Fluid conventional reactors

In one modification of this procedure, the starting material is pyroly2ed rice hulls in place of more conventional forms of sihcon dioxide (31). Another unique process involves chlorination of a combination of SiC and Si02 with carbon in a fluid-bed reactor (32). The advantages of this process are that it is less energy-intensive and substantially free of lower sihcon chlorides. 

The design of a fluid-bed reactor is difficult with conventional tools. For this reason, we will consider in this project the EDC from oxychlorination as an external source entering the purification system. 

In contrast with conventional reactor concepts, rules for the design of novel reactors are not always present. In order to come to a valuable assessment of these new reactors, additional information should become available to investigate the full potential. This information should concern e.g. data on mass/heat transfer characteristics and fluid mechanics. 

Randomly micropacked beds are mainly used for catalyst screening [31,32]. The advantage of a packed-bed MSR is the opportunity to employ catalysts developed previously for conventional reactors [33]. Furthermore, the use of fine particles greatly increases the rate of mass transfer between the fluid bulk and the catalyst surface. The external mass transfer coefficient can be estimated by using Equation (10) [34] ... 

Gas solid interactions are difficult to study systematically in conventional reactors but can readily be studied in a specialized type of temperature scanning reactor intended for this type of process, the stream swept reactor (SSR). In principle this is a batch reactor containing the solid through which the fluid phase flows sweeping out any desorbed material or reaction products to a detector at the outlet. Reactors of this type are also potentially applicable in adsorption studies and will be discussed in Chapter 5 under the heading TS-SSR. 

Nevertheless, there are several constraints hampering the use of microstruc-tured devices for fluid-solid reactions. In the catalytic reactions, the performance is very adversely affected by catalyst deactivation. Effective in situ catalyst regeneration thus becomes necessary, as the simple catalyst change practiced in conventional reactors is usually no longer an option. The thickness of the catalytic wall is often greater than the internal diameter of the channel and, therefore, may impede heat transfer for highly exothermic reactions leading to nonisothermal behavior. 

In this chapter, fluid-fluid flow patterns and mass transfer in microstructured devices are discussed. The first part is a brief discussion of conventionai fluid-fluid reactors with their advantages and disadvantages. Further, the ciassi-flcation of fluid-fluid microstructured reactors is presented. In order to obtain generic understanding of hydrodynamics, mass transfer, and chemical reaction, dimensionless parameters and design criteria are proposed. The conventional mass transfer models such as penetration and film theory as well as empirical correlations are then discussed. Finally, literature data on mass transfer efficiency at different flow regimes and proposed empirical correlations as well as important hydrodynamic design parameters are presented. 

In this chapter, different aspects of fluid-fluid systems in microstructured devices have been described. The disadvantages of conventional reactors have been clearly... 

Due to short diffusion pathways in the microsystem, the overall mass transport in the phases or the transfer via phase boundaries is often magnitudes higher than in conventional reactor systems. However, with regard to the desired high loadings with catalyst and low cost for fluid compression or pumping, the mass transfer to the catalyst and the mass transport within porous catalyst still has to be effective. As for the heat transport the differentiation between packed bed and wall-coated microreactor is necessary for mass transport considerations. The mass transport in packed bed microreactors is not significantly different to normal tubular packed bed reactors, so that equations like the Mears criteria (Eq. 6) can be used. 

In a multiphase stratified flow, the interfaces between immiscible fluids have several characteristics. Firstly, the specific interfacial area can be very large just as droplet-based flow. It can for example be about 10,000 m in a microchannel compared with only 100 m for conventional reactors used in chemical processes. Secondly, the mass transfer coefficient can be very high because of the small transfer distance and high specific interfacial area. It is more than 100 times larger than that achieved in typical industrial gas-liquid reactors. Thirdly, the interfaces of a stratified microchannel flow can be treated as nano-spaces. Simulation results show that the width of the interfaces of a stratified flow is in nanometers, and that diffusion-based mixing occurs at the interface. The interface width can be experimentally adjusted by adding surfactants. Finally, reactants only contact and react with each other at the interface. Therefore, the interfaces supply us with mediums to study interfacial phenomena, diffusion-controlled interfacial reactions and extraction. 

Even if the reactor temperature is controlled within acceptable limits, the reactor effluent may need to be cooled rapidly, or quenched, to stop the reaction quickly to prevent excessive byproduct formation. This quench can be accomplished by indirect heat transfer using conventional heat transfer equipment or by direct heat transfer by mixing with another fluid. A commonly encountered situation is... 

One disadvantage of fluidized heds is that attrition of the catalyst can cause the generation of catalyst flnes, which are then carried over from the hed and lost from the system. This carryover of catalyst flnes sometimes necessitates cooling the reactor effluent through direct-contact heat transfer hy mixing with a cold fluid, since the fines tend to foul conventional heat exchangers. 

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