1. Mechanical operations (active learning assignMent )
Abhisekh Chaudhary - ( )
Akbari Chirag – ( )
Divyanshi Bagrawala- ( )
Jaswinder Bhatti - ( )
Bhaumik Parikh - ( )

2. contents Introduction Regimes of fluidization
Conditions for fluidization
Calculating minimum velocity
Practical evaluation
Types of fluidization
Reference

3. introduction
Fluidization is a process in which solids are caused to behave like a fluid by blowing gas or liquid upwards through the solid-filled reactor.
Fluidization is widely used in commercial operations; the applications can be roughly divided into two categories, i.e. • Physical operations, such as transportation, heating, absorption, mixing of fine powder, etc. and  • Chemical operations, such as reactions of gases on solid catalysts and reactions of solids with gases etc. 

4. Fig:- Gas solid flidized bed

5. Fig2:- Flidized Bed reactor
(Handbook of Fluidization and Fluid-Particle Systems. Yang W
(Ed.). Marcel Dekker, Inc., NY, NY, USA, 53–113 (2003).)

6. The fluidized bed is one of the best known contacting methods used in the processing industry, for instance in oil refinery plants.
Among its chief advantages are that the particles are well mixed leading to low temperature gradients, they are suitable for both small and large scale operations and they allow continuous processing.
There are many well established operations that utilize this technology, including cracking and reforming of hydrocarbons, coal carbonization and gasification, ore roasting, Fisher-Tropsch synthesis, coking, aluminium production, melamine production, and coating preparations.
The application of fluidization is also well recognized in nuclear engineering as a unit operation for example, in uranium extraction, nuclear fuel fabrication, reprocessing of fuel and waste disposal.

7. The fluidization principle is straight forward, passing a fluid upwards through a packed bed of solids produces pressure drop due to fluid drag.
When fluid drag is equal to the bed weight the particles no longer rest on each other, this is the point of fluidisation.
The fluid velocity is sufficient to suspend the particles, but it is not large enough to carry them out of the vessel.
The solid particles swirl around the bed rapidly, creating excellent mixing among them.
The material “fluidized” is almost always a solid and the “fluidizing medium” is either a liquid or gas.
The characteristics and behavior of a fluidized bed are strongly dependent on both the solid and liquid or gas properties.

8. Fig3:- Typical fluidization9(

9. When fluidized, a bed of solid particles will behave as a fluid, like a liquid or gas.
The fluidic behavior allows the particles to be transported like a fluid, channeled through pipes, not requiring mechanical transport (e.g. conveyor belt).

10. REGImES OF FLuIDIZATION
When the solid particles are fluidized, the fluidized bed behaves differently as velocity, gas and solid properties are varied.
It has become evident that there are number of regimes of fluidization.
When the flow of a gas passed through a bed of particles is increased continually, a few vibrate, but still within the same height as the bed at rest. This is called a fixed bed.
With increasing gas velocity, a point is reached where the drag force imparted by the upward moving gas equals the weight of the particles, and the voidage of the bed increases slightly: this is the onset of fluidization and is called “minimum fluidization” with a corresponding minimum fluidization velocity.
Increasing the gas flow further, the formation of fluidization bubbles sets in. At this point, a bubbling fluidized bed occurs.

11. As the velocity is increased further still, the bubbles in a bubbling fluidized bed will coalesce and grow as they rise. If the ratio of the height to the diameter of the bed is high enough, the size of bubbles may become almost the same as diameter of the bed. This is called slugging.
If the particles are fluidized at a high enough gas flow rate, the velocity exceeds the terminal velocity of the particles. The upper surface of the bed disappears and, instead of bubbles, one observes a turbulent motion of solid clusters and voids of gas of various sizes and shapes. Beds under these conditions are called turbulent beds.
With further increases of gas velocity, eventually the fluidized bed becomes an entrained bed in which we have disperse, dilute or lean phase fluidized bed, which amounts to pneumatic transport of solids.

12. Fig2:- Regies of Flidization(

13. Geldart’s powder classification
combustion/
gasification
drying/
PE production
cat. reactions
hardly used

14. Group C Cohesive Difficult to fluidized, and channeling occurs
Interparticle forces greatly affect the fluidization behaviour of these
Powders.
dp ~ 0-30 μm
Example: flour, cement

15. Group A •Aeratable •Characterized by a small dp and small ρp
•Umb is significantly larger than Umf
•Large bed expansion before bubbling starts
There is a maximum bubble size
•dp ~ μm
•Examples: FCC, milk flour

16. Group B •Bubbling •Umb and Umf are almost identical
•Solids recirculation rates are smaller
Bubbles size is almost independent of the mean particle diameter and
the width of the particle size distribution
•No observable maximum bubble size
•dp ~ μm
•Example: sand

17. Group D •Spoutable •Either very large or very dense particles
•Bubbles coalesce rapidly and flow to large size
•Bubbles rise more slowly than the rest of the gas percolating through
the emulsion
•Dense phase has a low voidage
•dp ~ >1000 mm
•Examples: Coffee beans, wheat, lead shot

18. Conditions for fluidization
The minimum velocity at which a bed of particles fluidizes is a crucial parameter needed for the design of any fluidization operation.
The details of the minimum velocity depend upon a number of factors, including the shape, size, density, and poly-dispersity of the particles.
The density, for example, directly alters the net gravitational force acting on the particle, and hence the minimum drag force, or velocity, needed to lift a particle.
The shape alters not only the relationship between the drag force and velocity, but also the packing properties of the fixed bed and the associated void spaces and velocity of fluid through them.
To find the minimum fluidizing velocity, Umf , experimental and theoretical approaches can be used.

