granulation
DESCRIPTION
Topics on GranulationTRANSCRIPT
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Che5700 陶瓷粉末處理
造粒 Granulation
To produce free flowing particles for further processing; often after powder synthesis and before forming of products, may need to add binder/ wetting agent to keep small particles together, (but not to form hard agglomerate), semi-dry granule.
Principal methods: spray drying, extruding, simple pressing, mixing + perforated plate, etc;
Characteristics: critical range of liquid content for granulation (for each particle system); it affects granulate size, distribution, porosity; fine particles need more liquid.
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Direct Granulation
Che5700 陶瓷粉末處理
Sometimes referred as “pelletizing” process; e.g. pressing, extrusion, spray granulation etc. used to produce alumina, ferrite, clays, tile bodies, porcelain bodies, conventional refractory compositions, catalyst support, and feed materials for glass or metal refining; Granules may not be spherical, could be cylindrical; Spray granulation: spray (may contain binder) and stir to make pellets
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Formation of Granule
Che5700 陶瓷粉末處理
Can be viewed as nucleation & growth process; At first, binder solution droplet touch particle nucleus; capillary force and binder flocculation provide strength Growth by layering through contact and
adhesion; or by nuclei agglomeration;Rubbing between granules make granules surface smooth
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Spray granulation uniformity closely related to liquid content;
Hardness: mostly related to binder (and particle characteristics)
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Spray Granulation
Power demand = resistance to flowLiquid requirement is higher when specific surface area is high; Common liquid requirement: 20-36%
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Spray Granulation 2
There is a critical liquid content for each process; Granule may need to be dried before use;
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Spray DryingChe5700 陶瓷粉末處理
Main method of granulation: produce spherical particles (~20m), high productivity (e.g. ~ 10-100 kg/h); suitable for subsequent pressing process.
Use hot air (co-current or counter-current flow) to dry flowing solids
Droplet size ~ product size
Slurry viscosity: important operation variable, should be shear thinning, shear rate at nozzle ~104/sec
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Spray Drying (2)
•Atomization: large pressure drop at nozzle, significant wear; possibility of blockage; other variables: surface tension, feed rate
•Drying rate: gas temperature, contact time (usually less than 30 sec); avoid sticking to walls;
• Due to high temperature: should be aware of possible loss of material along with evaporation; polymer additives: possible cracking or decomposition;
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Taken from TA Ring, 1996
Droplet/particle: mean residence time ~ 30 sec
Three basic steps: (a)atomization, (b)droplet drying, (c) gas-droplet mixing
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Spray dried samples: donut particle, temperature rise too fast, surface dried (sealed), vaporization of internal liquid pores (viscous binder fluid may flow toward inside)
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Spraying Drying (3)
Foam index: bubbles in slurry low quality of granules, use foam index to represent bubbles in slurry: foam index (%) = [T – E] 100/T; T, E = theoretical and experimental density of slurry (the latter contain bubbles)
If necessary, add anti-foam agent; wall deposit problem
two-fluid nozzle: to lower pressure drop and to get smaller particles
Mass and heat transfer during drying, relative rate may get dry surface with some internal liquid
Che5700 陶瓷粉末處理
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Atomization
Che5700 陶瓷粉末處理
g
DuN dWe
max1
21 u1: interfacial velocity
between gas and liquid; Ddmax
= at critical Weber number, largest stable size
Some common techniques: high pressure nozzle, two-fluid nozzle, and high speed centrifugal disc; often need to remove large particles from slurry
Energy efficiency often low, also about 1% for new surface formation (breakup of steams into droplets), others for heating up the system;
Jet breakup mechanism: Rayleigh instability, one dimensionless parameter, Weber number; = aerodynamic force to surface tension force;
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Droplet Size
Depending on jet breakup mechanism different equations to estimate droplet sizeRayleigh breakup mechanism Dd = 1.89 Dj; for high viscous liquid, then Dd = 1.89 Dj (1 + 31/(1 Dj g) 1/2) 1/6; (Dj = jet diameter;)Gas / liquid interfacial velocity (u1) increase, breakup mechanism more complex; critical Weber number decide droplet size
Nv = dimension-less viscosity;
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Droplet Drying
3/1)(pp
dd
d
p
C
C
D
D
In theory, ideal drying (no crust), size of particle and size droplet relations (as follows): Cd & Cp: solid content in droplet and particle; (simple material balance)
During solvent evaporation: temperature should decrease;
Solvent evaporation concentration increase precipitation to get solid particles
If crust formation hollow particles
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• Gas-droplet mixing: maybe co-current or counter-current or even cross-current flow; decide contact time and heat and mass transfer effects.
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•Characteristics of dried particles: moisture adsorption; flow time; fill density; tap density/fill density ratio etc.
Che5700 陶瓷粉末處理
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ClassificationChe5700 陶瓷粉末處理
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Principle and Techniques
Che5700 陶瓷粉末處理
Wish to separate different particles according to its size, utilize the difference between differently sized particles: e.g. size (sieve opening), motion trajectory; (hydro-cyclone), or forces related to motion; gravity, drag, centrifuge); density, shape or even surface characteristics;
Sometimes: feed is separated into two streams (not many streams).
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Taken from TA Ring, 1996; can add some baffles, to separate large particles
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Size Selectivity
Che5700 陶瓷粉末處理
)()(
)()(
dFWdFW
dFWdSS
ffcc
cc
75.0)(
25.0)(
dSS
dSS
d
ds
To evaluate performance: size selectivity: SS(d), subscript c for coarse; f for fine; F(d) = cumulative distribution
Sharpness index s: ratio of size of particle entering coarse section at probability of 0.25 and 0.75
Cut size: particle over this size all enter coarse section; in reality not so ideal
Apparent bypass a: feed directly enter the coarse section
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取自 TA Ring 1996;
Cut size; bypass; Sharpness index
b-b’ curve: normal case
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Recovery & Yield
Che5700 陶瓷粉末處理
Classifier performance: recovery R & yield YIf fines are the product: following equation (if coarse is the product, one can write a corresponding equation)Classifier efficiency: E(d) = Rf(d) – Rc(d); difference between fine and coarse streams
cf
ff
ccff
fff
WW
WY
dFWdFW
dFWdR
)()(
)()(
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Che5700 陶瓷粉末處理
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phase transformation during calcinationGibbsite, Bayerite Al(OH)3; Boehmite AlOOHDiaspore α-AlOOH