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8/2/2019 Adsorption Vivek Neeri
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ADSORPTION EQUILIBRIA
AND REGENERATION
VIVEK KUMAR
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Adsorption is the process in which matter is extracted from one phase and concentrated at the surface of
a second phase. (Interface accumulation). This is a surface phenomenon as opposed to absorption where
matter changes solution phase, e.g. gas transfer. This is demonstrated in the following schematic.
ADSORPTION
d
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Adsorption Mechanism
ADSORPTION MECHANISM
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Exchange adsorption (ion exchange)electrostatic due to charged sites on the surface.
Adsorption goes up as ionic charge goes up and as hydrated radius goes down.
Physical adsorption: Van der Waals attraction between adsorbate and adsorbent. The attraction
is not fixed to a specific site and the adsorbate is relatively free to move on the surface. This is
relatively weak, reversible, adsorption capable of multilayer adsorption.
Chemical adsorption: Some degree of chemical bonding between adsorbate and adsorbent
characterized by strong attractiveness. Adsorbed molecules are not free to move on the surface.There is a high degree of specificity and typically a monolayer is formed. The process is seldom
reversible.
TYPES OF ADSORPTION
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SOME GENERAL ISOTHERMS
IRREVERSIBLEFAVORABLE
STRONGLY
FAVORABLE
UNFAVORABLE
FLUID PHASE CONCENTRATION, MASS/VOL
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Adsorption process
Adsorbent and adsorbate
Adsorbent(also called substrate) - The solid that provides surface for adsorption
high surface area with proper pore structure and size distribution is essential
good mechanical strength and thermal stability are necessary
Adsorbate- The gas or liquid substances which are to be adsorbed on solid
Surface coverage,
The solid surface may be completely or partially covered by adsorbed molecules
ADSORPTION ON SOLID SURFACES
define = = 0~1number of adsorption sites occupiednumber of adsorption sites available
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Rate of adsorption
Rate of desorption
)1( fpkr aa
fkr dd
At equilibrium
da
a
kpk
pkf
Mono-layer coverage fkm a ' ( m: mass of adsorbate adsorbedper unit mass of adsorbent)
Adsorption Isotherm: the mass of adsorbate per unit mass of adsorbent at equilibrium & at a
given temperature
(f: fraction of surface
area covered)
f
1-f
LANGMUIR ISOTHERM
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For the Langmuir model linearization gives:
0
a
e
0
ae
e C
QK
1
q
C
A plot of Ce/qe versus Ce should give a straight line with intercept :
and slope:
0
aQK
1
0aQ
1
e
0
a
0
ae QK
1
Q
1
q
1
Or:
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For the special case of heterogeneous surface energies (particularly good for mixed wastes) in
which the energy term, KF, varies as a function of surface coverage we use the Freundlich
model.
n and KF are system specific constants.
n1
eFe CKq
FREUNDLICH ISOTHERM
Here a plot of 1/qe versus 1/Ce should give a straight line with intercept 1/Qao
and slope 0aQK
1
For the Freundlich isotherm use the log-log version : ln
1Klogqlog Fe
A log-log plot should yield an intercept of log KF and a slope of 1/n.
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The BET isotherm
00a
0mm0a
p
psI
ppv
p
or
p
p
Cv
1C
Cv
1
ppv
p
Theoretical development based on several
assumptions: multimolecular adsorption
1st layer with fixed heat of adsorption H1
following layers with heat of adsorption
constant (= latent heat of condensation)
constant surface (i.e. no capillary
condensation) gives
OT fig1.3
BET method useful, but has limitationsmicroporous materials: mono - multilayer
adsorption cannot occur, (although BET surface
areas are reported routinely)
assumption about constant packing of N2molecules not always correct?
theoretical development dubious (recent
molecular simulation studies, statistical
mechanics) - value of C is indication o f the
shape of the isotherm, but not necessarily
related to heat of adsorption
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BS
e
0aB
B
eeS
e
K
1
C
C
QK
1K
q)CC(
C
0
aB QK
1
B
0
B a s
K 1
K Q C
For the BET isotherm we can arrange the isotherm equation to get:
Intercept =
Slope =
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Simplified method
1-point method simplefied BET assuming value of C 100 (usually the case), gives
usually choose p/p0 0,15
method underestimates the surface area by approx. 5%.
0
0a'm
0'm0mm0a
p
ppvv
pv
p
p
p
Cv
1C
Cv
1
ppv
p
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Summary of adsorption isotherms
Name Isotherm equation Application
Langmuir
Temkin =c1ln(c2P)
Freundlich
BET
ADSORPTION ON SOLID SURFACE
(11
1 0
0 /PcV
c
cV)P/P(V
P/P
mm
C
C
BP
B
s 0
01
21
1
C/pc
Chemisorption andphysisorption
Chemisorption
Chemisorption andphysisorption
Multilayer physisorption
Useful in analysis ofreaction mechanism
Chemisorption
Easy to fit adsorptiondata
Useful in surface areadetermination
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Five types of physisorption isotherms are found over all solids
Type I is found for porous materials with small pores e.g. charcoal.
It is clearly Langmuir monolayer type, but the other 4 are not
Type II for non-porous materials
Type III porous materials with cohesive force between adsorbate
molecules greater than the adhesive force between adsorbate moleculesand adsorbent
Type IV staged adsorption (first monolayer then build up of additionallayers)
Type V porous materials with cohesive force between adsorbatemolecules and adsorbent being greater than that between adsorbatemolecules
ADSORPTION ON SOLID SURFACE
I
II
III
IV
V
relative pres. P/P0
1.0
amountadsorb
ed
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To determine which model to use to describe theadsorption for a particular adsorbent/adsorbate
isotherms experiments are usually run. Data from
these isotherm experiments are then analyzedusing the following methods that are based on
linearization of the models.
