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ADSORPTION
Rushil Bhatt​
CONTENTS
Introduction
ADSORPTION CLASSIFICATION
INTRODUCTION
Molecular forces at the surface of a liquid are in a state of imbalance
or unsaturation. The same is true of the surface of a solid, where the
molecules or ions on the surface may not have all their forces satisfied
by union with other particles.
As a result of this unsaturation, solid and liquid surfaces tend to satisfy
their residual forces by attracting and retaining onto their surfaces
gases or dissolved substances with which they come in contact.
This phenomenon of the concentration of a substance on the surface
of a solid (or liquid) is called adsorption.
Thus, the substance attracted to a surface is said to be the adsorbed
phase or adsorbate, while the substance to which it is attached is the
adsorbent.
3
Adsorption should be carefully distinguished
from absorption, the later process being
characterized by a substance not only being
retained on a surface, but also passing
through the surface to become distributed
throughout the phase.
Presentation title 6
Presentation title
8
The study of adsorption of various gases (or vapors) on solid
surfaces has revealed that the forces operative in adsorption
are not the same in all cases.
Two types of adsorption are generally recognized: “physical”
or van der Waals adsorption and “chemical” or activated
adsorption
9
Physical adsorption (physisorption) is the result of intermolecular
forces of attraction between molecules of the solid and the substance
adsorbed.
When, for example, the intermolecular attractive forces between a solid
and a gas (or vapor) are greater than those existing between
molecules of the gas itself, the gas will condense upon the surface of
the solid even though its pressure may be lower than the vapor
pressure corresponding to the prevailing temperature.
The adsorbed substance does not penetrate within the crystal lattice of
the solid and does not dissolve in it but remains entirely upon the
surface.
10
Should the solid, however, be highly porous, containing many fine
capillaries, the adsorbed substance will penetrate these interstices if it
“wets” the solid.
The partial pressure of the adsorbed substance at equilibrium equals
that of the contacting gas phase, and by lowering the pressure of the
gas phase, or by raising the temperature, the adsorbed gas is readily
removed or desorbed in unchanged form.
Physical adsorption is characterized by low heats of adsorption
(approximately 40 Btu/lbmol of adsorbate) and by the fact that the
adsorption equilibrium is both reversible and established rapidly.
This latter point allows one to simplify calculations in most real-world
adsorption applications
Presentation title 11
Chemisorption, or activated adsorption, on the other hand, is the result of chemical interaction
between the solid and the adsorbed substance.
The strength of the chemical bond may vary considerably and identifiable chemical compounds in
the usual sense may not form.
Nevertheless, the adhesive forces are generally much greater than that found in physical
adsorption.
Chemisorption is also accompanied by much higher enthalpy changes (ranging from 80 to as high
as 400 Btu/lbmol) with the heat liberated being on the order of the enthalpy of chemical reaction.
The process is frequently irreversible, and, on desorption, the original substance will often have
undergone a chemical change.
DIFFERENCE
Presentation title 12
Physisorption Chemisorption
Although it is probable that all solids adsorb gases (or vapors)
to some extent, adsorption, as a rule, is not very
pronounced unless an adsorbent possesses a large surface
area for a given mass.
For this reason, such adsorbents as silica gel, charcoals, and
molecular sieves are particularly effective as adsorbing
agents.
These substances have a very porous structure and, with
their large exposed surface, can take up appreciable volumes
of various gases.
The extent of adsorption can be increased further by
“activating” the adsorbents in various ways. For example, wood charcoal can be “activated” by heating
between 350 and 1000 C in a vacuum or in air, steam,
and certain other gases to a point where the adsorption of
carbon tetrachloride, (e.g., at 248C) can be increased
from 0.011 g/g of charcoal to 1.48.
The activation apparently involves a distilling out of
hydrocarbon impurities and thereby leads to exposure of a
larger free surface for possible adsorption.
14
The amount of gas adsorbed by a solid depends on a host of factors, including the surface
area of the adsorbent, the nature of the adsorbent and gas being adsorbed, the temperature,
and the pressure of the gas.
Since the adsorbent surface area cannot always be readily determined, common practice is to
employ the adsorbent mass as a measure of the surface available and to express the amount
of adsorption per unit mass of adsorbing agent used.
A concept, which becomes especially important in determining adsorbent
capacity, is that of “available” surface, i.e., surface area accessible to the
adsorbate molecule.
It is apparent from pore size distribution data that the major contribution to
surface area is located in pores of molecular dimensions.
It seems logical to assume that a molecule, because of steric effects, will not
readily penetrate into a pore smaller than a certain minimum diameter—hence,
the concept that molecules are screened out.
This minimum diameter is the so-called
critical diameter and is a characteristic of
the adsorbate and related to molecular
size.
Thus, for any molecule, the effective
surface area for adsorption can exist only
in pores that the molecule can enter.
Presentation title 15
• The extent of adsorption of a given situation is reached once equilibrium is established between the
adsorbent and its contacting solution.
• In practice, adsorption performance is also strongly influenced by the mass transfer of the species
between the solution and the adsorbent surfaces and the adsorption reaction rate.
• Adsorption is, therefore, an equilibrium-diffusion-reaction process.
Presentation title 16
ADSORPTION AS A SORPTION PROCESS
• The basic operating principle of adsorption: the preferential concentration of species onto surfaces of
adsorbing solids also operates in two other processes; namely, chromatography and ion exchange.
• Similar to most adsorption operations, chromatography operates in fixed-bed mode, but is devised for
separating liquid mixtures through an intermittent feed of the solution to be separated, followed by the
passage of an elution solution.
• In ion exchange, the solid substance used contains charged groups that interact with the charged ions
present in the liquid solution.
• If one views adsorption as an exchange process involving a fictitious species, the equivalence between
adsorption and ion exchange becomes obvious.
Presentation title 17
ADSORPTION VERSUS ABSORPTION
• Gas absorption is an operation in which a gas mixture is brought into contact with a liquid for the
purpose of dissolving one or more components of the mixture into the liquid. Absorption, therefore, is a
bulk phenomenon, and the extent of separation is limited by the solubilities of the gases involved.
• Adsorption is a surface phenomenon, and the extent of adsorption is limited by the relevant adsorption
isotherm relationship.
• Absorption may be carried out by passing the gas and liquid streams through a packed column
concurrently or counter-currently. The operation consists of two moving phases (gas and liquid) and a
stationary phase (column packing), which provides the interfacial area for liquid/gas contact.
• In fixed-bed adsorption, the fluid to be treated passes through a bed packed with adsorbent. The
process involves two phases, a moving fluid and a stationary solid phase of adsorbents.
• Absorption, therefore, may be treated as a steady-state process, while adsorption in a fixed-bed
operation is an inherently nonsteady state.
• As a result, the computational effort required for the design of fixed-bed adsorption is more extensive
than that of absorption.
Presentation title 18
ADSORPTION VERSUS DISTILLATION
• Distillation also belongs to the equilibration-diffusion category of separation processes, and is used for
the separation of homogeneous liquid mixtures.
• However, unlike adsorption or absorption, separation by distillation is effected by using energy instead of
material as an agent of separation.
• In a hypothetical study comparing distillation versus adsorption, Ruthven (1984) showed that
• For separating an A-B mixture, the use of distillation becomes impractical if
the relative volatility of A to B is less than 1.2.
• To separate light gas mixtures, adsorption was found to be preferential to
cryogenic distillation, even when the relative volatility is high
Presentation title 19
ADSORPTION VERSUS DEEP-BED FILTRATION
• Deep-bed filtration is a process designed for the removal of fine particles from
diluted fluid suspensions.
• Its operation is carried out by passing the suspension to be treated through a
column packed with granular or fibrous substances (filter media).
• Generally speaking, deep-bed filtration and fixed-bed adsorption share many
common features, such as equipment configuration and modes of operation.
Because of their similarities, the words ‘adsorption’ and ‘filtration’ are often
used interchangeably.
• The removal of submicron colloidal particles from fluid to solid surfaces may
be described as either filtration or deposition (Hirtzel and Rajagopalan, 1985).
• Carbon columns used to remove dissolved organic solutes in water treatment
are often referred to as ‘carbon filters’ by water engineers. Similarly, the term
‘charcoal filter’ is used to denote cartridges filled with granular activated
carbon for personal protection
Presentation title 20
• A major difference between them resides in the fact that in deepbed filtration,
removal of particles from the suspension to be treated results in particle
deposition over the exterior surfaces of the filter media.
• In contrast, the adsorbed dissolved species in fixed-bed adsorption covers
mainly the interior surfaces of adsorbents.
• The extent of separation achieved in adsorption is limited by the adsorption
equilibrium relationship. On the other hand, particle retention in deep-bed
filtration depends strongly upon the nature of particle-collector interaction
forces, but there is no clear-cut limit on the extent of deposition (Tien and
Ramarao, 2007).
• As adsorption processes may cease operation once the adsorbents become
saturated, for deep-bed filtration, due to increasing particle retention, the
pressure drops required for maintaining a specified throughput increase with
time. The duration of operation is limited by the maximum allowable pressure
drop.
Presentation title 21
OPERATION MODES OF ADSORPTION PROCESSES
Separation by adsorption is effected by contacting
solutions to be treated with selected adsorbents.
a) Adsorption in agitated vessels: Batch adsorption in
agitated vessel represents perhaps the simplest way of
bringing about fluid/adsorbent contact. A fixed amount of
adsorbent of a known state is added to a volume of
solution of a known solute concentration in a closed
vessel.
Agitation is provided by rotating stirrers in order to insure
that adsorbent particles are fully suspended, and the
adsorbate concentration is kept uniform throughout the
solution.
The data collected are the temporal evolution of the solute
concentration of the solution.
Not suitable for treating large volumes of solution (such as
water supplies)
Presentation title 22
• B) Adsorption in continuous-flow tanks.: This type of operation is often
used in waste water treatment.
• Adsorbents in powdered form, such as activated carbon, are added directly to
a particular step of a treatment process (biological or physio-chemical) for the
purpose of removing a particular species of contaminant.
Presentation title 23
• C) Fixed-bed adsorption consists of passing a solution
through a column packed with adsorbents, and is
commonly applied for eliminating trace contaminants
from a liquid solution, or toxic and volatile vapors from
gas streams.
• Fixed-bed adsorption, by nature, is a batch process.
• Starting with a fresh or newly regenerated state,
adsorbents in fixed-bed operations become increasingly
saturated with adsorbate to a point that reactivation
(desorption) becomes necessary, and operation ceases.
• However, by employing a number of identical columns
and arranging the adsorption/reactivation sequence
properly, a continuous operation may be established.
• This, in fact, is the principle for the development of
pressure swing and thermal swing processes for gas
Presentation title 24
• Moving-bed adsorption: It is carried out with both solid (adsorbent) and fluid phases in
motion. The movements of both phases may be parallel, counter-current, or perpendicular
(cross-flow); although the counter-current pattern is used in practice.
• Counter-current moving-bed adsorption is similar to gas absorption in packed beds.
• It is a steady-state process, and has the advantage of a more favorable utilization of the
available driving force of mass transfer.
• However, maintaining a steady-state circulation of large quantities of solids provide serious
problems in design, operation, and maintenance.
Presentation title 25
• In order to overcome this difficulty, a rather ingenious design based on the use
of a rotation valve for directing and switching fluid flow and the addition of a
desorbent, which enables the imitation of moving-bed behavior of fixed-bed
adsorption, was developed.
• This simulated moving-bed (SMB) concept provides the basis for the
development of the SORBEX processes for aromatic isomers’ separation
• Generally speaking, both moving-bed and simulated moving-bed are more
costly in development, construction, and maintenance than fixed-bed; but offer
more efficient separation.
Presentation title 26
APPLICATIONS OF ADSORPTION
• Recorded application of charcoal for purifying water two millennia
ago was found in Sanskrit manuscripts: “It is good to keep water in
copper vessels to expose it in sunlight and to filter it through
charcoal”
• Modern use of adsorbents began with the discovery of the
decoloring effect of charcoal on solutions (Lowitz, 1786), followed
by the invention of a charcoal cartridge for personal protection
during the First World War.
• With the increasing availability of different types of adsorbents
nowadays, and the more recent interests in biotechnology and
green technology, there has been a great expansion in the
applications of adsorption in many areas
Presentation title 27
• For liquid-phase adsorption • Decoloring, drying, or degumming
of petroleum products • Removing of dissolved organic species
from water supplies • Removing odor, taste, and color from water
supplies • Advanced treatment of waste water (domestic and
industrial) • Decoloring of crude sugar syrup and vegetable oils •
Recovery and concentration of proteins, pharmaceuticals, and bio-
compounds from dilute suspensions • Bulk separation of paraffin
and isoparaffins
• For gas-phase adsorption • Recovering organic solvent vapors •
Dehydration of gases • Removing toxic agents and odor for
personal protection • Air separation • Separating normal paraffins
from isoparaffin aromatics • CO2 capture for addressing climate
Presentation title 28
ADSORBENTS
Presentation title 29
Presentation title 30
ACTIVATED CARBON ACTIVATED ALUMINA ACTIVATED SILICA ZEOLITES
• Most solid substances possess some adsorption capacity for gas and/or liquid, but only a few of
them meet the necessary requirements to qualify as adsorbents for practical use.
• Some of the most widely used adsorbents are:
• a. Activated carbon. It may be produced from carbonization of materials such as coconut
shells, coal, lignite, wood, and similar substances, followed by activation with air or steam. It is
used widely for recovery of organic vapor, decoloring liquid solutions, and treatment and
purification of water supplies and waste water.
• b. Activated alumina. Al2O3 is prepared by removing water from colloid alumina. It has a high
capacity for water vapor, and is used mainly as a desiccant for gasses and liquids. Activated
alumina can be reactivated for reuse.
• c. Activated silica. Activated silica is prepared from gel precipitated by acid treatment of a
sodium silicate solution. Activated silica is used primarily for dehydration of air and removing
toxic species from air for personal protection purposes (i.e., in gas masks).
Presentation title 31
• d. Molecular sieve zeolites. Most adsorbents, including those mentioned herein, have pores
covering a range of sizes. Molecular sieve zeolites, however, are an exception.
• Molecular sieve zeolites are crystalline inorganic polymers of aluminosilicate, alkali, or alkali-
earth elements such as Na, K, and Ca.
• The “cages” of the crystal cells can entrap adsorbed matter, and the diameter of the
passageways, controlled by the crystal composition, regulate the size of the molecules that
can enter.
• These “size-selection” characteristics differentiate molecular sieve zeolites from other
adsorbents and make molecular sieve zeolites particularly useful in the separation of
hydrocarbon mixtures.
• e. Polymeric adsorbents. These adsorbents are prepared from polymerizing styrene and
divinyl benzene or acrylic esters for adsorbing nonpolar organics from aqueous solutions or
polar solutes, respectively.
• Generally speaking, polymeric adsorbents are more expensive than other adsorbents, and
their use is, therefore, more selective.
Presentation title 32
Presentation title
• The structural adsorbents are a class of adsorbents fabricated in forms of simple
ordered geometry, such as monolith, laminate, or foam.
• Such adsorbents provide simple, identifiable flow channels, and offer low
resistance to fluid flow.
• Compared with granular adsorbents, they also offer low resistance to mass
transfer.
• These adsorbents are, therefore, of particular interest for applications in pressure
swing adsorption.
Presentation title 34
• Similar to commonly used zeolites, metal organic frameworks (MOFs) and
covalent organic frameworks (COFs) are porous crystalline solids.
• The metal organic frameworks are compounds consisting of metal ions or
clusters coordinated to organic ligands to form one-, two-, and three-
dimensional structures.
• Covalent organic frameworks are a new type of organic material constructed
with organic building units via covalent bonds.
• This well-defined crystalline porous structure, together with tailored
functionalities, are known for their unique features.
• Both MOFs and COFs supposedly have great potential for gas storage, gas
adsorption, and as catalysts as well as sensors
Presentation title 35
ACTIVATED CARBON
• Charcoal, an inefficient form of activated carbon, is obtained by
the carbonization of wood.
• Various raw materials have been used in the preparation of
adsorbent chars, resulting in the development of active carbon, a
much more adsorbent form of charcoal.
• Industrial manufacture of activated carbon is obtained from
shells or coal that is subjected to heat treatment in the absence
of air, in the case of coal, followed by steam activation at high
temperature.
• Other substances of a carbonaceous nature also used in the
manufacture of active carbons include wood, coconut shells,
peat, and fruit pits.
• Zinc chloride, magnesium chloride, calcium chloride, and
phosphoric acid have also been used in place of steam as
activating agents.
36
• Gas adsorbent carbons find primary application in gas
purification, solvent recovery (hydrocarbon vapor
emissions), and pollution control.
37
Presentation title
ACTIVATED ALUMINA
Activated alumina (hydrated aluminum oxide) is produced by special
heat treatment of the precipitate of native alumina or bauxite.
It is available in either granular or pellet form
Activated alumina is primarily used for the drying of gases and is
particularly useful for the drying of gases under pressure.
38
39
Presentation title
Presentation title
41
Presentation title
42
DIFFERENCE
• Activated Alumina
• Activated alumina is a white
spherical substance
• Because of its unique skeleton
structure, it has a strong affinity
with active components.
• The product has a uniform
micropore distribution, a suitable
pore size, and a large pore
volume.
• High, low bulk density, good
mechanical properties, and
good stability.
• for the dehydration and drying of
industrial gases such as
chemical, metallurgy,
electronics, petroleum, etc.,
such as air, oxygen, nitrogen
and other gases, smelting gas
and petroleum cracking gas,
• Molecular Sieve
• The molecular sieve has a large specific
surface area of 300~1000m2/g, and the
inner crystal surface is highly polarized.
• It is a kind of high efficiency adsorbent
and a kind of solid acid.
• The surface has a high acid concentration
and acid strength, which can cause
Catalytic reaction.
• When the metal ions in the composition
are exchanged with other ions in the
solution, the pore size can be adjusted to
change its adsorption and catalytic
properties, thereby preparing molecular
sieve catalysts with different properties.
• Because of strong moisture absorption
capacity and is used for gas purification
treatment.
• Molecular sieves that have been stored
for a long time and have absorbed
moisture should be regenerated before
use.
PROPERTIES AND PHYSICAL CHARACTERISTICS OF
ADSORBENTS
43
ADSORPTION
EQUILIBRIUM
ADSORBENT
CHARACTERISTICS
Presentation title
The equilibrium relationship between the solution
and the adsorbed phase, and the equilibrium data
of specific adsorbent-adsorbate systems give the
limit of the performance of adsorption operations.
• Adsorbent size
• Adsorbent density
• Adsorbent porosity
• Pore size and size distribution
• Specific surface area
ADSORPTION EQUILIBRIUM
Presentation title 44
ADSORBENT CHARACTERISTICS
Presentation title 45
Adsorbent size, dp.: Based on sieve analysis results.
Rate of adsorbate uptake is often dependent on it
Determine the pressure drop required to sustain a given throughput in a fixed-bed operation.
Adsorbent density, ρp.
Extent of adsorption is often expressed on the basis of unit adsorbent mass, ρp is a factor of determining the
height of fixed-beds in design.
A related quantity, ρb, is given as ρp (1- ε), where ε is the fixed-bed porosity
Adsorbent porosity, εp.
Indication of the internal structure of the adsorbent, gives the value of the fraction of the void space of an
adsorbent pellet (or granule).
An associated quantity of εp is the specific pore volume
Pore size and size distribution
Specific surface area, Sg (surface area per unit mass of adsorbent): critical importance to the adsorption
capacity of an adsorbent and, to a lesser degree, the uptake rate of the adsorbate
Presentation title 46
Presentation title 47
Some surrogate quantities (indices) have also been used to represent the
adsorption capacity of adsorbents.
The iodine number is a rough measure of the adsorption of small molecule
components, and correlates with the BET surface area. It is defined as the
milligrams of iodine adsorbed by one gram of material when the iodine residual
concentration of the filtrate is 0.02N (0.01 mol L-1) according to ASTM D4607
standard, which is based on a three-point isotherm.
