Sparged stirred tanks

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

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. 

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 evaluating k a 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 free liquid surface might be a considerable portion of total mass transfer rate. This can be prevented by passing an inert gas (e.g., nitrogen) over the free liquid. 

Estimate k a for oxygen absorption into water in the sparged stirred tank of Example 7.4. Operating conditions are the same as in Example 7.4. 

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 ... 

Figure 13. Sherwood number for liquid-solid mass transfer in sparged stirred tank slurry reactors (adapted from Asai et al. [119]).

Figure L2 Common reactor types with moving catalyst beds (a) fluid-bed reactor (b) bubble column with suspended catalyst (c) sparged stirred tank with suspended catalyst.

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. 

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. 

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. 

Figure 20-3 Polysulfide selectivity (maximum selectivity achieved during 100 min of reaction) versus impeller speed and oxygen flow rate. GeuCTation in a laboratory sparged stirred-tank reactor at T = 90°C using 2.0 g/L of Fisha- MnOi catalyst. Selectivity is the fraction of polysulfide (as g S°/L) formed divided by the hydrosulfide ion consumed during oxidation (as g S/L). (From Dobson and Bennington, 2002.)...

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