Agitators, power consumption

Interlock agitator power consumption to cutoff feed of reactants or catalyst or activate emergency cooling... 

Substitution of the generalized Reynolds number (Section IIB) for the simple Newtonian Reynolds number has been shown (06) to enable approximate prediction of agitator power consumption for non-Newtonian fluids at low Reynolds numbers. The conventional Newtonian power number-Reynolds number charts which have been drawn up by Rushton et al. (R9) were shown to be applicable in the laminar region. This laminar region, however, appeared to extend to Reynolds numbers of 20 to 25, as compared with critical values of 8 to 10 for Newtonian liquids. Above Reynolds numbers of about 70 the conventional Newtonian curve again appeared to be followed. 

In this chapter, we study various correlations for gas-liquid mass transfer, interfacial area, bubble size, gas hold-up, agitation power consumption, and volumetric mass-transfer coefficient, which are vital tools for the design and operation of fermenter systems. Criteria for the scale-up and shear sensitive mixing are also presented. First of all, let s review basic mass-transfer concepts important in understanding gas-liquid mass transfer in a fermentation system. 

Values for the constant K and the indices b and c for various types of agitator, tank-agitator geometries, and dimensions can be found in the literature see Rushton et al. (1999). A useful review of the published correlations for agitator power consumption... 

Correlations are next reported for gas-liquid mass transfer, interfacial area, bubble size, gas hold-up, agitation power consumption, and volumetric mass-transfer coefficients, which are vital tools for the design and operation of fermenter systems. 

The most important choices in the design of batch reactors are reactor volume selection of the agitator speed of the agitator power consumption geometry of the tank, including baffles and heat exchange area (internal and external). 

Pitched-blade turbines (Fig, 18-3) are used on top-entering agitator shafts instead of propellers when a high axial circulation rate is desired and the power consumption is more than 2,2 kW (3 hp), A pitched-blade turbine near the upper surface of liquid in a essel is effecth e for rapid submergence of floating particulate solids,... 

Based on the practical history of scale-up, most fermentation processes for alcohol and organic acid production have followed the concepts of geometric similarity and constant power per unit volume. From the above concept, and as a strong basis for translation of process criteria, only physical properties of the process were considered in the scale-up calculation. For power consumption in an agitated vessel, there is a fixed relation between impeller speed, N, and impeller diameter, l)t. The constant power per unit volume, for a mechanical agitated vessel is given by ... 

The power per unit volume is constant. From power consumptions in a bench-scale bioreactor, the necessary agitation rate is calculated for the scale up ratio, using Equation (13.2.1). The choice of criterion is dependent on what type of fermentation process has been studied. The following equation expresses relations for the impeller size and agitation rate in small and large bioreactors. 

Yoshida et al. (Y4) obtained data on power consumption in a 25-cm-diameter vessel. Four different types of vaned-disk impellers and one turbine were used, but for all impellers the ratio of impeller to tank diameter was maintained at 0.4. Their results, as would be expected, showed that the power dissipation increased rapidly with agitator speed and decreased with increasing gas rate for vaned-disk impellers. At certain agitator speeds, the amount of gas held below the vaned disk reaches a saturation value that apparently... 

A reaction is to be carried out in an agitated vessel. Pilot scale tests have been carried out under fully turbulent conditions in a tank 0.6 m in diameter, fitted with baffles and provided with a flat-bladed turbine, and it has been found that satisfactory mixing is obtained at a rotor speed of 4 Hz when the power consumption is 0.15 kW and the Reynolds number 160,000. What should be the rotor speed in order to achieve the same degree of mixing if the linear scale of the equipment if increased by a factor of 6 and what will be the Reynolds number and the power consumption ... 

The prediction of power consumption for agitation of a given non-Newtonian fluid in a particular mixer, at a desired impeller speed, may be evaluated by the following procedure. 

Rieger, F. and Novak, V. Trans. Inst. Chem. Eng. 52 (1974) 285. Power consumption for agitating viscoelastic liquids in the viscous regime. 

In the above equations, p, p. and a are the density, dynamic viscosity and surface tension respectively of the liquid PA, N and DA are the power consumption, rotational speed and diameter respectively of the agitator. 

Bohme G, Stenger M. Consistent scale-up procedure for power consumption in agitated non-Newtonian fluids. Chem Eng Technol 1988 11 199-205. 

The energy of dissipation per unit mass of liquid in a liquid-solid agitated vessel is related to the power consumption per unit volume of liquid (Ps) as follows (Kato et al., 2001) ... 

In summary, two of the principal approaches which were found useful for the calculation of pipe-line pressure drops have been extended to the problem of predicting power consumption for the agitation of non-Newtonian fluids. Extension of this work is required, but until further data become available, use of the standard power number-Reynolds number charts (with the generalized Reynolds number) is recommended. Between Reynolds numbers of 10 and 70 these charts will provide con-... 

Power Consumption Power consumption sometimes becomes important in industrial bioprocesses, because the power used for aeration and agitation can be highly expensive. The cost of power consumption occupies approximately 15-20% of total production cost in aerobic fermentation processes. 

Figure 5.3-5. Agitator rotation velocity vs. hydrogen flow and power consumption [15]. Agitator power...

When gas is bubbled below the impeller, the power drawn by the agitator decreases relative to the non-gassed liquid. This is because the gas/liquid mixture creates a zone of lower density below the stirrer, that strongly reduces the power consumption. From the point of view of design, however, it should be noted that the drive-motor- should be designed for the most demanding case, that is, the ungassed condition. 

Figure 10.11. Power consumption, (a) Ratio of power consumptions of aerated and unaerated liquids. Q is the volumetric rate of the gas (O) glycol ( X ) ethanol ( ) water. [After Calderbank, Trans. Inst. Chem. Eng. 36, 443 (1958)]. (b) Ratio of power consumptions of aerated and unaerated liquids at low values otQ/Nd3. Six-bladed disk turbine ( ) water ( ) methanol (10%) (A) ethylene glycol (8%) (A) glycerol (40%) Pg = gassed power input P = ungassed power input Q = gas flow rate IV = agitator speed d = agitator-impeller diameter. [Luong and Volesky, AIChE J. 25, 893 (1979)].

Fig. 9.5 A torque table to measure the power consumption for mechanical agitation.

Power consumption by agitation is a function of physical properties, operating condition, and vessel and impeller geometry. Dimensional analysis provides the following relationship ... 

The dimensionless group in the left-hand side of Eq. (9.53) is known as power number NP, which is the ratio of drag force on impeller to inertial force. The first term of the right-hand side of Eq. (9.53) is the impeller Reynolds number NRe. which is the ratio of inertial force to viscous force, and the second term is the Froude number NFr which takes into account gravity forces. The gravity force affects the power consumption due to the formation of the vortex in an agitating vessel. The vortex formation can be prevented by installing baffles. 

Since the oxygen is sparingly soluble gas, the overall mass-transfer coefficient KL is equal to the individual mass-transfer coefficient KL. Our objective in fermenter design is to maximize the oxygen transfer rate with the minimum power consumption necessary to agitate the fluid, and also minimum air flow rate. To maximize the oxygen absorption rate, we have to maximize KL, a, C - CL. However, the concentration difference is quite limited for us to control because the value of C L is limited by its very low maximum solubility. Therefore, the main parameters of interest in design are the mass-transfer coefficient and the mterfacial area. 

The power consumption by an agitator in an unbaffled vessel can be expressed as... 

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