Randomly Micro Packed Beds

The randomly micro packed bed reactors are used mainly for catalyst screening [3, 4]. The advantage of this type of catalytic reactor stems from the fact that the developed catalyst used in traditional reactors can be applied. In addition, the use of fine particles greatly increases the mass transfer between the fluid bulk and the catalyst surface. 

To avoid flow maldistribution in micro packed beds the particle diameter should be less than a 10th of the tube diameter dp df/lO). The residence time distribution in packed beds can be estimated with the following empirical relation valid 

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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. 

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]. 

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]. 

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