Nirma University
Institute of Technology
Chemical Engineering Department
2CH301 Mass Transfer Operations – I
Hand Outs /PS
HAND OUT :- 1
Lectures covered :- 1- 3
Topic:- Introduction and Classification of Mass Transfer Operations
There is scarcely any chemical process which does not require a preliminary purification
of raw materials or final separation of products from by-products, and for these the masstransfer operations are usually used. A substantial number of unit operations of chemical
engineering are concerned with the problem of changing the compositions of solutions
and mixtures through methods not necessarily involving chemical reactions. If the
operations involve changes in composition of solutions, they are known as mass transfer
operations. For mixtures, such separations may be entirely mechanical.
 What are the major points of contrast between mechanical operations and mass
transfer operations?
The major part of the cost of a process is that for the separations. These separation or
purifications costs depend directly upon the ratio of final to initial concentration of
separated substances, and if this ratio is large, the product costs are large.
 Defining MASS TRANSFER OPERATIONS…….
“defined by transfer of a substance through another on a molecular scale, wherein not
characterized by bulk transfer, in fact occurs as a result of concentration difference or
gradient, with the diffusing substance moving from a region of high to low
concentration.”
 Classification of mass transfer operations:
Governed by two aspects:
a. Direct contact of two immiscible phases
b. Phases separated by a Membrane
c. Direct contact of miscible phases
d. Use of surface phenomena
Categorization with examples under each of the above four categories shall be discussed
in detail.
Other discussions will involve:
a. Comparison between direct and indirect operations in terms of purity of product,
raw material requirements, further downstream processing and energy intensity.
b. Choice of separation method.
c. Methods of conducting mass-transfer operations involving solute recovery versus
fractionation, unsteady state versus steady state operation, stagewise operation,
differential (continuous contact) contact operation.
Nirma University
Institute of Technology
Chemical Engineering Department
2CH301 Mass Transfer Operations – I
Hand Outs /PS
d. Design principles underlying the diffusional operations such as number of
equilibrium stages, time requirement, permissible flow rate and energy
requirements.
HAND OUT :- 2
Lectures covered :- 4 - 7
Topic:- Molecular Diffusion in Fluids
Molecular diffusion is concerned with the movement of individual molecules through a
substance by virtue of their thermal energy. The kinetic theory of gases quantitatively
describes the diffusional phenomena. The phenomenon of molecular diffusion ultimately
leads to a completely uniform concentration of substances throughout a solution which
may initially have been nonuniform.
Eddy or turbulent diffusion is The method of solute transfer in which mechanical
agitation produces rapid movement of relatively large fluid chunks what is precisely
defined as “eddies”.
Types of molar fluxes: N-flux and J-flux.
What are the points of contrast between N and J fluxes?
What are the differences in applications of N and J fluxes?
Following topics shall be discussed in detail:
a. Fick’s first law
b. Equation of continuity
c. Fick’s second law
HAND OUT:- 3
Lectures covered:- 8 - 10
Topic :- Molecular Diffusion in Fluids
Following topics shall be discussed in detail:
Steady state molecular diffusion in fluids at rest and in laminar flow:
1. Molecular diffusion in Gases:
Special cases:
Nirma University
Institute of Technology
Chemical Engineering Department
2CH301 Mass Transfer Operations – I
Hand Outs /PS
a. Steady tate diffusion of A through nondiffusing B
b. Steady state equimolal counterdiffusion
c. Steady state diffusion in multicomponent mixtures
d. Diffusivity of gases: Wilke-Lee modification
2. Molecular diffusion in liquids:
Special cases:
a. Steady tate diffusion of A through nondiffusing B
b. Steady state equimolal counterdiffusion
c. Diffusivity of liquids
Numericals based on discussion of above concepts shall be solved in the class.
HAND OUT:- 4
Lectures covered:- 11 - 13
Topic:- Mass Transfer Coefficients
Rate at which a component is transferred from one phase to the other depends upon mass
transfer or rate coefficient and upon the degree of departure of the system from
equilibrium. The transfer stops when the equilibrium is attained.
Mass transfer coefficients are of great importance, since, as they regulate the rate at
which equilibrium is approached, they control the time required for separation and
therefore the size and cost of the equipment used. The transfer coefficients are also
important in governing the size of equipment used for entirely different purposes such as
carrying out chemical reactions.
Flux = coefficient * concentration difference
Types of MTCs:
a. F-type coefficient
b. k-type coefficient
Relation between mass-transfer coefficients shall be discussed with the resulting
conversions.
HAND OUT:- 5
Nirma University
Institute of Technology
Chemical Engineering Department
2CH301 Mass Transfer Operations – I
Hand Outs /PS
Lectures covered:- 14-16
Topic:- Theories related to the evolution of Mass Transfer Coefficients
Following theories related to the evolution of MTCs will be dealt with:
a. Film theory (turbulent fluid flow past a solid surface with mass transfer occurring
from solid surface to the fluid)
b. Penetration theory (mass transfer at fluid surfaces – gas/liquid contact)
c. Surface-renewal theories
d. Combination film-surface-renewal theory
e. Surface-stretch theory
 Analogy between mass, heat and momentum transfer…
 Analogous dimensionless groups in mass and heat transfer…
HAND OUT:- 6
Lectures covered:- 17- 22
Topic:- Interphase Mass Transfer
Interphase mass transfer is….
concerned with the simultaneous application of the diffusional mechanism for
each phase to the combined system. The rate of diffusion within each phase is dependent
upon the concentration gradient existing within it. At the same time, the concentration
gradients of the two-phase system are indicative of the departure from equilibrium which
exists between the phases. Should equilibrium be established, the concentration gradients
and hence the rate of diffusion will fall to zero.
 Curve: Equilibrium distribution of a solute between a gas and a liquid phase at
constant temperature.

