Multiple Impeller Systems

Multiple impeller systems are able to distribute energy throughout the reactor more efficiently, which leads to a more homogeneous shear rate distribution. Liquid circulation and gas dispersion are also improved, leading to longer gas-phase residence times. These factors lead to better gas utilization, higher gas-liquid mass transfer coefficients (Bouaifi and Roustan, 2001 Cabaret et al., 2008 Fujasova 

The impeller is the key component for proper STR operation, especially for multiple impeller systems. A proper selection procedure has to consider numerous options and their applicability to the particular process of interest. A mixed 

The mixed configuration efficiency and the declining increase in with increasing number of turbines are determined by the impeller loading. The bottom 

Bouaifi et al. (2001) found that the average bubble diameter was larger in the bottom section of the reactor than the upper section. They concluded that bubbles formed a distribution such that the larger bubbles were in a region outside the impeller stream and were up to four times larger than the bubbles entrained in the impeller stream. More specifically, gas in these setups would concentrate about the impeller shaft, impeller tip, and within the radial area between the impeller and reactor walls (Boden et al., 2008 McFarlane and Nienow, 1996a). 

These observations were made for an axial system, but are very similar to those made by Stenberg and Andersson (1988b) for a IRT setup, which produced a similar qualitative mass transfer behavior for these impeller types. 

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Gogate, P.R., Beenackers, A.A.C.M. and Prandit, A.B. (2000) Multiple impeller systems with a special emphasis on bioreactors a critical review. Biochemical Engineering Journal, 6, 109-144. 

In multiple impeller systems, it appears that conditions in the region around the impeller nearest to the point of gas injection have the greatest influence on system performance. In view of the work of Tereshkevitz, which showed a large influence of bubble size (as compared with liquid shear) on mass transfer, a tentative conclusion is that the average bubble size is determined mostly by the conditions near the gassed impeller. It follows that primary emphasis in process development and scale-up should be placed upon selection of the impeller and the gas-inlet conditions variations in the over-all system geometry, with the possible exception of liquid depth, may be of secondary importance. 

Fig. 8 k- Factor of multiple impeller system (Rushton -I- PBT) as a function of aeration number. (View this art in color at h>h>h>. dekker.com.)... 

Typical values for the impeller clearance off the vessel bottom are 7/4 for hydrofoil impellers and 7/3 for pitched-blade turbines. Hydrofoil impellers may be a poor choice when solid suspension is accompanied by other mixing duties, such as liquid-liquid dispersion or gas dispersion. For such cases, a multiple impeller system consisting of a high-efficiency impeller in combination with a 45"" pitched-blade impeller should be evaluated. Small pitched blade impellers with a... 

Vortex and large cavities on upper impellers (multiple impeller systems) develop when FlgFr X 0.2 (Figure 9.30(c))... 

Thus if this model is correct equal values of Pe give equal concentration profi les. Magelli et al. have extended this theory to multiple impeller systems in vessels of aspect ratio greater than one. 

Surface aeration is most common in multiple impeller systems and/or semibatch and batch processes (Lines, 1999). Multiple impeller designs place an impeller relatively close to the surface that can induce surface aeration at relatively low impeller speeds. Most authors do not check for this phenomenon, and it is often unclear as to which models are used for the mass balance in the gas phase. The exclusion of a dynamic gas holdup term (assuming dynamic conditions) does not affect the results if surface aeration is limited however, if surface aeration is significant, experimental errors could be large (Figueiredo and Calderbank, 1979). 

This discussion sheds light into the superior performance of a multiple impeller system with a radial-axial flow impeller combination, with a radial flow impeller in the lower position and an axial flow impeller in the upper position. The radial flow impeller is not affected by the sparger type and is able to efficiently disperse small bubbles. The upper impeller is loaded indirectly by the flow field, which it generates, and is able to provide proper mixing conditions. As such, the sparger choice does not affect the performance of the other impellers. If the impellers operate independently, impellers are optimally loaded for gas dispersion and liquid mixing such that progressive reduction in ki a is minimized and the desired process time can be reduced by >30% (Lines, 2000). 

