Catalytic wall micro-reactors

The theoretical foundation for this kind of analysis was, as mentioned, originally laid by Taylor and Aris with their dispersion theory in circular tubes. Recent contributions in this area have transferred their approach to micro-reaction technology. Gobby et al. [94] studied, in 1999, a reaction in a catalytic wall micro-reactor, applying the eigenvalue method for a vertically averaged one-dimensional solution under isothermal and non-isothermal conditions. Dispersion in etched microchannels has been examined [95], and a comparison of electro-osmotic flow to pressure-driven flow in micro-channels given by Locascio et al. in 2001 [96]. 

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. 

The catalytic wall reactor with channel diameter in the range of 50-1000 pm and a length dependent on the reaction time required circumvents the shortcomings of micro packed beds. This is discussed in more detail in Section 6.5.4. However, in most of the cases, the catalytic surface area provided by the walls alone is insufficient for the chemical transformation and, therefore, the SSA has to be increased by the chemical treatment of the channel walls, or by coating them with highly porous support layers. The thickness of the layer 5 3, depends on catalytic activity. In general, the layer thickness is sufficiently small to avoid internal heat and mass transfer influences. Catalytic layers can be obtained by using a... 

Kolb, G., Keller, S., Pecov, S., Pennemann, H., and Zapf, R. (2011) Development of micro-structured catalytic wall reactors for hydrogen production by methanol steam reforming over novel Pt/In203/Al203 catalysts. Chem. Eng. Trans., 24, 133-138. 

Classical heterogeneous catalytic reactor types used in various process technologies include packed beds, wall-catalyzed reactors, bubble columns, stirred tanks, risers, and fluidized beds. Monoliths and micro reactors have also made inroads in the last couple of decades. Novel designs attempt to... 

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. 

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... 

H. Nakamura, X. li, H. Wang, M. Uehara, M. Miyazaki, H. Shimizu, H. Maeda, A simple method of self assembled nanoparticles deposition on the micro-capillary inner walls and the reactor appUcation for photo-catalytic and enzyme reactions, Chem. Eng.]. 2004, 101, 261-268. 

B. Platzer, Y. Xu, T. Rossmann, R. Fddisch, D. Honicke, Consideration of the effect of irregular catalytic active component distributions in mesopores -extension of a model for wall-catalyzed reactions in micro channel reactors, Chem. Eng. Technol. 2003, 26, 765-773. 

---