Lab 1 Fluidized Bed

ABSTRACT\SUMMARY

This experiment is about fluidization of a bed of solid by passing a fluid, usually a

gas upwards through a bed of particles supported on a distributor. Fluidization or

fluidizing, converts a bed of solid particles into an expanded mass that has many

properties of a liquid. As a fluid is passed upward through a bed of particles, pressure loss

due to frictional resistance increases as fluid flow increases. At a point, whereby the

upward drag force exerted by the fluid on the particle equal to apparent weight of

particles in the bed, fluidization occurs.

The size of solid particle which can be fluidized varies greatly from less than 1m

to 6cm. It is generally concluded that particles distributed in sizes between 150m and

10m are the best for smooth fluidization (least formation of large bubbles). Large

particles cause instability and result in slugging or massive surges. Small particles (less

than 20m) frequently even though dry, act as if damp, forming agglomerates or fissures

in the bed, or spouting. Adding finer sized particles to a coarse bed or coarse sized

particles to a bed of fines usually results in better fluidization.

The upward velocity of the gas is usually between 0.15m/s and 6m/s. This

velocity is based upon the flow through the empty vessel as is referred to as the

superficial velocity. As the velocity of flow increases, the particles rearrange themselves

to offer less resistance to the fluid flow and the bed will tend to expand unless it is

composed of large particles (mean diameter > 1mm). The expansion continues until a

stage is reached where the drag force exerted on the particles will be sufficient to support

the weight of the particles in the bed. The fluid/particle systems then begin to exhibit

fluid like properties and it will flow under the influence of a hydrostatic head. This is the

point of incipient fluidization and the gas velocity needed to achieve this is referred to as

the minimum fluidization velocity, Umf.

Beyond this velocity, the pressure drop across the bed will be approximately

equal to the weight of the bed per unit area. The effective P excludes the hydrostatic

pressure drop across the bed which can be neglected in gas fluidized systems operating at

atmospheric pressure. It is likely, however that this pressure drop will be exceeded just

prior to fluidization with gas fluidized systems in order to overcome cohesive forces

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between the particles and break down the residual packing and interlocking of particles

within the bed.

The behavior of fluidization is depends on the types of the particles composed in

the vessel. Geldart (1973) classified powders into four groups according to their

fluidization properties at ambient condition. There are 4 stages of particles that are (A)

aerated, (B) bubble, (C) cohesive and (D) dense. In this experiment, we are considering

with a coarse sand which is in group B, Ballotini which is in group A and Glutinous flour

which is in group C.

From this experiment, we can obtain the bed expansion, bed pressure drop and the

flow rate of the fluid. By the equation given in the theory, superficial gas velocity, Umf

and mf for all cases can be calculated. Then only, we plotted two graphs which are bed

pressure drop against superficial gas velocity and bed expansion against superficial gas

velocity for all cases. The Umf predicted from the graph then is being compared with the

calculated one.

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INTRODUCTION

The upward flow of fluid through a bed of particles is a situation encountered

both in nature, as with the natural movement of ground water, crude petroleum or natural

gas, through porous media, and in industrial operations such as backwashing filters, ion-

exchange processes, extraction of soluble components from raw materials and for certain

types of chemical reactor. It is well known that if the particles are loosely packed and the

pressure drop due to the flow through the bed is equivalent to the weight of the bed, the

phenomenon of fluidization occurs. The fluidized state occurs naturally is so-called

‘quick sand’ and industrially, use is made of the high rate of solids mixing that

accompanies fluidization for various operations such as drying, coating, heat transfer and

chemical reaction.

This equipment is designed to allow the study of the characteristics of flow

through both fixed and fluidized bed of solid particles. Although the majority of fixed

and fluidized bed situations encountered by practicing engineers are three dimensional, in

order that students can readily observe the important phenomenon of bubbling that occurs

in gas-solid systems when the gas velocity is in the excess of that required for

fluidization. The transparent walls allow studies to be made of bubble behavior in the

gas-solid system.

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OBJECTIVES

There are three objectives of doing the fluidization experiment:

To determine the pressure drop and bed expansion through a fixed and

fluidized bed.

To verify the Ergun equation (1952), Wen & Yu equation (1966) and

Baeyens & Geldart equation (1977).

To observe the onset of fluidization

THEORY

a) Pressure drop across the bed, P

In order to determine the pressure drop through a fixed bed fro any flow

condition, the Ergun equation (1952) can be used:

Where:

Size of particles (m)

Height of bed (m)

Viscosity of air (N/m2s)

Average superficial velocity (m/s)

Bed voidage (-)

Density of air (kg/m3)

Pressure drop across the bed (N/m2)

Re = average Reynold’s number based on superficial velocity which is

dimensionless.

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Re = and if Re < 10 than laminar flow

Re > 2000 turbulent flow

If the flow rate Q is measured in liters/s and U is the average superficial velocity

in m/s, then:

Where A = Bed cross sectional area

Theoretically, at incipient fluidization;

Where is in cm water gauge.

