fluidization 1

8/12/2019 FLUIDIZATION 1

1/23

Fluidization has been a key technology in Fluid Catalytic

Cracking (FCC) to make gasoline in petroleum

industry in catalytic processes such as partial oxidation of

ammonia to acrylonitrile to prepare acrylic resin

in gas phase polymerization processes ofpolyethylene and polypropylene

in chlorination process of metals such as siliconfor purification in the semiconductor industry

in granulation process for the pharmaceuticalindustry

in fluidized bed combustion (FBC) of solid fuels(coal,waste and biomass) to generate steam forboilers

in waste incineration of solid and sludge

drying, dip powder coating , thermal treatment of

metals by hot or cold sand


2/23

Fluidized bed dryer

Temperature Sensor

Pressure Inlet Sensor

Pressure Outlet Sensor

Air Inlet


3/23

FLUIDIZATION A fluidized bed is formed

by passing a fluid usually a

gas upwards through abed of particles supportedon a distributor.

As a fluid is passedupward through a bed ofparticles, pressure lossdue to frictional resistanceincreases as fluid flowincreases.

At a point, upward dragforce exerted by the fluidon the particle equal toapparent weight ofparticles in the bed.

W

F F F = drag force

W = apparent

weight


4/23

FLUIDIZED BED
http://www.google.com/url?sa=i&rct=j&q=&esrc=s&frm=1&source=images&cd=&cad=rja&docid=q2Kyb4gX8f9h5M&tbnid=8GHItsXiYM6c-M:&ved=0CAUQjRw&url=http://www.umich.edu/~elements/12chap/html/12prof2b.htm&ei=Lj93UraWF8aOrgfUpYC4Cw&psig=AFQjCNF6B3oytTaeO1AYKDXNHNJaJeViyg&ust=1383632645400577http://www.google.com/url?sa=i&rct=j&q=&esrc=s&frm=1&source=images&cd=&cad=rja&docid=xa8SXdm_paSq6M&tbnid=4kiIHabcD0PCyM:&ved=0CAUQjRw&url=http://commons.wikimedia.org/wiki/File:Fluidized_Bed_Reactor_Graphic.JPG&ei=5z53UuWAGIGFrge0oIDwAg&psig=AFQjCNF6B3oytTaeO1AYKDXNHNJaJeViyg&ust=1383632645400577


5/23

Characteristics of Gas Fluidized Bed

Bed behaves like liquid of the same bulk

density can add or remove particles,

pressure-depth relationship, wave motion,

heavy objects sink, and light ones float.

Rapid particle motion good solid mixing

Very large surface area available 1m3of

100 m particles has a surface area ofabout 30,000 m2, and 1 m3of 50 m

particles 60,000 m2.


6/23

Characteristics of Gas Fluidized Bed

Good heat transfer from surface to bed,

and gas to particles.

Isothermal conditions radially and axially.

Pressure drop through bed depends only

on bed depth and particle density does

not increase with gas velocity.


7/23

Advantages of Fluidized Bed

High mobility Gives superb heat transfer, which usually always a problem

to powders.

Heavily used for drying eg: pharmaceutical industry.

Excellent reactors

Good temperature control A perfect gas/liquid mixing equipment.

Very flexible Can carry out many processes in a single vessel.

Mix, dry, granule, separate etc. in one vessel.

Less number of moving parts Easy to handle


8/23

DISADVANTAGES

Costly Blowing air into the system.

Trap air to make it fluidized.

Cleaning process

Some powders

costly in operation than others.

Not all particles fluidized Cohesive and large particles are difficult to

fluidize.

Difficult distributor design

Maldistribution of fluidizing gas

P across distributor = 30% of bed P.


9/23

PRESSURE DROP

The force balance;

Pressure drop=

Weight of particles - up thrust on particle

Bed cross - sectional area

For a bed of particle density, p, fluidized by a fluid

with fto form a bed of depth, Hand voidage, in a

vessel of cross sectional area,A;

A

gHAP

fp

1 gHP fp 1


10/23

PRESSURE DROP

For a flow of fluid through a packedbed, two distinct types of flow

involved. They are laminar and

turbulent flow. The pressure drop across a fluidized

bed is the only parameter which can

be accurately predicted:PFcm w.g.A

MgPF

AM1.0

where M in kg and A in m2.


11/23

mfis the bed voidage at Umfand a

close approximation to it can beobtained by measuring the aerated ormost loosely packed bulk density, bLP.

