Caterpillar micromixer

Hessel, V., Lowe, H., Hofmann, C., ScHONFELD, F., WeHLE, D., WeRNER, B., Process development of a fast reaction of industrial importance using a caterpillar micromixer/tubular reactor set-up, in Proceedings of the 6th International Conference on Microreaction Technology, IMRET 6, pp. 39-54 (11-14 March 2002), AIChE Pub. 

A nitroglycerine microprocessing plant, developed by IMM for the Chinese class-A company Xi an, is an example of the smart scaled-out concept, showing that just one caterpillar micromixer is sufficient for the continuous production of pharmaceutical-degree nitroglycerine, with a throughput of approximately 15 kg/h (Thayer 2006). 

Typical industrial process for the synthesis of phenyl boronic acid from phenylmag-nesium bromide and boronic acid trimethoxy ester requires strict temperature control (—25 to —55 °C) to minimize the formation of side products. Recently, Hessel and coworkers reported that a micromixer (width 40 pm and depth 300 pm)/ tubular reactor system gave the phenyl boronic acid at high yield (>80%) even at higher temperatures (22 or 50 °C) with minimum amounts of the side products (Scheme 4.48) [66]. They also achieved a pilot-scale production by employing a caterpillar minimixer (width range 600-1700 pm and depth range 1200-2400 pm). 

Figure4.44 Bas-relief micromixer with microstructured ramps in the channel floor and ceiling, termed caterpillar micromixer (by courtesy of IMM).

All other devices showed only the increasing part of such dependency that is, the highest performance was obtained at the longest residence time [318]. The best conversions of interdigital micromixers and caterpillar minimixers of 78 and 70%, respectively, still exceed notably the performance of a conventional mixing tee (1 mm inner diameter). 

The corresponding semibatch process is a rather slow reaction at 90 °C with simultaneous exothermic decomposition of DAST. Thus, the processed volume is restricted to laboratory scale (< 11) [ 50]. The transfer to production in a stirred tank is prohibited because of these safety reasons. A micromixer-tube reactor approach was chosen using the convective-flow-driven bas-relief caterpillar micromixer and tubes with diameters of 1-5 mm and lengths up to 20 m, respectively, and tube reactor volumes up to 500 ml (see Figure 5.16). 

The caterpillar micromixer consists of a number of serial oriented unit cells that repeat and complete the same type of mixing process. Eight such cells are serially combined in the standard version that is commercially available. Dependent on the mixing problem, however, more or less units may be appropriate, which, especially for production, needs to be optimized to reduce the pressure drop to the limit really needed and for efficient power dissipation. For this reason, caterpillar devices (600 pm width and depth) with 0, 2, 4, 6, and 8 mixing cells have been manufactured to test mixing efficiency by a standardized protocol (Fig. 6.3) ]27]. 

Evidently, mixing efficiency improves strongly with miniaturization of the mixing elements for the caterpillar micromixer [27]. For all three mixers tested, mixing efficiency improves with increasing flow rate, which is due to more intense recirculation patterns and thus interfacial stretching. The slope is steeper for the smaller caterpillar micromixers, i.e., the 800-pm device shows a more pronounced increase of mixing efficiency with flow rate. 

Part of the plant (e.g., the pumps and inlets for solids) is above a thermostat bath level, part (e.g., the melting pots for solids, micromixer, all tubing) is immersed into it to be exposed to a hot environment. Solid waxy materials like palmitic or stearic acid are molten in the pots encompassed by a water or oil bath. Being liquehed, these materials are pumped into an eight-component caterpillar micromixer where water and other ingredients are added (Fig. 6.16). In a second stage, another micromixer may feed a further component that is difficult to... 

Micromixer-tube reactor plants have been employed both at laboratory and pilot-scale [30]. For initial process development, a triangular ( focusing ) interdigital micromixer was used (Fig. 6.18), while for the pilot scale-out a caterpillar mixer was connected to four tubes of different hydraulic diameter by a five-port valve. 

