Mixing Using Micromixer

we consider mixing using a smaller flow reactor. One typical example is a Y-shaped reactor shown in Fig. 3.4. In this reactor, laminar flow should be dominant. When the width of the flow passage is in the order of 100 pm, the lime taken for diffusion would be in the order of seconds. In this case, fast mixing caimot be expected. When the width of the flow passage is in the order of 10 pm or less, the time taken for diffusion would be in the order of milliseconds. Such small tubes or channels, however, are not practically useful for synthesis because of only a small 

3 Fast Micromixing for High-Resolution Reaction Time Control 

As described above, the use of micromixers enables mixing in the order of milliseconds. A system consisting of a micromixer and a flow microreactor would enable a reaction with a residence time in the order of seconds or less. A macroflow 

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Yoshida and coworkers also developed a microreaction system for cation pool-initiated polymerization [62]. Significant control of the molecular weight distribution (Mw/Mn) was achieved when N-acyliminium ion-initiated polymerization of butyl vinyl ether was carried out in a microflow system (an IMM micromixer and a microtube reactor). Initiator and monomer were mixed using a micromixer, which was connected to a microtube reactor for the propagation step. The polymerization reaction was quenched by an amine in a second micromixer. The tighter molecular weight distribution (Mw/M = 1.14) in the microflow system compared with that of the batch system (Mw/M > 2) was attributed to the very rapid mixing and precise control of the polymerization temperature in the microflow system. 

From the viewpoint of dimensional analysis, the terms macro-mixing and micromixing used in the Theory of Turbulence are misleading, because they confuse the issue discussed above. In performing model experiments it does not matter whether the state of flow corresponds to the macro- or micro-mixing, but whether we succeeded in obtaining the working point of the same pi-space. 

In a previous section (Section 6.1.3), we discussed the problem of disguised chemical selectivity for extremely fast competitive consecutive reactions. This problem could be solved using micromixers, in which the mixing takes place in a very short period by virtue of a small diffusion path caused by the microstructure. Friedel-Crafts alkylation using N-acyliminium ion pools provides a nice example of the effectiveness of micromixing. 

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. 

Power per unit volume is frequently used as a scaleup criterion for gas liquid reactions, but there is disagreement about its merit for homogeneous reactions with imperfect mixing. If micromixing effects are most important, the selectivity should not change at scaleup at constant P/V, since e would be constant and (p should be the same at comparable positions in the reactors, giving the same value of parallel reactions in 1- and 20-liter tanks, Fournier and coworkers... 

Mixing in micromixers relies primarily on molecular diffusion or chaotic advection (laminar chaos) mechanisms. As discussed above, the diffusive mixing effect can be improved by increasing the interfacial contact area between the different fluids and reducing the diffusion length between them. The use of unstable electrokinetic flow fields to achieve chaotic mixing effect can also be adopted. 

Microreaction systems involve microreaction apparatuses that enable high controllability of chemical reactions. Such controllability results from efficient heat transfer, mass transport, and/or a larger surface/interface area. Recent studies have shown the potential benefits of using microfluidic reactors for various chemical reactions. Reactions using micromixing devices give better results than batchwise reactions. This is because a micromixer enables rapid mixing and therefore yields excellent controllability of rapid reactions. 

Several studies [1, 62-65] illustrated with special emphasis the enhancement of product selectivity, so that it has become a general idea that the better the mixing is, the higher is the selectivity. If it is often true in practice, this is not always the case, and there are some reports of no improvement with using micromixers [Ij cases with unfavorable effects are unfortunately not usually reported. 

An emulsion is a biphasic solution including dispersed small oil droplets in an aqueous phase. Because of the small channel size and high fluid velocity in micromixers, high shear force is applied to fluids, resulting in the generation of an emulsion in addition to enhancement of the mixing performance. The emulsion produced using micromixers has the characteristics of stability without surfadant. 

In what follows, both macromixing and micromixing models will be introduced and a compartmental mixing model, the segregated feed model (SFM), will be discussed in detail. It will be used in Chapter 8 to model the influence of the hydrodynamics on a meso- and microscale on continuous and semibatch precipitation where using CFD, diffusive and convective mixing parameters in the reactor are determined. 

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