Catalytically microstructure walls

We first present general criteria for the rational use of MSRs on the basis of fundamentals of chemical reaction engineering [21-24], The main characteristics of MSRs are discussed, and the potential gain in reactor performance relative to that of conventional chemical reactors is quantified (Section 2). Subsequently, the most important designs of fluid-solid and multiphase reaction systems are described and evaluated (Sections 3 and 4). Because microstructured multichannel reactors with catalytically active walls are by far the most extensively investigated MSRs for heterogeneous catalytic reactions, we present their principal design and recent synthetic methods separately in Section 5. 

Therefore, microstructured multichannel reactors with catalytically active walls are by far the most often used devices for heterogeneous catalytic reactions. Advantages are low pressure drop, high external and internal mass transfer performance and a quasi-isothermal operation. In most cases the reactors are based on micro heat exchangers as shown in Figure 15.2. Typical channel diameters are in the range of... 

Yube, K. and Furuta, M. and Aoki, N. and Mae, K. (2007). Control of selectivity in phenol hydroxylation using microstructured catalytic wall reactors. Applied Catalysis A General 327, 278-286... 

It is no surprise that monoliths are applied in many morphologies (cell sizes, wall thicknesses, channel shapes, materials of construction, microstructures (texture of the coating)) and overall dimensions. Monoliths are flexible to operate. They are well suited to optimal semi-batch, batch, continuous, and transient processing. Catalytic conversion can be... 

A major problem in using microstructured reactors for heterogeneously catalyzed gas-phase reactions is how to introduce the catalytic active phase. The possibilities are to (i) introduce the solid catalyst in the form of a micro-sized packed bed, (ii) use a catalytic wall reactor or (iii) to use novel designs. Kiwi-Minsker and Renken [160] have discussed in detail these alternatives. 

Advances in the technology of microstructured catalytic reactors depend crucially on the ability to generate appropriate catalyst layers. The activity of the catalyst determines the thickness of the layer that needs to be deposited on the structured support or the walls of the MSR. Relatively thick layers of up to several hundred micrometers are necessary for moderate reaction rates to achieve good reactor performance, whereas thin layers are desirable for very fast catalytic reactions to avoid internal mass transfer limitations (Section 3.2.3). 

The use of microstructured catalytic wall reactors offers an interesting option for the revamping of existing plants. The key idea of this so-called booster concept... 

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. 

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. 

Depending on the philosophy of the microdesign, the microstructure itself can be seen as a catalyst support or solely as a reactor wall. The catalytically active species can be used as raw material for microfabrication or can be deposited by conventional... 

On ceramic monoliths, foams or metaOic microchannels, catalytically active species can be deposited directly on the structured wall, when the microstructure aheady... 

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]. 

---