Transfer in Catalytic Microstructured Reactors

As described in Chapter 2, prior to catalytic heterogeneous reaction, which takes place on the surface of a solid catalyst, the reactant molecules have to first reach 

In the present chapter we focus our discussion on external mass transfer in MSR. Thus, we assume that the reactions occur on the outer surface of the catalyst particle or of the wall catalytic layer. The observable effective reaction rate is determined by the ratio of the characteristic mass transfer time, t, and the characteristic reaction time, commonly known as second Damkohler number Dull (see Section 2.6.1). 

The characteristic mass transfer time is given by the mass transfer coefficient in the fluid, k, and the specific outer surface area of the catalyst, a. 

Low values of Dali (t t ) correspond to a situation where the effect of mass transfer can be neglected and the observed reaction rate is close to the intrinsic 

At high values of Dull (t f,.) the rate of the transformation is completely controlled by mass transfer from the bulk of the fluid phase to the catalyst surface, with the surface concentration being nearly zero (cj S 0)  

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Mass Transfer in Catalytic Microstructured Reactors 247 Table 6.3 Mass transfer characteristics for different channel geometries [53],... 

Mkrostructured reactors (MSR) for heterogeneous catalytic processes mostly consist of a large number of parallel flow channels. At least one dimension of these channels is smaller than 1 mm, but rarely <100 pm. This leads to an increased heat transfer in the direction of the smallest dimension. The volumetric heat transfer performance in microstructured devices is several magnitudes higher than in conventional reactors. Therefore, even highly exothermic or endothermic reactions can be operated under near isothermal conditions and thermal runaway can be avoided (see Chapter 5). In addition, mass transfer between the bulk phase... 

Various parameters must be considered when selecting a reactor for multiphase reactions, such as the number of phases involved, the differences in the physical properties of the participating phases, the post-reaction separation, the inherent reaction nature (stoichiometry of reactants, intrinsic reaction rate, isothermal/ adiabatic conditions, etc.), the residence time required and the mass and heat transfer characteristics of the reactor For a given reaction system, the first four aspects are usually controlled to only a limited extent, if at aH, while the remainder serve as design variables to optimize reactor performance. High rates of heat and mass transfer improve effective rates and selectivities and the elimination of transport resistances, in particular for the rapid catalytic reactions, enables the reaction to achieve its chemical potential in the optimal temperature and concentration window. Transport processes can be ameliorated by greater heat exchange or interfadal surface areas and short diffusion paths. These are easily attained in microstructured reactors. 

Sample integrations similar to pharmaceutical approaches were already examined in 1997 [39]. Here, a chip-like microsystem was integrated into a laboratory automaton that was equipped with a miniaturized micro-titer plate. Microstructures were introduced later [40] for catalytic gas-phase reactions. The authors also demonstrated [41] the rapid screening of reaction conditions on a chip-like reactor for two immiscible liquids on a silicon wafer (Fig. 4.8). Process conditions, like residence time and temperature profile, were adjustable. A third reactant could be added to enable a two-step reaction as well as a heat transfer fluid which was used as a mean to quench the products. 

Advances in the technology of microstructured catalytic reactors depend crucially on the ability to generate appropriate catalyst layers. The activity of the catalyst determines the thickness of the layer that needs to be deposited on the structured support or the walls of the MSR. Relatively thick layers of up to several hundred micrometers are necessary for moderate reaction rates to achieve good reactor performance, whereas thin layers are desirable for very fast catalytic reactions to avoid internal mass transfer limitations (Section 3.2.3). 

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. 

Most of the reported microstructured gas-liquid-solid reactors concern catalytic hydrogenations (Table 8.2). This is because hydrogenation reactions represent about 20% of all the reaction steps in a typical fine chemical synthesis. Catalytic hydrogenations are fast and highly exothermic reactions. Consequently, reactor performance and product selectivity are strongly influenced by mass transfer, and heat evacuation is an important issue. Both problems may be overcome using microstructured devices. 

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... 

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