Characteristics of Aggregative Fluidization

The phenomena of rapid particle movement and the intimate contact between solids and at least a portion of the gas give rise to a series of characteristics of aggregative fluidization such as good mixing, near isothermal conditions and high rates of heat and mass transfer which are exploited in a wide range of unit operations. 

Fluidization quality in terms of material properties, particle characteristics, and particle group behavior thus needs to be assessed on three scales gross scale of the fluidized bed (macro scale), aggregate scale of gas bubbles, and particle clusters (meso scale), and scale of the discrete, individual particles (micro scale), as described in Chapter 4. 

Wall-to-Bed Heat Transfer. The wall-to-bed heat transfer coefficient increases with an increase in liquid flow rate, or equivalently, bed voidage. This behavior is due to the reduction in the limiting boundary layer thickness that controls the heat transport as the liquid velocity increases. Patel and Simpson [94] studied the dependence of heat transfer coefficient on particle size and bed voidage for particulate and aggregative fluidized beds. They found that the heat transfer increased with increasing particle size, confirming that particle convection was relatively unimportant and eddy convection was the principal mechanism of heat transfer. They observed characteristic maxima in heat transfer coefficients at voidages near 0.7 for both the systems. 

Three different flow patterns can be observed in a fluidized bed reactor (Figure 6.13). For a bubble flow, the solid particles are evenly distributed in the reactor. This flow pattern resembles fluidized beds where only a liquid phase and a solid catalyst phase exist. At high gas velocities, a flow pattern called aggregative fluidization develops. In aggregative fluidization, the solid particles are unevenly distributed, and the conditions resemble those of a fluidized bed with a gas phase and a solid catalyst phase. Between these extreme flow areas, there exists a slug flow domain, which has the characteristics typical of both extreme cases. An uneven distribution of gas bubbles is characteristic for a slug flow. 

Despite the fact that this hydraulic density is essential to many calculations involving fluidization and the suspension of particles, it is characteristic that in the related literature, authors use the terms particle density or solid density without specifying if the fluid in the pores has been taken into account. However, the subject of hydraulic density has been analyzed in studies of the behavior of impermeable aggregates in fluids. As these aggregates could be seen as porous particles, the relevant analysis is interesting and will be presented here. 

Early investigations were concerned mostly with physical properties, somewhat with particle characteristics but little with particle group behavior. Even so, significant results were obtained. For instance, the distinction between L/S fluidization and G/S fluidization, viz., particulate and aggregative, and the provision of criteria for such distinction (Wilhelm and Kwauk, 1948 Harrison et ai, 1961 Romero and Johanson, 1962), most of which were based on the Froude number. Other criteria were then proposed involving fluctuating parameters in fluidization (Rietema, 1967), for instance, pressure drop or voidage. 

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. 

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