an introduction to fluidization - saimm 2011 short-course on fluidization: concepts in industrial...

AN INTRODUCTION TO

FLUIDIZATION

BY

MILAN CARSKY

UNIVERSITY OF KWAZULU-NATAL

IFSA 2011 short-course on fluidization: Concepts in industrial fluidization

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AN INTRODUCTION TO FLUIDIZATION

SUMMARY

• Principle of fluidization (gas-solid fluidization, liquid-solid fluidization, properties of

fluidized beds)

• Components of a fluidized bed unit

• Fluidized bed materials (size, density, shape, strength; definition of particle size; bulk

and material density); classification of materials according to their fluidization

behaviour (Geldart’s diagram)

• Minimum fluidization velocity (experimental determination and literature predictions)

and fluidized bed hydrodynamics

• Special fluidized beds (spouted bed, vibro-fluidized beds)

• Fluidization regimes and transitions

• Use of fluidized bed technology (details later in the course):

� Fluidized bed (catalytic) reactors

� Ore roasting

� Fluidized bed combustion (for power generation)

� Drying of materials which are granular, free flowing, not agglomerating, not

fragile, not vulnerable to oxidation, neither the product nor the vapour released

is toxic or flammable

1. Principle of fluidization

By using gas (or liquid) flowing upwards through a layer of a particulate material supported

on a distributor, at certain fluid upwards velocity the particles start to move. Once that occurs

we have reached onset of fluidization and the fluid velocity at this point is called minimum

fluidization velocity (umf).


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Increasing fluid velocity

Properties of fluidized beds:

• Liquid-like behaviour


9

Hydrostatic pressure Particles flow on an Viscosity of a fluidized bed

inclined grid

• Rapid mixing of solids

Time=0 s Time = 3 s (top) and 5 s (bottom)

Tracer at the top


10

Average tracer concentration = 20%

• Uniform conditions and slow response to changing operating conditions

Fluidized bed combustion of coal. Temperature profile taken in different times. T1-T5

temperatures above the bed level, T6-T8 temperatures in the fluidized bed, T9 temperature of

the stagnant layer on the grid.

• High mass and heat transfer rates

• Pressure drop is independent of the fluid velocity


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• Erosion of internal parts because of moving particles

• Entrainment and attrition of the bed material

Liquid-solid fluidization

“Smooth” (particulate) fluidization. Uniform and large

expansion of the bed without presence of bubbles. Mixing by

diffusion of particles.

Gas-solid fluidization

“Aggregative” fluidization. Not

a uniform concentration of

particles in the bed. Presence of

gas bubbles (voids) in the bed

is the main factor of particle

mixing. The regimes may vary

from bubbling to slugging and

turbulent fluidized bed.


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2. Components of a fluidized bed unit

Typical industrial fluidized bed unit


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Typical multi-purpose laboratory fluidize bed unit. 1- blower, 2- gas tank, 3- rupture disc, 4-

filter, 5-dryer, 6-humidification column, 7- mist eliminator, 8-water circulation, 9- humidity

indicator, 10- pressure regulator, 11- rotameters, 12- control valves, 13- windbox, 14-

fluidized bed.

Gas distributors

Function of a distributor:

• To support bed material (and to prevent leaking of a bed material to the plenum

chamber below)

• To distribute gas uniformly into the fluidized bed

• Minimum pressure drop 10-30% of the bed pressure drop

Perforated (single, double) plates

Simple and cheap distributors, but difficult to achieve uniform gas distribution, prone to

weepage of solids, require mechanical support for larger diameters.


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Porous plate and “Sandwich” type distributors

Restricted to small-scale (laboratory) units. They prevent bed material from leaking through

the distributor and usually ensure uniform gas distribution. In case of sandwich type

distributors the operation is restricted to ambient temperature (filter cloth between two

perforated plates).

Cap type distributors

Limited weeping, good gas distribution but creating a stagnant region underneath, more

expensive, difficult to clean.

Spargers

Limited weeping, good gas distribution but creating a stagnant region underneath.


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Slot-type distributors

May achieve uniform gas distribution and promote particle mixing, no stagnant regions,

difficult to construct, require mechanical support for larger diameters.

Conical distributors

Promotes particle mixing, no stagnant regions, difficult to construct.

Cyclones, filters

Function: To separate solids from the gas at the exit (and to return them into the bed).


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Bag filter

Feeders (variety of constructions)

Laboratory screw feeder


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3. Fluidized bed materials

Particle size-characterisation

a) Sieve size (dp)

The width of the square aperture in a sieve.

b) Surface diameter (ds)

The diameter of a sphere having the same surface area as the particle.

c) Volume diameter (dv)

The diameter of a sphere having the same volume as the particle.

d) Surface/volume diameter (dsv)

The diameter of a sphere having the same surface to volume ratio as the particle.

e) Median particle diameter (dp50)

Particle size corresponding to the 50% value on particle size vs. wt% cumulative

undersize plot.

Particle shape

Sphericity:

� ��

��

For spherical particles: � � 1

For non-spherical particles: 0 � � � 1

dp = � dp, meas

dp = mean diameter of non-spherical particles

dp, meas = measured mean particle size


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Density

Particle density Bulk density

If a non-porous particle, particle density = skeletal density

Bed voidage

� �� ! ��

��

Particle strength: Impacts between particles and vessel internals lead to particle attrition

(and entrainment) and/or abrasion of the equipment

Classification of materials according to their fluidization behaviour (Geldart’s

diagram)


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Materials A are easy to fluidize. A typical example is

fluidized bed cracking catalyst. These particles can be

fluidized in a limited range of velocities in a regime similar

to liquid fluidization (i.e. “smooth” fluidization) without the

presence of gas bubbles in the bed.

