Optimization of reaction calorimetry for monitoring chemical reactions in supercritical fluids

The PhD thesis deals with the optimization of the reaction calorimetry technique to monitor chemical reactions in supercritical fluids. The aim is to develop this thermal analysis technique to monitor the heat released by a chemical reaction in a high pressure reactor and consequently discuss upon the reaction evolution. The experimental equipment used was previously developed in the group of chemical and physical safety at the EPFL. Additionally, a mass flow meter was installed during this thesis to measure the quantity of supercritical fluid inserted in the reactor. Initially, the core of the technique, meaning the heat flow equation, was examined in a term-by-term analysis to adapt and optimize each term for supercritical reaction systems. For this analysis a model reaction was chosen, namely the free-radical dispersion polymerization of methyl methacrylate in supercritical carbon dioxide, and a set of conditions was set as a reference. The particularities linked to the supercritical nature of the solvent were taken into consideration and more precisely the fact that the solvent occupies the entire available reactor volume. As a result it was found that every reactor part has to be thermally controlled and that special caution has to be paid on the estimation of the overall heat transfer coefficient between the reaction mixture and the temperature regulating fluid running in the reactor jacket and on the estimation of the specific heat capacity of the reaction mixture. Furthermore, the injection phase of the additional reactants, according to the reaction protocol, was found to be subject to considerable calorimetric errors; therefore an optimized injection pattern was designed to minimize the undesired temperature oscillations during this phase. The second step of the thesis consists of the discussion on the reaction evolution based on the optimized results obtained. First a direct comparison is presented between the calorimetric results before and after the analysis to highlight the points were significant improvements were introduced, mainly in terms of accuracy and reproducibility of the results. Then the discussion focuses on the nucleation phase of the polymer particles, where the data show that the reaction takes place almost exclusively in the continuous phase. This conclusion is also found to be in very good agreement with the results of other experimental techniques. Further, the role of the pressure on the reaction evolution was examined through a series of experiments, based on a small reaction deceleration observed, and was found to have a drastic effect on the creation of stable dispersion conditions. A key parameter in this investigation was the partitioning of the solvent, the monomer and the produced polymer in the two reaction phases. The latter also helped in the formulation of an explanation for the measured pressure profiles. Finally, the reaction heat rate data of the previous tests were used to discuss some safety aspects of the reaction, primarily through a cooling system failure scenario. Once the results on the model reaction were sufficiently confident, different reaction systems were tested to demonstrate the robustness and to explore the limits of the equipment and of the technique. It is shown that the calorimeter succeeds in detecting very small amounts of heat; therefore can monitor much less exothermic reactions, like the dispersion polymerization of styrene in supercritical carbon dioxide. On the other hand, the applicability of the technique is limited for very exothermic chemical reactions, like the precipitation polymerization of acrylic acid in supercritical carbon dioxide. Additionally, non-polymerization reaction systems can also be monitored and this is demonstrated with the example of the esterification of acetic anhydride with methanol in supercritical carbon dioxide. Finally, some preliminary tests were carried out to investigate the possibility of working with the high pressure reactor in a continuous mode but the results show that the developed set-up is not suited for such an application. The last part of the thesis deals with the use of the reaction calorimeter to produce polymeric foams using supercritical carbon dioxide as the blowing agent. The capability to control the reactor temperature with high accuracy and measure precisely the temperature and the pressure were exploited in this case. The equipment was initially calibrated in terms of the depressurization profiles that can be achieved and consequently polymer foams were produced. The effects on the final foam morphology of the polymer type, the temperature, the pressure, the depressurization rate and of the calorimeter operation were studied and various trends were identified.