Wall Microstructured Reactors

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 

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In a multiphase membrane reactor, the conversion of benzylpenicillin to 6-aminopenidllinic acid is performed. The type of microstructured reactor used is a fermentation reactor which contains the enzyme penicillin acylase immobilized on the wall of a hollow-fiber tube. The hollow-fiber tube extracts 6-aminopenicillinic acid at the same time selectively. Benzylpenicillin is converted at the outer wall of the hollow fiber into the desired product, which passes into the sweep stream inside the fiber where it can be purified, e.g. by ion exchange. The non-converted benzylpenicillin is recycled back through the reactor [84],... 

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. 

Alumina [23] Anodic oxidation of AIMg microstructured reactor wall... 

The main problem for controlling the flow pattern is its dependence on many experimental parameters such as flow velocity, flow ratio of phases, fluid properties, channel geometry, microchannel material, wall roughness, pressure, and temperature. All these parameters influence the relative importance of the different forces. Different flow regimes in gas-liquid flows in microstructured reactors are discussed in the following section. 

Heat transfer is promoted by thin walls between the reaction channels and the heating or cooling channels, which leads to an improved energy efficiency [17], excellent temperature control [18] often enables higher selectivity and catalyst lifetime [19], Another promising feature of microstructured reactors is their modularity, which facilitates the scale-up and the adaptation of the plant to the changing process needs. 

Of course, not all multiphase microstructured reactors are presented in Table 9.1. Either because they have attracted (too ) little interest, because they may have been qualified as microreactors in spite of their overall size but caimot be considered as microstmctured , or because they combine several contacting principles. Examples are a reactor developed by Jensen s group featuring a chaimel equipped with posts or pillars, thus resembling more a packed bed but with a wall-coated layer of catalyst [20], and a string catalytic reactor proposed by Kiwi-Minsker and Renken [21], that may applied to multiphase reactions. 

Rebrov EV, Ekatpure RP, de Croon MHJM, Schouten JC. Design of a thick-walled screen for flow equalization in microstructured reactors. J. Micromech. Microeng. 2007 17(3) 633-641. 

A reactor that combines the advantages of EMR technology with those of microstructured reactors is the microenzyme reactor (MER). Here the enzymes are bound to the reactor wall (enzyme withhold), while reactions can be run continuously at different temperature levels. 

As a result, scale layer was always formed on the smooth surface of reactor wall as well as irregular. Microstructure was blanketed as soon as the surface was coated with a Ti02 scale film. It could be deduced that once scales were formed, the effect of surface geometrical shape would be more important than its microstructure. 

Yube, K. and Furuta, M. and Aoki, N. and Mae, K. (2007). Control of selectivity in phenol hydroxylation using microstructured catalytic wall reactors. Applied Catalysis A General 327, 278-286... 

Laminar flow reactors are equipped with microstructured reaction chambers that have the desired low Reynolds numbers due to their small dimensions. Mass transport perpendicular to the laminar channel flow is dominated by diffusion, a phenomenon known as dispersion. Without the influence of diffusion, laminar flow reactors could not be used in heterogeneous catalysis. There would be no mass transport from the bulk flow to the walls as laminar flow, in contrast to turbulent flow, cannot mix the flow macroscopically. 

The reaction is also influenced by the heat of reaction developing during the conversion of the reactants, which is a problem in tubular screening reactors. In microstructures, the heat transport through the walls of the channels is facilitated by their small dimensions, which allows the development of isothermal reaction conditions. Thus, by decoupling the heat and mass balance, an analytical description of the flow in the screening reactor is achievable. 

We first present general criteria for the rational use of MSRs on the basis of fundamentals of chemical reaction engineering [21-24], The main characteristics of MSRs are discussed, and the potential gain in reactor performance relative to that of conventional chemical reactors is quantified (Section 2). Subsequently, the most important designs of fluid-solid and multiphase reaction systems are described and evaluated (Sections 3 and 4). Because microstructured multichannel reactors with catalytically active walls are by far the most extensively investigated MSRs for heterogeneous catalytic reactions, we present their principal design and recent synthetic methods separately in Section 5. 

FIGURE 9 Scheme of a wall-coated microstructured channel reactor. 

To ensure good performance of a microstructured wall reactor, the thickness of the catalyst layer <5cat/inax must not exceed the value specified by Equation (25). 

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