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7/29/2019 Yyzzzz Fluidization - J.R. Van Ommen & N. Ellis - 2010
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JMBC/OSPT course Particle Technology 2010
J. Ruud van Ommen* & Naoko Ellis**
*Delft University of Technology, the Netherlands
**University of British Columbia, Canada
Fluidization
Intro gas-solids fluidized bed
Fluidized bed:particlessuspended inan upwardgas stream;they move
drag force
equals
gravitational
force
Packed bed:particles are
stagnant
drag force
smaller than
gravitational
force
dense phase or
emulsion phase
gas bubble
particleinterstitialgas
gas
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Significance of Fluidized bedsAdvanced materials Chemical and Petrochemical
Combustion/pyrolysis
Physical operations
Silicon production forsemiconductor and solarindustryCoated nanoparticlesNano carbon tubes
Cracking of hydrocarbonsGas phase polymericreactions
Combustion/gasification of coalPyrolysis of wood waste
Chemical looping comubstion
Coating of metal andglass objectsDrying of solidsRoasting of foodClassify particles
http://www.chemsoc.org/timeline/pages/1961.html
http://physicsweb.org/article/world/11/1/9
www.unb.ca/che/che5134/ fluidization.html
http://www.niroinc.com/html/drying/fdfluidtype.html
http://www.dynamotive.com/biooil/technology.html
Pharmaceutical
Coating of pillsGranulationProduction of plantand animal cells
Gas-Solid Fluidized Bed
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Characteristics of Gas Fluidized Beds
Primary Characteristics:
Bed behaves like liquid of the same bulk density can add orremove particles.
Rapid particle motion good solids mixing.
Very large surface area available
What is the surface area of 1 m3 of 100 m particles?
1 m3 of 100 m particles has a surface area of about 30,000 m2.
Secondary Characteristics:
Good heat transfer from surface to bed, and gas to particles
Isothermal conditions radially and axially. Pressure drop depends only on bed depth and particle density
does not increase with gas velocity
Particle motion usually streamlines erosion and attrition
(Dis)advantages of fluid beds
Advantages
good G-S masstransfer in densephase
good heat transfer
easy solids handling
low pressure drop
Disadvantages
bypass of gas in bubbles
broad RTD gas and solids
erosion of internals
attrition of solids
difficult scale-up
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Basic Components
Yang W. Bubbling fluidized beds (Chapter 3). In: Handbook of Fluidization and Fluid-Particle Systems. Yang W(Ed.). MarcelDekker, Inc., NY, NY, USA, 53113 (2003).
1.56 m Diameter Column
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Industrial Scale
Solid offtake
A Fluid Catalytic Cracking Unit. Photo courtesyof Grace Davison.
Approaches to the Study of Particulate Systems
Totally empirical (leading to dimensional correlations)
Empirical guided by scientific principles (e.g. Buckingham PiTheorem to obtain dimensionally consistent correlations)
Semi-empirical, i.e. some mechanistic basis, but with one ormore empirical constants
Mechanistic physical model without any empiricism,numerical solutions of governing equations of motion andtransport
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Geldarts powder classification
hardly used
cat. reactions
drying/PE production
combustion/gasification
Geldarts powder classification
CCCohesive
0-30 m
flour
AAAeratable
30-100 m
milk powder
BBBubbling
100-1000 msand
DDSpoutable
>1000 mcoffee beans
Drag
Gravity
Attraction
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Group CCohesiveDifficult to fluidized, and channeling occursInterparticle forces greatly affect the fluidization behaviour of thesepowdersMechanical powder compaction, prior to fluidization, greatly affectedthe fluidization behaviour of the powder, even after the powder hadbeen fully fluidized for a whileSaturating the fluidization air with humidity reduced the formation ofagglomerates and greatly improved the fluidization quality. The watermolecules adsorbed on the particle surface presumably reduced thevan der Waals forces.dp ~ 0-30 mExample: flour, cement