A much simpler approach can be taken with systems having geometric and hydrodynamic similarities. It is often thought the gas-liquid mass transfer coefficient could be increased by increasing the amount of gas in the reactor volume. This idea has been in extensive use in multiple impeller systems because of the difficulty in determining (Moucha et al., 2003). Total gas holdup e was used to represent this concept (Moucha et al., 2003) and is defined as... 

Fig.6a-c Velocity fields at n = 140 min in different multiple impeller systems, a Four Rush-ton turbines, b two Rushton turbines and two pitched blade impellers, and c four pitched blade impellers... 

In Fig. 6 a velocity fields are shown for a system of four Rushton turbines. In addition to the velocity vector field, large arrows are used to illustrate the flow behavior. Each impeller creates a more or less independent symmetrical flow field. The multiple impeller system therefore shows very poor axial convection. The transport between the individual cells is performed mainly with the aid of axial turbulent dispersion. 

MULTIPLE-IMPELLER SYSTEMS USING 2-2 TYPE IMPELLER FOR GAS INDUCTION... 

The critical speed for gas induction is solely decided by the ability of the rotor to generate a suction higher than the sum of static head and other pressure losses. The rotor design obviously plays an important role. Even when a second impeller is employed for gas dispersion/solid suspension, its action is limited to the role assigned to it in a region substantially away from the gas-inducing device. Therefore, the presence of the second impeller should not have any effect on the critical speed for gas induction in a multiple-impeller system. This obvious fact was experimentally proved by Saravanan and Joshi (1995). Consequently, Equations 9.23 or 9.28, which yield similar predictions, can be used to predict The effect of liquid viscosity can be accounted through Equation 9.24. 

FIGURE 9.8 Flow patterns of (a) single PTD, (b) PTD-PTU, and (c) PTD-PTD multiple-impeller systems. (Reproduced from Patwardhan and Joshi, 1999 with permission from American Chemical Society. Copyright 1999 American Chemical Society.)... 

Type Multiple Impeller The bottom impeller should simultaneously play the role of gas dispersion and solid suspension. Solid suspension in this category has received very limited attention. The only study available is that of Saravanan et al. (1997). These investigators studied solid suspension characteristics of 2-2 type multiple-impeller systems. The top gas-inducing impeller used was 6 FID, while a variety of impellers that generated different flow patterns (Fig. 9.8) were used as the bottom impeller. The difference between multiple-impeller gas-inducing systems and stirred reactors vis-a-vis the location of the gas sparger has already been discussed in Section 9.4.3. Saravanan et al. (1997) found that the value of obeys the... 

There is not a single investigation on this important aspect for any type of gas-inducing impeller discussed in this chapter. Information on is particularly important for three-phase systems involving a solid phase as a catalyst. In the case of multiple-impeller system, the lower impeller is accorded the duty of solid suspension. As discussed in Sections 7A.5.4 and 7A.7, 6 PTU affords superior performance in three-phase systems. Therefore, the same is advisable in the present case. In the absence of snch information, afforded by 6 PTU in a geometrically similar three-phase stirred tank (Section 7A.7) configuration may be used. 

Multiple impeller systems with a large diameter ratio, such as Viscoprop impellers arranged one above another, are suitable for Re > 50. 

Video images from this technique have been used to show fluctuations in the discharge angle of impellers, the degree of flow compartmentalization and impeller interaction in multiple impeller systems, and to show the cavern size when mixing yield stress or highly shear thinning liquids. 

For examination of mixing behavior, light sheet visualization is important, particularly in multiple-impeller systems, to help the experimentahst think about suitable points of addition to study a mixing system, possible choice of feed location, and an initial estimate of suitable probe locations for mixing time experiments. Further tests should be performed to provide more detailed information on suitable choices of probe location. 

Adding a second impeller typically has a very small effect on the just-suspended speed. In multiple-impeller systems, zoning occurs when the impeller separation is too large. The most efficient solids mixing occurs just before the... 

Cooke et al. (1988) reported mixing times for a range of multiple-impeller systems. They found the time increased very significantly but did not distinguish whether the increase was due particularly to the increase in height or the extra number of impellers. The equation that they gave for multiple Rushton turbines is similar to eq. (9-2) ... 

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