Stirred impeller zone

Due to the liquid circulation in stirred tanks which transports all particles with a certain frequency through the impeller zone, they undergo the maximum shear stress. 

Phase-averaged values of 4 in a plane midway between two baffles of a stirred tank have been plotted in Fig. 1 (from Hartmann et al., 2004a) for two different SGS models (Smagorinsky and Voke, respectively) in LES carried out in a LB approach. The highest values, i.e., the strongest deviations from isotropy, occur in the impeller zone, in the boundary layers along wall and bottom of the tank, and at the separation points at the vessel wall from which the anisotropy is advected into the bulk flow. In the recirculation loops, the turbulent flow is more or less isotropic. 

This relationship will provide an upper bound on Eimpdfer as some energy will be made available to the mean flow. This mean flow energy is then being dissipated at the boundaries of the chamber. Further, energy will be dissipated outside of the impeller zone and impeller stream. However, laboratory and numerical studies have indicated that the vast majority of dissipation in stirred chambers takes place in the impeller zone and impeller stream (e.g. Cutter (1966), Bakker and Van den Akker (1994), Schafer et aL (1997)). 

How will micro-mixing times vary in the stirred tank described in example 4a, when it is known that local values of e vary from 0.05 to 50 times the mean value The micro-mixing time is inversely proportional to Te, according to both models expressed by eqs. (4.9) and (4.10), so we may expect variations from about 0.008 s (in the impeller zone) to about 0.27 s in the bulk of the liquid. 

The diffusion time is now the micro-mixing time in the zone near the inlet tube, and is related to the average energy dissipation rate. Of course, one would always mtroduce the reactor feed in the impeller zone. The most uncertain factor is c (= y/n), since it varies strongly in a stirred vessel in relation to the distance to the impeller blades. Note that these equations can only be used for scale-up as long as eq. (4.15) applies, i.e. that a purely laminar regime prevails. 

Some approximate methods have been applied previously to deal with the inhomogeneous nature of flow patterns in stirred tanks [67, 68]. Koh [67] divided the stirred tank into three compartments, the impeller zone, the bulk zone and a dead space, and assigned different shear rates for each compartment. Furthermore, Koh et al. [69] ignored the dead space, but split the impeller zone into impeller tip zone and impeller zone. Ducoste [68] essentially followed the same approach, dividing the suspension volume into two zones, the impeller discharge zone and the bulk zone. 

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. 

Stirred tank performance often is nearly ideal CSTR or the model may need to take into account bypassing, stagnant zones or other parameters associated with the geometry and operation of the vessel and the agitator. Sometimes the vessel can be visualized as a zone of complete mixing in the vicinity of the impellers followed by a plug flow zone elsewhere, thus a CSTR followed by a PFR. 

As discussed in Chapter 6, high-energy dissipation zones have been identified for certain stirred tank/ impeller configurations. These zones are often small, and they can move enough so that the exact location of and linear velocity from an addition dip pipe ate very difficult to optimize. When very intense micro- and/or mesomixing are required, stirred tanks are not the ideal type of equipment to catty out a robust, reproducible process. 

The solids are kept in suspension if the pumping capacity of the impeller causes strong enough circulation of the liquid. In most processes, complete suspension of the particles is not required. Often, so-called off-bottom suspension is sufficient, which means that all particles are moving above the bottom of the tank with some vertical velocity. Radial flow impellers are usually not very effective in suspending solid particles. Actually, about three times more power is required for a radial turbine to provide the same degree of uniformity compared to an axial turbine. This is because the radial turbines pick up particles from the bottom of the tank by the suction side of the impeller, which is only half of the total flow from the impeller. Due to the appearance of an upper and a lower circulation zone, the contents of the two zones are not sufficiently mixed. Axial impellers are therefore most frequently used for the suspension of solids in stirred tanks [65]. 

STR, stirred tank reactor consisting of a cylindrical vessel (diameter 30 cm, volume 21.2 1) with a flat bottom and four symmetrical baffles and a six-blade disc impeller with a diameter of 10 cm. Screw loop reactor, with the volume of the dispersing zone about 64 ml and a reactor volume of 1.67 1. 

Figure 4.6. Scheme of a two-compartment model for oxygen transfer in a stirred tank. The liquid phase is structured into a mixed zone (LJ and into a zone with freely rising bubbles in analogy to a bubble column (L2), with a connecting exchange (Fp) due to impeller pumping. (From Oosterhuis, 1984.)... 

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