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Basic Concepts in Fluidization

Britt HalvorsenTelemark University College

The phenomenon of fluidization

� Fluidization is a process whereby a granular material is converted from a static solid-like state to a dynamic fluid-like state.

� The process occurs when a fluid (liquid or gas) is passed up through the granular material (powder).

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Fluidized beds

� In a fluidized bed gas is passing upwards through a bed of particles. The earliest applications of fluidization were for the purpose of carrying out chemical reactions. Since that time there have been a number of successful chemical processes involving fluidized bed reactors, but also other applications.

� Fluidized beds are applied in industry due to their large contact area between phases, which enhances chemical reactions, heat transfer and mass transfer. The efficiency of fluidized beds is highly dependent of flow behaviour

Particle behaviour

� In a fluidized bed the frictional forces between particles are small� Gas/particle assembly behaves like a liquid

with a density equal to the bulk density. �Behaviour of particles in fluidized beds

depends on a combination of their mean particle size and density.

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Various stages of fluidization

Classification of particles

� Geldart fluidization diagram:� Identify characteristics

associated with fluidization of particular powders at ambient conditions.

Geldart (1973) classification of particles according to their fluidization behaviour.

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Group A powders

Most commercial fluidized bed catalytic reactors use Geldartgroup A powders. � Group A powders:

� Easily fluidized � Bed expands considerably before bubbles appear due to:

� Inter-particle forces that are present in this group of powders� Inter-particle forces are due to particle wetness, electrostatic charges and

van der Waals forces. � Bubble formation will occur when the gas velocity exceeds the minimum

bubble velocity. � The bubbles rice faster than the gas percolating through the emulsion.

Maximum bubble size usually less than 10 cm and independent of the bed size. Kunii and Levenspiel (1991)

Group B particles

� For group B particles� Inter-particle forces are negligible� Bubbles are formed as the gas velocity reaches the minimum

fluidization velocity. � Bed expansion is small compared to group A particles. Small

bubbles are formed close to the air distributor and the bubble size increase with distance above the distributor.

� Bubble size also increases with the excess gas velocity � Excess gas velocity: difference between the gas velocity and the

minimum fluidization velocity, Geldart (1986). � Coalescence is the dominating phenomena for group B powders

and bubble size is roughly independent of mean particle size. Most bubbles rise faster than the interstitial gas velocity.

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Group C and D powders

� Group C powders:� Cohesive. � Fluidization is

extremely difficult� Bubble formation will

not occur.

� Geldart group D powders:� Large and/or dense

particle powders � Undesirable for

fluidization operations. � Large gas flows are

needed to get these particles fluidized

Geldart C and D powders give a low degree of solid mixing and gas back-mixing.

Phenomenon of fluidization

Gas or liquid� Fixed and expanded bed (low fluid velocity)

� Fluid percolates through the void between theparticles.

� Expanded bed (higher fluid velocity)� Particles move apart

� Bed at minimum fluidization� Further increased velocity→particles are

suspended by the upward-flowing fluid� Frictional force between particle and fluid counter

balances. � Pressure drop equals weight of particles+fluid

� Minimum fluidization velocity

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� Liquid:� Smoothly fluidized bed

� Velocity above minimum fluidization velocity

� Gas and liquid� Lean phase fluidization

� High velocity, fine particles� Particles carried out of the

bed with the fluid� Pneumatic transport

� Gas� Bubbling fluidized bed

� Gas velocity increased aboveminimum fluidization velocity

� Bed height about minimum fluidization

� Bubbles growing with hight

� Slugging� Deep bed with small diameter

� Bubble diameter about bed diameter

� Fine particles, axialsluggs

� Coarse particles, flat slugs

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� Gas� Turbulent fluidization

