Over the last five years, many activities have focused on the unexploited field of carrying out reactions on small scales. Due to the rapid development of new components, this paper deals with recent developments only in a compressed form. An important point is the analysis of possible plant concepts for microreactors and whether these are a sensible option. Due to the enormous difference in size between the microchannels and the fluid periphery of possible components this is not just a technical question. It touches on the microtechnology concept as a whole. The direction in which the field should be developed and which measures can be taken to influence its development are questions that are addressed here with respect to the big industrial interest in microreactors.
Reactors for Photochemical and Electrochemical ProcessingMicroreactors may possibly be capable of reevaluating processes which so far have been poorly developed both in the lab and in the chemical industry. Among such processs certainly are electrochemical reactions and particularly photochemical reactions.
Photochemical and Electrochemical ProcessesIn view of the low quantum yield of photochemical processes even for respective lab-scale equipment, it would appear that guiding of thin fluid layers in transparent microreactors, e.g. mixer-reactors, in close proximity to a radiation source should be beneficial. At the Massachusetts Institute of Technology (MIT), Cambridge (USA), researchers carried out the pinacol synthesis using benzophenone as reactant in a pyrex/silicon microreactor equipped with a miniature UV lamp (k = 366 nm) (see Fig. 20) [98,99].An electrochemical microreactor was developed at IMM which combines the advantages of thin-layer cell technology (namely, current efficiencies up to 100 %, absence of conducting salt) and microchemical processing (e.g., isothermal processing, good mass transfer). The electrochemical microreactor has a parallel-plate architecture. Using this device, an increase in selectivity for the synthesis of anisaldehyde from 4-methoxytoluene was detected [100].
Heat Transfer
Fast Heating up for Chlorination of AlkanesThe chlorination of alkanes [102] performed by the Axiva company serves as a good and successful example based on the motto ªas much miniaturization as necessary, not as possibleº. The tube reactors used till now needed too much time to reach a certain temperature level or even exhibited temperature maxima above the target value, in effect similarly detrimental as a hot spot. To improve these features, IMM developed monolithic heating modules that are externally heated. The . UV absorption of reaction mixtures resulting from pinacol reaction using benzophenone in isopropanol at various flow rates and residence times, respectively. A comparison of these results to HPLC measurements at the same mixtures reveals that these UV in-line spectra are accurate and hence can be used to monitor the course of reaction. Source: K. Jensen, MIT, Department of Chemical Engineering, Cambridge (USA)
