Power requirement for agitation

The power requirements for agitation will depend on the degree of agitation required and will range from about 0.2 kW/m3 for moderate mixing to 2 kW/m3 for intense mixing. 

This group is referred to in Volume 1, Chapter 7 in the discussion of the power requirements for agitators. 

Several investigations have been carried out into the power requirements for agitation of aerated liquids including those of Yung et al.m> and Luong and Volesky(72) and it is generally concluded that the power required is less for an aerated system than for a non-aerated system. 

Ans. (a) Increasing aeration decreases power requirement for agitation (b) rpm has a larger effect on /f, than P, since power is consumed in shearing the air bubbles. 

Figure 3.24. Typical plot of power requirement for agitation in gassed reactors, expressed as the degree of power decrease PqIP versus aeration number as a function of different types of impellers [flat-blade turbines (FBT vaned disks (VD) with varying number paddle (P)]. (Adapted from Ohyama and Endho, 1955.)...

Example 4-8 Calculate the power required for agitation for a three-blade marine impeller of 0.610-m pitch and 0.610-m diameter operating at 100 rpm in an unbaffled tank containing water at 21.1°C. The tank diameter is 1.83 m, the depth of... 

Equipment suitable for reactions between hquids is represented in Fig. 23-37. Almost invariably, one of the phases is aqueous with reactants distributed between phases for instance, NaOH in water at the start and an ester in the organic phase. Such reac tions can be carried out in any kind of equipment that is suitable for physical extraction, including mixer-settlers and towers of various kinds-, empty or packed, still or agitated, either phase dispersed, provided that adequate heat transfer can be incorporated. Mechanically agitated tanks are favored because the interfacial area can be made large, as much as 100 times that of spray towers, for instance. Power requirements for L/L mixing are normally about 5 hp/1,000 gal and tip speeds of turbine-type impellers are 4.6 to 6.1 i7i/s (15 to 20 ft/s). 

Consider a stirred tank vessel having a Newtonian liquid of density p and viseosity p, is agitated by an impeller of diameter D, rotating at a rotational speed N. Let the tank diameter be D, the impeller width W, and the liquid depth H. The power P required for agitation of a single-phase liquid ean be expressed as ... 

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. 

Hyman (H21) reviewed the literature published up to about 1960 on power requirements for gas-liquid agitated systems. This section will report work published subsequent to his review. 

The power curve obtained on a pilot size unit can be used directly to obtain the power requirements for a geometrically similar production size unit once the agitator speed is known. 

The MSSR presents the same advantages as BSCR, such as high efficiency of heat-and mass-transfer and minimal intraparticle diffusional resistance, and are convenient for use in batch processes. For these reasons, the slurry-agitated reactors are also suitable for kinetic studies in the laboratory. Some of their major drawbacks are large power requirement for mechanical agitation,... 

Determine impeller size for required power input. The impeller required for the process must draw 7.91 hp per 1000 gal or 79.1 hp for the 10,000-gal batch. This power level must be achieved at the gas flow rate required by the process, which is, Qa = 1507 ft3/min. The power required to operate an agitator impeller for gas dispersion can be much less than the power required for a liquid without gas. The ratio of power with gas to power without P/Po is shown in Fig. 12.7 and is a function of the dimensionless aeration number NAe =Qa/ND3. 

It appears that the power requirements for liquid-liquid agitated systems can generally be dealt with in the same manner as for the case of one-liquid-phase systems. The appropriate effective mean viscosity of the two-phase mixture must be used as indicated above. Similarly, the average density must be used. In the power studies cited, a simple volume-average was used ... 

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 and Tip-speed Requirements for Agitation in Baffled Tanks... 

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