12.2 Centrifugation and sedimentation

Centrifugation and sedimentation are key separation techniques that rely on particle settling. These processes exploit differences in particle size, shape, and density to separate mixtures. Understanding the principles behind these methods is crucial for designing effective separation systems.

Calculations play a vital role in optimizing separation efficiency. From Stokes' Law to sigma theory, various equations help predict settling velocities and compare centrifuge performance. These tools enable engineers to design and select appropriate equipment for specific separation needs.

Principles and Processes

Principles of centrifugation and sedimentation

Top images from around the web for Principles of centrifugation and sedimentation

• 

  Motion of an Object in a Viscous Fluid · Physics View original

  Is this image relevant?

• 

  Motion of an Object in a Viscous Fluid | Physics View original

  Is this image relevant?

• 

  AMIT 145: Lesson 4 Gravity Separation – Mining Mill Operator Training View original

  Is this image relevant?

• 

  Motion of an Object in a Viscous Fluid · Physics View original

  Is this image relevant?

• 

  Motion of an Object in a Viscous Fluid | Physics View original

  Is this image relevant?

1 of 3

Top images from around the web for Principles of centrifugation and sedimentation

• 

  Motion of an Object in a Viscous Fluid · Physics View original

  Is this image relevant?

• 

  Motion of an Object in a Viscous Fluid | Physics View original

  Is this image relevant?

• 

  AMIT 145: Lesson 4 Gravity Separation – Mining Mill Operator Training View original

  Is this image relevant?

• 

  Motion of an Object in a Viscous Fluid · Physics View original

  Is this image relevant?

• 

  Motion of an Object in a Viscous Fluid | Physics View original

  Is this image relevant?

1 of 3

• Gravitational force drives sedimentation while centrifugal force powers centrifugation
• Particle settling velocity influenced by size, shape, density, fluid viscosity and density
• Stokes' Law describes settling velocity of particles in laminar flow $v = \frac{d^2(\rho_p - \rho_f)g}{18\mu}$
• Buoyancy opposes settling dependent on fluid density and particle volume
• Drag force resists particle motion through fluid increases with settling velocity

Centrifugation vs sedimentation processes

• Sedimentation uses gravity for slower separation suitable for larger particles (settling tanks)
• Centrifugation employs centrifugal force for faster separation of smaller particles (disc stack centrifuges)
• Centrifugation generally achieves higher separation efficiency
• Sedimentation applied in wastewater treatment while centrifugation used in biotechnology

Design of centrifuges and settlers

• Gravity settlers use rectangular or circular tanks with inclined plates to improve efficiency
• Hydrocyclones have conical shape with tangential inlet for particle separation
• Disc stack centrifuges contain series of conical discs for continuous high-throughput operation
• Tubular centrifuges feature long narrow bowl for high-speed batch or semi-continuous processing
• Decanter centrifuges use horizontal cylindrical bowl with screw conveyor for continuous solids discharge

Calculations for separation efficiency

• Terminal settling velocity calculated using Stokes' Law (laminar) or Newton's Law (turbulent)
• Centrifugal settling velocity given by $v_c = \frac{d^2(\rho_p - \rho_f)\omega^2r}{18\mu}$
• Sigma theory compares centrifuge performance to equivalent settling tank area
• Separation efficiency determined by $E = 1 - e^{-\frac{v_t A}{Q}}$
• Cut size represents particle size at which 50% separation occurs
• Residence time affects separation efficiency calculated as volume divided by flow rate