The molasses number was developed for cane sugar decoloration, and relates to
the adsorption of large molecules from liquid solutions. It is (DSTM 16) is a measure
of the decolorizing capacity and is an indicator of the macro pore and transport
pore structure of activated carbon. An amount of pulverised carbon is mixed and
heated with a standard molasses solution for a period of time. The Molasses
number is expressed as the relative amount of colour removed by an activated
carbon compared to a standard carbon. So the higher the molasses number, the
Presentation title
ADSORBENT SELECTION
In principle, an adsorbent may be selected based on a number of factors, including
the adsorbent’s adsorption capacity and selectivity, adsorbate uptake kinetics,
regenerability (for nowadays, discarding spent adsorbents is no longer an option),
compatibility with operating conditions, and not the least, cost.
While some of the information can be found in the literature and publications, other
information must be obtained from one’s own investigations, including experiments.
The circumstances one faces may not allow a thorough investigation.
Generally speaking, because of the large number of variables present, and their
incomplete information, there exists no general criteria for adsorbent selection.
Instead selection is often made on a case-by-case basis.
The decision is dictated largely by the type of application, and based on technical
considerations, in combination with subjective judgment and past experience.
ADSORBENT SELECTION
The moisture (water vapor) isotherm data of several common commercial
adsorbents are shown . Comment on their use in gas drying
• 3 stages as the adsorbate concentration increases.
• Firstly, a single layer of molecules builds up over the surface of the solid. This
monolayer may be chemisorbed and associated with a change in free energy
which is characteristic of the forces which hold it.
• As the fluid concentration is further increased, layers form by physical adsorption
and the number of layers which form may be limited by the size of the pores.
• Finally, for adsorption from the gas phase, capillary condensation may occur in
which capillaries become filled with condensed adsorbate, and its partial pressure
reaches a critical value relative to the size of the pore.
• In practice, all three may be occuring simultaneously in different parts of the
adsorbent since conditions are not uniform throughout.
• Generally, concentrations are higher at the outer surface of an adsorbent pellet
than in the centre, at least until equilibrium conditions have been established.
ADSORPTION
Adsorption equilibrium is a dynamic concept achieved when the rate at which
molecules adsorb on to a surface is equal to the rate at which they desorb
Most theories have been developed for gas–solid systems because the gaseous
state is better understood than the liquid.
Statistical theories are being developed which should apply equally well to gas–
solid and liquid–solid equilibria, though these are not yet at a stage when they
can be applied easily and confidently to the design of equipment.
ADSORPTION EQUILIBRIUM
The capacity of an adsorbent for a particular adsorbate involves the interaction of
three properties — the concentration C of the adsorbate in the fluid phase, the
concentration Cs of the adsorbate in the solid phase and the temperature T of the
system.
If one of these properties is kept constant, the other two may be graphed to
represent the equilibrium.
The commonest practice is to keep the temperature constant and to plot C against
Cs to give an adsorption isotherm.
When Cs is kept constant, the plot of C against T is known as an adsorption
isostere.
In gas–solid systems, it is often convenient to express C as a pressure of
adsorbate.
Single component adsorption
At very low concentrations the molecules adsorbed are widely spaced over the
adsorbent surface so that one molecule has no influence on another.
For these limiting conditions it is reasonable to assume that the concentration in
one phase is proportional to the concentration in the other
Concentration corresponding to any point on the
equilibrium curve may be obtained either by
adsorption of solute by a fresh adsorbent or by
desorption from an adsorbent having higher
adsorbate concentration.
In some cases, however, different equilibrium
curves, at least over a part of the isotherm, are
obtained depending upon whether the gas or
vapour is adsorbed or desorbed. This
phenomenon known as hysteresis
Equilibrium partial pressure corresponding to a
certain amount of solute concentration in the
adsorbent is high in case of adsorption than that in
case of desorption.
Hysteresis may be due to the shape of the
openings to the capillaries and pores of the
adsorbent or due to nonuniform wetting
58
59
Different types of adsorption isotherms are exhibited by different
adsorbate-adsorbent combinations
Moles adsorbed have been plotted as ordinate against (p′/p), p′ being the
partial pressure of the sorbate and p is its vapour pressure
Presentation title 60
Type-I is concave downwards
and represents systems in
which adsorption does not
proceed beyond the formation
of a monomolecular layer of
the adsorbate.
Common gas-solid adsorption
systems follow.
Adsorption of oxygen on carbon
at about -183°C is an example
Presentation title 61
Type-II is initially
concave downwards and
then near the middle of
the abscissa it becomes
concave upwards.
This point indicates the completion of
formation of monomolecular layer and
beginning of the formation of
multimolecular layer of the adsorbate.
Increasing tendency for the monolayer
to become complete before a second
layer starts.
Physical adsorption of water vapour by
carbon black at about 30°C.
62
Type-III is concave
upwards. This is a
relatively rare type.
The amount of gas or vapour adsorbed increases
indefinitely as the relative saturation (p′/p) approaches unity
The convex downward nature is caused by the heat of
adsorption of the first layer becoming less than the heat of
condensation due to molecular interaction in the monolayer
The solute loading is low at low concentration of the
adsorbate in the fluid. Bromine adsorption on silica gel at
20°C. , nitrogen on ice
63
Type-IV
Variation of a Type-II, but with a finite multilayer
formation corresponding to complete filling of the
capillaries
The concave region that occurs at low gas
concentrations is usually associated with the
formation of a single layer of adsorbate molecules
over the surface.
The convex portion corresponds to the build-up of
additional layers, whilst the other concave region
is the result of condensation of adsorbate in the
pores — so called capillary condensation
Adsorption of water vapour takes place on active
carbon at about 30°C.
64
Type-V
Variation of a Type-III
obtained
Adsorbing water
vapour on activated
carbon at about 100°C.
• Charcoal, with pores just a few molecules in diameter, almost always gives
a Type I isotherm.
• A non-porous solid is likely to give a Type II isotherm.
• If the cohesive forces between adsorbate molecules are greater than the
adhesive forces between adsorbate and adsorbent, a Type V isotherm is
likely to be obtained for a porous adsorbent and a Type III isotherm for a
non-porous one.
• In the activated aluminaair-water vapour system at normal temperature, the
isotherm is found to be of Type IV.
Presentation title 65
Freundlich Isotherm
The amount of gas adsorbed per given quantity of adsorbent increases relatively
rapidly with pressure and then much more slowly as the surface becomes covered
with gas molecules.
To represent the variation of the amount of adsorption per unit area or unit mass
with pressure, Freundlich proposed a equation
Although the requirements of the equation are met satisfactorily at lower
pressures, the experimental points curve away from the straight line at higher
pressures, indicating that this equation does not have general applicability in
reproducing adsorption of gases by solids.
Freundlich isotherm is particularly useful in cases where the actual identity of the
solute is not known as in the removal of coloured substances from mineral and
vegetable oils or from sugar solutions.
NE
The Langmuir isotherm
At higher gas phase concentrations, the number of molecules absorbed soon
increases to the point at which further adsorption is hindered by lack of space on
the adsorbent surface.
The rate of adsorption then becomes proportional to the empty surface available,
as well as to the fluid concentration.
At the same time as molecules are adsorbing, other molecules will be desorbing if
they have sufficient activation energy.
At a fixed temperature, the rate of desorption will be proportional to the surface
area occupied by adsorbate.
When the rates of adsorption and desorption are equal, a dynamic equilibrium
exists
NE
For adsorption of gases on to plane surfaces of glass, mica
and platinum.
As well as being limited to monolayer adsorption, the
Langmuir equation assumes that: (a) these are no
interactions between adjacent molecules on the surface. (b)
the energy of adsorption is the same all over the surface. (c)
molecules adsorb at fixed sites and do not migrate over the
surface
NE
• When adsorption first begins, every molecule colliding with the surface may
condense on it.
• However, as adsorption proceeds, only those molecules that occupy a part of
the surface not already covered by adsorbed molecules may be expected to be
adsorbed.
• The result is that the initial rate of condensation of molecules on a surface is
high and then falls off as the surface area available for adsorption is
decreased.
• On the other hand, a molecule adsorbed on a surface may, by thermal
agitation, become detached from the surface and escape into the gas.
• The rate at which desorption will occur will depend, in turn, on the amount of
surface covered by molecules and will increase as the surface becomes more
fully saturated.
• These two rates, condensation (adsorption) and evaporation (desorption), will
eventually become equal and when this happens, an adsorption equilibrium will
Presentation title 69
NE
Which vapor would be most readily adsorbed using activated
carbon?
H2O at 140 F
CH4 at 70 F
C4H10 at 140F
C4H10 at 70 F
C4H10 at 70 F
NE
The carbon dioxide
adsorption isotherms for
Columbia (“Columbia” is a
registered trademark of Union
Carbide Corporation)
activated carbon is presented
for temperatures of 30, 50,
and 95 C.
1) Determine the constants of
the Freundlich equation at a
temperature of 50C.
2) Determine the constants
of the Langmuir equation.
NE
For the Freundlich equation (1), the plot of (log Y ) vs
(log p) leads to the equation:
Y = 30p^0.7
NE
NE
Braunauer-Emmet-Teller Isotherm
The BET theory allows different numbers of adsorbed layers to build
up on different parts of the surface, although it assumes that the net
amount of surface which is empty or which is associated with a
monolayer, bilayer and so on is constant for any particular equilibrium
condition.
Monolayers are created by adsorption on to empty surface and by
desorption from bilayers.
Monolayers are lost both through desorption and through the
adsorption of additional layers.
The rate of adsorption is proportional to the frequency with which
NE
Spherical particles of 15 nm diameter and density 2290 kg/m3 are pressed together to form a pellet.
The following equilibrium data were obtained for the sorption of nitrogen at 77 K.
P/P0 M3 liq N2 * 10^6/kg
solid
0.1 66.7
0.2 75.2
0.3 83.9
0.4 93.4
0.5 108.4
0.6 130.0
0.7 150.2
0.8 202.0
0.9 348.0
Obtain estimates of the surface area of the pellet from the
adsorption isotherm and compare the estimates with the
geometric surface.
The density of liquid nitrogen at 77 K is 808 kg/m3.
Area occupied by one adsorbed molecule of nitrogen = 0.162
nm2
Avogadro Number = 6.02 × 10^26 molecules/kmol
NE
The Gibbs isotherm and H-J Model
Assume that adsorbed layers behave like liquid films, and that the adsorbed
molecules are free to move over the surface. It is then possible to apply the
equations of classical thermodynamics. The properties which determine the free
energy of the film are pressure and temperature, the number of molecules
contained and the area available to the film.
The H–J model may be criticised because the comparison between an adsorbed
film on a solid surface and a liquid film on a liquid surface does not stand
detailed examination
BET equation may be applied to low surface coverage when it is thought that adsorbed molecules do not move.
The H–J equation may be applied to the middle range of relative pressures (P /P0) corresponding to the
completion of a monolayer and the formation of additional mobile layers.
Neither theory, it seems, is equipped to deal with the filling of micropores that occurs with the onset of capillary
condensation at higher relative pressures
NE
The potential theory
In this theory the adsorbed layers are
considered to be contained in an adsorption
space above the adsorbent surface.
The space is composed of equipotential
contours, the separation of the contours
corresponding to a certain adsorbed volume
The theory was postulated by POLANYI, who
regarded the potential of a point in adsorption
space as a measure of the work carried out by
surface forces in bringing one mole of
adsorbate to that point from infinity, or a point at
such a distance from the surface that those
forces exert no attraction.
NE
The work carried out depends on the phases involved.
Polanyi considered three possibilities:
(a) that the temperature of the system was well below the critical temperature of the adsorbate
and the adsorbed phase could be regarded as liquid,
(b) that the temperature was just below the critical temperature and the adsorbed phase was a
mixture of vapour and liquid,
(c) that the temperature was above the critical temperature and the adsorbed phase was a
gas.
Only the first possibility, the simplest and most common, is considered here
The potential theory postulates a unique relationship between the adsorption potential εp and
the volume of adsorbed phase contained between that equipotential surface and the solid.
It is convenient to express the adsorbed volume as the corresponding volume in the gas phase
from adsorption data for one gas, data for other gases on the same adsorbent may be
NE
CHARACTERISTIC CURVE FOR THE ADSORPTION OF CARBON
DIOXIDE ON TO CHARCOAL
79
NE
NE
STRUCTURE OF ADSORBENTS
The equilibrium capacity of an adsorbent for different molecules is one factor affecting its selectivity.
Another is the structure of the system of pores which permeates the adsorbent
This determines the size of molecules that can be admitted and the rate at which different molecules
diffuse towards the surface.
Molecular sieves, with their precise pore sizes, are uniquely capable of separating on the basis of
molecular size.
In addition, it is sometimes possible to exploit the different rates of diffusion of molecules to bring
about their separation. A particularly important example concerns the production of oxygen and
nitrogen from air.
Although it is sometimes possible to view pores with an electron microscope and to obtain a
measure of their diameter, it is difficult by this means to measure the distribution of sizes and
impossible to measure the associated surface area.
NE
Surface area
The simplest measure of surface area is that obtained by the so-called point B method. Many isotherms of
Type II or IV show a straight section at intermediate relative pressures.
The more pronounced the section, the more complete is the adsorbed monolayer before multilayer
adsorption begins.
The constant of proportionality may be found by using the
same adsorbate on a solid of known surface area.
Since the equation was derived for mobile layers and makes
no provision for capillary condensation, it is most likely to fit
data in the intermediate range of relative pressures
NE
Pore sizes
Where there is a wide distribution of pore sizes and, possibly, quite separately developed pore
systems, a mean size is not a sufficient measure.
There are two methods of finding such distributions. In one a porosimeter is used, and in the
other the hysteresis branch of an adsorption isotherm is utilised.
Both require an understanding of the mechanism of capillary condensation.
The regions of Type IV and Type V isotherms which are concave to the gas-concentration axis at
high concentrations correspond to the bulk condensation of adsorbate in the pores of the
adsorbent.
An equation relating volume condensed to partial pressure of adsorbate and pore size, may be
found by assuming transfer of adsorbate to occur in three stages: (i) transfer from the gas to a
point above the meniscus in the capillary. (ii) condensation on the meniscus. (iii) transfer from a
plane surface source of liquid adsorbate to the gas-phase in order to maintain the first stage
NE
isotherm for nitrogen on activated alumina.
NE
KINETIC EFFECTS
It was assumed that equilibrium was achieved instantly between the concentrations of adsorbate
in the fluid and in the adsorbed phases.
Whilst it may be useful to make this assumption because it leads to relatively straightforward
solutions and shows the interrelationship between system parameters, it is seldom true in
practice.
In large-scale plant particularly, performance may fall well short of that predicted by the
equilibrium theory.
NE
There are several resistances which may hinder the movement of a molecule of adsorbate
from the bulk fluid outside a pellet to an adsorption site on its internal surface
Some of these are sequential and have to be traversed in series, whilst others derive from
possible parallel paths.
In broad terms, a molecule, under the influence of concentration gradients, diffuses from the
turbulent bulk fluid through a laminar boundary layer around a solid pellet to its external
surface. It then diffuses, by various possible mechanisms, through the pores or the lattice
vacancies in the pellet until it is held by an adsorption site. During desorption the process is
reversed
NE
In the process of being adsorbed, molecules lose degrees of freedom and a heat
of adsorption is released.
All the resistances to mass transfer also affect the transfer of heat out of the
pellet, though to a different extent.
If the heat cannot disperse fast enough, the temperature of the adsorbent
increases and the effective capacity of the equipment is reduced.
NE
a) Boundary film
Steady-state film-theory: It is assumed that the difference in concentration and
temperature between the bulk fluid and the external surface of a pellet is confined
to a narrow laminar boundary-layer in which the possibility of accumulation of
adsorbate or of heat is neglected.
b) Intra-pellet effects
After passing through the boundary layer, the molecules of adsorbate diffuse
into the complex structure of the adsorbent pellet, which is composed of an
intricate network of fine capillaries or interstitial vacancies in a solid lattice.
c) Adsorption
NE
c) Adsorption
The final stage in getting a molecule from the bulk phase outside a pellet on to
the interior surface is the adsorption step itself.
Where adsorption is physical in nature, this step is unlikely to affect the overall
rate.
An equilibrium state is likely to exist between adsorbate molecules immediately
above a surface and those on it.
NE
When chemisorption is involved, or when some additional surface chemical
reaction occurs, the process is more complicated.
Since the adsorption process results in the net transfer of molecules from the
gas to the adsorbed phase, it is accompanied by a bulk flow of fluid which keeps
the total pressure constant.
The effect is small and usually neglected.
As adsorption proceeds, diffusing molecules may be denied access to parts of
the internal surface because the pore system becomes blocked at critical points
with condensate.
NE
Simplifying by selecting what is termed the controlling resistance.
This relates to the step whose resistance is overwhelmingly large in comparison
with that for the other layers.
The whole of the difference in adsorbate concentration, from its value in the
bulk gas to its value at the adsorbent surface, occurs across the controlling
resistance.
It may be, however, that rate control is mixed, involving two or more
resistances.
ADSORPTION EQUIPMENT
The scale and complexity of an adsorption unit varies from a laboratory
chromatographic column a few millimeters in diameter, as used for analysis, to a
fluidised bed several metres in diameter, used for the recovery of solvent
vapours, from a simple container in which an adsorbent and a liquid to be clarified
are mixed, to a highly-automated moving-bed of solids in plug-flow.
All such units have one feature in common in that in all cases the adsorbent
becomes saturated as the operation proceeds.
For continuous operation, the spent adsorbent must be removed and replaced
periodically and, since it is usually an expensive commodity, it must be
regenerated, and restored as far as possible to its original condition.
In most systems, regeneration is carried out by heating the spent adsorbent in a
suitable atmosphere.
For some applications, regeneration at a reduced pressure without increasing
the temperature is becoming increasingly common.
The precise way in which adsorption and regeneration are achieved depends on
the phases involved and the type of fluid–solid contacting employed.
3 types of contacting:
(a) Those in which the adsorbent and containing vessel are fixed whilst the inlet
and outlet positions for process and regenerating streams are moved when
the adsorbent becomes saturated. The fixed bed adsorber is an example of
this arrangement. If continuous operation is required, the unit must consist
of at least two beds, one of which is on-line whilst the other is being
regenerated.
(b) Those in which the containing vessel is fixed, though the adsorbent moves
with respect to it. Fresh adsorbent is fed in and spent adsorbent removed for
regeneration at such a rate as to confine the adsorption within the vessel.
This type of arrangement includes fluidised beds and moving beds with
solids in plug flow.
(c) Those in which the adsorbent is fixed relative to the containing vessel which
moves relative to fixed inlet and outlet positions for process and regenerating
fluids. The rotary-bed adsorber is an example of such a unit.
Dual stationary-bed solvent recovery system with auxiliaries for collecting the
vapor–air mixture from various point sources, then transporting through the
particulate filter and into the on-stream carbon adsorber—in this case, bed 1.
The effluent air, which is virtually free of vapors, is usually vented outdoors.
The lower carbon adsorber (number 2) is regenerated during the service time of
bed 1.
A steam generator or other source of steam is required.
The effluent steam–solvent mixture from the adsorber is directed through the
condenser and the liquified mixture then passes into the decanter and/or
distillation column for separation of the solvent from the steam condensate.
2 designs of a stationary-bed solvent recovery system
The type employs vertical cylindrical bends wherein the solvent laden air flows
axially down through the bed.
This particular design is advertised for use in the recovery of solvents used in
degreasing and dry cleaning, although equally well-suited for recovery of
solvents from other industries.(trichloroethylene, tetrachloroethylene, toluene,
freon TF, and dichloromethane. )
Regeneration is accomplished with steam flowing upward through the bed and,
since the above-mentioned solvents are essentially immiscible in water,
decantation is used to separate the condensed solvent from the steam
condensate.
