AN INTRODUCTION TO FLUIDIZATION BY MILAN CARSKY UNIVERSITY OF KWAZULU-NATAL

Transcription

1 AN INTRODUCTION TO FLUIDIZATION BY MILAN CARSKY UNIVERSITY OF KWAZULU-NATAL

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3 AN INTRODUCTION TO FLUIDIZATION SUMMARY Principle of fluidization (gas-solid fluidization, liquid-solid fluidization, properties of fluidized beds) Components of a fluidized bed unit Fluidized bed materials (size, density, shape, strength; definition of particle size; bulk and material density); classification of materials according to their fluidization behaviour (Geldart s diagram) Minimum fluidization velocity (experimental determination and literature predictions) and fluidized bed hydrodynamics Special fluidized beds (spouted bed, vibro-fluidized beds) Fluidization regimes and transitions Use of fluidized bed technology (details later in the course): Fluidized bed (catalytic) reactors Ore roasting Fluidized bed combustion (for power generation) Drying of materials which are granular, free flowing, not agglomerating, not fragile, not vulnerable to oxidation, neither the product nor the vapour released is toxic or flammable 1. Principle of fluidization By using gas (or liquid) flowing upwards through a layer of a particulate material supported on a distributor, at certain fluid upwards velocity the particles start to move. Once that occurs we have reached onset of fluidization and the fluid velocity at this point is called minimum fluidization velocity (u mf ). 7

4 Properties of fluidized beds: Liquid-like behaviour Increasing fluid velocity 8

5 Hydrostatic pressure Particles flow on an Viscosity of a fluidized bed inclined grid Rapid mixing of solids Tracer at the top Time=0 s Time = 3 s (top) and 5 s (bottom) 9

6 Average tracer concentration = 20% Uniform conditions and slow response to changing operating conditions Fluidized bed combustion of coal. Temperature profile taken in different times. T1-T5 temperatures above the bed level, T6-T8 temperatures in the fluidized bed, T9 temperature of the stagnant layer on the grid. High mass and heat transfer rates Pressure drop is independent of the fluid velocity 10

7 Erosion of internal parts because of moving particles Entrainment and attrition of the bed material Liquid-solid fluidization Smooth (particulate) fluidization. Uniform and large expansion of the bed without presence of bubbles. Mixing by diffusion of particles. Gas-solid fluidization Aggregative fluidization. Not a uniform concentration of particles in the bed. Presence of gas bubbles (voids) in the bed is the main factor of particle mixing. The regimes may vary from bubbling to slugging and turbulent fluidized bed. 11

8 2. Components of a fluidized bed unit Typical industrial fluidized bed unit 12

9 Typical multi-purpose laboratory fluidize bed unit. 1- blower, 2- gas tank, 3- rupture disc, 4- filter, 5-dryer, 6-humidification column, 7- mist eliminator, 8-water circulation, 9- humidity indicator, 10- pressure regulator, 11- rotameters, 12- control valves, 13- windbox, 14- fluidized bed. Gas distributors Function of a distributor: To support bed material (and to prevent leaking of a bed material to the plenum chamber below) To distribute gas uniformly into the fluidized bed Minimum pressure drop 10-30% of the bed pressure drop Perforated (single, double) plates Simple and cheap distributors, but difficult to achieve uniform gas distribution, prone to weepage of solids, require mechanical support for larger diameters. 13

10 Porous plate and Sandwich type distributors Restricted to small-scale (laboratory) units. They prevent bed material from leaking through the distributor and usually ensure uniform gas distribution. In case of sandwich type distributors the operation is restricted to ambient temperature (filter cloth between two perforated plates). Cap type distributors Limited weeping, good gas distribution but creating a stagnant region underneath, more expensive, difficult to clean. Spargers Limited weeping, good gas distribution but creating a stagnant region underneath. 14

11 Slot-type distributors May achieve uniform gas distribution and promote particle mixing, no stagnant regions, difficult to construct, require mechanical support for larger diameters. Conical distributors Promotes particle mixing, no stagnant regions, difficult to construct. Cyclones, filters Function: To separate solids from the gas at the exit (and to return them into the bed). 15

12 Bag filter Feeders (variety of constructions) Laboratory screw feeder 16

13 3. Fluidized bed materials Particle size-characterisation a) Sieve size (d p ) The width of the square aperture in a sieve. b) Surface diameter (d s ) The diameter of a sphere having the same surface area as the particle. c) Volume diameter (d v ) The diameter of a sphere having the same volume as the particle. d) Surface/volume diameter (d sv ) The diameter of a sphere having the same surface to volume ratio as the particle. e) Median particle diameter (d p50 ) Particle size corresponding to the 50% value on particle size vs. wt% cumulative undersize plot. Particle shape Sphericity: For spherical particles: 1 For non-spherical particles: 0 1 d p = d p, meas d p = mean diameter of non-spherical particles d p, meas = measured mean particle size 17

14 Density Particle density Bulk density If a non-porous particle, particle density = skeletal density Bed voidage Particle strength: Impacts between particles and vessel internals lead to particle attrition (and entrainment) and/or abrasion of the equipment Classification of materials according to their fluidization behaviour (Geldart s diagram) 18

