ERT 313/4

BIOSEPARATION ENGINEERING

MASS TRANSFER & ITS APPLICATIONS

Prepared by:

Pn. Hairul Nazirah Abdul Halim

Mass Transfer and Its Applications

• Mass transfer – transfer of material from one

homogeneous phase to another.

• Based on differences in vapor pressure, solubility,

diffusivity.

• Driving force for transfer is a concentration

difference.

• Mass transfer operations – gas absorption,

distillation, extraction, leaching, adsorption,

crystallization, membrane separations, etc..

Gas Absorption

• A solute gas is absorbed from an inert gas into a

liquid.

• Example: Removal of ammonia from a mixture of

ammonia-air by means of liquid water.

• Ammonia is transferred from gas to liquid phase.

Distillation

• Separation of a liquid mixture of miscible and

volatile substances into individual components or

group of components by vaporization.

• Example:

1. Separation of ethanol and water into its

components.

2. Crude petroleum into gasoline, kerosene, fuel oil.

Liquid Extraction

• A mixture of two components is treated by solvent

that preferentially dissolves one or more of the

components in the mixture.

• Example;

1. recovery of penicillin from fermentation broth

solvent: butyl acetate

2. recovery of acetic acid (b.p 1180c) from dilute

aqueous (b.p 1000c) solutions

solvent: ethyl-acetate

Adsorption

• A solute is removed from either a liquid or a gas

through contact with solid adsorbent.

• Adsorbent has surface of which has a special

affinity for the solute.

• Example: Removal of dyes using activated carbon

as adsorbent.

Membrane separations

• Gas or liquid separations

• Such as Reverse osmosis, ultrafiltration,

nanofiltration.

• One component of liquid or gaseous mixture

passes through a selective membrane more readily

than the other components.

• Driving force – concentration or partial pressure.

ERT 313/4

BIOSEPARATION ENGINEERING

MASS TRANSFER THEORIES

Prepared by:

Pn. Hairul Nazirah Abdul Halim

Principles of Diffusion

• Diffusion – is the movement, under the influence of

a physical stimulus, of an individual component

through a mixture.

• Common cause of diffusion: concentration gradient

• Example: Removal of ammonia by gas absorption.

• Fick’s Law of Diffusion:

• JA = molar flux of comp. A (kg mol/m2.h)

• Dv = volumetric diffusivity (m2/h)

• cA = concentration (kg mol/m3)

• b = distance in direction of diffusion (m)

db

dcDJ

A

VA−=

• Turbulent flow is desired in most mass-transfer

operations:

1. to increase the rate of transfer per unit area

2. to help disperse one fluid in another

3. to create more interfacial area

• Mass transfer to a fluid interface is often unsteady-

state type.

Mass Transfer Theories

Mass Transfer Theories

Mass transfer coefficient, k

• Is defined as rate of mass transfer per unit area per

unit conc. difference.

• kc is molar flux divided by conc. difference

• kc has a unit of velocity in cm/s, m/s

• Concentration, c in moles/volume

Mass Transfer Theories

Mass transfer coefficient, k

• ky in mol/area.time (mol/m2.s)

• y or x are mole fractions in the vapor or liquid phase.

Gas phase coefficient, kg

Film Theory

• Basic concept – the resistance to diffusion can be

considered equivalent to that in stagnant film of a

certain thickness

• Often used as a basis for complex problems of

multicomponent diffusion or diffusion plus chemical

reaction.

Consider mass transfer from a turbulent gas stream

to the wall of a pipe;

• Laminar layer near the

wall

• Mass transfer is mainly

by molecular diffusion

• The conc. gradient

almost linear

• As the distance from the

wall increases, turbulent

become stronger.

• The resistance to mass

transfer is mainly in

laminar boundary layer.

Effect of one-way Diffusion

• When only component A is diffusing through a

stagnant film, the rate of mass transfer is greater

than if component B is diffusing in the opposite

direction.

• The rate of one-way mass transfer can be

expressed:

Boundary Layer Theory

• Mass transfer often take place in a thin boundary

layer near a surface where the fluid is in laminar

flow.

• The coefficient, kc depends on 2/3 power of

diffusivity and decreases with increasing distance

along the surface in the direction of flow

• Boundary layer theory can be used to estimate kcfor some situations,

• but exact prediction of kc cannot be made when the

boundary layer become turbulent.

Penetration Theory

• Makes use of the expression for the transient rate

of diffusion into a relatively thick mass of fluid with

a constant concentration at the surface.

Mass Transfer Between Phases

Two-Film Theory

• The rate of diffusion in both phases affect the

overall rate of mass transfer.

• Assumption in Two-Film Theory:

a) equilibrium is assumed at the interface

b) the resistance to mass transfer in the two

phases are added to get an overall

resistance.

• Use in most mass transfer operations such as gas

absorption, distillation, adsorption and extraction.

Mass Transfer Between Phases

• Nomenclature:

ky = mass-transfer coefficient in gas phase

kx = mass-transfer coefficient in liquid phase

Ky = Overall mass-transfer coefficient in gas

phase

Kx = Overall mass-transfer coefficient in liquid

phase

a = interfacial area per unit volume

The rate of transfer to the interface = the rate of

transfer from the interface

• The rate also equal to:

where;

Ky = overall mass transfer coefficient in gas phase

yA* = composition of the gas that equilibrium with the

bulk liquid of composition xA.

• 1/ Ky = overall resistance to mass transfer

• m/kx = the resistance in liquid film

• 1/ky = the resistance in gas film

• m = slope of the equilibrium curve