The valves are of disk type, opened and closed by air-driven pistons.
Water is used as coolant in the condenser. Steam, electric power to drive the
blower, and cooling water are three operating-cost items. The cost of steam and
cooling water increases with frequency of regeneration.
This particular unit is a Vic Model 572 with two adsorber beds used alternately, i.e.,
while one unit is on-stream adsorbing, the other is regenerating.
• Dual adsorber systems can also be operated with both beds on-stream
simultaneously, especially when solvent concentrations are low.
In this situation, regeneration is less frequent than a full work shift and may be
accomplished during off-work hours.
Operation in parallel almost doubles the air-handling capacity of the adsorption unit
and may be an advantage in terms of operating cost
At the inlet end of the bed, the adsorbent has become saturated and is in
equilibrium with the adsorbate in the inlet fluid.
At the exit end, the adsorbate content of the adsorbent is still at its initial value.
In between, there is a reasonably well-defined mass-transfer zone in which the
adsorbate concentration drops from the inlet to the exit value.
This zone progresses through the bed as the run proceeds.
At t1 the zone is fully formed, t2 is some intermediate time, and t3 is the breakpoint
time tb at which the zone begins to leave the column.
For efficient operation the run must be stopped just before the breakpoint
If the run extends for too long, the breakpoint is exceeded and the effluent
concentration rises sharply
The rotary component of the system consists of four coaxial cylinders.
The outer cylinder is impervious to gas flow except at the slots near the left end.
They serve as air inlet ports where they are shown uncovered and as steam–vapor outlet ports as
shown at the lower left end of the rotary.
The carbon bed is retained between two cylinders made of screen or perforated metal and also
segmented by partitions placed radially between the two cylinders.
The inner cylinder is again impervious to gas flow except at the slots near the right end of the
rotary.
These slots serve as outlet ports for the discharge air and as inlet ports for the regenerating steam.
On each rotation of the rotary, each segment of the bed undergoes adsorption and regeneration.
The desorbed solvent can then be separated from the steam by decantation or distillation.
Because of the continuous regeneration capability of the rotary bed, more efficient utilization of the
carbon is possible than with stationary-bed systems.
In most solvent recovery operations, the adsorption zone and the
saturated bed behind it are idle but add to the bulk of the system and
increase airflow resistance through the bed.
In deep beds of 12 to 36 inches, as required in the stationary-bed
systems, a large portion of the bed is idle at any one time.
By continuous regeneration, the regeneration time for each segment of
the bed is shortened, and thereby shorter bed lengths can be used.
Advantages: a more compact system and a reduced pressure drop.
Disadvantages are those associated with wear on moving parts and
maintaining seals in contact with moving parts.
The use of shorter beds also decreases the steam utilization efficiency.
Fluidized-bed solvent recovery system
The carbon is recirculated continuously through the adsorption–regeneration cycle.
The spent carbon, saturated with solvent, is elevated to the surge bin and then passed down into
the regeneration bed where it is contacted with an upward flow of steam.
The regenerated carbon is then metered into the adsorber, where the carbon traverses nine beds
while fluidized with the upward flow of the vapor-laden air.
An air velocity of approximately 240 ft/min is required to cause fluidization of the bed.
In both the regeneration and adsorption phases, the carbon is moved countercurrent to the gas or
vapor.
The countercurrent movement increases the efficiency of regeneration (i.e., the steam :solvent
ratio is less than for a stationary bed under otherwise comparable conditions).
In addition to the beneficial effects of the countercurrent movement, the bed length can be
increased to further improve steam utilization
Countercurrent flow also increases the effective use of the carbon; more solvent can
be recovered with less carbon than with stationary- or rotary-bed systems.
By adjustments or balance of the carbon and solvent input rates, the total carbon in the
adsorber can be made part of the adsorption zone reaching saturation in the lowest
bed just before it is discharged into the elevator.
Very little of the carbon is then idle; hence, maximum utilization is made of the carbon.
The fact that the carbon has reached saturation when delivered to the regeneration
phase is another factor in the reduced steam requirement; in this respect, the fluidized
bed system is most favorable.
In rotary-bed operations, the carbon moved into the regeneration phase has reached
the lowest state of saturation in the three systems.
When large air volumes are treated and available space for the installation is at a
premium, the smaller size and lower initial cost are definite advantages over the
stationary-bed system.
A serious disadvantage is that of high attrition losses of the carbon caused by the
fluidization of the beds.
Because of the attrition, filtration of the effluent airstream may be required.
The influent airstream need not be highly filtered because plugging of the fluidized
beds with particulate matter is minimal.
The major manufacturer of these units has given consideration to discontinue
operations.
No adsorber system could operate alone without sufficient auxiliary equipment and
the components required to collect, transport, and filter vapor-laden airstreams
being delivered to the adsorber.
The ducts and piping need to be sized properly for required air velocities to
optimize the efficiency of the adsorber.
If the air velocity is too high, the stationary-bed adsorber may become a fluidized-
bed adsorber, or low flows may create severe channeling through the beds.
The fan is the catalyst for forcing the gas stream in and out of the unit, so it is
important that careful attention to design and sizing is given to this equipment.
Because there are several adsorber configurations, the location of a
filter can be varied:
• before the inlet airstream (in a stationary bed) to reduce possible
contamination to the adsorbent,
• after the fluidized-bed adsorber to reduce particulate emissions.
Monitoring of the filter efficiency can be accomplished by measuring
the pressure drop across the filter using either a manometer or
pressure gauge.
DESIGN AND PERFORMANCE EQUATIONS
Presentation title 116
The gas stream containing the solute at an initial concentration C0 is passed
down through a (deep) bed of adsorbent material that is free of any solute.
Most of the solute is readily adsorbed by the top portion of the bed.
The small amount of solute that is left is easily adsorbed in the remaining section
of the bed.
The effluent from the bottom of the bed is essentially solute-free, denoted as C1.
After a period of time, the top layer of the adsorbent bed becomes saturated with
solute.
The majority of adsorption (approximately 95%) now occurs in a narrow portion of
the bed directly below this saturation section.
The narrow zone of adsorption is referred to as the mass transfer
zone (MTZ).
As additional solute vapors pass through the bed, the saturated
section of the bed becomes larger and the MTZ moves further down
the length of the adsorbent.
The actual length of the MTZ remains fairly constant as it travels
through the adsorbent bed.
Additional adsorption occurs as the vapors pass through the
“unused” portion of the bed.
The effluent concentration at C2 remains essentially zero, since
there is still an unsaturated section of the bed
Finally, when the lower portion of the MTZ reaches the bottom of the bed, the
concentration of solute in the effluent suddenly begins to rise.
This is referred to as the breakthrough point (or breakpoint)—where untreated
vapors are being exhausted from the adsorber.
If the air stream is not switched to a fresh bed, the concentration of the solute in the
outlet will quickly rise until it approaches the initial concentration
It should be noted that in most air pollution control applications even trace amounts
of contaminants in the effluent stream are undesirable.
To achieve continuous operation, adsorbers must be either replaced or cycled from
adsorption to desorption before breakthrough occurs.
In desorption or regeneration, the adsorbers solute vapors are removed from the
used bed in preparation for the next cycle.
In regard to regenerable adsorption systems, three important terms are used to
describe the equilibrium capacity (CAP) of the adsorbent bed. These capacity terms
are measured in mass of vapor per mass of adsorbent. First, the breakthrough
capacity (BC) is defined as the capacity of the bed at which unreacted vapors begin
to be exhausted. The saturation capacity (SAT) is the maximum amount of vapor that
can be adsorbed per unit mass of carbon (this is the capacity read from the
adsorption isotherm) and thus is equal to CAP. The working capacity or working
charge (WC) is the actual amount adsorbed in the adsorber. Thus, the working
capacity is a certain fraction of the saturation capacity. Working capacities can range
from 0.1 to 0.7 times the saturation capacity. (Note: A smaller capacity increases the
amount of carbon required.) This fraction is set by the designer for individual systems
by often balancing the cost of carbon and adsorber operation vs preventing
breakthrough while allowing for an adequate cycle time. Another factor in
determining the working capacity is that it is uneconomical to desorb all the vapors
from the adsorber bed. The small amount of residual vapors left on the bed is
referred to as the HEEL. This HEEL accounts for a large portion of the difference
between the saturation capacity and the working capacity. In some cases, the
In regard to regenerable adsorption systems, three important terms are used to
describe the equilibrium capacity (CAP) of the adsorbent bed. measured in mass of
vapor per mass of adsorbent.
First, the breakthrough capacity (BC) is defined as the capacity of the bed at which
unreacted vapors begin to be exhausted.
The saturation capacity (SAT) is the maximum amount of vapor that can be
adsorbed per unit mass of carbon (this is the capacity read from the adsorption
isotherm) and thus is equal to CAP.
The working capacity or working charge (WC) is the actual amount adsorbed in the
adsorber. Thus, the working capacity is a certain fraction of the saturation capacity.
Working capacities can range from 0.1 to 0.7 times the saturation capacity. (Note: A
smaller capacity increases the amount of carbon required.)
Another factor in determining the working capacity is that it is uneconomical to
desorb all the vapors from the adsorber bed.
The small amount of residual vapors left on the bed is referred to as the HEEL.
This HEEL accounts for a large portion of the difference between the saturation
capacity and the working capacity.
In some cases, the working capacity can be estimated by assuming it to be equal
to the saturation capacity minus the HEEL
The determination of the state of
the activated carbon (or any
adsorbent) is done by monitoring
the concentration of the
adsorbate through the column.
At the top of the column the
adsorbent is completely loaded,
further down it is partially loaded,
and towards the bottom it is
unladen (contains no adsorbate).
Presentation title 123
Over time, as more water and adsorbate pass through the
column, more of the adsorbent becomes loaded and the
concentration profile changes
At time t1 there is no adsorbate leaving the column (all gets
adsorbed in the adsorbent).
However, at time t2 there is some adsorbent leaving the column
with a concentration C2, which increases to C3 at time t3.
This outlet adsorbate concentration is extremely important in the
operation of an adsorption process, as it often has a maximum
allowable value imposed by process requirements or environmental
regulations
The evolution of this outlet concentration over time is called
the breakthrough curve of the column.
It can be generated by collecting the data (C1, C2, and C3) from the
curves
Alternatively, it is customary to rotate the concentration profiles by 90
degree counter-clockwise, plot them all in one graph and then use the
outlet concentration to develop the breakthrough curve
From a breakthrough curve,
the time of breakthrough can be
calculated.
The time of breakthrough is the
time t* when the outlet
concentration of the adsorbate
reaches the acceptable maximum
concentration C*
Once that point is reached, the
adsorption on this column must be
stopped and the adsorbent
regenerated, if possible, or
replaced.
Presentation title 128
The region of falling concentration is called the mass transfer zone (MTZ).
The gradient is called the adsorption wave front and is usually S-shaped.
When its leading edge reaches the exit, breakthrough is said to have been
attained.
Practically, the breakthrough is not regarded as necessarily at zero concentration
but at some low value such as 1% or 5% of the inlet concentration that is
acceptable in the effluent.
A hypothetical position, the average adsorbate content equals the saturation value,
is called the stoichiometric front.
The distance between this position and the exit of the bed is called the length of
unused bed (LUB).
The exhaustion time is attained when the effluent concentration
becomes the same as that of the inlet, or some practical high
percentage of it, such as 95 or 99%.
The shape of the adsorption front, the width of the MTZ, and the profile
of the effluent concentration depend on the nature of the adsorption
isotherm and the rate of mass transfer.
Practical bed depths may be expressed as multiples of MTZ, values of
5–10 multiples being economically feasible.
Systems that have linear adsorption isotherms develop constant MTZs
whereas MTZs of convex ones become narrower, and those of
concave systems become wider as they progress through
The narrower the MTZ, the greater the degree of utilization of the
Factors that play important roles in dynamic adsorption, and the length
and shape of the mass transfer zone are: (i) the type of the adsorbent
(ii) the particle size of the adsorbent, which is dependent on maximum
allowable pressure drop (iii) the depth of the adsorbent bed and the
fluid velocity (iv) the temperature of the fluid stream and the adsorbent
(v) the concentration of the contaminants to be removed and not to be
removed (including moisture) (vi) the pressure of the system (vii) the
removal efficiency required and (viii) possible decomposition or
polymerization of contaminants on the adsorbent
The ideal adsorption time is Is by which the solute concentration in the
fluid is reduced to 50% of the original value.
The movement of the adsorption zone through the adsorbent bed and
the effect of some process variables on the ideal adsorption time can be
obtained in the following manner:
The solute fed per unit time per unit area of bed cross section is given
by
Pressure swing adsorption (PSA)
It is based on the principle that the adsorbing capacity of an adsorbent increases
with increase in the solute pressure.
In a pressure swing adsorption system, adsorption is carried out at an elevated
pressure.
The flow of the gas is stopped as the breakthrough point is reached and the
adsorbent is regenerated by reducing the pressure.
Sometimes, a hot inert gas is blown through the solid bed to carry away the
desorbed solute molecules.
Hot air or steams are frequently used for the purpose.
A high dynamic adsorption capacity can be achieved in PSA by
reducing the pressure to near vacuum and the PSA system is
sometimes modified by putting a vacuum pump in the blow down line.
This is called vacuum swing adsorption (VSA) or Vacuum pressure
swing adsorption (VPSA).
Sometimes in a single bed system, the adsorber may be rapidly
pressurized and depressurized. It is called rapid pressure swing
adsorption (RPSA).
Some of the key industrial applications are as follows: (i) Gas drying (ii)
Solvent vapour recovery (iii) Fractionation of air to produce oxygen and
nitrogen enriched gases (iv) Production of hydrogen from steam-
methane reformer (SMR) off-gas and petroleum refinery off-gases (v)
Separation of methane and carbon dioxide from landfill gas (vi)
Separation of carbon monoxide and hydrogen (vii) Normal isoparaffin
separation (viii) Alcohol dehydration
Reasons for PSA:
(i) An extra degree of thermodynamic freedom for describing the
adsorption process introduces immense flexibility in the process
design as compared with the other conventional separation
processes such as distillation, extraction, absorption, etc.
(ii) Numerous micro-meso porous families of adsorbents exhibiting
different adsorptive properties for separation of gas mixtures are
available.
(iii) The adsorbate is recovered in a relatively concentrated form, and the
energy cost is low since no heating is involved.
(iv) The optimum combination between a material and a process in
designing PSA separation scheme promotes innovation.
The RPSA processes are designed to increase the productivity
(volume of product/volume of adsorbent/cycle) of separation devices
by an order of magnitude or more by using total process cycle times of
seconds instead of minutes as in the case of a conventional PSA
cycles.
It is achieved by simply running a conventional cycle faster using novel
hardware or by changing both the adsorber and the process cycle
designs.
However, separation efficiency and performance may be compromised
(lower product purity or recovery) due to time limitations in
incorporating the complimentary cyclic steps and limitations caused by
adsorption kinetics
Skarstrom cycle
In a single bed system, the output is not continuous as the bed is to be
regenerated separately.
However, in a twin bed adsorber the flow of output is kept continuous by
operating the twin beds in tandem, one bed acts on adsorption mode
while the other on regeneration mode.
This is the basic Skarstrom cycle
Each bed operates alternately in two half-cycles of equal duration: (i)
adsorption followed by pressurization, and (ii) purging followed by
depressurization or blow down
The feed gas is used for pressurization while a part of the effluent
product gas is used for purge.
Adsorption is taking place in bed 1 while a part of the gas leaving the bed
1 is routed to bed 2 to purge that bed in a direction counter-current to the
direction of flow of the feed gas during the adsorption step.
When moisture is to be removed from air, the dry air product is produced
during the adsorption step in each of the beds.
The adsorption and purge steps usually take less than 50% of the total
cycle time.
In many commercial applications of this technology, these two steps
consume a much greater fraction of cycle time because pressurization
and depressurization (blow down) can be completed rapidly.
Therefore, cycle times for PSA and VSA are short, typically seconds to
minutes. Thus, small beds have relatively large throughputs
Since the introduction of the Skarstrom cycle, numerous improvements have been
made to increase product purity, product recovery, adsorbent productivity and energy
efficiency
Among these modifications are the use of (i) three, four or more beds, (ii) a pressure
equalization step in which both beds are equalized in pressure following purge of
one bed and adsorption in the other, (iii) pretreatment or guard beds to remove
strongly adsorbed components that might interfere with the separation of other
components, (iv) purge with a strongly adsorbing gas, and (v) an extremely short
cycle time to approach isothermal operation, if a longer cycle causes an undesirable
increase in temperature during adsorption and an undesirable decrease in
temperature during desorption.
Separations by PSA and VSA are controlled by adsorption equilibrium
or adsorption kinetics, where the latter refers to mass transfer external
and/or internal to the adsorbent particle.
Both types of control are important commercially. For the separation of
air with zeolites, adsorption equilibrium is the controlling factor.
Nitrogen is more strongly adsorbed than oxygen and argon.
For air with 21% oxygen and 1% argon, oxygen of about 96% purity
can be produced. When carbon molecular sieves are used, oxygen and
nitrogen have almost the same adsorption isotherms, but effective
diffusivity of oxygen is much larger than that of nitrogen. Consequently,
nitrogen of very high purity (> 99%) can be produced.
Regeneration of the Adsorbent
Often it becomes necessary to regenerate the adsorbent and reuse the same.
This is particularly necessary if the adsorbent is costly or the adsorbate is valuable
and has to be recovered to the extent possible.
There are several methods for regeneration of adsorbents of which the pressure
swing and temperature swing methods are commonly used.
Temperature Swing Regeneration
An adsorbate loaded solid can be regenerated by passing a relatively
hot inert gas like hot air or steam through the bed.
The temperature of regeneration is decided on the basis of adsorption-
desorption equilibrium or isotherm and the characteristics of the
adsorbent.
Feed stream containing a small quantity of an adsorbate at a partial
pressure p′1 is passed through the adsorption bed at a temperature T1.
The initial equilibrium loading on the adsorbent is X1, generally
expressed in weight of adsorbate per unit weight of adsorbent.
After equilibrium between the adsorbate and the adsorbent is reached,
regeneration requires raising the temperature of the bed to T2.
Desorption occurs as more feed is passed through the bed and a new
equilibrium loading of X2 is established.
The net removal capacity of the adsorption bed is given by the difference
between X1 and X2.
When the bed is again cooled to T1, purification of the gas stream can be
started.
Typical adsorbate removal in a temperature swing cycle is generally in
excess of 1 kg/100 kg of adsorbent.
Since the regeneration time associated with the temperature swing cycle
is relatively long, the cycle is used almost exclusively to remove small
concentrations of adsorbate from feed streams.
This allows the on-stream time to be a significant fraction of the total
cycle time of the separation process.
In addition, temperature swing cycles may require a substantial
quantity of energy per unit of adsorbate.
However, if the adsorbate concentration in the feed is small, the
energy cost per unit of feed processed can be reasonable.
In the inert-purge cycle, the adsorbate during regeneration is removed
by passing a nonadsorbing gas containing no adsorbate through the
bed.
This lowers the partial pressure of the adsorbate and initiates the
desorption process.
If sufficient pure purge gas is directed through the bed, the adsorbate
will be completely removed and adsorption can be resumed.
Cycle times, in contrast to temperature swing processes are often only
a few minutes rather than hours.
However, because adsorption capacity is reduced as adsorbent
temperature rise, inert-purge processes are usually limited to 1 to 2 kg
The displacement purge cycle differs from the inert-purge cycle since
the cycle uses a gas or liquid in the regeneration step that adsorbs as
strongly as the adsorbate.
Thus, desorption is facilitated both by adsorbate partial pressure or
concentration reduction in the fluid, and by competitive adsorption of the
displacement medium. Typical cycle times are usually a few minutes.
