Microreactors mass transfer efficiency

This reaction serves as literature-known model reaction to characterize mass transfer efficiency in microreactors [318]. As it is a very fast reaction, solely mass transfer can be analyzed. The analysis can be done simply by titration the reactants are inexpensive and not toxic. 

The mass transfer efficiency of the falling-film microreactor was determined at various carbon dioxide volume contents (0.1,1.0 and 2.0 M) [318]. The molar ratio of carbon dioxide to sodium hydroxide was constant at 0.4 for all experiments, that is, the liquid reactant was in light excess. The higher the base concentration, the higher was the conversion of carbon dioxide. For all concentrations, complete absorption was, however, achieved at different carbon dioxide contents in the gas mixture. The results show the interdependency of the carbon dioxide content, the gas flow velocity and the sodium hydroxide concentration. 

The mass transfer efficiency of the falling-film microreactor and the microbubble column was compared quantitatively according to the literature reports on conventional packed columns (see Table 4.3) [318]. The process conditions were chosen as similar as possible for the different devices. The conversion of the packed columns was 87-93% the microdevices had conversions of 45-100%. Furthermore, the space-time yield was compared. Flere, the microdevices resulted in larger values by orders of magnitude. The best results for falling-film microreactors and the microbubble columns were 84 and 816 mol/(m3 s), respectively, and are higher than conventional packed-bed reactors by about 0.8 mol/(m3 s). 

Jahnisch et al. used an IMM falling-film microreactor for photochlorination of toluene-2,4-diisocyanate [38] (see also Chapter 4.4.3.3, page 161). As a result of efficient mass transfer and photon penetration, chlorine radicals were well distributed throughout the entire film volume, improving selectivity (side chain versus aromatic ring chlorination by radical versus electrophilic mechanism) and spacetime-based yields of l-chloromethyl-2,4-diisocyanatobenzene compared to those obtained using a conventional batch reactor. 

More recently, microreactor technology has entered the field of biocatalysis enzymes are used for synthesis rather than for diagnostics. The concept behind the use of biocatalytic microreactor systems is in fact twofold. First, a miniaturized reactor allows an efficient use of small amounts of enzyme, when enzyme kinetics determination is involved. Second, the classical advantages of microreactors in synthesis, namely, better control over heat- and mass-transfer... 

Besides using microreactor devices for efficient screening of biocatalysts, these can also be applied for enzymatic synthesis. Biocatalyzed reactions have recently received much attention because of the efficiency and selectivity that enzymes have to offer, combined with their ability to act under mild conditions [422,423]. Microreactors offer in this respect the advantage that enzymes can be more optimally used as a result ofthe higher mass-transfer properties of microchannels and the high surface-... 

Many existing chemistries can be studied over very wide variable ranges efficiently, safely and rapidly in microreactor equipment, especially when the reactor is interfaced to on-line analysis equipment. Unfortunately, a significant number of reactions are still difficult to study in this equipment. However, notably, since microreactors can also be numbered up to provide commercial scale production, new chemical routes to product production may be possible. Thus, reactions that would not have been deemed reasonable for large volume production in traditional processing equipment due to problems with heat and mass transfer issues may now be possible in microscale reactors. The good news is that it should be possible to explore these new chemistries much more rapidly. 

In batch processing aggressive reactants are, typically, diluted to prevent thermal overshooting and runaway. Even then they often are added drop-wise, to allow heat transfer to be adjusted to heat release. In some cases, this may take over half an hour or so. This unnecessarily prolongs processing time and, also, the reaction then is carried out for a considerable part under totally changing reactant concentrations (from zero to full-load content). Conversely, microreactors with their efficient heat and mass transfer have the potential to contact the full reactant load all at once . In addition, microreactors can cope with concentrated solutions or even piue liquid reactants. Several examples are known for which such all at once or solvent-free procediues are feasible in microreactors with reasonable selectivity, whereas the... 

Schwarz et al. [85] studied the efficiency of different microstructured mixers followed by microchannels and their influence on the space time for obtaining high product yields. With increasing mass transfer performance of the micromixer and decreasing channel diameter of the microchannel reactors, shorter reaction times of several minutes at lower reaction temperatures compared to conventional batch reactor were obtained. Similar observations are reported for the synthesis of biodiesel in capillary microreactors [86] and in zigzag microchannels [87]. 

The reaction progress is monitored ofF-Une by HPLC. Flow rates, residence times and initial concentrations of 4-chlorophenol are varied and kinetic parameters are calculated from the data obtained. It can be shown that the photocatalytic reaction is governed by Langmuir-Hinshelwood kinetics. The calculation of Damkohler numbers shows that no mass transfer limitation exists in the microreactor, hence the calculated kinetic data really represent the intrinsic kinetics of the reaction. Photonic efficiencies in the microreactor are still somewhat lower than in batch-type slurry reactors. This finding is indicative of the need to improve the catalytic activity of the deposited photocatalyst in comparison with commercially available catalysts such as Degussa P25 and Sachtleben Hombikat UV 100. The illuminated specific surface area in the microchannel reactor surpasses that of conventional photocatalytic reactors by a factor of 4-400 depending on the particular conventional reactor type. 

Most of these substitution reactions have already been investigated in microstruc-tured reactors, some of them more intensively than others. Researchers were in particular interested in finding routes to process optimization and process intensification compared with macroscopic processes by making use of the improved heat and mass transfer characteristic of microstmctured reactors. In this context, microreactors turned out to be efficient tools for systematic and fast parameter screenings under conditions of continuous processing, consuming only small amounts of chemicals. 

Comparison of a single-tube packed-bed reactor with a traditional batch reactor was also published in the case of o-nitroanisole hydrogenation, not for productivity purposes but rather as laboratory tools for kinetic studies (Scheme 9.11) [46]. It was shown that the better efficiency of mass transfer enables the microreactor to obtain intrinsic kinetic data for fast reactions with characteristic times in the range 1-100 s, under isothermal conditions, which is difficult to achieve with a stirred tank reactor. However, the batch reactor used in this study was not very well designed since a maximum mass transfer coefficient (kia) of only 0.06 s was measured at 800 rpm, whereas kia values of up to 2 s are easily achieved in small stirred tank reactors equipped with baffles and mechanically driven impellers [25]. This questions the reference used when comparing microstructured components with traditional equipment, with the conclusion that comparison holds only when the hest traditional technology is used. 

Some companies have started to announce the commercial availability of micro-reactor-based fuel processors in the past and present. Additionally, as more and more companies have the intention to explore the potential of microreaction technology for fuel conversion and small-scale electricity production, a commercial breakthrough may therefore be feasible in the future. Fast and efficient heat and mass transfer in microreactors and their operating strategies are prerequisites for such success. However, to obtain commercial breakthrough, a proof of principle in everyday life, i.e. long-term stability of catalysts under operation with real fuel and also of microreactor performance and material over several thousand hours, is still lacking - as for conventional fuel processors also. 

Microreactor technology (MRT) satisfies three basic requirements for a chemical reaction it can easily provide for an optimal reaction time (contact time), introduction or removal of heat into the reaction zone, and sufficient mass transfer. The reduced dimensions of MRT systems make them applicable to reactions that require good transport properties. An important feature is their high surface area-to-volume ratio. This is particularly important for reactions that require efficient heat transfer, that is, highly exothermic or endothermic reactions. In a traditional stirred tank reactor, the reaction rate can be compromised because of the limited heat transfer capacity and, in the case of hazardous reactions such as nitrations, a run-away might be induced by inefficient heat transfer. In the case of... 

---