Continuous-phase Microstructured Reactors

Yeong et al. [88, 92] used a microstructured film reactor for the hydrogenation of nitrobenzene to aniline in ethanol at 60 °C, 0.1-0.4MPa hydrogen pressure and a residence time of 9-17 s. Palladium catalyst was deposited as films or particles on the microstmctured plate. Confocal microscopy was used to measure the liquid film thickness. With increasing flow rate between 0.5 and 1.0 cm min , thicker liquid films between 67 and 92 pm were observed. The kia of this system was estimated to be 3-8 s with an interfadal surface area per reaction volume of 9000-15 000 m m. Conversion was found to be affected by both liquid flow rate and hydrogen pressure and the reactor operated between the kinetic- and mass transfer-controlled regimes. 

In a mesh microcontactor, the gas and liquid flow through separate channels. To provide stable operation, the fluid interface is immobilized by well-defined openings obtained with a thin mesh [94]. Interfacial forces help to stabilize the fluid interface within the openings, while fluid layers are thin enough to enhance mass 

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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. 

Continuous phase contacting, where the fluid phases are separated. Examples are microstructured falling film and mesh reactors. 

Basically, there are two possibilities for bringing two phases into contact The first (Type A) is to keep both fluid phases continuous in order to create a defined interface. Consequently, the reactor should embody microstructures that generate two stable continuous phases with a preferably high exchange area. The second pos sibility (Type B) is to disperse one phase into the other by using a suitable inlet or a micromixer upstream of the reactor section. The goal to create an exchange area is also a dominant aspect for this type. 

Another example is the use of a filamentous microstructured catalyst in a membrane reactor specifically developed for the continuous production of propene from propane via nonoxidative dehydrogenation. The catalytic filaments with a diameter of 7 pm consisted of a silica core covered by a y-Al203 porous layer, which served as a support for an active phase of platinum and tin [53,54]. 

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