Yang Fluidization, Solids Handling, and Processing

6.0SHALLOW FLUID BEDS

Another mode of bubbleless gas/solid contacting originated from the identification of a shallow region above the fluid bed distributor, before bubbles take shape. By far the major portion of gas/solid transfer takes place in this narrow region, and the contribution of the overlying solids is often only nominal. A critical factor in designing shallow beds is an understanding of the functioning of the fluid bed distributor to insure even gas distribution and minimal pressure drop.

6.1Dynamics for the Distributor Zone

Figure 28 (Yan, Yao and Liu, 1982) presents the results of turbulence measurement above a distributor plate by means of a hot-wire anemometer, indicating that turbulence intensity increases sharply near the distributor, especially for the perforated plates and the cap plate, and then diminishes beyond a certain distance (40 mm in the present case) to some nearly constant low values.

Figure 29 (Qin and Liu, 1982) shows the behavior of individual particles above the distributor recorded by video camera of small clusters of particles, coated with a fluorescent material and spot-illuminated by a pulse of ultra violet light from an optical fiber. The sequential images, of which Fig. 29 just represents exposures after stated time intervals, were reconstructed to form the track of motion of the particle cluster shown in Fig. 30. Neither this track nor visual observation of the shallow bed while fluidized, reveal any vestige of bubbles. Instead, the particles are thrown up by the high velocity jets issuing from the distributor orifices to several times their static bed height.

6.2Activated Solids Shallow Fluid Bed Heat Exchanger

Figure 31 (Liu, Liu, Li and Kwauk, 1986) shows a cylindrically shaped shallow fluid bed tubular heat exchanger. Solid particles are fluidized with a small stream of activating gas Ga, so as to insure maximal heat transfer between the particles and the exchanger tube wall. The waste gas Gw, from which heat is to be extracted, passes through the solid

particles already suspended by the activating gas, thus heating the solid particles. In merely percolating through the suspended particles, the waste gas experiences minimal pressure drop. The waste gas does not pass through the gas distributor, and therefore, cannot possibly clog the distributor holes for the carbonaceous materials the hot gas may contain. Figure 32 shows the variation of pressure of both gas streams, Ga and Gw, with their flow ratio. Obviously the pressure of Gw is much less than that of Ga, even at a flow ratio N = Gw/Ga of as high as 10.

6.3Cocurrent Multistage Shallow Fluid Bed

For slow reactions, the shallow fluid beds have been organized into a cocurrent multistage fluid bed (MSFB) reactor as shown in Fig. 33 (Yan, Yao, Wang, Liu and Kwauk, 1983). In this reactor, solids are carried up by the flowing gas stream, and once they reach the top, they are collected through a funnel and recirculated to the bottom by means of a pneumatically controlled downcomer.

Rather unexpected flow behavior was discovered while the shallow fluid beds were connected in series for cocurrent solids flow with the gas, as shown in Fig. 34. When a shallow dense bed is present at any stage, there is a positive increase of bed height to any increase in solids flow rate, at any given gas velocity. However, if the solids rate is reduced to some critical value, the dense bed slumps to practically nil height. If the solids rate is restored gradually, the dense bed does not build up until another (upper) critical value is reached. Then the dense bed reappears all of a sudden. Between these two critical solids flow rates, there exists a hysteretic region in which there are two stable bed heights corresponding to any solids rate: a dense bed and a dilute bed. This hysteresis suggests two different mechanisms of solids transport from a lower stage to an upper stage: elutriation in the dense phase, and “jetting” in the dilute phase. Above the upper critical solids flow rate, only dense bed exists, and below the lower critical rate, only dilute bed exists, both these extreme regions being monotonic in the relation between bed height and solids flow rate.