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Topics on Granulation

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  1. 1. 造粒 Granulation <ul><li>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. </li></ul><ul><li>Principal methods: spray drying, extruding, simple pressing, mixing + perforated plate, etc; </li></ul><ul><li>Characteristics: critical range of liquid content for granulation (for each particle system); it affects granulate size, distribution, porosity; fine particles need more liquid. </li></ul>Che5700 陶瓷粉末處理
  2. 2. Direct Granulation Che5700 陶瓷粉末處理 <ul><li>Sometimes referred as “pelletizing” process; e.g. pressing, extrusion, spray granulation etc. </li></ul><ul><li>used to produce alumina, ferrite, clays, tile bodies, porcelain bodies, conventional refractory compositions, catalyst support, and feed materials for glass or metal refining; </li></ul><ul><li>Granules may not be spherical, could be cylindrical; </li></ul><ul><li>Spray granulation: spray (may contain binder) and stir to make pellets </li></ul>
  3. 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 <ul><li>Growth by layering through contact and adhesion; or by nuclei agglomeration; </li></ul><ul><li>Rubbing between granules  make granules surface smooth </li></ul>
  4. 4. Spray granulation  uniformity closely related to liquid content; Hardness: mostly related to binder (and particle characteristics)
  5. 5. Spray Granulation <ul><li>Power demand = resistance to flow </li></ul><ul><li>Liquid requirement is higher when specific surface area is high; </li></ul><ul><li>Common liquid requirement: 20-36% </li></ul>
  6. 6. Spray Granulation 2 <ul><li>There is a critical liquid content for each process; </li></ul><ul><li>Granule may need to be dried before use; </li></ul>
  7. 7. Spray Drying <ul><li>Main method of granulation: produce spherical particles (~20  m), high productivity (e.g. ~ 10-100 kg/h); suitable for subsequent pressing process. </li></ul><ul><li>Use hot air (co-current or counter-current flow) to dry flowing solids </li></ul><ul><li>Droplet size ~ product size </li></ul><ul><li>Slurry viscosity: important operation variable, should be shear thinning , shear rate at nozzle ~10 4 /sec </li></ul>Che5700 陶瓷粉末處理
  8. 8. Spray Drying (2) <ul><li>Atomization: large pressure drop at nozzle, significant wear; possibility of blockage; other variables: surface tension, feed rate </li></ul><ul><li>Drying rate: gas temperature, contact time (usually less than 30 sec); avoid sticking to walls; </li></ul><ul><li>Due to high temperature: should be aware of possible loss of material along with evaporation; polymer additives: possible cracking or decomposition; </li></ul>
  9. 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. 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. 11. Spraying Drying (3) <ul><li>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) </li></ul><ul><li>If necessary, add anti-foam agent; </li></ul><ul><li>wall deposit problem </li></ul><ul><li>two-fluid nozzle: to lower pressure drop and to get smaller particles </li></ul><ul><li>Mass and heat transfer during drying, relative rate  may get dry surface with some internal liquid </li></ul>Che5700 陶瓷粉末處理
  12. 12. Atomization <ul><li>Some common techniques: high pressure nozzle, two-fluid nozzle, and high speed centrifugal disc; often need to remove large particles from slurry </li></ul><ul><li>Energy efficiency often low, also about 1% for new surface formation (breakup of steams into droplets), others for heating up the system; </li></ul><ul><li>Jet breakup mechanism: Rayleigh instability, one dimensionless parameter, Weber number; = aerodynamic force to surface tension force; </li></ul>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. 13. Droplet Size <ul><li>Depending on jet breakup mechanism  different equations to estimate droplet size </li></ul><ul><li>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;) </li></ul><ul><li>Gas / liquid interfacial velocity (u 1 ) increase, breakup mechanism more complex; critical Weber number  decide droplet size </li></ul>N v = dimension-less viscosity;
  14. 15. Droplet Drying <ul><li>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) </li></ul><ul><li>During solvent evaporation: temperature should decrease; </li></ul><ul><li>Solvent evaporation  concentration increase  precipitation to get solid particles </li></ul><ul><li>If crust formation  hollow particles </li></ul>
  15. 16. <ul><li>Gas-droplet mixing: maybe co-current or counter-current or even cross-current flow; decide contact time and heat and mass transfer effects. </li></ul>----------------
  16. 17. <ul><li>Characteristics of dried particles: moisture adsorption; flow time; fill density; tap density/fill density ratio etc. </li></ul>Che5700 陶瓷粉末處理
  17. 18. Classification Che5700 陶瓷粉末處理
  18. 19. Principle and Techniques <ul><li>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; </li></ul><ul><li>Sometimes: feed is separated into two streams (not many streams). </li></ul>Che5700 陶瓷粉末處理
  19. 20. Taken from TA Ring, 1996; can add some baffles, to separate large particles
  20. 21. Size Selectivity <ul><li>To evaluate performance: size selectivity: SS(d), subscript c for coarse; f for fine; F(d) = cumulative distribution </li></ul><ul><li>Sharpness index s: ratio of size of particle entering coarse section at probability of 0.25 and 0.75 </li></ul><ul><li>Cut size: particle over this size all enter coarse section; in reality not so ideal </li></ul><ul><li>Apparent bypass a: feed directly enter the coarse section </li></ul>Che5700 陶瓷粉末處理
  21. 22. 取自 TA Ring 1996; Cut size; bypass; Sharpness index b-b’ curve: normal case
  22. 23. Recovery & Yield Che5700 陶瓷粉末處理 <ul><li>Classifier performance: recovery R & yield Y </li></ul><ul><li>If fines are the product: following equation (if coarse is the product, one can write a corresponding equation) </li></ul><ul><li>Classifier efficiency: E(d) = Rf(d) – Rc(d); difference between fine and coarse streams </li></ul>
  23. 24. Che5700 陶瓷粉末處理
  24. 25. phase transformation during calcination Gibbsite, Bayerite Al(OH) 3 ; Boehmite AlOOH Diaspore α-AlOOH