Sedimentation Text book, Chapter 3 Reading Materials: Perry’s Handbook, Chapter 18 Coulson and Richardson’s Chemical Engineering, Volume 2, Chapter 5

What is Sedimentation? Sedimentation is a process for the gravity separation of a dispersed phase from a fluid due to density difference Hindered vs Free Settling Factors to be considered: Feed rate Settling rate of the materials Modes of operation Batch settler Continuous settler

For highly aggregates flocs

Sedimentation of Fine Particles

Settling velocity of a concentrated suspension Viscosity of suspension: OR: For a more concentrated suspension

Height of suspension

Concentration of suspension

Rate of sedimentation H0 H0

Batch Settler Occur in vessel or ponds Tap Points Solid Removal Occur in vessel or ponds Are simply filled and then allowed to stand until the desired degree of separation is achieved More tap points can be implemented

Continuous Settler To treat a stream for clarification or slurry to be thickened Feed and flow from thickener (or tap points) are continuous Solids removal – continuous or intermittent Horizontal, vertical, or inclined Tap Points Feed Solid Removal

Horizontal Separators Used for clarification operations Feed enters on one end and the clarified liquid overflows at the other Solid removal is done intermittently at the bottom of the vessel

Vertical Thickeners

Types of Settling Mechanism “Most common” “Rare”

Settling Mechanism

Settling Test To determine the solids settling velocity Type I

Type I I

If above correlation is given, How you can estimate C1 and C2 Particle volumetric Flux (1) If above correlation is given, How you can estimate C1 and C2

Sedimentation Theory Net Flow: Relative velocity between the fluid and solid: Batch So, we have:

Sharp Interfaces in Sedimentation Consider the above Figure, the interface between a suspension of concentration c1 containing particles settling at a velocity v1 and a suspension of concentration c2 containing particles settling at a velocity v2. The interface is falling at a velocity vint. All velocities are measured relative to the vessel walls. The mass balance over the interface gives Mass balance:  Define a particle volumetric flux,  = vc. We have

Kynch Analysis Batch Settling:

c = volume fraction (m3/m3)

Assume 1 kg total mass Density of water = 1000kg/m3=1.0kg/l)

Height (cm) Time (s)

Sizing of Gravity Settling Tank:

Sizing of Gravity Settling Tank:

Find the area of the gravity settling required the optimal condition

Sizing of industrial thickeners:

Sizing of industrial thickeners:

Sizing of thickeners: At any point in the tank, the volumetric downward flow rate of solids, q, is given by: is the interface settling velocity (from batch tests) C is the concentration corresponding to the is the relative velocity Specifically, at underflow point at concentration and

EQUATION (1)=(2) = q(feed)

Eliminating In general, at bottom is small in comparison to Then q ???? Note that feed solids volumetric flow rate q is equal where Q is the feed volumetric flow rate and is the volumetric feed solids Concentration.

If the mass flow rate is known, it can be simply be substituted: The area required for a feed rate of solids is then given by: We will require the maximum area since, for any smaller area the solids will not settle and will eventually overflow. This means, the calculation has to be repeated for each concentration and associated settling velocity to find maximum:

Formula one Formula two (max or min)

minimum

1 minimum (max or min)