Microtube reactors

Another advantage of microtube reactors is good heat exchange ability. Heat transfer can occur equally in all directions perpendicular to the flow direction through the inner surface and outer surface of the tube wall. 

Plastic microcapillary flow disk (MFD) reactors have been constructed from a flexible, plastic microcapillary film (MCF), comprising parallel capillary channels with diameters in the range of 80-250 jxm. MCFs are wound into spirals and heat treated to form solid disks. These reactors are capable of carrying out continuous flow reactions at elevated temperatures and pressures with a controlled residence time.  

Micromixers are the key elements of microflow systems for flash chemistry because extremely fast mixing is essential for conducting an extremely fast reaction between two reaction components As described in Chapter 6, the conventional approach to mixing often leads to disguised chemical selectivity. Fast mixing by virtue of a short diffusion path in 

Passive or active Mixing driving force Type 

Active mixing Diffusion and High-frequency vibration 

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The reaction scope is further enlarged to unsymmetrically disubstituted derivatives. Using one mole equivalent of two different aryllithium precursors at a time in a sequence of four micromixers and microtube reactors (MRi ), it was possible to obtain an unsymmetrically disubstituted final diarylethene. The yield was determined after solvent evaporation and sample purification on the column. 

Another kind of solid support that has gained more popularity is the surface-modified polypropylene, such as the Multipin crown and Microtubes. Reaction 3 was carried out on lightly and highly cross-linked polystyrene resins (1 % and >20% divinylbenzene, respectively), poly sty rene-PEG resin, and a surface-functionalized Microtube reactor.12 The reaction kinetics on these supports is compared in Figures 7.4 and 7.5. The reaction on the highly cross-linked polystyrene resin was shown to be slower than on other supports and the reaction on Microtube was faster than on polystyrene resins. 

Figure 7.4. Single-bead IR spectra of the product for the reaction 3 at various times on PS = polystyrene, AP = ArgoPore, Tube = Microtube reactor, and PS-PEG = polystyrene-polyethyleneglycol resin.

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. 

Figure 6.7 Microflow system for Friedel-Crafts alkylation of aromatic compounds with an N-acyliminium ion (M, micromixer R, microtube reactor)...

Figure 6.19 System for evaluation of heat transfer in microsystems (Ml, T-shaped mixer Rl, R2, R3, microtube reactors). The experiments in the absence of a monomer are conducted without using Ml and Rl...

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. 

Figure 7.5 Microflow system composed of microtube reactors and micromixers...

Microreactors containing a solid-supported organic catalyst have also been developed. For example, solid-supported l,5,7-triazabicyclo-[4.4.0] undec-3-ene is introduced to a microtube reactor (Figure 7.26).This packed-bed microreactor has been used for Knoevenagel condensation... 

For example, thermally induced radical chlorination of alkanes can be conducted in a microtube reactor having microheat transfer modules. 

When the reaction was performed on a 100 mg scale, the diene was obtained in 55% yield as a mixture of ( )- and (Z)-stereoisomers. However, when the scale was increased to lOOg, various by-products, such as cyclized products or alkyl group-migrated compounds, were produced presumably because of acid-catalyzed reactions of the diene. The formation of such by-products can be reduced using a microflow system composed of a micromixer and a microtube reactor. Thus, a solution of the allylic alcohol in tetrahydrofuran (THF) was mixed with a solution of p-toluenesulfonic acid (p-TsOH) in THF/toluene at 90 °C. After the reaction mixture was allowed to flow for 47 s, the reaction was quenched with a saturated NaHCOs solution at room temperature. In this case the desired diene was obtained in 80% yield. It is noteworthy that the acid-mediated by-products were not detected. This process was applied to the synthesis of pristane, a biologically important natural product that is widely used as an adjuvant for monoclonal antibody production. 

The use of a microflow system composed of a micromixer and a microtube reactor solves the diiodination problem, as shown in Scheme 8.10. The yield of monoiodo compound is 78%, whereas the yield of the diiodo compound is 4%. A significant increase in the product selectivity can also be accomplished for other highly reactive aromatic compounds. 

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. 

Figure 9.2 Microsystem for polymerization. Ml, M2, micromixers Rl, microtube reactor...

Radical polymerization can be conducted in microflow systems. A typical system for laboratory-scale radical polymerization that consists of a mixer and microtube reactors is shown in Figure 9.9. ... 

A monomer solution and an initiator solution are mixed at a T-shape micromixer Ml and microtube reactor Rl. In this case fast mixing of the two solutions is not important, because radical polymerization does not start until the temperature is elevated sufficiently for thermal decomposition of a radical initiator, such as AIBN. Therefore, the combination of a T-shape micromixer and a short microtube reactor is sufficient for producing a homogeneous solution before polymerization starts. 

Figure 9.9 Microflow system for polymerization. Ml, T-shape micromixer Rl, R2, R3, microtube reactors...

Polymerization occurs in microtube reactor R2, because its temperature is set high enough to decompose the initiator. Then, the mixture is introduced to the third microtube reactor R3, where polymerization is stopped by cooling. 

The successful synthesis of unsymmetrical diarylethenes using two different aryl bromides in a single flow speaks well for the potential of microfiow systems in making new functional materials, which would be otherwise difficult to synthesize in a conventional manner. Acceptable productivity from the viewpoint of industrial production of functional materials can be attained using microtube reactors with a relatively large inner diameter (800/rm) without numbering-up. 

The example described above indicates that a numbering-up microflow system consisting of several microtube reactors is quite effective for conducting radical polymerization. Precise temperature control by effective heat transfer, which is one of the inherent advantages of microflow systems, seems to be responsible for the effective control of the molecular-weight distribution. The data obtained with the continuous operation of the pilot plant demonstrate that the microflow system can be applied to relatively large-scale production, and speaks well for the potential of microchemical plants in the polymer industry. 

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