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  • Topic: Adsorption, Pressure, Langmuir equation
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JEVE 5760 Advanced Physico-Chemical Treatment Processes

Professor Xijun Hu

Adsorption
References:
D.D. Do, “Adsorption Analysis: Equilibrium and Kinetics”, 0Imperial College Press, 1998.
R.T. Yang, “Gas Separation by Adsorption Processes”, Butterworths, Boston, 1987.

D.M. Ruthven, “Principles of Adsorption and Adsorption
Processes”, John Wiley & Sons, New York, 1984.

In adsorption processes one or more components of a gas
or liquid stream (adsorbate or solute) are adsorbed on the
surface of a solid (adsorbent) and a separation is
accomplished.

235

JEVE 5760 Advanced Physico-Chemical Treatment Processes

Professor Xijun Hu

Adsorbents
Good adsorbents should have
 high surface area or micropore volume (for large
adsorption capacity)
 large pore network for the transport of molecules to
the interior (for fast kinetics)
The porous solid must have small pore size (micropore)
with a reasonable porosity to satisfy the first
requirement and have a network of large pore size
(macropore) for the second requirement.
Micropore: d < 2 nm
Mesopore: 2 < d < 50 nm
Macropore: d > 50 nm

236

JEVE 5760 Advanced Physico-Chemical Treatment Processes

Adsorbent

Activated
carbon

Silica gel

Activated
alumina

Molecular
sieve
zeolites

Manufacturing
method
by thermal
decomposition
of coal, wood,
vegetable shells,
etc.
by acid
treatment of
sodium silicate
solution & then
dried
hydrated
aluminum oxide
is activated by
heating to drive
off the water
porous
crystalline
aluminosilicate
containing
precisely
uniform pores

Surface
area
(m2/g)
3002000

340800

200500

Professor Xijun Hu

Pore
Usage
diameter
o
(A )
10-60
Removal of
organic
vapors;
H2
purification
20-140 dehydrate
gases &
liquids;
fractionate
hydrocarbon
20-140 drying;
gas

JEVE 5760 Advanced Physico-Chemical Treatment Processes

Properties to characterize adsorbents:






Specific pore volume (cm3/g).
Specific surface area (m2/g).
Particle density(g/cm-3).
Average pore diameter ( A ).
Pore size distribution
o

chromatograph
y

3001200

237

3-10

Professor Xijun Hu

drying,
separations
based on
molecular
size &
shape

238

JEVE 5760 Advanced Physico-Chemical Treatment Processes

Professor Xijun Hu

JEVE 5760 Advanced Physico-Chemical Treatment Processes

Professor Xijun Hu

Forces and Energies of Adsorption
Physical Adsorption
Low heat of adsorption
(2 or 3 times latent heat
of evaporation)

Non specific

Highly specific

Monolayer or multilayer.
No dissociation of
adsorbed species.
Only significant at
relatively low
temperatures.

Monolayer only
May involve dissociation.
Possible over a wide range
of temperature.

Rapid, non-activated,
reversible.
No electron transfer
although polarization of
sorbate may occur.

1. Van der Waals forces (Dispersion-Repulsion).
This is always present. By combining the attraction
potential and the repulsion between two isolated
molecules, we have the Lennard-Jones potential
function:

Activated, may be slow
and irreversible.
Electron transfer leading to
bond formation between
sorbate and surface.

  12
 6


  4      
 r 
 r 

which is sketched in the following figure. The force
constants  and  are characteristics of the particular
molecular species and available in the literature.

2. Electrostatic interactions (polarization, dipole, and
quadrupole).
This is significant only for adsorbents having ionic
structure.
239

240

JEVE 5760 Advanced Physico-Chemical Treatment Processes

Professor Xijun Hu

Entropy changes

JEVE 5760 Advanced Physico-Chemical Treatment Processes

Professor Xijun Hu

Adsorption equilibrium isotherm
(Single component)

Physical adsorption from the gas phase is
exothermic, why? Let us look at the thermodynamic
argument.
Since the adsorbed molecules is more regular
...
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