Mass Transfer Introduction
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Questions and Answers

What is mass transfer?

  • The transfer of momentum from one phase to another
  • The transfer of energy from one phase to another
  • The transfer of material from one homogeneous phase to another due to concentration difference (correct)
  • The transfer of heat from one phase to another

    • What is the driving force for transfer in mass transfer operations?

  • Volume difference
  • Temperature difference
  • Pressure difference
  • Concentration difference (correct)

    • What is achieved after a sufficiently long time of phase contact?

  • Equilibrium between phases (correct)
  • Steady state condition
  • Unstable condition
  • None of the above

    • What causes material transfer in mass transfer operations?

    <p>Combined effect of molecular diffusion and turbulence</p> Signup and view all the answers

    How is the rate of mass transfer evaluated?

    <p>By deviation from equilibrium concentration</p> Signup and view all the answers

    In which direction can mass transfer occur?

    <p>In one direction or in the opposite direction</p> Signup and view all the answers

    What type of mass transfer operation involves the absorption of carbon dioxide in an aqueous solution of KOH?

    <p>Gas absorption</p> Signup and view all the answers

    Which phase in contact is associated with crystallization?

    <p>Liquid-solid</p> Signup and view all the answers

    What is the term for the movement of individual molecules from an area of higher concentration to an area of lower concentration?

    <p>Diffusion</p> Signup and view all the answers

    What is the term for diffusion resulting from turbulent eddies?

    <p>Eddy diffusion</p> Signup and view all the answers

    What type of mass transfer occurs under the influence of motion in a fluid medium?

    <p>Convection</p> Signup and view all the answers

    What is Fick's Law of Diffusion related to?

    <p>Diffusion</p> Signup and view all the answers

    What is the direction of molecular diffusion?

    <p>From higher concentration to lower concentration</p> Signup and view all the answers

    Which of the following is an example of a liquid-gas mass transfer operation?

    <p>Gas absorption</p> Signup and view all the answers

    What is the unit of diffusivity?

    <p>m2/s</p> Signup and view all the answers

    What is the range of diffusivity values for gases?

    <p>5 Х 10-6 to 10-5 m2/s</p> Signup and view all the answers

    What is the unit of momentum diffusivity?

    <p>m2/s</p> Signup and view all the answers

    What is the formula for flux in terms of concentration and velocity?

    <p>Flux = (concentration) × (velocity)</p> Signup and view all the answers

    What is the definition of mole fraction?

    <p>The ratio of individual number moles to total number of moles</p> Signup and view all the answers

    What is the unit of molar flux in SI system?

    <p>kmol/m2s</p> Signup and view all the answers

    What is the unit of diffusion coefficient?

    <p>m2/s</p> Signup and view all the answers

    What is the ratio of mass flux to concentration gradient according to Fick's law?

    <p>diffusion coefficient</p> Signup and view all the answers

    Study Notes

    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.
      1. Unit of diffusion coefficient is m2/s.
      1. In molecular diffusion, according to Fick's law, the ratio of mass flux to concentration gradient is the diffusion coefficient.

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    Learn about the fundamentals of mass transfer, including the process of transfer of mass due to concentration differences.

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