Introduction to Fluidisation - NTU

8/20/2019 Introduction to Fluidisation - NTU

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What is Fluidization? Fluidization Regimes

Particle Properties

References:• Chapters 1,7, Introduction to Particle Technology, M Rhodes, Wiley, 2008 • Chapters 1,9.1-9.2, Principles of Gas-solid Flows, LS Fan and C Zhu, Cambridge, 1998 • Chapters 1-3, Fluidization Engineering, Kunii and Levenspiel, Butterworth, 1991 • Chapter 1, Handbook of Fluidization and Fluid-Particle Systems, Wen Ching Yang, CRC

Press, 2003

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Overview of Fluidization• Fluid is passed through a bed of particles, thereby particles are

transformed into a uid-like state. • Gas- Solid , Liquid- Solid and Gas-Liquid- Solid Fluidization

• Gas-solid is the most prevalent type of uidization in industrialapplications.

• Topics:• Advantages & disadvantages • Industrial applications • Fluidization regimes • Particle classication in Fluidization • Velocities denitions • Particle Properties

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Pros and Cons• Advantages

• Liquid-like ow of particles allows for continuous operations•

Excellent heat and mass transfer between uid and particles • Near-isothermal conditions throughout reactor • Bed of particles represent large thermal well, which responds slowly to

operational upsets, hence gives large margin of safety • Easy circulation of particles between two uidized beds enables vast

quantities of heat needed (or produced) to be added (or removed) • Suitable for large-scale operations

• Disadvantages

• Particle attrition or breakage

• Rapid mixing of particles leads to nonuniform residence time of particles,leading to nonuniform product and/or backmixing of gaseous reactant • Erosion of pipes and vessels through particle abrasion • High temperatures may lead to particle sintering and agglomeration

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Components in a Fluidized Bed 1/2

4

• Components include: plenum,gas distributor, cyclone, heatexchanger, expanded section,bafes


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Components in a Fluidized Bed 2/2• Plenum : Allows for distribution of gas before distributor

• Distributor : Ensures desired distribution of uidizing gas and supports particlesin bed • e.g., porous, cap types, perforated • initial bubble size strongly varies with distributor design

• Cyclone : separates solid particles from outlet gas • Internal or external to uidizing column • single or multistage

• Heat exchanger : removes generated heat or adds required heat • Immersed in dense bed or freeboard, or on wall of uidizing column

• Expanded section : reduces supercial gas velocity to reduce entrainment.

• Bafes : restrict ow, enhance bubble breakup, promote gas-solid contact,reduce particle entrainment • more effective for coarse (Groups B, D) than ner particles


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Industrial Applications 1/3• Biouidization : Cultivation of

microorganisms • Features of this bioreactor application

• Rotary agitator just above air distributor toprevent deuidization in lower portion of bed

• Rotating separator in the freeboard to returnelutriated particles to the bed

• Electrode to detect water content of particles • Fluidized cultivation reported to be superior in

terms of: • Large effective growing surface of

microorganisms • Efcient oxygen transfer leads to active

metabolism • Heat and CO 2 generated are efciently

removed • Temperature, moisture, pH levels easily and

automatically controlled

• Kikkoman Co. pioneered thisfor soy sauce production

• Fluidized particles = wheatbran + seed spores ofmicroorganisms

• Fluidizing gas = air


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Industrial Applications 2/3

•

Waste-to-energy incineration plants in Singapore: Tuas, Senoko, Tuas South,Keppel Seghers Tuas • Incineration of solid waste is inevitable in crowded areas like Singapore to reduce

waste volume - in this case, by 90% • High-capacity rotary crushers are used to break down bulky wastes so that they are

suitable for incineration • Operating temperatures of 850-1000 oC decomposes the organics in the bed and

freeboard • An efcient ue gas cleaning system comprising electrostatic precipitators, lime

powder dosing equipment and catalytic bag lters remove dust and pollutants fromthe ue gas before it is released into the atmosphere via 150m tall chimneys

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Industrial Applications 3/3• Advantages: large capacity, low cost

construction, high thermal efciency, easyoperability

• Single-stage (Fig 4a): when residence timeof solids do not matter; particles largely stayin vessel for a short time (bypassing)

• Multi-stage (Fig 4b,c): narrows residencetime distribution, eliminates bypassing

• Counterow contacting (Fig 4d) • Batch-continuous treatment (Fig 4e) • Two-stage drier for temperature-sensitive

materials (Fig 4f) • Heat supplied by heat exchanger instead of

uidizing gas (Fig 4g): suitable for very wetfeedstock

Fluidized bed dryer usedextensively in wide variety ofindustries • e.g., pharmaceutical, ne

chemical, iron & steel industries

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Fluidization Regimes 1/9 Gas-solid uidization

