Settling and sedimentation

In settling processes, particles are separated from a fluid by gravitational forces acting on the particles. The particles can be liquid drops or solid particles. The fluid can be a gas, vapor or liquid. 

When a particle falls under the influence of gravity, it will accelerate until the combination of the frictional drag in the fluid and buoyancy force balances the opposing gravitational force. If the particle is assumed to be a rigid sphere, at this terminal velocity, a force balance gives3,4,7,8 

Pf = density of dispersing fluid (kg m 3) d = particle diameter (m) g = the gravitational constant (9.81 m s 2) cD = drag coefficient (—) vT = terminal settling velocity (m s-1) 

Above a Reynolds number of around 2, Equation 8.5 will underestimate the drag coefficient and hence overestimate the settling velocity. Also, for Re 2, an empirical expression must be used7  

If the particles are assumed to be rigid spheres, then from Equations 8.2 and 8.33,4,7,8  

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At very low Reynolds numbers below about 1, the term creeping flow is used to describe flow at very low velocities. This type of flow applies for the fall or settling of small particles through a fluid. Stokes law is derived using this type of flow in problems of settling and sedimentation. 

In filtration the solid particles are removed from the slurry by forcing the fluid through a filter medium, which blocks the passage of the solid particles and allows the filtrate to pass through. In settling and sedimentation the particles are separated from the fluid by gravitational forces acting on the particles. 

In some processes of settling and sedimentation the purpose is to remove the particles from the fluid stream so that the fluid is free of particle contaminants. In other processes the particles are recovered as the product, as in recovery of the dispersed phase in liquid-liquid extraction. In some cases the particles are suspended in fluids so that the particles can be separated into fractions differing in size or in density. 

Batch Settling and Sedimentation Velocities. A batch settling test on a slurry... 

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