Impeller flooding

Flooding of Agitator Impellers A review of impeller flooding has been done by Sensei et al. [AlChE Symp. Series No. 283, 89 (1993)] and they have offered the following flooding correlation for a six-bladed disk-type turbine. 

Impeller flooding Effective mineral as catalysts Metabolites... 

Minimum rotational speed of impeller (flooding, solids suspension) ... 

Flooding of Agitator Impellers Impeller flooding correlations for six-blade disk (6BD) Rusthon turbines and six-blade disk Smith turbines (Chemineer designation CD-6) are presented by Bakker, Myers, and Smith [Chem. Eng., 101, 98 (Dec. 1994)] and a review of impeller flooding. The Bakker et al. (loc. cit.) correlation is... 

Impeller flooding is a phenomenon that limits the gas dispersion and mass transfer effectiveness of an impeller. Above the flood point (i.e., above the flooding gas sparging rate), gas effectively escapes the high shear zone around the impeller blade tips and is not dispersed effectively, and the mass transfer efficiency of the impeller decreases. In Bakker et al. (1994), a correlation is given for flooding of 6BD and the CD6 impellers. 

For high aeration values, gas expansion power Pgas can reach the magnitude of the aerated agitator power Pg. If gas expansion power is greater than or equal to the stirrer power, there is the likelihood of impeller flooding, which limits gas dispersion to the central region of the tank. 

In sparged agitated tanks, a considerable increase in interfacial area occurs and overall gas-liquid mass transfer is considerably higher. Many correlations indicate that the overall mass transfer increases approximately as volumetric rate to the 1/2 power. Studies of nonelectroljdic solutions as well as electrolytic solutions have been performed (78). Correlations are published for the critical agitation speed for impeller flooding (79). Correlations for multistage stirred tanks are also available. 

The analysis above shows that higher specific power input is necessary both to prevent impeller flooding and to achieve complete dispersion at the large scale. Also, from the flooding correlation, for constant aeration rate and a fixed vessel size,... 

Mechanical agitation is needed to break up the gas bubbles but must avoid rupturing the cells. The disk turbine with radial action is most suitable. It can tolerate a superficial gas velocity up to 120 m/h. (394 ft/h) without flooding, whereas the propeller is limited to about 20 i7i/h (66 ft/h). When flooding occurs, the impeller is working in a gas phase and cannot assist the transfer of gas to the liquid phase. Power input by agitation and air sparger is 1 to 4 W/L (97 to 387 Btu/[fF-h]) of liquid. 

Milk-of-lime transfer pumps should be of the open impeller centrifugal type. Pumps having an iron body and impeller with bronze trim are suitable for this purpose. Rubber-lined pumps with rubber-covered impellers are also frequently used. Makeup tanks are usually provided ahead of centrifugal pumps to ensure a flooded suction at all times. Plating out of lime is minimized by the use of soft water in the makeup tank and slurry recirculation. Turbine pumps and eductors should be avoided in transferring milk of lime because of scaling problems. 

With a draft tube inserted in a tank, no sidewall baffles are required, and, the flow into the axial impeller mounted inside the tube is flooded to give a uniform and high flow pattern into the inlet to the impeller. The upflow in the annulus around the tube has sufficient velocity to keep particles in suspension, if necessary. 

Raceway aquaculture systems, 3 193-194 Rack plating, 9 768 Ractopamine, 13 2, 14, 16 Radappertization, 3 655 Radar systems, vitreous silica in, 22 442 Radialene, 21 144 Radial flow impellers, 16 673, 684 flooding with, 16 701... 

Apart from the critical impeller speed for solid suspension and efficient gas dispersion, flooding is also a very important phenomenon in three-phase systems. Flooding may take place at low impeller speed or high gassing rate. Under these conditions, the gas is dispersed just around the central shaft of the tank, whereas the solids are settled at the bottom. Flooding characteristics are not affected by particles. Furthermore, high-viscosity liquids are able to handle more gas before flooding than low-viscosity liquids. 

Birch and Ahmed (1997) have shown that the location of the sparger has an important role in the flooding characteristics of impellers, and thus on efficient solids suspension. 

Below a critical power input the gas bubbles are not affected laterally but move upward with their natural buoyancy. This condition is called gas flooding of the impeller. At higher power inputs the gas is dispersed radially, bubbles impinge on the walls and are broken up, consequently with improvement of mass transfer. A correlation of the critical power input is shown as Figure 10.10. 

Figure 10.10. Minimum power requirement to overcome flooding as a function of superficial gas velocity and ratio of impeller and tank diameters, d/D. [Hicks and Gates, Chem. Eng., 141-148 (19 July 1976)).

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