FluidizationFluidization

Quak Foo LeeChemical and Biological Engineering

What is Fluidization?What is Fluidization?

The operation by which fine solids The operation by which fine solids are transformed into a are transformed into a fluidlike statefluidlike state through contact with a gas or liquid.through contact with a gas or liquid.

Some Key TerminologySome Key Terminology

AttritionAttrition: breakdown of particles: breakdown of particles ChokingChoking: collapse of a dilute-phase : collapse of a dilute-phase

suspension into a dense-phase flow as suspension into a dense-phase flow as the gas velocity is reduced at constant the gas velocity is reduced at constant solids flowsolids flow

Circulating fluidized bedCirculating fluidized bed: : configuration intended to send configuration intended to send particles around in a loop continuously, particles around in a loop continuously, with no upper interface within the bedwith no upper interface within the bed

Some Key TerminologySome Key Terminology

DownerDowner: column where particles are : column where particles are made to fall through under gravity, made to fall through under gravity, usually with cocurrent gas flowusually with cocurrent gas flow

Distributor or GridDistributor or Grid: support plate at : support plate at bottom which introduce the gas to the bottom which introduce the gas to the bottom of the bed and supports the bottom of the bed and supports the weight of the bed when it is shut downweight of the bed when it is shut down

ElutriationElutriation: tendency for fine : tendency for fine particles to be preferentially entrained particles to be preferentially entrained from the reactorfrom the reactor

Some Key TerminologySome Key Terminology

Fast fluidizationFast fluidization: flow regime whereby : flow regime whereby there is a relatively dense suspension, there is a relatively dense suspension, but no distinct upper surface, and a but no distinct upper surface, and a superficial velocity generally at least 3 superficial velocity generally at least 3 m/sm/s

FinesFines: generally particles smaller than : generally particles smaller than 37 µm in diameter (smallest regular sieve 37 µm in diameter (smallest regular sieve size)size)

FreeboardFreeboard: region extending from top of : region extending from top of bed surface to top of reactor vesselbed surface to top of reactor vessel

Some Key TerminologySome Key Terminology

Interstitial gasInterstitial gas: gas between the : gas between the particles in dense suspensionparticles in dense suspension

PorosityPorosity: fraction of gas in : fraction of gas in bed/given region as a whole or only bed/given region as a whole or only inside the particles; sometimes used inside the particles; sometimes used interchangeably with voidageinterchangeably with voidage

RiserRiser: column where particles are : column where particles are carried upwards by the gas, with no carried upwards by the gas, with no distinct bed surfacedistinct bed surface

Some Key TerminologySome Key Terminology

SegregationSegregation: tendency for particles : tendency for particles to gather in different zones to gather in different zones according to their size and/or according to their size and/or densitydensity

SolidsSolids: used synonymously with : used synonymously with particlesparticles

Superficial velocitySuperficial velocity: gas flow rate : gas flow rate divided by total column surface areadivided by total column surface area

Some Key TerminologySome Key Terminology

Transport disengaging zoneTransport disengaging zone: : region in freeboard beginning at bed region in freeboard beginning at bed surface in which particle flux surface in which particle flux decreases with height and above decreases with height and above which the entrainment is which the entrainment is independent of heightindependent of height

Voidage (or void fraction)Voidage (or void fraction): : fraction by volume of suspension or fraction by volume of suspension or bed which is occupied by the fluidbed which is occupied by the fluid

Elements of Fluidized Bed Elements of Fluidized Bed ReactorsReactors

Contacting MethodsContacting Methods Batch, cocurrent, backmix, crossflow, Batch, cocurrent, backmix, crossflow,

countercurrentcountercurrent Solids may often be represented by backmix Solids may often be represented by backmix

flowflow By using proper baffling and staging of units, By using proper baffling and staging of units,

and with negligible entrainment of solids, the and with negligible entrainment of solids, the contacting in fluidized beds can be made to contacting in fluidized beds can be made to approach closely the usually desirable approach closely the usually desirable extreme of cuntercurrent plug flowextreme of cuntercurrent plug flow

For good design, proper contacting of phases For good design, proper contacting of phases is essentialis essential

Advantages of Fluidized Advantages of Fluidized BedsBeds

The smooth, liquid-like flow of particles The smooth, liquid-like flow of particles allows continuous automatically controlled allows continuous automatically controlled operations with ease of handling.operations with ease of handling.

The rapid mixing of solids leads to nearly The rapid mixing of solids leads to nearly isothermal conditions throughout the isothermal conditions throughout the reactor, hence the operation can be reactor, hence the operation can be controlled simply and reliably.controlled simply and reliably.

It is suited to large-scale operations.It is suited to large-scale operations.

Advantages of Fluidized Advantages of Fluidized BedsBeds

The circulation of solids between two The circulation of solids between two fluidized beds makes it possible to transport fluidized beds makes it possible to transport the vast quantities of heat produced or the vast quantities of heat produced or needed in large reactors.needed in large reactors.

Heat and mass transfer rates between gas Heat and mass transfer rates between gas and particles are high when compared with and particles are high when compared with other modes of contacting.other modes of contacting.

