Rushton turbines

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. 

A general flow map of different hydrodynamic conditions (Fig. 23) consists of regions of flooding, dispersion, and recirculation on a plot of N vs for a Rushton turbine. For a low viscosity aqueous/air system, the gas flow numbers for the three conditions are given hy FI = 30Fr[D/TY for flooding, = 0.2Fr° (F/r)° for complete dispersion, and =13FF D/TY for recirculation. 

Gas holdup with Rushton turbine can be estimated from the following correlation ... 

At a given gas sparging rate, interfacial area a is constant at low mixer speeds. When mixer speed is increased above a critical speed a starts increasing and varies linearly with Ai For Rushton turbines this critical speed, as deterrnined in an sodium sulfate system, is given by the following ... 

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 8.7 Mean particle size versus specific energy input for different feed point positions if.p.p.) (CaOx, Rushton turbine, 40 min feed time, total concentration 0.008 M.) After Zauner and Jones, 2000b)...

Ranade, V.V., 1997. An efficient computational model for simulating flow in stirred vessels a case of Rushton turbine. Chemical Engineering Science, 52, 4473-4484. 

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...

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. 28. Porcine Fumarase subject to shearing, with and without air/liquid interface, with a stainless steel disc at a mean velocity gradient of 6490 s at 30 °C. The enzyme was also sheared, with and without air/liquid interface, with a 6 bladed Rushton turbine at a mean velocity gradient of 22700 s at 30 °C [107]...

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. 

Fig. 1. Phase-averaged plots of the anisotropy distanced in a plane midway between two baffles in a stirred vessel provided with a Rushton turbine, as obtained by means of LES, with two different SGS models (a) the Smagorinsky model (b) the Voke model. Reproduced with permission from Hartmann et al. (2004a).

Fig. 8. This is a snapshot of a spatial particle distribution. The plane shown is the horizontal cross-section just below the disc of a Rushton turbine in a flat-bottomed stirred tank. The impeller revolves in the counter clockwise direction. Particle size is some 0.468mm Re = 1.5- - x 105 volume fraction amounts to 3.6% number of particles tracked in the simulation just over 6.7 million. Reproduced with permission from Derksen (2003).

Fig. 10. Results of LES-based simulations of an agglomeration process in two vessels one agitated by a Rushton turbine (left) and one agitated by a Pitched Blade Turbine (right). The two plots show the agglomeration rate constant fl0 normalized by the maximum value, in a vertical cross-sectional plane midway between two baffles and through the center of the vessel. Each of the two plots consists of two parts the right-hand parts present instantaneous snapshots the left-hand parts present spatial distributions of time-averaged values after 50 impeller revolutions. Reproduced with permission from Hollander et al. (2003).

Fig. 11. The discrepancy between the original kinetic relation due to Mumtaz et al. (1997) and the observed relation between mean agglomeration rate constant fl0 and volume-averaged shear rate. Symbols refer to individual numerical simulations (LES). RT stands for Rushton Turbine, PBT for Pitched Blade Turbine. Reproduced with permission from Hollander et al. (2001b).

Note The respective impellers used are a classical Rushton turbine (DT), a hydrofoil impeller (A315) manufactured by Lightnin, and a Pitched Blade impeller (PBT). The cases 1 through 4 all relate to a superficial gas rate of 3.6mm/s only, with impeller speeds varying between 5 and 10/s (gas flow numbers between 0.01 and 0.02) cases 2 and 3 differ in sparger size and position. 

Fig. 14. Spatial distribution of the gas hold-up in a turbulent stirred vessel filled with a 0.075% Keltrol solution in water and agitated by a Rushton turbine (a) experimental data obtained by means of an optical probe (b) computational result from DAWN. Overall hold-up amounts to some 3.1% in the simulation and 3.7% in the experiment. Reproduced with permission from Venneker...

Gao, Z., Min, J., Smith J. M., and Thorpe, R. B., Large Eddy Simulation of Mixing Time in a Stirred Tank with Duals Rushton Turbines . 12th European Conference on Mixing, Bologna, Italy, pp. 431-438 (2006). 

Rousar, I., and Van den Akker, H. E. A., LDA Measurements of Liquid Velocities in Sparged Agitated Tanks with Single and Multiple Rushton Turbines . 8th European Conference on Mixing, Cambridge, UK IChemE Symp. Ser., 136, 89-96 (1994). 

Rushton impeller/turbine, 16 673, 701 gas holdup with, 16 703 Rushton turbine, 1 738 Russia... 

In the case of highly elastic liquids mixed by a Rushton turbine, flow reversal may occur in the low Reynolds number region, ReM< 30, leading to values of the power number as much as 60 per cent higher than for inelastic liquids. In the intermediate region, 50 1000, the power... 

Culture and bioconversion. The precultured cells were recovered by centrifugation at 4 °C and the cell pellet was inoculated into a 3 L fermenter (stirred tank with two Rushton turbine impellers and four baffles) containing 1.0 L of supplemented M9 medium. 

Ng and Assirelli have carried out a mixing study in batch stirred vessels with working volumes of 3 L to 20L using a fiber-optic UV-vis monitoring technique. Bromophenol blue sodium salt was used as a non-reactive tracer. The results on traditional Rushton turbines and 45° angled pitched blade turbines showed good agreement with a typical conductivity technique and a correlation proposed in literature. 

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