Microreactor

In recent years, research activities have paid increasing attention to reactions on a very small scale. The development of manufacturing technology has also enabled the production of miniature components for chemical reactor technology. Microstructured or microchannel reactors are called microreactors. They can also be defined as miniaturized reaction systems. The channel dimensions in microreactors are typically 50 Jim to 2 mm. Microreactor manufacturers provide microstructured mixers, heat exchangers. 

FIG u RE 9.18 General function principle of a catalytic membrane reactor, reversible reaction 

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 

FIGURE 9.19 Membrane reactor in practice. The application dehydrogenation of ethane, CH3CH3 CH2 = CH2 + H2. The ceramic tube consists of a multilayered composite Pt crystallites. (Data from Moulijn, J.A., Makkee, M., and van Diepen, A., Chemical Process Technology, Wiley, 2001.) 

FIGURE 9.20 Microstructured reactor setup for gas-phase studies (parts manufactured by Institut fur Mikrotechnik Mainz Gmbh). 

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Pileni M P 1993 Reverse micelles as microreactors J. Phys. Chem. 97 6961... 

Ethylene oxide catalyst research is expensive and time-consuming because of the need to break in and stabilize the catalyst before rehable data can be collected. Computer controlled tubular microreactors containing as Httle as 5 g of catalyst can be used for assessment of a catalyst s initial performance and for long-term life studies, but moving basket reactors of the Berty (77) or Carberry (78) type are much better suited to kinetic studies. 

Reproducible measurements of absolute activity for sulfur dioxide oxidation catalysts are very difficult to obtain for a number of reasons, including the fact that the reaction is extremely fast. In addition, there are differences in techniques and reporting methods used by the various workers. Pulse microreactors have been used to study quantities of these catalysts as small as 500 mg (83). 

There are many descriptions of various microreactors for hydrogenation. Requirements for these reactors are more exacting because of the need to measure accurately small amounts of consumed hydrogen (22,30,31,36,45,46, 49,59,69,70,91,92,104,111). 

Viable operating eonditions were identified experimentally for maximising the produetion of ethylene, propylene, styrene and benzene from the pyrolysis of waste produets. Data are given for pyrolysis temperature, produet reaetion time, and quench time using a batch microreactor and a pilot-plant-sized reactor. 26 refs. CANADA... 

Apart from the above described core-shell catalysts, it is also possible to coat active phases other than zeolite crystals, like metal nanoparticles, as demonstrated by van der Puil et al. [46]. More examples of applications on the micro level are given in Section 10.5, where microreactors and sensor apphcations are discussed. 

The following are some of the reasons that microreactors can be be used (i) reduced mass and heat transfer limitations, (ii) high area to volume ratio, (iii) safer operation, and (iv) ease of seating up by numbering out. The advantages of scaling down zeolite membranes are that it could be easier to create defect-free membranes and... 

The application of zeolite membranes in microreactors is still in an early stage of development, and suffers sometimes from unexpected problems arising from template removal [70]. However, several application examples of zeolite membranes in microstructured devices have been demonstrated yielding similar advantages as were to be expected from experiences on the macroscale. Because of the high surface to volume ratio of microreactors, the application of zeolite membranes in these systems has great potential. 

Microreactor scale-up is built upon the premise of numbering up channels. Figure 11.1. A single channel is demonstrated with the same geometry and fluid hydrodynamics as a full-scale reactor. Numbering up rehes on creating a massively... 

Degussa/Evonik (Germany) Falling film microreactors for ozonolysis and other chemical applications with IMM reactors [3] and DEMIS project collaboration [4, 5]... 

Jarosch, K., Tonkovich, A., Perry, S., Kuhlmann, D., and Wang, Y. (2005) Microreactor Technology and Process Intensification, in ACS Symposium Series, vol. 914, American Chemical Society, New York, pp. 258-273. 

