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Technische Universität Hamburg-HarburgInstitute of Solids Process Engineering
and Particle Technology
Fluidization- Fundamentals and Applications -
A TutorialJoachim Werther
Institute of Solids Engineering and Particle TechnologyHamburg University of Technology
D 21071 Hamburg, Germany
5th World Congress on Particle Technology, April 23-27, 2006Orlando, Florida, USA
Technische Universität Hamburg-HarburgInstitute of Solids Process Engineering
and Particle Technology
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Contents1. Introduction
1.1 Definitions1.2 Forms of fluidized beds1.3 Advantages and disadvantages of the fluidized bed as a reactor1.4 Comparison of the fluidized bed reactor with other types of gas-
solid reactors2. Typical fluidized bed applications
2.1 Historical development of the fluidization technique2.2. Technical applications of the fluidized bed
2.21 Physical processes2.2.11 Mechanical processes2.2.12 Processes with heat and mass transfer
2.22 Chemical processes2.2.21 Heterogeneous catalytic reactions2.2.22 Polymerizations reactions2.2.23 Solids as heat carriers2.2.24 Processes with reacting solids
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3. Fluid-mechanical principles3.1 Minimum fluidization velocity
3.11 Experimental determination of the minimum fluidization velocity3.12 Prediction of the minimum fluidization velocity
3.2 Fluidization properties of typical solids (Geldart‘s classification)3.3. The state diagram of fluidized beds according to Reh3.4 Gas distribution
3.4.1 Devices for gas distribution3.4.2 Minimum pressure drop3.4.3 Design of perforated plates
4. Local fluid mechanics of gas-solid fluidization4.1 Isolated bubbles in fluidized beds4.2 Bubble coalescence and splitting
5. Circulating fluidized beds5.1 Fluid mechanical characteristics5.2 Design characteristics
Technische Universität Hamburg-HarburgInstitute of Solids Process Engineering
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6. Entrainment6.1 Mechanisms6.2 Definitions and correlations
7. Solids mixing in fluidized beds7.1 Mechanisms7.2 Solids dispersion coefficients
8. Literature
Technische Universität Hamburg-HarburgInstitute of Solids Process Engineering
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What is fluidization?
Definition:A fixed bed may be brought into a liquid-like (fluidized) state by an upward flowing fluid once the flow exceeds a minimum value. In the fluidized state the fluid experiences a pressure drop which is equal to the weight of the particle bed minus its buoyancy divided by the bed’s cross-sectional area.
At = cross-sectional area of columnε = voidage of the bed
t s ffb
t
A H (1- ) ( ) gp A
ε ρ ρ−Δ =
Δp
Δp
V.
Δpfb
V.
Packed bed Fluidized bed
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Forms of gas-solid fluidized beds
state of minimumfluidization
bubblingfluidized bed
circulatingfluidized bed
turbulentfluidization
slugging fluidized bed
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Some properties of the fluidized bed
specific lighter objects are floating on the bed surface
upon tilting a horizontal adjustment of the bed surface occurs
through a hole in the wall the bed will flow out like a liquid
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Advantages and disadvantages of gas-solid fluidized beds
Advantages:- intense solids mixing by rising bubbles causes uniform temperature
distribution, even with highly exothermal reactions no hot spots- large transfer area between gas and solids- excellent heat transfer between fluidized bed and walls or internals- liquid-like behavior of fluidized bed makes solids handling easy
Disadvantages:- existence of bubbles causes bypass of reactant gas- intense solids mixing causes backmixing of reactant gas- intense movement of particles is responsible for particle attrition
(→ catalyst costs) and erosion of walls and bed internals- scale-up of fluidized bed processes is more difficult than for fixed beds
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Comparison of gas-solid reaction systems
Fine (0.02-0.5mm), with narrow particle-size distribution
Broad particle-size distribution (ca. 0.02-6mm); high fines content acceptable
Medium size (ca. 2-6mm) and uniform; no fines
Large pellets (ca. 8-20mm), as uniform as possible; no fines
Particle size
Properties intermediate between fluidized bed and moving bed
Very efficient exchange, good heat transport by solids
Poor heat exchange; due to high heat capacity of solids transport of large quantities of heat by way of circulating solids
Poor heat exchange; heat transport limits scale-up
Heat supply and removal, heat exchange
Axial temperature gradients can be held within limits by high solids circulation
High solids mixing ensures uniform temperature distribution in bed; temperature control by heat exchangers immersed in bed or by admission and removal of solids
Temperature gradients can be held within limits by virtue of high solids circulation and high gas throughput
Danger of hot spots with exothermic reactions
Temperature distribution
Possible for fast reactions; recycle of unreacted fines often difficult
No special requirements for feed particle-size distribution; high fines content also possible; continuous operation yields uniform product
For uniform feed particle size with low fines content; large reactor capacities possible
