Gas Sparging with Mechanical Agitation

Calderbank (1958) correlated the mterfacial areas for the gas-liquid dispersion agitated by a flat-blade disk turbine as follows  

The interfacial area for N (ND, / Vs)0 3 20,000 can be calculated from the interfacial area a0 obtained from Eq. (9.41) by using the following relationship. 

For Vs 0.02 m/s, Miller (1974) modified Eq. (9.41) by replacing the aerated power input by mechanical agitation Pm with the effective power input Pe, and the terminal velocity Vt with the sum of the superficial gas velocity and the terminal velocity Vs + Vt. The effective power input Pe combines both gas sparging and mechanical agitation energy contributions. The modified equation is 

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Gas Sparging with No Mechanical Agitation In a two-phase system where the continuous phase remains in place, the hold-up is related to superficial gas velocity Vs and bubble rise velocity V, (Sridhar and Potter, 1980) ... 

For gas absorption, the equipment possibilities are generally packed columns plate distillation towers, possibly with mechanical agitation on every plate deep-bed contactors (bubble columns or sparged lagoons) and mechanically agitated vessels or lagoons. Packed towers and plate distillation columns are discussed elsewhere. Generally these... 

The previous sections dealt with gas-liquid contacting without mechanical agitation in such devices as bubble-columns and airlift towers. To obtain better gas-liquid contacting, mechanical agitation is often required. The discussion is confined to baffled sparged stirred-tanks with impellers. Aeration by surfaction impellers are sometimes used in one wastewater treatment facilities (Zlokarnik. 1978). 

Forced convection can be achieved in a variety of ways, e.g. by agitating the solution (stirring mechanically, sparging with gas bubbles, ultrasonic radiation, etc.) or by moving or rotating the metal electrode, and this will result in more rapid transport of the reacting species to the electrode than when the solution is unagitated. 

In an airlift fermenter, mixing is accomplished without any mechanical agitation. An airlift fermenter is used for tissue culture, because the tissues are shear sensitive and normal mixing is not possible. With the airlift, because the shear levels are significantly lower than in stirred vessels, it is suitable for tissue culture. The gas is sparged only up to the part of the vessel cross section called the riser. Gas is held up, fluid density decreases causing liquid in the riser to move upwards and the bubble-free liquid to circulate through the down-comer. The liquid circulates in airlift reactors as a result of the density difference between riser and down-comer. 

FIGURE 11.2 Mechanically agitated vessel with gas sparging. 

A variety of gas-liquid contacting equipment with mechanical moving elements (e.g., stirred (agitated) tanks with gas sparging) are discussed in Chapters 7 and 12, including rotating-disk gas-liquid contactors and others. 

Figure CS11.2a shows a comparison of fractional gas holdup under sparging conditions, with and without the use of a gas-inducing impeller. In the absence of a gas-inducing impeller, such a system behaves like a conventional mechanically agitated contactor (MAC). The comparison is made in terms of gas holdup as a function of power consumption per unit volume for different superficial velocities of sparged gas (uq = 0, 6,18, and 29 mm/s). It can be seen that the fractional...

Surface reaction with pore diffusion dp, w, [ ], [5](, temperature Pore structure Type of impeller and stirring speed (mechanically agitated contactor), gas velocity (sparged contactor)... 

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