Generalized Fluidization

In liquid-liquid extraction, for instance, when a light liquid is the discontinuous phase, the rising droplets are actually fluidized by the heavy liquid flowing downward, forming an inverted fluidized bed of liquid droplets (Blanding and Elgin, 1942). Similarly, the rise of bubbles in liquid can be treated as sedimentation in a reversed sense. 

The distributor plate for fluidized solids is often a necessary stabilizing component—that is, for most fluidized systems, when the distributor is removed, the entire solids bed may fall off from its active region. Under certain 

The moving bed can also operate according to three cases of relative particle-fluid motion. 

Hindered sedimentation can operate in two modes when the fluid is essentially at standstill, and when an equal volume of fluid is displaced by the descending solids, such as for batch operation. 

Generalized fluidization refers to the overall field of particle-fluid motion represented by the 11 modes of operation shown in Fig. 10, the two types of hindered sedimentation just described, plus classical fluidization when the solids inventory possesses hardly any net motion up or down, totalling altogether 14 cases. It shall be described in the context of the concept of idealized fluidization (Kwauk, 1958, 1963a, 1973 Kwauk and Chong, 1963). 

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The PSD is an indicator of the fluidization properties of the catalyst. In general, fluidization improves as the fraction of the 0-40 micron particles is increased however, a higher percentage of 0-40 micron particles will also result in greater catalyst losses. 

Fluidized leaching and washing is a countercurrent operation with downflow of solids, called counter-down. The following equation for generalized fluidization... 

Figure 25. Charting G S generalized fluidization against experimental data for iron ore, A1203, and FCC. (Chen and Kwauk, 1985.)...

Chen, B., and Kwauk, M., Generalized Fluidization of Non-Ideal Systems, Proc. First Intern. Conf. Circulating Fluidized Bed, 127-132, Halifax, Canada (1985)... 

Fluidized-bed systems combust the fuel that has been fluidized by high-velocity air. In general, fluidized-bed systems are flexible in terms of fuel requirements. As a result, they are quite suitable for the simultaneous combustion of biomass and other fuels. 

If the relative velocity between the particles and the fluid is substituted for velocity u of classical fluidization, the general correlations of classical fluidization can be extended to include the entire realm of generalized fluidization. 

Equation (3.5) will be referred to as the equation of generalized fluidization for steady-state motion. Of the four quantities in this equation, n is an empirical function of Re, and the remaining three dimensionless variables are determined by operation and can be represented graphically for any constant value of n, as shown in Fig. 11 for n = 2.36, which corresponds to Re = 104. Figure 11 will be described in the following order ... 

Fig. 11. Constant-n chart for generalized fluidization. [After Kwauk, 1963.]... 

The simplest case for polydisperse systems is a binary mixture of a coarse and a fine size fraction of the same material, each considered to be uniform in size. The generalized fluidization equation for the coarse particles, with subscript 1, is (Kwauk, 1975 Huang et at., 1982)... 

Computer software is being developed to facilitate calculations dealing with various aspects of generalized fluidization. 

The equation for generalized fluidization, Eq. (3.5), can be rewritten in terms more adapted to the practitioner—that is, solids and fluid at their respective weight rates, S and L, added to the fluidized leacher/washer having a cross-sectional area of A ... 

For generalized fluidization, the diffusion flux Jm2 of Eq. (5.6) and the segregation flux Js2 of Eq, (5.7) will not be equal, their difference being the net flow rate of particles 2 ... 

In the circulating fluidized bed, solids are removed from the top and recycled to the bottom, to be carried up again by the flowing fluid, thus forming a co-up system. Generalized fluidization predicts a decrease in voidage, as shown in the constant-n chart. The high solids concentration,... 

Kwauk, M., Generalized Fluidization, I. Steady-State Motion," Scientia Sinica 12(4), 587 (1963a). 

Kwauk, M., Generalized Fluidization of Bidisperse Particles (in Chinese, unpublished), Inst. Chem. Metall., 1975-9-21 (1975). 

Kwauk, M. Generalized fluidization II Accelerative motion, Scientia Sinica 13(9), 1477 (1964). 

For generalized fluidization, Kwauk (1963) put forward the following relation among voidage, particles velocity, and fluid velocity for simultaneous particle-fluid motion ... 

The computer model is based upon a continuum description of fluidization in coal gasification reactors. In general, fluidized flows are dominated by specific physicochemical processes and, hence, require particular theoretical representations. For example, in the heavily loaded gas-particle regime appropriate to fluidization, the solid particles dominate the transport of momentum and energy. This aspect of fluidization is reflected in the mathematical descriptions which have been used in the fluidized bed model. 

In general, fluidized beds are of special interest when a high degree of gas to solid contact coupled with large throughput of gas at fairly low pressure drop is needed. 

Figure 24 Generalized fluidization diagram with homogeneous drag (air-FCC...

Figure 25 Generalized fluidization diagram with EMMS drag coefficient (air-FCC particle system, dp = 75(im, />p=1500 kg/m />g=1.3kg/m //g=1.8x 10 Pa s, Ut=0.2184 m/s) (Ullah et al, 2013b).

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