Reaction calorimetry is a useful tool to investigate chemical reaction kinetics, to determine required data for chemical processes safety and to access fundamental information about phases change and mixing behaviour. The fundamental study of the behaviour of the supercritical phase is still under intensive research and will need further investigations. Literature has proved to be very poor in the domain of calorimetry linked to supercritical phases and even more, to our knowledge, this work act as a pioneer using and developing a reaction calorimeter in supercritical field.

One of the main advantages of this reaction calorimeter is that it allows to study the effect of mixing, heat conduction and heat transfer at a reasonable scale where these phenomena could no longer be neglected in contrast to small-scale batch or tubular cells calorimeter. Moreover, this reactor size is an opportunity to add in-line probes as, for example, FTIR or ultrasonic. Therefore, it give access to fundamental phase and concentration evolution throughout the system, as it has been proved that even a small portion of a solute could alter the complete phase diagram of a supercritical mixture. The coupling of reaction calorimetry and in-line sensors could be very efficient and promising in order to understand fundamental aspects of supercritical fluids involved in reactions.

The calorimeter is able to work in three different operating modes: adiabatic, where the jacket temperature (Tj) is adjusted in such a manner that there is no heat transfer through the reactor wall; isoperibolic, where the jacket temperature is kept constant and the reaction temperature (Tr) follows the reaction profile; and isothermal, where the desired reaction temperature is set to a constant value and Tj is changed automatically to maintain Tr at the specified value. All the experiments presented in this paper are performed using the isothermal mode.

The “supercritical reaction calorimetry” has first to overcome some technical problems mainly due to the fact that the supercritical phase occupies all available space as illustrated in Figure 43. Thus, not only the jacket area has to be perfectly controlled but also the cover and the other parts have to be separately temperature-controlled in order to apply the heat balance equations without any additional heat transfer interferences. In our case, all the reactor parts in contact with the reacting media are adjusted to Tr in order to neglect the heat accumulation term.

Figure 43: Difference between classical liquid and supercritical reaction calorimetry.

To conclude, this new supercritical calorimeter system allows the evaluation of calorimetric measurements under supercritical conditions which is definitely not trivial. However, the complex phenomena of heat transfer with SCFs should be carefully taken into account in order to proceed with correct chemical reactions evaluation. Some preliminary results in the “supercritical reaction calorimetry” field have been presented: The Wilson plot study allowed the understanding of the fundamental behaviour of the internal film coefficient, which does not follow the same tendency as for classical liquids with respect to temperature, due to the very specific properties of scCO2 near its critical point. On the other hand, the linear trends of the Wilson plot regressions confirm the 2/3 value of the power of the Reynolds number in the Nusselt expression which equation was used to calculate the constant C for our system.