The use of two different types of purge fluid negatively affects the
contamination level of the less adsorbed product.
However, since the heat of adsorption of the displacement purge fluid is
approximately equal to that of the adsorbate as the two exchange
places on the adsorbent, the net heat generated or consumed is
essentially negligible and thus maintains nearly isothermal conditions
The latter condition makes it possible to increase the adsorbate loading of the
displacement purge cycle over that for the inert-purge cycle
One of the important considerations for a TSA system is the temperature of the
adsorbent bed during regeneration, others being quantity and flow of the
regenerating gas.
The regeneration temperature can be estimated by a simple graphical procedure
from the isosteres
The isosteres are a set of curves which may be drawn from the isotherms by
plotting the partial pressures or dew points or concentrations against the
corresponding temperature or reciprocal of temperature for a equilibrium loading
as parameter.
The particular form of the parameters, viz. p′, log p′, T, or 1/T is chosen so as to
make the isosteres almost linear for facilitating any interpolation.
154
155
Pressure Swing Regeneration
In this method, the adsorbent bed is regenerated by reducing the pressure of the
adsorbent bed to atmospheric or even lower pressure when the bed is stripped of
the adsorbed solute.
If required, the adsorbent bed may be further cleaned by passing hot air or steam
through the bed.
In the process, the partial pressure of an adsorbate can be reduced by lowering
the total pressure of the gas.
The lower the total pressure of the regeneration step, the greater is the adsorbate
loading during the adsorption step
156
The time required to load, depressurize, regenerate and repressurize a
bed is generally only a few minutes.
Thus, even though practical adsorbate loadings are almost always less
than 1 kg of adsorbate per 100 kg of adsorbent, to minimize thermal
gradient problems, the very short times makes the pressure swing cycle
a viable option for bulk gas separations
157
In the pressure swing process one can have 3-12 adsorber vessels,
one or more can be used in adsorption step while others are used for
various stages of regeneration.
Polybed pressure swing adsorption process for commercial production
of hydrogen with a purity of 99.99% by steam reforming of methane.
158
159
It is quite common in a pressure swing cycle to use a fraction of the less adsorbed
gas product as a low pressure purge gas.
Often the purge flow is in the opposite direction from that of the feed flow. The
simplest version of this process is designated as PSA.
Skarstrom has developed the rules for the minimum fraction of less adsorbed gas
product required for displacing the adsorbate. Although the concept of PSA is very
simple, the process in commercial application is fairly complex because it involves a
multicolumn design where the adsorbers operate under a cyclic steady-state using
a series of sequential nonisothermal, nonisobaric and nonsteady state steps.
These include adsorption, desorption and a multitude of complementary steps
which are designed to control the purity of product along with its recovery as well as
to optimize the overall separation performance.
160
Unique PSA cycles are also used to produce simultaneously two pure products
from a multicomponent feed, e.g. oxygen and nitrogen from air, carbon dioxide
and hydrogen from SMR off-gas, carbon dioxide and methane from landfill gas,
and to produce a product containing a component which is not present in the
feed, e.g. ammonia synthesis gas from SMR off-gas
161
The RPSA process for hydrogen purification developed by Questar Industries,
Canada utilizes the conventional Hydrogen-PSA process consisting of the
following steps:
Step 1: adsorption of feed gas over a packed bed of adsorbent at super-ambient
pressure in order to produce a hydrogen-enriched gas
Step 2: co-current depressurization of adsorber to produce additional amount of
hydrogen-enriched gas for pressurizing an accompanied vessel
Step 3: further co-current depressurization to produce another stream of
hydrogen-enriched gas for counter-currently purging an accompanied vessel
Step 4: counter-current depressurization to desorb a part of the feed-gas
impurities Step 5: counter-current purge with hydrogen from Step 3 to further
desorb the impurities
Step 6: counter-current pressurization with hydrogen from Step 2,
and Step 7: counter-current pressurization to feed pressure level with hydrogen
from Step 1.
162
The cycle runs rapidly using a total cycle time of ~1 min compared to cycle times of
10-30 minutes in a conventional process.
Six parallel adsorbers packed with layers of activated carbons and zeolites are
used in conjunction with two rotating flow control valves for introduction and
removal of gases to and from the adsorbers.
The process produces 99.95% pure hydrogen with a recovery of 67–73% from the
feed gas at 8.5 atm pressure.
163
The peculiarity of a novel adsorber design shows the use of two or more (multiples
of two) layers of a zeolite adsorbent inside a single vessel separated by a
perforated plate (similar to the arrangement of packings or trays in gas/vapour-liquid
contacting equipment).
The RPSA process consists of the following steps: (i) simultaneous pressurization
and adsorption of nitrogen from air by one layer of the adsorbent in order to produce
the oxygen-enriched gas (ii) simultaneous depressurization and purging with
oxygen product of the other adsorbent layer of the pair for its regeneration.
Using 5A zeolite as the adsorbent and total cycle time of 10 s (compared to 60-240
s. for a conventional PSA-oxygen process) the process can produce 27.5% oxygen
with recovery of 64.1% at feed air pressure of 2.22 atm
164
High Temperature PSA Process
A chemisorbent, Na2O supported on alumina, is used in a PSA process for direct removal and
recovery of bulk carbon dioxide from a high temperature feed gas (wet) without pre-drying or
cooling of the feed.
The adsorbent reversibly chemisorbs carbon dioxide in presence of steam in the temperature
range 200-300°C
A gas stream at ~200°C saturated with water, is treated for simultaneous production of CO2
depleted stream and CO2 enriched stream without removing water.
The PSA cycle consists of the following steps: Step 1: adsorption at ~125 psia Step 2: co-current
CO2 rinse at ~125 psia with recycle of effluent gas as feed Step 3: counter-current evacuation to
~2.5 psia with simultaneous steam purge at feed gas temperature and Step 4: counter-current
pressurization with a part of CO2 depleted product gas from Step 1.
165
The effluent gas from Step 3 is partly used as the purge gas in Step 2 after
recompression, and the balance is withdrawn as the recovered CO2 product gas.
The inert gas and CO2 recoveries from the feed gas are ~100 and 78%.
This application may be attractive for greenhouse gas emission control, i.e. CO2
sequestration from a hot gas or CO2 removal from combustion gases for power
generation without cooling the gas and removing the water.
This technology is a promising one for future development and possesses high
potential for tackling many practical problems in the fields of natural gas supply and
conservation, reduction of greenhouse gas emissions, production and conservation
of hydrogen, etc.
166
Choice between the adsorption cycles
The total quantity of adsorbent required for a particular application is equal to the
total load of the adsorbate or contaminant in the cycle divided by the dynamic
adsorption capacity.
The higher is the dynamic adsorption capacity, the less is the size of the bed and
hence, less is the capital cost of the equipment as well as the cost of the adsorbent
replacement.
Thus, prima facie PSA appears to be a better choice than TSA.
But if the throughput is large, the capital cost of the equipment and the cost of the
compression would be very high.
Also, in PSA a purge is required to drive off the residual gas from the void spaces of
the bed for improving the product quality, and often this purge has to be the product
gas itself or a high purity inert gas. Since this cannot be reused due to its level of
In TSA too, a regenerating gas is required and that has to be heated to a sufficiently
high temperature for regenerating the adsorbent bed.
In large industries TSA is usually in operation due to having spare resources for use
in regeneration.
Very large throughputs cannot be handled by PSA.
If the feed rate is low or moderate but the feed contains contaminants in large
quantity, PSA system is suitable.
PSA and VSA cycles have been modelled successfully for both equilibrium and kinetic
controlled processes.
Of particular importance in PSA and TSA is the determination of the cyclic steady
state.
In TSA, following the desorption step the regenerated bed is usually clean. Thus, a
cyclic steady state is closely approached in one cycle.
In PSA and VSA, this does not often happen where complete regeneration is seldom
achieved or necessary.
It is only required to attain a cyclic steady-state whereby the product obtained during
the adsorption step has the desired purity and at cyclic steady-state, the difference
between the loading profiles after adsorption and desorption is equal to the solute
entering in the feed.
Starting with a clean bed, the attainment of a cycle steady-state for a fixed cycle
time may require tens or hundreds of cycles.
For example, ethane and carbon dioxide are separated from nitrogen using 5A
zeolite at ambient temperature with adsorption and desorption for 3 min each at 4
atm and 1 atm, respectively, in beds of 0.25 m in length
The computation of the loading and gas concentration profiles at the end of each
adsorption step for ethane starting with a clean bed, shows that at the end of the
first cycle the bed is still clean beyond about 0.11 m and by the end of the 10th
cycle, a cyclic steady-state has almost been attained with a clean bed existing only
near the very end of the bed.
Experimental data for ethane loading at the end of 10th cycle agree reasonably well
with the computed profile
The cycle time in PSA is very short as the adsorbent bed becomes saturated quickly
due to large concentration of the contaminant (adsorbate).
So the adsorbent must be rugged enough to withstand millions of pressurization and
depressurization cycles without disintegration.
The carbon molecular sieve in nitrogen generation plant suffers more than 100
million such cycles in its life of 10 years.
On the other hand, TSA handles a very large quantity of feed and its cycle time is long
because of the sufficient time required to heat and cool the bed.
The bed is to be cooled after heating, and brought to adsorption temperature
otherwise the adsorption capacity would be less.
Naturally, an adsorbent suffers several thousand cycles of thermal expansion and
cooling in its life time, and has to be mechanically and chemically rugged enough to
withstand such repeated thermal shocks
For adsorbent like silica gel, the regeneration temperature is very low, usually less
than 150°C.
But for molecular sieves and activated carbons, the regeneration temperatures are
quite high depending on the application. Normally the molecular sieves require
regeneration temperatures more than 220°C because of the higher heat of adsorption
Gas phase Liquid phase
Process Condition Temperature swing Inert purge Displacement
purge
PSA TSA
Feed: gas or vapour y y y y n
Feed: liquid, vaporise at < 200°C n y y y y
Feed: liquid, vaporise at > 200°C n n n n y
Adsorbate concentration in feed,
< 3%
y y n n y
Adsorbate concentration in feed,
3-10%
y y y y n
Adsorbate concentration in feed,
> 10%
n y y y n
High product purity Required y y y Y (hAp,
lDp)
y
Thermal generation Required y y n n n
Difficult purge/ adsorbate
separation
Y (no recovery of
adsorbate)
n n NA y
Adsorption Equipment
Stage-wise Contact
Adsorption from gases
Fluidized beds are widely used for adsorption from gases like recovery of vapours
from vapour-gas mixtures, desorption, fractionation of light hydrocarbon vapours and
several other applications.
Adsorption is usually carried out at gas velocities just above the quiescent fluidizing
velocity when the bed expands and looks like boiling liquid.
The solid particles move about freely and are thoroughly circulated. Well defined
bubbles of gas rise through the bed.
Only fine particles below 20 mesh are suitable for use in fluidized beds and the
superficial gas velocity has to be kept below 0.60 m/s.
They can, however, retain large range of particle sizes.
Teeter beds are more frequently used for adsorption from gases since
they provide better gas-solid contact by accommodating coarser
particles, up to 10 mesh at superficial gas velocities in the range of 1.5
to 3 m/s.
In these beds, however, there is little gas bubbling and the size range
of particles retained is relatively small.
Multistage operations are generally not practised for gas-liquid contact
in commercial scale
Adsorption from liquids
Agitated vessels are commonly used for adsorption from liquids.
The liquid depths vary between 0.75 to 1.5 tank diameters. The tanks are provided
with wall baffles to avoid vortex formation and lift the solid particles through vertical
liquid motion.
Flat blade or pitched blade turbines, fitted to an axial shaft are generally used for
efficient mixing.
Multiple turbines on the same shaft are sometimes used to get more uniform slurry
throughout the tank. The clearance of the lowest impeller from the tank bottom should
be more than the settled height of the bed, so that the impeller is not embedded if it
stops.
A typical batch equipment for liquid-solid contact consists of an agitated
vessel as above but having a filter press and a collection tank.
The liquid to be treated and the adsorbent particles are thoroughly mixed
in the tank, at the desired temperature for the required length of time.
The slurry is then filtered in a filter press to separate the clear liquid from
the solid adsorbent containing the adsorbed solute.
The operation can be readily converted to multistage one by using
additional tanks and filter presses as required.
The operation can also be made continuous by using continuous rotary
filters or centrifuges instead of a filter press.
Alternatively, the adsorbent may be allowed to settle and continuously
Multistage contact:
Removal of a given amount of solute can be done with greater
economy of adsorbent by treating the solution separately with small
batches of adsorbent and filtering between stages rather than
employing the entire adsorbent in a single stage.
This can be practiced both in batch and continuous operations.
As in other mass transfer operations, greater economy of adsorbent
can be achieved by counter-current operation where the solution to be
treated and the adsorbent flow in opposite directions, either from stage
to stage or in a single equipment.
Continuous Contact
Steady-state moving bed adsorbers
Adsorption from gases: Steady state operation requires continuous
movement of both fluid and adsorbent through the equipment at constant
rates with no change in conditions at any point with passage of time.
In order to have better performance, it is necessary to have counter-
current operation so that several stages can be accommodated within a
single device, since parallel flow of solids and fluids can at best produce
a single stage.
In view of the difficulties in moving solids through equipment continuously
and uniformly, it has not been possible to successfully develop
Adsorption from liquids: The difficulties in moving solids continuously
and uniformly through equipment, as in case of adsorption from gases,
stand in the way of developing successful continuous counter-current
equipment for adsorption from liquids also.
In case of liquids, however, much less elaborate arrangements are
required to overcome the difficulties.
For instance, a screw feed or simple gravity flow from a bin may introduce
the adsorbent at the top of the tower and the same may be removed from
the bottom by a revolving compartmented valve.
In the pulsed or moving bed, the
liquid enters the bottom and leaves
through the top of the column.
The flow of liquid is stopped
periodically, spent adsorbent is
withdrawn (pulsed) from the bottom,
and virgin or regenerated adsorbent
is added into the top of the adsorber.
The Higgins contactor, originally
developed for ion-exchange has
been more or less successfully used
for adsorption from liquids.
The equipment though intermittent
and cyclic fashion made by the action
of piston, approaches counter-current
operation in its performance.
Adsorption takes place in the upper
part of the equipment where the
solution flows down through a
stationary bed of the adsorbent.
At the end of adsorption, the solid particles are transferred to the
lower part for regeneration and the upper part is filled with
regenerated adsorbent when the cycle is repeated.
Adsorption, regeneration and material shifting from adsorption to
regeneration zone are carried out in an intermittent and cyclic way by
regulating the valves.
Out of three valves in the equipment, one valve remains open for
each operation.
Unsteady-state fixed bed adsorbers
The inconvenience and relatively high cost of moving solid particles as required in
steady-state operations often make it more economical to use fixed bed adsorbers
where the gas or liquid is made to pass through a stationary bed of solid adsorbent.
Fixed bed adsorbers are frequently used in recovery of solvent vapours from air or
other gases, purification of air in gas masks, dehydration of air or other gases, etc.
Adsorber vertically placed with six trays.
A thin bed of adsorbent particles is
spread on a screen or perforated plate.
The depth of the bed usually varies from
0.3 to 1.3 m.
Filtered feed gas is admitted into the
chamber and flows through the bed.
If the effluent gas is substantially free
from solvent vapour, it is discharged into
the atmosphere.
If, on the other hand, the gas contains
appreciable amount of vapour, the same
is diverted to a subsequent adsorber in
series. When the adsorbent particles in
any absorber become nearly saturated
The enclosures for fixed-bed adsorbers may be horizontal or vertical, conical or
cylindrical shells
Conical fixed-bed adsorbers are used when a low pressure drop through the cone-
shaped bed is less than one-half of that through a conventional flat-bed adsorber,
while the air volume is more than double of that through the flat bed.
When multiple fixed beds are needed, the usual
configuration is a vertical cylindrical shell
The type of enclosure used is normally dependent
on the gas volume and the permissible pressure
drop.
The gas flow can be either down or up.
Downflow allows for the use of higher gas
velocities, while in upflow the gas velocity must be
maintained below the value which causes flooding
of the bed.
When large volume of gas is to be handled,
cylindrical horizontal vessels are suitable.
For the continuous operation of fixed bed adsorbers, it is desirable to have two or
three units.
With two adsorber units, one unit adsorbs while the other regenerates or desorbs.
Under most situations, two units are sufficient if the regeneration and cooling of the
second bed can be completed prior to the breakthrough of the first unit.
The move to three units makes it possible for one bed to be adsorbing, one cooling
and the third regenerating.
The vapour-free air from the first bed is used to cool the unit which was just
regenerated.
Occasionally a fourth bed is used, when two units adsorbing in parallel, discharge the
exhaust to a third unit on the cooling cycle and a fourth unit is being regenerated.
Regeneration is done by hot inert gas or low pressure steam, the latter being
preferred if the solvent is not miscible with water or if presence of small amount of
water is not objectionable.
The size of the adsorbent bed is determined on the basis of gas flow rate and
desired cycle time.
The cross sectional area of the bed is fixed to give a superficial gas velocity of 0.15
to 0.45 m/s.
For very large flow rates, rectangular beds in horizontal cylinders are generally
used.
The operation can be made more or less continuous by placing the adsorbent
particles in several small chambers contained in an inner drum rotating inside a
stationary compartmented outer shell.
As the inner drum rotate, the adsorbent is successively subjected to adsorption,
regeneration, drying and cooling.
Moist gases can be dried by passing them through beds of silica gel, alumina,
bauxite or molecular sieves.
If the gases are under pressure, towers may be 10 m or more in height, but in such
cases, the adsorbent should be supported on trays at intervals of 1 to 2 m in order
to minimize compression of the bed
Adsorption from liquids
Fixed bed adsorbers are often more economical in case of adsorption from liquids also.
Liquids such as gasoline, kerosene, transformer oil, etc., can be dehydrated by activated alumina in
fixed bed adsorbers
High temperature steam along with vacuum can be used for regenerating the adsorbent.
For the bulk separation of liquid mixtures, continuous counter-current systems are preferred over
batch systems because they maximize the mass transfer driving force.
This advantage is particularly important for difficult separations where selectivity is not high and/or
where mass transfer rates are low.
Ideally, a continuous counter-current system involves a bed of adsorbent moving downward in plug
flow and the liquid mixture flowing upward in plug flow through the void space of the bed.
Unfortunately, such a system has not been successfully developed because of problems of
adsorbent attrition, liquid channeling and nonuniform flow of adsorbent particles.
Successful commercial systems are based on a simulated counter-current system using a fixed bed.
Chromatographic separation
It is based on the partition coefficient between a mobile phase and a
stationary phase.
Chromatography techniques have long been used for chemical
analysis and separation.
The reasons for finding increasing use of chromatographic separation
can be as follows: (i) Highest number of theoretical plates it offers per
unit length at perhaps the lowest cost per plate (ii) Maximum flexibility
in designing separation factor for a given separation through selectivity
engineering in the form of infinitely possible surface modifications (iii)
Give multicomponent separation in a single operation
Chromatography is the technique for the separation, purification, and testing of compounds
The term “chromatography” is derived from Greek, chroma meaning, “colour,” and graphein
meaning “to write.”
In this process, we apply the mixture to be separated on a stationary phase (solid or liquid) and a
pure solvent such as water or any gas is allowed to move slowly over the stationary phase, carrying
the components separately as per their solubility in the pure solvent.
Chromatography separates mixtures into components by passing a fluid mixture
through a bed of adsorbent material called stationary phase.
The two most common adsorbents used in chromatography are porous alumina and
porous silica. Sometimes carbon, magnesium oxide and various carbonates are also
used.
Alumina is a polar adsorbent and is preferred for the separation of components that
are weakly or moderately polar. In addition, alumina is a basic adsorbent preferentially
retaining acidic compounds.
Silica gel is less polar than alumina and is an acidic adsorbent preferentially retaining
basic components such as amines.
