Granulation

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Information about Granulation
Technology

Published on January 18, 2009

Author: elfoxy99

Source: slideshare.net

Description

Topics on Granulation

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. Che5700 陶瓷粉末處理

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.

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

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

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

Growth by layering through contact and adhesion; or by nuclei agglomeration;

Rubbing between granules  make granules surface smooth

Spray granulation  uniformity closely related to liquid content; Hardness: mostly related to binder (and particle characteristics)

Spray Granulation Power demand = resistance to flow Liquid requirement is higher when specific surface area is high; Common liquid requirement: 20-36%

Power demand = resistance to flow

Liquid requirement is higher when specific surface area is high;

Common liquid requirement: 20-36%

Spray Granulation 2 There is a critical liquid content for each process; Granule may need to be dried before use;

There is a critical liquid content for each process;

Granule may need to be dried before use;

Spray Drying Main method of granulation: produce spherical particles (~20  m), 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 ~10 4 /sec Che5700 陶瓷粉末處理

Main method of granulation: produce spherical particles (~20  m), 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 ~10 4 /sec

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;

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;

Taken from TA Ring, 1996 Droplet/particle: mean residence time ~ 30 sec Three basic steps: (a)atomization, (b)droplet drying, (c) gas-droplet mixing

Spray dried samples: donut particle, temperature rise too fast, surface dried (sealed), vaporization of internal liquid  pores (viscous binder fluid may flow toward inside)

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

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

Atomization 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; Che5700 陶瓷粉末處理 u 1 : interfacial velocity between gas and liquid; D d max = 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;

Droplet Size Depending on jet breakup mechanism  different equations to estimate droplet size Rayleigh breakup mechanism  D d = 1.89 D j ; for high viscous liquid, then Dd = 1.89 Dj (1 + 3  1 /(  1 D j g) 1/2 ) 1/6 ; (Dj = jet diameter;) Gas / liquid interfacial velocity (u 1 ) increase, breakup mechanism more complex; critical Weber number  decide droplet size N v = dimension-less viscosity;

Depending on jet breakup mechanism  different equations to estimate droplet size

Rayleigh breakup mechanism  D d = 1.89 D j ; for high viscous liquid, then Dd = 1.89 Dj (1 + 3  1 /(  1 D j g) 1/2 ) 1/6 ; (Dj = jet diameter;)

Gas / liquid interfacial velocity (u 1 ) increase, breakup mechanism more complex; critical Weber number  decide droplet size

 

Droplet Drying In theory, ideal drying (no crust), size of particle and size droplet relations (as follows): C d & C p : 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

In theory, ideal drying (no crust), size of particle and size droplet relations (as follows): C d & C p : 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

Gas-droplet mixing: maybe co-current or counter-current or even cross-current flow; decide contact time and heat and mass transfer effects. ----------------

Gas-droplet mixing: maybe co-current or counter-current or even cross-current flow; decide contact time and heat and mass transfer effects.

Characteristics of dried particles: moisture adsorption; flow time; fill density; tap density/fill density ratio etc. Che5700 陶瓷粉末處理

Characteristics of dried particles: moisture adsorption; flow time; fill density; tap density/fill density ratio etc.

Classification Che5700 陶瓷粉末處理

Principle and Techniques 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). 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).

Taken from TA Ring, 1996; can add some baffles, to separate large particles

Size Selectivity 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 Che5700 陶瓷粉末處理

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

取自 TA Ring 1996; Cut size; bypass; Sharpness index b-b’ curve: normal case

Recovery & Yield Che5700 陶瓷粉末處理 Classifier performance: recovery R & yield Y If 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

Classifier performance: recovery R & yield Y

If 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

Che5700 陶瓷粉末處理

phase transformation during calcination Gibbsite, Bayerite Al(OH) 3 ; Boehmite AlOOH Diaspore α-AlOOH

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