Settling and Sedimentation in Particle-Fluid Separation

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. 

When a particle is at a sufficient distance from the walls of the container and from other particles so that its fall is not affected by them, the process is called free settling. Interference is less than 1% if the ratio of the particle diameter to the container diameter is less than T.200 or if the particle concentration is less than 0.2 vol % in the solution. When the particles are crowded, they settle at a lower rate and the process is called hindered settling. The separation of a dilute slurry or suspension by gravity settling into a clear fluid and a slurry of higher solids content is called sedimentation. 

We will consider a particle of mass m kg falling at a velocity v m/s relative to the fluid. The density of the solid particle is kg/m solid and that of the liquid is pkg/m liquid. The buoyant force in N on the particle is 

The drag force on a body in N may be derived from the fact that, like in flow of fluids, the drag force or frictional resistance is proportional to the velocity head v /2 of the fluid displaced by the moving body. This must be multiplied by the density of the fluid and by a significant area A, such as the projected area of the particle. This was defined previously in Eq. (3.1-1). 

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Settling and sedimentation. In settling and sedimentation the particles are separated from the fluid by gravitational forces acting on the various size and density particles. 

Elutriation differs from sedimentation in that fluid moves vertically upwards and thereby carries with it all particles whose settling velocity by gravity is less than the fluid velocity. In practice, complications are introduced by such factors as the non-uniformity of the fluid velocity across a section of an elutriating tube, the influence of the walls of the tube, and the effect of eddies in the flow. In consequence, any assumption that the separated particle size corresponds to the mean velocity of fluid flow is only approximately true it also requires an infinite time to effect complete separation. This method is predicated on the assumption that Stokes law relating the free-falling velocity of a spherical particle to its density and diameter, and to the density and viscosity of the medium is valid... 

Gravitational sedimentation is a well-known separation method. It was empirically used for thousands of years, for example, to separate gold particles from sand. Particles that are in suspension in a fluid are influenced by different forces. If the particle has a higher density than the fluid in which it is immersed, it tends to migrate downward, following the gravity force direction. The frictional force acts to resist the movement (Fig. 13.1). The rate of this sedimentation may depend on the size as well the shape and the mass of the particles, and the viscosity and density of the fluid. Back diffusion, a force that relies on the concentration gradient, acts to counterbalance the tendency to settle. 

Sedimentation is the separation of suspended solid particles from a fluid stream by the action of a body force on the settling behavior of the particle. The body force may be either gravitational or centrifugal force. This section covers gravity sedimentation, represented by clarification and thickening. The equipment used for these two operations are called clarifiers and thickeners, respectively. Centrifugal sedimentation will be discussed in the next section. 

The theory of filtration is largely concerned with the size and shape of the separating medium, with the way in which particles are trapped by the medium (at its upstream surface or in its depth), and with the way that a cake of trapped solids builds up on the medinm. By contrast, the theory of sedimentation is entirely concerned with the way in which a particle (or droplet) settles in a continuous fluid. The design of sedimentation equipment is then a matter of giving the particle enough time to settle out of suspension, either to the bottom of the settling tank, or to the nearest solid surface (e.g. to the nearest one of an array of parallel plates). 

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