Drying and evaporation are key processes in separation, relying on heat and mass transfer to remove moisture from materials. These techniques involve complex interactions between air properties, material characteristics, and moisture types, impacting drying rates and efficiency.

Understanding equilibrium moisture content is crucial for optimizing drying processes and storage conditions. Factors like air temperature, humidity, and velocity, along with material properties, influence drying rates and the transition between constant and falling rate periods.

Fundamentals of Drying and Evaporation

Principles of drying and evaporation

• Heat and mass transfer drive moisture removal through conduction (direct contact), convection (fluid movement), and radiation (electromagnetic waves)
• Vapor pressure difference between material and surrounding air propels moisture evaporation
• Psychrometry relates air temperature and humidity affecting drying capacity (wet bulb, dry bulb temperatures)
• Fick's law of diffusion describes moisture movement within solids based on concentration gradients
• Latent heat of vaporization quantifies energy required for liquid-to-vapor phase change (2257 kJ/kg for water at 100℃)

Types of moisture in materials

• Free moisture easily removed during initial drying unbound to material structure (surface water)
• Bound moisture held in small capillaries (physically) or attached through chemical bonds (chemically)
• Equilibrium moisture content represents minimum achievable moisture level under given conditions
• Unbound moisture exceeds equilibrium moisture content removable through drying
• Moisture types impact drying process altering rates and energy requirements throughout stages

Factors affecting drying rates

• Air properties influence drying capacity and efficiency
  • Temperature increases drying rate by enhancing vapor pressure difference
  • Humidity affects air's moisture-holding capacity (lower humidity accelerates drying)
  • Velocity improves convective heat and mass transfer (faster air movement)
• Material properties affect moisture removal and heat transfer
  • Particle size impacts surface area for evaporation (smaller particles dry faster)
  • Porosity influences internal moisture movement (higher porosity facilitates faster drying)
  • Thermal conductivity affects heat transfer within material (higher conductivity improves drying)
• Critical moisture content marks transition between constant and falling rate periods
• Constant rate period characterized by steady surface evaporation dominating drying process
• Falling rate period controlled by internal moisture diffusion resulting in decreasing drying rate

Equilibrium moisture content concept

• Equilibrium moisture content (EMC) represents moisture in balance with surrounding air
• EMC influenced by relative humidity, temperature, and material composition
• Sorption isotherms graphically depict EMC vs relative humidity relationship
• Hysteresis in sorption isotherms shows difference between adsorption and desorption curves
• EMC applications include:
  1. Determining minimum achievable moisture content
  2. Optimizing drying process endpoints
  3. Designing appropriate storage conditions for dried materials (preventing moisture regain)