Yang Fluidization, Solids Handling, and Processing

546 Fluidization, Solids Handling, and Processing

Figure 36. Comparison of rate of reduction of iron oxide with and without coMSFB (Yan, Yao, Wang, Liu, and Kwauk, 1983).

7.0FLUIDIZATION WITH NO NET FLUID FLOW

Certain reactions between solids go through intermediate steps in the gas phase. For instance, the segregating roasting of oxidic copper ores with addition of salt and coal, and the reduction of metal oxides with solid carbon. These reactions, known as pseudo-solid-solid reactions, may sometimes be carried out advantageously in the fluidized state. Solid fluidization would enhance transport of the gaseous intermediates, and would facilitate the addition and withdrawal of a circulating thermal carrier in the case of an endothermic reaction. Inasmuch as no gaseous reactant needs to be introduced, it would be logical to seek means of achieving fluidization, even on a periodic basis, with no net fluid flow.

7.1Levitation of Discrete Particles

Levitation is a stable condition in which a particle responds to the oscillating fluid in such a way that the influence of finite buoyancy, or gravity, forces is completely neutralized so that the particle oscillates about a fixed position (Houghton, 1963, 1964, 1966, 1968; Krantz, Carley and Al-taweel, 1973; Tunstall and Houghton, 1968; Van Oeveren and Houghton, 1971). Figure 37 (Liu, 1983) shows the levitation of solid particles in air under oscillation caused by a sonic generator located at the bottom of the column.

Figure 37. Levitation of solid particles in vertically oscillating air. (a) Dilute suspension of solids. (b) Solids concentration at node. (Liu, Keling, 1983.)

548 Fluidization, Solids Handling, and Processing

A simplified analysis (Deng and Kwauk, 1990) of solids levitation led to the following ordinary differential equation involving three parameters, ux, dux/dx, and d2ux/d2x, to be determined from experiments:

creating oscillation, and ux is the function of x in the variable-separated form of the fluid velocity u(t,x).

Experiments for verifying the above equation were carried out in a U- shaped plexiglas tube, shown in Fig. 38a (Deng and Kwauk, 1990), one leg of which is filled with particles for oscillation tests and voids itself into a large-diameter tank at the top which both serves as a hydraulic seal for the system and also dissipates the kinetic energy of the oscillating fluid. Fluid oscillation was generated by a mechanical device, shown in Fig. 38b. Ionexchange resin particles, 0.29 to 0.44 mm in diameter, were tested in water, and 0.44 mm glass beads in glycerol-water mixtures.

Figure 39 shows the visual appearance of resin particles in oscillating water at different frequencies. Figure 39a shows that at low frequency the particles merely jump up and then fall back to the supporting wire net. Figure 39b shows that as frequency increases, the particles may remain in the liquid for some time before falling down. Figure 39c shows that at some still higher frequency, the particles remain levitated, executing vertically oscillating motion. At even higher frequencies the particles will rise against gravity (not shown in Fig. 39).

The frequency of fluid oscillation at which levitation takes place is plotted in Fig. 40 against the corresponding amplitude A of oscillation, the asymmetry factor ko or ratio of the duration of the downstroke to that of the upstroke, and the resin particle diameter dp. From these experimental data, the three parameters of the equation were correlated to the particle diameter:

ux = 11.1dp + 0.536

du x = −39 .5d p + 2.20 dx

d 2ux = −1.15×106 dp + 7.25×104 dx2

Curves computed from the equation using these empirically correlated parameters are also shown in Fig. 40, indicating acceptable agreement.

Trajectories of a single resin particle computed from the equation are shown in Fig. 41, indicating that at f = 4.5 Hz the particle sinks, while at f = 7.5 Hz rising levitation takes place.

7.2Semi-Fluidization through Oscillatory Flow

Both Fig. 37 and Fig. 39 show solid particles highly dispersed as a dilute phase in the oscillating fluid, either gas or liquid, without evidence of any bubbles. When a packed bed of solid particles was subjected to the action of an oscillating liquid, however, only the upstroke portion of the periodic fluid motion was capable of dispersing the solid particles against the action of gravity, while during downstroke they fell back onto the distributor plate.

The dynamics of such a mode of semi-fluidization is similar to that of jigging in ore dressing, which is a common operation in coal or ore dressing, though little used in the design of chemical reactors. Thus, at least on a periodic basis, jigging yields the same advantages as other modes of fluidization with no net fluid flow.