Microreactors for catalytic reactions

This chapter focuses on description of microstructured reactors for gas-solid, gas-liquid, and three-phase processes, flow regimes, mass transfer considerations for various configurations of microchannels, design criteria, and evaluation of each reactor type. Special attention is devoted to Taylor flow in microchannels, as this flow regime is the most adapted for practical engineering applications. 

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Applications of microreactors to biphasic catalytic reactions constitute a topac of interest. The benefits of having an exceedingly high surface-to-volume ratio and efficient mass-transfer in microchannels have led many researchers to study continuous flow systems using microreactors for catalytic reactions. The excellent mass transfer characteristics within and between the catalyst carrier phase and reaction medium, together with the minimal catalytic pore diffusion resistances at the micrometer scale, make such biphasic catalysis an attractive alternative to conventional catalysis operation (Wieflmeier, 1996 Rahman et al., 2006). 

A related effort is FORSiM (Fast Oxidation Reaction in Si-technology-based Microreactors) which is funded by the Dutch Technology Foundation and is a cooperative venture between the University of Twente and the Technical University of Eindhoven. The objective of this work is to build and operate the first microreactor for catalytic partial oxidation for small-scale and on-demand hydrogen production61. 

As Table 2-3 shows, imprinted polymers have been mainly used as separation media (mostly in chromatography). Of special interest is the enantiomeric resolution of race-mates. Further applications are as immunosorbents and chemosensors. The cavities in the imprinted polymers have also been used as microreactors for selective reactions and, more interestingly, as the active sites of catalytically active polymers. In 1998 nearly 100 papers appeared in the literature on molecular imprinting, together with one book [114] another book is imminent [115]. 

J.F. (2007) Microreactor technology and process miniaturization for catalytic reactions - A perspective on recent developments and emerging technologies. Chem. Eng. Sci., 62 (24), 6992-7010. 

The application of solid catalysts in microreactors has been studied for different processes. Automated laboratory systems were applied for catalyst screenings [53,54]. Ag/Al and Ag/Al203 were applied in microflow-through reactors for the partial oxidation of ethylene [55]. For catalytic applications, a microflow-through arrangement with a static micromixer was used to prepare Au/Ag nanoparticles [56]. Microfluid segments are also of interest for catalytic reactions in microreactors [57]. 

For catalytic reactions many multiphase microreactors contain catalysts coated on walls, incorporated in thin nonporous films or in packed beds... 

This section starts with a classification of phase-contacting principles according to the type of catalytic bed. Advantages and disadvantages of the reactor types are explained, followed by a discussion of criteria for reactor selection and an overview of purchasable microreactors for catalytic gas-phase reactions. 

Multiphase catalytic reactors are employed in nearly 80% of industrial processes with annual global sales of about 1.5 trillion, contributing around 35% of the world s GDP [17]. Microreactors for multiphase reactions are classified based on the contact principles of gas and liquid phases continuous-phase contacting and dispersed-phase contacting [18]. In the former type, the two phases are kept in continuous contact with each other by creating an interface. In the latter case, one fluid phase is dispersed into another fluid phase. In addition, micro trickle bed operation is reported following the path of classical chemical engineering. The study of mass and heat transfer in two-phase flow in micro trickle bed reactors still remains as a less... 

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. 

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. 

Multiphase catalytic reactions, such as catalytic hydrogenations and oxidations are important in academic research laboratories and chemical and pharmaceutical industries alike. The reaction times are often long because of poor mixing and interactions between the different phases. The use of gaseous reagents itself may cause various additional problems (see above). As mentioned previously, continuous-flow microreactors ensure higher reaction rates due to an increased surface-to-volume ratio and allow for the careful control of temperature and residence time. 

The catalytic activity of SBA and AISBA samples toward cumene cracking were tested in a continuous flow fixed-bed microreactor system with helium (25 mL min 1) as carrier gas. The catalyst load for the tests was 100 mg and the catalyst was preheated at 573 K under helium flow for 3 h. For the reaction, a stream of cumene vapor in helium was generated using a saturator at room temperature. The reaction products were analyzed by gas chromatography. 

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