� Very high gas velocity� Terminal velocity of solids

exceeded

� Upper surface of bed disappears

� Entrainment of particlessignificant

� Turbulent motion of solid clusters and voids of gas withvarious size and shape

When will the bed start to fluidize?� At the velocity where the buoyant forces equals the drag

forces. (wall friction and solids stress are neglected) � Minimum fluidization velocity developed from the :

( )( )

[ ][ ]

[ ]2sg

2

3gs

g

sgg

sggsg

m/st coefficien drag particleGas

m/sgravity ofon accelerati g

kg/mdensity gas and solid,

fraction (gas) void

)uu(g1

−=Φ

=

=ρρ

=ε

−ε

Φ=⋅ρ−ρε−

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Minimum fluidization velocity

( )( )sg

gsmfmfmf

gU

Φ−−

=ρρεε 12

� At minimum fluidization:� Solid velocity is zero:

� Calculated minimum fluidization velocity:� Deviation from experimental;

� Void fraction� Particle size distribution� Mean particle diameter

Drag models

� Different drag models are developed�Ergun equation :

( )

velocitysolid and Gasv,v

viscosityGasµ

factor Formψ

diameter Particled

fraction volumeSolidε

0.8εfor ,ψd

εvvρ75.1

)ψ(dε

µε1150Φ

sg

g

s

s

s

gss

ssgg

2ssg

g2

gsg

=

====

≤−

+−

=

9

sggg2t

gDs

sg vv)ε1(εR

1ρC

d4

3Φ −−=

( )

0.85εfor , εB

0.85εfor ,0.8εB

A

AA)(2B0.12Re0.0036Re0.06ReA5.0R

g2.65g

g1.28g

14.4

2s

2sst

>=

<=

=

+−++−=

gε

( )g

psgggs

s

tD

dvvRe

Re

R4.80.63C

µ⋅−⋅ρ⋅ε

=

+=

Syamlal & O’Brien drag model� Empirical model

Calculated and experimental minimum fluidizationvelocities

0

0.1

0.2

0.3

0.4

0.5

0.6

0 200 400 600 800 1000 1200

Mean particle diameter [µµµµm]

Min

imum

flu

idiz

atio

n v

eloci

ty [m

/s]

Ex. Small+mix small

Ex. Medium + mix medium

Ex. Large

Calc., void=0.37

Calc., void=0.38

Calc., void=0.40

Calc., void=0.42

Calc., void=0.44

•Particle size distribution influences on:Minimum fluidization velocity, Bubble

formation, Segregation, Pressure drop

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Pressure drop

a. Uniform distributed sand( Figure from: Kuuni and Levenspiel, Fluidization Engineering)

Wide distribution of particle sizes (180-1400 µm)( Figure from: Kuuni and Levenspiel, Fluidization Engineering)

Pressure drop(Experiments performed at TUC)

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Efficiency

Bubble Distribution

Bubble velocity

Bubble frequency

Bubble size

Efficiency

V=dy/dt

dy

dx

� Experiment: Mixture of two powders.

Particle range: 100-200 µm (153) and 750-1000 µm (960)

� Simulation: Two particle phases: Mean diam. 153 µm and 960 µm

Particle segregation

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Experimental set-up, 3-D bed

2.0 m

0.35 m

0.75 m

0.25 m

Fibre optical probe

-2-1.5

-1-0.5

00.5

11.5

2

0 0.1 0.2 0.3 0.4 0.5

Time [s]

Sig

nal

[V]

Signal 1

Signal 2

Probe head

distance between lights: 2.7 mm

Light reflection

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Labview program

Pneumatic transfer Test facility [Akilli H et al., 2001]

Lab scale set-up for experimental study of pneumatic transport. Characteristic for industrial transport systems:

•Long pipes with rather small diameters.

•Pipe walls influence significantly on the flow behaviour.

