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Topics on Granulation

Topics on Granulation

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  • 1. 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
  • 2. 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
  • 3. 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
  • 4. Spray granulation  uniformity closely related to liquid content; Hardness: mostly related to binder (and particle characteristics)
  • 5. Spray Granulation
    • Power demand = resistance to flow
    • Liquid requirement is higher when specific surface area is high;
    • Common liquid requirement: 20-36%
  • 6. Spray Granulation 2
    • There is a critical liquid content for each process;
    • Granule may need to be dried before use;
  • 7. 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
  • 8. 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;
  • 9. Taken from TA Ring, 1996 Droplet/particle: mean residence time ~ 30 sec Three basic steps: (a)atomization, (b)droplet drying, (c) gas-droplet mixing
  • 10. Spray dried samples: donut particle, temperature rise too fast, surface dried (sealed), vaporization of internal liquid  pores (viscous binder fluid may flow toward inside)
  • 11. 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
  • 12. 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
  • 13. 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;
  • 14.  
  • 15. 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
  • 16.
    • Gas-droplet mixing: maybe co-current or counter-current or even cross-current flow; decide contact time and heat and mass transfer effects.
  • 17.
    • Characteristics of dried particles: moisture adsorption; flow time; fill density; tap density/fill density ratio etc.
    Che5700 陶瓷粉末處理
  • 18. Classification Che5700 陶瓷粉末處理
  • 19. 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
  • 20. Taken from TA Ring, 1996; can add some baffles, to separate large particles
  • 21. 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
  • 22. 取自 TA Ring 1996; Cut size; bypass; Sharpness index b-b’ curve: normal case
  • 23. 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
  • 24. Che5700 陶瓷粉末處理
  • 25. phase transformation during calcination Gibbsite, Bayerite Al(OH) 3 ; Boehmite AlOOH Diaspore α-AlOOH