Yang Fluidization, Solids Handling, and Processing

Recirculating and Jetting Fluidized Beds 247

Effect of Draft Tube and Downcomer Area Ratio. When a draft tube of 9.55 cm I.D. (downcomer/draft tube area ratio = 7.8) was changed to a draft tube of 5 cm I.D. (downcomer/draft tube area ratio = 30) with other design parameters being the same, the gas bypassing reversed direction, as shown in Fig. 4. With the smaller draft tube (D/dD = 1), the gas bypasses from the draft tube side into the downcomer side for most experimental conditions, except for jet velocities in excess of 76 m/s at the concentric solids feeder; with the larger draft tube (D/dD = 1.9), the gas bypasses from the downcomer side into the draft tube side in most experiments.

Effect of Distance between the Distributor Plate and the Draft Tube Inlet. Figure 4 clearly indicates that the gas bypassing phenomenon depends not only on the design parameters but also on the operating conditions. For the conical plate at a distance from the draft tube inlet of L = 21.7 cm, gas bypasses from the draft tube side to the downcomer side at a high flow ratio and reverses the direction at a low flow ratio. When the conical plate was moved closer to the draft tube inlet at L = 14.1 cm, the gas bypassing direction was exclusively from the downcomer side to the draft tube side.

Gas Bypassing with a Conical Distributor Plate. Further gas bypassing results (Yang and Keairns, 1983) obtained for a conical distributor plate are shown in Figs. 5–7. The data are expressed with respect to the flow ratio (FR) as well. The flow ratio is similarly defined as the total gas flow supplied through the draft tube gas supply (No. 8 flow) and the concentric gas nozzle (No. 7 flow) divided by the total gas flow supplied through the downcomer gas supply (No. 3 flow). When the downcomer gas supply (No. 3 flow) is absent, the flow ratio FR is evaluated from the ratio of No. 7 flow and No. 8 flow. Then the total flow passing through the downcomer is calculated as Y/(X + Y). The X and Y are the actual amount of gas passing up the draft tube and the downcomer, respectively, determined from the tracer gas injection studies.

When only the No. 3 and No. 7 flows are present, gas bypasses from the draft tube side to the downcomer side at a high flow ratio and reverses the direction at a low flow ratio at a distance of L1 = L2 = 21.7 cm between the conical plate and the draft tube inlet (see Fig. 5). The angle of the conical plate (a = 45° and 60°) does not seem to affect the gas bypassing characteristics greatly. When the conical plate was moved closer to the draft tube inlet at L1 = L2 = 14.1 cm, the gas bypassing direction was exclusively from the downcomer side to the draft tube side.

250 Fluidization, Solids Handling, and Processing

The gas bypassing characteristics for the cases with all three flows (No. 3, No. 7, and No. 8) are presented in Fig. 6. The gas bypassing is exclusively from the downcomer side to the draft tube side when L1 = L2. A more favorable gas bypassing characteristics can be created even for the cases where L1 = L2 = 14.7 cm by holding the No. 7 flow constant and increasing the No. 8 flow as shown for Run No. 58-i and 61-i in Fig. 6. The gas bypassing characteristics in those runs are similar to that obtained when L1 = L2 = 21.7 cm. With L1 = 29.3 cm and L2 = 21.7 cm, the gas bypassing is exclusively from the draft tube side to the downcomer side, the most favorable gas bypassing characteristics. This accounts for the high solid circulation rates observed during the experiments to be discussed later in “Solids Circulation Rate.”

