Microheat exchanger

In a detailed process optimization study, the impact of the type of micromixers and process parameters was determined [56]. As a result, a pilot with a Toray Hi-mixer connected to a shell and tube microheat exchanger was constructed. Continuous operation for 24 h was carried out to obtain pentafluorobenzene (PFB) after protonation (92% yield). In this time, 14.7 kg of the product was produced, that is, about 5 t/a. Thus, the industrial-scale production carried out using a batch reactor (10 m3) can be replaced by adding only four microflow systems of the scale investigated. The pilot plant produces 0.5 kg in 6h continuous operation, thus about 730kg/a (see Figure 5.19). The name of the industrial company was not disclosed. 

Figure 7.20 (a) Photograph of the cross flow microheat exchanger and (b) schematic... 

Figure 7.21 Shell and tube microheat exchanger. Copyright 2005 American Chemical Society...

It is crucial to increase the capacity of the reactor for industrial applications, and to increase the capacity of a heat exchanger in a microflow system a shell and tube microheat exchanger, which contains 55 microtubes ((f) = 490 /xm) in the shell has been developed (Figure 10.6, see also Chapter 7). This is an example of internal numbering-up. The heat exchanger has only one inlet and one outlet, although it contains 55 microtubes inside. 

A microflow system for relatively large-scale production (i.e., a pilot plant) composed of a Toray Hi-mixer connected to a shell and tube microheat exchanger has been constructed (Figures 10.8 and 10.9). The... 

Fig. 14.6 Schematic diagram showing cross section of microheat exchangers (a) without and (b) with manifold [33. 34]...

Figure 3.9 Schematic drawing of a countercurrent microheat exchanger.

Stief, T., Langer, O.-U., Schubert, K. Numerical investigations of opimal heat conductivity in microheat exchangers, Chem. Eng. Technol. 22, (1999) 297-302. 

Figure 11.19 Staged bubble column reactor with integrated structured catalyst layers and microheat exchanger (a) vertical cut through the heat exchanger element (b) and sintered metal fiber catalyst (c) [53]. (Adapted with permission from Elsevier.)...

The H5 boundary condition can be used to approximate parallel and counterflow microheat exchangers in which the fluid bulk temperature varies exponentially along the microchaimel. 

Until recently, the temperature control of highly exothermic reactions using the microreaction systems was mainly based on the removal of heat in order to prevent hot spot formation and thermal runaway [29]. More recently, however, research has focused on techniques that enable microreactors to be heated because they can efficiently dissipate the heat. If a microheat exchanger is integrated into a microreactor, both effects can be combined, that is, either enabling fast heat supply in the reactor or heat removal from the reactor [30]. In practice, strongly exothermic reactions such as nitration, oxidation, chlorination, and even fluorination with elementary fluorine (in microreactors made of nickel) can be carried out in microreactor systems under nearly isothermal conditions [31]. 

Besides microstructured heat exchangers/reactors constructed in the form of plates, as shown in Figure 2.2, shell and tube microheat exchangers are available. An example is shown in Figure 2.6. The heat transfer within the reactor tubes can be estimated with Equation 2.22. The outer heat transfer coefficient depends on the flow regime, the arrangement of the tubes, and the baffles [12,18]. For small-scale systems, capillaries submerged in constant temperature baths are commonly used. In this case, the main heat transfer resistance is mostly located at the outer side of the reactor. 

The microstructured multichannel reactors with catalytically active walls are by far the most used devices for heterogeneous catalytic reactions. Advantages are low pressure drop (Equation 2.7), high external and internal mass transfer performance, and a near-isothermal operation. In most cases, the reactors are based on microheat exchangers with typical channel diameters in the range from 50 to 500 xm and a length between 20 and 100 mm (see Figure 2.2). 

Figure 3.5 Schematic representation of modular microreactor setup employed for the vitamin A acetate synthesis (1 pressure probe 2 micromixer 3 microheat exchanger 4 capillary reactor with electrical heating 5 thermal liquid inlet for heat exchanger).

Equipment Hewlett Packard 6890 gas chromatograph equipped with a HP-5 0.25 gm, 30 mx 0.32 mm capillary column, a thermostat, three flasks for the reaction educts and products, microreactor setup according to Figure 12.15 with two microheat exchangers, a microreactor, three inlet/outlet modules, and a micromixer. Extra equipment for the experiments in a batch reactor, such as round-bottom flask equipped with a thermometer, a reflux cooler, a dropping funnel, and a mechanical stirrer, are necessary. 

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