SEDIMENTATION 9/11/2018

Definition Sedimentation is the tendency for particles in suspension or molecules in solution to settle out of the fluid in which they are entrained, and come to rest against a wall. This is due to their motion through the fluid in response to the forces acting on them: these forces can be due to gravity or centrifugal acceleration. 9/11/2018

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Mechanism behind Sedimentation Mechanics of fluid motion Terminal velocity Drag coefficient & Reynolds number Motion of spherical particles 9/11/2018

Mechanics of fluid motion Three forces acting on a particle moving through the fluid The external force The buoyancy force The drag force 9/11/2018

Terminal velocity The velocity at which the driving forces (external forces) are cancelled out by the resistive forces (drag forces). Once an object has reached terminal velocity, the object is not accelerating (a=0), therefore it is not speeding up or slowing down. It is a constant velocity unless the driving forces or the resistive forces change. 9/11/2018

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Drag coefficient & Reynolds number Drag Coefficient is a dimensionless quantity which is used to quantify the resistance of an object in a fluid environment Reynolds number gives the condition in which one type of flow changes to other type 9/11/2018

Settling Free Settling When the particle is at sufficient distance from the boundaries of the container or from other particles so that, its fall is not affected by them. Hindered Settling If the motion of the particles is impeded by other particles which will happen when the particles are near each other, though they may not be colliding, the process is called hindered settling. 9/11/2018

Types of Sedimentation processes Gravity settling processes Centrifugal settling processes Why we need to apply the centrifugal force? Because of Brownian Motion 9/11/2018

Gravity Settling Processes Particles heavier than the suspending fluid may be removed from a gas or a liquid in a large settling tank, in which the fluid velocity is low and the particles have ample time to settle out. 9/11/2018

Types of gravity settlers A settler that removes virtually all the particles from a liquid is known as a clarifier. A device that separates the solids into two fractions is called as a classifier. 9/11/2018

1. Gravity Classifier Separate particles on the basis of size, in situation in which the density of the fine particles is the same as that of the larger ones. 9/11/2018

Classifier 9/11/2018

2. Clarifier Clarifiers are used for gravity separation under hindered settling conditions to convert a dilute slurry of fine particles into a clarified liquid and a concentrated suspension. 9/11/2018

Gravity Clarifier 9/11/2018

Flocculation Process by which clumps of solids in water aggregate through biological or chemical action so they can be separated from water OR The process of forming floc particles when a chemical coagulant or flocculent such as alum or ferric chloride is added to the water The gathering together of fine particles in water by gentle mixing after the addition of coagulant chemicals to form larger particles 9/11/2018

Flocs Particles finer than 0.1 µm (10-7m) in water remain continuously in motion due to electrostatic charge (often negative) which causes them to repel each other. Once their electrostatic charge is neutralized by the use of coagulant chemical, the finer particles start to collide and agglomerate (combine together) under the influence of Vander wall's forces. These larger and heavier particles are called flocs. 9/11/2018

Flocculants Flocculants, or flocculating agents, are chemicals that promote flocculation by causing colloids and other suspended particles in liquids to aggregate, forming a floc. Flocculants are used in water treatment processes to improve the sedimentation or filterability of small particles. For example, a flocculent may be used in swimming pool or drinking water filtration to aid removal of microscopic particles which would otherwise cause the water to be turbid (cloudy) and which would be difficult or impossible to remove by filtration alone. 9/11/2018

Centrifugal settling processes To increase the settling rate, the force of gravity acting on the particle may be replaced by a much stronger centrifugal force. 9/11/2018

Centrifugal settling devices For the separation of solids from gases, cyclones are used. Cyclones used for separating solids from liquids are called hydro cyclones. 9/11/2018

Cyclones Cyclones operate to collect relatively large size particulate matter from a gaseous stream through the use of centrifugal forces. Dust laden gas is made to rotate in a decreasing diameter pathway forcing solids to the outer edge of the gas stream for deposition into the bottom of the cyclone. Efficiencies is 90% in particle sizes of 10 microns or greater are possible. 9/11/2018

Advantages & Disadvantages Reduces internal access needs Operation at elevated temperatures possible Few moving parts, few mechanical / electrical ignition sources Optimal flow rate difficult to adjust Disadvantages Prone to internal erosion / corrosion Low efficiency for small diameter material Hopper recirculation / flow distribution problems High energy costs for volumetric flow requirements Dew point agglomeration, bridging, and plugging 9/11/2018

Hydrocyclone A hydro cyclone is a device to classify, separate or sort particles in a liquid suspension based on the densities of the particles. A hydro cyclone may be used to separate solids from liquids or to separate liquids of different density. A hydro cyclone will normally have a cylindrical section at the top where liquid is being fed tangentially, and a conical base. The angle, and hence length of the conical section, plays a role in determining operating characteristics. 9/11/2018

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Advantages ► Excellent abrasion resistance  ► Excellent corrosion resistance  ► Excellent chemical resistance /acids, bases, oils/  ► Excellent temperature resistance  ► Very good resistance to hydrolysis and micro- organisms  ► Very good pressure resistance  ► High resilience  ► Successful application by sorting aggressive and high temperature gases and gas mixture  ► Successful application in flue gas desulphurization 9/11/2018