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1

l d


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Fl idi i R i 4/9


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Bubbling

Slugging

Slugging

Cyclone12

Fl idi i R i 5/9


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• Largely depending on gas velocity, as gas velocity increases, extent ofuidization increases

• Fluidization regime chosen based on application needs • Some terms commonly used:

• Fixed bed : uid percolates through void spaces between stationary particles • Expanded bed : particles vibrate and move apart

• Incipiently uidized bed or minimum uidization : All particles justsupported by upward owing uid • Bubbling (or aggregative or heterogeneous) uidized bed : Bubbling and

channeling of gas • Dense-phase uidized bed : Fairly clear bed surface

• Slugging bed : when bubble size exceeds column size • Axial slug for ne particles; Flat slug for coarse particles

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Fl idi ti R i 6/9


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14

• Turbulent uidized bed : When terminal velocity of the solids is exceeded,upper surface of bed disappears and entrainment occurs; turbulent motion of

solid clusters and voids of gas of varying sizes/shapes • Dilute- or lean-phase uidization : particles are carried out of the bed with

the gas • Fluid bed, fast uidized bed, circulating uidized bed fall under this category

• Channeling bed : Small Geldart Group C or A particles

• Spouted bed : Larger Geldart Group D particles

Fl idi ti R i 7/9


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Fl idi ti R gi 8/9


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1


Fl idi i R i 9/9


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Fluidization Regimes 9/9

V particle = V total (1 ! ! o )V

void

= V total

! o

m bed = " pV total (1 ! ! o ) + " f V total ! o

• What is ! o ?

Bed voidage,!

o = Volume fraction of particlebed not occupied by solid particles

U U U


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• U = Supercial velocity (m/s)• U mf = Minimum Fluidization Velocity (m/s)

• U mb = Minimum Bubbling Velocity (m/s)

U , U mf , U mb

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Minimum Fluidization Velocity, U mf 2/7

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• At U # U mf , ! P = H(1- " o )( # p - # f )g • In other words, ! P = W / A . (W =weight of bed; A=cross-sectional area of

column) • Usually, in practice, ! P < W / A because small percentage of the bedparticles is supported by the wall owing to

• the less than perfect design of the gas distributor • the nite dimension of the containing vessel

• the possibility of channeling.• Ergun equation: used to calculate pressure drop for ow through a packed

bed! P

H mf = 150

(1" ! mf )2

! mf 3

µ U mf

" 2

d p2

+ 1.751" ! mf

! mf 3

# f U mf 2

" d p

• ! mf is ! o at U = U mf


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! mf = Volume fraction not occupied by solid particles at U = U mf



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Minimum Fluidization Velocity, U mf 4/7 • Re-expressing Ergun equation in terms of Re and Ar

•

• Substitute•

1.75

! mf 3

" Re p,mf

2+

150(1 # ! mf )

! mf 3

" 2 Re p,mf = Ar

K 1 =1.75

! mf 3 "

; K 2 =150(1 # ! mf )

! mf 3 " 2

K 1 Re p ,mf 2

+ K 2 Re p,mf = Ar

Recall: Re p,mf =

! f d pU mf

µ

Ar =! f (! p " ! f )gd p

3

µ 2

l d l U


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• Further simplifying for easy usage• For very small particles

•

• For very large particles

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U mf =d p

2 (! p ! ! f )g150 µ

" mf 3 # 2

1 ! " mf for Re p,mf < 20

U mf 2

=d p (! p ! ! f )g

1.75 ! f " mf

3 # for Re p,mf > 1000

1.75

! mf

3 " Re p,mf

2+

150(1 # ! mf )!

mf

3 " 2 Re p,mf = Ar

Mi i Fl idi i V l i U 6/7


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• For very small particles (Re p,mf < 20):

• For very large particles (Re p,mf > 1000): • Effect of Pressure ( P )

• Note: % $ f(P ) • For small particles, none of the terms

changes signicantly with P ! henceU mf $ f(P )

• For large particles, U mf % (1/ # f )1/2 ! P " , U mf #

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Minimum Fluidization Velocity, U mf 6/7 U mf =

d p2 (! p ! ! f )g

150 µ

" mf 3 # 2

1 ! " mf

U mf 2 = d p (! p "

! f )g1.75 ! f

# mf 3 $

Mi i Fl idi i V l i U 7/7


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• For very small particles (Re p,mf < 20):

• For very large particles (Re p,mf > 1000): • Effect of Temperature ( T )

• Note: as T " , # f # and % " • For small particles, U mf % (1/ % ) ! T

" , U mf # • For large particles, U mf % (1/ # f )1/2 !

