Impeller Rushton-type

A comparison between the two last correlations and experimental data for small laboratory reactors (25 to 300 cm3) equipped with a magnetic stirrer or a traditional six-blade impeller (not Rushton-type) and using a catalytic hydrogenation in organic solvent has been published (Fig. 45.5) [55]. 

Figure 6.2 Cavity types for the Rushton-type impellers where N is the impeller speed and Qg(F ) is the volumetric gas flow rate (Doran, 2013).

The observed cavity structures have inspired improvements to the Rushton-type turbine to increase the gas-handling capacity of the impeller to allow more gas and, hence, higher gas concentrations in the reactor. To accomplish this, impellers shonld create smaller cavities in similar structures while minimizing flooding. The concave blade disk tnrbine, discussed in Section 6.3, has been shown to accomplish these goals. 

Figure 6.3 Cavity structures for the Rushton-type impeller. Adapted from Nienow et al. (1985).

Concave blade disc turbines are of interest in gas-liquid dispersion because they are able to handle more gas than Rushton-type turbines before flooding (Smith et al., 1977 Vasconcelos et al., 2000). The mass transfer capacity for the concave blade disc turbine is very similar to the Rushton-type turbine, but Chen and Chen (1999) found that the blade curvature could be optimized for a certain power input to produce higher gas-liquid mass transfer coefficients. Unlike the Rushton-type turbine, the concave blade disc turbine requires the cup orientation to be in the direction of impeller rotation (Tatterson, 1994). 

Cabaret et al. (2008) and Gagnon et al. (1998) concluded that better mixing and higher product conversion can be achieved if a close clearance impeller, such as the helical ribbon, is used in conjunction with a radial flow impeller such as the RT in a highly viscous system. The Rushton-type turbine provides proper gas dispersion, while the close clearance impeller attempts to contact most of the reactor volume and provides proper bulk mixing, shear distribution, lower apparent viscosity, and minimal stagnant zones (Tecante and Choplin, 1993). These effects also lead to higher reactor utilization and can decrease power requirements. 

Figure 6.22 Sample mass transfer results for an STR with a single Rushton-type impeller...

Rushton-type impeller (radial flow impeller)... 

For large vessels, the height becomes critical as the hydrostatic pressure in the lower sections of the vessel may directly impact cell viability, or increased gas solubility may become relevant for the metabolic activities possibly impacting productivity and product quality. This is of particular importance considering that Rushton-type impellers provide only very limited axial movement of the broth. 

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). 

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