Unsuitable for continuous processes; batchwise operation yields nonuniformproduct
Suitability for gas-solid reactions
Gas in virtually plug flow; high conversion possible
Backmixing of gas due to mixing motion of solids and bubble-gas bypass lead to lower conversion
Plug flow gas ensures high gas conversion
Catalyst attrition may be critical, depending on operating conditionsCatalyst attrition negligible
can also be used with catalyst that is rapidly deactivatedonly for catalyst that is deactivated very slowly
Suitability for heterogeneous catalytic gas-phase reactions
Entrained flowFluidized bedMoving bedFixed bedCharacteristics
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Fluidized bed - applications
The first patent was issued in 1922 to BASF in Germany for a fluidized-bed gasifier for lignite.Inventor: Fritz Winkler
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Winkler‘s gasifier
airoxygen
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FCC–Fluid Catalytic Cracking - the most successful fluid bed process
The problem: carbon deposition from cracking deactivates catalyst
The solution: cycling a fluidized catalyst between reactor and regenerator, use hot regenerated catalyst as heat carrier for supplying heat to endothermalcracking reaction.
1940 development work by Essoand MIT
1942 13,000 barrels/day plant in Baton Rouge
C CH H
H HC CH H
H HC CH H
H HCH
H
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Reactor: oil vapors react in presence of catalyst
Regenerator: coke is burned off to regenerate the catalyst
FCC process: Kellogg-Orthoflow system
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The riser cracking process: the UOP system
reactor
stripper regenerator
air gridriser
slidevalve
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Fluidization technology: physical processes
air slide conveyorfor the transport of solids
elutriatorfines are elutriated fromcoarse particle fluidized bed
solid-liquid suspension
classification watercoarse
fluidized bed
fines
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Fluidization technology: processes with heat and mass transfer
fluidized bed cooler
for alumina particles
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Fluidization technology: processes with heat and mass transfer
fluidized-bed drying fluidized-bed spray granulationSprühflüssigkeit
gas gas
product product
solution
solution
bottom spray top spray
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Fluidization technology: processes with heat and mass transfer
Coating of glass beads with paraffin in the supercritical fluidized bed
fluidizing medium: CO2
fluidized bed 80 bar, 40°C
supercritical solution before
expansion: 160bar, 70°C
20 µm
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Fluidized bed chemical processes – solid is a catalyst
Example 1: Phtalic anhydride from naphtalene (Badger/Sherwin-Williams process)
problem:highly exothermal reaction,explosion risk limits inletconcentration with fixed bed reactors
solution:-naphtalin is injected in the liquid form → mixing occurs in the fluidized bed, no explosion possible, no separate evaporator-temperature homogeneity avoids hot spots-in-bed heat exchanger extracts heat of reaction
naphtalene
product gas
filter
steam
air
Dowtherm
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Fluidized bed chemical processes – solid is a catalyst
3 6 3 2 2 23C H NH O CH CH CN 3H O2
+ + → = − +
Example 2: Synthesis of acrylonitrile(Sohio process)Ammoxidation of propylene
- precise adjustment of reaction temperature leads to optimum yield
- mixing of reactants inside the fluidized bed avoids risk of explosion
- steam raising via in-bed heat exchanger tubes
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Fluidized-bed chemical processes – solid is the product in a catalytic process
reactor
cooler
catalyst
separator
compressor
Gas-phase polymerization of ethylene (Unipol process)
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Fluidized-bed chemical processes: solid particles act as heat carriers
Fluid Coking Process
for the thermal cracking of heavy residues,
cracking leads to coke
deposition on bed particles (petroleum coke)
coke is partially burned and heated in the heater hot, particles supply heat to the reactor
a) Slurry recycle; b) Stripper; c) Scrubber;d) Reactor; e) Heater; f) Quench elutriator
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Fluidized-bed chemical processes: solid particles are reactants
Calcination of aluminiumhydroxide (Lurgi process)
- endothermal reaction- countercurrent flow
of gas and solids through the process saves energy
a) Venturi fluidized bedb) Cyclonec) Fluidized-bed furnaced) Fluidized-bed coolere) Recycle cyclonef) Electrostatic precipitator
- low NOx by staged combustion
- in-situ desulphurization with limestone dosing:CaCO3 → CaO + CO2CaO + SO2 + 1/2O2→CaSO4
Coal combustion in the (circulating) fluidized bed
a) Circulating fluidized-bed reactor
b) Recycle cyclonec) Siphond) Fluidized-bed heat
exchangere) Convective passf) Dust filterg) Turbineh) Stack
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Fluid-mechanical principles – the minimum fluidization velocity
Measurement in the laboratory:
At : cross-sectionalarea of column
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0
250
0 300
Δp
u
Minimum fluidization velocity – evaluation of measurement
- segregation occurs around umf→ avoid measuring here!
