Turbines Rushton type

Fig. 45.4 Left Schematic diagram of a continuous stirred tank reactor with gas and liquid inlet and outlet. Right A Rushton-type turbine (adapted from [3]).

With a gas sparger and a radial turbine of the Rushton type, gas loads of up to 500 m3/(m2h) can be achieved with reasonable energy consumption. This method of dispersing the gas phase is usually employed for fast reactions or in situations where the hourly demand for the gaseous reactant is high, as in industrial fermenters. With such high gas-feed rates, the gas reactant may not react completely, so that eventually the unreacted gas may be recycled externally. 

Turbines are smaller multi-blades mixers, which can be hub-mounted flat-blade type, straight or curved, or disc-mounted flat blade (Rushton) type, of generally six blades. 

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. 

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). 

Rushton-type turbine + Techmix 335 up pumping Rushton-type turbine + Techmix 335 down pumping -H- 2 Rushton-type turbines... 

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. 

Two different types of impellers for turbulent flow exist, radial impellers and axial impeller, as sketched in Fig. 7.2. The radial impellers project the fluid radially out from the blade towards the tank wall. The flow splits at the tank wall, and approximately 50 % of the fluid circulates towards the surface while the rest to the bottom, creating two regions of low mean velocity. The flow exiting the impeller blade forms a sharp velocity gradient with a strong peak at the impeller blade horizontal center line. Examples of radial flow impellers are disk style flat blade turbines (Rushton... 

Rushton turbine A type of impeller used for gas dispersion such as in biochemical reactors and consists of a flat disc with six vertical flat blades mounted on the circumference (see Fig. 54). It therefore provides radial-flow mixing. It is named after American chemical engineer John Hetuy Rushton (1905-85), who was noted for his work on mixing. There have been many subsequent modiflcations and improvements to the basic design. 

Loiseau et al. (100) have proposed to use empirical Michel and Miller (103) types of correlations to fit e qperiinental data concerning water and aqueous, organic and foaming liquid systems inside two fully baffled small vessels (5.5 and 8.9 litres) agitated by a six flat-blade Rushton types of disk turbine. 

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