Draft Tubes

In the Davy-Powergas unit (118—120), shown in Figure 13c, the Hquids mn through a draft tube and are pumped by an impeller mnning directly above the draft tube. The dispersion flows out from the top of the mixer and down through a channel into a rectangular settler. Large units of this type are used for copper extraction (7). 

Fig. 36. Examples of specially designed mixers (a) draft tube circulator, (b) airlift with draft tube, (c) Fluidics vortex mixer, and (d) mixer emulsifier.

If severe heat-transfer requirements are imposed, heating or cooling zones can be incorporated within or external to the CSTR. For example, impellers or centrally mounted draft tubes circulate Hquid upward, then downward through vertical heat-exchanger tubes. In a similar fashion, reactor contents can be recycled through external heat exchangers. 

Eig. 23. Airlift reactors (a) spHt cylinder internal loop, (b) draft tube internal loop, and (c) external loop (94). 

Fig. 23. Schematic diagram of draft-tube-baffle crystallizer.

The fluidfoil impellers in large tanks require only two baffles, but three are usually used to provide better flow pattern asymmetiy. These fluidfoil impellers provide a true axial flow pattern, almost as though there was a draft tube around the impeller. Two or three or more impellers are used if tanks with high D/T ratios are involved. The fluidfoil impellers do not vortex vigorously even at relatively low coverage so that if gases or solids are to Be incorporated at the surface, the axial-flow turbine is often required and can be used in combination with the fluidfoil impellers also on the same shaft. 

Using a draft tube in the tank for solids suspension introduces another, different set of variables. There are other relationships that are veiy much affected by scale-up in this type of process, as shown in Fig. 18-22. Different scale-up problems exist whether the impeller is pumping up or down within the draft tube. 

FIG. 18-22 Typical draft tube circulator, sbown bere for down-pumping mode for tbe impeller in tbe draft tube. 

When pumping down the draft tube, flow normally makes a more troublefree velocity change to a flow going up the annulus. Since the area of the draft tube is markedly less than the area of the annulus, pumping up the draft tube requires less flow to suspend sohds of a given settling velocity than does pumping down the draft tube. 

Draft Tube Designs and Spouted Beds A draft tube is often employed to regnlate particle circnlation patterns. The most common design is the Wurster draft tnbe flnidbea employed extensivelvin the pharmacentical industry, nsnally for coating and layered growtJi applications. The Wurster coater nses a bottom positioned spray, bnt other variations are available (Table 20-47). 

A basic stirred tank design is shown in Fig. 23-30. Height to diameter ratio is H/D = 2 to 3. Heat transfer may be provided through a jacket or internal coils. Baffles prevent movement of the mass as a whole. A draft tube enhances vertical circulation. The vapor space is about 20 percent of the total volume. A hollow shaft and impeller increase gas circulation (as in Fig. 23-31). A splasher can be attached to the shaft at the hquid surface to improve entrainment of gas. A variety of impellers is in use. The pitched propeller moves the liquid axially, the flat blade moves it radially, and inclined blades move it both axially and radially. The anchor and some other designs are suited to viscous hquids. 

FIG. 23-30a A basic stirred tank design, not to scale, showing a lower radial impeller and an upper axial impeller boused in a draft tube. Four equally spaced baffles are standard. H = beigbt of liquid level, Dj = tank diameter, d = impeller diameter. For radial impellers, 0.3 < d/Dt < 0.6. 

In the reactor shown in Fig. 23-41, a stable fluidized bed is maintained by recirculation of the mixed fluid through the bed and a draft tube. An external pump sometimes is used instead of the built-in... 

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