A triangular interdigital or a caterpillar micromixer followed by a tube were used (Fig. 6.30) [45]. The micromixer-tube reactor was submersed into a thermostat bath for temperature setting. The temperature was set to -10 °C to room temperature. Piston and HPLC pumps fed the bromine and aromatic flows, respectively. The use of bromine demands special materials that are stable under such harsh conditions. Fluorinated reactor materials like PVDF or glass turned out to be suitable. 

This flow configuration has been used in the so-called caterpillar micromixer (Figure 4.19b) that is commercially available. This micromixer exists in different configurations with standard mixing size ranging from 150 to 2400 pm and a flow rate up to 250 lh . ... 

The micromixer with structured internal surfaces (e.g., caterpillar mixer, discussed in the later section) show different trends compared to the other microchannels. It shows quadratic dependency of the pressure loss on the flow velocity according to the following equation [16] ... 

An example for microchannels with structured internals are caterpillar micromixer, where the surfaces consist of ramp-Hke structures, moving the fluid constantly up and down and the fluid contact is achieved by a sequence of repeated sphtting-reshaping-recombination processes [5]. Sometimes simple foam stacks are used as static mixer for dispersing two immiscible phases. 

To create fine dispersion in microcapillaries, a micromixer (e.g., caterpillar mixer) needs to be attached upstream. At elevated flow velocity the static internals create dispersion, and as a result, part of the continuous phase flows in the form of small droplets in the dispersed phase. 

The mass transfer performance of a caterpillar micromixer under slug-dispersed flow regime is given in Table 7.10. The fine dispersion results in very high specific interfacial area leading to as high as 2.25 s. ... 

Figure 20.10 Schematic drawings of typical micromixers used for emulsification V-Type MicroMixer (FZK, left), P-Type MicroMixer (FZK, middle) and Caterpillar Split-Recombine Microstructured Mixer [66] (IMM, right)...

Concerning mixing of highly viscous media, it may worth mentioning the type of split-and-recombine micromixers exemplified on the so-called Caterpillar Micro... 

Figure 22.12 Experimental set-up at IMM for surfactant dispersion. All heated parts beside the supply vessel are embedded in a thermostated bath, (a) Complete view of the setup with control units at the left side and the thermostat at the right side with heatingjacketforthe supply vessel and pumps, (b) View inside the thermostatic bath. Visible is especially the caterpillar micromixer and the pump heads [3].

Figure 22.13 Micromixers employed in the experimental setups for surfactant dispersion. The V-type mixer of FZK (a) is a parallel multilamination micromixer whereas the caterpillar mixer of IMM (b) is a split-and-recombine micromixer (serial multilamination) [3].

I MM [17]. (a) On a plate a thermostat inset, eight PMMA version of a multicomponent caterpillar fed pumps and four supply vessels are fixed. The micromixer of I MM allowing mixing of up to shown part will be seton/in a thermostat bath, eight different components. Source Institutfur Parts of the feed vessels and also the pump Mikrotechnik Mainz GmbH, heads are inside the thermostated bath, allowing... 

Bas-relief micromixers induce transversal motion, when miscible liquids are considered, to mix by convection [92,93]. In the gas-liquid case to be considered here, the mechanism for bubble formation is yet unclear, but likely related to shear forces coming from a similar liquid motion in the dispersed flow. Caterpillar mixers induce such transversal motion by ramp-like microstructures, lifted up and down, placed in one channel at the bottom and ceiling [4] (see also Ref. [S6]). Caterpillar mixers were developed as family of devices with grouped capacity using smart enlargement of the internal chaimel and have high volume flows, for example,... 

The mass transfer efficiency of different gas-liquid contactors as a function of residence time was compared, including an interdigital micromixer, a caterpillar minimixer, a mixing tee, and three microbubble column with microchannels of varying diameter (Figure 9.35) [141]. The two microbubble columns comprising the... 

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