19. Calculating minimum velocity
The point at which the fluid or gas flow causes the bed of particles to expand and lift into the vertical column is marked by a conceptually simple balance.
At Umf , the hydrodynamic drag force on the particles Fd, due to the flow of gas through the packed bed of particles, matches (or just exceeds), the net gravitational forces Fg,
0 = Fg + Fd

20. The net gravitational forces on the bed of particles must consider the weight W of the particles and the buoyancy forces Fb,
Fg = W − Fb
= (dp − df ) gVp
Where, dp is the density of the particles,
df is the density of the fluid,
g is the gravitational acceleration constant
Vp is the total volume of particles within the fluidized bed.

21. If the weight and density of the particles is known, then the particle volume can be calculated.
Using the definition of bed voidage Em, the volume of the particles can be written as
Vp = AH(1 − Em)
Where, A is the cross sectional area of the fluidized bed
H is the height of the bed of the particles prior to the onset of fluidization.

22. Hydrodynamic Drag Forces:- The local pressure drop through a porous medium is a function of the bed voidage E, the flow velocity U, and details of the particles,
∇P = f (E, U, De, Os)
The velocity U is the superficial velocity, or volumetric flow rate of the fluid normalized by the cross-sectional area of the column.
The equivalent volume diameter De and sphericity factor (Os) account for the details of the particle size and shape.
NOW,
The pressure drop across the bed must be equal to the effective weight per unit area of the particles at the point of incipient fluidization. This is expressed in mathematical form as
∆P = (dp − dg )(1− E)gL ……..(1)

23. Where, ΔP is the pressure drop, dp and dg are the densities of the particle and gas respectively.
Em is the porosity(voidage) at minimum fluidization,
and L is the height of the bed.
The Ergun equation can be used to calculate the pressure drop in packed beds.
…..(2)

24. Substituting (2) in (1), we obtain a quadratic equation for the minimum fluidization velocity (u)

25. The pressure gradient, or drag force, depends on the flow velocity in a non-simplistic manner. However, different regimes of flow can be easily identified, much like the well- known case of the drag force on a single particle. The regimes are defined in terms of the Reynolds number,

26. Practical evaluation
Fig3:- pressure drop P as a function of the superficial velocity (

27. Measurements of the pressure drop across the bed of particles can be used to identify the minimum velocity of fluidization.
As diagrammed in Figure 3, the pressure drop increases with flow rate until the bed expands and increases the porosity (point A).
Note that the velocity and pressure drop relationship is not necessarily linear as shown, depending upon the range of “Re” covered.
Upon further increasing the velocity, the pressure drop attains a maximum value.
Between points A and B, the frictional drag force causes the particles to rearrange, which can alter the voidage.

28. Upon rearrangement, the pressure decreases and point B lies above point C as a result.
As U is increased beyond point C, the pressure drop remains approximately constant until some point D where the velocity is not significantly greater than at point C.
If the process is reversed by steadily lowering the velocity U, point E will be found instead of point B due to the different voidage resulting from the rearrangement of the particles, and line EF is the process for reforming the fixed bed of particles.
The minimum fluidization velocity is the velocity at which these two lines(AB and EF) intercept.

29. Types of flUidization
Particulate fluidization
Bubbling fluidization

30. Bubbling Fluidization
This type of fluidization has been called ‘aggregative fluidization’, and under these conditions, the bed appears to be divided into two phases, the bubble phase and the emulsion phase.
The bubbles appear to be very similar to gas bubbles formed in a liquid and they behave in a similar manner. The bubbles coalesce as they rise through the bed.
Fig4:- aggregative fluidization

31. Particulate fluidization
High solid hold-ups (typically % by volume).
High velocity of gas.
Uniform expansion of the bed.
Limited axial mixing of gas.
Suitable for exothermic and fast reactions.
Good gas-solid contact and hence, favors reactant conversion.
high gas flow-rates operation and good for isothermal operation.
Favorable bed to surface heat transfer.
Fig5:- Particulate fluidization

32. Significance of Fluidized beds
Pharmaceutical
•Coating of pills
•Granulation
•Production of plant
and animal cells
Combustion/pyrolysis
Combustion/gasification of coal
•Pyrolysis of wood waste
•Chemical looping comubstion

33. Silicon production for semiconductor and solar industry
Chemical and Petrochemical
•Cracking of hydrocarbons
•Gas phase polymeric
Reactions
Advanced materials
Silicon production for
semiconductor and solar
industry
•Coated nanoparticles
•Nano carbon tubes

34. Physical operations
Coating of metal and
glass objects
•Drying of solids
•Roasting of food
•Classify particles

35. Reference
W.L. McCabe, J. C. Smith and P. Harriot (1985), “Unit Operations of Chemical Engineering”, McGraw Hill, New York 7e Page No. 177 to 187.
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