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Adsorption Processes
Ali Ahmadpour
Chemical Eng. Dept.
Ferdowsi University of Mashhad
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Contents
Introduction
Principles of adsorption
Types of adsorption
Definitions
Brief history
Adsorption isotherms
Mechanism of separation
Industrial adsorbents
Adsorption applications
Adsorber
Desorption methods
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Introduction
Separation is a process that transforms a mixtureof substances into two or more products thatdiffer from each other in composition.
Separation steps account for the major productioncosts in chemical and petrochemical industries.
There are several industrial separation processes.
Adsorption is one of the separation techniqueused for separating gas mixtures based on thesolid surface forces.
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Principles of adsorption
Adsorption is a surface phenomenon and
defined as the increase in concentration of a
particular component at the surface or interface
between two phases.
Adsorption mechanisms are generally
categorized as either physisorption,
chemisorption, or electrostatic adsorption.
Only physical adsorption is encountered in gas
separation.
5
Cont.
Weak molecular forces, such as Van der Waalsforces, provide the driving force for physicaladsorption.
A chemical reaction forms a chemical bondbetween the compound and the surface of thesolid in chemisorption.
Electrostatic adsorption involves the adsorptionof ions through Coulombic forces, and isnormally referred to as ion exchange.
In liquids, interactions between the solute and thesolvent also play an important role in establishingthe degree of adsorption.
6
Cont.
The amount of adsorption that occurs is dependent
on particular characteristics of the adsorbate and
adsorbent. The amount of adsorption that takes
place on the solid follows various isotherms or
kinetic rates.
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Types of adsorption
Physical adsorption (Physisorption): The adsorbate is
weakly bound onto the adsorbent by a combination of Van
der Waals forces and electrostatic forces. No covalent bonds
are formed and heat is released upon adsorption.
Chemical adsorption (Chemisorption): There is covalent
interaction of gas molecule and the surface of the adsorbent
which gives scope for much larger increases in adsorption
capacity.
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General features of physical
& chemical adsorptions
Physisorption Chemisorption
Low heat of adsorption (< 2 or 3
times latent heat of evaporation).
Non-specific.
Monolayer or multilayer.
No dissociation of adsorbed species.
Only significant at relatively low
temperatures.
Rapid, non-activated, reversible.
No electron transfer, although
polarization of sorbate may occur.
High heat of adsorption (> 2 or 3
times latent heat of evaporation).
Highly specific.
Monolayer only.
May involve dissociation.
Possible over a wide range of
temperatures.
Activated, may be slow and
irreversible.
Electron transfer leading to bond
formation between sorbate and
surface.
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Definitions
Adsorbate: The complex of adsorptive and adsorbent, i.e., the
adsorbing agent in loaded state.
Adsorbent: Porous solid having lattice vacancies of uniform size and
of molecular dimensions. They selectively adsorb molecules of a
certain shape.
Adsorption: The reversible attachment of small particles (molecules,
atoms, ions) to a solid, the adsorbent. Examples are the attachment
of water vapor to a drying agent or the attachment of organic
molecules to a zeolite for the purpose of exhaust treatment.
Adsorptive: The substance that is to be adsorbed, still in the fluid
phase, e.g., solvent molecules.
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Cont.
Load: The amount of material adsorbed in the adsorbate, usually
expressed in g per g of non-loaded adsorbent or as a percentage
figure. For example, zeolite 3A typically can take up water up to
25% of its weight. The maximum load depends on the nature of
the adsorbing material and the adsorbed material, the total
pressure, the temperature and the competition by other adsorbed
substances, e.g., water.
Desorption: The release of attached particles from the surface of
the adsorbing agent and the transition to the fluid state; reversal of
adsorption.
Adsorption and desorption are in a state of relative equilibrium;
the two processes always occur together.
11
Cont.
Fluid phase: The fluid medium around the adsorbing material which
contains adsorbates (in most cases it is water or air).
Kinetics: Mathematical description of the speed of a process, in
connection with adsorption, mainly the description of the speed at
which a particle moves from the fluid phase to the place of
adsorption.
Isotherm: Mapping the state of equilibrium of an adsorbing agent
with an adsorbed substance against the partial pressure of the
adsorbed substance in the fluid phase at constant temperature
(isotherm). The isotherm is the most important parameter for design
but it alone is not sufficient.
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Brief history
1930s: Recognizing the ability of porous solids toreversibly adsorb large volumes of vapor and gas.
1950s: Investigating the adsorptive properties ofcoals and carbonized polymers, introducing processfor recovery of aromatic hydrocarbons.
1960s: Purification of air and industrial vent gasesby shape selective molecular sieves.
1970s: Significant increase in the range and scale ofgas separation processes.
1980s-now: A vast growth in separation processtechnology, mathematical modeling and newadsorbents.
