Phase Microstructured Reactors

In a medi microcontactor, the gas and hquid flow through separate channels. To provide stable operation the fluids are separated by a thin mesh of typically 5 pm thickness. The fluids are in contact through holes with diameters of about 

5 pm [8]. In contrast to microstructured falling film reactors, the velocity of the fluids can be varied without changing the interfacial area, which is given by the porosity of the membrane. Interfacial forces help to stabilize the fluid interface within the openings, while fluid layers are thin enough to enhance mass transfer. 

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Kashid, M., Gupta, A., Renken, A., Kiwi-Minsker, L, (2010). Numbering-up and mass transfer studies of liquid-liquid two-phase microstructured reactors. Chemical Engineering Journal, 158, 233-240. 

Figure 8.8 Mass transfer in dispersed phase microstructured reactors where gas solute diffuses through liquid toward solid surface, (a) Schematic representation, (b) Resistance model.

Markowz G, Schirrmeister S, Albrecht J, Becker F, Schiltte R, Caspary KJ, Klemm E (2005) Microstructured reactors for heterogeneously catalysed gas-phase reactions on an industrial scale. Chem Eng Technol 28 459 -64 Muller A, Cominos V, Hessel V, Horn B, Schiirer J, Ziogas A, Jahnisch K, Hillmann V, GroBer V, Jamc KA, Bazzanella A, Rinke G, Kraut M (2005) Fluidic bus system for chemical process engineering in the laboratory and for small-scale production. Chem Eng J 107 205-214 Pennemann H, Watts P, Haswell S, Hessel V, Lowe H (2004) Org Proc Res Dev 8 422... 

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. 

G. Markowz, S. Schirrmeister, J. Albrecht, F. Becker, R. Schiitte, K.J. Caspary, E. Klemm, Microstructured reactors for heterogeneously catalyzed gas-phase reactions on an industrial scale, Chem. Eng. Technol. 28 (2005) 459. 

Integration of various components is an important issue for the DCF systems. The simple scale-up of microreactors is not enough as the DCF system. A DCF system should consist of not only reactors but also other factory parts like a mixer, separator, and temperature controller. Many integrated microreaction systems have been reported and some of these are commercially available. For example, K. F. Jensen s group has reported an integrated microreactor system for gas-phase catalytic reactions using microstructured reactors and other devices on a computer board [13]. They have achieved computer control over the reaction system through this device as shown in Fig. 6. 

Figure 6.13 Mass transfer effectiveness for different microstructured reactors. Gas-phase, physical properties of air at 20 °C, 0.1 MPa.

Carbon-coated microstructured reactors for heterogeneously catalyzed gas phase reactions influence of coating procedure on catalytic activity and selectivity. Chem. Eng. /, 101 (1-3), 11-16. 

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. 

Often microstructured reactors are used for high flow velocities where the inertial forces dominate the surface forces. In this case, a separation principle identical to conventional equipment is used. The gravity based separation, based on the density difference between two phases, is the most commonly used method of separation. 

A two-phase capillary reactor was used for the oxidation of aromatic alcohols and Baeyer-Villiger oxidation of ketones using elemental fluorine [80]. The substrate in an appropriate solvent (acetonitrile or formic acid) was injected at a controlled rate by a syringe pump into the reaction channel. Compared to batchwise operation the yield and conversion was comparable or better using microstructured devices. 

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. 

Sulfonations are a further important type of electrophilic substitution reaction. However, only very few examples can be found in the literature describing the use of microstructured reactors for the strongly exothermic liquid-phase sulfonation of aromatics (sulfonation of toluene wdth gaseous SO3 was described by Jaehnisch et al. [34]). Burns and Ramshaw [25, 35] claimed that their concept of performing liquid/liquid nitration reactions in a slug-flow capillary-microreactor can be also... 

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