There are also interesting gas bypassing phenomena observed when only No. 7 and No. 8 flows are present and the flow specifically supplied to the downcomer (No. 3 flow) is absent. When L1 = L2, the fraction of the total flow passing through the downcomer has a maximum as shown in Fig. 7. At very low and very high flow ratio, defined in this case as FR = No. 7 flow/No. 8 flow, downward gas flow in the downcomer was observed. In these cases, the solids circulation rate depends primarily on the entrainment capability of the jets. With L1 = 29.3 cm and L2 = 21.7 cm, no maximum in the flow split was observed. The fraction of the total flow passing through the downcomer is linearly and inversely proportional to the flow ratio FR. The total flow passing through the downcomer can be substantially higher. The No. 7 flow in Run No. 81-i is approximately twice that in Run No. 83-i. A stronger sink created by the No. 7 flow in Run No. 81-i tends to draw the gas toward the draft tube.

Solids Circulation Mechanisms and Solids Circulation Rate. Both solids circulation mechanisms and solids circulation rate are important aspects in designing and operating a recirculating fluidized bed with a draft tube. For commercial applications in the area of coating and encapsulation of solid particles, such as in coating of pharmaceutical tablets and in coating seeds for delayed germination and controlling the release rate of fertilizers, the particle residence time and cycle time are important considerations. The performance based on cycle time distribution analysis for coating and granulation was studied by Mann and Crosby (1973, 1975) and Mann (1983). Further discussions on this subject can be found in Ch. 6.

Solids Circulation Mechanisms. Two mechanisms for solids circulation have been observed experimentally (Yang and Keairns, 1978a).

Recirculating and Jetting Fluidized Beds 251

High-speed movies (1000 to 1500 frames per second) taken at the inlet and the midsection of the draft tube with a sand bed revealed that solids transport inside the draft tube was not a conventional pneumatic transport, where uniform solid suspension prevailed, but a slugging-type transport. The high-speed movies taken at the outlet of a 23 m/s air jet showed that the air jet issuing from the jet nozzle supplying air to the draft tube was comprised of bubble rather than of a steady jet. The bubble grew from the mouth of the nozzle until its roof reached the draft tube; then the sudden suction from the draft tube punctured the roof. A continuous stream of dilute solids suspension passed through the roof into the draft tube. Simultaneously, another bubble was initiated. As this bubble grew, it pushed a slug of solids into the draft tube. The high-speed movies taken at the midsection of the draft tube exhibited alternate sections of dilute solid suspension and solids slug occupying the total cross section of the draft tube.

A steady jet without bubbling can be maintained in a sand bed between the jet nozzle and the draft tube inlet with high jet velocities of the order of 60 m/s and without downcomer aeration. Once the downcomer is aerated, the solids circulation rate increases dramatically and the steady jet becomes a bubbling jet. Apparently, the inward-flowing solids have enough momentum to shear the gas jet periodically into bubbles.

When the polyethylene beads (density = 907 kg/m3, average size = 2800 μm) and the hollow epoxy spheres (density = 210 kg/m3, average size = 2800 μm) were used as the bed material, a steady jet between the jet nozzle and the draft tube was always observed for all experiments conducted.

Solids Circulation Rate. The solids circulation rate was obtained from the particle velocity measurements at the downcomer side by following visually the tracer particles at the wall with a stop watch. The data reported here by Yang and Keairns (1983) are for polyethylene beads (907 kg/m3 in density and 2800 μm in average particle size) and hollow epoxy spheres (210 kg/cm3 in density and 2800 μm in average particle size). The experiments were carried out in a semicircular transparent Plexiglas apparatus, 28.6 cm in diameter and 610 cm in height.

The effect of downcomer aeration, of distance between the distributor plate and the draft tube inlet, and of the distributor plate design configuration on solid circulation rate is discussed below. For ease of presentation for materials of different densities, the solid particle velocity in the downcomer rather than the solid circulation rate is used.

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Solids circulation rate was found to be strongly affected by the design configuration at the bottom of the draft tube and the downcomer due to changes in gas bypassing characteristics. This coupling effect indicates that an understanding of the gas bypassing characteristics is essential. Except for simple cases, the dependency of the gas bypassing characteristics on design and operating parameters are still not amenable to theoretical treatment as discussed earlier. A recent study by Alappat and Rane (1995) on the effects of various design and operational parameters on solids circulation rate essentially affirms the above conclusions.

