Residence times, microreactor technology

Reduction of residence time is the issue to high selectivity, as the product degrades and side products also form. In contrast to the conventional technology that needs 14 h to accomplish the reaction, a microreactor that enhances selectivity accomplishes the same in about 18 min. The reason for the better microreactor performance is not detailed and if the process parameters (e.g. temperature) were changed. 

Reactions occurring in microfluidic systems have characteristic benefits for polymerization reactions. First, microreactors provide accurate control of the reaction conditions such as temperature, reaction (residence) time and mixing. Second, microfluids based on laminar flow provide precise manipulation. This technology only allows the production of various solid polymeric materials which are synthesized under microfluidic control conditions. Third is ease in parallel operation and on-line processing. This advantage imparts upon the microreactor great potential as a... 

At this point, one may add a second substance to the pool at low temperature to react with the carbocation [8]. The more versatile cation flow strategy involves pumping the cold solution of the cation into a mixing chamber of a microreactor into which the other reactant is simultaneously being pumped. The flow rates of the two species can be controlled such that reaction is complete within the residence time of the microreactor chamber. This mode of operation can be elaborated, further mixing the product with a third reactant downstream of the microieactor chamber. The technology can be readily adapted to combinatorial synthesis of families of related substances [8]. 

For heterogeneous catalysts, tandem reactor technology also relies on the fact that each polymer particle is in fact a microreactor operated in semibatch mode, into which monomers and chain-transfer agents are fed continually, while the polymer formed never leaves the microreactor. In this way, polymer populations with different average properties are produced in each reactor and accumulate in the polymer particle microreactor, as illustrated in Figure 8.37. In theory, an optimal balance does exist between the fractions of these different populations to meet certain performance criteria. This creates a truly fascinating reactor and product design problem because the fractions of the different polymer populations per particle will be a function of the residence time distribution in the individual reactors in the reactor train. 

It is now well established that microreactor technology enables the residence time of chemical reactions to be substantially reduced [35-37]. Fundamentally, this improvement in reaction efficiency is because the mixing [38,39] within the microreactors is highly efficient and furthermore the reaction itself may be intensified by application of high-temperature conditions [40]. Furthermore for PET imaging since only nanogram quantities of radiopharmaceutical are actually needed, consequently these volumes of product are very easily made within even the smallest microreactors on the market. 

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