Scale agitated vessels

Scale-Up of Mixers For the details associated with the design and scale-up of agitated vessels, the reader is referred to Section 18 which covers this topic in great detail. The intention here is to provide only some of the generalprinciples involved which have particular apphcation to liquid-hquid extraction. 

This chapter reviews the various types of impellers, die flow patterns generated by diese agitators, correlation of die dimensionless parameters (i.e., Reynolds number, Froude number, and Power number), scale-up of mixers, heat transfer coefficients of jacketed agitated vessels, and die time required for heating or cooling diese vessels. 

Tipnis, S. K., Penny, W. R., and Fasano, J. B., An experimental investigation to determine a scale-up method for fast competitive parallel reactions in agitated vessels, AIChE Annual Meeting, St. Louis, November 1993. 

Mersmann, A. and Laufliiitte, H.D., 1985. Scale-up of agitated vessels for different mixing processes. 5th European Conference on Mixing 1985, Wurzburg, pp. 273-284. 

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 ... 

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 ... 

Although experts in agitator design are loath to admit to using such a simplistic rule, most scaleups of conventionally agitated vessels are done at or near constant power per unit volume. The consequences of scaling in this fashion are explored in Example 4.7... 

Although the scale-up of agitated vessels is mainly based on geometrical similarity, there are various cases where this target is difficult to achieve. Furthermore, the application of geometrical similarity does not ensure the similarity of dynamic and kinematic phenomena. Experience and intuition have to be employed in the scale-up procedure (McCabe et al., 1983). 

In relatively small-scale operations, where frequent shutdowns may be desirable, as at night or during week ends, the mixer-settler plant offers the advantage that concentration gradients are maintained within the plant indefinitely even though operations have stopped. On start up, steady-state conditions are established immediately, product specifications are met with the first effluents, and recycling of the first products withdrawn is unnecessary. The agitated vessel is also eminently suitable for purely batch processes, which are likely to be on a small scale. 

As a result, if we consider scale-up from a 20-gallon to a 2,500-gallon agitated vessel, the scale ratio is equal to 5, and the impeller speed of the prototype will be... 

Miller, D. N., "Scale-Up of Agitated Vessels Gas-Liquid Mass Transfer," AIChE. ]. 20 (1974) 442-453. 

Mechanically agitated reactors have been used for cultivation of plant cells (Kato et al., 1972 Tanaka, 1981). Ajar fermenter with a six fiat-blade turbine and a modified paddle has been used by Tanaka (1981) and a similar jar fermenter with two disk turbine impellers has been used by Kato et al. (1972) at about 50 rpm with no significant shear damage to plant cells. Paddle-type impellers were found to be more appropriate (less shear damage) than flat-blade turbine type impellers (Kato et al., 1972). The only production-scale reactor used for shikonin production in Japan is also an agitated vessel. 

There is an intimate and often very complex interrelation between the performance of an agitated vessel and the system geometry, the properties of the processed materials, and the operating conditions. In some cases the true complexity of a mixing situation may not be appreciated. Particularly in scale-up problems, the physical nature of the mixing in a process may be as important a factor as the proper reaction temperature or reactant ratios. Thus, another objective of this review is to demonstrate that agitation is a process variable of major significance. 

For agitated vessels, it is most important that the proper laboratory experimental program be implemented so that effective scale-up is accomplished. A more complete and comprehensive discussion (than the discussion presented here) of the proper laboratory experimental program is given by Penney and Fasano (1991). The key features of the recommended experimental program are ... 

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