Introduction toFluidized Bed Technology

J. Ruud van OmmenReactor & Catalysis EngineeringDelft University of Technology

E-mail: j.r.vanommen@tnw.tudelft.nl

Glatt Seminar 18 March 2003

Introduction

Fluidized bed:particles suspendedin an upwardgas stream

drag force

equals

gravitational force

Outline

Classification: Particle sizeHydrodynamic regime (~ gas velocity)Lay-out

DesignApplicationsTypical properties: Low pressure drop

Heat and mass transferErosion / measurementsSegregation and agglomeration

ModellingSummaryReferences

Geldart’s powder classification

Geldart’s powder classification

CCCohesive

0-30 µm

flour

AAAeratable

30-100 µm

milk powder

BBBubbling

100-1000 µm

sand

DDSpoutable

>1000 µm

coffee beans

Drag

Gravity

Attraction

Geldart’s powder classification

A powder (30-100 µm)

maximum bubble size

B powder (100-1000 µm)

bubbles keep on growing

From CD-ROM: Laboratory Demonstrations in Particle Technology, M. Rhodes

Geldart’s powder classification

C powder (<30 µm)

very cohesive

D powder (>1000 µm)

spouted fluidization

From CD-ROM: Laboratory Demonstrations in Particle Technology, M. Rhodes

Influence of particle size distribution

wide size distribution

narrow size distribution

0 4 8Dimensionless kinetic rate constant [-]

1.0

0.5

0.0

Con

vers

ion

[-]

0 50 100 150particle diameter [micron]

20

15

10

5

0

20

15

10

5

0

mas

s%m

ass%

Adapted from Sun & Grace (1990)

Fluidization Regimes

gas gas gas

solids returns

solids returns

solids returns

gasgasgas

only A powdersat low gas velocity

only narrow beds

gas

fixed bed

homogeneous

bubbling

slugging

turbulent fastfluidization

pneumatictransport

gas velocity

Fluidized bed lay-outs

twin bed

bubblingbed

riser

turbulentbed

circulatingbed

downer

laterally staged bed

verticallystaged bed

spouted bed

floating bed

Fluidized bed design

windbox/plenum

freeboard

cyclones

reactant gas

heatexchangetubes

product gas

Applications

Physical processes:heat exchangedryingcoatinggranulationgas purification via adsorption

Chemical processes:Fluid Catalytic Cracking (FCC)Synthesis reactions (e.g., vinyl acetate, phtalic anhydride, acrylonitrile)Polymerization of olefines (ethylene, propylene)Silicon productionFischer-Tropsch synthesis of gasolineFluid Coking and Flexi-CokingCoal / biomass / waste combustionCoal / biomass / waste gasification

Low pressure drop

pressuredrop

gas velocity

pressuredrop

gas velocity

Packed bed

Fluidized bed

onset of fluidization

Lower pressure drop

lower power costs

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 bubbles decrease the mass transferResearch decrease bubble size

Ways to decrease the bubble size

Rosensweig, Exxon

1

0.6

rel.bubble

size

0 5field strength [kV/cm]Kleijn van Willigen et al.,

TU Delft

Magnetic fieldElectric fieldVibration

Mori et al., Nagoya Inst of Techn.

Coppens and Lems,TU Delft

Fractal injector Optimizingparticle properties

Van Ommen et al.,TU Delft

Pulsed gasinjection

Coppens et al.,TU Delft

Erosion / measurements

Fluidized bed:

• often high temperature• often chemically aggressive• large mechanical stress

High erosion rateOpaque nature

Only few measurement techniques are available!

Industrial fluidized beds: only pressure and temperature measurements on a routinely base.

Erosion

Cross-sectional pictureof thermocouple

(Sethi et al., Kentucky Energy Cabinet Lab.)

Segregation and agglomeration

More informationpresentation ‘Agglomerationdetection’ this afternoon

Difference in size and/or density can lead to

segregation of the particles.

Hoomans, Kuipers, et al., Twente University

Agglomeration problems occur in various fluidized

bed processes

Modelling

Simple engineering models Computational Fluid Dynamics (CFD)

‘Two fluid’ model Discrete particle model

Levenspiel,Oregon State Univ. Van Wachem, Van den Bleek,

et al., Delft Univ. of Techn.Hoomans, Kuipers, et al.,

Twente Univ.

Two-region model

Models still show shortcomings scaling-up remains troublesome

Summary

Fluidized bed: particles suspended in a gas streamParticle size and gas velocity strongly influence the

fluidized bed behaviourLarge range of application and many different lay-out+ Low pressure drop+ Heat transfer+/- Mass transfer- Erosion- Segregation & agglomerationImprovement of models is still continuing

More information

Books:Fluidization Engineering, Kunii & Levenspiel, ISBN 0409902330Gas Fluidization, Mell Pell, ISBN 0444883355 Circulating Fluidized Beds, Grace, ISBN 0751402710

Articles:Review turbulent fluidization, Bi et al., Chem.Eng.Sci. (2000) 55, pp. 4789Measurement techniques, Werther, Powder Technol., 102 (1999) pp. 15

Web-sites:Tutorials: www.erpt.org/technoar/fluidbed.htmThis presentation: www.dct.tudelft.nl/~vanommen