Fluidization

Question 1

Advantages of fluidized bed over packed bed

Answer 1

1. increased gas-solid contact = great mixing
2. heat transfer in fluidized beds = 5-10 times better

Question 2

Disadvantages of fluidized beds

Answer 2

1. attrition (or particles)
   - often need cyclone at end to capture the particles
2. scale-up is difficult
   - hydrodynamics of bubbles/fluid changes with size (small size = high wall effects)

Question 3

Design parameters for a fluidized bed

Answer 3

1. bed diameter
2. height of column
3. gas velocity
4. flux of fine particles (particles carried out of bed)

Question 4

Important parameters to know for fluidized bed

Answer 4

1. particle characteristics (dp + size distribution, density)
2. fluid properties (viscosity, density)
3. operating T and P (T changes gas velocity and P changes empirical correlations)

Question 5

Minimum fluidization velocity

Answer 5

velocity where the packed bed changes to a fluidized bed
- 3 cases:

1. perfectly spherical equal sized particles - fluidize normally
2. small particles - agglomerate together and cause a slight hump when reaching umf
3. wide size distribution, causes a not so straight line to umf (takes longer because the small particles fluidize first while the large particles take longer to fluidize)

Question 6

How does viscosity change with temperature and pressure for gases?

Answer 6

• negligible change with pressure
• linear increase with temperature

Question 7

How does viscosity change with temperature and pressure for liquids?

Answer 7

• viscosity decrease with temperature
• viscosity increases with pressure

Question 8

Minimum bubbling velocity

Answer 8

• minimum velocity where first bubble appears

Question 9

What happens to bubble sizes for type A and B particles?

Answer 9

Type A: the continue to split and coalesce, causing bubble size to reach a stable point

Type B: bubbles continue to coalesce, causing bubbles to grow

Question 10

Fluidized Bed 2 phase theory

Answer 10

2 phase in a fluidized bed:

• Emulsion phase (with properties of a packed bed at umf)
• gas phase (just a bubble)

Question 11

Parts of a bubble?

Answer 11

1. emulsion (stuff around bubble)
2. cloud (a lighter solid-gas mixture at interface of bubble and emulsion)
3. bubble (gas, less than 0.1% solids by volume)
4. Wake (a dense solid region carried up by bubble)

Question 12

How does bubble shape change in fluidization?

Answer 12

• the bubble angle increase with viscosity (for gas-solid mixtures)

Question 13

Rise velcoity

Answer 13

velocity at which bubble rises through column (like particles, the bubble velocity can be hindered)

Question 14

Bubble drift/Particle movement in fludized beds

Answer 14

as particles rise, it pushes particles above it away (thus moving particles above bubble around)

• also carries particles up in its wake, even as it rises above the bed, will continue to carry particles until shedding occurs (particle terminal velocity is greater than the bubble’s pull)

*these motions cause great particle mixing in fluidized beds
**while this occurs, gas can permeate into bubble and from bubble out into the emulsion (but this doesn’t’ always occur)

Question 15

Gas movement with bubble in fluidized beds

Answer 15

2 cases:

1. Bubble rise velocity slower than umf (gas velocity)
   - emulsion gas penetrated into bubble then exits bubble from top
2. Bubble rise velocity is faster than umf
   - the pull of the bubble causes gas to flow downward (increasing mixing)
   - creates a cloud (contain gas) around it where the fast gas in the bubble exits at the top but then in drawn back into the faster bubble

Question 16

Why do bubble grow in fluidized beds?

Answer 16

1. hydrostatic P decreases as rise up bed (less P = bubbles reach each other since bubble rise velocity increases)
2. bubbles catch up to one another (drag from wake to higher bubble draws the lower bubble into it)
3. bubbles are side by side (and then merge into one another)

**when bubbles coalesce, gain some gas from emulsion. Similarly when bubbles break apart, loose some gas to emulsion

Question 17

How are particles carried out of a fluidized bed?

Answer 17

when the particle terminal velocity is less than the gas velocity (often the case for the smallest particles)

Question 18

How does flow regime influence particle entrainment?

Answer 18

• laminar flow has the higher spite while other regions are lower, causes particles to move in circles

-turbulent flow has a higher region of fast moving velocity (a flatter peak)