Mass transfer as Unit Process

1. Mass Transfer
Dr: Fatehalrahman Magbool, PhD
Assist. Professor of Pharmaceutics
Department of Pharmaceutics
Al Neelain University

2. • When a system contains two or more components whose concentrations vary
from point to point, there is a natural tendency for mass to be transferred,
minimizing the concentration differences within the system and moving it
towards equilibrium.
• The transport of one component from a region of higher concentration to that of
a lower concentration is called mass transfer.
• Mass transfer plays an important role in many pharmaceutical processes.

3. • A group of operations for separating the components of mixtures is based on the
transfer of material from one homogenous phase to another.
• These methods-covered by the term mass transfer operations include such
techniques as;
• Distillation, gas absorption, humidification, liquid extraction, adsorption, and
others.
• The driving force for transfer in these operations is a concentration gradient.
Similar to a temperature gradient provide the driving force for heat transfer.

4. • Do Not confuse mass transfer with movement of mass.
• There must be a change in concentration in order to have a mass transfer
phenomena.
• Types of Mass Transfer
A- Solid/fluid mass transfer
B- Fluid/fluid mass transfer

5. • When a crystal is immersed in a solvent in which it is soluble , the crystal is
surrounded by a stationary boundary layer of conc. solution, with the bulk of the
fluid being able to move.
• Hence, transport of the molecules of the dissolving solid will take place in two
stages. First, the molecules move through the boundary layer by molecular
diffusion, with no mechanical mixing or movement, a process that is analogous to
heat transfer by conduction.
• Once material has passed through the boundary layer, mass transfer takes place
by bulk movement of the solution, known as eddy diffusion, and analogous to
heat transfer by convection.

6. • Such a movement could be:
• a) a natural convection (temperature or density changes)
• b) forced convection (agitation).
• Hence the dissolving molecules are transported in two stages:
• a) molecular diffusion, through the boundary layer.
• b) eddy diffusion, by bulk movement of the solution.
• The rate controlling factor is the molecular diffusion through the boundary layer.

7. • Eddy diffusion will not be considered further since, in general, molecular
diffusion, is the controlling process. As would be expected, mass transfer by the
latter process can be represented in a similar manner to conduction heat
transfer, but with a concentration gradient instead of a temperature gradient.

8. • It will be recalled that the term fluid includes and vapours as well as liquids. and
the preceding discussion can refer equally to mass transfer from a solid to a gas.
• As an example, if a solid is drying in air, the vapour molecules must diffuse
through the air boundary layer to the atmosphere.
• The driving force in this case will be the partial vapour pressure gradient through
the air boundary layer.

9. • Mass transfer by molecular diffusion can be represented by an equation, similar
to conduction heat transfer in which:
W= D.A(C1. –C2) Ө
L
w = D.A(C1. –C2)
L
Where; W = weight of solute diffusing; w = weight ·of solute diffusing in unit time;
D = diffusion coefficient; A = area.; Ө = time; C1 = concentration of solute at
interface; C2 = concentration of solute in bulk; L = film thickness.

10. • A similar equation can be written for a vapour:
W= D.A(P1 – P2)
L
Where;
P1 = partial pressure of vapour at interface; P2= partial pressure of vapour in the
atmosphere.

11. • As in heat transfer, films of unknown thickness can be dealt with as film
coefficients, and for multiple layers an overall coefficient of mass transfer can be
derived.
• An equivalent situation occurs when mass transfer takes place between two
immiscible fluids, which may two liquids or a liquid and a gas (or vapour).
• In this case there will be boundary layers of both fluids on each side of the
interface, where the slope of the concentration gradients depends on the
diffusion coefficients in the two materials.

12. • Mass transfer theory outlined briefly above can be applied to any operation in
which material changes phase, whether it is solid/liquid, solid/vapour (or gas),
liquid/liquid, or liquid/vapour (or gas).
• The effect can be seen in simple operations, such as the making of a solution of a
solid in liquid, where the rate of solution can be increased by:
• Agitation, which reduces the thickness of the boundary layers and disperses any
local concentrations of solution, so increasing the concentration gradient.
• Elevated temperatures (which will increase the solubility of most materials) but
which increase the diffusion coefficient and decrease the viscosity of the liquid,
so reducing boundary layer thickness.
• Size reduction of the solid, which increases the area over which diffusion can
occur.

13. • Design of mass transfer equipment must take account, therefore, of similar
considerations to heat transfer, that is;
• Turbulent flow conditions,
• Maximum concentration or partial pressure gradients, and
• The largest possible surface area.

14. THANKS