have come to notice that employees can be partitioned into two camps. There are those who wait to be trained and there are those who will not wait to for it. As a rule, scientists and engineers are driven by curiosity and not a small portion of competitive spirit. This group will engage in self-study to acquire the necessary skills to push back the limits of their abilities.
An instrument like the RC1 requires that the user be familiar with the intimate details of the chemical transformation. It is possible to alter the experiment profile on the fly, and that is not the work of a pure analyst following SOP’s. A chemist experienced in experimental synthesis with a broad background in material phenomena and descriptive chemistry is one who can steer the instrument and tease out key subtleties.
I recently had a reaction mixture in the RC1 that formed a slush at low temperature. At this temperature the heat flow trace was extremely irregular. The reaction mass showed little visible sign of mixing. Addition of reactant was followed by large magnitude, short coupled, exothermic swings. Apparently the heat of reaction was being released on a relatively small top portion of the reaction mass and eventually swirling towards the heat sensor strip with little dilution, giving exaggerated heat flow indications. With a Tr increase of 20 ºC and a higher mixing frequency, the mass began to thin a bit giving a vortex. The wild heat excursions disappeared.
What I take from this experience is that control problems might arise as a result of poor mixing leading to temperature or feed control inputs that are exaggerated as a result of being out of phase with the state of the reaction mass. An economic consequence might arise in the form of overly conservative metering of reactant, adding extra plant hours to the cost.
The concentration effects due to poor mixing can lead to localized enthalpic overheating and potential disturbances in the composition profile. A reaction mixture with high viscosity or density in a solvent with low tensile strength (i.e., diethyl ether) can lead to cavitation and further exacerbation of mixing problems.
A poorly mixing slurry of reactive components in a low boiling solvent is a bad combination. Especially when the reactor is filled to afford low headspace. A temperature excursion can exceed the boiling point and cause the thick mixture to develop into a foam which can expand into the headspace or beyond. This is the realm of heterogeneous flow and your emergency venting system may not be designed for such flows. This is one of the many reasons that some operations define an operating temperature policy relating to the reaction temperature and the boiling point of the reaction solvent.
It is worth pointing out that process intensification is likely to lead to higher power densities (W/kg) in the reaction mass as well as solubility problems that can cause poor mixing and heat transfer. The RC1 can help the process chemist flesh out the merits of process intensification.