Microflow systems transfer

Short residence time The length of time that the solution remains inside the reactor is called the residence time. In a flow system, the residence time can be adjusted by changing the length of the channels and flow speed. In microflow systems, the residence time can be greatly reduced because of the small size of the channel. This feature of microflow systems is extremely useful in controlling reactive species. In short, unstable reactive species can be transferred to another location to be used in the next reaction before they decompose. Therefore, chemical conversions that are impossible in... 

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. 

Figure 8.12 Phase transfer diazo-coupling reaction in a microflow system...

In summary, microflow systems are quite effective for molecular-weight distribution control of very fast, highly exothermic free-radical polymerizations. The superior heat transfer ability of the microflow system in comparison with conventional macrobatch systems seems to be responsible for the high molecular-weight distribution controllability. It should be noted that the controllability is much lower than is achieved by conventional living free-radical polymerization, because residence time control does not work for controlling radical intermediates. The lifetime of a radical intermediate is usually much shorter than the residence time in a microflow system. It is also noteworthy that the more rapid and exothermic the polymerization is, the more effective the microflow system is. These facts speak well for the potentiality of microflow systems in the control of highly exothermic free-radical polymerization without deceleration by reversible termination. 

The concept of flash chemistry can be applied to polymer synthesis. Cationic polymerization can be conducted in a highly controlled manner by virtue of the inherent advantage of extremely fast micromixing and fast heat transfer. An excellent level of molecular weight control and molecular-weight distribution control can be attained without deceleration caused by equilibrium between active species and dormant species. The polymerization is complete within a second or so. The microflow system-controlled cationic polymerization seems to be close to ideal living polymerization within a short residence time. 

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. 

The principles and examples of flash chemistry using microflow systems have been discussed in the previous chapters. Microflow systems serve as an effective method for the control of fast reactions. Extremely fast reactions can be conducted without deceleration in a highly controlled manner by virtue of characteristic features of microsystems, such as fast mixing, fast heat transfer, and short residence time. Synthetic reactions can be much faster when they are released from the restriction of a flask. 

Recently, microflow systems have attracted significant research interest from both academia and industry [28, 29]. Microflow systems are expected to serve as a much better reaction environment than conventional macrobatch reactors because of the inherent advantages of microspaces, such as fast molecular diffusion by virtue of small sizes and fast heat and mass transfer by virtue of large surface-to-volume ratios. In electroorganic synthesis, the use of a microflow reactor serves as a solution to the problems with conventional macrobatch electrochemical reactors, such as difficulty in mass transfer on the surface of the electrodes and high ohmic drop between the electrodes. 

Oxidation and reduction are fundamental processes in the synthesis of organic and inorganic compounds. Some oxidation and reduction reactions are difficult to control in macro-scale batch reactors and in such cases microflow reactors serve as powerful tools for accomplishing the reactions in a highly controlled manner. This is especially true for many oxidation reactions because of their exothermic nature. It should also be noted that the danger of unexpected explosions can be avoided by the use of microflow reactors because of the small volume and highly efficient heat transfer ability of microflow systems. This chapter provides an overview of oxidation and reduction reactions using chemical, electrochemical and biochemical methods in microflow reactors. 

Oxidation of primary, secondary, cyclic and benxylic alcohols and cyclohexanol also takes place smoothly to give the corresponding carbonyl compounds in good yields and selectivities (Table 7.1). A dramatic effect of the microflow system seems to be attributable to precise temperature control and extremely fast mixing by virtue of a short diffusion path. A short residence time by fast transfer of the reactive intermediate to the next reactor also seems to be essential for the success of the... 

Kim, Grubbs, and coworkers reported microflow cross-methathesis of methyl oleate with ethylene [59]. The reaction in the microflow system gave the desired product in higher sdectivity compared to the reaction in bulk batch system due to the high surface area-to-volume ratio suitable for fast mass transfer of the gaseous ethylene in to the solution phase. 

The use of phase-transfer catalysts in biphasic microflow systems has grown largely in the last few years [28-30]. Phase-transfer catalysts are used to allow reactants in two immiscible phases to undergo a more efficient reaction by allowing insoluble materials to pass into their originally immiscible solvent This has been used extensively in batch conditions for many years and been shown to increase productivity in industrial appUcations such as polymerization of polyesters. Coupling this technique with microreactor chemistry has also been shown in many recent publications. 

A microreaction system was developed for the carbonylation of nitrobenzene as well [28]. Under lower CO gas pressure [9.5 bar much lower than those in conventional ones (>100 bar)], phenylisocyanate was produced. A gas-liquid slug flow of the reactant mixture was formed in the microchannel for efficient mass transfer across the gas-liquid interfaces. The isocyanate yield of the microflow reaction was shown to be three to six times higher than that of the batch reaction, depending on the inner diameter (i.d.) of the microtube. A higher isocyanate yield was obtained in a narrow-bore tube (0.5 mm i.d.) than in a wide-bore tube (1.0 mm i.d.). The catalyst they applied was Pd(py)2Cl2 and pyridine system. 

Flow control systems are critical components of most of the energy systems involving fluid flow and heat transfer. These systems are essential for performance optimization of both macroscale and microscale devices. Micropumps, microvalves, microshear stress sensors, and microflow sensors are integral components of flow control systems. Capillary micropump, MHD micropump, thermocapillary micropump, and electrokinetic micropump have been presented in earlier chapters. The present chapter reports various microactuators and shear stress sensors for flow control systems. More details on microvalves and microflow sensors can be found in other references (Nguyen and Wereley, 2006). 

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