Flow microstructured reactors, conventional

In this chapter, fluid-fluid flow patterns and mass transfer in microstructured devices are discussed. The first part is a brief discussion of conventionai fluid-fluid reactors with their advantages and disadvantages. Further, the ciassi-flcation of fluid-fluid microstructured reactors is presented. In order to obtain generic understanding of hydrodynamics, mass transfer, and chemical reaction, dimensionless parameters and design criteria are proposed. The conventional mass transfer models such as penetration and film theory as well as empirical correlations are then discussed. Finally, literature data on mass transfer efficiency at different flow regimes and proposed empirical correlations as well as important hydrodynamic design parameters are presented. 

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. 

Illg, T., P. Lob, and V. Hessel. 2010. Flow chemistry using milli- and microstructured reactors—From conventional to novel process windows. Bioorg. Med. Chem. 18 3707-3719. 

M 39] [P 37] Using an azo-type competitive reaction, the selectivities were compared for the P- and V-type micro mixers having straight and oblique fluid injection, respectively [41]. In this way, laminar- and turbulent-flow mixing achieved by vertical interdigital microstructured mixers can be compared. The selectivities of the turbulent V-type mixer are better to some extent as compared with the P-type device however, neither approaches the characteristics of the ideal tubular reactor. The micro devices, however, are better than a conventional jet mixer. 

The economic benefit is one of the dominant problems if a micro structured reactor plant is used for chemical production. Without any doubt, an overall flow rate through a micro structured device can be achieved that is comparable to that with a conventional batch process. However, the residence time is very short because of the dimensions of a microstructured device. If the reaction kinetics are slow, an additional device is necessary to increase a dwell time. Hence, much effort should be devoted to increasing the reaction rate instead of transferring the standard protocol to a micro structured reactor [13]. 

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... 

The flow phenomena in microfluidic devices such as microchannels have been studied extensively during the last few decades (Ehrfeld et al., 2000 Hessel et al. 2005b Papautsky et al., 2001). Fluid flow in microstructured or microchannel reactors is quite different from that in conventional macro reactor systems, due to the smaller hydraulic diameter of the... 

A microstructured monolith for autothermal reforming of isooctane was fabricated by Kolb et cd. from stainless steel metal foils, which were sealed to a monohthic stack of plates by laser welding [73]. A rhodium catalyst developed for this specific application was coated by a sol-gel technique onto the metal foils prior to the sealing procedure. The reactor carried a perforated plate in the inlet section to ensure flow equi-partition. At a weight hourly space velocity of 316 L (h gcat). S/C 3.3 and O/C 0.52 ratios, more than 99% conversion of the fuel was achieved. The temperature profile in the reactor was relatively flat. It decreased from 730 °C at the inlet section to 680 °C at the outlet. This was attributed to the higher wall thickness of the plate monolith compared with conventional metallic monolith technology. The reactor was later incorporated into a breadboard fuel processor (see Section 9.5). 

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