Wall-coated microreactors

Scheme 37 The use of a wall-coated microreactor in the epoxidation of 1-pentene 143.

For the negligible heat transport limitations in the case of the wall-coated microreactor the equations have to be different due to the geometric changes for derivation of the criteria. Looking at Fig. 1 it becomes dear that two cases have to be distinguished with regard to the microreactor stacking scheme A) the strictly cooled case and B) the monolithic system, where most of the channels with reaction are located next to each other. 

Due to short diffusion pathways in the microsystem, the overall mass transport in the phases or the transfer via phase boundaries is often magnitudes higher than in conventional reactor systems. However, with regard to the desired high loadings with catalyst and low cost for fluid compression or pumping, the mass transfer to the catalyst and the mass transport within porous catalyst still has to be effective. As for the heat transport the differentiation between packed bed and wall-coated microreactor is necessary for mass transport considerations. The mass transport in packed bed microreactors is not significantly different to normal tubular packed bed reactors, so that equations like the Mears criteria (Eq. 6) can be used. 

Lopes JP, Cardoso SSS, Rodrigues AE. Interplay between channel and catalyst operating regimes in wall-coated microreactors. Chemical Engtneering Journal 2012J227 42-55. 

Lopes JP, Alves MA, Oliveira MN, Cardoso SS, Rodrigues AE. Regime mapping and the role of the intermediate region in wall-coated microreactors. Chemical Engineering Science 2013 94 166-184. 

Table 29 Illustration of the effect of channel depth on the illuminated specific surface areas per unit of liquid in a microreactor (constant channel width of 500 pm) and the yield of N-ethylbenzylamine 195 obtained using a Ti02 wall coating...

Since 2004, many articles on preparation of zeolite MMs have been published, on such areas as MFI or Sil-1 zeolite etched on the Si substrate for gas separation applications, and MMRs for KCR and fine chemical synthesis (Coronas and Santamaria, 2004 Kwan et al, 2010 Wan et al., 2001 Yeung et al, 2005). Coronas and Santamaria (2004) have reported on the use of zeolite films and interfaces in micro-scale and portable applications, including the removal of volatile organic compounds from indoor air, recovery of catalysts in homogeneous reactions, zeolitic microreactors and microseparators, for example. Moreover, zeolite coated microreactors and microseparators exhibit high surface-to-volume ratio, and are capable of high productivity as a result of the good contact between reactants and catalyst wall. 

Both reactor types R3 and R4 use the segmented flow (Taylor) principle. They are divided into two categories R3 has very small channels (<1 mm) and R4 are monolith reactors (honeycomb), well developed on the laboratory scale with at least one example of industrial application. Category R3 includes single-channel and multi-ple-channel reactors [10], etched in silicon [10] or glass [10,11], with wall-coated or immobilized catalysts in the case of gas-liquid-solid additions [12], and capillary microreactors for gas-liquid-liquid systems [13]. 

Of course, not all multiphase microstructured reactors are presented in Table 9.1. Either because they have attracted (too ) little interest, because they may have been qualified as microreactors in spite of their overall size but caimot be considered as microstmctured , or because they combine several contacting principles. Examples are a reactor developed by Jensen s group featuring a chaimel equipped with posts or pillars, thus resembling more a packed bed but with a wall-coated layer of catalyst [20], and a string catalytic reactor proposed by Kiwi-Minsker and Renken [21], that may applied to multiphase reactions. 

The superiority of microreactors for reduced deactivation due to less thermal load on the catalyst has been proven several times (e.g. [47]), even benchmarked to a fixed bed [18] or to a foam [19]. However, one should be careful when operating the catalyst in a microreactor at its kinetic limit. The deactivation behavior of a specific catalyst could be more visible than in a conventional fixed bed or even coated foam. The mass of thin wall-coated catalysts operated free from mass transfer limitations could be much less for reaching 100% conversion. However, there is no backup catalyst mass. Hence additional catalyst mass has to be considered for process layout [20]. Also, leaks and the parallelizing of microstructures (concerning their feed distribution and heat distribution) are a challenge that can influence the catalyst stability and thus the operation of a microreactor [40]. 

Table 1. Categories for decision on p>acked bed microreactor versus various options of catalyst wall coatings in microreactors for heterogeneous catalysis...

Lopes et al. [24, 43] presented a conceptual analysis of the different operating regimes that may appear in a wall-coated monolith/microreactor. There are several particularities in the proposed methodology compared with other studies in the... 

Rebrov, E.V., Berenguer-Murcia, A., Skelton, H.E., Johnson, B.F.G., Wheatley, A.E.H., and Schouten, J.C. (2009) Capillary microreactors wall-coated with mesoporous titania thin film catalyst supports. Lab Chip, 9 (4), 503-506. 

Various reactor types have been used as the foundation for microreactor designs, including coated wall reactors, packed-bed reactors, structured catalyst reactors, and membrane reactors. 

A measure that has been the subject of extensive publication is that of microreactors with catalytically coated walls (7,8). A microreactor has been defined as a miniaturized reaction vessel with characteristic dimensions in the range 10-300 pm which has been fabricated using state-of-the-art high-precision engineering (7). Such reactors exhibit well-defined laminar-flow patterns and permit facile scale-up by simple numbering up of the number of channels and flexible... 

Figure 3.2 GPMR used for biocatalytic transformations with immobilized enzymes [22]. (a) the fully assembled microreactor, (b) microstructured multichannel plate, and (c) electron micrograph of the wash-coat layer of y-aluminum oxide covering the microchannel walls.

In addition to flow maldistribution, small deviations in the channel diameter (which typically originate from imperfect manufacturing) cause a broadening of the RTD. The deviations may also be the result of nonuniform coating of the channel walls with catalyst material (Section 5). If the number of parallel channels is large (i.e., N > 30), a normal distribution of the channel diameters with a standard deviation a can be assumed. The effect of the diameter variation on the pressure drop in the microreactor can be estimated on the basis of the relative standard deviation, G = ffd/ t [81 ] ... 

In general, the geometric surface area of the microchannels in a typical microreactor is insufficient to carry out catalytic reactions at high performance. Consequently, the specific surface area must be increased, either by chemical treatment of the channel walls or by coating them with a porous layer. The porous layer may serve directly as a catalyst or as a support for the catalytically active components. Various techniques to introduce the catalyst have been developed and are summarized in the following sections [147,148]. 

A method for coating microchannel walls with layers as thick as 25 pm was developed by Stefanescu et al. [181]. The microreactor was built from FeCrAl (Aluchrom ). The metal surface was first chemically treated in several steps and afterward annealed at 1200 °C for 1 h to trigger the segregation of aluminum and the formation of an alumina layer on the metallic surface. An alumina washcoat was subsequently deposited from a slurry onto the microstructure and characterized by various physical methods. The authors varied the properties such as viscosity, particle size, and pH of the slurry. Acrylic acid, a component used as dispersant and binder, was found to be particularly important for the adhesion of the alumina layer. 

Microreactors can be used for either gas-phase or liquid-phase reactions, whether catalyzed or uncatalyzed. Heterogeneous catalysts (or immobilized enzymes) can be coated onto the channel wall, although on occasion the metal wall itself can act as the catalyst. Gas-liquid contacting can be effected in the microchannels by either bubbly or slug flow of gas, an annular flow of liquid, or falling liquid films along the vertical channel walls. Contact between two immiscible liquids is also possible. The use of microreactor systems in the area of biotechnology shows much promise, not only for analytical purposes but also for small-scale production systems. 

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