Abstract:

The total efficiency of gravity settlers is usually calculated by taking the product of grade efficiency and weight fraction of particles over a series of particle size intervals and then summing them up. The design of this kind of devices is based on specifying the efficiency required for given size of particles. The accuracy of this method depends on number of particle size interval and width of individual intervals. There are over 30-35 units in operation in whole over United States, Mexico and South Africa.

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Introduction:

Gravity Settlers are normally used for clarification or thickening. They operate with slow settling solids generally finer than 50 microns which a thickened slurry which can be pumped out of the equipment.
This device is simply a particulate collection device using the principle of gravity to settle the particulate matter in a gas stream passing through its long chamber. The primary requirement of this device is a chamber in which the carrier gas velocity is lessened so as to allow the particulate matter to settle out of the moving gas stream under the action of gravity.

Formulation of model:
Lets consider a stationary elemental volume of unit area (L × W) and Thickness within the dispersion band, at a height z above the passive interface and at an arbitrary distance from the settler inlet. Characterizing the size of the drops in the band by their volume (v) a balance for the number of drops over the stationary volume element can be written as:
No. of drops of size v to (v + dv) entering the element per unit time- No. of drops of size v to (v + dv) leaving the element per unit time- No. of drops of size v to v + dv disappearing by drop-drop× coalescence within the element per unit time+ No. of drops of size v to v + d v appearing by drop-drop× coalescence within the element per unit time = 0

Design:

Lamella gravity settler is a sedimentation unit. The most significance of its design is its settling area. It is basically an inclined plane with a shallow depth. Using series of inclined parallel plates can reduce required area. When compared to conventional clarifier, it uses 20% of area but provides same total effective area. Rule of thumb method was used previously to design horizontal gravity settlers but it resulted in higher than necessary capital costs. Now a simplified design procedure has been developed which is more accurate than rule of thumb method and require no trial and error calculations.

Types:

Gravity settler has three basic types as follows
1) Thickener
2) Clarifier
3) Lamella

Thickener:

Conventional Circular Thickener is simple but very large. It contains a central feed well for introducing the feed slurry.The top of the thickener acts as a clarifier and the clarified liquid overflows a peripheral weir at the top.
A rake in the bottom consolidates the thickened solids and moves them towards a central outlet where they are pumped out.
The floor usually has a shallow slope towards the outlet but some have flat bottom. Gravity thickeners have a long residence time so they consume large area. Long residence means they accept short term fluctuation in feed slurry.
Underflow may contain 20-60% solids and thickeners may contain 5-20% solids.
Other designs such as lamella thickener and tray thickener are designed to reduce equipment size.

Clarifier:
A clarifier is generally used to remove solid particles and suspended particulates from liquid for clarification or thickening.
Circular clarifier is same as circular thickeners but with lighter rakes because of less concentrated slurry. Spirals at the bottom consolidates the thickened solids and moves them out through central outlet. Conventional clarifiers may be operated without or with flocculants. Overflow quality can be improved by ensuring efficient fluctuation.

Lamella:

A lamella also known as inclined plate separator can be used both of thickener and clarifier to remove particulates from liquids.
Unlike conventional sedimentation equipment, they use series of inclined plates. They provide a large effective area for small footprints. Solid particles begin to settle on plates and begin to accumulate in collection hoppers at bottom of clarifier. The clarified liquid exits the unit at the top of weir and sludge is then drained from the bottom of hopper. The main advantage of lamella clarifiers over other clarifying systems is the large effective settling area caused by the use of inclined plates, which improves the operating conditions of the clarifiers in a number of ways. The unit is more compact usually requiring only 65-80 % of the area of clarifiers operating.

Working principle:
Gravity settlers are the oldest and simplest means of removing suspended particles from a gas. In principle, pollutants are removed by reducing the velocity of the gas stream sufficiently to allow particles to settle out under the influence of gravity. High efficiency settling chambers are often fitted with baffles or deflectors to change the gas flow direction.

Applications:
The advantages of gravity settler is stated as follows:
1) useful for large particulates that are greater than almost 50micrometers.
2) Low energy cost.
3) Low capital cost.