Mechanical Separations Lecture_03

Lecture # 3 : 

Lecture # 3 September 29, 2010 1 MECHANICAL SEPARATIONS Prepared by: Engineer A.N.Tabish Lecturer MECHANICAL SEPARATIONS (Ch.E-207) Department of Chemical Engineering University of Engineering & Technology, Lahore -Pakistan Introduction and Fundamentals of Sedimentation

Slide 2: 

September 29, 2010 MECHANICAL SEPARATIONS 2 Course Outline: Ch.E-207 MECHANICAL SEPARATIONS Introduction to separation processes; Role of mechanical separations in chemical processing; Fundamentals of the following Mechanical Separations; Filtration, Sedimentation, Clarification, Flotation, Centrifugal Separations, Jigging, Tabling, Magnetic and Electrostatic Separations, Sonic Agglomeration; Different type of Filters, Cyclones, Centrifuges, Flotation cells, Electrostatic Precipitators, Magnetic Separators and settling tanks with emphasis on the followings; Mechanical Construction, Operation and the working principle, design and the controlling parameters, efficiency, economic evaluation, applications and the selection criteria.

Slide 3: 

September 29, 2010 MECHANICAL SEPARATIONS 3 Sedimentation : Sedimentation is the separation of particles from fluids due to the effect of a body force, which may be either gravity or centrifugal on the buoyant mass of the particle.

Slide 4: 

September 29, 2010 MECHANICAL SEPARATIONS 4 Sedimentation : A particle, falling from rest, accelerates under the force of gravity. The drag force increases so the acceleration decreases (liquid viscosity is important here). Acceleration eventually becomes zero – the terminal velocity is reached.

Slide 5: 

September 29, 2010 MECHANICAL SEPARATIONS 5 Sedimentation : A Typical Waste Water Treatment Plant

Slide 6: 

September 29, 2010 MECHANICAL SEPARATIONS 6 Nature of Solid Suspension in Slurry:

Slide 7: 

September 29, 2010 MECHANICAL SEPARATIONS 7 Sedimentation of Solid Suspensions: Rate of sedimentation remains constant until The interface corresponds the top of zone “C” Particles of varying size are present, Larger particles settle at greater rate that makes the suspension of variable composition.

Slide 8: 

September 29, 2010 MECHANICAL SEPARATIONS 8 Types of Settling/Sedimentation: Four types may be defined as a function of the solids concentration in suspension and the tendency of the solids to interact with one another as they settle.

Slide 9: 

5/1/2010 SEDIMENTATION 9 1. Free / Discrete Settling: Occurs in settling of dilute suspensions < 500 mg/L of particles with little or no interaction. Each particle settles as an individual entity. Settling of sand particles in grit chamber in waste water treatment process. No interaction between particles as they settle. Types of Settling/Sedimentation:

Slide 10: 

5/1/2010 SEDIMENTATION 10 2. Flocculent Settling: Occurs in dilute suspensions < 500 mg/L of flocculent particles. Particles coalesce or flocculate during settling so that their mass increases and settling velocity increases as they fall. Occurs in Primary sedimentation tank during waste water treatment process. Interaction between particles as they settle. Types of Settling/Sedimentation:

Slide 11: 

5/1/2010 SEDIMENTATION 11 3. Hindered Settling: Occurs in suspensions of intermediate concentration 500 to 2000 mg/L. Inter-particle forces hinder settling of neighboring particles. Particles tend to remain in fixed positions (relatively) with respect to one another and the mass of particles settles as a unit. A distinct "solid-liquid interface" develops at the top of the settling mass with clarified liquid above and the particle mass below. Types of Settling/Sedimentation: Particle interaction

Slide 12: 

5/1/2010 SEDIMENTATION 12 4. Compression Settling: Particles are sufficiently concentrated that they form a "structure" . Settling occurs by compression of the structure (water gets squeezed out) by the weight of additional solids settling from above. e.g. The solids (sludge) in the bottom layer of a secondary sedimentation tank. Types of Settling/Sedimentation:

Slide 13: 

September 29, 2010 MECHANICAL SEPARATIONS 13 Types of Settling/Sedimentation:

Slide 14: 

5/1/2010 SEDIMENTATION 14 Consider settling of discrete particle, Modeling of Free/Discrete Settling:

Slide 15: 

5/1/2010 SEDIMENTATION 15 From Newton's Second law, Where “v” is the velocity of particle and “m” is the mass, Gravity Force “FG” is given by, Where “ρP” is the density of particle and “V” is the Volume, Buoyant Force “FB” is given by, Where “ρf” is the density of fluid Modeling of Free/Discrete Settling:

Slide 16: 

5/1/2010 SEDIMENTATION 16 The frictional drag “FD” depends on particle velocity “v” , fluid density “ρf”, projected area “AP” and drag coefficient “CD”. Following empirical expression is used, From (1), (2), (3) and (A), For steady state system Particle attains a specific speed known as “terminal velocity”. At steady state (constant settling velocity): FD + FB = FG Modeling of Free/Discrete Settling:

Slide 17: 

5/1/2010 SEDIMENTATION 17 Where vt is the particle terminal velocity, Assuming: Geometry of particle is Solid Spherical. The particle is sufficiently far from other particles and the vessel wall so that the flow pattern around the particle is not distorted. Particle has attained its terminal velocity with respect to the fluid. Modeling of Free/Discrete Settling:

Slide 18: 

5/1/2010 SEDIMENTATION 18 And Where “µ” is the dynamic viscosity of Fluid. Modeling of Free/Discrete Settling:

Slide 19: 

5/1/2010 SEDIMENTATION 19 For Laminar Flow (NRe < 1) Stokes’ Newton’s Modeling of Free/Discrete Settling:

Slide 20: 

5/1/2010 SEDIMENTATION 20 Combining (4), (5) and (B), The drag always increases with velocity. The acceleration decreases with time and approaches zero. The particle quickly reaches a constant velocity which is the maximum attainable under the circumstances. This maximum settling velocity is called terminal velocity. (Stokes Law) Modeling of Free/Discrete Settling: Larger the diameter of particle greater will be the Sedimentation velocity

Slide 21: 

September 29, 2010 MECHANICAL SEPARATIONS 21 Factors Effecting the Rate of Sedimentation: 1. Height of Suspension:

Slide 22: 

September 29, 2010 MECHANICAL SEPARATIONS 22 Factors Effecting the Rate of Sedimentation: 2. Diameter of Vessel: For Plug flow is achievable and the walls of the container do not effect on the rate of sedimentation. The sedimentation rate may be reduced because of the retarding influence of the walls. For

Slide 23: 

September 29, 2010 MECHANICAL SEPARATIONS 23 Factors Effecting the Rate of Sedimentation: 3. Concentration of Suspension:

Slide 24: 

September 29, 2010 MECHANICAL SEPARATIONS 24 Factors Effecting the Rate of Sedimentation:

Slide 25: 

September 29, 2010 MECHANICAL SEPARATIONS 25 Factors Effecting the Rate of Sedimentation:

Slide 26: 

September 29, 2010 MECHANICAL SEPARATIONS 26 Factors Effecting the Rate of Sedimentation: 4. Effect of Flocculation:

Slide 27: 

September 29, 2010 MECHANICAL SEPARATIONS 27 tabish288@yahoo.com