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Fluidization JMBC Particle Technology 2019 Fluidization Ruud van Ommen* With material from Naoko Ellis** *Delft University of Technology, the Netherlands **University of British Columbia, Canada Powder Flow versus Gas-Particle Flow • First particle-particle • First gas-particle interaction, then gas- interaction, then particle- particle interaction particle interaction • Research area: • Research area: granular matter fluid mechanics Chemical Engineering / Particle Technology Intro gas-solids fluidized bed Packed bed: drag force gas ‘bubble’ dense phase or particles are smaller than emulsion phase stagnant gravitational force Fluidized bed: drag force interstitial particles gas particle equals suspended in an upward gravitational gas gas stream; force they move Significance of Fluidized beds Advanced materials Chemical and Petrochemical •Silicon production for •Cracking of hydrocarbons semiconductor and solar •Gas phase polymeric industry reactions •Coated nanoparticles •Nano carbon tubes Physical operations •Coating of metal and Combustion/pyrolysis glass objects •Drying of solids •Combustion/gasification of coal •Roasting of food •Pyrolysis of wood waste •Classify particles Pharmaceutical •Chemical looping comubstion http://www.chemsoc.org/timeline/pages/1961.html •Coating of pills http://physicsweb.org/article/world/11/1/9 •Granulation www.unb.ca/che/che5134/ fluidization.html •Production of plant http://www.niroinc.com/html/drying/fdfluidtype.html and animal cells http://www.dynamotive.com/biooil/technology.html Gas-Solid Fluidized Bed Characteristics of Gas Fluidized Beds Primary Characteristics: – Bed behaves like liquid of the same bulk density – can add or remove particles – Rapid particle motion – good solids mixing – Very large surface area available Question What is the surface area of 1 m3 of 100 m particles? 2 3 Specific surface area [ m / m particle ] 3 2 3 Area for m bulk: take voidage into account: [ m / m bulk ] Assuming a voidage of 0.5: 1 m3 of 100 m particles has a surface area of ~30,000 m2 Characteristics of Gas Fluidized Beds Secondary Characteristics: – Good heat transfer from surface to bed, and gas to particles – Isothermal conditions radially and axially – Pressure drop depends only on bed depth and particle density – does not increase with gas velocity (ideal case) – Particle motion usually streamlines erosion and attrition (Dis)advantages of fluid beds Advantages Disadvantages • good G-S mass bypass of gas in bubbles transfer in dense broad RTD gas and solids phase erosion of internals • good heat transfer attrition of solids • easy solids handling difficult scale-up • low pressure drop Basic Components Yang W. Bubbling fluidized beds (Chapter 3). In: Handbook of Fluidization and Fluid-Particle Systems. Yang W (Ed.). Marcel Dekker, Inc., NY, NY, USA, 53–113 (2003). 1.56 m Diameter Column Industrial Scale Solid offtake A Fluid Catalytic Cracking Unit. Photo courtesy of Grace Davison. Approaches to the Study of Particulate Systems • Totally empirical (leading to dimensional correlations) • Empirical guided by scientific principles (e.g. Buckingham Pi Theorem to obtain dimensionally consistent correlations) • Semi-empirical, i.e. some mechanistic basis, but with one or more empirical constants • Mechanistic physical model without any empiricism, numerical solutions of governing equations of motion and transport Geldart’s powder classification combustion/ gasification drying/ PE production cat. reactions hardly used Geldart’s powder classification C Drag A Attraction Cohesive Aeratable 0-30 m 30-100 m flour Gravity milk powder B D Bubbling Spoutable 100-1000 m >1000 m sand coffee beans Group C •Cohesive •Difficult to fluidized, and channeling occurs •Interparticle forces greatly affect the fluidization behaviour of these powders •Mechanical powder compaction, prior to fluidization, greatly affected the fluidization behaviour of the powder, even after the powder had been fully fluidized for a while •Saturating the fluidization air with humidity reduced the formation of agglomerates and greatly improved the fluidization quality. The water molecules adsorbed on the particle surface presumably reduced the van der Waals forces. •dp ~ 0-30 m •Example: flour, cement Group A •Aeratable •Characterized by a small dp and small p •Umb is significantly larger than Umf •Large bed expansion before bubbling starts •Gross circulation of powder even if only a few bubbles are present •Large gas backmixing in the emulsion phase •Rate at which gas is exchanged between the bubbles and the emulsion is high •Bubble size reduced by either using a wider particle size distribution or reducing the average particle diameter •There is a maximum bubble size •dp ~ 30-100 m •Examples: FCC, milk flour Group B •Bubbling •Umb and Umf are almost identical •Solids recirculation rates are smaller •Less gas backmixing in the emulsion phase •Rate at which gas is exchanged between bubbles and emulsion is smaller •Bubbles size is almost independent of the mean particle diameter and the width of the particle size distribution •No observable maximum