Group AAeratableCharacterized by a small dp and small pUmb is significantly larger than UmfLarge bed expansion before bubbling startsGross circulation of powder even if only a few bubbles are presentLarge gas backmixing in the emulsion phase
Rate at which gas is exchanged between the bubbles and the emulsion ishighBubble size reduced by either using a wider particle size distribution orreducing the average particle diameterThere is a maximum bubble sizedp ~ 30-100 mExamples: FCC, milk flour
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Group BBubblingUmb and Umfare almost identicalSolids recirculation rates are smallerLess gas backmixing in the emulsion phaseRate at which gas is exchanged between bubbles and emulsion is smaller
Bubbles size is almost independent of the mean particle diameter andthe width of the particle size distributionNo observable maximum bubble sizedp ~ 100-1000 mExample: sand
Group D
SpoutableEither very large or very dense particlesBubbles coalesce rapidly and flow to large sizeBubbles rise more slowly than the rest of the gas percolating throughthe emulsionDense phase has a low voidage
dp ~ >1000 mmExamples: Coffee beans, wheat, lead shot
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Influence of particle size distribution
wide size distribution
narrow size distribution
0 4 8Dimensionless kinetic rate constant [-]
1.0
0.5
0.0
Conversion[-]
0 50 100 150particle diameter [micron]
20
15
10
5
0
20
15
10
5
0
mass%
mass%
Adapted from Sun & Grace (1990)
Implication of Umb/Umf
Umb/Umfcould be used as an important index forthe fluidization performance of fine particlefluidized beds on local hydrodynamics
Geldart particle classification:
Group A powders
with Umb/Umf>1
Group B powders
with Umb/Umf=1
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Demarcation
1U
U
mf
mb
Demarcation between Group A and B powders
pmb dKU =
For air at room T and P, K = 100 (Yang, W.-C., 2003)
( ) 1K
dg108 pp4
Demarcation between Group B and D powders
For Group D powdersmf
mfB UU
Pressure and temperature effect: (Grace, 1986)
( )425.0
p
AB
*p 101d
=
If : powder belongs to Group A or CIf : powder belongs to Group B or D
) )AB
*p
*p dd
Demarcation
1U
U
mf
mb
Demarcation between Group A and B powders
Demarcation between Group B and D powders
For Group D powdersmf
mfB
UU
Demarcation between Group C and A powders (Molerus, 1982)
310( ) / 0.01p f p H
d g F =
FH is the adhesion force determined experimentally.
(FH=8.76x10-8 N for glass beads and FCC catalysts)
10 100 1000100
1000
10000
C(cohesive)
A(aeratable)
D
(spoutable)B(bubble readily)
s-f,
(kg/m
3)
Mean particle diameter, dp
(m)
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Characteristics of single bubble: Slow vs. fast bubbles(Davidson model)
fast bubble
slow bubble
group A & Bgroup D
Simple demarcation criteria accounting for T, Peffects
Goosens classification:
C/A boundary: Ar=9.8 A/B boundary:
Ar=88.5
B/D boundary:Ar=176,900
Graces classification:
A/B boundary: Ar=125
B/D boundary:Ar=145,000
( )23
pgpg dgAr =
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Correlations for Umb
Abrahamsen and Geldart(1980)
Where F45 is the fraction of solids which are less than 45m.
( )( ) 9340934080
45
52301260 71602300.
gp
..
p
.
g
.
g
mf
mb
gd
F.exp
U
U
=
Flow Regimes
gas gas gas
solids
returns
solids
returns
solids
returns
gas
gasgas
only A powdersat low gas velocity only narrow beds
gas
fixed bed
homogeneous
bubbling
slugging
turbulent fastfluidization
pneumatictransport
gas velocity
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Flow RegimesU range Regime Appearance and Principal Features
Fixed Bed Particles are quiescent; gas flows through interstices
Particulateregime
Bed expands smoothly in a homogeneous manner;top surface well defined, small-scale particle motion
Bubblingregime Gas voids form near distributor, coalesce and grow;rise to surface and break through
Slug flowregime
Bubble size approaches column cross-section. Topsurface rises and collapses with regular frequency.
