Foam Microstructured Reactors

The mass transfer coefficient in solid open foams can be estimated based on relations similar to those presented for packed beds. 

For anisotropic foams a further variation of Equation 6.17 was proposed [49]. For this relationship the form-factor was modified by introducing three axis describing the pore geometry. But the measurement of all pore dimensions is quite demanding and, therefore, is not presented. 

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This chapter primarily covers microstructured reactors (MSRs). Furthermore, the use and the advantages of microstructured catalysts such as fabrics, grids, and foams are discussed. 

Figure 6.5 Microstructured reactor with integrated exchangeable catalytic foam plates. (Courtesy Fraunhofer ICT-IMM, Germany.)...

The comparion of pressure drop in three different types of microstructured reactors, foam reactor, square channels and packed bed, is shown in Example 6.2. 

Aligned multiwall CNT arrays were synthesized as a basis for a microstructured catalyst, which was then tested in the Fischer-Tropsch reaction in a microchannel reactor [269]. Fabrication of such a structured catalyst first involved MOCVD of a thin but dense A1203 film on a FeCrAlY foam to enhance the adhesion between the catalyst and the metal substrate. Then, multiwall CNTs were deposited uniformly on the substrate by controlled catalytic decomposition of ethene. Coating the outer surfaces of the nanotube bundles with an active catalyst layer results in a unique hierarchical structure with small interstitial spaces between the carbon bundles. The microstructured catalyst was characterized by the excellent thermal conductivity inherent to CNTs, and heat could be efficiently removed from the catalytically active sites during the exothermic Fischer-Tropsch synthesis. 

Although MSRs have been shown to be suitable for the optimization of many synthetic procedures, they have not yet received enough attention for catalytic chemistry. The main reason for the reluctance to apply them is the difficulty of introducing a solid catalyst into the microchannels of the reactor. Micropacked-bed reactors are easy to fabricate, but they usually produce a high pressure drop during the flow of gases. To overcome this problem, microstructured packings such as foams or fibrous supports may be used instead. 

Yu et al. [36] used metal foams as catalyst supports for a microstructured methanol reformer and investigated the infiuence of the foam material on the catalyst s selectivity and activity. Some electrically conducting materials such as silicon carbide (SiC) [37] are used as a foam as well as internal heating element, and the reactor can be heated very rapidly to temperatures in the range 800-1000 °C. Moreover, as a result of the high thermal conductivity of metal or SiC foams, axial and radial temperature profiles are minimized and nearly isothermal reactor operation is facihtated. 

Foams were proved to be highly suitable as catalytic carrier when low pressure drop is mandatory. In comparison to monoliths, they allow radial mixing of the fluid combined with enhanced heat transfer properties because of the solid continuous phase of the foam structure. Catalytic foams are successfully used for partial oxidation of hydrocarbons, catalytic combustion, and removal of soot from diesel engines [14]. The integration of foam catalysts in combination with microstructured devices was reported by Yu et al. [15]. The authors used metal foams as catalyst support for a microstructured methanol reformer and studied the influence of the foam material on the catalytic selectivity and activity. Moritz et al. [16] constructed a ceramic MSR with an inserted SiC-foam. The electric conductive material can be used as internal heating elements and allows a very rapid heating up to temperatures of 800-1000°C. In addition, heat conductivity of metal or SiC foams avoids axial and radial temperature profiles facilitating isothermal reactor operation. 

Example 6.2 Comparison of pressure drop in microstructured packed bed reactor, microchannel reactor and foam reactor... 

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