T " , U mf "

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Minimum Fluidization Velocity, U mf 7/7 U mf =

d p2 (! p ! ! f )g

150 µ

" mf 3 # 2

1 ! " mf

U mf 2

=

d p (! p " ! f )g1.75 ! f #

mf 3

$

Terminal Velocity 1/3


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Free settling for rigid sphere

Gravitation, F g

Buoyancy, F B

Drag Force, F D

F g

F B+F Dmg F g =

g V g m F p p

B ! ! ! ==

AuC F D D !

2

2

=


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Geldart Classication 2/6


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Group

C

GroupB

Group

A

GroupD

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• Group C : Very ne, cohesive particles• Inter-particle contact force (e.g., van der Walls,capillary, electrostatics) dominate hydrodynamic force • Gas channeling common; no bubbles • Difcult to uidize; uidization may be assisted bymechanical means (e.g., stirrer or vibration)

• Group A : Fine, somewhat cohesive particles • Both inter-particle and hydrodynamics forces important • Can be operated in both particulate (no bubbles) and

bubbling regime since U mf < U mb • A maximum stable bubble size exist




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• Group B : Coarse, non-cohesive particles • No particulate uidization regime since U mf = U mb • No maximum bubble size, bubble size increases

with bed height

• Group D : Very coarse particles • Usually operated as a spouting bed • Low bed expansion compared to Groups A and B• Particle mixing poor compared to Groups A and B




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• Demarcation between Groups A and C (Molerus, 1982) :

• F H is the cohesive force between particles, with a value ranging from 8.8 x10 -8 (for hard material) and 3.7 x 10 -7 (for soft material)

• Demarcation between Groups A and B (Geldart, 1973):

• For elevated pressures and temperatures (Grace, 1986):

• Demarcation between Groups B and D (Grace, 1986):

• for (# p - # f )/ # f > 21934

Ar =! f ( ! p " ! f )gd p

3

µ 2


Particle Properties


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Particle Properties• Flow characteristics in uidized bed systems vary signicantly with geometric

and material properties of particles

• Geometric properties : Affect particle ow behavior through interaction with gasmedium • e.g., drag force, distribution of boundary layer on particle surface, generation and

dissipation of wake vortices

• Material properties : Affect long- and short-range inter particle forces, particleattrition, erosion behaviors in gas-solid ows • e.g., physical adsorption, elastic & plastic deformation, ductile & brittle fracturing, solid

electrication, magnetization, heat conduction & thermal radiation, optical transmission

• Equivalent diameter : dened in relation to a specic sizing method developedbased on a certain equivalency criterion • Needed because particles in uidized bed applications are non-spherical and polydisperse • More than 1 equivalent diameter can be dened • Selection of a desired equivalent diameter depends on specic application

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Equivalent Diameters of Non-spherical Particle 1/2


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q p• Sieve diameter = width of minimum square aperture through which the particle

will pass • Sieving is a very popular particle sizing method

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• Surface diameter, Volume diameter, Sauter s diameter = reect athree-dimensional geometric characteristic of an individual particle

• Surface diameter = diameter of sphere having the same surface area asparticle

• Volume diameter = diameter of sphere having the same volume as particle • Sauter’s diameter = diameter of sphere having the same external surface

to volume ratio as particle

Equivalent Diameters of Non-spherical Particle 2/2


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q p• Martin

s diameter, Feret

s diameter, Equivalent circle diameter (orProjected area diameter) = based on the projected image on a single particle

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• Dynamic diameter = diameter of a sphere having the same density and terminalvelocity as the particle in a uid of the same density and viscosity •

• In Stokes’

regime, C D=24/Re t, hence


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Particle Sizing Methods 2/4


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g

39

• Imaging : Direct measurement of particle dimensions from enlarged images • Optical microscope: 1-150 &m • scanning electron microscope (SEM), transmission electron microscope (TEM):

0.001-5 &m • reveal surface morphology • SEM: uses a ne beam of electrons of medium energy (5-50 keV) to scan across

the sample in a series of parallel tracks. These scanning electrons producesecondary electron emission, back scattered electrons, light, and X-rays whichcan be detected.

