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 1µ m to 6cm. It is generally concluded that particles distributed in sizes between 150µ m and 10µ m 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 20µ m) 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 ε
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, ionexchange 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: 1.75 ρ g U (1 − ε ) − ∆P 150 µU (1 − ε ) + = 3 d pε d pε 3 H 2
2
Where: dp =
Size of particles
H = Height of bed µ = Viscosity of air
(µ m) (m) (N/m2s)
U = Average superficial velocity (m/s)
ε=
Bed voidage
ρg = Density of air
(-) (kg/m3)
∆P = Pressure drop across the bed (N/m2) Re =
average Reynold’s number based on superficial velocity which is
dimensionless.
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∴
Re =
ρg Ud p µ(1 − ε )
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: U =
Q A
Where A = Bed cross sectional area
Theoretically, at incipient fluidization; ∆P =
0.1M A
Where ∆P is in cm water gauge.
Pressure drop, ∆ P
∆P =
Superficial gas velocity, U
0.1M A
Umf The pressure drop at fluidization can also be predicted by using the equation: ∆P = H (1 − ε ) ( ρ g − ρ g ) g
Where ρg g
=
density of the particle
=
gravitational force
b) Minimum Fluidization velocity, Umf For particles, dpi > 100 µ m, Wen and Yu (1966) can be used to find Umf Ar = 1060Remf + 159
[(
1.687 mf
Or
Re mf = 33 .7 1 + 3.59 x10 −5 Ar
)
0.5
]
−1
For spheres ranging from 0.01 < Remf < 1000
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ρ g ( ρ p − ρ g ) gd sv3
Where
Ar =
And
Re mf =
µ2 ρg U mf d sv µ
For particles, dpi < 100µ m, Baeyens and Geldart (1977) can be used;
U mf =
(ρ
p
− ρg )
0.934
.g 0.934 .d p
1110.µ 0.87 .ρ g
1.8
0.066
For angular (quartz) shape of particles, from Abrahamsen and Geldart (1984), d v =1.13 d p
PROCEDURE 6
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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Column
manometer
Fluidize bed apparatus
RESULTS Experiment 1: Coarse grade sand Flow rate,
Superficial gas
Bed pressure
Bed expansion,
Bed state
Q (l/min)
velocity, U
drop, ∆ P
Ht (mm)
(mm)
1.0 2.0 3.0 4.0 5.0 6.0
(cm/s) 0.00000 1.20286 1.80429 2.40572 3.00716 3.60859
(mm water) 0 106 152 173 184 187
150 150 150 154 155 160
150 150 150 150 150 150
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Bed pressure drop,AP (mm water)
Bed pressure drop against superficial gas velocity 200 180 160 140 120 100 80 60 40 20 0 0.000
1.000
2.000
3.000
4.000
superficial gas velocity, U (cm/s)
Bed expansion against superficial gas velocity Bed expansion (mm)
162 160 158 156 154 152 150 148 0.000
1.000
2.000
3.000
4.000
superficial gas velocity, U (cm/s)
Experiment 2: Ballotini Flow rate,
Superficial gas
Bed pressure
Bed expansion,
Bed state
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Q (l/min)
velocity, U
drop, ∆ P
Ht (mm)
(mm)
1.0 2.0 3.0 4.0 5.0 6.0 7.0 8.0 9.0 10.0
(cm/s) 0.00000 1.20286 1.80429 2.40572 3.00716 3.60859 4.21002 4.81145 5.41288 6.01431
(mm water) 0 340 520 720 880 112 137 158 171 191
150 150 150 150 150 150 150 150 150 150
150 150 150 150 150 150 150 150 150 150
Bed pressure drop, AP (mm water)
Bed pressure drop against superficial gas velocity 250 200 150 100 50 0 0.000
1.000
2.000
3.000
4.000
5.000
6.000
7.000
superficial gas velocity, U (cm/s)
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Bed expansion (mm)
Bed expansion against superficial gas velocity 160 140 120 100 80 60 40 20 0 0.000
1.000
2.000
3.000
4.000
5.000
6.000
7.000
superficial gas velocity,U (cm/s)
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 Πd 2 4 Π(59 .4 E −3 ) 2 Area = 4 Area = 2.771 E −3 m 2 Area =
Q A 3.3333 E −5 U = 2.771 E −3 U = 0.01203 m / s U =
Calculation of minimum fluidization velocity, Umf : ρ p: 2677.9 kg/m3 dp: 287.5E-6m Bed diameter: 59.4mm ρ g: 1.2 kg/m3 µ 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 329ml 1000g
x 1000 ml x 1000 L 1L 1 m3
= 1455.26 kg/m3 Because of dpi is > 100µ m, Wen & Yu (1966) can be used to find Umf 12
ρg ( ρ p − ρ g ) gdp 3 Ar = µ2 Ar =
1.2( 2677 .9 −1.2 )( 9.81) ( 287 .5 E −6 )
(1.8E )
−5 2
Ar = 2311 .099
[ = 33 .7[(1 + 3.59 E
Re mf = 33 .7 (1 + 3.59 E −5 Ar ) Re mf Re mf = 1.3702
Re mf = 1.3702 = U mf
( 1- εmf )( ρp – ρf )g
3
0 .5
]
−1
× 2311 .099 )
−5
0 .5
]
−1
ρg U mf dp µ
(1.2)U mf
(287 .5E ) −6
1.8 E −5 = 0.0715 m / s
= 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-εmf )² + 1.75(1.2)(0.0715)2(1–εmf) (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 13
Πd 2 4 Π(59 .4 E −3 ) 2 Area = 4 Area = 2.771 E −3 m 2 Area =
Q A 3.3333 E −5 U = 2.771 E −3 U = 0.01203 m / s U =
Calculation of minimum fluidization velocity, Umf : ρ p = 2945.6 kg/m3 dp = 267.5E-6m Bed diameter: 59.4mm ρ g: 1.2 kg/m3 µ 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 320ml 1000g
x 1000 ml x 1000 L 1L 1 m3
= 1803.75 kg/m3
Because of dpi is > 100µ m, Wen & Yu (1966) can be used to find Umf
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ρg ( ρ p − ρg ) gdp 3 Ar = µ2 Ar =
1.2( 2945 .6 −1.2 )( 9.81) ( 267 .5 E −6 )
(1.8E )
−5 2
Ar = 2047 .73
[ = 33 .7[(1 + 3.59 E
Re mf = 33 .7 (1 + 3.59 E −5 Ar ) Re mf Re mf = 1.2167
Re mf = 1.2167 = U mf
( 1- εmf )( ρp – ρf )g
3
−5
0.5
]
−1
× 2047 .73 )
0.5
]
−1
ρg U mf dp µ
(1.2)U mf
(267 .5E ) −6
1.8 E −5 = 0.0682 m / s
= 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-εmf )² + 1.75(1.2)(0.0682)2(1–εmf) (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.
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Other than that, it is very difficult to fluidize because the inter particle forces is higher 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, ε while ε
3.
mf
mf
for coarse grade sand is 0.321387
for Ballotini is 0.383097.
The bed expansion and the pressure drop of the particle are proportional to the superficial velocity of the gas supply.
RECOMMENDATIONS
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∗
Students should be able to utilize appropriate conversion factors to ensure
consistency of units when making calculations. ∗
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
19
20
21
