Fluid is at Ion

8/2/2019 Fluid is at Ion

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FLUIDISATIONMechanism of Fluidization

Low velocities: Packed Bed Region. No movement of particles.

Possible equations for Pressure Drop: CK, Generalized, Ergun.

Entrainment

Gas Superficial Velocity u (v gas would have in tube if there was no bed)


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Graph 1: As fluctuation occurs, the Pressure drop levels off. Bubbles then begin to form in bed

centre and then pressure begins to drop.

As velocity increases, slight movement of particles is seen (especially at walls), rate ofrise ofP begins to drop off towards B. Above B, small bubbles begin to form and

burst at the top of the bed. P moves to be a constant value with increasing u (i.e. at C).

Increasing u further increases bubble size and the number of bubbles, and the bedresembles a boiling liquid. (Because of increased friction between gas and particles).

There will be a slight increase in P as u increases. When at E, it is possible for

particles to be carried away at high u. [Choose u in between C and E when

designing fluidized bed.]


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In decreasing the u, there is the HYSTERIS effect. As bed u decreases, bed dropsback into packed bed and may fall into a configuration of higher porosity i.e. higher. (It

falls into looser packing and P decreases). Bed height may be slightly higher.

Minimum Fluidizing Curve

Hump in curve when increasing u, is only for the larger particles. As the gas flows up, ittends to push the particles together to form a bridge. Bubbles form under bridge with

Minimum Fluidizing Velocity

Normal Operating Velocity = 1.5 2.5 Uo

Uo

C


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pressure build up. Eventually particles pushed up a side releasing the pressure (hence the

DROP).

Types of Fluidization

a) Aggregative Fluidization (See handout #5 overside)Normally: Gas Fluidized Beds

Bubbles characterize the bed operation. In A.F. the particles operate / move in a group /

bulk.

b) Particulate FluidizationNormally the fluid is a liquid. There are no bubbles. The particles tend to move of their

own nature, spacing themselves evenly through the bed. Increasing the velocity simply

pushes the particles apart: VOIDAGE INCREASES. Particles operate individually.

Aggregative: Gas Fluidized Bed normally, however Particulate fluidization has beenobserved. Lead shot fluidized by water.

Particulate: Liquid Fluidized normally. However light resin particles fluidized by CO2has been shown to exhibit PARTICULATE FLUIDIZATION.

Observation from this: difference in density between particles and fluid is the key issue:

High difference: fluidize in aggregative. Low difference: particulate.

Determination ofP and Uo at Incipient Fluidization

Uo

P P Fluidization

Pressure force must be related to the weight of

particles in fluidization region.

o = Porosity of bed at initial fluidization.

01

sPA g AL

Volume of particles

in bed

Incipient fluidization

conditions


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To calculate u:

At the transition point between packed and fluidized bed i.e. uo, the pressure drop is the

same, but only at that point.

In packed region CK equation:

Put (2.20) and (2.27) together to eliminate P:

For higher Re use Gen / Ergun, easier to use.

L

A

01sP g L Pressure drop at fluidization.

2

2

3 2

11 8 0

s

U LP

D

(2.20)

2 3

00 2

0180 1

ss

Du g

(2.28)

2

2

0 3 2

11 180

s

s

ULg L

D

Equation for Minimum

Fluidizing Velocity


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Behavior of Freely Bubbling Beds

How to get data on bubble sizes and bubble size development.a) Assessment of bubble eruption diameter (difficult).b) Wall observations on circular beds (difficult).c) Observations of fluidization in 2 dimensional beds. Problem is influence of walls.

d) Circular bed: X ray, rays.No wall influence.

Measurement of bubble sizes has given rise to empirical equation:

Db = Bubble diameter.

Ds = Particle diameter.

s = Solid density.

L = Depth of bed.

Results from many investigations incorporated.

Rectangular Section.

Thickness of section < Normal size of bubbles.

As bubbles comes up, you will see voids.

X rays

Collect emergent beam using

photographic techniques.

0

1.4 2.2b s su

D D Lu


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Aspects of gas fluidized beds

Channeling and spouted beds

Channeling is the phenomenon observed when a disproportionately large amount of thefluidizing fluid follows one or two particular parts through the bed. This is often a marked

characteristic of a bed of very fine particles, or of sticky or waxy particles which tend toagglomerate.

A spouted bed may be seen as an extreme form channeling in which the fluidizing fluid takes

one (usually centralized) path up the bed.

Distributor plates

The general functions of the distributor plates are1. Hold up the bed2. Distribute the gas evenly under the bed3. To create small bubbles (the smaller the bubbles the more efficient the transfer process

becomes)

Need to form small bubbles to start with, since we know they will increase in size.Aim: as small bubbles as possible to improve the transfer processes (i.e. heat, mass,

catalytic processes). Large bubbles are voids (hard to get transfer activity in voids).


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Distributor design

Choice of distributor Experiments by Grohse and other experiments by Zenz and Othmerhaveshown that the quality of bubbling fluidization is strongly influenced by the type of gasdistributor.

