Structured Catalytic Micro-Beds

The drawback of randomly packed microreactors is the high pressure drop. In multitubular micro fixed beds, each channel must be packed identically or supplementary flow resistances must be introduced to avoid flow maldistribution between the channels, which leads to a broad residence time distribution in the reactor system. Initial developments led to structured catalytic micro-beds based on fibrous materials [8-10]. This concept is based on a structured catalytic bed arranged with parallel filaments giving identical flow characteristics to multichannel microreactors. The channels formed by filaments have an equivalent hydraulic diameter in the range of a few microns ensuring laminar flow and short diffusion times in the radial direction [10]. 

In a second example, a filamentous microstructured catalyst was used in a membrane reactor specially developed for the continuous production of propene from propane via nonoxidative dehydrogenation. The catalytic filaments with a diameter of about 7 pm consisted of a siUca core covered by a y-alumina porous layer on which an active phase of Pt/Sn is supported [8]. 

The catalytic filaments were introduced into the tubular reactor in the form of threads. A bundle of 100 filaments with a diameter of 7 pm each formedthreads of diameter of about 0.5 mm. The catalytic threads were placed in parallel into the tube to form a cylindrical catalytic bed of several centimeters length. This arrangement gives about 300 threads per cm within the tube cross section with a porosity of = 0.8. The specific surface per volume is in the order of 10 m m and, thus, about 50 times higher compared to washcoated tubes of the same inner diameter [8]. The performance comparison under identical experimental conditions with randomly packed beds with particles of silica and y-alumina of different shapes and sizes showed significantly broader residence time distribution compared to the structured filamentous packing with about five times lower pressure drop for the same hydraulic diameter and comparable gas flow rates. 

Reactor channels filled with metallic or ceramic foams as catalyst supports demonstrate several advantages compared to randomly packed beds [12]. Open cell foams consist of a network of interconnected solid stunts-building cavities 

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. 

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Third, there may be a concentration gradient of reactants and products along the length of the catalyst bed. If the structure of the catalyst depends upon the composition of the gas phase, then an average of the various structures will be measured. There is little discussion of this topic in the literature of XAFS spectroscopy of working catalysts. An extreme example of structural variations within a sample is discussed in Section 6, where there is a discussion of XAFS spatially resolved spectra recorded to allow direct observation of the axial distribution of phases present. If the XAFS data are not measured with spatial resolution, then it is recommended that XAFS data be measured under differential conversion conditions. However, if the aim of the experiment is to relate the catalyst structure directly to that in some industrial catalytic processes, then differential conversion conditions will only reflect the structure of the catalyst at the inlet of the bed. To learn about the structure of the catalyst near the outlet of the bed, the reaction has to be conducted at high conversions. If it is anticipated that this operation will lead to variations in the catalyst structure along the bed, then the feed to the micro-reactor should be one that mimics the concentration of reactants toward the downstream end of the bed (i.e., products should be added to the reactants). 

To avoid high-pressure drop and clogging problems in randomly packed micro-structured reactors, multichannel reactors with catalytically active walls were proposed. The main problem is how to deposit a uniform catalyst layer in the microchannels. The thickness and porosity of the catalyst layer should also be enough to guarantee an adequate surface area. It is also possible to use methods of in situ growth of an oxide layer (e.g., by anodic oxidation of a metal substrate [169]) to form a washcoat of sufficient thickness to deposit an active component (metal particles). Suzuki et al. [170] have used this method to prepare Pt supported on nanoporous alumina obtained by anodic oxidation and integrate it into a microcatalytic combustor. Zeolite-coated microchannel reactors could be also prepared and they demonstrate higher productivity per mass of catalyst than conventional packed beds [171]. Also, a MSR where the microchannels are coated by a carbon layer, could be prepared [172]. 

An easy way to design catalytic MSR consists of introducing the catalytic active phase within the microchannels in the form of powders creating a micro packed bed. Besides randomly packed beds, the use of structured catalysts is proposed and typical examples are presented here. 

Figure 5.13 Comparison between conventional fixed-bed technology (conv) and a combined micro-structured plate heat-exchanger/catalytic afterburner (micro) according to Delsman et al. [386] power range, lOOWei-...

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