Fluidization and Fluidized Bed

1. Fluidization

2. WASEEM AKHTAR
NFC IET MULTAN

3. What is Fluidization?
The operation by which fine solids are
transformed into a fluidlike state through
contact with a gas or liquid.

4. Some Key Terminology
 Attrition: breakdown of particles
 Choking: collapse of a dilute-phase suspension
into a dense-phase flow as the gas velocity is
reduced at constant solids flow
 Circulating fluidized bed: configuration
intended to send particles around in a loop
continuously, with no upper interface within the
bed

5. Some Key Terminology
 Downer: column where particles are made to
fall through under gravity, usually with cocurrent
gas flow
 Distributor or Grid: support plate at bottom
which introduce the gas to the bottom of the
bed and supports the weight of the bed when it
is shut down
 Elutriation: tendency for fine particles to be
preferentially entrained from the reactor

6. Some Key Terminology
 Fast fluidization: flow regime whereby there is
a relatively dense suspension, but no distinct
upper surface, and a superficial velocity generally
at least 3 m/s
 Fines: generally particles smaller than 37 µm in
diameter (smallest regular sieve size)
 Freeboard: region extending from top of bed
surface to top of reactor vessel

7. Some Key Terminology
 Interstitial gas: gas between the particles in
dense suspension
 Porosity: fraction of gas in bed/given region as
a whole or only inside the particles; sometimes
used interchangeably with voidage
 Riser: column where particles are carried
upwards by the gas, with no distinct bed surface

8. Some Key Terminology
 Segregation: tendency for particles to gather in
different zones according to their size and/or
density
 Solids: used synonymously with particles
 Superficial velocity: gas flow rate divided by
total column surface area

9. Some Key Terminology
 Transport disengaging zone: region in
freeboard beginning at bed surface in which
particle flux decreases with height and above
which the entrainment is independent of height
 Voidage (or void fraction): fraction by volume
of suspension or bed which is occupied by the
fluid

10. Elements of Fluidized Bed Reactors

11. Contacting Methods
 Batch, cocurrent, backmix, crossflow, countercurrent
 Solids may often be represented by backmix flow
 By using proper baffling and staging of units, and
with negligible entrainment of solids, the contacting
in fluidized beds can be made to approach closely
the usually desirable extreme of cuntercurrent plug
flow
 For good design, proper contacting of phases is
essential

15. Advantages of Fluidized Beds
 The smooth, liquid-like flow of particles allows
continuous automatically controlled operations with
ease of handling.
 The rapid mixing of solids leads to nearly isothermal
conditions throughout the reactor, hence the operation
can be controlled simply and reliably.
 It is suited to large-scale operations.

16. Advantages of Fluidized Beds
 The circulation of solids between two fluidized beds
makes it possible to transport the vast quantities of heat
produced or needed in large reactors.
 Heat and mass transfer rates between gas and particles
are high when compared with other modes of
contacting.
 The rate of heat transfer between a fluidized bed and an
immersed object is high, hence heat exchangers within
fluidized beds require relatively small surface areas.

17. Disadvantages of Fluidized Beds
 The difficult-to-describe flow of gas, with its large
deviation from plug flow and the bypassing of solids by
bubbles, represents an inefficient contacting system.
 The rapid mixing of solids in the bed leads to
nonuniform residence times of solids in the reactor.
 Friable solids are pulverized and entrained by the gas.
 Erosion of pipes and vessels from abrasion by particles.
 For noncatalytic operations at high temperature the
agglomeration and sintering of fine particles can
necessitate a lowering in temperature of operation,
reducing the reaction rate.

18. Commercial Applications
 Solid-Catalysed Gas-Phase Reactions:
 Fluid catalytic cracking, reforming
 Fischer-Tropsch synthesis
 Phthalic and maleic anhydride
 Acrylonitrile and aniline
 Chlirination and bromination of hydrocarbons
 Polyethylene and polypropylene
 Oxidation of SO2 to SO3

19. Commercial Applications
 Gas-Solid Reactions:
 Roasting or ores (ZnS, Cu2S, nickel sulphides, etc.)
 Combustion and incineration
 Gasification, coking and pyrolysis/carbonization
 Calcination (limestone, phosphates, aluminium
hydroxide)
 Flurination of uranium oxide
 Fluid coking
 Reduction of iron oxide
 Catalyst regeneration

20. Commercial Applications
 Gas-Phase Non-Catalytic Reactions:
 Natural gas combustion
 Gas-Liquid-Solid:
 Hydrotreating, hydroprocessing
 Biochemical processes

21. Commercial Applications
 Physical Processes:
 Drying of particles
 Coating of surfaces
 Granulation (growing particles)
 Heat treatment (e.g. annealing, quenching)
 Medical beds
 Filtration
 Back-purging of filters
 Blending
 Classification

22. Flow Regimes for Upward Flow of Gas
through Solid Particulate Materials

23. Various Kinds of Contacting of a
Batch of Solids by Fluid

24. Classification of Fluidized Beds

25. Classification of Dense-Phase
Fludized Beds

26. Industrial Applications
of Fluidized Beds

27. Winkler Gas Generator

28. Large Scale Fluid Bed Catalytic
Cracking Pilot Plant

29. Two-Stage Fluidized Salt Dryer

30. Pilot Plant for Fluidized Drying of
Air with Adsorbent
The drying of air by circulation of large
(3.2 to 4.8 mm) silica gel beads of
multistage fluidized adsorption.
To reduce the humidity from 0.00191 to
0.0015 kg/kg pilot plant uses a five-stage
fluidized absorber 1.2 m square in cross
section, 6.1 m high, a pressure drop of
127 cm H2O.
A perforated plate distributor with rubber
flaps at the lower end of the downcomers
to assure steady flow of particles from
stage to stage.

31. Typical Flow Regimes Observed

32. Qualitative Fluidization Map for
Fine Solids

33. Solids Mixing by a Single Rising
Bubble in a Bed of Small Particles

34. Solids Mixing by a Single Rising
Bubble in a Bed of Large Particles