Gas-sparged stirred tanks

Gas-liquid mass transfer in fermentors is discussed in detail in Section 12.4. In dealing with in gas-sparged stirred tanks, it is more rational to separate and a, because both are affected by different factors. It is possible to measure a by using either a light scattering technique [9] or a chemical method [4]. Ihe average bubble size can be estimated by Equation 7.26 from measured values of a and the gas holdup e. Correlations for have been obtained in this way [10, 11], but in order to use them it is necessary that a and d are known. 

It would be more practical, if A in gas-sparged stirred tanks were to be directly correlated with operating variables and liquid properties. It should be noted that the definition of k a for a gas-sparged stirred tank (both in this text and in general) is based on the clear liquid volume, without aeration. 

The empirical Equation 7.36a, b [3] can be used for very rough estimation of in aerated stirred tanks with accuracy within 20-40%. It should be noted that in using Equation 7.36a, b, Pq must be estimated by the correlations given in Section 7.4.2. For an air-water system, a water volume V 2.6 m, and a gas- 

It is worth remembering that the power requirement of gas-sparged stirred tanks per unit liquid volume at a given superficial gas velocity Uq is proportional to L where N is the rotational speed of the impeller (T ) and L is the tank size (L), such as the diameter. Usually, k a values vary in proportion to (Pq/V) and Uq , where m = 0.4-0.7 and = 0.2-0.8, depending on operating conditions. 

in some cases, such as scaling-up of geometrically similar stirred tanks, the estimation of power requirement can be simplified using the above relationship. 

In evaluating kLa in gas-sparged stirred tanks, it can usually be assumed that the liquid concentration is uniform throughout the tank. This is especially true with small experimental apparatus, in which the rate of gas-liquid mass transfer at the 

Estimate kLa for oxygen absorption into water in the sparged stirred tank of Example 7.4. The operating conditions are the same as in Example 7.4. 

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Ford, J.J., Heindel, T.J., Jensen, T.C., and Drake, J.B. (2008), X-ray computed tomography of a gas-sparged stirred-tank reactor, Chemical Engineering Science, 63 2075-2085. 

R Gas huhhle swarm in sparged stirred tank reactor with solids present... 

Values of kj a for viscoelastic liquids in aerated stirred tanks are substantially smaller than those in inelastic liquids. Moreover, less breakage of gas bubbles in the vicinity ofthe impeller occurs in viscoelastic liquids. The following dimensionless equation [8] (a modified form of Equation 7.37) can be used to correlate kj a in sparged stirred tanks for non-Newtonian (including viscoelastic) liquids ... 

Bubble columns are most widely spread, followed by sparged stirred tanks. Often both are used for a certain application. Stirred tanks and bubble columns have similar characteristics with respect to mass transfer (see Table 8.1). In both reactors the liquid is well mixed. The gas phase in the bubble column shows plug-flow behavior, while in stirred tanks it is well mixed. 

The first major decision in the choice of a reactor for gas-liquid reactions taking place in the liquid phase is based on the optimal usage of the total reactor volume, i.e. the choice of the parameter P, which is the ratio of the liquid-phase volume to the volume of the diffusion layer (see Section 8.4.2). When reactions are slow compared to the mass transfer from the gas to the liquid, sparged stirred tanks and bubble columns are preferred, as these reactors have the largest bulk liquid volume. On the other hand, fast reactions for a large part take place in the diffusion layer, so in this case spray columns and packed columns are more suitable. 

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). 

Stirred-tank bioreactors mechanically agitate the gas-liquid dispersion, and the resulting power draw is an important parameter in these bioreactors. The measured power draw is used to quantify two dimensionless numbers in air-sparged stirred-tank bioreactors, the ungassed and gassed power numbers. 

Garcia-Ochoa, F.F., and Gomez, E. (2004), Theoretical prediction of gas-liquid mass transfer coefficient, specific area and hold-up in sparged stirred tanks, Chemical Engineering Science, 59(12) 2489-2501. 

Garcia-Ochoa F, Gomez E. (2005) Prediction of gas-liquid mass transfer in sparged stirred tank bioreactors. BiotechnoL Bioeng., 92 761-772. 

Stirred tanks are often used for gas-liquid reactions. The usual geometry is for the liquid to enter at the top of the reactor and to leave at the bottom. The gas enters through a sparge ring underneath the impeller and leaves through the vapor space at the top of the reactor. A simple but effective way of modeling this and many similar situations is to assume perfect mixing within each phase. 

The reactor is operated in the semibatch mode with component A being sparged into the stirred tank. Unreacted A and the reaction products leave through the gas phase so that the mass of liquid remains constant. To the extent that these assumptions are true and the catalyst does not deactivate, a pseudo-steady-state can be achieved. Find (flg)o j. Assume that Henry s law is valid throughout the composition range and ignore any changes in the gas density. 

As in boundary-layer flows, smaller reference floe diameters are found with gas sparging than with the same average power input in a baffled stirred tank 127] or [44,45]. This can be explained if it is assumed that the floes come into close contact with the gas phase and find their way into the zones of higher stress. 

It can be seen that for the same average power input, greater stresses are produced by gas sparging than by many impellers. Fig. 17. According to the comparison in Fig. 17, evidently zones exist in bubble columns in which the energy densities are 20 times higher than in a stirred tank. But the comparison on the basis of average power input in Fig. 16 shows that also impeller (for example small inclined blade impellers) exist which produce more shear than bubble columns. 

For bubble columns the value of knq Am strongly depends on the superficial gas velocity. For the stirred tank reactor gas sparged is immediately sucked into the gas cavities behind the stirred blades. The value of k)iq depends strongly on both the superficial gas velocity, the pressure at the stirrer level and the liquid volume. 

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. 

The ratio of the power requirement of gas-sparged (aerated) liquid in a stirred tank, Pq, to the power requirement of ungassed liquid in the same stirred tank, Pq, can be estimated using Equation 7.34 [7]. This is an empirical, dimensionless equation based on data for six-flat blade turbines, with a blade width that is one-fifth of the impeller diameter d, while the liquid depth Hp is equal to the tank diameter. Although these data were for tank diameters up to 0.6 m. Equation 7.34 would apply to larger tanks where the liquid depth-to-diameter ratio is typically... 

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