M Fluidized Bed Reactors

Fluidized bed reactors offer the advantages of excellent solid mixing and heat transfer characteristics. These reactors find widespread applications in the chemical, petrochemical, metallurgical and energy industries. Some typical applications involving gas-solid fluidized bed reactors are listed in Table 12.1. Flow regimes prevailing in these fluidized bed reactors are also listed in this table. Despite the widespread use, the complex hydrodynamics of fluidized bed reactors is still not completely understood. Several different flow regimes may exist in the reactor as detailed in Table 12.1. Depending on these flow regimes and different configurations, several varieties of fluidized bed reactors are used in practice, as shown in Fig. 12.1. Application of computational flow modeling to the reactor engineering of these fluidized bed reactors is briefly discussed in this chapter.

Before we discuss reactor-engineering issues of fluidized bed reactors, we briefly describe flow regimes and their identification. A discussion on the basics of modeling multiphase flows and possible approaches was given in Chapter 4. With reference to that discussion, we include here a brief review of CFD modeling of fluidized bed reactors and guidelines for selection/development of appropriate flow models to simulate the most commonly encountered flow regimes in fluidized bed reactors. Despite the significant progress made in recent decades, several questions and issues concerning the rigorous modeling of gas-solid fluidized beds remain unanswered. The consequences and implications of not understanding some of these issues on reactor engineering applications are discussed. Despite these, ways of fruitfully using

TABLE 12.1 Some Industrial Applications of Fluidized Bed Reactors

Processes/applications

Operating regime

Fluidized bed catalytic cracking (FCC)

Maleic anhydride/phthalic anhydride Acrylonitrile Ethylene dichloride Polymerization of olefins

(polyethylene/polypropylene) Coal gasification Fischer-Tropsch synthesis Acrylonitrile/metacrylonitrile Calcination/roasting of ores Incineration of solid waste

Riser reactor: fast-fluidized bed regime Regenerator: bubbling bed/turbulent fluidized bed regime Turbulent fluidized bed regime Bubbling/turbulent bed regime Bubbling/turbulent bed regime Bubbling/turbulent fluidized bed regime

Turbulent fluidized bed regime Dense phase/fast-fluidized bed regime Bubbling/turbulent bed regime Bubbling/turbulent bed regime Bubbling/turbulent bed regime computational fluid dynamic models of gas-solid fluidized bed reactors are presented with the help of some examples.

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