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Drying

This guide covers essential drying methods for wet, porous solids such as granules, focusing on achieving optimal moisture content to prevent degradation while ensuring excellent powder flow and tablet properties. Key concepts such as psychrometry, wet and dry bulb temperatures, relative humidity, and various types of dryers, including static and fluidized bed dryers, are discussed. We also present drying equations for tray/truck and spray dryers, along with a practical example to illustrate the drying process of wet granules under specific conditions.

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Drying

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  1. Drying • We are primarily concerned with drying wet, porous solids (granules). • Important to ensure proper moisture content in the granules: • low enough to prevent product degradation and ensure good powder flow, but high enough to provide good tablet properties CHEE 440

  2. Definitions • psychrometry • determination of the vapor concentration and carrying capacity of drying gas • wet bulb T • dry bulb T • relative humidity (RH) • moisture content CHEE 440

  3. Psychrometric Chart • Determine the relative humidity and moisture content in the air if the wet bulb T is 35 ºC and the dry bulb T is 50 ºC. CHEE 440

  4. Types of Dryers CHEE 440

  5. Static Dryers Tray/truck driers CHEE 440

  6. Drying behavior for static bed CHEE 440

  7. Fluidized Bed CHEE 440

  8. Spray Dryer CHEE 440

  9. Drying Equations • Tray/Truck Dryers • constant rate drying stage only • parallel flow over a static bed • perpendicular flow through a bed CHEE 440

  10. Drying Equations • Fluidized beds and spray driers • use correlation for flow past an individual spherical particle CHEE 440

  11. 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 60ºC(dry bulb) and a wet bulb temp of 25ºC 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 %. CHEE 440

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