Carbon is a nonpolar stationary phase with the highest attraction for larger nonpolar
molecules.
Adsorbent type sorbents are better suited for the separation of a mixture on the basis
of chemical type (e.g. olefins, esters, acids, aldehydes, alcohols, etc.) than for
For the separation of individual members of a homologous series, partition
chromatography is preferred wherein an inert solid support like silica gel, is coated
with a liquid phase.
For application to gas chromatography the liquid must be nonvolatile.
For liquid chromatography, the stationary liquid phase must be insoluble in the
mobile phase.
The stationary liquid phase may however be bonded to the solid support if required.
An example of a bonded phase is the result of reacting silica with a chlorosilane. If
the resulting stationary phase is more polar than the mobile phase, the technique is
referred to as normal-phase chromatography.
Otherwise, the name reverse-phase chromatography is used.
Adsorption (mass transfer operations 2) ppt
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Adsorption (mass transfer operations 2) ppt

  • 3. INTRODUCTION Molecular forces at the surface of a liquid are in a state of imbalance or unsaturation. The same is true of the surface of a solid, where the molecules or ions on the surface may not have all their forces satisfied by union with other particles. As a result of this unsaturation, solid and liquid surfaces tend to satisfy their residual forces by attracting and retaining onto their surfaces gases or dissolved substances with which they come in contact. This phenomenon of the concentration of a substance on the surface of a solid (or liquid) is called adsorption. Thus, the substance attracted to a surface is said to be the adsorbed phase or adsorbate, while the substance to which it is attached is the adsorbent. 3
  • 4. Adsorption should be carefully distinguished from absorption, the later process being characterized by a substance not only being retained on a surface, but also passing through the surface to become distributed throughout the phase.
  • 5.
  • 8. 8 The study of adsorption of various gases (or vapors) on solid surfaces has revealed that the forces operative in adsorption are not the same in all cases. Two types of adsorption are generally recognized: “physical” or van der Waals adsorption and “chemical” or activated adsorption
  • 9. 9 Physical adsorption (physisorption) is the result of intermolecular forces of attraction between molecules of the solid and the substance adsorbed. When, for example, the intermolecular attractive forces between a solid and a gas (or vapor) are greater than those existing between molecules of the gas itself, the gas will condense upon the surface of the solid even though its pressure may be lower than the vapor pressure corresponding to the prevailing temperature. The adsorbed substance does not penetrate within the crystal lattice of the solid and does not dissolve in it but remains entirely upon the surface.
  • 10. 10 Should the solid, however, be highly porous, containing many fine capillaries, the adsorbed substance will penetrate these interstices if it “wets” the solid. The partial pressure of the adsorbed substance at equilibrium equals that of the contacting gas phase, and by lowering the pressure of the gas phase, or by raising the temperature, the adsorbed gas is readily removed or desorbed in unchanged form. Physical adsorption is characterized by low heats of adsorption (approximately 40 Btu/lbmol of adsorbate) and by the fact that the adsorption equilibrium is both reversible and established rapidly. This latter point allows one to simplify calculations in most real-world adsorption applications
  • 11. Presentation title 11 Chemisorption, or activated adsorption, on the other hand, is the result of chemical interaction between the solid and the adsorbed substance. The strength of the chemical bond may vary considerably and identifiable chemical compounds in the usual sense may not form. Nevertheless, the adhesive forces are generally much greater than that found in physical adsorption. Chemisorption is also accompanied by much higher enthalpy changes (ranging from 80 to as high as 400 Btu/lbmol) with the heat liberated being on the order of the enthalpy of chemical reaction. The process is frequently irreversible, and, on desorption, the original substance will often have undergone a chemical change.
  • 13. Although it is probable that all solids adsorb gases (or vapors) to some extent, adsorption, as a rule, is not very pronounced unless an adsorbent possesses a large surface area for a given mass. For this reason, such adsorbents as silica gel, charcoals, and molecular sieves are particularly effective as adsorbing agents. These substances have a very porous structure and, with their large exposed surface, can take up appreciable volumes of various gases. The extent of adsorption can be increased further by “activating” the adsorbents in various ways. For example, wood charcoal can be “activated” by heating between 350 and 1000 C in a vacuum or in air, steam, and certain other gases to a point where the adsorption of carbon tetrachloride, (e.g., at 248C) can be increased from 0.011 g/g of charcoal to 1.48. The activation apparently involves a distilling out of hydrocarbon impurities and thereby leads to exposure of a larger free surface for possible adsorption.
  • 14. 14 The amount of gas adsorbed by a solid depends on a host of factors, including the surface area of the adsorbent, the nature of the adsorbent and gas being adsorbed, the temperature, and the pressure of the gas. Since the adsorbent surface area cannot always be readily determined, common practice is to employ the adsorbent mass as a measure of the surface available and to express the amount of adsorption per unit mass of adsorbing agent used. A concept, which becomes especially important in determining adsorbent capacity, is that of “available” surface, i.e., surface area accessible to the adsorbate molecule. It is apparent from pore size distribution data that the major contribution to surface area is located in pores of molecular dimensions. It seems logical to assume that a molecule, because of steric effects, will not readily penetrate into a pore smaller than a certain minimum diameter—hence, the concept that molecules are screened out.
  • 15. This minimum diameter is the so-called critical diameter and is a characteristic of the adsorbate and related to molecular size. Thus, for any molecule, the effective surface area for adsorption can exist only in pores that the molecule can enter. Presentation title 15
  • 16. • The extent of adsorption of a given situation is reached once equilibrium is established between the adsorbent and its contacting solution. • In practice, adsorption performance is also strongly influenced by the mass transfer of the species between the solution and the adsorbent surfaces and the adsorption reaction rate. • Adsorption is, therefore, an equilibrium-diffusion-reaction process. Presentation title 16
  • 17. ADSORPTION AS A SORPTION PROCESS • The basic operating principle of adsorption: the preferential concentration of species onto surfaces of adsorbing solids also operates in two other processes; namely, chromatography and ion exchange. • Similar to most adsorption operations, chromatography operates in fixed-bed mode, but is devised for separating liquid mixtures through an intermittent feed of the solution to be separated, followed by the passage of an elution solution. • In ion exchange, the solid substance used contains charged groups that interact with the charged ions present in the liquid solution. • If one views adsorption as an exchange process involving a fictitious species, the equivalence between adsorption and ion exchange becomes obvious. Presentation title 17
  • 18. ADSORPTION VERSUS ABSORPTION • Gas absorption is an operation in which a gas mixture is brought into contact with a liquid for the purpose of dissolving one or more components of the mixture into the liquid. Absorption, therefore, is a bulk phenomenon, and the extent of separation is limited by the solubilities of the gases involved. • Adsorption is a surface phenomenon, and the extent of adsorption is limited by the relevant adsorption isotherm relationship. • Absorption may be carried out by passing the gas and liquid streams through a packed column concurrently or counter-currently. The operation consists of two moving phases (gas and liquid) and a stationary phase (column packing), which provides the interfacial area for liquid/gas contact. • In fixed-bed adsorption, the fluid to be treated passes through a bed packed with adsorbent. The process involves two phases, a moving fluid and a stationary solid phase of adsorbents. • Absorption, therefore, may be treated as a steady-state process, while adsorption in a fixed-bed operation is an inherently nonsteady state. • As a result, the computational effort required for the design of fixed-bed adsorption is more extensive than that of absorption. Presentation title 18
  • 19. ADSORPTION VERSUS DISTILLATION • Distillation also belongs to the equilibration-diffusion category of separation processes, and is used for the separation of homogeneous liquid mixtures. • However, unlike adsorption or absorption, separation by distillation is effected by using energy instead of material as an agent of separation. • In a hypothetical study comparing distillation versus adsorption, Ruthven (1984) showed that • For separating an A-B mixture, the use of distillation becomes impractical if the relative volatility of A to B is less than 1.2. • To separate light gas mixtures, adsorption was found to be preferential to cryogenic distillation, even when the relative volatility is high Presentation title 19
  • 20. ADSORPTION VERSUS DEEP-BED FILTRATION • Deep-bed filtration is a process designed for the removal of fine particles from diluted fluid suspensions. • Its operation is carried out by passing the suspension to be treated through a column packed with granular or fibrous substances (filter media). • Generally speaking, deep-bed filtration and fixed-bed adsorption share many common features, such as equipment configuration and modes of operation. Because of their similarities, the words ‘adsorption’ and ‘filtration’ are often used interchangeably. • The removal of submicron colloidal particles from fluid to solid surfaces may be described as either filtration or deposition (Hirtzel and Rajagopalan, 1985). • Carbon columns used to remove dissolved organic solutes in water treatment are often referred to as ‘carbon filters’ by water engineers. Similarly, the term ‘charcoal filter’ is used to denote cartridges filled with granular activated carbon for personal protection Presentation title 20
  • 21. • A major difference between them resides in the fact that in deepbed filtration, removal of particles from the suspension to be treated results in particle deposition over the exterior surfaces of the filter media. • In contrast, the adsorbed dissolved species in fixed-bed adsorption covers mainly the interior surfaces of adsorbents. • The extent of separation achieved in adsorption is limited by the adsorption equilibrium relationship. On the other hand, particle retention in deep-bed filtration depends strongly upon the nature of particle-collector interaction forces, but there is no clear-cut limit on the extent of deposition (Tien and Ramarao, 2007). • As adsorption processes may cease operation once the adsorbents become saturated, for deep-bed filtration, due to increasing particle retention, the pressure drops required for maintaining a specified throughput increase with time. The duration of operation is limited by the maximum allowable pressure drop. Presentation title 21
  • 22. OPERATION MODES OF ADSORPTION PROCESSES Separation by adsorption is effected by contacting solutions to be treated with selected adsorbents. a) Adsorption in agitated vessels: Batch adsorption in agitated vessel represents perhaps the simplest way of bringing about fluid/adsorbent contact. A fixed amount of adsorbent of a known state is added to a volume of solution of a known solute concentration in a closed vessel. Agitation is provided by rotating stirrers in order to insure that adsorbent particles are fully suspended, and the adsorbate concentration is kept uniform throughout the solution. The data collected are the temporal evolution of the solute concentration of the solution. Not suitable for treating large volumes of solution (such as water supplies) Presentation title 22
  • 23. • B) Adsorption in continuous-flow tanks.: This type of operation is often used in waste water treatment. • Adsorbents in powdered form, such as activated carbon, are added directly to a particular step of a treatment process (biological or physio-chemical) for the purpose of removing a particular species of contaminant. Presentation title 23
  • 24. • C) Fixed-bed adsorption consists of passing a solution through a column packed with adsorbents, and is commonly applied for eliminating trace contaminants from a liquid solution, or toxic and volatile vapors from gas streams. • Fixed-bed adsorption, by nature, is a batch process. • Starting with a fresh or newly regenerated state, adsorbents in fixed-bed operations become increasingly saturated with adsorbate to a point that reactivation (desorption) becomes necessary, and operation ceases. • However, by employing a number of identical columns and arranging the adsorption/reactivation sequence properly, a continuous operation may be established. • This, in fact, is the principle for the development of pressure swing and thermal swing processes for gas Presentation title 24
  • 25. • Moving-bed adsorption: It is carried out with both solid (adsorbent) and fluid phases in motion. The movements of both phases may be parallel, counter-current, or perpendicular (cross-flow); although the counter-current pattern is used in practice. • Counter-current moving-bed adsorption is similar to gas absorption in packed beds. • It is a steady-state process, and has the advantage of a more favorable utilization of the available driving force of mass transfer. • However, maintaining a steady-state circulation of large quantities of solids provide serious problems in design, operation, and maintenance. Presentation title 25
  • 26. • In order to overcome this difficulty, a rather ingenious design based on the use of a rotation valve for directing and switching fluid flow and the addition of a desorbent, which enables the imitation of moving-bed behavior of fixed-bed adsorption, was developed. • This simulated moving-bed (SMB) concept provides the basis for the development of the SORBEX processes for aromatic isomers’ separation • Generally speaking, both moving-bed and simulated moving-bed are more costly in development, construction, and maintenance than fixed-bed; but offer more efficient separation. Presentation title 26
  • 27. APPLICATIONS OF ADSORPTION • Recorded application of charcoal for purifying water two millennia ago was found in Sanskrit manuscripts: “It is good to keep water in copper vessels to expose it in sunlight and to filter it through charcoal” • Modern use of adsorbents began with the discovery of the decoloring effect of charcoal on solutions (Lowitz, 1786), followed by the invention of a charcoal cartridge for personal protection during the First World War. • With the increasing availability of different types of adsorbents nowadays, and the more recent interests in biotechnology and green technology, there has been a great expansion in the applications of adsorption in many areas Presentation title 27
  • 28. • For liquid-phase adsorption • Decoloring, drying, or degumming of petroleum products • Removing of dissolved organic species from water supplies • Removing odor, taste, and color from water supplies • Advanced treatment of waste water (domestic and industrial) • Decoloring of crude sugar syrup and vegetable oils • Recovery and concentration of proteins, pharmaceuticals, and bio- compounds from dilute suspensions • Bulk separation of paraffin and isoparaffins • For gas-phase adsorption • Recovering organic solvent vapors • Dehydration of gases • Removing toxic agents and odor for personal protection • Air separation • Separating normal paraffins from isoparaffin aromatics • CO2 capture for addressing climate Presentation title 28
  • 30. Presentation title 30 ACTIVATED CARBON ACTIVATED ALUMINA ACTIVATED SILICA ZEOLITES
  • 31. • Most solid substances possess some adsorption capacity for gas and/or liquid, but only a few of them meet the necessary requirements to qualify as adsorbents for practical use. • Some of the most widely used adsorbents are: • a. Activated carbon. It may be produced from carbonization of materials such as coconut shells, coal, lignite, wood, and similar substances, followed by activation with air or steam. It is used widely for recovery of organic vapor, decoloring liquid solutions, and treatment and purification of water supplies and waste water. • b. Activated alumina. Al2O3 is prepared by removing water from colloid alumina. It has a high capacity for water vapor, and is used mainly as a desiccant for gasses and liquids. Activated alumina can be reactivated for reuse. • c. Activated silica. Activated silica is prepared from gel precipitated by acid treatment of a sodium silicate solution. Activated silica is used primarily for dehydration of air and removing toxic species from air for personal protection purposes (i.e., in gas masks). Presentation title 31
  • 32. • d. Molecular sieve zeolites. Most adsorbents, including those mentioned herein, have pores covering a range of sizes. Molecular sieve zeolites, however, are an exception. • Molecular sieve zeolites are crystalline inorganic polymers of aluminosilicate, alkali, or alkali- earth elements such as Na, K, and Ca. • The “cages” of the crystal cells can entrap adsorbed matter, and the diameter of the passageways, controlled by the crystal composition, regulate the size of the molecules that can enter. • These “size-selection” characteristics differentiate molecular sieve zeolites from other adsorbents and make molecular sieve zeolites particularly useful in the separation of hydrocarbon mixtures. • e. Polymeric adsorbents. These adsorbents are prepared from polymerizing styrene and divinyl benzene or acrylic esters for adsorbing nonpolar organics from aqueous solutions or polar solutes, respectively. • Generally speaking, polymeric adsorbents are more expensive than other adsorbents, and their use is, therefore, more selective. Presentation title 32
  • 34. • The structural adsorbents are a class of adsorbents fabricated in forms of simple ordered geometry, such as monolith, laminate, or foam. • Such adsorbents provide simple, identifiable flow channels, and offer low resistance to fluid flow. • Compared with granular adsorbents, they also offer low resistance to mass transfer. • These adsorbents are, therefore, of particular interest for applications in pressure swing adsorption. Presentation title 34
  • 35. • Similar to commonly used zeolites, metal organic frameworks (MOFs) and covalent organic frameworks (COFs) are porous crystalline solids. • The metal organic frameworks are compounds consisting of metal ions or clusters coordinated to organic ligands to form one-, two-, and three- dimensional structures. • Covalent organic frameworks are a new type of organic material constructed with organic building units via covalent bonds. • This well-defined crystalline porous structure, together with tailored functionalities, are known for their unique features. • Both MOFs and COFs supposedly have great potential for gas storage, gas adsorption, and as catalysts as well as sensors Presentation title 35
  • 36. ACTIVATED CARBON • Charcoal, an inefficient form of activated carbon, is obtained by the carbonization of wood. • Various raw materials have been used in the preparation of adsorbent chars, resulting in the development of active carbon, a much more adsorbent form of charcoal. • Industrial manufacture of activated carbon is obtained from shells or coal that is subjected to heat treatment in the absence of air, in the case of coal, followed by steam activation at high temperature. • Other substances of a carbonaceous nature also used in the manufacture of active carbons include wood, coconut shells, peat, and fruit pits. • Zinc chloride, magnesium chloride, calcium chloride, and phosphoric acid have also been used in place of steam as activating agents. 36 • Gas adsorbent carbons find primary application in gas purification, solvent recovery (hydrocarbon vapor emissions), and pollution control.
  • 38. ACTIVATED ALUMINA Activated alumina (hydrated aluminum oxide) is produced by special heat treatment of the precipitate of native alumina or bauxite. It is available in either granular or pellet form Activated alumina is primarily used for the drying of gases and is particularly useful for the drying of gases under pressure. 38
  • 42. 42 DIFFERENCE • Activated Alumina • Activated alumina is a white spherical substance • Because of its unique skeleton structure, it has a strong affinity with active components. • The product has a uniform micropore distribution, a suitable pore size, and a large pore volume. • High, low bulk density, good mechanical properties, and good stability. • for the dehydration and drying of industrial gases such as chemical, metallurgy, electronics, petroleum, etc., such as air, oxygen, nitrogen and other gases, smelting gas and petroleum cracking gas, • Molecular Sieve • The molecular sieve has a large specific surface area of 300~1000m2/g, and the inner crystal surface is highly polarized. • It is a kind of high efficiency adsorbent and a kind of solid acid. • The surface has a high acid concentration and acid strength, which can cause Catalytic reaction. • When the metal ions in the composition are exchanged with other ions in the solution, the pore size can be adjusted to change its adsorption and catalytic properties, thereby preparing molecular sieve catalysts with different properties. • Because of strong moisture absorption capacity and is used for gas purification treatment. • Molecular sieves that have been stored for a long time and have absorbed moisture should be regenerated before use.
  • 43. PROPERTIES AND PHYSICAL CHARACTERISTICS OF ADSORBENTS 43 ADSORPTION EQUILIBRIUM ADSORBENT CHARACTERISTICS Presentation title The equilibrium relationship between the solution and the adsorbed phase, and the equilibrium data of specific adsorbent-adsorbate systems give the limit of the performance of adsorption operations. • Adsorbent size • Adsorbent density • Adsorbent porosity • Pore size and size distribution • Specific surface area
  • 45. ADSORBENT CHARACTERISTICS Presentation title 45 Adsorbent size, dp.: Based on sieve analysis results. Rate of adsorbate uptake is often dependent on it Determine the pressure drop required to sustain a given throughput in a fixed-bed operation. Adsorbent density, ρp. Extent of adsorption is often expressed on the basis of unit adsorbent mass, ρp is a factor of determining the height of fixed-beds in design. A related quantity, ρb, is given as ρp (1- ε), where ε is the fixed-bed porosity Adsorbent porosity, εp. Indication of the internal structure of the adsorbent, gives the value of the fraction of the void space of an adsorbent pellet (or granule). An associated quantity of εp is the specific pore volume Pore size and size distribution Specific surface area, Sg (surface area per unit mass of adsorbent): critical importance to the adsorption capacity of an adsorbent and, to a lesser degree, the uptake rate of the adsorbate
  • 47. Presentation title 47 Some surrogate quantities (indices) have also been used to represent the adsorption capacity of adsorbents. The iodine number is a rough measure of the adsorption of small molecule components, and correlates with the BET surface area. It is defined as the milligrams of iodine adsorbed by one gram of material when the iodine residual concentration of the filtrate is 0.02N (0.01 mol L-1) according to ASTM D4607 standard, which is based on a three-point isotherm. The molasses number was developed for cane sugar decoloration, and relates to the adsorption of large molecules from liquid solutions. It is (DSTM 16) is a measure of the decolorizing capacity and is an indicator of the macro pore and transport pore structure of activated carbon. An amount of pulverised carbon is mixed and heated with a standard molasses solution for a period of time. The Molasses number is expressed as the relative amount of colour removed by an activated carbon compared to a standard carbon. So the higher the molasses number, the
  • 49. ADSORBENT SELECTION In principle, an adsorbent may be selected based on a number of factors, including the adsorbent’s adsorption capacity and selectivity, adsorbate uptake kinetics, regenerability (for nowadays, discarding spent adsorbents is no longer an option), compatibility with operating conditions, and not the least, cost. While some of the information can be found in the literature and publications, other information must be obtained from one’s own investigations, including experiments. The circumstances one faces may not allow a thorough investigation. Generally speaking, because of the large number of variables present, and their incomplete information, there exists no general criteria for adsorbent selection. Instead selection is often made on a case-by-case basis. The decision is dictated largely by the type of application, and based on technical considerations, in combination with subjective judgment and past experience.