15 Materials A are easy to fluidize. A typical example is fluidized bed cracking catalyst. These particles can be fluidized in a limited range of velocities in a regime similar to liquid fluidization (i.e. smooth fluidization) without the presence of gas bubbles in the bed. Materials B are also very common for fluidization but unlike group A materials bubbles are always present in the bed (aggregative fluidization). Typical example of materials B is sand. Materials C are very fine particles (eg. flour) which are difficult and often impossible to fluidize because of large surface (cohesive) forces holding particles together. Materials D consist of large particles. Their fluidization is uneven, vigorous, fountainlike, results in equipment shaking vigorously. 19

16 4. Minimum fluidization velocity and fluidized bed hydrodynamics Experimental determination of minimum fluidization velocity Ergun equation for a pressure drop in a static bed: P h 2 ( 1 ε ) µ U ( 1 ε ) = ε d ε GU p d p where P is pressure drop (Pa), h height of the bed (m), ε bed voidage, U gas velocity (m/s), and G gas mass velocity (kg/(m 2.s). 20

17 Pressure drop for a fluidized bed: P = ( 1 ε )( ρ ρ p f ) hg Correlations for minimum fluidization velocity (examples) Broadhurst and Becker: R e m f 2 m f A r = U = = D A r ρ A r ( ) ρ 3 p f p f p ρ ( ρ ρ ) g µ R e D p ρ m f f µ f Geldart: U mf 0.5 [( ) 33.7] µ = Ar ρ d f p Kunii and Levenspiel: U mf = Ar ( ρ ρ ) p f Ar A correlation given below (see table below for values for C 1 and C 2 ) was developed by multiple researchers: 2 Remf = C1 + C2 Ar C1 Re mf = mf ρ g where: Re mf Reynolds number at the onset of fluidization Ar Archimedes number ρ p Particle density (kg/m 3 ) ρ f or ρ g Fluid density (kg/m 3 ) D p or d p Particle size (diameter) (m) g Acceleration due to gravity (9.81 m/s 2 ) µ Fluid viscosity (Pa.s) u mf Minimum fluidization velocity (m/s) d p U µ 21

18 Reference C 1 C 2 Wen&Yu Richardson Saxena&Vogel Babu et al Grace Chitester et al Fluidized bed hydrodynamics A = static bed, B = bed at minimum fluidization conditions, C = fluidized bed u A < u B < u c h 0 (A) < h(b) < h(c) p(a) < p(b) = p(c) 22

19 Pressure drop increases with gas velocity in the static bed; is constant in the fluidized bed and decreases as the particles are entrained from the bed. Bed height is constant for the static bed and increases in the fluidized bed to the column height h. Bed voidage is constant for the static bed and increases to 1 in the fluidized bed. Particle concentration is constant for the static bed and decreases to zero in the fluidized bed. Packed (static) bed Fluidized bed 23

20 Onset of fluidization for a mixture A = Fine (light) particles start to fluidize B = Minimum fluidization velocity determined experimentally for a mixture C = All bed material fluidizes Mixing vs. segregation for a binary mixture of particles in a fluidized bed 5. Special fluidized beds Vibro-fluidized beds Used for materials which are difficult to fluidize otherwise (namely materials C ). Mechanical vibrations imposed on fluidized bed units. Vibrations either vertical (mostly) or horizontal. 24

21 Support for the vibrated fluidized bed unit with motors. Vibro-fluidized bed unit 25

22 Spouted beds A distributor with a single aperture. The material starts moving even before reaching its minimum fluidization velocity. Suitable for operations with materials which are difficult to fluidize otherwise. Intensive particle circulation. 6. Fluidization regimes and transitions A... Static bed. Gas velocity is below minimum fluidization velocity. B... Particulate regime. Occurs between minimum fluidization and minimum bubbling velocities for Geldart group A materials. Fluidized bed without bubbles. C... Bubbling fluidized bed. Distinct bubbles in the bed. Occurs at velocities higher than minimum bubbling velocity for Geldart group A materials; and higher than minimum fluidization velocity for Geldart group B and D materials. D... Turbulent bed. No distinct bubbles. Highly turbulent mixing of solids and gas. Large particle entrainment. E... Slugging bed. Occurs in small diameter column where a bubble diameter reaches the column diameter. High pressure fluctuations, poor particle-solids contact. Unwanted regime. 26

23 Development of slugs and their different types. a) Axisymmetric slug, b) asymmetric slug, c) plug slug. Correlations for bubble and slug velocity: F... Channelling in the bed. Typical for Geldart group C materials. Poor particle-solids contact. Unwanted regime 27

24 Transitions between regimes: 7. Use of fluidized bed technology (details later in the course): a. Fluidized bed (catalytic) reactors b. Ore roasting c. Fluidized bed combustion (for power generation) d. Drying of materials which are granular, free flowing, not agglomerating, not fragile, not vulnerable to oxidation, neither the product nor the vapour released is toxic or flammable Fluidized bed dryer 28