•Elbows highly influence the flow behaviour

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Conveying system

� Horizontal pipes� Vertical pipes� Bend

� Horizontal-vertical� Vertical-horizontal� Horizontal-horizontal� U-bend� Distance between bends

� Ratio diameter/length

� Different flow regimes can arise

� Flow regime can change during the transport

� Change in velocity direction influence the flow behaviour

� Particle collisions with walls

Particle motion

� Influenced by:�Gravitational settling in horizontal pipes� Inertial behaviour in bends and branches�Turbulent dispersion�Lift forces, friction forces�Particle-wall collisions

� Rough walls/smooth walls

�Particle-particle collisions

(Huber and Sommerfeld, 1998)

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Flow regimes

� Homogeneous flow: Dilute flow transport particles are well mixed homogeniousstate by turbulent mixing

� Dune flow: Particles start to settle� Slug flow: Includes regions with settled particles – and regions with particles in

suspension. Slug flow is used in dense flow transport. Higher pressure drop and solid loading. Low velocity less material degradation and line erosion.

� Packed bed: Lower velocity. No transport

Flow regimes in horizontal pipes, Crowe et al.(1997)

Flow regimes

Pressure drop as a function of gas velocity, Kunii and Levenspiel (1991)

Saltation velocity: Superficial gas velocity at which particles begin to separate from the gas phase and slide or roll along the bottom of the pipe. →Changes from suspended to non-suspended transport

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Wall roughness

� Smooth pipes� Particles settle� Stronger coupling with gas phase

� Deformation of stream-wise velocity profile (ref)

� Rough walls� Increase of rebound angle compared to compact

angle� Particles re-suspended� Gravitational settling reduced� Secondary flow� Increasing pressure loss

Huber and Sommerfeldt, 1998

Wall roughness

Figure: Particle mass flux in smooth and rough pipe. (Huber and Sommerfeld, 1998)

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Rope

Figure: Rope formation and dispersion [Akilli et al., 2001]

Formation and disintegration:•Centrifugal forces•Secondary flows•Gas velocity•Bend radius•Solid loading•Pipe orientation•Particle size•Particle density

Mc Clusky et al. (1989)Levy and Mason (1998)Yilmaz and Levy (2001)

Rope

� Centrifugal forces in the elbow cause gas and particles to segregate� solid particles impinging on the outer wall of the bend, forming relatively dense

phase structure (rope). � The rope formation and dispersion is dependent of

� Centrifugal forces� Secondary flows� Gas velocity� Bend radius� Solid loading� Pipe orientation� Particle size� Particle density

� The particles are decelerated in the bend due to particle-particle collisions and particle-wall collisions, and have to be re-accelerated downstream the bend to the conveying velocity.

Mc Clusky et al. (1989), Levy and Mason (1998), Yilmaz and Levy (2001)

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Pressure drop� Pressure drop influenced by:

� Gas velocity� Dictates flow regime� Great effect on total pressure drop� Increase in gas velocity strongly increases the total pressure drop

� cause high energy losses. � Operation at low gas velocity

� cause problems in suspending particles.

� Particle density� Increasing slip velocity� Increasing total pressure drop

� Particle size� Particle concentration

� Slight decrease in slip velocity� Increase in total pressure drop

� Particle-wall collision� Bend, radius ratio� Pipe dimensions

Hidayat and Rasmuson, (2005)

Turbulence

� Particle influence �Displacement of flow field�Generation of wakes�Dissipation to the motion of dispersed phase�Modification of velocity gradients�Additional length scales�Particle-particle interaction (0.1)

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Concluding remarks� Fluidization behaviour depends on:

� Particle size and particle size distribution� Particle density� Fluid properties

� Efficiency of fluidsized bed depends on:� Contact between particles and fluid

� Bubble size� Bubble frequency� Bubble velocity� Bubble distribution

� Economy� Low pressure drop� Good mixing� High degree of conversion� Low degree of energy loss

� Challenges(A lot of research still remaining)�Develop good models

� (Empirical)� Physical

�CFD simulations� Scaling� Reduce simulation time