Mass transfer is the net movement of a component in a mixture from one location to another due to a concentration gradient. It occurs through mechanisms such as diffusion, where molecules move through random motion, and convection, where bulk fluid motion enhances diffusion. Mass transfer plays a key role in chemical reactions by bringing reactants together. It is quantified by mass transfer coefficients and can be modeled by theories like two film theory. Mass transfer is important in many industrial processes like distillation, drying, and absorption.

2. Lecture 7
Mass transfer

3. What is mass transfer?
 Mass transfer is the net movement of a component in mixture from one location to another where
the component exist at different concentration.
 Mass transfer is mass in transit due to a species concentration gradient in a mixture.
 Mass transfer occurs in many processes, such as absorption, evaporation, drying, precipitation, membrane
filtration, and distillation.
 In food processing, we often create
conditions to encourage chemical reactions
that produce desirable end-products in the
most efficient manner.
 Mass transfer plays a key role in creating
favorable conditions for reactants to
physically come together, allowing a
reaction to occur.

4. Example of mass transfer operations
 Distillation, evaporation, Extraction processes as indicates in figure below.

5. Example of mass transfer operations
 Drying, absorption, adsorption and membrane filtration as indicated below.

6. Mechanisms of mass transfer
 There are basically two modes of mass transfer:
 Mass Transfer by Diffusion - the transport of mass by random molecular
motion in sluggish or laminar flowing fluids.
 Mass transfer by diffusion occurs due to:
 Concentration gradient,
 Temperature gradient, and
 Hydrostatic pressure difference.
 Convective Mass Transfer - the rate of molecular
diffusion of mass can be accelerated by the bulk
motion of the fluid.

7. Where does Mass transfer can occur?
 Mass transfer can also occur in liquids and solids as well as in
gases.

8. Diffusion
 Diffusion is the process of movement of molecules under a concentration gradient.
 When transfer of the component from one phase to the other occurs due to
concentration gradient, the phenomenon is called diffusion. The diffusion stops
once equilibrium is attained.
 There are two types of diffusion,
 Molecular diffusion
 Eddy diffusion or Turbulent diffusion

9. Molecular diffusion
 Molecular diffusion can be defined as the movement of individual molecules in a
highly zigzag manner through another fluid.
 The movement of molecules is imagined to be in a straight line at uniform velocity.
However, the velocity and direction change when they are bombarded with other
molecules.
 Molecular diffusion can also be called as Random-Walk process since the molecular
movement is in a random path.
 If a colored solution is introduced in a pool
of water, it begins slowly to diffuse into the
entire liquid which is termed as molecular
diffusion.

10. Eddy diffusion or Turbulent diffusion
 Eddy diffusion is the movement of a molecule with the help of stirring (external
force); therefore, it does not require the actual migration of the molecule.
 To enhance its rate of mixing, a mechanical agitation is provided and this will
cause a turbulent motion. This method of mass transfer is known as eddy
or turbulent or convective diffusion.

11. Diffusivity or diffusion coefficient
 Diffusion mainly depends upon the concentration gradient. In other words, the
driving force for diffusion to occur is concentration gradient.
 This mass transfer phenomenon is defined by Fick’s first law of diffusion, which
states that molar flux is directly proportional to the concentration gradient.
 Mathematically, it can be expressed as:
 Mass transfer involves both mass diffusion occurring at a molecular scale and bulk transport of mass
due to convection flow.

12. Mass transfer coefficient
 The mass transfer coefficient is a diffusion rate constant that relates the mass transfer rate,
mass transfer area, and concentration change as a driving force.
 Note that:
Rate of mass transfer ∝ concentration gradient.
Rate of mass transfer ∝ Area of contact between the phases.

13. Mass transfer coefficient
 The specific definition and calculation of mass transfer coefficients can vary depending
on the type of mass transfer process and the system involved.
 Here are some common scenarios:
 Gas – liquid mas transfer
 Liquid – liquid mass transfer
 Solid – gas mass transfer
 Packed bed mass transfer
 Determining mass transfer coefficients often involves experimental measurements or complex
simulations, as they depend on various factors, including the physical properties of the
substances, fluid dynamics, and the specific geometries of the systems.
 Accurate values of mass transfer coefficients are essential for designing and optimizing
chemical/food processes and equipment.

14. Interphase mass transfer
 Interphase mass transfer is governed by Fick's law of
diffusion, which relates the rate of mass transfer to
the concentration gradient and the mass diffusivity of
the species.
 Different theories involved:
 Two film theory
 Penetration theory
 Surface renewal theory
 Interphase mass transfer refers to the transport of a particular species across the
interface or boundary between two phases.
 Examples: distillation, liquid – liquid extraction

15. End of lecture 7