ERT 313/4 BIOSEPARATION ENGINEERING MASS TRANSFER &...
Transcript of ERT 313/4 BIOSEPARATION ENGINEERING MASS TRANSFER &...
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ERT 313/4
BIOSEPARATION ENGINEERING
MASS TRANSFER & ITS APPLICATIONS
Prepared by:
Pn. Hairul Nazirah Abdul Halim
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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..
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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.
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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.
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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
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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.
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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.
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ERT 313/4
BIOSEPARATION ENGINEERING
MASS TRANSFER THEORIES
Prepared by:
Pn. Hairul Nazirah Abdul Halim
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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.
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• 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−=
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• 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
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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
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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.
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Gas phase coefficient, kg
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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.
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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.
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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:
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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.
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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.
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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.
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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
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The rate of transfer to the interface = the rate of
transfer from the interface
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• 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.
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• 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