Impeller down-pumping

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 the layer of graphite and corrosion products is impervious to the solution, corrosion wdl cease or slow down. If the layer is porous, corrosion will progress by galvanic behavior between graphite and iron. The rate of this attack will be approximately that for the maximum penetration of steel by pitting. The layer of graphite formed may also be effective in reducing the g vanic action between cast iron and more noble alloys such as bronze used for valve trim and impellers in pumps. 

Down-pumping axial flow impellers are more appropriate for most particle-suspension operations (Perry and Green, 1984). Typical dimensional ratios are DT/Da = 2-3.3, and Za/DT = 0.25-0.5. The power can be calculated as in the case of liquid agitation (see eq. (3.107)) (Perry and Green, 1984). 

A modern high efficiency gas-liquid STR commonly consists of a radial gas dispersing impeller in combination with one or more axial down-pumping impellers, as seen in Fig. 2. The type and number of open impellers used in the tank depend on the gassed liquid height and the media viscosity (see Table 4). Recommended axial impellers are 45° four-bladed pitch blade turbines... 

Hari-Prajitno, H. Mishra, V.I. Takemaka, K. Bujalski, W. Nienow, A.W. McKemmie, J. Gas-liquid mixing studies with multiple up and down pumping hydrofoil impellers power characteristics and mixing times. Can. J. Chem. Eng. 1998, 76, 1056-1068. 

Injection downward into the impeller at about Z>/4 from the centerline of the impeller shaft and G/D above the impeller midplane. High-efficiency three-blade down pumping turbine. 

Extensive analysis and experimentation on dip-pipe design has been carried out by Penney and co-workers (Jo et al. 1994), and the reader is referred to this work for specific design recommendations for diameter and flow rate. Their recommendations are summarized in Table 6-3 for six-blade flat-blade and three-blade high-efficiency down-pumping impellers. These recommendations should be followed to prevent dip-pipe back-mixing, enhance local blending, and prevent solids plugging. 

Jaworski Z., Nienow A.W., Oyster K.N.. An LDA Study of the turbulent Flow Field in a baffled Vessel Agitated by an Axial, Down-Pumping Hydrofoil Impeller, Can. J. chem. eng. 74 (1996)... 

Down-pumping axial flow impellers are more appropriate for most paiticle-suspension operations (Peiiy and Green, 1984). Typical dimensional ratios are = 2-3.3, and... 

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