Equations are used to predict thetheoretical pressure drop comparing toexperimental one.

gH

Pgpmf

F 1


12/23

Laminar flowThrough the work of

Darcy and Poiseuille, ithas been known formore than 120 yearsthat the averagevelocity through a

packed bed, orthrough a pipe, isproportional to thepressure gradient.

Pressure gradient fluid velocity

Based on Carmen-Kozeny (1927, 1933and 1937),

U

H

P

32

21180

pd

U

H

P

Carmen-

Kozeny equationfor laminar flow.


13/23

TURBULENT FLOW

Burke Plumme equation for turbulentflow through a randomly packed bed

of monosized spheres of diameter, dp.

3

2 175.1

p

g

d

U

H

P


14/23

Laminar and turbulent flow

Based on experimental data covering awide range of size and shape of

particles, Ergun (1952) suggested the

following general equation for any flowconditions;

Ergun equation

3

2

32

2 175.11150

p

g

p d

U

d

U

H

P

Laminar

component

Turbulent

component


15/23

Reynold number,

For Re* < 10, laminar flowFor Re* > 2000, turbulent flow

Ergun also expressed flow through a

packed bed in terms of friction factor;

Friction factor,

Compare this friction factor with

Fanning friction factor.Then it

becomes

1Re

* Ud gp

1*

3

2U

d

H

Pf

g

p

75.1Re*

150* f


16/23

with for Re* < 10

and for Re* > 2000

For non-spherical particles; dpis

replaced by dsv,then,

Re*

150* f

75.1* f

3

2

32

2 175.11150

sv

g

sv d

U

d

U

H

P

The surface/volume size, dsv is used: if only sieve sizes are

available, depending on the particle shape, an approximation

can be used for non-spherical particles;

psv dd 87.0 pv dd 13.1Recalling,

where dpis the mean sieve size.

Minim m Fl idi ation Velocit U


17/23

Minimum Fluidization Velocity, Umf

.

A plot of pressure drop across the bed vs. fluid velocity

Line OA packed bed region. Solid

particles do not move relative to one anotherand their separation is constant.

Region BC: fluidized bed region

Point A: Phigher than predicted value n.

ABed pressure

drop, p

Gas velocity, U

B

O

C

Umf

ppp


18/23

This is due to powders, which have beencompacted to some extent before thefluidization process takes place.

Higher Pis associated with the extraforce required to overcome inter particleattractive forces.

Minimum fluidization velocity, Umf:superficial fluid velocity at packed bedbecomes a fluidized bed (as marked ongraph).

Also known as incipient fluidizationvelocity.


19/23

Umfincreases with particle size and particle

density and affected by fluid properties.

Recalling Ergun (1952) for any flowcondition;

(1)

and (2)

substituting (1) into (2),

Rearrange:

3

2

32

2 175.11150

sv

g

sv d

U

d

U

H

P

gHP fp 1

3

2

32

2175.11150

1

sv

mfg

sv

mf

fpd

U

d

Ug

2

222

3

2

3

3

2

3

2

..175.1

..1150

1

fsvmf

svf

fsvmf

svf

fp

dU

d

dU

dg


20/23

2,3

,3

2

2

3

.175.1

.1150

1

mfe

mfe

svf

fp

R

Rd

g

2,3,3

2

.175.1

.1150

mfemfe RRAr

2

3

svfpf gdAr

svmff dURe

or

where,

Archimedes number

- Reynolds number


21/23

Wen and Yu (1966) correlation

for Umf.

for spheres ranging 0.01 < Re,mf< 1000

used for particles larger than 100 m

use dvinstead of dsvfor Wen and Yu

Please check the Wen & Yu correlation in determining Umffrom Data Booklet.

687.1

,, 1591060 mfemfe RRAr

11059.317.33 5.05, ArR mfe

or

B d G ld t


22/23

Baeyens and Geldart

for particles, dp< 100 m;

066.0

f

87.0

f

8.1

p

934.0934.0

fp

mf1110

dg

U


23/23

Assignment

A bed of angular sand of mean sieve size778 m is fluidized by air. The particledensity is 2540 kg/m3, g(air) = 18.4 10-6kg/ms, g= 1.2 kg/m

3and 24.75 kg

of the sand are charged to the bed 0.216m in diameter. The bed height atincipient fluidization is 0.447 m. Find;

mf

The pressure drop across the bubblingbed in cm water gauge. The incipient fluidization velocity, Umf.

fluidization 1

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