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Pore structure of adsorbent
Based on IUPAC classification:
Micropore d <2 nm
Mesopore 2 < d <50 nm
Macropore d >50 nm
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The difference between Adsorption & Absorption
Adsorption is the attraction between the outer surface
of a solid particle and a fluid molecule, whereas
absorption is the uptake of the fluid molecule into the
physical structure of the solid.
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Adsorption of particles from a gas
to a solid surface
The adsorbate is in a state of equilibrium with the fluid
phase, i.e., gas or liquid.
The more the adsorbing material is loaded, the higher is its
vapor pressure.
Very low loads represent a single-place adsorption. In this
range, the vapor pressure of the adsorption substance is
proportional to the load, which formally corresponds to
Henry's law.
If pore condensation occurs, the vapor pressure of the
adsorption substance enters the range of the vapor pressure
of the liquid phase.
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17
Cont.
How much substance can be taken up by the adsorbing substance, depends on
three factors:
Temperature: Higher temperatures reduce the load because the adsorption
process releases heat. Therefore, it is possible to release the adsorbed material
by increasing the temperature.
Chemical interaction: The properties of the adsorbing material and the adsorbed
substance (polarity), determine the degree of interaction between both.
Partial pressure: The higher the concentration of the adsorbed material in the
gaseous phase the more material is adsorbed. Reversely, if the concentration of
the adsorbed material is low in the gaseous phase, some adsorbed material is
released from the adsorbing surface. This means that the adsorbing material
can be regenerated with pure gas.
18
Cont.
The mathematical presentation of concentration versus pressure is
referred to as adsorption isotherm.
The adsorption isotherm does not describe time-related factors because
it reflects an equilibrium state. It is generated by determining the
amount of molecules attaching to the adsorbing material after hours of
establishing the equilibrium in a static gas atmosphere.
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Adsorption isotherms
The mathematical presentation of the context between the
adsorptive and the load is referred to as adsorption isotherm.
It is plotted at constant temperature.
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Working capacity
The working capacity for a gas is the difference in adsorption capacity
and desorption capacity.
The adsorbent needs to have a good working selectivity (ratio of one
gas working capacity to another gas working capacity).
21
Cont.
Initially there is a linear relationship between the partial pressure
and the amount of adsorbed substance and that the adsorbing
material is saturated and no more molecules are attached from a
certain concentration level.
The best-known theoretical prediction is Langmuir's isotherm.
This isotherm is one of the most frequently used models.
Under practical conditions, the curves take on most different
shapes and can even become a hysteresis for desorption, at the
onset of pore condensation. The progression of the isotherm
depends on the adsorbing material and temperature. A rule of
thumb is that the adsorption capacity drops to one half if the
temperature goes up by 10 - 20 K .
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Forces and energies in adsorption
Dispersion (attractive) forces
Adsorbate-Adsorbate interaction
Adsorbate-Adsorbent interaction
Repulsion forces
Adsorbate-Adsorbate interaction
Adsorbate-Adsorbent interaction
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Mechanism of separation
The adsorptive separation is achieved by one of three
mechanisms: steric, kinetic, or equilibrium effect.
The steric effect derives from the molecular sieving properties of
zeolites and molecular sieves. In this case only small and
properly shaped molecules can diffuse into the adsorbent,
whereas other molecules are totally excluded.
Kinetic separation is achieved by virtue of the differences in
diffusion rates of different molecules. In this case, the pore size
needs to be precisely tailored to lie between the kinetic diameters
of the two molecules that are to be separated.
Equilibrium separations are based on the differences in adsorbed
amounts on the equilibrium.
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Steric separation
Steric adsorption is unique with zeolites
because of the uniform pore size in the
structure.
Two largest applications are:
Drying with 3A zeolite
Separation of n-paraffin from iso-paraffin with
5A zeolite
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Kinetic separation
Kinetic separation is possible with carbon
molecular sieve because of distribution of
pore sizes.
The kinetic selectivity is measured by the
ratio of the micropores or intracrystalline
diffusivities for the components considered.
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Equilibrium separation
The separation factor in equilibrium
adsorption is:
XA= mole fraction of component A in adsorbed
phase at equilibrium
YA= mole fraction of component A in fluid phase at
equilibrium
BA
BA
ABY/Y
X/ X
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Adsorption at equilibrium
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Kinetic separation
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Selective separation
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Industrial adsorbents
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The past two decades have shown an explosion in the
development of new nanoporous materials:
mesoporous molecular sieves, zeolites, pillared clays,
sol gel-derived metal oxides, and new carbon
materials (carbon molecular sieves, super-activated
carbon, activated carbon fibers, carbon nanotubes,
fullerenes and heterofullerenes, microporous glasses
and graphite nanofibers).