The rate of heat transfer between a fluidized The rate of heat transfer between a fluidized bed and an immersed object is high, hence bed and an immersed object is high, hence heat exchangers within fluidized beds require heat exchangers within fluidized beds require relatively small surface areas.relatively small surface areas.

Disadvantages of Disadvantages of Fluidized BedsFluidized Beds

The difficult-to-describe flow of gas, with its large The difficult-to-describe flow of gas, with its large deviation from plug flow and the bypassing of deviation from plug flow and the bypassing of solids by bubbles, represents an inefficient solids by bubbles, represents an inefficient contacting system.contacting system.

The rapid mixing of solids in the bed leads to The rapid mixing of solids in the bed leads to nonuniform residence times of solids in the nonuniform residence times of solids in the reactor.reactor.

Friable solids are pulverized and entrained by the Friable solids are pulverized and entrained by the gas.gas.

Erosion of pipes and vessels from abrasion by Erosion of pipes and vessels from abrasion by particles.particles.

For noncatalytic operations at high temperature For noncatalytic operations at high temperature the agglomeration and sintering of fine particles the agglomeration and sintering of fine particles can necessitate a lowering in temperature of can necessitate a lowering in temperature of operation, reducing the reaction rate.operation, reducing the reaction rate.

Commercial ApplicationsCommercial Applications

Solid-Catalysed Gas-Phase Reactions:Solid-Catalysed Gas-Phase Reactions: Fluid catalytic cracking, reformingFluid catalytic cracking, reforming Fischer-Tropsch synthesisFischer-Tropsch synthesis Phthalic and maleic anhydridePhthalic and maleic anhydride Acrylonitrile and anilineAcrylonitrile and aniline Chlirination and bromination of Chlirination and bromination of

hydrocarbonshydrocarbons Polyethylene and polypropylenePolyethylene and polypropylene Oxidation of SOOxidation of SO22 to SO to SO33

Commercial ApplicationsCommercial Applications

Gas-Solid Reactions:Gas-Solid Reactions: Roasting or ores (ZnS, CuRoasting or ores (ZnS, Cu22S, nickel S, nickel

sulphides, etc.)sulphides, etc.) Combustion and incinerationCombustion and incineration Gasification, coking and Gasification, coking and

pyrolysis/carbonizationpyrolysis/carbonization Calcination (limestone, phosphates, Calcination (limestone, phosphates,

aluminium hydroxide)aluminium hydroxide) Flurination of uranium oxideFlurination of uranium oxide Fluid cokingFluid coking Reduction of iron oxideReduction of iron oxide Catalyst regenerationCatalyst regeneration

Commercial ApplicationsCommercial Applications

Gas-Phase Non-Catalytic Reactions:Gas-Phase Non-Catalytic Reactions: Natural gas combustionNatural gas combustion

Gas-Liquid-Solid:Gas-Liquid-Solid: Hydrotreating, hydroprocessingHydrotreating, hydroprocessing Biochemical processesBiochemical processes

Commercial ApplicationsCommercial Applications

Physical Processes:Physical Processes: Drying of particlesDrying of particles Coating of surfacesCoating of surfaces Granulation (growing particles)Granulation (growing particles) Heat treatment (e.g. annealing, Heat treatment (e.g. annealing,

quenching)quenching) Medical bedsMedical beds FiltrationFiltration Back-purging of filtersBack-purging of filters BlendingBlending ClassificationClassification

Flow Regimes for Upward Flow Regimes for Upward Flow of Gas through Solid Flow of Gas through Solid

Particulate MaterialsParticulate Materials

Various Kinds of Contacting Various Kinds of Contacting of a Batch of Solids by Fluidof a Batch of Solids by Fluid

Classification of Classification of Fluidized BedsFluidized Beds

Classification of Dense-Classification of Dense-Phase Fludized BedsPhase Fludized Beds

Industrial Industrial Applications of Applications of Fluidized BedsFluidized Beds

Winkler Gas GeneratorWinkler Gas Generator

Large Scale Fluid Bed Large Scale Fluid Bed Catalytic Cracking Pilot Catalytic Cracking Pilot

PlantPlant

Two-Stage Fluidized Salt Two-Stage Fluidized Salt DryerDryer

Pilot Plant for Fluidized Pilot Plant for Fluidized Drying of Air with Drying of Air with

AdsorbentAdsorbentThe drying of air by circulation of large (3.2 to 4.8 mm) silica gel beads of multistage fluidized adsorption.

To reduce the humidity from 0.00191 to 0.0015 kg/kg pilot plant uses a five-stage fluidized absorber 1.2 m square in cross section, 6.1 m high, a pressure drop of 127 cm H2O.

A perforated plate distributor with rubber flaps at the lower end of the downcomers to assure steady flow of particles from stage to stage.

Typical Flow Regimes Typical Flow Regimes ObservedObserved

Qualitative Fluidization Qualitative Fluidization Map for Fine SolidsMap for Fine Solids

Solids Mixing by a Single Solids Mixing by a Single Rising Bubble in a Bed of Rising Bubble in a Bed of

Small ParticlesSmall Particles

Solids Mixing by a Single Solids Mixing by a Single Rising Bubble in a Bed of Rising Bubble in a Bed of

Large ParticlesLarge Particles