Development of reaction calorimetry applied to supercritical CO2 and methanol-CO2 critical mixture -heat transfer, heat flow and hydrodynamics

The combination of reaction calorimetry and supercritical fluids as reactants and/or solvent holds great potential for the optimization of chemical processes using high pressure near-critical and/or supercritical mixtures. The very specific properties of supercritical fluids and fluid mixtures have been under investigation already from the nineteenth century and some are quite well understood. But there is still a lack of knowledge in macroscopic properties and raw data for pure fluids and fluid mixtures. This cumulates with the difficulty to apply simple equations of state for the prediction of phase behavior and the calculation of the thermodynamic and the derived transport properties. As a whole, the relatively poor engineering information about supercritical fluids is one of the major reasons that explains the slow industrial development of supercritical processing. The work presented here concerns the complete development of the new "supercritical reaction calorimetry field", which is in fact the application of reaction calorimetry to supercritical fluids media. As far as we know we are pioneers in this domain as only relatively small scale calorimeters have been successfully used with SCFs and only scarcely. The fact is that the use of reaction calorimetry provides essential information for scale-up purposes. Moreover it allows additional in-line equipment to be used, in order to get complementary information. This renders the reaction calorimeter much lesser specialized and potentially attractive as a tool for industrial application developments. In order to correctly position this project in the scale of laboratory and industrial researches, the first chapter is dedicated to the review of SCFs and SCF mixtures properties, their potential as volatile organic solvent relievers and their actual industrial applications. As a result there is still a great need in engineering data for pure SCFs and their mixtures, although their unique properties make them suitable for several processes, mostly driven by environmental considerations. Classical reaction calorimetry is also shortly discussed with the determination of the technical challenge that arose from the use of a supercritical fluid, as it basically occupies all the available space. Thus we point out that both cover and flange should be separately controlled and adjusted to the inner temperature in order to avoid side heat transfer from the reaction media to those elements. Moreover, the regulation parameters for the jacket and the additional cover and flange should be optimized. This has proved to be difficult as these parameters have been shown to depend strongly on the supercritical carbon dioxide pairs, density and temperature. Nevertheless satisfactory pairs of P, I parameters for all three parts of the reactor have been found using the Ziegler-Nichols approach for the jacket and a pure trial-and-error approach for the two others. The heat transfer with SCFs should be carefully taken into account in order to proceed with correct chemical reactions' evaluation. Heat transfer in scCO2 and CO2-methanol mixtures can be described in stirred tank reactors using the well known Wilson plot methods. The behavior of the internal film heat transfer coefficient with the temperature is drastically different from the one observed for classical liquid systems. In contrast to classic liquids, in supercritical CO2, the lower the temperature (above the critical point) the higher the internal film heat transfer coefficient. It shows a clear divergence when approaching the critical point. This tendency could be explained by the evolution of the thermodynamical and transport properties of scCO2 around the critical point. This confirms some observations in the literature with some continuous flow heat transfer apparatuses working with SCFs. The heat transfer coefficients for the cover and the flange have been evaluated and show the same general tendency as the one of the jacket in regard to the temperature. The hydrodynamics of scCO2 in a stirred tank reactor have been investigated using Laser Doppler Anemometry measurements and indicating that the stirred scCO2 has a similar behavior with liquid water. However, the turbulent kinetic energy and the mean velocities are 20% lower for scCO2, which can be explained by its lower density. Information about the phase behavior during a reaction is essential in order to proceed homogeneously. Under this condition, the system allows for higher reaction rates and for easier process control. This has motivated the complementary development of a video imaging surveillance system for the inside of the reactor, as well as the development of a high pressure variable-volume view cell for the screening of mixtures' phase behavior. As the "top-view" observation is not ideal for phase change observation (no meniscus visualisation), the further development of an in-situ high pressure ultrasonic probe is also provided in this work. Sound speed measurements have been proven to be not only useful as a "phase-change" detector, but also allowed determining the critical point of pure fluids and mixtures. This work also pointed out the real difficulty to combine a homogeneous supercritical reaction media to a sufficiently high heat produced signal. To access the limit of detection of the calorimetric system and to find out the characteristic time constant of the reactor and its variation with temperature and density, we created an amplifier system based on a Labview® interface program. Using some defined functions for the calibration heater released heat, it has been possible to simulate some typical reaction curves and thus validate the heat flow measurements with the supercritical reaction calorimeter.

Meyer, Thierry
Lausanne, EPFL
Other identifiers:
urn: urn:nbn:ch:bel-epfl-thesis3228-3

Note: The status of this file is: EPFL only

 Record created 2005-04-06, last modified 2018-03-17

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