Micromixer microstructured

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

There is another way to solve the problem of disguised chemical selectivity of extremely fast competitive reactions, which does not need to slow the reactions, i.e. micromixing, based on microstructures that makes the diffusion path very short. 

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. 

As shown in Table 6.3, when k = 10 L/(mol s), for ideal mixing, where A and B are mixed completely immediately after being introduced to the microstructure, the selectivity P1/P2 = 97/3. This is the selectivity based on the kinetics. The use of a micromixer of 100 im lamination width leads to P1/P2 = 75/25. A decrease in the lamination width causes an increase in the selectivity, which becomes very close to the selectivity for ideal mixing. 

A microstructured fluidic device that is used for conducting chemical reactions is called a microreactor. A microreactor is a reactor containing microstructures for chemical reactions. The size of the microstructure inside a microreactor usually ranges from several micrometers to several hundred micrometers. Various types of microstructured fluidic devices, such as microchip reactors and microtube reactors, have been developed for chemical reactions. Micromixers often serve as microreactors because reactions take place immediately after mixing. In some cases, external energies, such as thermal, photo, and electric energies, are provided in the microspace to promote the chemical reactions. For such purposes, special microreactors have been developed. 

The use of micromixers having microstructures sometimes suffers from clogging problems because of the small channel size. If the reaction yields a solid product or a solid side product, the channels of micromixers, which are micrometer-sized, might get clogged. Anti-fouling micromixers, such... 

K. Schubert, W. Bier, J. Brandner, M. Fichtner, C. Franz, G. Linder, Realization and testing of microstructure reactors, micro heat exchangers and micromixers for industrial application in chemical engineering, in W. Ehrfeld, I.H. Rinard, R.S. Wegeng (Eds.), Proceedings of 2nd International Conference on Microreaction Technology (IMRET 2), AIChE, New Orleans, 1998, p. 88. 

Micromixers have been developed for flows ranging from < 1 mL h up to 10 000 L h At the low flow range, only single elements of the micromixer are required. These are often used in integrated systems for credit card sized fluidic chips [65]. For flows > 1 L h microstructured mixers are used for process intensification in pilot- and conventional-scale applications [65]. 

Che-Hsin L, Chien-Hsiung T, Lung-Ming F (2005) A rapid three-dimensional vortex micromixer utilizing self-rotation effect under low Reynolds number conditions. J Micromech Microeng 15 935. http //dx.doi.Org/10.1088/0960-1317/15/5/006 Bessoth F, deMello A, Manz A (1999) Microstructure for efficient continuous flow mixing. Anal Commun 36(6) 213-215... 

Microstructured devices for fluid-fluid systems exist in a number of configurations. They can be roughly classified into three types based on the contacting principles [ 1] micromixer, microchannels, and falling film microreactors. The first two types of devices are used for all fluid-fluid applications while the microstructured falling film reactor is used only for gas-liquid systems. Depending on the application, these microstructured devices can also be used in series, for example. 

Interdigital mixers and micromixer with the split-recombine technique Single microchannel operating in annular flow regime Microstructured falling film reactor Mesh microreactor... 

Schwarz et al. [85] studied the efficiency of different microstructured mixers followed by microchannels and their influence on the space time for obtaining high product yields. With increasing mass transfer performance of the micromixer and decreasing channel diameter of the microchannel reactors, shorter reaction times of several minutes at lower reaction temperatures compared to conventional batch reactor were obtained. Similar observations are reported for the synthesis of biodiesel in capillary microreactors [86] and in zigzag microchannels [87]. 

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)...

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