Turbulentfluidizationflow regime
Pressure fluctuations gradually decrease untilturbulent fluidization flow regime is reached.
(TurbulentRegime)
Small gas voids and particle clusters dart to and fro.Top surface is difficult to distinguish.
FastFluidization
No upper surface to bed, particles transported outthe top in clusters and must be replaced.
Pneumaticconveying
No bed. All particles fed in are transported out thetop as a lean phase.
mfUU0
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Unifying Fluidization Regime Diagram
Fluidized bed lay-out
twin bed
bubblingbed
riser
turbulentbed
circulatingbed
downer
laterally staged bed
vertically
staged bed
spouted bed
floating bed
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Static Head of Solids
DP
L
When acceleration, friction and gas head are negligible
areasectionalcrossbed
particlesonupthrustparticlesofweightP
=
( )( )g1LP p =
Time-averaged pressure measurementsMost industrial and pilot plant fluidized beds have pressure taps.There should be at least 2 or 3 taps within the fluidized bed.Pressure measurement from plenum chamber must be from where itwill not be affected by either the gas expansion or the contractionFor hot units, back flushed taps are often used.
(gas flowrate must be regulated)
For other measurement techniques in fluidized beds:van Ommen & Mudde, Int J Chem Reactor Eng 6 (2008) R3
Fluidization Curve
U, m/s
P/L
Umf
Umb
FluidizedNon-bubblingregion
Packedbed
Fluidized
BubblingRegion
U, m/s
P/L
Umf
Ucf
Packedbed
Fluidized BubblingRegion
Group A particles Group B particles
Wide psd
narrow psd
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Minimum Fluidization
The frictional pressure drop at the point of minimum fluidizationequalizes the bed mass per unit of cross-sectional area.
( ) ( )
( )( )mfpmf
3
mfsv
mfmf
2
mf
3
mf
2
sv
mf
2
mfmffriction
1xg
D
x1U75.1
D
x1U150P
=
+
=
The frictional pressure drop at the point of minimum fluidization(Umf, mf, xmf), can be considered equal to the frictionalpressure drop in a fixed bed (Ergun)
Minimum Fluidization Velocity (Umf)
Dimensionless relationship following from equation onprevious slide
122
1Re CArCCmf +=
svmfmf
DURe = ( ) 23 svp DgAr =
713001 mfC = 75.13
2 mfC =
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Minimum Fluidization
Estimation ofmf 0.4 < mf< 0.55 mf fixed bed mf (14 )-1/3 where is the particle sphericity
Authors C1 C2
Wen and Yu (1966)
Richardson (1971)
Saxena and Vogel (1977)
Babu et al. (1978)
Grace (1982)
Chitester et al. (1984)
33.7
25.7
25.28
25.25
27.2
28.7
0.0408
0.0365
0.0571
0.0651
0.0408
0.0494
Freely Bubbling Beds: Bubble GrowthWhy grow?1) The hydrostatic pressure on the bubbles decreases
as they rise up the bed;2) Bubbles may coalesce by one bubble catching up
with another;3) Bubbles which are side by side may coalesce;4) Bubbles may grow by depleting the continuous phase
locally.
