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Phase separation overview

DOI: 10.1615/AtoZ.p.phase_separation_overview

The best overview of phase separation is provided by the chart shown in Figure 1 [Weast (1972)] . Most of this section will discuss what is on this chart and show how it can be used.

For dispersions, the most important characterizing parameters are the size and density of the dispersed phase and the density and viscosity of the continuous phase. The particle size is one of the coordinates of Figure 1 . A single dispersed phase specific gravity of 2 is assumed. The behavior of these particles suspended in two different continuous phases, air and water, are displayed on this chart. Starting at the top of Figure 1, the sizes in various unit systems are given. Of particular importance, this chart shows the relation of particle sizes to several different screen sizes.

Particles that are familiar to all of us, of various sizes are given as examples for dispersions in both air and water. This helps us to relate the diameters of particles in general to the diameters of particles that we know. The range of particles that is displayed goes all the way from large, single molecules to gravel.

Methods of analyzing particles various sized are also suggested. These methods include all the methods also available for separating particles from the suspending fluid. Included are impingers, sieves, centrifuges, electrostatic precipitators, filters and gravity (see also Particle Size Measurement).

Many porous or fluffy particles like ash, soot, or flocculent precipitates of various kinds have an effective density which is much closer to that of the continuous phase than that of the pure compound of which they are nominally composed. The average porosity of the particle and its diameter are the critical parameters in determining the relative velocity between these particles and the continuous phase.

Phase separation is a concern for a number of pairs of phases. These include:

  • Liquid-Gas

  • Liquid-Solid

  • Gas-Solid

  • Liquid-Liquid

  • Solid-Solid Systems.

The separation of all these combinations of phases is discussed in detail in Perry (1973). Of the five combinations given above, the liquid-liquid and solid-solid systems are not mentioned in Figure 1, so a brief introduction to the separation of these systems will be included here. (See also Vapor-Liquid Separation, Liquid-Solids Separation and Gas-Solids Separation.)

Characteristics of particles and particle, dispersolids.

Figure 1. Characteristics of particles and particle, dispersolids.

Liquid-Liquid Systems

Separation in liquid-liquid systems is most commonly accomplished by means of either gravity or a centrifuge (see Cyclones). The key difference is whether the droplets, which constitute the dispersed phase are large enough so that they rise or fall at a useful velocity. Figure 1 shows how rapidly a particle of specific gravity 2 will settle in water. The settling velocity in Figure 1 is proportional to the density difference (see Stokes' Law), so we are speaking of very small settling velocities in a one g field when the particles are small.

The settling velocity can be greatly enhanced by use of a centrifuge, some of which have effective accelerations of several thousand g's.

Solid-Solid Systems

Separation of solids can be accomplished by a variety of processes working on density differences, size differences, electrostatic forces and magnetic forces. (See Electrostatic Separation.) Figure 2 [Perry (1973)] shows the size ranges for which different kinds of sorting methods are appropriate. The chart is for mixtures of solids suspended in gases or liquids.

Particle size as a guide to the range of applications [Perry (1973)].

Figure 2. Particle size as a guide to the range of applications [Perry (1973)].

REFERENCES

Weast, R. C. (1972) Handbook of Chemistry and Physics, CRC Press, Cleveland, OH, PF 230.

Perry, R. H. and Chilton, Cecil H. (1973) Chemical Engineers Handbook, Fifth Edn., Chapters 18, 19, 20, 21.

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