Precise Temperature Control in Microflow Systems

It is reasonable to consider that heat transfer through the reactor wall is very fast in microsystems in comparison with macrosystems because of the high surface-to-volume ratio of microspaces as described above. However, it is rather difficult to evaluate the interior temperature of microsystems. Physical methods for temperature measurement seem to be problematic. The introduction of a thermometer inside a microsystem might change the nature of the flow, because the volume of a thermometer is not negligible in comparison with the volume of the microreaction space. It is also important to note that the heat capacity of the thermometer and heat transfer through the thermometer cannot be ignored because the volume of the solution in the microreaction space is very small. 

Chemical methods can also be applied for the measurement of the interior temperature of microsystems. A fluorescent molecular thermometer using temperature-induced phase transitions seems to be quite attractive,but it needs a transparent window on the wall of the microchannel for the optical measurement. However, this may change the heat transfer ability of the wall. 

A solution of azobisisobutyronitrile (AIBN), which is one of the most popular radical initiators, in toluene is introduced to a microflow system, which is heated in an oil bath (80 or 100°C). Aliquots of the outlet solution are collected and the amount of unchanged AIBN in the outlet solution is determined by high-performance liquid chromatography (HPLC). 

If such an unstable reactive species can be transferred to another location to be used in the subsequent reaction before it decomposes, the reaction can be conducted without lowering the temperature. If it is possible to do so, chemical conversions that are impossible in a conventional way should be made possible in some cases. In order to accomplish chemical reactions in such a way, we need a flow reactor with a short residence time. 

It should be kept in mind, however, that the residence time calculated in this way is the mean residence time. There is a distribution of residence times in a reactor. The flow speed of the fluid near the channel wall surface 

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