- just take measurements in the fully fluidized state and in the fixed bed state
- umf is then determined by extrapolation
- a reproducible fixed bed is obtained by shutting the gas supply suddenly off in the fully fluidized state
Δpfb
umf
Δpfb
bed +distributor
bed
distributor
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Minimum fluidizing velocity - calculation
fluidized bed pressure drop: Δpfb(u≥umf) = (1-ε)(ρs-ρf)gHfixed bed pressure drop: Δpfix(u≤umf) = function of u, dp, ε, gas conditions
(e.g. Ergun‘s equation)
umf from Δpfb = Δpfix (u=umf)Good approximation: Remf=33.7 {(1+3.6•10-5Ar)0.5-1} (Wen + Yu, 1966)
If a sample of the bed solids is available, the following procedure is recommended:
1. Measure umf with air under ambient conditions in the lab2. Calculate the Sauter diameter of the bed solids from Ergun‘s equation3. Convert umf (air, ambient conditions) to umf (gas at process
conditions) by using Ergun‘s equation (Werther, Chem.-Ing.-Techn., 1976)
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Minimum fluidization velocity – Calculation of umf under process conditions
Measured and calculated minimum fluidization velocities as function of pressure and temperature. (Measurements by Knowlton, 1974 with nitrogen (T=293K) and by Janssen, 1973 with air (p= 1bar)).
Comparison between measured and calculated minimum fluidization velocities for different gases (Measurements by Singh, Rigby and Callcott, 1973).
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Geldart (1973): The existence of types of powders with characteristic behaviors
Group C:fine, cohesive materials, difficult to fluidize, particles are sticking to each other, „rat holes“ are formed, mechanically stirring of the bed may be needed
Group A:typical is FCC catalyst, good fluidization, above umf first homogeneous fluidization which breaks down at umb, upon shutting off the gas supply the bed is slowly collapsing.
Group B:typical is sand of 0.1-0.3mm, bubbling occurs immediately above umf, upon shutting off the gas supply the bed is rapidly collapsing.Group D: large particles, typical are wheat grains, formation of very large bubbles.
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purpose:to characterize the state of fluidization for a given system by an (average) voidage ε
abscissa:particle Reynolds number
ordinate:
auxiliary grid with
Fluid-mechanical principles- Reh‘s status diagram
pudRe
ν=
2f
s f p
3 uFr with Fr4 gd
ρρ ρ−
=
3 3p s f f2
f s f
gd uAr , Mg
ρ ρ ρν ρ ν ρ ρ
−= = ⋅−
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Reh‘s status diagram
- its backbone is the force balance on a single particle (ε→ o)
- the lines ε = const for gas-solid (bubbling, „aggregative“) fluidization are based on experiments
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Reh‘s status diagram
may be used to locate different fluidized bed (and even fixed bed) processes
a) Circulating fluidized bed
b) Fluidized-Bed roaster
c) Bubbling fluidized bed
d) Shaft furnace
e) Moving bed
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Reh‘s status diagram
can answer a number of practical questions:
- which voidage is expected for given solids (dp,ρs), gas (ν,ρ,g) and gas velocity u?→ calculate Ar, Re → status S
- particles of which size will be elutriated?→ use M = const → S1
- if particle agglomeration occurs: for which size fluidization will break down?→ use M = const → S2
- find the minimum fluidization velocity→ use Ar = const → S3
- where is a (theoretical) upper limit of fluidization?→ use Ar = const → S4
S
S1 S4
S2
S3
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The role of the gas distributor in the fluidized bed
The distributor shall- ensure uniform fluidization over the entire cross-section of the bed- provide complete fluidization of the bed without dead spots
(where, for example, deposits can form)- maintain a constant pressure drop over long operation periods
(outlet holes must not become clogged)- prevent solids from raining through the grid both during operation
and after the bed has been shut off
Distributor types:- porous plates in the laboratory- perforated plates, nozzles, bubble caps, spargers in technical units
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Gas distribution devices in large-scale fluidized bed combustors
1 Nozzle7 bubble cap2-6 and 8combined types with mixed characteristics of bubble caps and nozzles,10 sparger9,11,12, special designs (after VGB_MerkblattM218 H)
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Gas distributor design
