Multilamination Micromixers

Reducing the solution of the convection-diffusion equation for the concentration field to a ID problem in the comoving frame-of-reference clearly reaches its limits if the fluid lamellae get deformed and are no longer arranged in parallel. Such a situation may occur if the channel depth is not small compared to its width and the design comprises a sudden expansion or contraction of the flow 

For the channel design shown in Fig. 3.19 it could be shown that lamellae multiplication works in a close-to-ideal manner for Reynolds numbers below about 30 [70]. For higher Reynolds numbers inertia forces become important and cause a deformation and finally a merging of lamellae of the same type. Altogether, experiments indicate that at comparatively small Reynolds numbers the design shown enables an efficient liquid mixing at low pressure drops, thus 

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

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...

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. 

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. 

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). 

Figure 4.18 Different types of multilamination micromixers, (a) liquid flows lO-IOOOmI h", (b) 138 microchannels flow 3501 h" at 3.5 bar, (c) 20 ml h", and (d)...

From the geometric point of view, the mixing in single-channel micromixers is not sufficient for fast reactions and, therefore, it needs to be improved further. In multilamination micromixers, the lamination improves mixing by reducing the striation thickness however, it requires additional mechanical power to create... 

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].

The Friedel-Crafts alkylation of aromatic and heteroaromatic compounds often suffers from a polyalkylation problem owing to competitive, consecutive alkylation reactions. Owing to the large exothermicity of the reaction, the product distribution has proved difficult to control on the macroscale, resulting in synthesis of a large proportion of dialkylated products (the monoalkylated dialkylated ratio may be 1 1). Yoshida et al. carried out an alkylation reaction in a microchannel at —78 °C as per Fig. 5 [3]. 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 xm) and a T-mixer (500 xm). 

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