Materials B are also very common for

fluidization but unlike group A materials

bubbles are always present in the bed

(aggregative fluidization). Typical example of

materials B is sand.

Materials C are very fine particles (eg. flour) which are difficult and often impossible to

fluidize because of large surface (cohesive) forces holding particles together.

Materials D consist of large particles. Their

fluidization is uneven, vigorous, fountain-

like, results in equipment shaking

vigorously.


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4. Minimum fluidization velocity and fluidized bed hydrodynamics

Experimental determination of minimum fluidization velocity

Ergun equation for a pressure drop in a static bed:

where ∆P is pressure drop (Pa), h height of the bed (m), ε bed voidage, U gas velocity (m/s),

and G gas mass velocity (kg/(m2.s).

( ) ( )

ppd

GU

d

U

h

P

−+

−=

∆

ε

εµ

ε

ε 175.1

1150

2

2


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Pressure drop for a fluidized bed:

Correlations for minimum fluidization velocity (examples)

Broadhurst and Becker:

Geldart:

Kunii and Levenspiel:

A correlation given below (see table below for values for C1 and C2) was developed by

multiple researchers:

where:

Remf Reynolds number at the onset of fluidization

Ar Archimedes number

ρp Particle density (kg/m3)

ρf or ρg Fluid density (kg/m3)

Dp or dp Particle size (diameter) (m)

g Acceleration due to gravity (9.81 m/s2)

µ Fluid viscosity (Pa.s)

umf Minimum fluidization velocity (m/s)

hgP fp ))(1( ρρε −−=∆

1 . 8 52

0 . 8 5 0 . 1 3

3

2

R e

3 7 . 7 2 4 2 0 0 0 ( )

( )

R e

m fp

f

p f p f

m f

m f

p f

A r

A r

D gA r

UD

ρ

ρ

ρ ρ ρ

µ

µ

ρ

=

+

−=

=

( ) 85.013.05

85.1

7.371042.2 Ar

ArU

fp

mf

+−×=

ρρ

µ

ρgmfp

mf

mf

Ud

CArCC

=

−+=

Re

Re 12

2

1

( )[ ]7.330408.07.11355.0

−+= Ard

Upf

mfρ

µ


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Reference C1 C2

Wen&Yu 33.7 0.0408

Richardson 25.7 0.0365

Saxena&Vogel 25.3 0.0571

Babu et al. 25.25 0.0651

Grace 27.2 0.0408

Chitester et al. 28.7 0.0494

Fluidized bed hydrodynamics

A = static bed, B = bed at minimum

fluidization conditions, C = fluidized

bed

uA < uB < uc

h0(A) < h(B) < h(C)

∆p(A) < ∆p(B) = ∆p(C)


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Pressure drop increases with gas velocity in

the static bed; is constant in the fluidized bed

and decreases as the particles are entrained

from the bed.

Bed height is constant for the static bed and

increases in the fluidized bed to the column

height h’.

Bed voidage is constant for the static bed and

increases to 1 in the fluidized bed.

Particle concentration is constant for the

static bed and decreases to zero in the

fluidized bed.

Packed (static) bed Fluidized bed


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Onset of fluidization for a mixture

A = Fine (light) particles start to fluidize

B = Minimum fluidization velocity

determined experimentally for a mixture

C = All bed material fluidizes

Mixing vs. segregation for a binary mixture of particles in a fluidized bed

5. Special fluidized beds

Vibro-fluidized beds

Used for materials which are difficult to fluidize otherwise (namely materials “C”).

Mechanical vibrations imposed on fluidized bed units.

Vibrations either vertical (mostly) or horizontal.


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Support for the vibrated fluidized bed unit with motors.

Vibro-fluidized bed unit


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Spouted beds

A distributor with a single aperture. The material starts moving even

before reaching its minimum fluidization velocity. Suitable for

operations with materials which are difficult to fluidize otherwise.

Intensive particle circulation.

6. Fluidization regimes and transitions

A ... Static bed. Gas velocity is below minimum fluidization velocity.

B ... Particulate regime. Occurs between minimum fluidization and minimum bubbling

velocities for Geldart group A materials. Fluidized bed without bubbles.

C ... Bubbling fluidized bed. Distinct bubbles in the bed. Occurs at velocities higher than

minimum bubbling velocity for Geldart group A materials; and higher than minimum

fluidization velocity for Geldart group B and D materials.

D ... Turbulent bed. No distinct bubbles. Highly turbulent mixing of solids and gas. Large

particle entrainment.

E ... Slugging bed. Occurs in small diameter column where a bubble diameter reaches the

column diameter. High pressure fluctuations, poor particle-solids contact. Unwanted regime.


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Development of slugs and their different types. a) Axisymmetric slug, b) asymmetric slug, c)

“plug” slug.

Correlations for bubble and slug velocity:

F ... Channelling in the bed. Typical for Geldart group C materials. Poor particle-solids

contact. Unwanted regime


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Transitions between regimes:

7. Use of fluidized bed technology (details later in the course):

a. Fluidized bed (catalytic) reactors

b. Ore roasting

c. Fluidized bed combustion (for power generation)

d. Drying of materials which are granular, free flowing, not agglomerating, not

fragile, not vulnerable to oxidation, neither the product nor the vapour released

is toxic or flammable

Fluidized bed dryer

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