Power requirements agitated vessels

This chapter reviews the various types of impellers, die flow patterns generated by diese agitators, correlation of die dimensionless parameters (i.e., Reynolds number, Froude number, and Power number), scale-up of mixers, heat transfer coefficients of jacketed agitated vessels, and die time required for heating or cooling diese vessels. 

For the agitated and aerated vessel, the ratio of power requirements for aerated versus non-aerated systems is expressed by a dimensionless number known as the aeration rate the value is obtained from Figure 6.7. 

For continuous operation with two liquids, where the vessel is operated full and consequently in the absence of vortex, very limited tests with flat-bladed turbines (F3) have tentatively indicated that (a) the effect of extent of baffling on power requirements may be different from that found in open vessels, but has not yet been established (b) at all except low agitator speeds the ratio of phases contained within the vessel is the same as the ratio in the feed mixture and (c) the power is independent of rate of flow through the vessel. But these conclusions are all based on measurements with small vessels and with only two systems. They need confirmation under a much wider variety of conditions. 

Although, as shown in this monograph, mechanical agitation is provided in a number of different ways, the most common method is by a stirrer in a standard vessel. In many mechanically agitated reactors, the vessel contains internals such as baffles (particularly for low-viscosity fluids), feed and drain pipes, heat transfer coils, and probes (e.g., thermometers or thermocouples, pressure transducers, level indicators). The degree of mixing and power requirement depend on the nature of the internals present in the vessel. 

Hirsekorn and Miller (H2) made visual qualitative observations of the suspension of solids by paddle agitation in very viscous liquids (to about 50,000 cp.). For low impeller Reynolds numbers (about 10) in geometrically similar systems (6-, 12-, 18-in. vessels) the major factor in effecting particle suspension appeared to be power input per unit volume. In any given case the power required for complete suspension of all the particles was affected by system geometry and the settling velocity of the solids. No detailed correlation of the observations was presented. 

POWER CONSUMPTION. An important consideration in the design of an agitated vessel is the power required to drive the impeller. When the flow in the tank is turbulent, the power requirement can be estimated from the product of the flow q produced by the impeller and the kinetic energy (. per unit volume of the fluid. These are... 

Example 9.5. An agitated vessel 6 ft (1.8 m) in diameter with a working depth of 8 ft (2.44 m) is used to prepare a slurry of 150-mesh fluorspar in water at 70 F. The solid, has a specific gravity of 3.18, and the slurry is 25 percent solids by weight. The impeller is a four-blade pitched-blade turbine 2 ft (0.61 m) in diameter set 1.5 ft above the vessel floor, (a) What is the power required for complete suspension (b) What is the critical stirrer speed ... 

Example 9.8. A pilot-plant vessel 1 ft (305 mm) in diameter is agitated by a six-blade turbine impeller 4 in. (102 mm) in diameter. When the impeller Reynolds number is I0 , the blending time of two miscible liquids is found to be 15 s. The power required is 2 hp per 1000 gal (0.4 kW/m ) of liquid. ( ) What power input would be required to give the same blending time in a vessel 6 ft (1830 mm) in diameter (b) What would be the blending time in the 6-fl (1830-mm) vessel if the power input per unit volume was the same as in the pilot-plant vessel ... 

The power per unit volume required in the 6-ft vessel is then 2 x 36 = 72 hp per 1000 gal 14.4 kW/m ). This is an impractically large amount of power to deliver to a low-viscosity liquid in an agitated vessel. 

Although it is impractical to achieve the same blending time in the full-scale unit as in the pilot-plant vessel, a moderate increase in blending time in the larger vessel reduces power requirement to a reasonable level. Such trade-offs are often necessary in scaling up agitation equipment. 

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