Micro-multichannel reactor

Thus, the mass transfer coefficient in micro-multichannel reactors depends, besides the molecular diffusion coefficient, on the channel diameter and the second Damkbhler number Dan ... 

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]. 

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... 

The design of multichannel micro reactors for gas-phase reactions is typically based on a stack of micro structured platelets. For strongly endothermic or exothermic reactions, it lends itself to alternate between layers of reaction channels and heat-... 

In consequence, the plug flow behavior in a multichannel micro-reactor > 100) can be assumed only if the relative standard deviation is < 0.07. 

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]. 

The results are plotted as the function of the interstitial velocity in Figure 6.13. Whereas the multichannel and the foam reactor reaches transfer effectiveness of 035> t] >0.1, the values for micro packed beds is found to be between 0.006 >t > 0.002. The highest effectiveness can be obtained with microchannel... 

Owing to the high degree of parallelization, the scale-up of the micro-reactor is relatively simple provided that equipartition of the fluid flow through the chaimels is achieved. It was shown by Walter et al. that even at low Reynolds numbers Re of around 30, vortices can be formed in distribution chambers upstream of multichannel geometries, which lead to deviations from the mean flow rate inside the channels of about 20% [5]. 

The Tamao-Kumada-Corriu reaction has also been carried out with supported catalytic systems. One interesting recent example has been the use of a multichannel microreactor to perform this reaction [169], The glass micro reactor was designed so as to increase the catalytic surface area and ensure a uniform distribution of the velocity/temperature field. The sol-gel procedure was used to immobilize the nickel catalyst to the channel walls. The Tamao-Kumada-Corriu reaction was conducted using bromobenzene and phenylmagnesium bromide, and was also carried out in the batch configuration for comparison, and it was observed that the reaction in-flow was four orders of magnitude more rapid than that performed under batch conditions and there was a threefold increase in the yield of the biaryl compound. 

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