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.

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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.

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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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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.

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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).

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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.

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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.

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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

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Cont.

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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.

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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.