HAND OUT :- 1

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 OperationsThere 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 mass-transfer 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 phenomenaCategorization 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.

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. Ficks first law

b. Equation of continuity

c. Ficks second law

HAND OUT:- 3

Lectures covered:- 8 - 10

Topic :- Molecular Diffusion in FluidsFollowing 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:

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

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.

Diffusion between phases:

a. Local two-phase mass transfer two resistance concept

b. Local overall mass transfer coefficients

c. Use of local overall coefficients

MATERIAL BALANCES:

a. Steady-state Cocurrent process

b. Steady-state Countercurrent process Concept of stage and cascade.

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.Concept of Raoults Law and Henrys Law:

PA* = PA * xAWhere PA* = Equilibrium Partial Pressure of A

PA= Vapor Pressure of A

xA = Mole fraction of A

HAND OUT:- 8Lectures 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

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

Kremsers Equation:

For dilute gas mixtures Kremsers equation are used to determine the number of trays for an absorption column.

HAND OUT:- 10

Lectures covered:- 30-32Tray 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