Mass Transfer Introduction

Mass Transfer

• Mass transfer occurs due to concentration differences between two or more components in a mixture.
• It involves the transfer of material from one homogeneous phase to another.

Mass Transfer Operations

• Separation techniques: distillation, gas absorption, liquid extraction, drying, leaching, etc.
• Material transfer occurs due to molecular diffusion and turbulence.
• Rate of mass transfer is evaluated by deviation from equilibrium concentration.

Types of Mass Transfer Operations

• One direction: gas absorption
• Opposite direction: distillation
• With simultaneous heat transfer: drying
• With simultaneous chemical reactions: absorption of carbon dioxide in an aqueous solution of KOH

Classification of Mass Transfer Operations

Phase in Contact

• Liquid-vapour (gas): distillation
• Liquid-gas: gas absorption, stripping, humidification
• Liquid-solid: leaching, crystallization
• Liquid-liquid: extraction
• Solid-gas: adsorption

Diffusion

• Movement of individual molecules from higher concentration to lower concentration.
• Types of diffusion:
  • Molecular diffusion: from random molecular motion.
  • Eddy diffusion: from turbulent eddies.

Convective Mass Transfer

• Occurs under the influence of motion in a fluid medium.
• Similar to convective heat transfer.

Fick's Law of Diffusion

• Molar flux due to diffusion is proportional to the concentration gradient.
• Rate of molecular diffusion can be expressed as a molar flux for the component diffusing.
• Diffusion occurs in the direction of decreasing concentration.

Diffusivity

• Unit of diffusivity is m2/s.
• Range of diffusivity values:
  • Solids: 10-14 to 10-10 m2/s.
  • Liquids: 10-10 to 10-9 m2/s.
  • Gases: 5 × 10-6 to 10-5 m2/s.

Other Types of Diffusivity

• Momentum diffusivity: same as kinematic viscosity, unit is m2/s.
• Thermal diffusivity: same unit as momentum diffusivity and mass diffusivity.

Newton's Law of Viscosity, Fourier's Law of Heat Conduction, and Fick's Law of Diffusion

• Analogous laws for momentum, heat, and mass transfer.

Flux and Concentration

• Flux can be expressed in terms of concentration and velocity.
• Formula: Flux = (concentration) × (concentration) × (velocity).

Mole Fraction and Molar Concentration

• Mole fraction: ratio of individual number of moles to total number of moles.
• Molar concentration: moles per unit volume.

Mass Concentration and Molar Concentration

• Mass concentration: equivalent to mass density, ratio of mass to volume.
• Molar concentration: ratio of number of moles to volume.

Questions and Answers

• 1. Unit of molar flux in SI system is kmol/m2s.
• 2. Unit of diffusion coefficient is m2/s.
• 3. In molecular diffusion, according to Fick's law, the ratio of mass flux to concentration gradient is the diffusion coefficient.