Mean bubble size =f(type of distributor,distance above the distributor
plate, excess gas velocity)Initial bubble size
effect of wall
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Freely Bubbling Beds: Bubble Size
Darton et al. (1977) ( )0.8
0.40.2( ) 0.54 4b mf
or
AD z g U U z
N
= +
where A/Nor is the area of distributor plate per orifice
Mori & Wen (1975)
( )0.4
1.64be mf
D A U U =
0
exp 0.3be b
be b T
D D z
D D D
=
where: ( )
( )
0.4
0 0.2
2
0
1.38
0.376
b mf
or
b mf
AD U U
g N
D U U
=
=
for perforated plates
for porous plates
Ranges of key variables:
m3.1D
s/m48.0UU
m450d60
s/mm200U5
T
mf
p
mf
Q=UA
QBQmf
UB
H
Two-phase theory
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Two-phase theory
Total gas flow rate:
Flow rate in dense phase:
Gas passing through the bed as bubbles:
Fraction of the bed occupied by bubbles:
UAQ =AUQ mfmf =
( )AUUQQ mfmf =
mf mf mf b
B
H H Q Q U U
H AU U
= = =
B
In practice, the two-phase theory overestimates the volume of gaspassing through the bed as bubbles
Visible bubble flow rate:where 0.8 < Y< 1.0 for Group A powderswhere 0.6 < Y< 0.8 for Group B powderswhere 0.25 < Y< 0.6 for Group D powders
( )mfB UUY
A
Q=
The distribution of the gas between the bubbles and dense phase is ofinterest because it influences the degree of chemical conversion.
21.027.2 = ArY
Baeyens and Geldart (1985)
Homogeneous fluidization
Non-bubbling fluidizationParticulate or homogeneous fluidization
Bubbling fluidizationAggregative or heterogeneous fluidization
Mechanism???Delay caused in the adjustment of themean particle velocity to a change inthe local concentration resulting fromthe larger particle to fluid phase inertia(Didwania, 2001)
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Steady-state expansion of fluidized beds
A)1(LV BB =
constantgVP BgpB ==
Homogeneous bed expansion: Richardson-Zaki relationn
tU
U=
HA)1(M
:bedtheinpaticlesofmass
p=B
VB: total volume of particles
Log U/Ut
Log 0.4 1.0
1.0
Viscous regime: n=4.8
Inertial regime: n=2.4
1
2
12 H
)1(
)1(H
=
Heat and mass transfer
Bubble:shortcutof gas
Interstitial gas:effective
Heat transfer: particle to wall or internalMass transfer: gas to particle
Fluidized beds show an excellent heat transfer
Mixing of solids by (large) bubbles almost constant temperature throughout the reactor
However, large bubblesdecrease the mass transfer
Research decrease bubble size
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Ways to structure a fluidized bed
Particles
Gas
GeometryDynamics
live electrodes
ground
electrodes
Qp
Qs
van Ommen et al., Ind. Eng. Chem. Res., 46 (2007) 4236
0.00
0.05
0.10
0.15
0 50 100 150 200
Particle diameter [m]
Fraction
[-]
Tailoring the particle size distribution
0.00
0.05
0.10
0.15
0 50 100 150 200
Particle diameter [m]
Fraction
[-]
0 % Fines
15 % Fines
30 % Fines
0-30%
50 m
wide size distribution
narrow size distribution
0 4 8
Dimensionless kinetic rate constant [-]
1.0
0.5
0.0
Conversion[-]
0 50 100 150particle diameter [micron]
20
15
10
5
0
20
15
10
5
0
mass%
m
ass%
narrow size distribution
O3 decomposition
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air
PC
pressuredrop
sensors
pressurefluctuationsensors
Measurement techniques:
pressure drop pressure fluctuations optical probes
High Throughput Experimentation
Fines effect on bubble size
0.4
0.6
0.8
1.0
0.00 0.05 0.10 0.15 0.20 0.25 0.30
Fines w eight fraction [-]
Relativebubblesize
[-]
6 cm/s
24 cm/sBased onpressure
fluctuations
Fines are added to a powder with a D50of 70 m
Beetstra et al., AIChE J. 55 (2009) 2013
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AC electric field: CFD simulations
No Field Vertical
0.7 kV/mm, 30Hz
Horizontal
0.7 kV/mm, 30Hz
Discrete particlemodel with 10000particles.Uniform AC electricfields.
At experimental fieldstrength: ~50 %reduction of bubblearea.