Pressure drop, P Superficial gas velocity, U

Umf

The pressure drop at fluidization can also be predicted by using the equation:

Where = density of the particle

g = gravitational force

b) Minimum Fluidization velocity, U mf

For particles, dpi > 100 m, Wen and Yu (1966) can be used to find Umf

Ar = 1060Remf +

Or

For spheres ranging from 0.01 < Remf < 1000

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Where

And

For particles, dpi < 100m, Baeyens and Geldart (1977) can be used;

For angular (quartz) shape of particles, from Abrahamsen and Geldart (1984),

PROCEDURE

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1. Firstly, identify the apparatus used for the experiment whether it is in good

condition or else it might affect the data taken from the experiment.

2. Then, the column of the fluidized bed is filled to a height of 150 mm with the

coarse grade sand.

3. Next, switch on the air pump and the control valve is adjusted to give the flow rate

of 2.0 l/min.

4. The flow rate is then increased by 1.0l/min.

5. The bed expansion, manometer reading and the state of the bed is recorded in each

of the increasing flow rate.

6. The entire experiment is repeated by using Ballotini and Glutinous flour.

7. Lastly, the volume of the particles is taken by weighing it, in order to determine the

bulk particle density.

8. All data are recorded in tables form for easier observations.

APPARATUS

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Fluidize bed apparatus

RESULTS

Experiment 1: Coarse grade sand

Flow rate,

Q (l/min)

Superficial gas

velocity, U

(cm/s)

Bed pressure

drop, P

(mm water)

Bed expansion,

Ht (mm)

Bed state

(mm)

1.0 0.00000 0 150 150

2.0 1.20286 106 150 150

3.0 1.80429 152 150 150

4.0 2.40572 173 154 150

5.0 3.00716 184 155 150

6.0 3.60859 187 160 150

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manometerColumn

Bed pressure drop against superficial gas velocity

020406080100120140160180200

0.000 1.000 2.000 3.000 4.000

superficial gas velocity, U (cm/s)

Bed

pre

ssu

re d

rop

,AP

(m

m w

ater

)

Bed expansion against superficial gas velocity

148

150

152

154

156

158

160

162

0.000 1.000 2.000 3.000 4.000

superficial gas velocity, U (cm/s)

Bed

exp

ansi

on

(m

m)

Experiment 2: Ballotini

Flow rate, Superficial gas Bed pressure Bed expansion, Bed state

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Q (l/min) velocity, U

(cm/s)

drop, P

(mm water)

Ht (mm) (mm)

1.0 0.00000 0 150 150

2.0 1.20286 340 150 150

3.0 1.80429 520 150 150

4.0 2.40572 720 150 150

5.0 3.00716 880 150 150

6.0 3.60859 112 150 150

7.0 4.21002 137 150 150

8.0 4.81145 158 150 150

9.0 5.41288 171 150 150

10.0 6.01431 191 150 150

Bed pressure drop against superficial gas velocity

0

50

100

150

200

250

0.000 1.000 2.000 3.000 4.000 5.000 6.000 7.000

superficial gas velocity, U (cm/s)

Bed

pre

ssu

re d

rop

, A

P

(mm

wat

er)

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Bed expansion against superficial gas velocity

0

2040

6080

100

120140

160

0.000 1.000 2.000 3.000 4.000 5.000 6.000 7.000

superficial gas velocity,U (cm/s)

Bed

exp

ansi

on

(m

m)

Experiment 3: Glutinous Flour

Fluidization did not occur. As the velocity of gas increases, half of the bed lifted as a

piston. Then it dropped and built a “rathole”, the gas opens channels that extend from the

gas distributor to the surface.

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CALCULATIONS

Experiment 1: Coarse grade sand

Calculation of superficial gas velocity

Calculation of minimum fluidization velocity, Umf :

p: 2677.9 kg/m3

dp: 287.5E-6m

Bed diameter: 59.4mm

g: 1.2 kg/m3

g: 1.8E -5N/m2s

Mass of particle: 478.78g

Volume of particle: 329ml

g: 9.81m/s2

Bulk particle density, ρb = weight Volume

= 478.78 g x 1 kg x 1000 ml x 1000 L 329ml 1000g 1 L 1 m3

= 1455.26 kg/m 3

Because of dpi is > 100m, Wen & Yu (1966) can be used to find Umf

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( 1- εmf )( ρp – ρf )g = 150 µ Umf ( 1 – εmf )² + 1.75 ρf Umf² ( 1 – εmf ) dsv² εmf ³ dsv εmf ³

(1-εmf)(2677.9 – 1.2)(9.81) = 150(1.8 x 10 -5 )(0.0715)(1-ε mf )² + 1.75(1.2)(0.0715) 2 (1–ε mf) (287.5 x 10-6)2 εmf ³ 287.5 x 10-6 εmf ³

