Fluidization curves

Figure 1. Ideal fluidization curve, showing typical test chamber and relevant parameters Ah, Ap, l and Vmf.

Note the mean diameter used by Geldart (1973) is actually a surface volume mean diameter dsvm, based on Eq. (3). However, comparing the fluidization curves of several materials with the Geldart (1973) diagram (e.g., fly ash, pulverized coal, coarse ash, PVC powder and screened coke), Wypych (1989b) found the following ... 

Particulate solid matter can exist over a wide range of bulk densities and, therefore, exhibits substantial differences between incipient buoyancy and incipient bubbling. This is illustrated qualitatively in Figure 172 the curves for Dpi and Dp2 represent the most typical shape of fluidization curves (bed expansion versus gas flow rate). Because there is no way to predict precisely a powder s range of bulk densities and the optimum operating gas velocity does not have narrow limits, it is always safe to select a higher gas velocity if in... 

IIL Vcr 11/ >7 lY Distribution Partial fluidization indicaling particle segregation ACDEFG Normal fluidization curve FG Fully fluidized region... 

A study by Chiba et al. (1979) on the minimum fluidization velocity of binary particle mixtures indicated that the fluidization curve shown in Fig. 27 was... 

The gas flowing upward relative to the solids generates a frictional pressure drop. The relationship between the pressure drop per unit length (AP/Lg) and the relative velocity for a particular material is determined by the fluidization curve for that material. Normally, this fluidization curve is generated in a fluidization column where the solids are not flowing. However, the relationship also applies for solids flowing in a standpipe. 

Figure 3 Fluidization curves—group A and group B solids.

The onset of fluidization described previously (Fig. 7.22) may be conveniently represented through the use of the fluidization curve shown in Fig. 7.30. In this graph the logarithm of the pressure drop across the bed is plotted against gas velocity on a logarithmic scale here the line AB corresponds to the pressure drop across the fixed bed before fluidization takes place the region BC represents the rearrangement of the bed to provide the minimum... 

Fig. 8. (a) Schematic for an FCC unit showing where the various fluidization regimes are found and (b) a corresponding phase diagram for Group A powder (FCC catalyst) where the numbers on the curves represent the superficial soHd velocity in m/s. A represents the bubbling regime B, the turbulent ... 

Fig. 10. Expansion curve for FCC catalyst in a 0.15-m inner diameter column showing the fluidization regimes where the numbers on the lines correspond...

Fig. 5. Sum of gas and liquid holdup in gas-liquid fluidized bed. Experimental data of Turner (T4) and theoretical curves of 0stergaard (03).

Adsorption equilibrium of CPA and 2,4-D onto GAC could be represented by Sips equation. Adsorption equilibrium capacity increased with decreasing pH of the solution. The internal diffusion coefficients were determined by comparing the experimental concentration curves with those predicted from the surface diffusion model (SDM) and pore diffusion model (PDM). The breakthrough curve for packed bed is steeper than that for the fluidized bed and the breakthrough curves obtained from semi-fluidized beds lie between those obtained from the packed and fluidized beds. Desorption rate of 2,4-D was about 90 % using distilled water. 

Fig. 3 shows the breakthrough curves for the packed, semi-fluidized, and fluidized beds. It is seen that the breakthrough curve obtained from the semi-fluidized bed lies between those obtained from the packed and fluidized beds, since the semi-fluidized bed possesses the features of both the fluidized and packed beds. This figure also shows that the shape of the breakthrough curve for the packed bed is steeper than that for the fluidized bed. 

The shape of the breakthrough curve for a packed bed is steeper than that for the fluidized bed and the breakthrough curves obtained from semi-fluidized bed lies between those obtained from the packed and fluidized beds. 

Different reactor networks can give rise to the same residence time distribution function. For example, a CSTR characterized by a space time Tj followed by a PFR characterized by a space time t2 has an F(t) curve that is identical to that of these two reactors operated in the reverse order. Consequently, the F(t) curve alone is not sufficient, in general, to permit one to determine the conversion in a nonideal reactor. As a result, several mathematical models of reactor performance have been developed to provide estimates of the conversion levels in nonideal reactors. These models vary in their degree of complexity and range of applicability. In this textbook we will confine the discussion to models in which a single parameter is used to characterize the nonideal flow pattern. Multiparameter models have been developed for handling more complex situations (e.g., that which prevails in a fluidized bed reactor), but these are beyond the scope of this textbook. [See Levenspiel (2) and Himmelblau and Bischoff (4).]... 

Figure 9 shows comparison of this model with experimental data obtained at three different pressures. The solid curve represents the relationship for fixed beds, while the dashed lines represent the behavior for fluidized beds (i.e., Eq. 10) upon exceeding minimum fluidization. 

The attrition rate, i.e., the rate of generation of fines, 0-d microns, at the submerged jets in a fluidized bed, tends to fall off asymptotically with time to a steady-state rate as shown in Fig. 9. Initially the attrition rate is high due to the wearing off of angular comers. Typically, it takes long time, hours to days, for the particles to reach steady-state (equilibrium) where the particles tend to be more rounded. For most catalytic fluidized bed processes, the bed operates at equilibrium. That means the most significant part of the attrition rate curve is the steady-state rate. 

This equation is plotted in the middle set of curves in Fig. 2 as Z versus with TV as a parameter. It should be noted from these curves thatZ is always less than unity, signifying the fact that the congregation of particles in fluidization reduces the rate of fall of the particles, thus prolonging their residence time. This same set of curves could, therefore, be interpreted to... 

Phase Diagram (Zenz and Othmer) As shown in Fig. 17-2, Zenz and Othmer, (Fluidization and Fluid Particle Systems, Reinhold, New York, 1960) developed a gas-solid phase diagram for systems in which gas flows upward, as a function of pressure drop per unit length versus gas velocity with solids mass flux as a parameter. Line OAB in Fig. 17-2 is the pressure drop versus gas velocity curve for a packed bed, and line BD is the curve for a fluidized bed with no net solids flow through it. Zenz indicated that there was an instability between points D and H because with no solids flow, all the particles will be... 

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