bubble size •dp ~ 100-1000 m •Example: sand Group D •Spoutable •Either very large or very dense particles •Bubbles coalesce rapidly and flow to large size •Bubbles rise more slowly than the rest of the gas percolating through the emulsion •Dense phase has a low voidage •dp ~ >1000 mm •Examples: Coffee beans, wheat, lead shot Influence of particle size distribution 1.0 20 15 10 mass% 5 0 0 50 100 150 particle diameter [micron] 0.5 20 Conversion [-] Conversion wide size distribution 15 narrow size distribution 10 mass% 5 0 0.0 0 4 8 Adapted from Sun & Grace (1990) Dimensionless kinetic rate constant [-] Implication of Umb/Umf •Umb/Umf could be used as an important index for the fluidization performance of fine particle fluidized beds on local hydrodynamics • Geldart particle classification: – Group A powders with Umb/Umf>1 – Group B powders with Umb/Umf=1 Demarcation Demarcation between Group A and B powders Umb 1 Umb K dp Umf For air at room T and P, K = 100 (Yang, W.-C., 2003) 8 104 gd p p 1 K Pressure and temperature effect: (Grace, 1986) 0.425 * * p If : d d powder belongs to Group A or C d* 101 p p AB p AB If : d * d * powder belongs to Group B or D p p AB Demarcation between Group B and D powders Umf For Group D powders UB mf Demarcation Demarcation between Group C and A powders (Molerus, 1982) 3 10(pfpH )dg / F 0.01 FH is the adhesion force determined experimentally. -8 (FH=8.76x10 N for glass beads and FCC catalysts) Demarcation between Group A and B powders U D mb 10000 1 B (spoutable) (bubble readily) Umf ) 3 1000 , , (kg/m f A Demarcation between Group B and D powders - (aeratable) s C (cohesive) 100 Umf 10 100 1000 For Group D powders U Mean particle diameter, d (m) B p mf Characteristics of single bubble: Slow vs. fast bubbles (Davidson model) slow bubble fast bubble group D group A & B Simple demarcation criteria accounting for T, P effects Goosen’s classification: Grace’s classification: • C/A boundary: Ar=9.8 • A/B boundary: • A/B boundary: Ar=125 Ar=88.5 • B/D boundary: • B/D boundary: Ar=145,000 Ar=176,900 g d 3 Ar g p g p 2 Correlations for Umb Abrahamsen and Geldart (1980) U 2300 0.126 0.523 exp0.716F mb g g 45 0.8 0.934 0.934 U mf d p g p g Where F45 is the fraction of solids which are less than 45m. Flow Regimes fixed bed bubbling turbulent fast pneumatic fluidization transport homogeneous slugging solids returns solids returns solids returns gas gas gas gas gas gas gas only A powders only narrow beds at low gas velocity gas velocity Flow Regimes U range Regime Appearance and Principal Features Fixed Bed Particles are quiescent; gas flows through interstices 0 U Umf Particulate Bed expands smoothly in a homogeneous manner; Umf U Umb regime top surface well defined, small-scale particle motion Bubbling Gas voids form near distributor, coalesce and grow; Umb U Ums regime rise to surface and break through Slug flow Bubble size approaches column cross-section. Top Ums U Uc regime surface rises and collapses with regular frequency. Turbulent Pressure fluctuations gradually decrease until fluidization turbulent fluidization flow regime is reached. Uc U Uk flow regime (Turbulent Small gas voids and particle clusters dart to and fro. Uk U Utr Regime) Top surface is difficult to distinguish. U U Fast No upper surface to bed, particles transported out tr Fluidization the top in clusters and must be replaced. U U Pneumatic No bed. All particles fed in are transported out the tr conveying top as a lean phase. Regime Transition Flow Chart D <0.66D Bubble free Umb B,max fluidization Fixed Bed Bubbling Slugging Turbulent U fluidization fluidization fluidization se U mf Ums Uc Bubble-free Dense D <0.66D Vmb B,max transport Fixed Bed Turbulent Core-annular Dense-phase Bubbly Slug flow Flow Dilute-phase transport transport transport transport flow V V V Vmp mf ms c VCA Homogeneous Increasing gas velocity Dilute-phase Gs = constant flow Bi & Grace, 1995 Unifying Fluidization Regime Diagram Fluidized bed lay-out laterally staged bed circulating bed vertically turbulent staged bed bed bubbling bed spouted bed twin bed riser downer floating bed Static Head of Solids When acceleration, friction and gas head are negligible L weight of particles upthrust on particles P bed cross sectional area DP P 1 g L p Time-averaged pressure measurements •Most industrial and pilot plant fluidized beds have pressure taps. •There should be at least 2 or 3 taps within the fluidized bed. •Pressure measurement from plenum chamber must be from where it will not be affected by either the gas expansion or the contraction •For hot units, back flushed taps are often used. (gas flowrate must be regulated) For other measurement techniques in fluidized beds: van Ommen & Mudde, Int J Chem Reactor Eng 6 (2008) R3 Fluidization Curve Group A particles Group B particles Fluidized narrow psd Non-bubbling Fluidized Bubbling region Fluidized Region Packed Bubbling Packed bed Region bed P/L P/L Wide psd Umf Umb Umf Ucf U, m/s U, m/s Minimum Fluidization The frictional pressure drop at the point of minimum fluidization equalizes the bed mass per unit of cross-sectional area.
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