Effect of Downcomer Aeration. When only the central gas flows (No. 7 and No. 8 flows) were employed without downcomer aeration, the solids circulation rate depended primarily on the entrainment rate of the jets. The linear relationship for both bed materials (hollow epoxy and polyethylene) in Fig. 8 shows that the volumetric concentration of the solids inside the draft tube after acceleration (or the gas voidage) is approximately constant, independent of particle density. This can be readily realized by expressing the volumetric solid loading in the draft tube as follows:

φ = U pr Ar (1− εr )

Eq. (16)

Gr

A straight line relationship between Upr and Gr as shown in Fig. 8 implies that the volumetric solid loading φ is approximately constant because Ar is constant and ε r can be assumed to be approximately constant when the downcomer is not fluidized. More than 85% of the gas supplied through the central No. 7 and No. 8 flows in those experiments ends up in the draft tube as can be seen from the gas bypassing data presented in Fig. 7.

Aeration of the downcomer can also be provided with a conical distributor plate (No. 3 flow) with greatly increased solids circulation rate as shown in Fig. 8. At lower downcomer aeration, the solids circulation rate is essentially similar to that without downcomer aeration at a distributor plate location of L = 21.7 cm. At higher downcomer aeration, however, a substantial increase in solids circulation rate is realized with the same total gas flow rate. Apparently, a minimum aeration in the downcomer is required in order to increase substantially the solids circulation rate. For polyethylene beads, this critical aeration rate is at a downcomer superficial

254 Fluidization, Solids Handling, and Processing

gas velocity of 0.42 m/s as compared to a minimum fluidization velocity of 0.76 m/s. This critical aeration velocity in the downcomer where the solid circulation rate starts to increase substantially was obtained by tracer gas injection. Data for hollow epoxy spheres did not extend to enough gas flow rate to permit extraction of the critical aeration velocity. However, it is evident from Fig. 8 that much less gas flow is required for the hollow epoxy spheres. The minimum fluidization velocity for the hollow epoxy spheres is 0.35 m/s.

The same kind of phenomenon was not observed when distributor plate was located closer to the draft tube inlet at L = 14.1 cm and when only No. 7 and No. 8 or No. 7 and No. 3 flows were used. When all three flow injection locations were used, substantial improvement in solids circulation rate is possible even at L = 14.1 cm as shown in Fig. 9. The critical downcomer aeration velocities (superficial velocities based on downcomer area) for the data shown in Fig. 9 were determined through tracer gas injection experiments to be 0.29 m/s at L = 21.7 cm and 0.22 m/s at L = 14.1 cm.

Effect of Distributor Plate Design. Both conical distributor plates of included angles of 60° and 90° were used. They do not seem to affect the solids circulation rate as shown in Fig. 10. Proper location of the distributor plate and the gas nozzle, however, substantially increased the solids circulation rate.

When the distributor plate was located at L1 = 29.3 cm and the concentric jet was located at L2 = 21.7 cm, No. 7 and No. 8 flows alone are enough to create high solids circulation rate as though the downcomer was separately aerated by the No. 3 flow (compare Figs. 8 and 10). This design configuration changes the gas bypassing characteristics sufficiently to provide enough aeration in the downcomer. The critical aeration in the downcomer required to promote high solids circulation rate shown in Fig. 10 was determined to be 0.25 m/s (a superficial velocity based on the downcomer area) through tracer gas injection studies. This design configuration has decisive advantages in that location of the central jet at L2 = 21.7 cm from the draft tube inlet minimizes the start-up problem (discussed later in the section “Start-up and Shutdown Considerations), and location of the distributor plate at L1 = 29.3 cm eliminates the necessity of supplying separate aeration to the downcomer through an additional location such as No. 3 flow. In some industrial applications, this design feature may prove to be a critical advantage.