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Technische Universität Hamburg-Harburg Institute of Solids Process Engineering and Particle Technology Fluidization - Fundamentals and Applications - A Tutorial Joachim Werther Institute of Solids Engineering and Particle Technology Hamburg University of Technology D 21071 Hamburg, Germany 5th World Congress on Particle Technology, April 23-27, 2006 Orlando, Florida, USA

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Page 1: Werther, 2006

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

Page 2: Werther, 2006

Technische Universität Hamburg-HarburgInstitute of Solids Process Engineering

and Particle Technology

2

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

Page 3: Werther, 2006

Technische Universität Hamburg-HarburgInstitute of Solids Process Engineering

and Particle Technology

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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

Page 4: Werther, 2006

Technische Universität Hamburg-HarburgInstitute of Solids Process Engineering

and Particle Technology

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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

Page 5: Werther, 2006

Technische Universität Hamburg-HarburgInstitute of Solids Process Engineering

and Particle Technology

5

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

Page 6: Werther, 2006

Technische Universität Hamburg-HarburgInstitute of Solids Process Engineering

and Particle Technology

6

Forms of gas-solid fluidized beds

state of minimumfluidization

bubblingfluidized bed

circulatingfluidized bed

turbulentfluidization

slugging fluidized bed

Page 7: Werther, 2006

Technische Universität Hamburg-HarburgInstitute of Solids Process Engineering

and Particle Technology

7

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

Page 8: Werther, 2006

Technische Universität Hamburg-HarburgInstitute of Solids Process Engineering

and Particle Technology

8

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

Page 9: Werther, 2006

Technische Universität Hamburg-HarburgInstitute of Solids Process Engineering

and Particle Technology

9

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

Page 10: Werther, 2006

Technische Universität Hamburg-HarburgInstitute of Solids Process Engineering

and Particle Technology

10

Fluidized bed - applications

The first patent was issued in 1922 to BASF in Germany for a fluidized-bed gasifier for lignite.Inventor: Fritz Winkler

Page 11: Werther, 2006

Technische Universität Hamburg-HarburgInstitute of Solids Process Engineering

and Particle Technology

11

Winkler‘s gasifier

airoxygen

Page 12: Werther, 2006

Technische Universität Hamburg-HarburgInstitute of Solids Process Engineering

and Particle Technology

12

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

Page 13: Werther, 2006

Technische Universität Hamburg-HarburgInstitute of Solids Process Engineering

and Particle Technology

13

Reactor: oil vapors react in presence of catalyst

Regenerator: coke is burned off to regenerate the catalyst

FCC process: Kellogg-Orthoflow system

Page 14: Werther, 2006

Technische Universität Hamburg-HarburgInstitute of Solids Process Engineering

and Particle Technology

14

The riser cracking process: the UOP system

reactor

stripper regenerator

air gridriser

slidevalve

Page 15: Werther, 2006

Technische Universität Hamburg-HarburgInstitute of Solids Process Engineering

and Particle Technology

15

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

Page 16: Werther, 2006

Technische Universität Hamburg-HarburgInstitute of Solids Process Engineering

and Particle Technology

16

Fluidization technology: processes with heat and mass transfer

fluidized bed cooler

for alumina particles

Page 17: Werther, 2006

Technische Universität Hamburg-HarburgInstitute of Solids Process Engineering

and Particle Technology

17

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

Page 18: Werther, 2006

Technische Universität Hamburg-HarburgInstitute of Solids Process Engineering

and Particle Technology

18

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

Page 19: Werther, 2006

Technische Universität Hamburg-HarburgInstitute of Solids Process Engineering

and Particle Technology

19

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

Page 20: Werther, 2006

Technische Universität Hamburg-HarburgInstitute of Solids Process Engineering

and Particle Technology

20

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

Page 21: Werther, 2006

Technische Universität Hamburg-HarburgInstitute of Solids Process Engineering

and Particle Technology

21

Fluidized-bed chemical processes – solid is the product in a catalytic process

reactor

cooler

catalyst

separator

compressor

Gas-phase polymerization of ethylene (Unipol process)

Page 22: Werther, 2006

Technische Universität Hamburg-HarburgInstitute of Solids Process Engineering

and Particle Technology

22

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

Page 23: Werther, 2006

Technische Universität Hamburg-HarburgInstitute of Solids Process Engineering

and Particle Technology

23

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

Page 24: Werther, 2006

- 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

Page 25: Werther, 2006

Technische Universität Hamburg-HarburgInstitute of Solids Process Engineering

and Particle Technology

25

Fluid-mechanical principles – the minimum fluidization velocity

Measurement in the laboratory:

At : cross-sectionalarea of column

Page 26: Werther, 2006

Technische Universität Hamburg-HarburgInstitute of Solids Process Engineering

and Particle Technology

26

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

Page 27: Werther, 2006

Technische Universität Hamburg-HarburgInstitute of Solids Process Engineering

and Particle Technology

27

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)

Page 28: Werther, 2006

Technische Universität Hamburg-HarburgInstitute of Solids Process Engineering

and Particle Technology

28

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).

Page 29: Werther, 2006

Technische Universität Hamburg-HarburgInstitute of Solids Process Engineering

and Particle Technology

29

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.

