Gravity settling density

For very small particles or low density solids, the terminal velocity may be too low to enable separation by gravity settling in a reasonably sized tank. However, the separation can possibly be carried out in a centrifuge, which operates on the same principle as the gravity settler but employs the (radial) acceleration in a rotating system (o r) in place of the vertical gravitational... 

Dilute This is a fully expanded condition in which the solids particles are so widely separated that they exert essentially no influence upon each other. Specifically, the solids phase is so fully dispersed in the gas that the density of the suspension is essentially that of the gas phase alone (Fig. 12-29). Commonly, this situation exists when the gas velocity at all points in the system exceeds the terminal settling velocity of the solids and the particles can be lifted and continuously conveyed by the gas however, this is not always true. Gravity settling chambers such as prilling towers and countercurrent-flow spray dryers are two exceptions in which gas velocity is insufficient to entrain the solids completely. 

Equation (2.17) is analogous to Eq. (2.14) for a gravity settling tank. It shows that ) (, the radius of the neutral zone, is sensitive to the density ratio, especially when the ratio is nearly unity. If the densities of the fluids are too nearly alike, the neutral zone may be unstable even if the speed of rotation is sufficient to separate the liquids quickly. The difference between and pg should not be less than approximately 3 percent for stable operation. 

Definition The aerodynamic diameter of a particle is the diameter of a fictitious sphere of unit density which, under the action of gravity, settles with the same velocity as the particle in question. 

It is reasonable to aim to separate by unassisted gravity settling globules of about 15 p.m (0.015 cm) diameter. These have a rate of rise or fall in fresh water of about 1.4(p — 1) cm/s, where p is the density of the dispersed phase (in g/cm ). A positive figure indicates downward movement. Since the settling speed is fairly slow it is important to have ... 

Gravity settling For immiscible liquids the density difference may yield sufficient separation Oil-water separation... 

Descriptions and discussions of gravity sedimentation in textbooks (and this one is no exception) are usually dominated by water treatment and mineral processing applications. One must not lose sight, however, of the many chemical, pharmaceutical, nuclear, petrochemical or petroleum applications where gravity settling is used to resolve emulsions or to separate other liquid-liquid dispersions. As the density difference in such cases is nearly always low, the benefits of coalescence are usually sought. The present book, as per its title, is concerned primarily with solid-liquid systems and a reader interested in separation of liquid-liquid dispersions is referred to an excellent review of such applications (and of electrostatic coalescence)... 

Two different principles have been used to develop equipment for removing suspended sohds from water. Gravity settling uses the density difference between the solid particle and the water to remove the solids filtration traps the solids within a filter medium that allows water to pass. 

Since the radial acceleration functions simply as an amplified gravitational acceleration, the particles settle toward the bottom -that is, toward the circumference of the rotor-if the particle density is greater than that of the supporting medium. A distance r from the axis of rotation, the radial acceleration is given by co r, where co is the angular velocity in radians per second. The midpoint of an ultracentrifuge cell is typically about 6.5 cm from the axis of rotation, so at 10,000, 20,000, and 40,000 rpm, respectively, the accelerations are 7.13 X 10, 2.85 X 10 , and 1.14 X 10 m sec" or 7.27 X 10, 2.91 X 10, and 1.16 X 10 times the acceleration of gravity (g s). 

---