Yang Fluidization, Solids Handling, and Processing

point to the middle section of a three-section reducer. A considerable amount of CO is generated between reaction of C and FeO, which fluidizes the two top sections by continuous flow. Division into the two fluidized sections helps the establishment of a solids concentration gradient (dilute in the top section, and dense in the lower section) which tends to suppress longitudinal mixing. Only the lowest of the three-section reducer is semifluidized through jigging, in which only the last traces of CO are evolved.

All the CO resulting from the pseudo solid-solid reaction is conducted, together with entrained char, from the top fluidized section through a constriction, in which the high-velocity gas flow prevents backflow, to a transport combustor, where the CO is burned to CO2 with preheated air, along with as much of the char as is called for by heat balance to maintain the endothermic FeO-C reaction. The heated recycled char is separated from the off gas at the top of this transport combustor in a hot cyclone and is returned as a thermal carrier to the lower part of the lowest jigged section, while the hot flue gas from the transport combustor is used to preheat the incoming air in a recuperator.

In the top fluidized section, where the first portion of FeO reduction, that is, prereduction, takes place, the lighter coke particles are elutriated from the heavier ore particles. The lower fluidized section is sized for adequate FeO-C mixing in order to maximize contacting of the reacting solids. Then, in the lowest section, where vestigial amounts of CO are evolved, solids are semi-fluidized through jigging to effect an additional stage of coke upflow in order to denude the powdered metallic iron product of its accompanying char.

Jigging is produced by a gas oscillation generator, which could be either mechanical or electromagnetic, separated from the bottom of the jigged section first by a solids knockout hopper and then by a thermal break, which is essentially a packed-bed heat regenerator. To protect further the gas oscillation generator from hot gases, a cooler is interposed between it and the thermal break.

The process proposed in Fig. 42 uses powdered iron ore directly without prior treatment such as sintering or pelletization as called for in the case of the blast furnace, and powdered coke or coal without prior gasification. Jigging through gas oscillation replaces expensive gas recycling which calls for first cooling and then reheating, often accompanied by unavoidable carbon deposition through the Bouduord reaction.

560 Fluidization, Solids Handling, and Processing

Figure 46. Typical bed collapse curve traced by optical-fiber probe tracking instrument (solids:alumina A66, 140–280 microns). (Yang, Tung, and Kwauk, 1985.)

Not all powders exhibit all the three stages described above, as shown in Fig. 47. Depending on the presence or absence of the above stages, a three-digit qualitative designation has been formulated:

100 Bubble-escape stage only

123Three stages: bubble escape, hindered sedimentation and solids consolidation

020 Hindered sedimentation only

023 Hindered sedimentation plus solids consolidation

562 Fluidization, Solids Handling, and Processing

Mathematical modeling of the three-stage bed collapsing process led to a more quantitative characterization of powders, in the form of a dimensionless number called the dimensionless subsidence time:

To test the viability of Θ in quantifying fluidizing characteristics, it is plotted against the ratio of incipient bubbling velocity to incipient fluidization velocity, umb/umf, the latter being calculated after Geldart (1972). Figure 48 shows that a linear relation exists between ln(umb/umf) and θ¼ as represented by the following empirical relation

ln(umb /umf) = 4 Θ¼

The straight line starts from Θ = 0 and umb /umf = 1, and extends without limit towards the upper right-hand corner. The value of umb /umf = 1 obviously signifies bubbling. The corresponding value of Θ is zero, for which stages 1 and 2 of the bed collapsing process take place almost instantaneously. As the fluidizing characteristics improve, the value of umb/umf becomes progressively greater than unity, signifying particulate expansion. The corresponding value of Θ also increases, showing a slower bed collapsing process. As the curve tends toward even larger values of Θ, the characteristics of particulate fluidization becomes more predominant.

8.2Improving Fluidization by Particle Size Adjustment

Catalyst powders with carefully specified particle size distribution have been known to possess good fluidization characteristics. Generally, addition of fine particles to coarse particles tends to improve the latter’s fluidization characteristics. Experiments were thus conducted on binary particle mixtures, each consisting of a fairly close particle size distribution.

Figure 49 shows a set of bed collapsing curves for a Geldart Group A- A (for Geldart’s classification of solid particles, see Geldart, 1972, 1973) binary solids mixture, two closely sized alumina powders, of average particle diameter 104 and 66 microns, respectively. The curve on the extreme left with 0% fines represents the pure coarse component, which is