The Microreactor a systematic and efficient tool for the transition from Batch to Continuous Process Chem. Eng. Res. Des., 84 (5), 363-369. 

Microwave technology has now matured into an established technique in laboratory-scale organic synthesis. In addition, the application of microwave heating in microreactors is currently being investigated in organic synthesis reactions [9-11] and heterogeneous catalysis [12, 13]. However, most examples of microwave-assisted chemistry published until now have been performed on a... 

The advantages of microreactors, for example, well-defined control of the gas-liquid distributions, also hold for photocatalytic conversions. Furthermore, the distance between the light source and the catalyst is small, with the catalyst immobilized on the walls of the microchannels. It was demonstrated for the photodegradation of 4-chlorophenol in a microreactor that the reaction was truly kinetically controlled, and performed with high efficiency [32]. The latter was explained by the illuminated area, which exceeds conventional reactor types by a factor of 4-400, depending on the reactor type. Even further reduction of the distance between the light source and the catalytically active site might be possible by the use of electroluminescent materials [19]. The benefits of this concept have still to be proven. 

J. (2000) Experiences with the use of microreactors in organic synthesis, in Microreaction Technology - IMRET 3 Proceedings of the 3rd International Conference on Microreaction Technology (ed. W. Ehrfeld), Springer, Berlin, p. 181. 

In another investigation, ELP[V5L2G3-90] with three lysines in the N-terminal region was immobihzed on a glass surface in a microreactor to enable temperature-controlled positioning of ELP fusion proteins. For this purpose, the glass surface was first functionalized with A -2-(aminoethyl)-3-aminopropyltrimethoxysilane, followed by glutaraldehyde treatment and reductive amination to immobilize the biopolymer on the surface (Fig. 17b) [132]. 

Static mixing catalysts Operation Monolithic reactors Microreactors Heat exchange reactors Supersonic gas/liquid reactor Jet-impingement reactor Rotating packed-bed reactor... 

Several reactions have been demonstrated using microreactors. One of the potentially more important is the direct synthesis of MIC from oxygen and methyl formamide over a silver catalyst. Dupont have demonstrated this process using a microreactor cell similar to that described above in which the two reactants are mixed, then heated to 300 °C in a separate layer and subsequently passed through another tube coated with the silver catalyst. The estimated capacity of a single cell with tube diameters of a few millimetres is 18 tpa. 

Recently, microstructured reactors have stepped into chemical production [4] and thus microreactor process and plant design, including economic incentives, is the issue at this time. For this purpose, large-capacity microstructured apparatus is needed ( micro inside, fist- to shoebox size outside ) and plant concepts have to be proposed which include all process steps. 

Temperature profile of the phenyl boronic acid synthesis along the major steps of the process flow scheme. The difference in the temperatures of the conventional batch and the microreactor processes stand for the reduction in energy consumption and respective heat-transfer equipment when using the latter [10]... 

In this way, the operational range of the Kolbe-Schmitt synthesis using resorcinol with water as solvent to give 2,4-dihydroxy benzoic acid was extended by about 120°C to 220°C, as compared to a standard batch protocol under reflux conditions (100°C) [18], The yields were at best close to 40% (160°C 40 bar 500 ml h 56 s) at full conversion, which approaches good practice in a laboratory-scale flask. Compared to the latter, the 120°C-higher microreactor operation results in a 130-fold decrease in reaction time and a 440-fold increase in space-time yield. The use of still higher temperatures, however, is limited by the increasing decarboxylation of the product, which was monitored at various residence times (t). 

The authors developed a multi-layered microreactor system with a methanol reforma- to supply hydrogen for a small proton exchange membrane fiiel cell (PEMFC) to be used as a power source for portable electronic devices [6]. The microreactor consists of four units (a methanol reformer with catalytic combustor, a carbon monoxide remover, and two vaporizers), and was designed using thermal simulations to establish the rppropriate temperature distribution for each reaction, as shown in Fig. 3. 

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