Agitation, Gas-Liquid Contact

Both open and disk flat-blade turbines are used extensively, particularly because of the high discharge velocities normal to the flow of gas which they maintain. Especially in the larger sizes, the disk type is preferred. They best specified with dJT — 0,25 to 0.4 and set off the bottom of the vessel a distance equal to the impeller diameter. In some cases, especially designed impellers can be used to induce the gas flow from the space above the liquid down into the agitated mass [81, 115]. 

For production of effective gas dispersions with disk flat-blade turbines, the impeller speed should exceed that given by [122] 

If the gas flow rate is too large, especially for liquids of high viscosity [20], bubbles of gas become trapped below the eye of the impeller, thus blocking the flow of liquid from below to the impeller. Such an effect should be minimized for impeller speeds exceeding those of Eq. (6.18), In any event, for most installations the superficial gas velocity based on the vessel cross-sectional area does not exceed = 0.08 m/s (0.25 ft/s). 

The presence of gas in the vessel contents results in lowering the power required to turn an impeller at a given speed, probably because of the lowered mean density of the mixture. Of the many correlations which have been attempted [86], that shown in Fig. 6.7, despite some shortcomings [20], is recommended for disk flat-blade turbines [24] in water and aqueous solutions of nonelectrolytes. It has been found applicable to vessels ranging in capacity up to 4 m (140 ft ) and larger [5]. The broken lines of the figure adequately represent the data, and 

I It is anticipated that the superficial gas velocity rather than Qa, would better 

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Let us first examine the main variables that affect the absorption rate in agitated gas-liquid contacting. Some of the more important ones are ... 

In addition, it was concluded that the liquid-phase diffusion coefficient is the major factor influencing the value of the mass-transfer coefficient per unit area. Inasmuch as agitators operate poorly in gas-liquid dispersions, it is impractical to induce turbulence by mechanical means that exceeds gravitational forces. They conclude, therefore, that heat- and mass-transfer coefficients per unit area in gas dispersions are almost completely unaffected by the mechanical power dissipated in the system. Consequently, the total mass-transfer rate in agitated gas-liquid contacting is changed almost entirely in accordance with the interfacial area—a function of the power input. 

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