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ADSORPTION
Outline
1) Adsorption Phenomena Adsorption Forces,
Definitions and Types 2) Adsorbents 3)
Adsorption Equilibrium 4) Characterization of
adsorbents 5) Rate proccesses in adsorption and
simple design methods for fixed bed adsorption 6)
Adsorption Process Cycles 7) Applications
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Characterization of Adsorbents

• Texture
• a)granularity size and shape of individual
  particles
• b) Character of its porosity pore volume,
  pore size distribution, pore area, surface area
  
• 2. Densities true, apparent and bulk

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Particle Size Determination
Some other indirect methods
A)Sedimentation B)Elutriation C)
Scattering Laser, X-ray
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DENSITIES
mass of particlesm ?ssolid density m / VS
synonoms solid density true density
skeleton density
??p apparent density m / (VSVg)m/VP VP
VSVgparticle volume
??bbulk density m / (VSVgVb) V
VSVgVbtotal bed volume Bulk density can be
measured by measuring the volume and weight of
particles in a graduated cylinder
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Mercury and Helium pycnometry
VSVHe volume of helium displaced
by the sample
Helium is a small non-adsorbable gas at room
T VpVS VgVHg volume of mercury displaced
by the sample at near atmospheric P VHg- VHe
Vg pore volume
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Bed porosityvoidage ?b 1- ?b / ?a
Vb / (VSVgVb)
Particle porosity ?p 1- ?a / ?s Vg /
(VSVg)
Specific pore volume Vg/m ?p/ ?a
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Pore Classification

• According to IUPAC definition pores can be
  classified in three groups with respect to their
  dimensions.
• Macropores Pores with diameters larger than 50
  nm (500 ?).
• Mesopores Pores with diameters between 2 nm and
  50 nm (20-500 ?).
• Micropores Pores with diameters less than 2 nm
  (20 ?).

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• It may be also desirable to subdivide
• micropores into two groups
• The micropores smaller than about 0.7 nm (7 ?)
  can be defined as narrow micropores or
  ultramicropores.
• The micropores in the range of 0.7-2.0 nm
  (7-20 ?) can be defined as super micropores.

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Typical Pore Size distribution of oxidic
adsorbents(differential or incremental )
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Pore structure of activated carbon(schematic)
external surface
internal surface
external surface
Narrow micropores
micropores
mesopores
macropores
internal surface
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Methods for Determination of Pore Size
Distribution

• Two common methods
• Mercury Penetration Method
• Nitrogen Desorption Method

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Mercury Porosimetry
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If the contact angle between liquid and solid is
greter than 90o , the at equilibrium, the
pressure on the convex side of the meniscus must
be greater then on the concave side. Thus if a
porous solid is immersed in a nonwetting liquid
such as mercury, there will be no penetration of
pores untill the applied pressure reaches the
equilibrium value.
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Wetting / Contact Angles
Wetting ? lt 90?
Non-wetting ? gt 90?
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Capillarity
Capillary rise ? lt 90?
Capillary depression ? gt 90?
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Mercury Porosimetry 2

• Force balance
• r2 P -2 ? r ?cos ?
• ?? contact angle
• ?? surface tension
• r pore radius
• ??

Washburn equation
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Mercury porosimetry

• ?? 140 o (contact angle may very depending
  on type of solid)
• ?? 480 dyne/cm ( surface tension )
• Then substitting these values in Washburn
  equation
• r (Å) 8.75 X 105 / P(psi) (working
  equation)
• Pressure limits( minumum poresizes) for
  commercial mercury porosimeters
• P 30 000 psi r30 Å (diameter60 Å)
• P 60 000 psi r 15 Å (diameter 30 Å

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Mercury Porosimeter
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Mercury porosimeter is simply an instrument
designed to apply a controlled mercury pressure
to the adsorbent and record the volume of mercury
penetrating the pore structure.
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Mercury Porosimeter
Mercury porosimeter is simply an instrument
designed to apply a controlled mercury pressure
to the adsorbent and record the volume of mercury
penetrating the pore structure.
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Sample Cell
The sample cell or penetrometer is used both to
contain the sample and to facilitate the
measurement of intrusion and extrusion volumes
via metal sheath and electrode cap.
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Pressure gtgt Intrusion
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Intrusion-Extrusion Profiles
Extrusion
Intrusion
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Use of Kelvin Equation for Pore size Distibution
Measurement of an isotherm under conditions of
capillary condensation provides a simple means of
determining the pore size distribution of the
adsorbent. Applying the Kelvin equation to the
desorption branch of the isotherm gives the value
of (r) corresponding to known relative pressure
and the corresponding to adsorption loading (q).
If adsorption on the pore walls is neglected,
q/?L would correspond to the total pore
volume made up of pores of radius less than or
equal r. ?L density of liquid sorbate at the
sorption temperature
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Use of Kelvin Equation for Pore size Distibution
For nitrogen at 195.8 oC (nbp) With ?0 cos
?1 ? Kelvin equation becomes
r(Å) 4.15 / log P0/P Thickness of
adsorbed layer is neglected.
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Use of Kelvin Equation for Pore size Distibution
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Cumulative pore volume
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Average pore radius from surface area and pore
volume measurements ( Wheeler method)
Assumption all pores are straight and
cylindrical of the same radius r and length L
(not interconnected) Total surface area of
particle m Sg(2?rL)n (eq 1) m mass of
particle L length of pore n number of
pores Sg specific surface area Total pore
volume of particle m Vg (?r2L) n (eq2) Vg
specific pore volume Dividing eq 1 by eq 2 and
rearranging average pore radius r 2 Vg /
Sg or d2r 4 Vg / Sg