Crystallisation

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Crystallisation

• CHEE4020/7302

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Reasons for crystallisation

• High purity
• Facilitates drying and filtering
• Improves product appearance

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Types of Crystals

• Isometric e.g. cubic, octahedron dodecahedron
• Tetragonal e.g. tetragonal prism, dipyramids
  (square based)
• Orthorhombic e.g. dipyramids (triangular based)
• Monoclinic
• Triclinic
• Hexagonal
• Trigonal

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Batch Crystallisation equipment
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Continuous Crystallisation Equipment
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Purity of Crystals

• Simple relationship

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Depends on solubility changes
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Types of Nucleation

• Homogenous as a result of supersaturation only
• Heterogenous as a result of seeds
• Attritive as a result of breaking apart of
  existing crystals
• Secondary contact nucleation

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Rate of Nucleation

• Mass transfer limited

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Single crystal growth

• Described by
• where
• k is a surface reaction rate constant which is
  function of temperature
• k is a mass transfer coefficient which is a
  function of agitation and vessel conditions

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Characteristic length

• Ensures that all shapes can be treated the same
  mathematically
• where f is a shape factor for the different
  crystal shapes. Thus

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Scale up of crystallisation

• Difficult
• Needs many careful experiments.
• Some techniques include
• Constant tip Speed
• Constant power to volume ratio
• Constant agitation
• Minimum agitation to maintain suspension

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Reasons for drying

• Stabilise product
• Preserve activity
• Recover solvent

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Undesirable effects of drying

• Case hardening
• Chemical dehydration
• Protein denaturation

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Conduction Driers

• Tumble driers
• Tray driers
• Fraction water remaining after time (t)
• Freeze driers
• Rotary drum driers

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Adiabatic driers

• Spraydriers
• Employs atomisation and spraying of product
  solution into a heated chamber through a nozzle.
• Hot gas inside the chamber provides heat for
  evaporation of the liquid.
• Dried particles are separated from hot gases
  using cyclones.
• Expensive to purchase.
• Preferred method for heat-sensitive materials.

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Adiabatic driers

• Pneumatic Conveyor Drying
• Use a hot air stream to suspend and transport
  particles.
• Retention time of a particle in the gas stream is
  usually short (a few seconds).
• Work well when surface drying is critical.
• Exposure time is insufficient for drying large
  porous particles where water removal is
  diffusion-controlled.
• Well suited for heat-sensitive and easily
  oxidised materials

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Two different periods

• Constant rate
• Falling rate
• Total time
• Where