Micromixing multilamination

The reason behind such a low energetic efficiency is the mismatch of flow fields and concentration fields. The mechanical energy provided to the mixer is used to achieve the flow in device, but in zones of pure component with no interface with another component, this energy does not contribute in mixing. In the case of micromixers, multilamination improves mixing by reducing the striation thickness, but it requires additional mechanical power to create fine multilamellae before contacting. 

Figure 4 Micromixers, (a) Interdigital structure of a multilamination micromixer. (b) Principle of split-and-recombine static micromixers. (Source IMM.)...

As industrial relevant Friedel-Crafts reaction, the synthesis of Bisphenol-F, a material for epoxy resin, from phenol and formaldehyde was chosen [57]. This reaction involves formation of higher order condensates such as tris-phenols. To minimize the latter, the molar ratio of phenol to formaldehyde is set to a very high value (30-40), which is more than 15 times larger than the amount theoretically necessary. Three types of micromixers were used. These are a T-shaped mixer with 500 pm inner diameter, a multilaminating interdigital micromixer with 40 pm channels and a so-called self-made K-M micromixer with center collision mixing. 

When a T-shaped mixer is used, the product selectivity is essentially the same as for the macrobatch reactor (Scheme 6.4). The use of the YM-1 mixer, a splitting-and-recombination-type micromixer (see Chapter 7), increases the selectivity, however, a significant amount of dialkylation product is still produced. The use of the IMM multilamination-type micromixer results in excellent selectivity of the monoalkylation product. The amount of dialkylation product is very small. Therefore, the product selectivity strongly depends on the manner of mixing. 

In the case of Y- or T-shaped micromixers, a decrease in the channel width to shorten mixing time leads to a decrease of production volume per unit time. To solve this problem interdigital multilamination micromixers have been developed. In this type of micromixer, two fluids are separated into many small narrow streams, which are arranged to contact each other alternately. The mixing takes place at interfaces of such sub-streams by molecular diffusion. The IMM (Institute of Microtechnik Mainz) single... 

In interdigital multilamination micromixers, the small thickness of the lamellae leads to short diffusion paths, resulting in fast mixing. Further thinning of the liquid lamellae should lead to shorter diffusion paths and faster mixing. The IMM single mixer applied this concept by shrinking the channel width in the slit. A further extension of this concept leads to the... 

Figure 7.9 Principle of a multilamination micromixer with triangular-shaped mixing...

Therefore, the observed selectivity is the disguised chemical selectivity caused by an extremely fast reaction. The reaction using a microflow system, however, gives rise to a dramatic increase in the product selectivity. The monoalkylation product was obtained in excellent selectivity and the amount of dialkylation product was very small. In this case, a solution of the N-acyliminium ion and that of trimethoxy-benzene are introduced to a multilamination-type micromixer at —78°C and the product solution leaving the device was immediately quenched with triethylamine in order to avoid the consecutive reactions. Extremely fast 1 1 mixing using the micromixer and efficient heat transfer in the microflow system seem to be responsible for the dramatic increase in the product selectivity. 

However, the use of a microflow system composed of a multilamination micromixer and a microtube reactor gives rise to a significant increase in the yield of the cycloadduct (79%) at the expense of the amount of the polymer (ca. 20 % based on styrene). The fast and efficient 1 1 mixing by a micromixer seems to be responsible. The extremely fast mixing might cause the cationic product to be formed at a very low concentration of styrene, which leads to the effective formation of the neutral cycloadduct. Similar mixing effects have also been observed for p-chloro- and p-methylstyrenes. 

Numerical simulations of styrene free-radical polymerization in micro-flow systems have been reported. The simulations were carried out for three model devices, namely, an interdigital multilamination micromixer, a Superfocus interdigital micromixer, and a simple T-junction. The simulation method used allows the simultaneous solving of partial differential equations resulting from the hydrodynamics, and thermal and mass transfer (convection, diffusion and chemical reaction). 

Superfocus interdigital multilamination micromixer can achieve better control than a macrobatch reactor, and the PDI obtained is very close to the theoretical limiting value of 1.5. As the characteristic dimension of the microdevice increases the reactive medium cannot be fully homogenized by diffusion transport before leaving the system, resulting in a high PDI and a loss in control of the polymerization. 

In the chemical industry (on the mega- as well as the micro-scale) fine emulsions have many useful applications in, e.g., extraction processes or phase transfer catalysis. Additionally, they are of interest for the pharmaceutical and cosmetic industry for the preparation of creams and ointments. Micromixers based on the principle of multilamination have been found to be particularly suitable for the generation of emulsions with narrow size distributions [33]. Haverkamp et al. showed the use of micromixers for the production of fine emulsions with well-defined droplet diameters for dermal applications [38]. Bayer et al. [39] reported on a study of silicon oil and water emulsion in micromixers and compared the results with those obtained in a stirred tank. They found similar droplet size distributions for both systems. However, the specific energy required to achieve a certain Sauter mean diameter was 3-1 Ox larger for the macrotool at diameters exceeding 100 pm. In addition, the micromixer was able to produce distributions with a mean as low as 3 pm, whereas the turbine stirrer ended up with around 30 pm. Based on energy considerations, the intensification factor for the microstirrer appears to be 3-10. 

Fig. 6.32 Comparison of the polydispersity index (DPI) obtained in a multilamination micromixer (open symbols) and in a tube reactor (filled symbols) as a function of the radial Peclet number. (Courtesy of the Royal Society of Chemistry [48].)...

Hessel, V., Numerical simulation of polymerization in interdigital multilamination micromixers. Lab Chip 5(9) (2005) 966-973. 

Cha et al. [68] presented a novel micromixer design relying on a concept not far from that of the multilamination mixer, named a chessboard mixer. The mixer was able to complete the mixing in only 1.400 mm and the author claimed that the flow rate can be increased easily by using different arrays without affecting the performance (Fig. 3c). 

A further interesting concept for the creation of multilaminated streams is that applied in circular micromixers [69, 80, 81]. Circular micromixers rely on the formation of a vortex due to the self-rotation of the fluid stream injected in a quasitangential orientation to the circular mixing chamber (Fig. 3d). Excellent... 

Fig. 4 Flow microreactor system for controlledAiving cationic polymerization of vinyl ether initiated by SnCL. M interdigital multilamination micromixer, R microtube reactor...

The effects of mixing in radical polymerization of MMA are interesting [168]. The use of a 5 mm static mixer leads to fouling in the reactor. In contrast, the use of an interdigital multilamination micromixer with 36 lamellae of 25 pm thickness results in a reduction in fouling. This numbering-up approach enables production of 2,000 tons per year without the fouling problem [169]. 

Continuous nitroxide-mediated block copolymerization of n-butyl acrylate (first monomer) and styrene (second monomer) can be performed using two serial 900-p m inner diameter stainless steel microtube reactors (Fig. 29) [215]. For the second polymerization process, the influence of mixing was examined by changing micromixers. The use of a high-pressure interdigital multilamination micromixer (HPIMM) provided by the Institut fiir Mikrotechnik Mainz (Mainz, Germany), can significantly reduce the polydispersity index = 1.36, 120°C) compared... 

Yoshida et al. also demonstrated the effect of mixing on alkylation yields and selectivity by using an efficient multilamination micromixer (supplied by IMM channel width = 25 pm) and a T-mixer (500 pm). 

Adeosun J, Lawal A (2010) Residence-time distribution as a measure of mixing in T-junction and multilaminated/elongational flow micromixers. Chem Eng Sci 65(5) 1865-1874... 

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