What is Fluidization?!Reference: !

Chapter 7, Introduction to Particle Technology, M Rhodes, Wiley, 2008!Chapter 9, Principles of Gas-solid Flows, LS Fan and C Zhu, Cambridge, 1998!

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

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

•  Gas-solid is the most prevalent type of fluidization in industrial applications."

•  Topics: "•  Advantages & disadvantages"•  Components in a Fluidized Bed"•  Particle classification"•  Fluidization regimes"•  Industrial applications!

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Pros and Cons!•  Advantages: "

•  Liquid-like flow of particles allows for continuous operations "•  Excellent heat and mass transfer between fluid 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 fluidized 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!•  Plenum: Allows for distribution of gas before

distributor"•  Distributor: Ensures desired distribution of fluidizing

gas and supports particles in 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 fluidizing column"•  single or multistage"

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

column"•  Expanded section: reduces superficial gas velocity to

reduce entrainment."•  Baffles: restrict flow, enhance bubble breakup,

promote gas-solid contact, reduce particle entrainment"•  more effective for coarse (Groups B, D) than finer particles!

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•  Components include: plenum, gas distributor, cyclone, heat exchanger, expanded section, baffles!

Geldart Classification 1/4!

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•  Developed empirically in 1973, mostly widely used classification to date"•  Particles classified in terms of density difference between particle and gas (ρp-ρ) and

average particle diameter (dp) into four groups - A,B,C, and D"•  Different fluidization behavior exhibited by each group; Provides predictive capability for

fluidization properties of particles"•  Applicable for fluidizing by air at ambient conditions"

•  Some caution needed in using under conditions of high temperature, high pressure, different fluidizing gas.!

Geldart Classification 2/4!Group !

C!

Group !B!

Group !A!

Group !D!

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Geldart Classification 3/4!•  Group C: Very fine, cohesive particles "

•  Inter-particle contact force (e.g., van der Walls, capillary, electrostatics) dominate hydrodynamic force"

•  Gas channeling common; no bubbles"•  Difficult to fluidize; fluidization may be assisted by mechanical 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 Umf < Umb"•  A maximum stable bubble size exist"

•  Group B: Coarse, non-cohesive particles"•  No particulate fluidization regime since Umf = Umb"•  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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Geldart Classification 4/4!•  Demarcation between Groups A and C (Molerus, 1982):"

•  FH is the cohesive force between particles, with a value ranging from 8.8 x 10-8 (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 (ρp-ρf)/ρf > 219 !13!

Ar =ρ f (ρp − ρ f )gdp

3

µ 2

Fluidization Regimes 1/3!Gas-solid fluidization!

•  Largely depending on gas velocity, as gas velocity increases, extent of fluidization increases"

•  Fluidization regime chosen based on application needs"•  Fluidization Jargon"

•  Fixed bed: fluid percolates through void spaces between stationary particles"•  Expanded bed: particles vibrate and move apart"•  Incipiently fluidized bed or minimum fluidization: All particles just supported by

upward flowing fluid! 14!

Fluidization Regimes 2/3!Gas-solid fluidization!

•  Aggregative or heterogeneous or bubbling fluidized bed: Bubbling and channeling of gas"

•  Dense-phase fluidized bed: Fairly clear bed surface"•  Slugging bed: when bubble size exceeds column size"

•  Axial slug for fine particles; Flat slug for coarse particles"•  Turbulent fluidized 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 fluidization: particles are carried out of the bed with the gas"•  Fluid bed, fast fluidized bed, circulating fluidized bed fall under this

category"•  Spouted bed: Coarse Geldart Group D particles"•  Channeling bed: Fine Geldart Group C or A particles!

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Fluidization Regimes 3/3!Gas-solid fluidization!

Bubbling!

Slugging!

Slugging!

Cyclone!16!

Industrial Applications 1/3!•  Biofluidization: Cultivation of microorganisms"•  Features of this bioreactor application"

•  Rotary agitator just above air distributor to prevent defluidization in lower portion of bed"

•  Rotating separator in the freeboard to return elutriated 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"•  Efficient oxygen transfer leads to active

metabolism"•  Heat and CO2 generated are efficiently

removed"•  Temperature, moisture, pH levels easily and

automatically controlled!17!

•  Kikkoman Co. pioneered this for soy sauce production"

•  Fluidized particles = wheat bran + seed spores of microorganisms"

•  Fluidizing gas = air!

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 850-1000oC decomposes the organics in the bed and freeboard"

•  An efficient flue gas cleaning system comprising electrostatic precipitators, lime powder dosing equipment and catalytic bag filters remove dust and pollutants from the flue 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 efficiency, easy operability"

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

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

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

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

fluidizing gas (Fig 4g): suitable for very wet feedstock!

•  Fluidized bed dryer used extensively in wide variety of industries"•  e.g., pharmaceutical, fine

chemical, iron & steel industries!19!