DRYING

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DRYING

• we are primarily concerned with drying wet porous
  solids (granules)
• important in ensuring proper moisture content
• low enough to prevent product degradation and
  ensure good powder flow, but high enough to
  provide good tablet properties

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• General process

heated air
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• drying is a result of simultaneous heat and mass
  transfer
• mass is transferred as a result of
• diffusion
• capillary flow
• drying rate is determined by T, gas flow rate,
  humidity of gas

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definitions

• psychrometry
• determination of the vapor concentration and
  carrying capacity of drying gas
• wet bulb T
• dry bulb T
• relative humidity (RH)
• moisture content

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Psychrometric Chart
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Example

• using psychrometric chart, determine the relative
  humidity and moisture content in the air if the
  wet bulb T is 35 C and the dry bulb T is 50 C.

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Types of Dryers
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Tray/Truck Dryers

• Static - Bed
• common

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Drying Time profile
drying behavior for static bed
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Fluidized Bed
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Spray Dryer
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Drying Equations

• Tray/Truck Dryers
• constant rate drying stage only
• parallel flow over a static bed
• perpendicular flow through a bed

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Drying Equations

• Fluidized beds and spray driers
• use correlation for flow past an individual
  spherical particle

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Example

• Wet granules (density 1.5 g/cm3) are spread
  onto a screen at an amount of 10 kg/m2 of screen.
  The bed porosity is 45 and the wet granules
  have an average diameter of 300 mm. Air at
  60C(dry bulb) and a wet bulb temp of 25 C is
  passed through the bed at a velocity of 0.15 m/s.
  How long will it take to dry the granules from
  20 to 10 moisture? The critical moisture
  content of the granules is 9 .