Cont.
32
Adsorptive gas separation
Gas separations are a major production cost in the chemical
industry today. Production of industrial gases by pressure
swing adsorption (PSA), is expected to grow much faster
than by the conventional method, cryogenic distillation
(because of much lower cost for low capacity plants).
An economic gas separation process depends primarily on the
adsorbent with:
High selectivity
High capacity
Long life
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Adsorbents
• The porous solid of a given adsorption process is a critical
variable.
• The success or failure of the process depends on how the
solid performs in both equilibria and kinetics.
• A solid with good capacity but slow kinetics is not a good
choice as it takes adsorbate molecules too long a time to reach
the particle interior.
• A solid with fast kinetics but low capacity is not good either
as a large amount of solid is required for a given throughput.
• Thus, a good solid is the one that provides good adsorptive
capacity as well as good kinetics.
34
Cont.
• To satisfy these two requirements, the following aspects must be
satisfied:
the solid must have reasonably high surface area or micropore volume
the solid must have relatively large pore network for the transport of
molecules to the interior (macropores)
Most practical solids commonly used in industries do satisfy these two criteria,
with solids such as activated carbon, zeolite, alumina and silica gel.
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Alumina
• Alumina adsorbent is normally used in industries requiring the removal of
water from gas stream. This is due to the high functional group density on
the surface, providing active sites for polar molecules (such as water).
• There are a variety of alumina available, but the common solid used in
drying is γ-alumina. The characteristic of a typical γ-alumina is given
below.
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Silica gel
• Silica gel is a hard glassy substance and is milky white in color which is
made from the coagulation of a colloidal solution of silicic acid.
• The term gel simply reflects the conditions of the material during the
preparation step, not the nature of the final product.
• This adsorbent is used in most industries for water removal due to its strong
hydrophilicity of surface towards water.
• Some applications of silica gel are:
water removal from air
drying of non-reactive gases
drying of reactive gases
adsorption of hydrogen sulfide
oil vapor adsorption
adsorption of alcohols
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Activated carbon
• Activated carbon is one of the most complex solids but it is the most
versatile because of its extremely high surface area and micropore volume.
Moreover, its bimodal (sometimes trimodal) pore size distribution
provides good access of sorbate molecules to the interior.
• The structure of AC is complex and it is basically composed of an
amorphous structure and a graphite-like microcrystalline structure.
• The graphitic structure is important from the capacity point of view as it
provides "space" in the form of slit-shaped channel to accommodate
molecules. Because of the slit shape the micropore size for activated
carbon is reported as the micropore half-width rather than radius as in the
case of alumina or silica gel.
• Although the basic configuration of the graphitic layer in activated carbon
is similar to that of pure graphite, there are some deviations, for example
the interlayer spacing ranges from 0.34nm to 0.35nm.
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Cont.
• Commercial activated carbon has a very wide range of properties depending
on the application.
• For liquid phase applications, due to the large molecular size of adsorbate,
activated carbons used should have larger mesopore volume and larger
average pore radius.Typical characteristics of activated carbon used in gas separation
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Zeolite
• Zeolite, another widely adsorbent, can be found naturally or made
synthetically.
• Application of natural zeolite is not as widely as that of synthetic zeolite
because of the more specificity of the synthetic zeolite. There are many
types of synthetic zeolite, such as type A, X, Y, mordenite, ZSM, etc.
The typical characteristics of the zeolite A
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Adsorption applications
Gas separationN2 from air Gas sweetening
O2 from air CO2 from NG
n-Paraffin separation Xylene separation
Gas dryingAir drying
Solvent drying
Drying of cracked gas
Gas recoveryH2 recovery
Solvent recovery from air
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Commercial adsorption
processes and sorbents used
42
Cont.
43
Cont.
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Some separation and purification
applications by new sorbents
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Adsorber
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Objectives for the development of adsorbents and adsorbent processes
can be summarized as follows:
Cont.
Adsorbents
Process
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Desorption methods
TSA – Thermal Swing Adsorption: the desorption is
triggered by an increase in temperature. This is energy
intensive and slow since the entire mass of adsorbent must
be heated.
VSA – Vacuum Swing Adsorption: the desorption is
triggered by creating a near-vacuum. One advantage is that
this system will operate at near ambient temperature, so
require less energy. Another advantage is that the energy
used is applied only to adsorbed molecule and so it is
thermodynamically more efficient than TSA.
PSA – Pressure Swing Adsorption: the desorption is
triggered by a decrease in pressure, usually from an
elevated level to near atmospheric pressure.
48
Cont.
ESA – Electrical Swing Adsorption: the desorption is
triggered by an applied voltage. This method has the
advantage of being fast and require less energy.
Different separation process configurations are possible
depending on the temperature and pressure of the effluent
gas stream.