Reactors Rushton turbine

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

Correlations are available for mixing times in stirred-tank reactors with several types of stirrers. One of these, for the standard Rushton turbine with baffles [13], is shown in Figure 7.9, in which the product of the stirrer speed N (s ) and the mixing time t (s) is plotted against the Reynolds number on log-log coordinates. For (Re) above approximately 5000, the product N t (-) approaches a constant value of about 30. 

A stirred-tank reactor equipped with a standard Rushton turbine of the following dimensions contains a hquid with density p = 1.0 g cm and viscosity... 

Mechanically agitated slurry reactors are widely used in three-phase catalytic and noncatalytic reactions. In aerated slurry reactors, the three regimes outlined in Table V prevail. These regimes are schematically illustrated in Fig. 11. The gas flow rate and stirrer speed where the transition from regimes a to b or b to c with a Rushton turbine stirrer occurs can be estimated from the relationships described in Table VI. 

The essential feature of these reactors, though, is the stirring devise used to keep the solid catalyst uniformly suspended in the liquid reaction medium to maximize the gas/liquid contact area and to promote interfacial mass transport. Good agitation is also needed for optimum temperature control of the reaction. A commonly used stirrer is the six blade Rushton turbine shown in Fig. 6.3. This... 

Fig 6.3. Reactor agitation as provided by a six blade Rushton turbine. Gas dispersion is enhanced by holes in the hollow shaft. (Courtesy Autoclave Engineers.)... 

Jenne, M. and Reuss, M. (1999), A critical assessment on the use of k-e turbulence model for simulation of the turbulent liquid flow induced by a Rushton turbine in baffled stirred tank reactor, Chem. Eng. ScL, 54, 3921-3941. 

Schafer, M., Hofken, M. and Durst, F. (1997), Detailed LDV measurements for visualization of the flow field within a stirred tank reactor equipped with a Rushton turbine, Chem. Eng. Res. Des., 75, 729-736. 

A stirred-tank reactor equipped with a standard Rushton turbine of the following dimensions contains a liquid with density p = 1.000 gcm 3 and viscosity p = 0.013 gcm 1s 1. The tank diameter D = 2.4m, liquid depth HL = 2.4m, the impeller diameter d = 0.8m, and liquid volume = 10.85 m3. Estimate the stirrer power required and the mixing time, when the rotational stirrer speed N is 90r.p.m., that is, 1.5 s 1. 

Figure 5 shows some impellers typically used for low-viscosity mixtures. The six-blade Rushton turbine [8] is commonly used for gas dispersion and to promote gas-liquid mass transfer and heat transfer. It is, however, not often used for the suspension of solids. This stirrer provides mainly radial fluid flow within the reactor. 

Nocentini, M., Pinelli, D., and MageUi, F. (1998), Analysis of the gas behavior in sparged reactors stirred with multiple Rushton turbines Tentative model validation and scale-up, Industrial Engineering Chemistry Research, 37(4) 1528-1535. 

A critical analysis of the many publications concerning the simulation of liquid flow in baffied stirred tank reactors equipped with a Rushton turbine reveals several discrepancies. The most important differences between the simulations concern the dimensionality of the simulations (three-dimensional or axisym-metric), turbulence modeling, the modeling approaches for the Rushton turbine as well as the accuracy of the numerical predictions, which depends on the grid size. 

In principle it is also possible to extend the simulations for multiple impellers to gassed systems. Figure 7 shows an example of the distribution of the specific gas hold-up for a reactor equipped with two Rushton turbines. 

FIGURE 7A.3 Some commonly used impellers for two-/three-phase stirred reactors, (a) Standard six-blade Rushton turbine, (b) Six-blade 45° pitched turbine, (c) Lightnin A315 . (d) SC ABA 6SRGT. (Reproduced from Middleton 2000 with permission from Elsevier. 1992.)... 

AssireUi M, Bujalski W, Eaglesham A, Nienow AW. (2005) Intensifying micromixing in a semi-batch reactor using a Rushton turbine. Chem. Eng. Sci., 60 2333-2339. 

Figure 2.2 Characteristic times in a stirred-tank reactor (STR) at the 8m scale with a Rushton turbine. Mass transfer time (A) and (O) at a superficial gas velocity of 2 X 10 m s , ( ) without aeration. Short...

This is the reaction system used by Bourne et al. (1981) and Middleton et al. (1986). The first reaction is much faster than the second reaction Ki = 7300 m / mol s versus K2 = 3.5 m /mol s. The experimental data published by Middle-ton et al. were used to determine the Magnussen model constants. Two reactors were studied, a 30 L reactor equipped with a D/T = 5 Rushton turbine and a 600 L reactor with a D/T = Rushton turbine. In the CFD analysis, a converged flow field was computed first for each reactor, using experimental data for the impeller boundary conditions. The reactants A and B were then introduced to the tank on an equimolar basis. The reactant A was assigned a weak but uniform concentration throughout the vessel. The reactant B was added in a high concentration in a small region. The calculation of the flow fleld variables was disabled after the addition of the reactants, and the species calculations alone... 

In the example a gas-liquid reaction with particulate solids (e.g., a catalyst) operating in regime 11 in a stirred reactor with a Rushton turbine is to be scaled up. The primary process requirement is for the same degree of reaction conversion at each scale, which means the same number of moles of gas transferred per mole of liquid fed ... 

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