Voidage profile axial

Figure 10.10. Typical axial voidage profiles for Group A particles (after Li and Kwauk, 1980 Yang, 1992).

Fig. 5. Types of apparatus and their respective effects on axial voidage profile. 1, fast fluidized bed 2, intermediate hopper 3, cyclone 4, slow fluidized bed 5, downcomer 6, solids rate controlling device 7, solids rate measurement device 8, suspension section.

As Fig. 5b (Li et ai, 1984) shows, there is no intermediate solids hopper as for type A, and the solids rate device is used only for measurement rather than for control. In operation, solids circulation rate is determined essentially by gas velocity. Therefore, gas velocity and solids circulation rate could not be adjusted independently. Solids circulation rate follows what an operating velocity produces at whatever solids inventory the system was initially filled with. Owing to the absence of an intermediate hopper, the initial solids inventory could influence directly axial voidage profile, that is, the position of the inflection point changes with the initial solids inventory in the system. 

Figure 15 shows typical results of axial voidage profile measurements (Li and Kwauk, 1980 Li et al, 1981). Experiments were conducted by using different solids materials in a Type A apparatus, 90 mm i.d. and 8 m high. Voidages were calculated from pressure gradient measurements. Solids circulation rate was controlled by a pneumatically actuated pulse feeder (see Section III in Chapter 7). 

Fig. 15. Axial voidage profile Tor four powders in a Type A fast fluidized bed (after Li and...

Fig. 16. Axial voidage profile for different inventories of pulverized coal in a Type B fast fluidized bed (after Li et al., 1984).

To describe the axial voidage profile, a one-dimensional model for steady-state operation has been developed as follows. 

When the inlet and outlet effects are minimal to the extent of being negligible, a set of correlations of the state parameters on the axial voidage profiles, as... 

The S-shaped axial voidage profile can be described by using the diffusion-segregation model (Li and Kwauk, 1980) as follows ... 

Boundaries in fast fluidization refer mainly to the column wall as well as the inlet and outlet. Effect of the wall on pressure drop due to friction between the fluidized solids and the wall surface is minimal (Li et al, 1978), although it is the very cause of radial distribution of parameters. The configuration of the inlet and the outlet often strongly affect gas-solids flow, especially with regard to axial voidage profile. 

Fig. 45. Comparison of axial voidage profiles for FCC catalyst with one (O) or two (4 gas inlets—inlet (4) (after Xia and Tung, 1989).

Li, J., Tung, Y., and Kwauk, M. Axial voidage profiles of fast fluidized beds, in Circulating Fluidized Bed Technology II (P. Basu and J. F. Large, eds.), p. 193. Pergamon Press, 1988. 

Figure 6 shows the dependence of axial voidage profiles on gas velocity. If gas velocity increases without changing solids inventory, the difference between the bottom dense region and the top dilute region will be reduced, accompanied by an increase of solids flow rate, until the difference disappears. It is evident that solids flow rate is dependent on gas velocity. 

Figure 7 shows the dependence of axial voidage profiles on solids inventory, which was first recognized by Weinstein et al. (1983), for system FCC/air (pp = 1,450 kg/m3, dp = 59 /im). Two kinds of voidage profile curves are shown. The curves for / = 15, 20 kg in Fig. 7a and / = 15, 20, 22 kg in Fig. 7b are S-shaped, with the co-existence of two regions, and transition occurring inside the unit, while solids can be fed at the bottom of the bed at saturation flow rates. Variation of solids inventory / does not result in any change of... 

Fig. 6. Dependence of axial voidage profiles on gas velocity at a constant solids inventory (FCC/air).

Fig. 7. Axial voidage profiles for system FCC/air. ( ) Data used in Fig. 5.

Figure 8 shows the dependence of axial voidage profile on gas velocity... 

Fig. 8. Variation of axial voidage profile with gas velocity at constant solids flow rate.

Experiments also indicated that at any constant gas velocity, S-shaped profiles can exist only within certain limits of I, as represented by the boundary of zone PFC/FD in Fig. 10a. Thus, to realize a given mode of operation, the necessary global conditions need to be satisfied in addition to intrinsic hydrodynamics. Bed height can also affect axial voidage profiles, as already noted. The smaller the bed height is [Z, < Z2), the shorter the horizontal section of Gs = f Ug, I) is in Fig. 10a, and the more difficult it is to develop the S-shaped profile. Additional data in Fig. 10b for hollow glass beads further identify the three operating modes just enumerated. 

Then the entire axial voidage profile can be calculated from the model mentioned earlier in the present section. 

Fig. 31. Axial voidage profiles with and without ring internals (O, without ring internals , with ring internals) (Zheng et at, 1992).

Whether axially or radially, gas-solid flow is heterogeneous in structure. Axial voidage profile characterizes a dilute region at the top and a dense region at the bottom, i.e., an S-shaped distribution curve. 

Figure 26 compares the computed axial voidage profiles with the experimental, indicating that they agree well. The axial gaseous compositions... 

Fig. 26. Comparison of axial voidage profile between predicted and experimental values (after Lin, 1991).

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