Page 30: Werther, 2006

Technische Universität Hamburg-HarburgInstitute of Solids Process Engineering

and Particle Technology

30

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

−= = ⋅−

Page 31: Werther, 2006

Technische Universität Hamburg-HarburgInstitute of Solids Process Engineering

and Particle Technology

31

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

Page 32: Werther, 2006

Technische Universität Hamburg-HarburgInstitute of Solids Process Engineering

and Particle Technology

32

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

Page 33: Werther, 2006

Technische Universität Hamburg-HarburgInstitute of Solids Process Engineering

and Particle Technology

33

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

Page 34: Werther, 2006

Technische Universität Hamburg-HarburgInstitute of Solids Process Engineering

and Particle Technology

34

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

Page 35: Werther, 2006

Technische Universität Hamburg-HarburgInstitute of Solids Process Engineering

and Particle Technology

35

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)

Page 36: Werther, 2006

Technische Universität Hamburg-HarburgInstitute of Solids Process Engineering

and Particle Technology

36

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

Δ = ⋅ ⋅

Problems with gas distributor:

- open jets will cause attrition of bed solids- pressure fluctuations may cause backflow of solids into the windbox

Page 37: Werther, 2006

Technische Universität Hamburg-HarburgInstitute of Solids Process Engineering

and Particle Technology

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

Page 38: Werther, 2006

Technische Universität Hamburg-HarburgInstitute of Solids Process Engineering

and Particle Technology

38

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

Page 39: Werther, 2006

Technische Universität Hamburg-HarburgInstitute of Solids Process Engineering

and Particle Technology

39

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)

Page 40: Werther, 2006

Technische Universität Hamburg-HarburgInstitute of Solids Process Engineering

and Particle Technology

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)

Page 41: Werther, 2006

Technische Universität Hamburg-HarburgInstitute of Solids Process Engineering

and Particle Technology

41

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:

Page 42: Werther, 2006

Technische Universität Hamburg-HarburgInstitute of Solids Process Engineering

and Particle Technology

42

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

⎧ ⎫⎛ ⎞ ⎛ ⎞⎪ ⎪= −⎜ ⎟ ⎜ ⎟⎨ ⎬⎜ ⎟ ⎜ ⎟⎪ ⎪⎝ ⎠ ⎝ ⎠⎩ ⎭

⎛ ⎞ ⎛ ⎞= −⎜ ⎟ ⎜ ⎟⎜ ⎟− ⎝ ⎠⎝ ⎠

Page 43: Werther, 2006

Technische Universität Hamburg-HarburgInstitute of Solids Process Engineering

and Particle Technology

43

Circulating fluidized beds

most important applications:catalytic cracking (FCC process) fluidized-bed combustion

Page 44: Werther, 2006

Technische Universität Hamburg-HarburgInstitute of Solids Process Engineering

and Particle Technology

44

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

Page 45: Werther, 2006

Technische Universität Hamburg-HarburgInstitute of Solids Process Engineering

and Particle Technology

45

CFB fluid mechanics

Q3 = cumulative mass distribution, ut = single particle terminal velocity

→ circulating fluidized beds are operated well above the single particles‘terminal velocities!

Page 46: Werther, 2006

Technische Universität Hamburg-HarburgInstitute of Solids Process Engineering

and Particle Technology

46

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

Page 47: Werther, 2006

Technische Universität Hamburg-HarburgInstitute of Solids Process Engineering

and Particle Technology

47

CFB local fluid mechanics (combustion systems)

Flensburg combustor 105 MWth, 100 % load, u = 6.3 m/ss = thickness of hydrodynamical boundary layer

Page 48: Werther, 2006

Technische Universität Hamburg-HarburgInstitute of Solids Process Engineering

and Particle Technology

48

Pressure distribution in the CFB system

a = fluidized bed, b = return leg

Page 49: Werther, 2006

Technische Universität Hamburg-HarburgInstitute of Solids Process Engineering

and Particle Technology

49

CFB: Design options for the pressure seal

Siphon L-valve

Page 50: Werther, 2006

Technische Universität Hamburg-HarburgInstitute of Solids Process Engineering

and Particle Technology

50

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)

Page 51: Werther, 2006

Technische Universität Hamburg-HarburgInstitute of Solids Process Engineering

and Particle Technology

51

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

Page 52: Werther, 2006

Technische Universität Hamburg-HarburgInstitute of Solids Process Engineering

and Particle Technology

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

Page 53: Werther, 2006

Technische Universität Hamburg-HarburgInstitute of Solids Process Engineering

and Particle Technology

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:

Page 54: Werther, 2006

Technische Universität Hamburg-HarburgInstitute of Solids Process Engineering

and Particle Technology

54

Solids circulation in fluidized beds

radial distribution of the visible bubble flow

bubble-induced solids circulation pattern

(Werther, 1974)

Page 55: Werther, 2006

Technische Universität Hamburg-HarburgInstitute of Solids Process Engineering

and Particle Technology

55

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

Page 56: Werther, 2006

Technische Universität Hamburg-HarburgInstitute of Solids Process Engineering

and Particle Technology

56

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!

Page 57: Werther, 2006

Technische Universität Hamburg-HarburgInstitute of Solids Process Engineering

and Particle Technology

57

Heat transfer to internals / walls in fluidized beds

Page 58: Werther, 2006

Technische Universität Hamburg-HarburgInstitute of Solids Process Engineering

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Bed-to-wall heat transfer depends on particle size

Page 59: Werther, 2006

Technische Universität Hamburg-HarburgInstitute of Solids Process Engineering

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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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Technische Universität Hamburg-HarburgInstitute of Solids Process Engineering

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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)