Marie-Agnès Schneider
Following several tragic chemical accidents that have occurred in recent years, some directives concerning chemical process safety have been issued. The Seveso directive, for example, requires a precise description of the consequences of a possible chemical accident assuming a worst-case scenario. However, to build such scenarios and, hence, make chemical processes safe, profound knowledge of the kinetic and thermal parameters of the reactions in question is necessary. These parameters are usually determined by calorimetric methods. However, in the existing commercial calorimeters the characterization of fast, exothermal reactions raises several problems. Indeed, on the one hand, it is difficult to maintain the isothermal conditions required to determine the kinetics simply and precisely. On the other hand, the calorimeter should be able to measure the heat flow generated as soon as the reactants are brought into contact. It should be able to do this without the need for a period of thermal equilibration that would perturb the measurements, and without any limitations due to the mixing of the reactants. The aim of this work is to develop a calorimetric method that is particularly adapted to studying fast exothermal reactions. The proposed system combines a microreactor with a commercially available microcalorimeter. In the first part of this project a flexible technique for constructing microreactors was chosen. The technique had to be precise, reproducible and inexpensive. The geometry of the microreactor had to fit into the cavity of the commercially available microcalorimeter. The technique chosen was the silk-screen printing of a thick film dielectric. This method makes it possible to build quickly and at low cost numerous microreactors that can be regarded as disposables. The geometry of the reaction channel can be varied and can be adapted to the type of reaction. Once the microreactors were built, the degree of mixing obtained in the microchannels was estimated first by simulation and then by experiment. The flow in the reaction channel was found to be purely laminar and the mixing time corresponded to the time for radial diffusion. Due to the small size of the channels, the mixing time was found to be adequate and not limiting for the characterization of fast reactions. The microreactor was then inserted into the cavity of the commercial calorimeter. The resulting microsystem was calibrated using the neutralization reaction of sulphuric acid with NaOH. This system was further modified to optimize the thermal transfer between the reaction channel and the sensor of the microcalorimeter and to increase its thermal efficiency. Finally, an electrical preheating system of the incoming liquids was put in place and tested. Once the quality of the thermal signal had been optimized, kinetic studies of chemical reactions could be undertaken. First, a model reaction was studied in order to validate the results obtained with the microsystem and to avoid the risk of systematic errors. In the second stage, a previously unknown fast exothermal reaction was characterized. The heat flows measured during the reaction reached 160'000 W·kg-1 but the conditions, however, remained completely isothermal. The global kinetics of this reaction as well as its activation energy were determined.
EPFL2004