For a few air inlets the bed density fluctuates appreciably at all flow rates of 20% - 50% aroundthe mean value, Though more severely at high flow rates. The bed density varies with height and

gas channeling may be severe. For many inlet air openings the fluctuations in bed density is

negligible at low gas flow rates but again becomes appreciable at high gas flow rates. Beddensity is more uniform through out and , bubbles are smaller and gas solid contacting is more

intimate with less channeling of gas.

(Grohse 1955) found that the point of incipient fluidization was more reproducible with a porous

plate distributor than if the bed was supported by a BS 300 mesh screen or a multi- orifice plate.Generally the most appropriate type of plate is the perforated plate

A sintered plate tends to becomes clogged with minute particles, it is also not very easily cleaned

The perforated plate is a flat plate with holes drilled in it, it is cheap and works well withparticles larger than the size of the holes


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If there is a problem with particles passing through the holes alternative devices have to be used

to prevent flow back. Other possible methods of promoting even fluidization is to use bubblecap, conical or porous plates. The use of baffles may also be an option to improve even

fluidization although information on its use is sparse.


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Twophasetheoryoffluidization

This theory postulates that the bed is made up of (2) distinct phases

1.

Dense phase contains the particles2. Bubble phase contains no particlesIt also stipulates that the gas velocity in the dense phase is at its minimum fluidizing condition

i.e. uo and excess gas passes through as

bubbles

Average bubble size

& average velocity =

b

b

V

u

From the principle of continuity

0 bQ uA u A V (2.30)

Where

uoA represents the dense phase

Vb represents the bubble phase is the frequency at which bubbles pass up through the bed

Expansion of bed over the minimum fluidizing conditions

The bed undergoes expansion as bubbles appear. Increasing Q will expand the bed further since

bubble volume increases accordingly.

Bubble volume 0A L L

Lo

L

A

u


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0 2.31

Time for onebubbleto pass through thebed

b

bVolumetricflow

LA L L V n

u

Combine equations (2.30) and (2.31) and substituting for nVb

0 0b

LA L L A u u

u

0 0b

b

L L u u

L u

Bubble porosity (fraction of the bed as bubbles)

The value for

1

20.711b bu gD

Where

Db is the bubble diameter

However gas flow patterns in fluidized beds are more complex. The difference in behavior is as aresult of the following 2 conditions

Slow moving bubbles gas flows in the dense phases are higher than the bubble velocity Fast moving bubbles gas flows in the dense phase is less than the bubble velocity

(1) Slow moving bubble ub


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As visible flow in the form of bubbles As gas flow through bubbles

Unit cross-sectional area

0 01 2.34

b b b

Bubble BubbleDenseFlow Through

Phase FlowFlow

Au Au A u Abu

(2.34)

b is a constant which we will vary with conditions

(2) Fast moving bubbles where ub>uo

The flow of gas is downwards relative to bubble. However, gas just below the base of the bubble

sees a void. It shoots up through the bubble and as it emerges and as it comes off at the top of thebubble all of a sudden it encounters high resistance (i.e. particles). The gas gets pushed aside by

fast moving bubble eventually finding itself back at the base. It will continue to re-circulate as it

moves through the denser phase.

Application of fluidized bedsAdvantages of fluidized beds

1) Smooth liquid like flow of the particles allows for ease of handling2) Rapid mixing of particles can lead to near isothermal conditions3) Well suited to large operations4) Heat and mass transfer between particles and gas are high

Disadvantages

1) Difficult to describe fluid/particle contacting can give rise to inefficient contacting2) Friable particles can be broken up and entrained3) It is possible to get erosion of the vessel4)

There is a restricted size range of particles uo is increased the particles will be entrained.

There are a wide range of operations using fluidization

Physical operations Transportation Mixing soap powders e.g. Breeze Heat exchange Particle size enlargement/coatings (e.g. Urea) Drying

Reactions

Catalytic cracking Catalytic reforming Thermal cracking Calcination Al2O3.3H2O => Al2O3+3H2O CaCO3=>CaO +CO2

[Fluidization Engineering Kunni & Levenspiel]


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Heat Transfer in fluidized beds

Cooling of Urea and Al2O3

Assumptions

I. Perfect mixing of particles in vertical planeII.

No internal temperature gradientsIII. Uniform distribution of gas under bed

IV. No heat loss from side of vessel

A heat balance over a differential section

1s s s g go gMassflowofair

m C dt uWdl C t t

WhereW is the width of the bed

Need to find a relationship for tgo and ts

Consider heat transfer between particles and the gas in the differential section [handout 7]

A heat balance in the differential section in the vertical plane


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.

sec

g g s g

volumetric Volume flow of diff rate tion

udLW c dt ha t t dLWdz

Integrating between the limits

z = 0 z = z

tg = tgi tg = tg0

0 0

lnsi gi

s g g

t t hdz

t t uc

Rewriting this equation as

1 exp 2.36go gi s gig

haZt t t t

uc

Insert (2.36) into (2.35)

0

exp 1 exp 2.37s gi g g

s gi s s g

t t M c haZt t M c uc

Integrate between the limits of

l = 0 ts = ts0

l = l ts = tsi

For the special case of where the bed depth is large and the particle size is small and there are

high values of aZ

2.37 becomes


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exp 0g

haZ

uc

0

exps gi g g

s gi s s

t t M c

t t M c

Alternatively place tgo = ts into 2.35 and integrate directly

Fluid is at Ion

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