  • 50. ADSORBENT SELECTION The moisture (water vapor) isotherm data of several common commercial adsorbents are shown . Comment on their use in gas drying
  • 51. • 3 stages as the adsorbate concentration increases. • Firstly, a single layer of molecules builds up over the surface of the solid. This monolayer may be chemisorbed and associated with a change in free energy which is characteristic of the forces which hold it. • As the fluid concentration is further increased, layers form by physical adsorption and the number of layers which form may be limited by the size of the pores. • Finally, for adsorption from the gas phase, capillary condensation may occur in which capillaries become filled with condensed adsorbate, and its partial pressure reaches a critical value relative to the size of the pore. • In practice, all three may be occuring simultaneously in different parts of the adsorbent since conditions are not uniform throughout. • Generally, concentrations are higher at the outer surface of an adsorbent pellet than in the centre, at least until equilibrium conditions have been established. ADSORPTION
  • 52. Adsorption equilibrium is a dynamic concept achieved when the rate at which molecules adsorb on to a surface is equal to the rate at which they desorb Most theories have been developed for gas–solid systems because the gaseous state is better understood than the liquid. Statistical theories are being developed which should apply equally well to gas– solid and liquid–solid equilibria, though these are not yet at a stage when they can be applied easily and confidently to the design of equipment. ADSORPTION EQUILIBRIUM
  • 53. The capacity of an adsorbent for a particular adsorbate involves the interaction of three properties — the concentration C of the adsorbate in the fluid phase, the concentration Cs of the adsorbate in the solid phase and the temperature T of the system. If one of these properties is kept constant, the other two may be graphed to represent the equilibrium. The commonest practice is to keep the temperature constant and to plot C against Cs to give an adsorption isotherm. When Cs is kept constant, the plot of C against T is known as an adsorption isostere. In gas–solid systems, it is often convenient to express C as a pressure of adsorbate.
  • 54.
  • 55.
  • 56.
  • 57. Single component adsorption At very low concentrations the molecules adsorbed are widely spaced over the adsorbent surface so that one molecule has no influence on another. For these limiting conditions it is reasonable to assume that the concentration in one phase is proportional to the concentration in the other
  • 58. Concentration corresponding to any point on the equilibrium curve may be obtained either by adsorption of solute by a fresh adsorbent or by desorption from an adsorbent having higher adsorbate concentration. In some cases, however, different equilibrium curves, at least over a part of the isotherm, are obtained depending upon whether the gas or vapour is adsorbed or desorbed. This phenomenon known as hysteresis Equilibrium partial pressure corresponding to a certain amount of solute concentration in the adsorbent is high in case of adsorption than that in case of desorption. Hysteresis may be due to the shape of the openings to the capillaries and pores of the adsorbent or due to nonuniform wetting 58
  • 59. 59 Different types of adsorption isotherms are exhibited by different adsorbate-adsorbent combinations Moles adsorbed have been plotted as ordinate against (p′/p), p′ being the partial pressure of the sorbate and p is its vapour pressure
  • 60. Presentation title 60 Type-I is concave downwards and represents systems in which adsorption does not proceed beyond the formation of a monomolecular layer of the adsorbate. Common gas-solid adsorption systems follow. Adsorption of oxygen on carbon at about -183°C is an example
  • 61. Presentation title 61 Type-II is initially concave downwards and then near the middle of the abscissa it becomes concave upwards. This point indicates the completion of formation of monomolecular layer and beginning of the formation of multimolecular layer of the adsorbate. Increasing tendency for the monolayer to become complete before a second layer starts. Physical adsorption of water vapour by carbon black at about 30°C.
  • 62. 62 Type-III is concave upwards. This is a relatively rare type. The amount of gas or vapour adsorbed increases indefinitely as the relative saturation (p′/p) approaches unity The convex downward nature is caused by the heat of adsorption of the first layer becoming less than the heat of condensation due to molecular interaction in the monolayer The solute loading is low at low concentration of the adsorbate in the fluid. Bromine adsorption on silica gel at 20°C. , nitrogen on ice
  • 63. 63 Type-IV Variation of a Type-II, but with a finite multilayer formation corresponding to complete filling of the capillaries The concave region that occurs at low gas concentrations is usually associated with the formation of a single layer of adsorbate molecules over the surface. The convex portion corresponds to the build-up of additional layers, whilst the other concave region is the result of condensation of adsorbate in the pores — so called capillary condensation Adsorption of water vapour takes place on active carbon at about 30°C.
  • 64. 64 Type-V Variation of a Type-III obtained Adsorbing water vapour on activated carbon at about 100°C.
  • 65. • Charcoal, with pores just a few molecules in diameter, almost always gives a Type I isotherm. • A non-porous solid is likely to give a Type II isotherm. • If the cohesive forces between adsorbate molecules are greater than the adhesive forces between adsorbate and adsorbent, a Type V isotherm is likely to be obtained for a porous adsorbent and a Type III isotherm for a non-porous one. • In the activated aluminaair-water vapour system at normal temperature, the isotherm is found to be of Type IV. Presentation title 65
  • 66. Freundlich Isotherm The amount of gas adsorbed per given quantity of adsorbent increases relatively rapidly with pressure and then much more slowly as the surface becomes covered with gas molecules. To represent the variation of the amount of adsorption per unit area or unit mass with pressure, Freundlich proposed a equation Although the requirements of the equation are met satisfactorily at lower pressures, the experimental points curve away from the straight line at higher pressures, indicating that this equation does not have general applicability in reproducing adsorption of gases by solids. Freundlich isotherm is particularly useful in cases where the actual identity of the solute is not known as in the removal of coloured substances from mineral and vegetable oils or from sugar solutions. NE
  • 67. The Langmuir isotherm At higher gas phase concentrations, the number of molecules absorbed soon increases to the point at which further adsorption is hindered by lack of space on the adsorbent surface. The rate of adsorption then becomes proportional to the empty surface available, as well as to the fluid concentration. At the same time as molecules are adsorbing, other molecules will be desorbing if they have sufficient activation energy. At a fixed temperature, the rate of desorption will be proportional to the surface area occupied by adsorbate. When the rates of adsorption and desorption are equal, a dynamic equilibrium exists NE
  • 68. For adsorption of gases on to plane surfaces of glass, mica and platinum. As well as being limited to monolayer adsorption, the Langmuir equation assumes that: (a) these are no interactions between adjacent molecules on the surface. (b) the energy of adsorption is the same all over the surface. (c) molecules adsorb at fixed sites and do not migrate over the surface NE
  • 69. • When adsorption first begins, every molecule colliding with the surface may condense on it. • However, as adsorption proceeds, only those molecules that occupy a part of the surface not already covered by adsorbed molecules may be expected to be adsorbed. • The result is that the initial rate of condensation of molecules on a surface is high and then falls off as the surface area available for adsorption is decreased. • On the other hand, a molecule adsorbed on a surface may, by thermal agitation, become detached from the surface and escape into the gas. • The rate at which desorption will occur will depend, in turn, on the amount of surface covered by molecules and will increase as the surface becomes more fully saturated. • These two rates, condensation (adsorption) and evaporation (desorption), will eventually become equal and when this happens, an adsorption equilibrium will Presentation title 69 NE
  • 70. Which vapor would be most readily adsorbed using activated carbon? H2O at 140 F CH4 at 70 F C4H10 at 140F C4H10 at 70 F C4H10 at 70 F NE
  • 71. The carbon dioxide adsorption isotherms for Columbia (“Columbia” is a registered trademark of Union Carbide Corporation) activated carbon is presented for temperatures of 30, 50, and 95 C. 1) Determine the constants of the Freundlich equation at a temperature of 50C. 2) Determine the constants of the Langmuir equation. NE
  • 72. For the Freundlich equation (1), the plot of (log Y ) vs (log p) leads to the equation: Y = 30p^0.7 NE
  • 73. NE
  • 74. Braunauer-Emmet-Teller Isotherm The BET theory allows different numbers of adsorbed layers to build up on different parts of the surface, although it assumes that the net amount of surface which is empty or which is associated with a monolayer, bilayer and so on is constant for any particular equilibrium condition. Monolayers are created by adsorption on to empty surface and by desorption from bilayers. Monolayers are lost both through desorption and through the adsorption of additional layers. The rate of adsorption is proportional to the frequency with which NE
  • 75. Spherical particles of 15 nm diameter and density 2290 kg/m3 are pressed together to form a pellet. The following equilibrium data were obtained for the sorption of nitrogen at 77 K. P/P0 M3 liq N2 * 10^6/kg solid 0.1 66.7 0.2 75.2 0.3 83.9 0.4 93.4 0.5 108.4 0.6 130.0 0.7 150.2 0.8 202.0 0.9 348.0 Obtain estimates of the surface area of the pellet from the adsorption isotherm and compare the estimates with the geometric surface. The density of liquid nitrogen at 77 K is 808 kg/m3. Area occupied by one adsorbed molecule of nitrogen = 0.162 nm2 Avogadro Number = 6.02 × 10^26 molecules/kmol NE
  • 76. The Gibbs isotherm and H-J Model Assume that adsorbed layers behave like liquid films, and that the adsorbed molecules are free to move over the surface. It is then possible to apply the equations of classical thermodynamics. The properties which determine the free energy of the film are pressure and temperature, the number of molecules contained and the area available to the film. The H–J model may be criticised because the comparison between an adsorbed film on a solid surface and a liquid film on a liquid surface does not stand detailed examination BET equation may be applied to low surface coverage when it is thought that adsorbed molecules do not move. The H–J equation may be applied to the middle range of relative pressures (P /P0) corresponding to the completion of a monolayer and the formation of additional mobile layers. Neither theory, it seems, is equipped to deal with the filling of micropores that occurs with the onset of capillary condensation at higher relative pressures NE
  • 77. The potential theory In this theory the adsorbed layers are considered to be contained in an adsorption space above the adsorbent surface. The space is composed of equipotential contours, the separation of the contours corresponding to a certain adsorbed volume The theory was postulated by POLANYI, who regarded the potential of a point in adsorption space as a measure of the work carried out by surface forces in bringing one mole of adsorbate to that point from infinity, or a point at such a distance from the surface that those forces exert no attraction. NE
  • 78. The work carried out depends on the phases involved. Polanyi considered three possibilities: (a) that the temperature of the system was well below the critical temperature of the adsorbate and the adsorbed phase could be regarded as liquid, (b) that the temperature was just below the critical temperature and the adsorbed phase was a mixture of vapour and liquid, (c) that the temperature was above the critical temperature and the adsorbed phase was a gas. Only the first possibility, the simplest and most common, is considered here The potential theory postulates a unique relationship between the adsorption potential εp and the volume of adsorbed phase contained between that equipotential surface and the solid. It is convenient to express the adsorbed volume as the corresponding volume in the gas phase from adsorption data for one gas, data for other gases on the same adsorbent may be NE
  • 79. CHARACTERISTIC CURVE FOR THE ADSORPTION OF CARBON DIOXIDE ON TO CHARCOAL 79 NE
  • 80. NE
  • 81. STRUCTURE OF ADSORBENTS The equilibrium capacity of an adsorbent for different molecules is one factor affecting its selectivity. Another is the structure of the system of pores which permeates the adsorbent This determines the size of molecules that can be admitted and the rate at which different molecules diffuse towards the surface. Molecular sieves, with their precise pore sizes, are uniquely capable of separating on the basis of molecular size. In addition, it is sometimes possible to exploit the different rates of diffusion of molecules to bring about their separation. A particularly important example concerns the production of oxygen and nitrogen from air. Although it is sometimes possible to view pores with an electron microscope and to obtain a measure of their diameter, it is difficult by this means to measure the distribution of sizes and impossible to measure the associated surface area. NE
  • 82. Surface area The simplest measure of surface area is that obtained by the so-called point B method. Many isotherms of Type II or IV show a straight section at intermediate relative pressures. The more pronounced the section, the more complete is the adsorbed monolayer before multilayer adsorption begins. The constant of proportionality may be found by using the same adsorbate on a solid of known surface area. Since the equation was derived for mobile layers and makes no provision for capillary condensation, it is most likely to fit data in the intermediate range of relative pressures NE
  • 83. Pore sizes Where there is a wide distribution of pore sizes and, possibly, quite separately developed pore systems, a mean size is not a sufficient measure. There are two methods of finding such distributions. In one a porosimeter is used, and in the other the hysteresis branch of an adsorption isotherm is utilised. Both require an understanding of the mechanism of capillary condensation. The regions of Type IV and Type V isotherms which are concave to the gas-concentration axis at high concentrations correspond to the bulk condensation of adsorbate in the pores of the adsorbent. An equation relating volume condensed to partial pressure of adsorbate and pore size, may be found by assuming transfer of adsorbate to occur in three stages: (i) transfer from the gas to a point above the meniscus in the capillary. (ii) condensation on the meniscus. (iii) transfer from a plane surface source of liquid adsorbate to the gas-phase in order to maintain the first stage NE
  • 84. isotherm for nitrogen on activated alumina. NE
  • 85. KINETIC EFFECTS It was assumed that equilibrium was achieved instantly between the concentrations of adsorbate in the fluid and in the adsorbed phases. Whilst it may be useful to make this assumption because it leads to relatively straightforward solutions and shows the interrelationship between system parameters, it is seldom true in practice. In large-scale plant particularly, performance may fall well short of that predicted by the equilibrium theory. NE
  • 86. There are several resistances which may hinder the movement of a molecule of adsorbate from the bulk fluid outside a pellet to an adsorption site on its internal surface Some of these are sequential and have to be traversed in series, whilst others derive from possible parallel paths. In broad terms, a molecule, under the influence of concentration gradients, diffuses from the turbulent bulk fluid through a laminar boundary layer around a solid pellet to its external surface. It then diffuses, by various possible mechanisms, through the pores or the lattice vacancies in the pellet until it is held by an adsorption site. During desorption the process is reversed NE
  • 87. In the process of being adsorbed, molecules lose degrees of freedom and a heat of adsorption is released. All the resistances to mass transfer also affect the transfer of heat out of the pellet, though to a different extent. If the heat cannot disperse fast enough, the temperature of the adsorbent increases and the effective capacity of the equipment is reduced. NE
  • 88. a) Boundary film Steady-state film-theory: It is assumed that the difference in concentration and temperature between the bulk fluid and the external surface of a pellet is confined to a narrow laminar boundary-layer in which the possibility of accumulation of adsorbate or of heat is neglected. b) Intra-pellet effects After passing through the boundary layer, the molecules of adsorbate diffuse into the complex structure of the adsorbent pellet, which is composed of an intricate network of fine capillaries or interstitial vacancies in a solid lattice. c) Adsorption NE
  • 89. c) Adsorption The final stage in getting a molecule from the bulk phase outside a pellet on to the interior surface is the adsorption step itself. Where adsorption is physical in nature, this step is unlikely to affect the overall rate. An equilibrium state is likely to exist between adsorbate molecules immediately above a surface and those on it. NE
  • 90. When chemisorption is involved, or when some additional surface chemical reaction occurs, the process is more complicated. Since the adsorption process results in the net transfer of molecules from the gas to the adsorbed phase, it is accompanied by a bulk flow of fluid which keeps the total pressure constant. The effect is small and usually neglected. As adsorption proceeds, diffusing molecules may be denied access to parts of the internal surface because the pore system becomes blocked at critical points with condensate. NE
  • 91. Simplifying by selecting what is termed the controlling resistance. This relates to the step whose resistance is overwhelmingly large in comparison with that for the other layers. The whole of the difference in adsorbate concentration, from its value in the bulk gas to its value at the adsorbent surface, occurs across the controlling resistance. It may be, however, that rate control is mixed, involving two or more resistances.
  • 92. ADSORPTION EQUIPMENT The scale and complexity of an adsorption unit varies from a laboratory chromatographic column a few millimeters in diameter, as used for analysis, to a fluidised bed several metres in diameter, used for the recovery of solvent vapours, from a simple container in which an adsorbent and a liquid to be clarified are mixed, to a highly-automated moving-bed of solids in plug-flow. All such units have one feature in common in that in all cases the adsorbent becomes saturated as the operation proceeds. For continuous operation, the spent adsorbent must be removed and replaced periodically and, since it is usually an expensive commodity, it must be regenerated, and restored as far as possible to its original condition.
  • 93. In most systems, regeneration is carried out by heating the spent adsorbent in a suitable atmosphere. For some applications, regeneration at a reduced pressure without increasing the temperature is becoming increasingly common. The precise way in which adsorption and regeneration are achieved depends on the phases involved and the type of fluid–solid contacting employed.
  • 94. 3 types of contacting: (a) Those in which the adsorbent and containing vessel are fixed whilst the inlet and outlet positions for process and regenerating streams are moved when the adsorbent becomes saturated. The fixed bed adsorber is an example of this arrangement. If continuous operation is required, the unit must consist of at least two beds, one of which is on-line whilst the other is being regenerated. (b) Those in which the containing vessel is fixed, though the adsorbent moves with respect to it. Fresh adsorbent is fed in and spent adsorbent removed for regeneration at such a rate as to confine the adsorption within the vessel. This type of arrangement includes fluidised beds and moving beds with solids in plug flow. (c) Those in which the adsorbent is fixed relative to the containing vessel which moves relative to fixed inlet and outlet positions for process and regenerating fluids. The rotary-bed adsorber is an example of such a unit.
  • 95.
  • 96. Dual stationary-bed solvent recovery system with auxiliaries for collecting the vapor–air mixture from various point sources, then transporting through the particulate filter and into the on-stream carbon adsorber—in this case, bed 1. The effluent air, which is virtually free of vapors, is usually vented outdoors. The lower carbon adsorber (number 2) is regenerated during the service time of bed 1. A steam generator or other source of steam is required. The effluent steam–solvent mixture from the adsorber is directed through the condenser and the liquified mixture then passes into the decanter and/or distillation column for separation of the solvent from the steam condensate.
  • 97. 2 designs of a stationary-bed solvent recovery system The type employs vertical cylindrical bends wherein the solvent laden air flows axially down through the bed. This particular design is advertised for use in the recovery of solvents used in degreasing and dry cleaning, although equally well-suited for recovery of solvents from other industries.(trichloroethylene, tetrachloroethylene, toluene, freon TF, and dichloromethane. ) Regeneration is accomplished with steam flowing upward through the bed and, since the above-mentioned solvents are essentially immiscible in water, decantation is used to separate the condensed solvent from the steam condensate.
  • 98.