Kleijn van Willigen et al., Powder Technol. 183 (2008) 196
Textbooks
Fluidization EngineeringKunii, D. & Levenspiel, O.ISBN: 8131200353
Pub. Date: Jan 2005 , 2nd ed.Publisher: Elsevier
Handbook of Fluidization and Fluid-Particle SystemsEd. Wen-Ching Yang
ISBN: 978-0-8247-0259-5Pub. Date: March 2003Publisher: Routledge, USA
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Course Material
Additional Resource Material http://www.erpt.org/ http://www.erc.ufl.edu/ http://www.chml.ubc.ca/fluidization/ Rhodes, M., Introduction to Particle Technology, Wiley, Sussex England,
ISBN: 0471984833, 1999. Yang, W.-C., Marcel Dekker, Handbook of Fluidization and Fluid-particle
Systems, New York ISBN: 082470259, 2003. Fan, L.-S., Gas-Liquid-Solid Fluidization Engineering, Butterworth-
Heinemann, Boston, 1989. Perry, R.H. and D.W. Green, Perrrys Chemical Engineers Handbook, 7th
Ed., McGraw-Hill, New York, 1997. Fan, L.S and C. Zhu, "Principles of gas-solid flows", Cambridge Press,
Cambridge, 1998. Grace, J.R., A. Avidan and T.M. Knowlton, "Circulating Fluidized Beds,"
Blackie Academic press, Boston, 1997.
Kunii, D. and O. Levenspiel, "Fluidization Engineering", 2nd edition,Butterworth-Heinemann, Boston, 1991.
Geldart, D., "Gas Fluidization Technology", John Wiley & Sons, New York,1986.
References Rhodes, M., and Zakhari, A., Laboratory Demonstrations in Particle
Technology, CD, Monash University, Melbourne, Australia (1999). Geldart, D., Types of Gas Fluidization, Powder Tech., 7, 285-292 (1973). Yang, W.-C., Bubbling Fluidized Beds, in Handbook of Fluidization and Fluid-
Particle Systems, Ed. Yang, W.-C., Marcel Dekker, N.Y., pp. 53-111 (2003). Ellis, N., Hydrodynamics of Gas-Solid Turbulent Fluidized Beds, Ph.D.
Dissertation, UBC (2003). Bi, H.T., and Grace, J.R., Flow Regime Diagrams for Gas-Solid Fluidization
and Upward Transport, Int. J. Multiphase Flow, 21, 1229-1236 (1995). Grace, J.R., Contacting Modes and Behaviour Classification of Gas-Solid and
Other Two-Phase Suspensions, Can. J. Chem. Eng., 64, 353-363 (1986). Gidaspow, D., Multiphase Flow and Fluidization: Continuum and Kinetic
Theory Descriptions, Academic Press, San Diego (1994). Kunii, D., and Levenspiel, O., Fluidization Engineering, Butterworth-
Heinemann, Newton, MA (1991).
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References
Bird, R.B., Stewart, W.E., and Lightfoot, E.N., Transport Phenomena, JohnWiley & Sons, N.Y., Chapter 6: Interphase Transport in isothermal Systems(2002).
Wen, C.Y., and Yu, Y.H., Mechanics of Fluidization, Chem. Eng. Prog. Symp.Series, 62, 100 - 111 (1966).
Gidapsow, D., and Ettehadieh, B., Fluidization in Two-Dimensional Beds witha Jet, Part II: Hydrodynamics Modeling, I&EC Fundamentals, 22, 193-201(1983).
Didwania, A.K., A New Instability Mode in Fluidized Beds, EurophysicsLetters, 53, 478-482 (2001).
DallaValle, J.M., Micromeritics, the technology of fine particles, Pitman Pub.,N.Y. (1948).
McCabe W.L., Smith, J.C., and Harriot P., Unit operations of ChemicalEngineering, McGraw-Hill, New York (1993)
Rhodes, M., Introduction to Particle Technology, Wiley, Chichester (1998) Grace, J.R., Fluidized-Bed Hydrodynamics, in Handbook of Multiphase
Systems, Ed. Hetsroni, G., Hemisphere, Washington, pp., 8:5-64 (1982).