distributor
bed
p 0.1 ... 0.3p
Δ ≈Δ
Basic requirement:
Design procedure:
- (index o relates to conditions in orifice, drag coefficient CD from measurement)
- with uo calculate number no of orifices from continuity
2o d D 0p C u
2ρ
Δ = ⋅ ⋅
Problems with gas distributor:
- open jets will cause attrition of bed solids- pressure fluctuations may cause backflow of solids into the windbox
Technische Universität Hamburg-HarburgInstitute of Solids Process Engineering
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37Local fluid mechanics: Bubble formation
• Gas-solid fluidized beds are characterized by the presence of bubbles• bubbles are responsible for the temperature homogeneity of fluidized-bed reactors
(bubbles are „stirring“ the bed) and for the excellent heat transfer between bed and walls or intervals (bubbles account for „surface renewal“ at the heat transfer surfaces)
• but: bubbles are also responsible for drawbacks of the fluidized-bed reactor:- bubbles cause a bypass of reaction gas which limits the conversion of a catalytic
gas-phase reaction- bubble-induced solids movement leads to attrition of the bed particles and
erosion of walls and internals• the ultimate cause of bubble formation is the universal tendency of gas-solid flows
to segregate.Stability theories (Jackson, Molerus etc.) indicate that disturbances induced in an initially homogenous gas-solid suspension do not decay but always lead to the formation of macroscopic voids
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Local fluid mechanics: Gas flow in and around a rising bubble
pressure outside bubble is higher than inside → gas will flow into the bubble
Davidsons‘s bubble model:
streamlines of fluid (broken lines) and particles (solid lines) around a spherical bubble
p
h
( )( )s f mfdp 1 gdh
ρ ρ ε= − −
pressure insidebubble is constant
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Visualization of bubble fluid dynamics
Injection of an NO2 bubble into an incipiently fluidized 2 D bedDavidson and Harrison, 1971)
X-ray photo of a 3 D bubble(Rowc, 1971)
Technische Universität Hamburg-HarburgInstitute of Solids Process Engineering
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40Coalescence and splitting of bubbles
Coalescence of bubbles from Toei et al. (1965)(X-ray photo anddimensionless correlation)
Splitting of a single bubble (X-ray sequence, Rowe, 1971)
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Calculation of bubble growth
b
v/
bvud
dhdd
λ−⎟
⎠⎞
⎜⎝⎛
πε=
392 31
1/ 3b
0.2v,o 20
0
0.008 porous plated
industrial gas distributor withVm 1.3g V volumetric gas flow through a single orifice
ε⎧ ⋅⎪⎛ ⎞
= ⎛ ⎞⎨⎜ ⎟⋅⎝ ⎠ ⎜ ⎟⎪ =⎝ ⎠⎩
&
&
for Geldart group A and B solids (Hilligardt and Werther, 1987)
dv = diameter of volume-equivalent sphere
h = height above distributorεb = bubble volume fractionub = bubble rise velocitycoalescence splitting
λ = mean bubble lifetime
mfu 280 (typically 0.05 ... 0.15 s)
gλ =
at h=h0:
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Gas jets in fluidized beds
(after Karri and Werther, 2003)correlations suggested by Merry(1974):
( )
0.3 0.2 2
up f o o
o s p p
0.4 0.22phor o 0 f
o s p s o
L d u 5.2 1.3 1d d gd
dL u 5.25 4.5d 1 gd d
ρρ
ρ ρε ρ ρ
⎧ ⎫⎛ ⎞ ⎛ ⎞⎪ ⎪= −⎜ ⎟ ⎜ ⎟⎨ ⎬⎜ ⎟ ⎜ ⎟⎪ ⎪⎝ ⎠ ⎝ ⎠⎩ ⎭
⎛ ⎞ ⎛ ⎞= −⎜ ⎟ ⎜ ⎟⎜ ⎟− ⎝ ⎠⎝ ⎠
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Circulating fluidized beds
most important applications:catalytic cracking (FCC process) fluidized-bed combustion
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Operating characteristics of FCC risers and CFB combustors
<1%(0.1-0.3 %)
>1 %(1-10 %)
mean solids volume concentration in upper dilute zone of riser
approx. 20-40 sapprox. 4saverage solids residence time per single pass
5-8 m/s>300 kg/m2s(external) solids circulation rate Gs
5-8 m/sbetween 4.5 – 6 m/s
(min. velocity at bottom) and 15-20 m/s (at riser exit)
operating characteristics:superficial gas velocity
approx. 0.2 mm broad size distribution
approx. 0.06 mmbed particle size distribution:Sauter diameter dps
membrane walls(vertical tubes/fins)
flatwalls of riser
<5(10)>20height-to-diameter ratio
4-8m (hydraulic diameter)0.7 – 1.5 mriser diameter
mostly rectangular or square
circulargeometry:cross-section of riser
CFB combustorFCC riser
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CFB fluid mechanics
Q3 = cumulative mass distribution, ut = single particle terminal velocity
→ circulating fluidized beds are operated well above the single particles‘terminal velocities!
