Microreactors for processing of hazardous and explosible reactions

Microstructured reactors are well known to provide far better heat exchange characteristics than attainable in macroscopic batch or flow-through reactors due to their high surface-to-volume ratios. In the last decade, a large number of studies have impressively demonstrated that the accumulation of strong reaction heats and hot spots, which result in unwanted side, subsequent and decomposition reactions, can be successfully surpressed in microreactors. Consequently, the use of microreactors greatly reduces the hazardous potential associated with reactions that are highly exothermic or potentially explosive. Greater safety is also attained with toxic substances due to the small hold-up of microfluidic devices. Here we report on the use of microreactors for the safe processing of strong exothermic reactions in the liquid and liquid/liquid regime, such as nitrations, oxidations, esterifications, etc. The hazardous potential of such reactions often arises from the huge reaction enthalpy and/or the thermolability of the reaction products or intermediates. Microreactors have been particularly used in our studies to systematically investigate strong exothermic reactions under unusual process conditions such as higher temperatures, higher concentrations or varied stoichiometries which are not possible to apply on a macroscopic scale. Such parameter screenings provide valuable routes for process intensification in terms of yield and selectivity but also with respect to energy savings and improved safety. Hence, microreactors have been deliberately used as tools for safety analyses to investigate experimentally worst case scenarios at the threshold of decomposition and runaway reactions. Moreover, we use microreactors also as measurement tools to quantify the heat release under strong exothermic process conditions. For this purpose, we have developed a continuous microL-flowthrough calorimeter which consists of a microreactor embedded between thermoelectric modules (Seebeck and Peltier elements). This new mirol-calorimeter has a very small time constant of about 2 s which is by a factor of 20 - 30 smaller than that of conventional reaction calorimeters. Hence, it is ideally suited to measure enthalpies of fast and highly exothermic reactions under both isothermal and continuous process conditions.