Fast fluidized beds hydrodynamics

The interaction of parametric effects of solid mass flux and axial location is illustrated by the data of Dou et al. (1991), shown in Fig. 19. These authors measured the heat transfer coefficient on the surface of a vertical tube suspended within the fast fluidized bed at different elevations. The data of Fig. 19 show that for a given size particle, at a given superficial gas velocity, the heat transfer coefficient consistently decreases with elevation along the bed for any given solid mass flux Gs. At a given elevation position, the heat transfer coefficient consistently increases with increasing solid mass flux at the highest elevation of 6.5 m, where hydrodynamic conditions are most likely to be fully developed, it is seen that the heat transfer coefficient increases by approximately 50% as Gv increased from 30 to 50 kg/rrfs. 

On the basis of the observations in the macroscale, the flow of a fast fluidized bed can be represented by the core-annulus flow structure in the radial direction, and coexistence of a bottom dense region and a top dilute region in the axial direction. Particle clusters are an indication of the heterogeneity in the mesoscale. A complete characterization of the hydrodynamics of a CFB requires the determination of the voidage and velocity profiles. There are a number of mathematical models accounting for the macro- or mesoaspects of the flow pattern in a CFB that are available. In the following, basic features of several types of models are discussed. 

Li, Y., and Wang, F. Experiments of hydrodynamics for roasted magnesite in a fast fluidized bed, Research report (in Chinese), Institute of Chemical Metallurgy, Academia Sinica (1983). 

In this chapter, emphasis will be given to heat transfer in fast fluidized beds between suspension and immersed surfaces to demonstrate how heat transfer depends on gas velocity, solids circulation rate, gas/solid properties, and temperature, as well as on the geometry and size of the heat transfer surfaces. Both radial and axial profiles of heat transfer coefficients are presented to reveal the relations between hydrodynamic features and heat transfer behavior. For the design of commercial equipment, the influence of the length of heat transfer surface and the variation of heat transfer coefficient along the surface will be discussed. These will be followed by a description of current mechanistic models and methods for enhancing heat transfer on large heat transfer surfaces in fast fluidized beds. Heat and mass transfer between gas and solids in fast fluidized beds will then be briefly discussed. 

Inasmuch as heat transfer depends on the hydrodynamic features of fast fluidization, if the fast fluidized bed is equipped with an abrupt exit, the axial distribution of solids concentration will have a C-shaped curve (Jin et al., 1988 Bai et al., 1992 Glicksman et al., 1991. See Chapter 3, Section III.F.l). The heat transfer coefficient will consequently increase in the region near the exit, as reported by Wu et al. (1987). 

Another hydrodynamic complication found in fast fluidized beds is the tendency for particles to aggregate into strands or clusters, as reported by Horio et al. (1988) and Chen (1996). The concentration of solid particles in such clusters is significantly greater than in the bed itself, and it increases with increasing radial position and with increasing total solid flux (see Soong et al., 1993). This characteristic also directly affects heat transfer at walls. 

The above is a very condensed discussion of the hydrodynamic characteristics of fast fluidized beds. As presented below, heat transfer is strongly dependent on the time-averaged local concentration of solid particles and is therefore influenced by these hydrodynamic characteristics. Almost all the heat transfer models require information on solid concentration. The reader is referred to Chapter 19 of this book for more detailed discussion of the hydrodynamics, and... 

The core-annulus models treated in the previous section improve on the one-dimensional models covered in the preceding section by making some allowance for the difference in behavior between the relatively dense wall region and the dilute core of fast fluidized beds. However, the hydrodynamics are represented in a relatively crude manner. As illustrated in Fig. 33, experimental results show that reactant concentration varies continuously across the entire cross section of the riser, rather than there being a sharp discontinuity at a coreannulus boundary. Hence models are needed that provide for continuous variation across the riser, or at least a greater number of intervals in the lateral direction. Such models include the following ... 

As in the Orcutt model, for a slow reaction, i.e. a small kvs, the overall conversion of the fluidized bed is less sensitive to mass transfer and thus to bed hydrodynamics than for a fast reaction, i.e. large kws. 

Saraiva et al [121] presented an extended model for a circulating atmospheric fluidized bed combustor (CAFBC) which included hydrodynamics for the fast section at the top of the bed as well a bubbling bed section at the bottom of the CAFBC. For the fast section of the bed, one dimensional momentum and energy balances were used to predict the temperature and velocity profiles for gas and particles throughout the reactor. The model contain species mass balances for five gas species including SO2, as well as a model of SO2 retention by limestone particles. A bubbling bed model was considered to simulate the chemical process at the bottom of the combustor. 

Whenever the difiusional limitation is broken through the use of fine catalyst powder in a bubbling fluidized bed, a new limitation arises related to the hydrodynamics of the system. In the bubbling fluidized bed, it is not possible to fully exploit the very intrinsic kinetics of the powdered catalyst. Fast fluidization (transport) reactor configuration offers excellent potential to break this limitation. 

For fast reactions, the conversion is determined by the bed hydrodynamics. These results are typical for a gas-phase catalytic reaction in a fluidized bed. 

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