ERT 313/4BIOSEPARATION 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 313/4BIOSEPARATION 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 AVA

• 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 TheoriesMass 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 TheoriesMass 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 kc for 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 PhasesTwo-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 phaseyA* = 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