26258.427 = 2335.577 (1-εmf ) + 37.342 εmf ³ εmf ³

26258.427 εmf ³ = 2335.577 – 4671.154 εmf + 37.342

26258.427 εmf ³ + 4671.154 εmf = 2372.919

By trial and error, εmf = 0.321387

Experiment 2: Finer sand

Calculation of superficial gas velocity

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Calculation of minimum fluidization velocity, Umf :

p = 2945.6 kg/m3

dp = 267.5E-6m

Bed diameter: 59.4mm

g: 1.2 kg/m3

g: 1.8E -5 N/m2s

Mass of particle: 577.20g

Volume of particle: 320ml

g: 9.81m/s2

Bulk particle density, ρb = weight Volume

= 577.20 g x 1 kg x 1000 ml x 1000 L 320ml 1000g 1 L 1 m3

= 1803.75 kg/m 3

Because of dpi is > 100m, Wen & Yu (1966) can be used to find Umf

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( 1- εmf )( ρp – ρf )g = 150 µ Umf ( 1 – εmf )² + 1.75 ρf Umf² ( 1 – εmf ) dsv² εmf ³ dsv εmf ³

(1-εmf)(2945.6 – 1.2)(9.81) = 150(1.8 x 10 -5 )(0.0682)(1-ε mf )² + 1.75(1.2)(0.0682) 2 (1–ε mf) (267.5 x 10-6)2 εmf ³ 267.5 x 10-6 εmf ³

28884.564 = 2573.360 (1-εmf ) + 36.514 εmf ³ εmf ³

28884.564 εmf ³ = 2573.360 – 2573.360 εmf + 36.514

28884.564 εmf ³ + 2573.360 εmf = 2609.87

By trial and error, εmf = 0.383097

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DISCUSSIONS

Fluidization is a process when a fluid is passed upward trough a bed of particles

the pressure loss in the fluid due to the frictional resistance with increases with increasing

fluid flow. A point is reached when the upward drag force exerted by the fluid on the

particles is equal to the apparent weight of particles in the bed. At this point the particles

are lifted by the fluid, the separation of the particle increases, and the bed become

fluidized. The superficial fluid velocity at which the packed bed becomes a fluidized bed

is known as the minimum fluidization velocity. This velocity increases with particle size

and particle density and is affected by fluid properties.

For the first types of particles which is coarse grade sand, the graph shows a little

increasement in the pressure drop when the superficial velocity gas also increase. The

fluidization starts when it reaches minimum fluidization velocity which is about

0.0715m/s.

The second type is finer sand or ballotini, from the graph we can saw that the

pressure drop also increase as the superficial gas velocity increased. For this case, the

minimum fluidization velocity is 0.0682m/s. For this two types of particles, bubbles

continue to grow, never achieving a maximum size.

Lastly is the glutinous flour, fluidization did not occur in this case. The bed not

expanding and resist aeration. This is because the flour is cohesive and the structure is so

strong upon fluidization. Beside that, it also because the interaction force between the

particles is strong if compared to the hydronamic force by the fluiding gas.

For Glutinous Flour, fluidization did not occur because group C particles exhibit

cohesive tendencies. The structures are so strong which upon fluidization, cracks and rat

hole is form and at a given pressure different, the bed not expanding and resist aeration.

Other than that, it is very difficult to fluidize because the inter particle forces is higher

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than hydrodynamic forces exerted on the particles by the fluidizing gas. However, group

C fluidization can be improved by mechanical help such as include a vibrator or a mixer.

Lastly, for bed voidage at minimum fluidization velocity, εmf for sand is 0.321387

while εmf for Ballotini is 0.383097. This shows that bed voidage for Ballotini is higher and

makes it more porous than sand.

CONCLUSIONS

1. The minimum fluidizing velocity, Umf for coarse grade sand is 0.0715m/s while Umf

for Ballotini is 0.0682m/s.

2. The voidage at minimum fluidizing velocity, mf for coarse grade sand is 0.321387

while mf for Ballotini is 0.383097.

3. The bed expansion and the pressure drop of the particle are proportional to the

superficial velocity of the gas supply.

RECOMMENDATIONS

Students should be able to utilize appropriate conversion factors to ensure consistency

of units when making calculations.

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Students should read and have a brief idea of what is going on in the experiment by

reading the lab manual or other reference book for better understanding.

Make sure that the apparatus is in good condition for better operation.

REFERENCES

1. Fluidized beds Combustion and Applications.

Edited by J.R.Howard,

Department of Mechanical Engineering, the University of Aston in Birmingham,

United Kingdom.

Applied Science Publishers London and New York.

2. Perry’s Chemical Engineers’ Handbook, Seventh Edition

Edited by Robert H. Perry and Don W. Green,

McGraw-Hill International Edition,

Chemical Engineering Series.

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APPENDICES

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