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CFB fluid mechanics
→ CFB is characterized by very high slip velocity!
cv,mf
pneumatic conveying
CFB bubbling (stationary) fluidized bed
usl = slip velocityGs = solids circulation
rate kg/m2s
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CFB local fluid mechanics (combustion systems)
Flensburg combustor 105 MWth, 100 % load, u = 6.3 m/ss = thickness of hydrodynamical boundary layer
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Pressure distribution in the CFB system
a = fluidized bed, b = return leg
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CFB: Design options for the pressure seal
Siphon L-valve
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Entrainment from fluidized beds
- bubble eruptions shed particles into the freeboard
- entrained particles disengage in the freeboard: coarser particles sink back into the bed, finer particles are elutriated
- the disengagement process is finished after TDH(= transport disengaging height)
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Calculation of entrainment
the specific mass flow rate of solids leaving the CFB at the trop (above TDH) is
xi = mass fraction of the (entrainable) particle size fraction in the bed material
χi* = elutriation rate constant for this size fraction, kg/m2s
obtainable from various empirical correlations
∑ ∗χ⋅=i
iis xG
Technische Universität Hamburg-HarburgInstitute of Solids Process Engineering
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52Estimation of TDH for FCC catalyst type particles
(after Zenz and Othmer,1960; the parameter is the bed diameter)
0,1 1
0,1
1
10
0.3 m7.5 m
3 m
1.5 m0.6 m
0.15 m
0.075 m
D = 0.025 m
trans
port
dise
ngag
ing
heig
ht T
DH
, m
U - Umf, m/s
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53
Solids mixing in fluidized beds
Solids are displaced by rising bubbles
→ particle drift effect causes
particle mixing → dispersion process
Solids are carried upward in the wakes of rising bubbles
→ convective transport
The consequence: mixing in the vertical direction is much better than in thehorizontal direction!
The mechanism:
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Solids circulation in fluidized beds
radial distribution of the visible bubble flow
bubble-induced solids circulation pattern
(Werther, 1974)
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Lateral solids mixing in a bubbling fluidized bed
Measurements by Bellgardt and Werther (1986)
Solid CO2 (dry ice) was injected through the side wall of the bed.Sublimation cooling led to a steady-state temperature distribution in the bed with a distinct temperature gradient in the horizontal direction
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Vertical dispersion of solids in fine-particle fluidized beds
→ better mixing with increasing fluidizing velocity and in beds of larger diameters→ horizontal dispersion coefficients are two orders of magnitude lower!
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Heat transfer to internals / walls in fluidized beds
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Bed-to-wall heat transfer depends on particle size
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Maximum heat transfer coefficient as a function of particle size
αmax decreases because heat capacity of small particles is rapidly exhausted
heat conduction in the gas-filled gap between particle and wall is limiting
gas convection is increasingly contributing
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Literature:
[1] D. Kunii and O. Levenspiel:Fluidization Engineering – Second Edition.Butterworth-Heinemann, Boston 1991
[2] J.R. Grace, A.A. Avidan, T.M. Knowlton (Eds): Circulating fluidized bedsBlackie Academic and Professional, London 1997
[3] W.C. Yang (Ed.):Handbook of Fluidization and Fluid-Particle Systems.Marcel Dekker, New York 2003.
The current state of the art is documented in the proceedings of three conference series:
„Fluidization“ (Engineering Foundation Conference, 11th was 2004)„International Conference on Circulating Fluidized Beds“ (8th was 2005)„Fluidized Bed Combustion Conference“ (18th was 2005)