Supercritical calorimetry: an emerging field

Calorimetry (adiabatic, isothermal, differential, oscillating or acoustic) is generally based on heat-flow measurements of the studied system. Most of its applications are dedicated to kinetic-parameter detn., safety studies and process optimization, phase equil. and phase transition studies. Heat flow calorimetry on the lab. scale is currently limited to low viscosity fluids. An emerging new field is concerned with the use of calorimetry in the presence of supercrit. fluids as solvent reaction, which will be named supercrit. calorimetry. Supercrit. carbon dioxide (CO2s.c.) represents an increasingly interesting media for a wide variety of reactions. To fulfill this need, a special supercrit. calorimeter has been developed in collaboration with Mettler-Toledo, Schwerzenbach, CH and some preliminary results are presented. This paper explores supercrit. calorimetry applied to the intrinsic properties of carbon dioxide in the liq., gas and esp. supercrit. phase as well as applications and theory related to reaction calorimetry. The CO2s.c. heat capacity (cp) is measured over the range of 33-112 DegC and 77-206 bar using a reaction calorimeter (RC1e, Mettler-Toledo) coupled with a high-pressure HP350 metallic reactor. Measured values are compared to theor. values obtained from Wagner and Span's equation of state. 3D representations of the predicted values for heat capacity, d. and sound speed of carbon dioxide in the fluid phase are presented.

Development of reaction calorimetry applied to supercritical CO2 and methanol-CO2 critical mixture

Frédéric Lavanchy

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.