  • 99. The valves are of disk type, opened and closed by air-driven pistons. Water is used as coolant in the condenser. Steam, electric power to drive the blower, and cooling water are three operating-cost items. The cost of steam and cooling water increases with frequency of regeneration. This particular unit is a Vic Model 572 with two adsorber beds used alternately, i.e., while one unit is on-stream adsorbing, the other is regenerating. • Dual adsorber systems can also be operated with both beds on-stream simultaneously, especially when solvent concentrations are low. In this situation, regeneration is less frequent than a full work shift and may be accomplished during off-work hours. Operation in parallel almost doubles the air-handling capacity of the adsorption unit and may be an advantage in terms of operating cost
  • 100. At the inlet end of the bed, the adsorbent has become saturated and is in equilibrium with the adsorbate in the inlet fluid. At the exit end, the adsorbate content of the adsorbent is still at its initial value. In between, there is a reasonably well-defined mass-transfer zone in which the adsorbate concentration drops from the inlet to the exit value. This zone progresses through the bed as the run proceeds.
  • 101. At t1 the zone is fully formed, t2 is some intermediate time, and t3 is the breakpoint time tb at which the zone begins to leave the column. For efficient operation the run must be stopped just before the breakpoint If the run extends for too long, the breakpoint is exceeded and the effluent concentration rises sharply
  • 102.
  • 103.
  • 104.
  • 105.
  • 106.
  • 107. The rotary component of the system consists of four coaxial cylinders. The outer cylinder is impervious to gas flow except at the slots near the left end. They serve as air inlet ports where they are shown uncovered and as steam–vapor outlet ports as shown at the lower left end of the rotary. The carbon bed is retained between two cylinders made of screen or perforated metal and also segmented by partitions placed radially between the two cylinders.
  • 108. The inner cylinder is again impervious to gas flow except at the slots near the right end of the rotary. These slots serve as outlet ports for the discharge air and as inlet ports for the regenerating steam. On each rotation of the rotary, each segment of the bed undergoes adsorption and regeneration. The desorbed solvent can then be separated from the steam by decantation or distillation. Because of the continuous regeneration capability of the rotary bed, more efficient utilization of the carbon is possible than with stationary-bed systems.
  • 109. In most solvent recovery operations, the adsorption zone and the saturated bed behind it are idle but add to the bulk of the system and increase airflow resistance through the bed. In deep beds of 12 to 36 inches, as required in the stationary-bed systems, a large portion of the bed is idle at any one time. By continuous regeneration, the regeneration time for each segment of the bed is shortened, and thereby shorter bed lengths can be used. Advantages: a more compact system and a reduced pressure drop. Disadvantages are those associated with wear on moving parts and maintaining seals in contact with moving parts. The use of shorter beds also decreases the steam utilization efficiency.
  • 110. Fluidized-bed solvent recovery system The carbon is recirculated continuously through the adsorption–regeneration cycle. The spent carbon, saturated with solvent, is elevated to the surge bin and then passed down into the regeneration bed where it is contacted with an upward flow of steam. The regenerated carbon is then metered into the adsorber, where the carbon traverses nine beds while fluidized with the upward flow of the vapor-laden air. An air velocity of approximately 240 ft/min is required to cause fluidization of the bed. In both the regeneration and adsorption phases, the carbon is moved countercurrent to the gas or vapor. The countercurrent movement increases the efficiency of regeneration (i.e., the steam :solvent ratio is less than for a stationary bed under otherwise comparable conditions). In addition to the beneficial effects of the countercurrent movement, the bed length can be increased to further improve steam utilization
  • 111.
  • 112. Countercurrent flow also increases the effective use of the carbon; more solvent can be recovered with less carbon than with stationary- or rotary-bed systems. By adjustments or balance of the carbon and solvent input rates, the total carbon in the adsorber can be made part of the adsorption zone reaching saturation in the lowest bed just before it is discharged into the elevator. Very little of the carbon is then idle; hence, maximum utilization is made of the carbon. The fact that the carbon has reached saturation when delivered to the regeneration phase is another factor in the reduced steam requirement; in this respect, the fluidized bed system is most favorable. In rotary-bed operations, the carbon moved into the regeneration phase has reached the lowest state of saturation in the three systems.
  • 113. When large air volumes are treated and available space for the installation is at a premium, the smaller size and lower initial cost are definite advantages over the stationary-bed system. A serious disadvantage is that of high attrition losses of the carbon caused by the fluidization of the beds. Because of the attrition, filtration of the effluent airstream may be required. The influent airstream need not be highly filtered because plugging of the fluidized beds with particulate matter is minimal. The major manufacturer of these units has given consideration to discontinue operations.
  • 114. No adsorber system could operate alone without sufficient auxiliary equipment and the components required to collect, transport, and filter vapor-laden airstreams being delivered to the adsorber. The ducts and piping need to be sized properly for required air velocities to optimize the efficiency of the adsorber. If the air velocity is too high, the stationary-bed adsorber may become a fluidized- bed adsorber, or low flows may create severe channeling through the beds. The fan is the catalyst for forcing the gas stream in and out of the unit, so it is important that careful attention to design and sizing is given to this equipment.
  • 115. Because there are several adsorber configurations, the location of a filter can be varied: • before the inlet airstream (in a stationary bed) to reduce possible contamination to the adsorbent, • after the fluidized-bed adsorber to reduce particulate emissions. Monitoring of the filter efficiency can be accomplished by measuring the pressure drop across the filter using either a manometer or pressure gauge.
  • 116. DESIGN AND PERFORMANCE EQUATIONS Presentation title 116
  • 117. The gas stream containing the solute at an initial concentration C0 is passed down through a (deep) bed of adsorbent material that is free of any solute. Most of the solute is readily adsorbed by the top portion of the bed. The small amount of solute that is left is easily adsorbed in the remaining section of the bed. The effluent from the bottom of the bed is essentially solute-free, denoted as C1. After a period of time, the top layer of the adsorbent bed becomes saturated with solute. The majority of adsorption (approximately 95%) now occurs in a narrow portion of the bed directly below this saturation section.
  • 118. The narrow zone of adsorption is referred to as the mass transfer zone (MTZ). As additional solute vapors pass through the bed, the saturated section of the bed becomes larger and the MTZ moves further down the length of the adsorbent. The actual length of the MTZ remains fairly constant as it travels through the adsorbent bed. Additional adsorption occurs as the vapors pass through the “unused” portion of the bed. The effluent concentration at C2 remains essentially zero, since there is still an unsaturated section of the bed
  • 119. Finally, when the lower portion of the MTZ reaches the bottom of the bed, the concentration of solute in the effluent suddenly begins to rise. This is referred to as the breakthrough point (or breakpoint)—where untreated vapors are being exhausted from the adsorber. If the air stream is not switched to a fresh bed, the concentration of the solute in the outlet will quickly rise until it approaches the initial concentration It should be noted that in most air pollution control applications even trace amounts of contaminants in the effluent stream are undesirable. To achieve continuous operation, adsorbers must be either replaced or cycled from adsorption to desorption before breakthrough occurs. In desorption or regeneration, the adsorbers solute vapors are removed from the used bed in preparation for the next cycle.
  • 120. In regard to regenerable adsorption systems, three important terms are used to describe the equilibrium capacity (CAP) of the adsorbent bed. These capacity terms are measured in mass of vapor per mass of adsorbent. First, the breakthrough capacity (BC) is defined as the capacity of the bed at which unreacted vapors begin to be exhausted. The saturation capacity (SAT) is the maximum amount of vapor that can be adsorbed per unit mass of carbon (this is the capacity read from the adsorption isotherm) and thus is equal to CAP. The working capacity or working charge (WC) is the actual amount adsorbed in the adsorber. Thus, the working capacity is a certain fraction of the saturation capacity. Working capacities can range from 0.1 to 0.7 times the saturation capacity. (Note: A smaller capacity increases the amount of carbon required.) This fraction is set by the designer for individual systems by often balancing the cost of carbon and adsorber operation vs preventing breakthrough while allowing for an adequate cycle time. Another factor in determining the working capacity is that it is uneconomical to desorb all the vapors from the adsorber bed. The small amount of residual vapors left on the bed is referred to as the HEEL. This HEEL accounts for a large portion of the difference between the saturation capacity and the working capacity. In some cases, the
  • 121. In regard to regenerable adsorption systems, three important terms are used to describe the equilibrium capacity (CAP) of the adsorbent bed. measured in mass of vapor per mass of adsorbent. First, the breakthrough capacity (BC) is defined as the capacity of the bed at which unreacted vapors begin to be exhausted. The saturation capacity (SAT) is the maximum amount of vapor that can be adsorbed per unit mass of carbon (this is the capacity read from the adsorption isotherm) and thus is equal to CAP. The working capacity or working charge (WC) is the actual amount adsorbed in the adsorber. Thus, the working capacity is a certain fraction of the saturation capacity. Working capacities can range from 0.1 to 0.7 times the saturation capacity. (Note: A smaller capacity increases the amount of carbon required.)
  • 122. Another factor in determining the working capacity is that it is uneconomical to desorb all the vapors from the adsorber bed. The small amount of residual vapors left on the bed is referred to as the HEEL. This HEEL accounts for a large portion of the difference between the saturation capacity and the working capacity. In some cases, the working capacity can be estimated by assuming it to be equal to the saturation capacity minus the HEEL
  • 123. The determination of the state of the activated carbon (or any adsorbent) is done by monitoring the concentration of the adsorbate through the column. At the top of the column the adsorbent is completely loaded, further down it is partially loaded, and towards the bottom it is unladen (contains no adsorbate). Presentation title 123
  • 124. Over time, as more water and adsorbate pass through the column, more of the adsorbent becomes loaded and the concentration profile changes At time t1 there is no adsorbate leaving the column (all gets adsorbed in the adsorbent). However, at time t2 there is some adsorbent leaving the column with a concentration C2, which increases to C3 at time t3.
  • 125.
  • 126. This outlet adsorbate concentration is extremely important in the operation of an adsorption process, as it often has a maximum allowable value imposed by process requirements or environmental regulations The evolution of this outlet concentration over time is called the breakthrough curve of the column. It can be generated by collecting the data (C1, C2, and C3) from the curves Alternatively, it is customary to rotate the concentration profiles by 90 degree counter-clockwise, plot them all in one graph and then use the outlet concentration to develop the breakthrough curve
  • 127.
  • 128. From a breakthrough curve, the time of breakthrough can be calculated. The time of breakthrough is the time t* when the outlet concentration of the adsorbate reaches the acceptable maximum concentration C* Once that point is reached, the adsorption on this column must be stopped and the adsorbent regenerated, if possible, or replaced. Presentation title 128
  • 129.
  • 130.
  • 131. The region of falling concentration is called the mass transfer zone (MTZ). The gradient is called the adsorption wave front and is usually S-shaped. When its leading edge reaches the exit, breakthrough is said to have been attained. Practically, the breakthrough is not regarded as necessarily at zero concentration but at some low value such as 1% or 5% of the inlet concentration that is acceptable in the effluent. A hypothetical position, the average adsorbate content equals the saturation value, is called the stoichiometric front. The distance between this position and the exit of the bed is called the length of unused bed (LUB).
  • 132. The exhaustion time is attained when the effluent concentration becomes the same as that of the inlet, or some practical high percentage of it, such as 95 or 99%. The shape of the adsorption front, the width of the MTZ, and the profile of the effluent concentration depend on the nature of the adsorption isotherm and the rate of mass transfer. Practical bed depths may be expressed as multiples of MTZ, values of 5–10 multiples being economically feasible. Systems that have linear adsorption isotherms develop constant MTZs whereas MTZs of convex ones become narrower, and those of concave systems become wider as they progress through The narrower the MTZ, the greater the degree of utilization of the
  • 133. Factors that play important roles in dynamic adsorption, and the length and shape of the mass transfer zone are: (i) the type of the adsorbent (ii) the particle size of the adsorbent, which is dependent on maximum allowable pressure drop (iii) the depth of the adsorbent bed and the fluid velocity (iv) the temperature of the fluid stream and the adsorbent (v) the concentration of the contaminants to be removed and not to be removed (including moisture) (vi) the pressure of the system (vii) the removal efficiency required and (viii) possible decomposition or polymerization of contaminants on the adsorbent
  • 134. The ideal adsorption time is Is by which the solute concentration in the fluid is reduced to 50% of the original value. The movement of the adsorption zone through the adsorbent bed and the effect of some process variables on the ideal adsorption time can be obtained in the following manner: The solute fed per unit time per unit area of bed cross section is given by
  • 135. Pressure swing adsorption (PSA) It is based on the principle that the adsorbing capacity of an adsorbent increases with increase in the solute pressure. In a pressure swing adsorption system, adsorption is carried out at an elevated pressure. The flow of the gas is stopped as the breakthrough point is reached and the adsorbent is regenerated by reducing the pressure. Sometimes, a hot inert gas is blown through the solid bed to carry away the desorbed solute molecules. Hot air or steams are frequently used for the purpose.
  • 136. A high dynamic adsorption capacity can be achieved in PSA by reducing the pressure to near vacuum and the PSA system is sometimes modified by putting a vacuum pump in the blow down line. This is called vacuum swing adsorption (VSA) or Vacuum pressure swing adsorption (VPSA). Sometimes in a single bed system, the adsorber may be rapidly pressurized and depressurized. It is called rapid pressure swing adsorption (RPSA).
  • 137. Some of the key industrial applications are as follows: (i) Gas drying (ii) Solvent vapour recovery (iii) Fractionation of air to produce oxygen and nitrogen enriched gases (iv) Production of hydrogen from steam- methane reformer (SMR) off-gas and petroleum refinery off-gases (v) Separation of methane and carbon dioxide from landfill gas (vi) Separation of carbon monoxide and hydrogen (vii) Normal isoparaffin separation (viii) Alcohol dehydration
  • 138. Reasons for PSA: (i) An extra degree of thermodynamic freedom for describing the adsorption process introduces immense flexibility in the process design as compared with the other conventional separation processes such as distillation, extraction, absorption, etc. (ii) Numerous micro-meso porous families of adsorbents exhibiting different adsorptive properties for separation of gas mixtures are available. (iii) The adsorbate is recovered in a relatively concentrated form, and the energy cost is low since no heating is involved. (iv) The optimum combination between a material and a process in designing PSA separation scheme promotes innovation.
  • 139. The RPSA processes are designed to increase the productivity (volume of product/volume of adsorbent/cycle) of separation devices by an order of magnitude or more by using total process cycle times of seconds instead of minutes as in the case of a conventional PSA cycles. It is achieved by simply running a conventional cycle faster using novel hardware or by changing both the adsorber and the process cycle designs. However, separation efficiency and performance may be compromised (lower product purity or recovery) due to time limitations in incorporating the complimentary cyclic steps and limitations caused by adsorption kinetics
  • 140. Skarstrom cycle In a single bed system, the output is not continuous as the bed is to be regenerated separately. However, in a twin bed adsorber the flow of output is kept continuous by operating the twin beds in tandem, one bed acts on adsorption mode while the other on regeneration mode. This is the basic Skarstrom cycle Each bed operates alternately in two half-cycles of equal duration: (i) adsorption followed by pressurization, and (ii) purging followed by depressurization or blow down
  • 141.
  • 142. The feed gas is used for pressurization while a part of the effluent product gas is used for purge. Adsorption is taking place in bed 1 while a part of the gas leaving the bed 1 is routed to bed 2 to purge that bed in a direction counter-current to the direction of flow of the feed gas during the adsorption step. When moisture is to be removed from air, the dry air product is produced during the adsorption step in each of the beds. The adsorption and purge steps usually take less than 50% of the total cycle time.
  • 143. In many commercial applications of this technology, these two steps consume a much greater fraction of cycle time because pressurization and depressurization (blow down) can be completed rapidly. Therefore, cycle times for PSA and VSA are short, typically seconds to minutes. Thus, small beds have relatively large throughputs
  • 144. Since the introduction of the Skarstrom cycle, numerous improvements have been made to increase product purity, product recovery, adsorbent productivity and energy efficiency Among these modifications are the use of (i) three, four or more beds, (ii) a pressure equalization step in which both beds are equalized in pressure following purge of one bed and adsorption in the other, (iii) pretreatment or guard beds to remove strongly adsorbed components that might interfere with the separation of other components, (iv) purge with a strongly adsorbing gas, and (v) an extremely short cycle time to approach isothermal operation, if a longer cycle causes an undesirable increase in temperature during adsorption and an undesirable decrease in temperature during desorption.
  • 145. Separations by PSA and VSA are controlled by adsorption equilibrium or adsorption kinetics, where the latter refers to mass transfer external and/or internal to the adsorbent particle. Both types of control are important commercially. For the separation of air with zeolites, adsorption equilibrium is the controlling factor. Nitrogen is more strongly adsorbed than oxygen and argon. For air with 21% oxygen and 1% argon, oxygen of about 96% purity can be produced. When carbon molecular sieves are used, oxygen and nitrogen have almost the same adsorption isotherms, but effective diffusivity of oxygen is much larger than that of nitrogen. Consequently, nitrogen of very high purity (> 99%) can be produced.
  • 146. Regeneration of the Adsorbent Often it becomes necessary to regenerate the adsorbent and reuse the same. This is particularly necessary if the adsorbent is costly or the adsorbate is valuable and has to be recovered to the extent possible. There are several methods for regeneration of adsorbents of which the pressure swing and temperature swing methods are commonly used.
  • 147. Temperature Swing Regeneration An adsorbate loaded solid can be regenerated by passing a relatively hot inert gas like hot air or steam through the bed. The temperature of regeneration is decided on the basis of adsorption- desorption equilibrium or isotherm and the characteristics of the adsorbent.
  • 148. Feed stream containing a small quantity of an adsorbate at a partial pressure p′1 is passed through the adsorption bed at a temperature T1. The initial equilibrium loading on the adsorbent is X1, generally expressed in weight of adsorbate per unit weight of adsorbent. After equilibrium between the adsorbate and the adsorbent is reached, regeneration requires raising the temperature of the bed to T2. Desorption occurs as more feed is passed through the bed and a new equilibrium loading of X2 is established.
  • 149. The net removal capacity of the adsorption bed is given by the difference between X1 and X2. When the bed is again cooled to T1, purification of the gas stream can be started. Typical adsorbate removal in a temperature swing cycle is generally in excess of 1 kg/100 kg of adsorbent. Since the regeneration time associated with the temperature swing cycle is relatively long, the cycle is used almost exclusively to remove small concentrations of adsorbate from feed streams.
  • 150. This allows the on-stream time to be a significant fraction of the total cycle time of the separation process. In addition, temperature swing cycles may require a substantial quantity of energy per unit of adsorbate. However, if the adsorbate concentration in the feed is small, the energy cost per unit of feed processed can be reasonable.
  • 151. In the inert-purge cycle, the adsorbate during regeneration is removed by passing a nonadsorbing gas containing no adsorbate through the bed. This lowers the partial pressure of the adsorbate and initiates the desorption process. If sufficient pure purge gas is directed through the bed, the adsorbate will be completely removed and adsorption can be resumed. Cycle times, in contrast to temperature swing processes are often only a few minutes rather than hours. However, because adsorption capacity is reduced as adsorbent temperature rise, inert-purge processes are usually limited to 1 to 2 kg
  • 152. The displacement purge cycle differs from the inert-purge cycle since the cycle uses a gas or liquid in the regeneration step that adsorbs as strongly as the adsorbate. Thus, desorption is facilitated both by adsorbate partial pressure or concentration reduction in the fluid, and by competitive adsorption of the displacement medium. Typical cycle times are usually a few minutes. The use of two different types of purge fluid negatively affects the contamination level of the less adsorbed product. However, since the heat of adsorption of the displacement purge fluid is approximately equal to that of the adsorbate as the two exchange places on the adsorbent, the net heat generated or consumed is essentially negligible and thus maintains nearly isothermal conditions
  • 153. The latter condition makes it possible to increase the adsorbate loading of the displacement purge cycle over that for the inert-purge cycle One of the important considerations for a TSA system is the temperature of the adsorbent bed during regeneration, others being quantity and flow of the regenerating gas. The regeneration temperature can be estimated by a simple graphical procedure from the isosteres The isosteres are a set of curves which may be drawn from the isotherms by plotting the partial pressures or dew points or concentrations against the corresponding temperature or reciprocal of temperature for a equilibrium loading as parameter. The particular form of the parameters, viz. p′, log p′, T, or 1/T is chosen so as to make the isosteres almost linear for facilitating any interpolation.
