Downcomer aeration

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. 

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. 

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 ... 

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 ofL = 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... 

The same kind of phenomenon was not observed when distributor plate was located closer to the draft tube inlet atL = 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. 

When all three draft tubes were operated at similar velocities, the pressure drops across all draft tubes and downcomers were comparable. However, solid particle velocities in outside downcomers close to the walls were substantially less due to wall effect and redistribution of downcomer aeration flow. Smooth operations under these conditions were possible. The solid particle velocities in outside downcomers can be increased by enlarging the downcomer cross-section or by increasing downcomer aeration through separate plenums to minimize wall effects. 

The downcomer aeration factor < tdc is defined by Eq. (6.18). It describes the fractional volumetric liquid holdup in the downcomer. 

Figure .I2 Downcomer aeration, (a) Structure of two phase mixture in the downcomer (b) depen-denco of downcomer and downcomer underflow gas fraction on downcomer liquid velocity, air-water tests. (Data for part b based on M. J. Lockett and A. A, W, Gharani, /. Chem. E. Symp. Ser, 56, p. 2.3/43, The Institution of Chemical Engineers UK, 1979.)...

Downcomer aeration factor prediction. The fractional liquid holdup varies from about 0.3 in the froth zone to close to unity in the clear liquid zone (Fig. 6.12a). The height of each zone is a complex function of system properties, operating conditions, and downcomer geometry. This makes it practically impossible to theoretically predict the average downcomer aeration factor <(>. . Correlations in the literature (e.g., 46) are based on limited data obtained in atmospheric pressure simulator work with small downcomers. It is therefore difficult to recommend them for commercial-size applications. Zuiderweg (17) presented a plot of downcomer aeration factors derived theoretically from commercial-scale high-pressure flood data. However, the plot is based on a handful of data and is therefore difficult to recommend for general aeration factor prediction. 

At high pressures, the difference between vapor and liquid density becomes smaller, and separation of vapor from liquid in the downcomer becomes difficult. Because of the more difficult separation, downcomer aeration increases, raising both downcomer frictional losses and froth backup in the downcomer. High liquid flow rates also increase tray pressure drop, tray liquid level, and frictional losses in the downcomer. For this reason, downcomer flooding is favored at high pressures and high liquid rates. 

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