CHEE2945 – Lecture 16
Fluidisation:
- For particles in a container with a porous base, when fluid is forced up
through the base, there will be three forces on the particle:
o Gravity,
o Buoyancy,
o Drag.
- When F b+ F d ≥ F g, the particles are lifted, so they are no longer touching each
other and remain suspended by the drag force. This creates a fluidised bed.
- Entrainment conveying occurs when the upward fluid flow is greater than the
terminal settling velocity. This causes particles to flow out of the container.
- Fluidised beds have the properties of liquids:
o They don’t have a fixed shape; their shape depends on the shape of
the vessel.
o Lighter objects float on top, and heavier objects sink to the bottom.
o The bed surface stays horizontal, even if the vessel and bed are tilted.
o Solids can flow through an opening in the vessel.
o Fluidised beds have a static head.
- Some advantages of fluidised beds include:
o Liquid like behaviour.
o Rapid mixing.
o Efficient solid-fluid contacting.
o High heat and mass transfer rates.
o Resists temperature changes.
- Some disadvantages of fluidised beds are:
o Breakdown due to collisions and aggregation can occur.
o Erosion of pipe and vessel walls.
o Bubbling is difficult to control.
- Fluidized beds have many applications:
o Reactors:
Catalytic cracking of hydrocarbons.
Coal gasification.
Bioreactors.
o Combustion of coal or biomass.
o Drying of materials; polymers, fertilisers, pharmaceuticals, etc.
o Granulation.
o Density or size separation in minerals and coal industries.
o Coating polymers onto metals.
o Cooling solids.
Fluidisation:
- For particles in a container with a porous base, when fluid is forced up
through the base, there will be three forces on the particle:
o Gravity,
o Buoyancy,
o Drag.
- When F b+ F d ≥ F g, the particles are lifted, so they are no longer touching each
other and remain suspended by the drag force. This creates a fluidised bed.
- Entrainment conveying occurs when the upward fluid flow is greater than the
terminal settling velocity. This causes particles to flow out of the container.
- Fluidised beds have the properties of liquids:
o They don’t have a fixed shape; their shape depends on the shape of
the vessel.
o Lighter objects float on top, and heavier objects sink to the bottom.
o The bed surface stays horizontal, even if the vessel and bed are tilted.
o Solids can flow through an opening in the vessel.
o Fluidised beds have a static head.
- Some advantages of fluidised beds include:
o Liquid like behaviour.
o Rapid mixing.
o Efficient solid-fluid contacting.
o High heat and mass transfer rates.
o Resists temperature changes.
- Some disadvantages of fluidised beds are:
o Breakdown due to collisions and aggregation can occur.
o Erosion of pipe and vessel walls.
o Bubbling is difficult to control.
- Fluidized beds have many applications:
o Reactors:
Catalytic cracking of hydrocarbons.
Coal gasification.
Bioreactors.
o Combustion of coal or biomass.
o Drying of materials; polymers, fertilisers, pharmaceuticals, etc.
o Granulation.
o Density or size separation in minerals and coal industries.
o Coating polymers onto metals.
o Cooling solids.