EPFL2005

Reaction calorimetry in supercritical fluids

Sophie Fortini

This thesis is devoted to the study of the dispersion polymerization of methyl methacrylate in supercritical carbon dioxide (scCO2), using a poly(dimethylsiloxane) macromonomer (PDMS macromonomer) as stabilizer. Supercritical fluids (SCF) and SCF mixtures are characterized by a temperature and a pressure above their critical point(s), which is the last point on the vaporization line of a pure component. This means that these fluids operate from moderate to high pressure. They can be used in various processes ranging from extractions, nanoparticle formation for controlled drug release, chemical reactions and polymer processing. Nowadays, the best candidate for SCF processing is carbon dioxide. The fundamental motivation of using scCO2 as a solvent is based on its potential to replace harmful chemical organic volatile compounds (VOCs) in order to develop more sustainable and environmentally friendly chemical processes. At this point, the crucial role of CO2 in the development of the so-called "green chemistry" comes on the stage. CO2 is a natural abundant compound with low toxicity exhibiting no inflammability. This last property is very advantageous considering the cost investments spent by the chemical industry to control the safety of the chemical processes using highly flammable compounds like VOC solvents. As expected, environmental arguments are not sufficient to motivate the development of new chemical process routes. Therefore, additional arguments to use SCFs have to be found, and they do exist. As supercritical fluids are compressible fluids they can exhibit liquid-like and gas-like properties, which can be tuned easily by varying the operating conditions, like pressure and temperature. This fundamental behavior of SCF is their main asset and demonstrates their superiority to develop more flexible processes. The polymer industry is one of the industries that uses the largest volumes of organic solvents and sometimes halogenated ones, well known to destroy the ozone layer. The use of scCO2 gives to chemists and engineers the opportunity to develop more sustainable polymer processes, considering the numerous chemical and physical advantages of carbon dioxide. The processing of scCO2 for polymer production is no more than fifteen years-old. This means that a certain quantity of knowledge has been acquired but still a lot of unknowns hinder their promotion at industrial level. This work is inserted in this context and finds there its main motivations. This thesis is composed of two different but intrinsically connected approaches of the dispersion polymerization of the methyl methacrylate (MMA) in scCO2. A part of this thesis is devoted to the development of techniques allowing the on-line monitoring of polymerizations in scCO2 at "larger" scale, conducted from an engineering approach. The intrinsically connected part is devoted to the understanding of the fundamental phenomena that govern the dispersion polymerization of MMA in scCO2, its kinetic features and the product characteristics of the polymer produced, being conducted from a chemical approach of the subject. Up to now, most of the studies dealing with polymerization reactions in scCO2 are realized in small autoclaves between 2 and 60 ml allowing pertinent fundamental analysis but with poor similarities with an industrial reactor. A keystone of this work is the development of a supercritical reaction calorimeter composed of a 1.3 liters high pressure reactor allowing the kinetic study of the dispersion polymerization, which in turn gives a direct insight into the parameters that control the dispersion polymerization stability, efficiency and mechanism. Based on an adapted heat balance, the calorimeter can give the profile of monomer conversion (thermal conversion). Furthermore, the volume of the reactor allows inserting on-line sensors inside the reactor. This possibility led to the development of an ultrasonic sensor to monitor the polymerization process. The combination between the calorimetric information and the sensor signal shows the potential of these sensors to monitor polymerization reactions in scCO2. By measuring the speed of sound evolution during the course of the polymerization, it is possible to calculate the composition of the medium and thus evaluate the monomer conversion. The analysis of the effects of temperature, stirring speed and impeller types demonstrate that the dispersion polymerization of MMA can be effective under a wide range of operating conditions using the PDMS macromonomer as stabilizer. The experiments point out that the stability of the dispersion polymerization and in turn the rate of polymerization, the polymerization loci1 and the polymer quality depend strongly on the stabilizer concentration but more fundamentally on its degree of solubility in the carbon dioxide. Phase behavior measurements demonstrate that the 5'000 g/mol PDMS macromonomer exhibits a relative good solubility in carbon dioxide and that this solubility can be greatly improved by the presence of the monomer in the mixture. In fact, the monomer acts as a cosolvent for the PDMS macromonomer improving the solvency of the scCO2 (polarity, density). This study demonstrates that the concentration of the monomer in the reacting medium is a key parameter to control the stability of the dispersion throughout the polymerization process. A dispersion polymerization is characterized by polymer-rich particles dispersed in a continuous phase, i.e. the CO2-rich continuous phase. The results give evidence of the fact that the polymerization can take place in both phases depending on the concentration of stabilizer and its solubility in the reacting medium. In this case, bimodal molecular weight distributions and intermediate rate of polymerization are observed. When only one reaction locus is active, monomodal molecular weight distributions are obtained. Furthermore, the locus of the polymerization influences directly the rate at which the polymer is produced and the degree of polymerization. In order to complete the study, a model has been developed demonstrating that diffusion limitations are operative in the dispersion polymerization of MMA in scCO2. A gel effect is present in the polymer-rich phase that leads to the increase of the molecular weight of the polymer produced, as observed also experimentally. Moreover, the presence of this gel effect occurring inside the polymer-rich particles explains the auto-acceleration of the polymerization rate as the conversion increases. The model demonstrates that in the case of an effective dispersion polymerization of the methyl methacrylate in scCO2 the main reaction loci are the polymer-rich particles. ______________________________ 1 A dispersion polymerization is composed of polymer-rich particles dispersed in the CO2-rich continuous phase. The polymerization locus is referred to the phase into which the polymerization can take place.

EPFL2006

Reaction calorimetry in supercritical carbon dioxide - methodology development

Sophie Fortini, Frédéric Lavanchy, Thierry Meyer

Reaction calorimetry is an efficient tool used to obtain kinetic, thermodn. and safety data. A reaction calorimeter, RC1e-HP350, developed in collaboration with Mettler-Toledo GmbH, allows investigating chem. reactions under supercrit. conditions. The main tech. difference, compared with a classical liq. system, is that the whole reactor vol. is occupied by the media. Heat transfer anal. in supercrit. carbon dioxide (scCO2) by the Wilson plot method shows that the behavior of the internal heat transfer coeff. in scCO2 is the opposite of the one obsd. for classical liq. In scCO2 the lower the temp. (above the crit. point) the better the internal heat transfer coeff. The evolution of scCO2 thermodynamical and transport properties near the crit. point are responsible for this behavior. The dispersion polymn. of Me methacrylate in scCO2, with the polydimethylsiloxane monomethacrylate as stabilizer, is used as a model reaction. A polymn. reaction enthalpy of -56.9 +- 2.2 kJ.mol-1 is detd., being in good agreement with previously reported data. The results presented illustrate the accuracy of the heat balance model used and emphasize the potential of reaction calorimetry for the promotion of supercrit. fluids technologies.