Adsorption and its models

1. ADSORPTION
By Poonam Kamlekar
Department of Chemical Engineering

2. CONTENTS
● Adsorption basics
● Physisorption & chemisorption
● Comparing physical and chemical adsorptions
● Adsorption Isotherms
● Freundlich Adsorption Isotherm
● Langmuir Adsorption isotherm
● Brunauer–Emmett–Teller (BET) theory
● Kinetics
● Desorption
● Batch Adsorption
● Industrial Adsorbents
● Applications
● References
2

3. Adsorption basics
● Adsorption is the adhesion of atoms, ions or
molecules from a gas, liquid or dissolved solid to a
surface.
● An adsorbate is any substance that has a substance
that is adsorbed.
● An Adsorbent is usually a solid substance that adsorbs
another substance.
● adsorption is more exothermic than liquefaction.
● Adsorption is spontaneous in nature
ΔG= negative
ΔH= negative
3

4. Physical adsorption or physisorption
4
● low interaction between molecules and solid surfaces.
● physisorption attains quickly the equilibrium, and thus
it is reversible
● The physisorption of gases on solids is frequently used
for textural analyses of catalysts and solids, such as
surface area and pore distribution and sizes.
Chemical adsorption or chemisorption
● the interaction is specific between molecules and solids, the adsorbed molecules form a
monolayer.
● The chemisorption processes need longer times to attain the equilibrium condition, in particular
at low temperatures

5. 5
Comparing physical
and chemical
adsorptions

6. ADSORPTION ISOTHERMS
6
❖ Adsorption process is usually studied through graphs known as Adsorption
Isotherm
OR
❖ The amount of gas adsorbed with the equilibrium pressure at constant
temperature.
[n = f(P, T, gas, solid)]

7. FREUNDLICH
ISOTHERM
7
● x/m = adsorption per gram of adsorbent which is obtained by
dividing the amount of adsorbate (x) by the weight of the
adsorbent (m).
● P is Pressure, k and n are constants whose values depend
upon adsorbent and gas at particular temperature .
● Taking the logarithms of Freundlich Isotherm : Freundlich
Isotherm correctly established the relationship of adsorption with
pressure at lower values,
Limitation : Failed to predict adsorption capacity at higher pressure
Empirical equation for representing the isothermal variation of adsorption of
a quantity of gas adsorbed by unit mass of solid adsorbent with pressure is
Freundlich Adsorption Isotherm
log(x/m) = log K + (1/n) log C…………. Freundlich Adsorption
Isotherm

8. 8
Langmuir Adsorption isotherm
★ Limitation : Valid for Low Pressures only
Explains adsorption by assuming an adsorbate behaves as an ideal gas at
isothermal conditions.
1. Adsorption of adsorbates takes place at well-defined local sites. 30
Introduction to Adsorption
2. All adsorption sites are the same (energetically), and each site
accommodates only one adsorbate molecule.
3. There is no lateral interaction between adsorbed molecules.
Adsorption rate = kap (qm- q)
Desorption rate = kd q
● ka and kd are the adsorption and desorption
rate constants,
● qm is the maximum adsorption (when all
sites are filled).
● p is the adsorbate pressure
q

9. Brunauer–Emmett–Teller (BET) theory
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BET model of multilayer adsorption, that is,
● gas molecules physically adsorb on a solid in layers infinitely;
● gas molecules only interact with adjacent layers; and
● the Langmuir theory can be applied to each layer.
● the enthalpy of adsorption for the first layer is constant and greater
than the second (and higher).
● the enthalpy of adsorption for the second (and higher) layers is the
same as the enthalpy of liquefaction
------------------------- BET Adsorption
Isotherm BET PLOT
★ Limitation : It violates the Gibbs' phase rules

10. 10
Single site
Dual site
Kinetics
A + S AS
the adsorption of A on a site S is represented by

11. Desorption
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the products of the surface reaction adsorbed on the surface are subsequently desorbed
into the gas phase
the rate of desorption of C is
where KDC is the desorption equilibrium constant with units of atm. the desorption step for C is just the reverse of
the adsorption step. Consequently, the rate of desorption of C, rDC, is just opposite in sign to the rate of
adsorption of C, rADC
the desorption equilibrium constant KDC is just the reciprocal of the adsorption equilibrium constant for C, KC
rate of desorption of C can be written

12. Batch Adsorption
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● In batch adsorption, for good adsorbent/solution contact, adsorbents
should remain in suspension and not settle out due to gravity.
● t that the concentration of the solution be kept uniform. To insure the
presence of this so-called “just suspension”
● t that the concentration of the solution be kept uniform. To insure the
presence of this so-called “just suspension”
● The empirical correlation of the “just suspension” stirrer speed (rev/s),
njs, is found to be
● D is the stirrer diameter (m);
● ν, the solution kinematic viscosity (m2 /s)
● dp, the adsorbent particle diameter (m);
● g, the gravitational acceleration (m/s2 );
● B, the percentage (by weight) of adsorbent of
the adsorbent-solution mixture

13. Industrial Adsorbents
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Chitosan
Natural Adsorbent
Graphene
Nano -
Adsorbents
Layered double hydroxides
Ionic Adsorbents
Activated
alumina
Polymer
Adsorbent
Silica gel
Synthetic
Adsorbent

14. Applications
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Purification of water
Adsorption chromatography
Metallurgy
Air pollution mask
Protein Adsorption

15. References
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● Chemical Reaction Engineering by Martin SCHMAL
● INTRODUCTION TO ADSORPTION Basics, Analysis, and Applications BY CHI TIEN
● Elements of Chemical Reaction Engineering by H Scott Fogler( 5th edition)