a.
b.
c.
Diffusion between phases:
Local two-phase mass transfer – two resistance concept
Local overall mass transfer coefficients
Use of local overall coefficients
 MATERIAL BALANCES:
a. Steady-state Cocurrent process
b. Steady-state Countercurrent process
 Concept of “stage” and “cascade”.
Nirma University
Institute of Technology
Chemical Engineering Department
2CH301 Mass Transfer Operations – I
Hand Outs /PS
HAND OUT:- 7
Lectures covered:- 22-23
Gas Absorption:
Gas absorption is an operation in which a gas mixture is contacted with a liquid for the
purpose of preferentially dissolving one or more components of the gas and to provide a
solution of them in the liquid.
For example: Gas from by product coke ovens is washed with water to remove ammonia
and again with an oil to remove benzene and toluene vapours. Objectionable hydrogen
sulphide is removed from such a gas or from naturally occurring hydrocarbon gases by
washing with various alkaline solutions in which it is absorbed.
Equilibrium Solubility of gases in liquids:
The rate at which a gaseous constituent of a mixture will dissolve in an absorbent liquid
depends upon the departure from equilibrium which exists and therefore it necessary to
consider the equilibrium characteristics of gas- liquid systems.
Multicomponent Systems:
If a mixture of gases is brought into contact with a liquid, under certain conditions the
equilibrium solubilities of each gas will be independent of the others, provided that the
equilibrium is described in terms of the partial pressures in the gas mixture.
Concept of Ideal Liquid Solutions
There are four significant characteristics of ideal solution:
The average intermolecular forces of attraction and repulsion in the solution are
unchanged on mixing the constituents.
The volume of the solution varies linearly with composition.
There is neither absorption nor evolution of heat in mixing the constituents.
The total vapour pressure of the solution varies linearly with composition expressed as
mole fractions.
Nirma University
Institute of Technology
Chemical Engineering Department
2CH301 Mass Transfer Operations – I
Hand Outs /PS
Concept of Raoult’s Law and Henry’s Law:
PA* = PA * xA
Where
PA* = Equilibrium Partial Pressure of A
PA= Vapor Pressure of A
xA = Mole fraction of A
HAND OUT:- 8
Lectures covered:- 24-26
Choice of solvent for Absorption
Following factors must be considered while making choice of solvent
 Gas solubility:
Gas solubility should be high, thus increasing the rate of absorption.

Volatility
The solvent should have a low vapour pressure since the gas leaving an absorption
operation is ordinary saturated with the solvent and much may thereby be lost.

Corrosiveness
The materials of construction required for the equipment should not be unusual or
expensive.

Cost
The solvent should be inexpensive, so that losses are not costly and should be
readily available.

Viscosity
Low viscosity is preferred for reasons of rapid absorption rates

Miscellaneous
Non toxic, non flammable and chemically stable and should have a low freezing
point.
One Component Transferred: Material Balances
 Counter current Flow
Gs(Y1-Y2) = Ls(X1-X2)
-------- (1)
Gs = molar flow rate of non diffusing component of gas
Nirma University
Institute of Technology
Chemical Engineering Department
2CH301 Mass Transfer Operations – I
Hand Outs /PS
Ls= molar flow rate of non volatile solvent
Y = mole ratio of soluble component to insoluble in gas phase
X= mole ratio of soluble component to insoluble in liquid phase.
‘1’ and ‘2’ represents the top and bottom of tower
Concept of Minimum Liquid to Gas ratio
Discussion on significance of (L/G) min for operation of absorbers.
HAND OUT:- 9
Lectures covered:- 27-29
 Concurrent flow
 Numerical Problems based on the concept of Minimum liquid to gas ratio and
operating gas to liquid flow rate.
 Counter current Multistage operation: One component transferred
Ideal or Theoretical Tray
Tray is defined as one where average composition of all the gas leaving the tray is in
equilibrium with the average composition of all the liquid leaving the tray.
 Using graphical method to estimate Number of Theoretical trays for an
absorption tower.
 Dilute Gas mixtures
Kremser’s Equation:
For dilute gas mixtures Kremser’s equation are used to determine the number of
trays for an absorption column.
Nirma University
Institute of Technology
Chemical Engineering Department
2CH301 Mass Transfer Operations – I
Hand Outs /PS
HAND OUT:- 10
Lectures covered:- 30-32
Tray Efficiency
 Murphree Efficiency
Continuous Contact Equipment
 Absorption of one component:
 Concept of HETP, NTU and HTU
 Overall Coefficients and Transfer units
 Absorption with Chemical Reaction
Equipment for Gas liquid Operations

Gas dispersed
(a) Sparged vessels
(b) Tray Towers
 Discussion on the operating characteristics
 Concept of Flooding, weeping, coning, dumping
 Tray Efficiency