Temperature control microflow systems

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. 

In this section, we discuss the cases where the use of microflow reactors allows us to conduct reactions under unconventional conditions, such as high temperatures, which significantly accelerate the rates of the reactions. In the macrobatch processes, however, the use of such reaction conditions may cause undesirable side reactions, such as the decomposition or subsequent reactions of products. Precise temperature control and short residence time, which minimize the consecutive side reactions, are responsible for successful reactions in microflow systems. 

As shown in Figure 8.13, the reaction of an acid fluoride and an amine, which are derived from the corresponding amino acids, in the presence of N-methylmorpholine has been carried out at several temperatures. The reaction was complete in 3 min at 90 °C, and the yield of the dipeptide was excellent. Reaction at higher temperatures and/or for a prolonged period leads to a decrease in the yield of the dipeptide and an increase in the yield of the tripeptide. Residence-time control in a microflow system is quite effective in reducing the amount of undesired tripeptide byproducts. Some other examples, which are combined with fluorous tag technology, " are shown in Scheme 8.12. In the first case, the reaction was complete in 3 min at 90 °C, whereas the corresponding conventional... 

Polymerization of benzyl methacrylate (BMA) is much slower than that of BA. Although the yield of the polymer increased with an increase in the residence time, the polymerization did not complete within 12 min. The value of M /Mn was much smaller than that for BA, both in the microflow system and the macrobatch system. The effect of the microflow system on molecular-weight distribution control is, however, smaller than for the BA case. Probably, temperature control for BMA polymerization is better than that for BA polymerization, even in the macrobatch system, because heat generation per unit time for BMA polymerization seems to be much less than that for BA polymerization. 

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. 

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... 

Although the applications to A-acyliminium ions, alkoxycarbeniumions, and benzylic cations were successful, it seems to be difficult to apply the method to less stabilized cations. The applicability of the cation pool method inebitably depends upon the stability of the cation that is accumulated. The cation flow method [20, 21] which involves generation of carbocations in a microflow electrochemical system should be much more favorable because of short residence times and efficient temperature control. 

This is necessary both for process control as well as the reliabihty of the system. The integration of sensors into the microreactor or building a multisensor module for the four functions of state is easy for a microreactor made of sUicon. For the process pressure, the piezoresistive principle is used often. With diEFerential pressure measurements, the flow rate can be determined. Alternatively, calorimetric principles are used widely. These are easy to implement technically, but a calibration is needed for eatii new medium. The most robust sensors are the Coriolis mass flow sensors. In process engineering, they are very common, but in terms of micro process engineering, there is still a need for research. In Ref. [26], sensors of this type are described. Ref [25] is a good summary of other microflow sensors. For measurement of temperature, there are many equivalent principles but will not be discussed here. Substantially, it is more difficult to measure the concentration in the reactor. In addition to optical principles, the impedance spectroscopy is often used. See Ref [27-31] for more details. 

---