Rushton type turbine impellers

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). 

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. 

Figure 7.7 shows three commonly used types of impellers or stirrers. The six-flat blade turbine, often called the Rushton turbine (Figure 7.7a), is widely used. The standard dimensions of this type of stirrer relative to the tank size are as follows ... 

The overall mixing pattern in a stirred tank is a function of the impeller type, and the phase ratios. Figs 2 shows mixing patterns derived from particle tracking techniques for (Fig. 2a) a pitched blade turbine and (Fig 2b) a Rushton turbine in the same vessel (100 mm diameter), with the same impeller diameter and rotation speed. It is evident that the overall patterns are very different. These patterns are in fact very time averaged and the actual trace of the "flow follower" particle indicates signifrcant randomness and chaos in... 

The impeller used in the stirrer is of a type for which hydrodj namic data is relatively lacking. However, a considerable amount of data is available for Rushton turbines and flat bladed paddle stirrers. For this reason, it is necessar to use a theoretical analysis to predict the shear rather than drawing directly on the results of previous studies. This analysis follows. 

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