(209f) A Microfluidic Calorimeter for Reaction Enthalpy and Kinetics Measurements

Kinetics and thermodynamics characterisation of chemical reactions is required for chemical process development and safety. We know that both phenomena, heat transfer and chemical kinetics, are present in chemical reactions. One way to control and measure those phenomena is calorimetry. The characterisation of exothermic chemical reaction is well developed and different kinds of calorimeter exist. Nevertheless, they are not simultaneously adapted to the use of microvolumes, high thermal sensitivity, good control of the reagent mixing and quasi isothermal conditions. To this end, the recent development of MEMS (Micro-Electro-Mechanical Systems) allows the investigation of new approaches to measure the enthalpy of chemical reactions. The principal advantage is the small size of the microchannels which permits the mixing control of the reactants with very low Reynolds laminar flows. However, the measurement of temperature at a small scale represents the main difficulty. In fact, the temperature information in such devices is among the most important parameters. Today, only a few studies deal with this. One method consists in developing localised temperature microsensors. The method, applied here, measures a global heat flux around the microchannel. A new microfluidic differential flow calorimeter working in quasi isothermal conditions is developed. The main idea is to implement a microfluidic chip with integrated flow channels (area = 100x100 µm²) taken as a microreactor and a thermopile (Seebeck effect) used to measure the global heat flux produced by the chemical reaction. The advantages of this device are the following ones: the use of small reagent volumes, the thermal properties of the microfluidic chip which offers quasi isothermal conditions, and the low cost of the microfluidic chip as well as the thermopile adapted for industrial tests. Firstly, a thermal modelisation of the system is realized using an analytical development in order to analyze the thermal behaviour. These results allow to choose the optimised microfluidic wafer that offers the higher isothermal conditions, and they show that less than 2 % of the generated heat flux is lost (figure 1). Thus, the microcalorimeter gives an excellent control of the thermal transfer. Secondly, this device is calibrated when using a heat source produced by Joule effect. For that purpose, a calibration chip including an electrical resistance is manufactured. Various electrical powers are applied to the system and the measured heat flux is compared to the injected heat flux which determines the conversion coefficient (figure 2). Thus, the voltage signal can be converted into a heat flux by this coefficient. Then, the enthalpy estimation of a well known reaction (strong acid-base) is performed in order to validate this device. The experiment comes out the following way: when the reagents are injected, the heat flux increases until raising a steady state and then decreases when injection is stopped. This experiment is realized at different flow rates, so that the reaction enthalpy can be determined (figure 3). The enthalpy value obtained is closed to the literature value (56 kJ/mol) which validates the microcalorimeter. After calibration, the work consists in explorating the possibilities offered by this microcalorimeter. The estimation of mixing time in the reaction microchannel comes through the microcalorimeter and is compared to the calculated time for radial diffusion. A specific microfluidic chip is designed, and a quasi-instantaneous reaction has to be chosen to have the mixing step as the limiting step. Afterwards, the kinetics study of chemical reactions is realized through determination of the activation energy and the kinetics parameters. This study reveals how kinetics parameters can be deduced from the microcalorimeter raw data, and is validated by characterizing a well-known reaction. Some experiment sets are performed at different flow rates and different setting temperatures. Finally, the same experiments are realized with a droplet flow and the kinetics results will be compared to the results obtained previously. The droplet flow shows advantages: the analogy of a droplet with a single nanolitre batch reactor, an efficient mixing, the isothermal condition increase, the absence of axial dispersion, and the possibility to use viscous product reaction such as polymerization. To conclude, it is important to note that the developed micro-calorimeter allows the measurement of many chemical data, including enthalpy. Experiments can be done in quite good isothermal conditions, under continuous flow and in thermal steady state. The required volumes of products go from 1 nL to 1 µL, and the detectable minimum flux is around 50 µW. The thermopiles used (Seebeck effect) offer a great sensitivity of measurement (3 and 1 µW) and are perfectly adapted to carry out measurement in steady state.