  • 154. 154
  • 155. 155 Pressure Swing Regeneration In this method, the adsorbent bed is regenerated by reducing the pressure of the adsorbent bed to atmospheric or even lower pressure when the bed is stripped of the adsorbed solute. If required, the adsorbent bed may be further cleaned by passing hot air or steam through the bed. In the process, the partial pressure of an adsorbate can be reduced by lowering the total pressure of the gas. The lower the total pressure of the regeneration step, the greater is the adsorbate loading during the adsorption step
  • 156. 156 The time required to load, depressurize, regenerate and repressurize a bed is generally only a few minutes. Thus, even though practical adsorbate loadings are almost always less than 1 kg of adsorbate per 100 kg of adsorbent, to minimize thermal gradient problems, the very short times makes the pressure swing cycle a viable option for bulk gas separations
  • 157. 157 In the pressure swing process one can have 3-12 adsorber vessels, one or more can be used in adsorption step while others are used for various stages of regeneration. Polybed pressure swing adsorption process for commercial production of hydrogen with a purity of 99.99% by steam reforming of methane.
  • 158. 158
  • 159. 159 It is quite common in a pressure swing cycle to use a fraction of the less adsorbed gas product as a low pressure purge gas. Often the purge flow is in the opposite direction from that of the feed flow. The simplest version of this process is designated as PSA. Skarstrom has developed the rules for the minimum fraction of less adsorbed gas product required for displacing the adsorbate. Although the concept of PSA is very simple, the process in commercial application is fairly complex because it involves a multicolumn design where the adsorbers operate under a cyclic steady-state using a series of sequential nonisothermal, nonisobaric and nonsteady state steps. These include adsorption, desorption and a multitude of complementary steps which are designed to control the purity of product along with its recovery as well as to optimize the overall separation performance.
  • 160. 160 Unique PSA cycles are also used to produce simultaneously two pure products from a multicomponent feed, e.g. oxygen and nitrogen from air, carbon dioxide and hydrogen from SMR off-gas, carbon dioxide and methane from landfill gas, and to produce a product containing a component which is not present in the feed, e.g. ammonia synthesis gas from SMR off-gas
  • 161. 161 The RPSA process for hydrogen purification developed by Questar Industries, Canada utilizes the conventional Hydrogen-PSA process consisting of the following steps: Step 1: adsorption of feed gas over a packed bed of adsorbent at super-ambient pressure in order to produce a hydrogen-enriched gas Step 2: co-current depressurization of adsorber to produce additional amount of hydrogen-enriched gas for pressurizing an accompanied vessel Step 3: further co-current depressurization to produce another stream of hydrogen-enriched gas for counter-currently purging an accompanied vessel Step 4: counter-current depressurization to desorb a part of the feed-gas impurities Step 5: counter-current purge with hydrogen from Step 3 to further desorb the impurities Step 6: counter-current pressurization with hydrogen from Step 2, and Step 7: counter-current pressurization to feed pressure level with hydrogen from Step 1.
  • 162. 162 The cycle runs rapidly using a total cycle time of ~1 min compared to cycle times of 10-30 minutes in a conventional process. Six parallel adsorbers packed with layers of activated carbons and zeolites are used in conjunction with two rotating flow control valves for introduction and removal of gases to and from the adsorbers. The process produces 99.95% pure hydrogen with a recovery of 67–73% from the feed gas at 8.5 atm pressure.
  • 163. 163 The peculiarity of a novel adsorber design shows the use of two or more (multiples of two) layers of a zeolite adsorbent inside a single vessel separated by a perforated plate (similar to the arrangement of packings or trays in gas/vapour-liquid contacting equipment). The RPSA process consists of the following steps: (i) simultaneous pressurization and adsorption of nitrogen from air by one layer of the adsorbent in order to produce the oxygen-enriched gas (ii) simultaneous depressurization and purging with oxygen product of the other adsorbent layer of the pair for its regeneration. Using 5A zeolite as the adsorbent and total cycle time of 10 s (compared to 60-240 s. for a conventional PSA-oxygen process) the process can produce 27.5% oxygen with recovery of 64.1% at feed air pressure of 2.22 atm
  • 164. 164 High Temperature PSA Process A chemisorbent, Na2O supported on alumina, is used in a PSA process for direct removal and recovery of bulk carbon dioxide from a high temperature feed gas (wet) without pre-drying or cooling of the feed. The adsorbent reversibly chemisorbs carbon dioxide in presence of steam in the temperature range 200-300°C A gas stream at ~200°C saturated with water, is treated for simultaneous production of CO2 depleted stream and CO2 enriched stream without removing water. The PSA cycle consists of the following steps: Step 1: adsorption at ~125 psia Step 2: co-current CO2 rinse at ~125 psia with recycle of effluent gas as feed Step 3: counter-current evacuation to ~2.5 psia with simultaneous steam purge at feed gas temperature and Step 4: counter-current pressurization with a part of CO2 depleted product gas from Step 1.
  • 165. 165 The effluent gas from Step 3 is partly used as the purge gas in Step 2 after recompression, and the balance is withdrawn as the recovered CO2 product gas. The inert gas and CO2 recoveries from the feed gas are ~100 and 78%. This application may be attractive for greenhouse gas emission control, i.e. CO2 sequestration from a hot gas or CO2 removal from combustion gases for power generation without cooling the gas and removing the water. This technology is a promising one for future development and possesses high potential for tackling many practical problems in the fields of natural gas supply and conservation, reduction of greenhouse gas emissions, production and conservation of hydrogen, etc.
  • 166. 166 Choice between the adsorption cycles The total quantity of adsorbent required for a particular application is equal to the total load of the adsorbate or contaminant in the cycle divided by the dynamic adsorption capacity. The higher is the dynamic adsorption capacity, the less is the size of the bed and hence, less is the capital cost of the equipment as well as the cost of the adsorbent replacement. Thus, prima facie PSA appears to be a better choice than TSA. But if the throughput is large, the capital cost of the equipment and the cost of the compression would be very high. Also, in PSA a purge is required to drive off the residual gas from the void spaces of the bed for improving the product quality, and often this purge has to be the product gas itself or a high purity inert gas. Since this cannot be reused due to its level of
  • 167. In TSA too, a regenerating gas is required and that has to be heated to a sufficiently high temperature for regenerating the adsorbent bed. In large industries TSA is usually in operation due to having spare resources for use in regeneration. Very large throughputs cannot be handled by PSA. If the feed rate is low or moderate but the feed contains contaminants in large quantity, PSA system is suitable.
  • 168. PSA and VSA cycles have been modelled successfully for both equilibrium and kinetic controlled processes. Of particular importance in PSA and TSA is the determination of the cyclic steady state. In TSA, following the desorption step the regenerated bed is usually clean. Thus, a cyclic steady state is closely approached in one cycle. In PSA and VSA, this does not often happen where complete regeneration is seldom achieved or necessary. It is only required to attain a cyclic steady-state whereby the product obtained during the adsorption step has the desired purity and at cyclic steady-state, the difference between the loading profiles after adsorption and desorption is equal to the solute entering in the feed.
  • 169. Starting with a clean bed, the attainment of a cycle steady-state for a fixed cycle time may require tens or hundreds of cycles. For example, ethane and carbon dioxide are separated from nitrogen using 5A zeolite at ambient temperature with adsorption and desorption for 3 min each at 4 atm and 1 atm, respectively, in beds of 0.25 m in length The computation of the loading and gas concentration profiles at the end of each adsorption step for ethane starting with a clean bed, shows that at the end of the first cycle the bed is still clean beyond about 0.11 m and by the end of the 10th cycle, a cyclic steady-state has almost been attained with a clean bed existing only near the very end of the bed. Experimental data for ethane loading at the end of 10th cycle agree reasonably well with the computed profile
  • 170. The cycle time in PSA is very short as the adsorbent bed becomes saturated quickly due to large concentration of the contaminant (adsorbate). So the adsorbent must be rugged enough to withstand millions of pressurization and depressurization cycles without disintegration. The carbon molecular sieve in nitrogen generation plant suffers more than 100 million such cycles in its life of 10 years.
  • 171. On the other hand, TSA handles a very large quantity of feed and its cycle time is long because of the sufficient time required to heat and cool the bed. The bed is to be cooled after heating, and brought to adsorption temperature otherwise the adsorption capacity would be less. Naturally, an adsorbent suffers several thousand cycles of thermal expansion and cooling in its life time, and has to be mechanically and chemically rugged enough to withstand such repeated thermal shocks For adsorbent like silica gel, the regeneration temperature is very low, usually less than 150°C. But for molecular sieves and activated carbons, the regeneration temperatures are quite high depending on the application. Normally the molecular sieves require regeneration temperatures more than 220°C because of the higher heat of adsorption
  • 172. Gas phase Liquid phase Process Condition Temperature swing Inert purge Displacement purge PSA TSA Feed: gas or vapour y y y y n Feed: liquid, vaporise at < 200°C n y y y y Feed: liquid, vaporise at > 200°C n n n n y Adsorbate concentration in feed, < 3% y y n n y Adsorbate concentration in feed, 3-10% y y y y n Adsorbate concentration in feed, > 10% n y y y n High product purity Required y y y Y (hAp, lDp) y Thermal generation Required y y n n n Difficult purge/ adsorbate separation Y (no recovery of adsorbate) n n NA y
  • 173. Adsorption Equipment Stage-wise Contact Adsorption from gases Fluidized beds are widely used for adsorption from gases like recovery of vapours from vapour-gas mixtures, desorption, fractionation of light hydrocarbon vapours and several other applications. Adsorption is usually carried out at gas velocities just above the quiescent fluidizing velocity when the bed expands and looks like boiling liquid. The solid particles move about freely and are thoroughly circulated. Well defined bubbles of gas rise through the bed. Only fine particles below 20 mesh are suitable for use in fluidized beds and the superficial gas velocity has to be kept below 0.60 m/s.
  • 174. They can, however, retain large range of particle sizes. Teeter beds are more frequently used for adsorption from gases since they provide better gas-solid contact by accommodating coarser particles, up to 10 mesh at superficial gas velocities in the range of 1.5 to 3 m/s. In these beds, however, there is little gas bubbling and the size range of particles retained is relatively small. Multistage operations are generally not practised for gas-liquid contact in commercial scale
  • 175. Adsorption from liquids Agitated vessels are commonly used for adsorption from liquids. The liquid depths vary between 0.75 to 1.5 tank diameters. The tanks are provided with wall baffles to avoid vortex formation and lift the solid particles through vertical liquid motion. Flat blade or pitched blade turbines, fitted to an axial shaft are generally used for efficient mixing. Multiple turbines on the same shaft are sometimes used to get more uniform slurry throughout the tank. The clearance of the lowest impeller from the tank bottom should be more than the settled height of the bed, so that the impeller is not embedded if it stops.
  • 176. A typical batch equipment for liquid-solid contact consists of an agitated vessel as above but having a filter press and a collection tank. The liquid to be treated and the adsorbent particles are thoroughly mixed in the tank, at the desired temperature for the required length of time. The slurry is then filtered in a filter press to separate the clear liquid from the solid adsorbent containing the adsorbed solute. The operation can be readily converted to multistage one by using additional tanks and filter presses as required. The operation can also be made continuous by using continuous rotary filters or centrifuges instead of a filter press. Alternatively, the adsorbent may be allowed to settle and continuously
  • 177. Multistage contact: Removal of a given amount of solute can be done with greater economy of adsorbent by treating the solution separately with small batches of adsorbent and filtering between stages rather than employing the entire adsorbent in a single stage. This can be practiced both in batch and continuous operations. As in other mass transfer operations, greater economy of adsorbent can be achieved by counter-current operation where the solution to be treated and the adsorbent flow in opposite directions, either from stage to stage or in a single equipment.
  • 178. Continuous Contact Steady-state moving bed adsorbers Adsorption from gases: Steady state operation requires continuous movement of both fluid and adsorbent through the equipment at constant rates with no change in conditions at any point with passage of time. In order to have better performance, it is necessary to have counter- current operation so that several stages can be accommodated within a single device, since parallel flow of solids and fluids can at best produce a single stage. In view of the difficulties in moving solids through equipment continuously and uniformly, it has not been possible to successfully develop
  • 179. Adsorption from liquids: The difficulties in moving solids continuously and uniformly through equipment, as in case of adsorption from gases, stand in the way of developing successful continuous counter-current equipment for adsorption from liquids also. In case of liquids, however, much less elaborate arrangements are required to overcome the difficulties. For instance, a screw feed or simple gravity flow from a bin may introduce the adsorbent at the top of the tower and the same may be removed from the bottom by a revolving compartmented valve.
  • 180. In the pulsed or moving bed, the liquid enters the bottom and leaves through the top of the column. The flow of liquid is stopped periodically, spent adsorbent is withdrawn (pulsed) from the bottom, and virgin or regenerated adsorbent is added into the top of the adsorber.
  • 181. The Higgins contactor, originally developed for ion-exchange has been more or less successfully used for adsorption from liquids. The equipment though intermittent and cyclic fashion made by the action of piston, approaches counter-current operation in its performance. Adsorption takes place in the upper part of the equipment where the solution flows down through a stationary bed of the adsorbent.
  • 182. At the end of adsorption, the solid particles are transferred to the lower part for regeneration and the upper part is filled with regenerated adsorbent when the cycle is repeated. Adsorption, regeneration and material shifting from adsorption to regeneration zone are carried out in an intermittent and cyclic way by regulating the valves. Out of three valves in the equipment, one valve remains open for each operation.
  • 183. Unsteady-state fixed bed adsorbers The inconvenience and relatively high cost of moving solid particles as required in steady-state operations often make it more economical to use fixed bed adsorbers where the gas or liquid is made to pass through a stationary bed of solid adsorbent. Fixed bed adsorbers are frequently used in recovery of solvent vapours from air or other gases, purification of air in gas masks, dehydration of air or other gases, etc.
  • 184. Adsorber vertically placed with six trays. A thin bed of adsorbent particles is spread on a screen or perforated plate. The depth of the bed usually varies from 0.3 to 1.3 m. Filtered feed gas is admitted into the chamber and flows through the bed. If the effluent gas is substantially free from solvent vapour, it is discharged into the atmosphere. If, on the other hand, the gas contains appreciable amount of vapour, the same is diverted to a subsequent adsorber in series. When the adsorbent particles in any absorber become nearly saturated
  • 185. The enclosures for fixed-bed adsorbers may be horizontal or vertical, conical or cylindrical shells Conical fixed-bed adsorbers are used when a low pressure drop through the cone- shaped bed is less than one-half of that through a conventional flat-bed adsorber, while the air volume is more than double of that through the flat bed.
  • 186. When multiple fixed beds are needed, the usual configuration is a vertical cylindrical shell The type of enclosure used is normally dependent on the gas volume and the permissible pressure drop. The gas flow can be either down or up. Downflow allows for the use of higher gas velocities, while in upflow the gas velocity must be maintained below the value which causes flooding of the bed. When large volume of gas is to be handled, cylindrical horizontal vessels are suitable.
  • 187. For the continuous operation of fixed bed adsorbers, it is desirable to have two or three units. With two adsorber units, one unit adsorbs while the other regenerates or desorbs. Under most situations, two units are sufficient if the regeneration and cooling of the second bed can be completed prior to the breakthrough of the first unit. The move to three units makes it possible for one bed to be adsorbing, one cooling and the third regenerating. The vapour-free air from the first bed is used to cool the unit which was just regenerated. Occasionally a fourth bed is used, when two units adsorbing in parallel, discharge the exhaust to a third unit on the cooling cycle and a fourth unit is being regenerated.
  • 188. Regeneration is done by hot inert gas or low pressure steam, the latter being preferred if the solvent is not miscible with water or if presence of small amount of water is not objectionable. The size of the adsorbent bed is determined on the basis of gas flow rate and desired cycle time. The cross sectional area of the bed is fixed to give a superficial gas velocity of 0.15 to 0.45 m/s.
  • 189. For very large flow rates, rectangular beds in horizontal cylinders are generally used. The operation can be made more or less continuous by placing the adsorbent particles in several small chambers contained in an inner drum rotating inside a stationary compartmented outer shell. As the inner drum rotate, the adsorbent is successively subjected to adsorption, regeneration, drying and cooling. Moist gases can be dried by passing them through beds of silica gel, alumina, bauxite or molecular sieves. If the gases are under pressure, towers may be 10 m or more in height, but in such cases, the adsorbent should be supported on trays at intervals of 1 to 2 m in order to minimize compression of the bed
  • 190. Adsorption from liquids Fixed bed adsorbers are often more economical in case of adsorption from liquids also. Liquids such as gasoline, kerosene, transformer oil, etc., can be dehydrated by activated alumina in fixed bed adsorbers High temperature steam along with vacuum can be used for regenerating the adsorbent. For the bulk separation of liquid mixtures, continuous counter-current systems are preferred over batch systems because they maximize the mass transfer driving force. This advantage is particularly important for difficult separations where selectivity is not high and/or where mass transfer rates are low. Ideally, a continuous counter-current system involves a bed of adsorbent moving downward in plug flow and the liquid mixture flowing upward in plug flow through the void space of the bed. Unfortunately, such a system has not been successfully developed because of problems of adsorbent attrition, liquid channeling and nonuniform flow of adsorbent particles. Successful commercial systems are based on a simulated counter-current system using a fixed bed.
  • 191. Chromatographic separation It is based on the partition coefficient between a mobile phase and a stationary phase. Chromatography techniques have long been used for chemical analysis and separation. The reasons for finding increasing use of chromatographic separation can be as follows: (i) Highest number of theoretical plates it offers per unit length at perhaps the lowest cost per plate (ii) Maximum flexibility in designing separation factor for a given separation through selectivity engineering in the form of infinitely possible surface modifications (iii) Give multicomponent separation in a single operation
  • 192. Chromatography is the technique for the separation, purification, and testing of compounds The term “chromatography” is derived from Greek, chroma meaning, “colour,” and graphein meaning “to write.” In this process, we apply the mixture to be separated on a stationary phase (solid or liquid) and a pure solvent such as water or any gas is allowed to move slowly over the stationary phase, carrying the components separately as per their solubility in the pure solvent.
  • 193. Chromatography separates mixtures into components by passing a fluid mixture through a bed of adsorbent material called stationary phase. The two most common adsorbents used in chromatography are porous alumina and porous silica. Sometimes carbon, magnesium oxide and various carbonates are also used. Alumina is a polar adsorbent and is preferred for the separation of components that are weakly or moderately polar. In addition, alumina is a basic adsorbent preferentially retaining acidic compounds. Silica gel is less polar than alumina and is an acidic adsorbent preferentially retaining basic components such as amines. Carbon is a nonpolar stationary phase with the highest attraction for larger nonpolar molecules. Adsorbent type sorbents are better suited for the separation of a mixture on the basis of chemical type (e.g. olefins, esters, acids, aldehydes, alcohols, etc.) than for
  • 194. For the separation of individual members of a homologous series, partition chromatography is preferred wherein an inert solid support like silica gel, is coated with a liquid phase. For application to gas chromatography the liquid must be nonvolatile. For liquid chromatography, the stationary liquid phase must be insoluble in the mobile phase. The stationary liquid phase may however be bonded to the solid support if required. An example of a bonded phase is the result of reacting silica with a chlorosilane. If the resulting stationary phase is more polar than the mobile phase, the technique is referred to as normal-phase chromatography. Otherwise, the name reverse-phase chromatography is used.

Editor's Notes

  1. (a) zeolites, (b) Activated carbon, (c) mesoporous silica, (d) carbon nanotubes, (e) cyclodextrins, and (f) chitosan beads