Rushton impeller

RADIAL FLOW IMPELLERS Rushton turbine (DT) is the most widely used in this category. Therefore, the following discussion is limited to its performance. As shown in Figure 7A.13, for the standard [CIT=DIT=1/3] configuration, DT discharges a liquid stream directed radially to the vessel wall. This stream divides into two portions at the vessel wall—one portion directed down toward the base, while the rest travels upward. The lower flow loop has to travel considerable distance comprising three sections (1) outward along the vessel radius from the impeller tip, (2) downward at the wall, and... 

Rotor-Stator-Ruhrsystem rotor-stator impeller, Rushton-turbine impeller... 

Until recently most industrial scale, and even bench scale, bioreactors of this type were agitated by a set of Rushton turbines having about one-thind the diameter of the bioreactor (43) (Fig. 3). In this system, the air enters into the lower agitator and is dispersed from the back of the impeller blades by gas-fiUed or ventilated cavities (44). The presence of these cavities causes the power drawn by the agitator, ie, the power requited to drive it through the broth, to fall and this has important consequences for the performance of the bioreactor with respect to aeration (35). k a has been related to the power per unit volume, P/ U, in W/m and to the superficial air velocity, in m/s (20), where is the air flow rate per cross-sectional area of bioreactor. This relationship in water is... 

Each equation is independent of impeller type. As pointed out eadier, the absolute kpi values vary considerably from Hquid to Hquid. However, similar relationships have been found for other fluids, including fermentation broths, and also for hold-up, 8. Therefore, loss of power reduces the abiHty of the Rushton turbines to transfer oxygen from the air to the broth. 

The Oldshue-Rushton column (Eig. 15d) was developed (162) in the early 1950s and has been widely used in the chemical industry. It consists essentially of a number of compartments separated by horizontal stator-ring baffles, each fitted with vertical baffles and a turbine-type impeller mounted on a central shaft. Columns up to 2.74 m in diameter have been reported in service (162—167). Scale-up is reported to be reliably predictable (168) although only limited performance data are available (169). A detailed description and review of design criteria are available (170). 

M. 5-26-J, haffled tank with standard hlade Rushton impeller... 

The Oldshue-Rushton (Mixco) extractor is similar in construction to the RDC in the fact that it is a relativelv open design, consisting of a series of compartments separated by horizontal stator baffles. The major difference from the RDC is that the height/diameter ratio of the compartments is greater, each compartment is fitted with vertical baffles, and the mixing is accomplished by means of a turbine impeller rather than a disc. 

Prochazka and Landau [19] developed a mixing time conelation for a single Rushton turbine impeller in a baffled tank in the standard configuration for > 10" ... 

In Figure 8.6, the results for the referenee eonditions (Rushton turbine, 40-min feed time, feed point position elose to the impeller, total eoneentration 0.008 M) for ealeium oxalate eonfirm this observation. 

Figure 5-13. Power consumption of impellers. By permission, Rushton, J. H., Costich, E. W. and Everett, H. L., Chem. Engr. Prog., V. 46, No. 8 and No. 9,1950 [18].

Since the process is more complex, the proposed method may not be valid for scale-up calculation. The combination of power and Reynolds number was the next step for correlating power and fluid-flow dimensionless number, which was to define power number as a function of the Reynolds number. In fact, the study by Rushton summarised various geometries of impellers, as his findings were plotted as dimensionless power input versus impeller... 

Rushton (R11) in 1954 presented a graph showing contacting efficiency as a function of impeller diameter at constant power input. He found that the rate of mass transfer between phases increased to a maximum and then decreased as the impeller diameter increased. The optimum occurred at a ratio of impeller to tank diameter of about 0.25, a ratio which is much smaller than that found for liquid blending. 

Karow et al. (Kl), Rushton et al. (R12), and Oldshue (02) studied rates of absorption with multiple impellers, and their results indicate that incorrect spacing and operating conditions result in decreased capacities. Rushton et al. (R12) found that for ratios of liquid depth to tank diameter less than 2.5 it was not possible to alter the absorption coefficients more than +10% by the use of multiple impellers. They achieved a 25 % increase in the absorption coefficient by the use of multiple impellers for a system with liquid depth equal to 4 tank diameters, at a power input of about 3.5 hp/1000 gal. In the same system, the coefficient could be decreased by 50% if the impellers were not properly positioned. Advantages of multiple turbines accrue only at high air flows or at high power levels (1.8-3.6 hp/1000 gal). Disadvantages of multiple impellers may come about at low air flows, low power inputs, and with improper spacing of impellers. 

Figure 7,20. Commonly used impellers (a) Three-bladed propeller ( >) Six-bladed disc turbine (Rushton turbine) (c) Simple paddle (d) Anchor impeller (e) Helical ribbon (/) Helical screw with draft tubs...

Tests on a small scale tank 03 m diameter (Rushton impeller, diameter 0.1 m) have shown that a blending process between two miscible liquids (aqueous solutions, properties approximately fee same as water, i.e. viscosity 1 mN s/m2, density 1000 kg/m3) is satisfactorily completed after 1 minute using an impeller speed of 250 rev/min, It is decided to scale up fee process to a tank of 2.5 m diameter using fee criterion of constant tip-speed. 

An agitated tank with a standard Rushton impeller is required to disperse gas in a solution of properties similar to those of water. The tank will be 3 m diameter (1 m diameter impeller), A power level of 0.8 kW/m3 is chosen. Assuming fully turbnlent conditions and feat the presence of fee gas does not significantly affect the relation between fee Power and Reynolds numbers ... 

Figure 15.1 Instantaneous PIV measurement of velocity and vorticity in the impeller outflow from a Rushton turbine in a 1-1 laboratory reactor.

Fig. 14. Calcium response of Sf-9 insect cells subjected to different values of e in a stirred bioreactor equipped with a 5.1 cm diameter 6-bladed Rushton impeller (closed circles) or in the capillary flow system (open squares). Error bars for stirred bioreactor are standard deviation for each experiment but for the capillary, data are hard to discern [99]...

The three basic types of impeller which are used at high Reynolds numbers (low viscosity) are shown in Figures 10.55a, b, c. They can be classified according to the predominant direction of flow leaving the impeller. The flat-bladed (Rushton) turbines are essentially radial-flow devices, suitable for processes controlled by turbulent mixing (shear controlled processes). The propeller and pitched-bladed turbines are essentially axial-flow devices, suitable for bulk fluid mixing. 

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