• TEM: generates an image of a particle sample on a photographic plate by meansof an electron beam, through the transmissibility of the electron beam on thesample

SEM – 5-100 " m pollen grains, wikipedia.org TEM – 30 nm polio virus, wikipedia.org


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• Coulter Principle : electrical sensing technique • typically 1-50 &m • particles to be analyzed are rst suspended and

homogenized in an electrolyte and then passedthrough a cylindrical orice placed between twoelectrodes

• passages of particles through the orice generatesvoltage pulses that are amplied, recorded, andanalyzed to produce a particle size distribution.

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• Cascade Impactor :• samples and classies particle sizes by their inertia • measurement size range: 0.1 - 100 &m

• Laser Doppler Phase Shift • Sedimentation • Gas Adsorption

Particle Size Distributions (PSDs) 1/6


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( )• For polydisperse systems, various averaged diameters can be dened depending

on specic needs in industrial applications • An averaged diameter depends on both the type of PSD and choice of weighing

factor • Differential vs Cumulative frequency distributions

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• Rosin Rammler Distribution

• Representative of broken coal, moon dust, and many irregular particles

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• Arithmetic Mean Diameter = averageddiameter based on the number density functionof sample

• Surface Mean Diameter = diameter of ahypothetical particle having the same averagedsurface area as that of the given sample

• Volume Mean Diameter = diameter of ahypothetical particle having the same averagedvolume as that of the given sample

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• Sauter

s Mean Diameter = diameter of a

hypothetical particle having the same specicsurface area per unit volume as that of thegiven sample

• DeBroucker

s Mean Diameter = averageddiameter based on the mass density functionof the sample

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d SV

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Patent: J.W. Chew, B. Zou, MEMC-SunEdison, “Improving Operation ofFluidized Bed Reactors by Optimizing Temperature Gradients viaVarying Particle Size Distributions”, U.S. Patent Application PCT/

US2013/078062, Publication PCT/US2013/078062, Filing Date27Dec2013, Publication Date 3Jul2014.

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• Empirical descriptions of particle shape are usually based on identifying two of the followingcharacteristic parameters: (i) volume of the particle, (ii) surface area of the particle, (iii)projected area of the particle, and (iv) projected perimeter of the particle

• All proposed shape factors to date are open to criticism, because a range of bodies withdifferent shapes may have the same shape factor.

• Sphericity : measure of deviation from a spherical particle

• Wadell, 1933:

• caveat: difcult to obtain surface area of an irregular particle

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Density 1/4


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Bulk density

where V pp is the packed volume of the particles.

Particle density

where V p is the volume particles would displace if surfacewere nonporous.

! b =m ppV pp

! p =

m pV p

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Density 2/4


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What is Porosity?

Enclosed pores and open poresPore size and volume (pore volume could be up to 70 –80%)Internal surface area (e.g. Zeolite catalyst: 150-250 m 2/g)

SEM of dissolved serpentine SEM of precipitated MgCO 3

Sk l l d i ( d i )Density 3/4


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Skeletal density (true density)

where V s is the skeleton volume of the particles.

! s =

m pV s

! b = ! p (1 " # o ) + !# o = ! s 1 " # p( ) 1 " # o( )+ !# o

Bed voidage Pore fraction

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Bulk density of porous particles

! P

= V poreV particle

=Volume of

Volume of

= 0 for non porous particle

Assume the fluid does not enter the pores

Porousparticle

Density 4/4


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Volume ofParticle density = Mass of

Skeletal density =

Mass of

Volume of

Bulk density =Mass of

Volume of

Assume the particle is soaked in fluid, but the fluid doesn’t get into

the pores

Particle Density: Summary

Mechanical Property 1/2


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• Hardgrove Grindability Index (HGI) • originally developed to measure relative ease of pulverizing coal • does not directly relate to hardness• higher HGI = higher grindability; usually in the range 15 to 40 • 50 g of 600 - 1180 &m particles milled for 60 revolutions. Then,

amount of material ( W 200 ) passing the 75mm screen is thenmeasured. HGI is calculated from the equation HGI = 13 + 6.93 W 200

• Abrasiveness Index • in pulverized coal combustion, abrasiveness of particles severely

limits life of pulverizer grinding elements • weight loss of metal coupons measured after specied contact with

particles

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Mechanical Property 2/2 • Erosiveness Index


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• Erosiveness Index• weight loss of the coupon is an indication of the erosiveness of the

particular coal and the potential damage to the processing and handling

equipment, and other boiler components • Attrition Index

• Dictates design of uidized bed operation, beause attrition affectsentrainment

• Two techniques: Solids impaction on a plate and Davidson jet cup

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Introduction to Fluidisation - NTU

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