(338b) Supercritical Reaction Calorimetry: a Novel Route to Supercritical Fluid Reaction Monitoring

Supercritical fluids are intensively investigated during the past decade as alternative, environmentally benign reaction solvents, in order to eliminate the use of toxic and flammable organic solvents and the generation of large volumes of aqueous waste. The most common among them is carbon dioxide, due to its easily attainable critical temperature (71.1°C) and pressure (7.38MPa). A new analytical technique is derived from the merge of classical heat flow reaction calorimetry and the supercritical fluid technology, namely the supercritical reaction calorimetry.

The experimental set-up used, consists of a liter scale autoclave (approx. 1.3liter), coupled with a Mettler-Toledo RC1e calorimeter. The whole system is fully computer controlled and able to operate up to 300°C and 35MPa. In addition an ultrasonic sensor is inserted in the reactor, and a digital camera is used to observe reaction conditions through a sapphire window. These two devices allow us to couple calorimetric measurements with optical observations and speed of sound information.

The primary challenge in supercritical reaction calorimetry is that the reaction medium occupies not only a part of the reactor as in classical calorimetry, but all the available volume. For that both the reactor cover and the reactor flange have to be thermally controlled, and a special micro-heat exchanger is used to refine the heat signal during reactants' dosing. Such developments result in changes in the heat balance equation, which is used to calculate the heat of reaction and consequently monitor the reaction evolution. To validate the calorimeter, the calibration heater was used to simulate an exothermic reaction by introducing a known amount of heat, and the response of the system was monitored. Results show that the calorimeter reacts accurately to heat flows and that the hypothetical reaction is better monitored with the modified heat balance equation. An example of chemical reaction monitoring will be presented.

Furthermore when working with supercritical fluids one has to take into account that the thermodynamical and transport properties of these fluids in the vicinity of the critical point, demonstrate drastic changes. These changes greatly influence heat transfer through the reactor wall, which is crucial information for the development of supercritical fluids' processes. Results of a non-dimensional analysis show that the Nusselt correlation is valid for supercritical carbon dioxide despite the particular behavior of its properties. Furthermore, the film heat transfer coefficient of supercritical carbon dioxide was measured and the results demonstrate a decrease with increasing temperature. Such behavior is completely opposite to that of classic liquids, and leads to interesting conclusions on the heat transfer of supercritical fluids' processes.

Finally, optical observations of phase transitions are realized and coupled with speed of sound measurements, indicating clearly the passage over or near the critical point and into the supercritical region.