Microreactor catalytic wall

This intermediate scale affords a preliminary validation of the intrinsic kinetics determined on the basis of microreactor runs. For this purpose, the rate expressions must be incorporated into a transient two-phase mathematical model of monolith reactors, such as those described in Section III. In case a 2D (1D+ ID) model is adopted, predictive account is possible in principle also for internal diffusion of the reacting species within the porous washcoat or the catalytic walls of the honeycomb matrix. 

For three types of microstructured devices - the multichannel catalytic wall microreactor, the micro packed bed, and the catalytic metallic foam - the mass transfer effectiveness was calculated with the relations for mass transfer and pressure loss given in the previous sections. For the metallic foam, characteristic data were taken from [46] and [48]. The effectiveness is not dependent on the size or length of the device. 

From a design point of view, it is important to understand how to introduce two separate flows into one microchannel. In addition, the relative velocities of the flows have a significant influence on the resulting pattern of the multiphase flow. Another important aspect is how to introduce the catalysts active phase for a heterogeneous reaction where the solid catalyst is coated on the wall and/or placed as a packed bed inside a reactor. Even though the packed bed reactors are easier to fabricate than catalytic wall microreactors (CWM), CWMs are still favoured in most cases due to lower pressure drop and as they exhibit higher heat transfer rates (Kin et al, 2006). 

To combine the advantages of packed-bed and catalytic wall microreactors, catalytic bed microreactors were proposed recently. In this novel reactor design, the catalyst is applied on metallic filaments or wires which are incorporated in a microreactor, leading to a low pressure drop and a nanow residence time distribution [87-89]. By insertion of metallic wires a uniform gas distribution and a reduced risk of temperature gradients is obtained. However, similarly to catalytic wall microreactors, an increase in the specific surface area of the grid or wire is required. In addition to metallic wires and grids, modified ceramic tapes can also be used as a catalyst support [90]. 

The highest pressure drops usually occur in packed-bed microreactors with axial flow design. A reduced pressure drop while maintaining catalytic area can be achieved by a cross-flow design packed-bed microreactor [82]. To keep pressure drops down, the use of catalytic wall and catalytic bed microreactors is recommended. 

Several methods for the incorporation of catalysts into microreactors exist, which differ in the phase-contacting principle. The easiest way is to fill in the catalyst and create a packed-bed microreactor. If catalytic bed or catalytic wall microreactors are used, several techniques for catalyst deposition are possible. These techniques are divided into the following parts. For catalysts based on oxide supports, pretreatment of the substrate by anodic or thermal oxidation [93, 94] and chemical treatment is necessary. Subsequently, coating methods based on a Uquid phase such as a suspension, sol-gel [95], hybrid techniques between suspension and sol-gel [96], impregnation and electrochemical deposition methods can be used for catalyst deposition [97], in addition to chemical or physical vapor deposition [98] and flame spray deposition techniques [99]. A further method is the synthesis of zeoUtes on microstructures [100, 101]. Catalysts based on a carbon support can be deposited either on ceramic or on metallic surfaces, whereas carbon supports on metals have been little investigated so far [102]. 

This kind of reactor is easy to fabricate, commonly operates with laminar flow, and is used for catalyst screening and for the production of chemicals. However, micropacked bed reactors (MPBRs) usually have a high pressure drop dimng the passage of gases. Therefore, catalytic wall microreactors are more suitable. 

There are two main ways to incorporate the catalyst in a microreactor as a packed bed [57] or as a coating [58]. The advantage of the second method is that industrial catalysts direct in the desired sieve group can be used. Also, the accessible catalyst contents are much higher than those in catalytic wall reactors. Because of the high-pressure loss in the microchannel and a less efficient heat removal as in coated reactors, the packed-bed microreactors can play only a minor role [19]. 

The use of MRs, particularly catalytic wall microreactors, for the SR of methane/NG allows supply of the heat necessary to drive the endothermic reaction by the catalytic oxidation of methane or any other fuel in a series of channels parallel to those in which reforming occurs [192]. Parametric analysis of the thermal behaviour suggests that whatever the nature of the catalyst used in the exothermic channels, the strongest influence comes from the activation energies of both exothermic and endothermic reactions [192]. Unbalancing heat consumed... 

Flat plate microreactor with porous wall and a catalytic wall opposite to it high conversion... 

Fig. 8. a) 3D representation of the aluminum plates fixing the ionic liquid-supported catalyst, b) 2D scheme of the structured catalytic wall microreactor. Adapted from Kiwi-Minsker et al. (2010). Reproduced by permission of Elsevier. 

Kiwi-Minsker, L., Ruta, M., Eslanloo-Pereira, T., Bromley, B. (2010). Structured catalytic wall microreactor for efficient performance of exothermic reactions. Chem. Eng. Process., 49, 9, (September 2010) 973-978, ISSN 0255-2701... 

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. 

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... 

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