Drying

Drying Process Overview

• Drying involves heat and mass transfer operations to remove moisture from materials.
• Heat is needed to provide latent heat for moisture vaporization.
• Mass transfer describes the diffusion of water through the material to its surface and then into the air.

Important Temperature Concepts

• Wet-bulb temperature indicates the balance of heat transfer between air and liquid, represented by a thermometer covered with a water-saturated wick.
• Dry-bulb temperature measures the actual air temperature using a standard thermometer.

Rate of Evaporation

• The rate of evaporation is directly proportional to the heat transfer:
  • Formula: ( \frac{dW}{d\Theta} = \frac{q}{\lambda} )
  • ( dW/d\Theta ): rate of evaporation
  • ( q ): overall heat transfer rate
  • ( \lambda ): latent heat of vaporization.

Rate of Diffusion

• The rate of moisture diffusion into the air is expressed as:
  • Formula: ( \frac{dW}{d\Theta} = k A(H_s - H_g) )
  • ( k ): mass transfer coefficient
  • ( A ): area of evaporating surface
  • ( H_s ) and ( H_g ): humidity values.

Factors Affecting Drying Rate

• Initial adjustments equalize heat and mass transfer rates.
• Heat transfer methods include convection, radiation, and conduction.
• Drying can be accelerated by:
  • Increasing air flow and inlet air temperature for better convection.
  • Enhancing radiation and conduction by reducing material thickness and maximizing surface area.

Drying Rate Stages

• B: Constant moisture film on surface maintains temperature stability.
• C: Critical moisture content reached, dry spots appear; evaporation matches diffusion.
• C-D: First falling rate period, drying rate decreases as more dry spots emerge.
• D: Complete evaporation of surface film; drying predominantly relies on diffusion.
• D-E: Second falling rate period, where the drying rate falls rapidly.
• E: Equilibrium moisture level reached; drying rate becomes zero, indicating optimal stopping point.

Classification of Solids by Drying Behavior

• Crystalline solids have water in accessible pores, enabling easier drying.
• Amorphous solids trap moisture in molecular structures, making them harder to dry.

Fluidized-Bed Drying Systems

• Solids are suspended in gas streams, mimicking liquid behavior for efficient drying.
• This technique ensures uniform conditions for temperature, composition, and particle size distribution.
• Granules are kept partially moist to prevent cracking.

Advantages of Fluidized-Bed Dryers

• High drying rates achieved through effective heat and mass transfer.
• Consistent drying rate as all particle surfaces receive uniform heat.
• Produces spherical, free-flowing particles, minimizing aggregation and color migration.

Pneumatic Drying Systems

• Spray dryers handle fluid materials like solutions and thin pastes.
• Liquid droplets are fed into hot gas streams; quick evaporation forms a solid shell.
• The diffusion rate of liquid is slower than the rate of heat transfer.

Learn about wet-bulb temperature, which is the equilibrium temperature between air and liquid during the drying process. Understand how heat is transferred between air and liquid based on temperature differences. Discover how the difference in vapor pressure affects the evaporation rate of liquid materials in the drying process.