Mixers power coefficient

FIG. 18-26 IXpk al curve for ma.s.s tran.sfer coefficient K.,a as i mixer power and superficial gas velocity. 

Comment. Note that for small bubbles and large bubbles alike, the mass transfer coefficients are independent of both the mixer power consumption and the gas flowrate. 

It may be that the power level for the mixer may be reduced since the energy from the gas going through the tank is higher in order to maintain a particular mass transfer coefficient, KqO-, however, this changes the relative power level compared to the gas and other mass transfer rates, such as the liquid-solid mass transfer rate. The capacity for the blending type flow pattern is not affected in the same way with changes in the mixer power level as is the gas-liquid mass transfer coefficient. 

FIG. 18-29 Typical curve for mass transfer coefficient K /i as a function of mixer power and superficial gas velocity. 

FIGURE 7-38 Power coefficient versus Reynolds number. The top curve is typical of flat-blade mixers with wide blades. The middle curve is typical of flat-blade mixers with narrow blades. The bottom curve is typical of pitched-blade mixers. (Reproduced by permission of Hayward Gordon.)... 

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. 

The power input, required for calculation of the mass transfer coefficient, is calculated from Eqs. (36) and (37). For a motionless mixer, the power comes from the gas and liquid phases for the ejector, power comes from the liquid only. 

Cas-Liquid Mass Transfer Gas-liquid mass transfer normally is correlated by means of the mass-transfer coefficient K a versus power level at various superficial gas velocities. The superficial gas velocity is the volume of gas at the average temperature and pressure at the midpoint in the taiik divided by the area of the vessel. In order to obtain the partial-pressure driving force, an assumption must be made of the partial pressure in equihbrium with the concentration of gas in the liquid. Many times this must be assumed, but if Fig. 18-26 is obtained in the pilot plant and the same assumption principle is used in evaluating the mixer in the full-scale tank, the error from the assumption is limited. 

Mass-Transfer Models Because the mass-transfer coefficient and interfacial area for mass transfer of solute are complex functions of fluid properties and the operational and geometric variables of a stirred-tank extractor or mixer, the approach to design normally involves scale-up of miniplant data. The mass-transfer coefficient and interfacial area are influenced by numerous factors that are difficult to precisely quantify. These include drop coalescence and breakage rates as well as complex flow patterns that exist within the vessel (a function of impeller type, vessel geometry, and power input). Nevertheless, it is instructive to review available mass-transfer coefficient and interfacial area models for the insights they can offer. 

The power requirecT to accomplish mixing in a motionless mixer is provided by the pump used to force the fluid through the mixer. The pressure drop through a motionless mixer is usually expressed as a multiplier K of the open pipe loss or as a valve coefficient Cy- The value of the multiplier is strongly dependent on the detail geometry of the mixer, but is usually available through information from the supplier. Fluid properties are taken into account by the value of the Reynolds... 

We find then that the coefficient P is between 2 and 4 for propellers and pitched blade impellers, and between 1 and 2 for flat disc turbines. That would mean that the latter are the most power-effective mixers. 

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