Cd drag coefficient d drop diameter g gravitational constant Re Reynolds number Sd drop parameter u drop velocity pc continuous phase density Ap density difference lie continuous phase viscosity O interfacial tension

Figure 15. Dimensionless drop velocity vs. dimensionless drop diameter.

Figure 15. Dimensionless drop velocity vs. dimensionless drop diameter.


Figure 16. Drop velocity vs. drop diameter.

It is generally desirable to provide as uniform a drop size as possible. A wide range in drop sizes may allow the smaller drops to attain equilibrium, but they are en route longer, while the larger drops zip through, not attaining equilibrium.

It is also considered desirable to allow drops to coalesce and be redispersed, as mass transfer from a forming drop is always higher than it is from a stagnant drop.

Backmixing caused by flow patterns induced in the equipment can also deleteriously affect performance by reducing the driving force gradient, as illustrated in Fig. 17. Sleicher[6] presents procedures for calculating the consequences of backmixing on overall extraction results.

Figure 17. Effect of backmixing on extraction driving force.


Extraction equipment can be classified by function as providing discrete stages or continuous differential contact. Separation may be by gravity alone or by centrifugal force. Additional energy may be applied to control drop size, either by mechanical agitation or pulsation. This classification is shown in Table 1, along with major examples of available equipment.

6.1 Non-Agitated Gravity Flow Extractors

Spray Column. The simplest differential extractor is the spray column (Fig. 18a.), which depends upon the initial dispersion of the dispersed phase to create favorably sized droplets. There is no means provided to redisperse this phase if any coalescence occurs. Although the equipment is simple and inexpensive, it is difficult to obtain more than one stage extraction. The passage of the dispersed phase ind uces considerable backmixing of the continuous phase, particularly in larger diameter columns.

Table 1. Classification of Industrial Extraction Equipment

Flow by

Drop Size control by


Continuous Differential

Gravity alone

Gravity alone

Perforated Place Column

Spray Column Packed Column

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