DETERMINATION OF APPROPRIATE MODEL
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ADSORPTION-DESORPTION HYSTERESIS
Hysteresis is classified by IUPAC (seefig.)
Traditionally desorption branch usedfor calculation
H1: narrow distribution of mesopores
H2: complex pore structure, networkeffects, analysis of desorption loopmisleading H2: typical for activated carbons
H3 & 4: no plateau, hence no well-defined mesopore structure, analysisdifficult H3: typical for clays
Handbook
fig 2 s 431
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Pore sizes
micro pores dp
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POROSITY AND PORE SIZE
The pore structure (porosity, pore diameter, pore shape) is important for the
catalytic properties
pore diffusion may influence rates
pores may be too small for large molecules to diffuse into
Measurement techniques:
Hg penetration
interpretation of the adsorption - desorption isotherms
electron microscopy techniques
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Hg PENETRATION
Based on measuring the volume of a non-wetting liquid forced into the pores bypressure (typically mercury)
Surface tension will hinder the filling of the pores, at a given pressure anequilibrium between the force due to pressure and the surface tension isestablished:
where P = pressure of Hg, is surface tension and is the angle of wetting
Common values used: = 480 dyn/cm and = 140 give average poreradius
valid in the range 50 - 50000
cr2rP 2
]cm/kp[P
7500r 2
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PORE SIZE DISTRIBUTION
If the Hg-volume is recorded as a function of pressure and this curve isdifferentiated we can find the pore size distribution function V(r)=dV/dr
OT fig 2.3.
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FACTORS WHICH AFFECT ADSORPTION EXTENT (AND
THEREFORE AFFECT ISOTHERM) ARE:
Adsorbate:
SolubilityIn general, as solubility of solute increases the extent of adsorption decreases. This is known
as the Lundelius Rule. Solute-solid surface binding competes with solute-solvent attraction
as discussed earlier. Factors which affect solubility include molecular size (high MW- low
solubility), ionization (solubility is minimum when compounds are uncharged), polarity (as
polarity increases get higher solubility because water is a polar solvent).
pHpH often affects the surface charge on the adsorbent as well as the charge on the solute.
Generally, for organic material as pH goes down adsorption goes up.
Temperature
Adsorption reactions are typically exothermic i.e., Hrxnis generally negative. Here heat is
given off by the reaction therefore as T increases extent of adsorption decreases.Presence of other solutes
Ingeneral, get competition for a limited number of sites therefore get reduced extent of
adsorption or a specific material.
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If the adsorbent and adsorbate are contacted long enough an equilibrium will be established between the
amount of adsorbate adsorbed and the amount of adsorbate in solution. The equilibrium relationship is
described by isotherms.
Define the following:
qe = mass of material adsorbed (at equilibrium) per mass of adsorbent
Ce = equilibrium concentration in solution when amount adsorbed equals qe.
qe/Ce relationships depend on the type of adsorption that occurs, multi-layer, chemical, physical adsorption,
etc.
Adsorption heat:
The increase in enthalpy when 1 mole of a substance is adsorbed upon another at constant
pressure.
Adsorption is usually exothermic (in special cases dissociatedadsorption can be endothermic)
The heat of chemisorption is in the same order of magnitude of reaction heat; the heat of
physisorption is in the same order of magnitude of condensation heat.
DEFINITION
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ADSORPTION EQUILIBRIAIf the adsorbent and adsorbate are contacted long enough an equilibrium will be
established between the amount of adsorbate adsorbed and the amount of adsorbate
in solution. The equilibrium relationship is described by isotherms.
Heats of Adsorption
Gas adsorption to a solid is exothermic.
The magnitude and variation as a function of coverage may reveal informationconcerning the bonding to the surface.
Calorimetric methods determine heat, Q evolved.
V
in
qi = integral heat of adsorption
V
D
n
,
qD = differential heat of adsorption
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3/22/2012
HEAT OF ADSORPTION .. CONTINUED
Since G=H-TS, it is clear that for Gto be negative,
Hof adsorption process must be negative. That is, theadsorption is an exothermic process.
the amount of gas adsorbed will decrease as thetemperature is increased.
The molar enthalpy, adsHm, of adsorption in reversiblesystem will adhere to the Clausius-Clapeyron equation
The subscript nrepresents an isosteric adsorption.adsHm is called the molar isosteric enthalpy ofadsorption.
ads
2
ln
n
p
T RT
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ENTHALPY OF ADSORPTION
Heats of adsorption change as a function of surface coverage
2
0
0
00
0
0000
Kln
Kln
Kln
RT
H
T
RS
RTH
STHRTG
SMSM
AD
ADAD
AADAD
surfacesurfaceg
differentiate
Vant Hoff equation
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DESORPTION AND REGENERATION OF ADSORBENTS
Adsorbent particles have finite capacity for fluid phase molecules. An extended
contact with the feed fluid will ultimately lead to the creation of a
thermodynamic equilibrium between the solid adsorbent and the fluid phases. At
this equilibrium condition the rates of adsorption and desorption are equal and
the net loading on the solid cannot increase further, It is now becomes necessary
either to regenerate the adsorbent or to dispose of it.
In certain applications it may be more economical to discard the adsorbent after
use. Disposal would be favoured when the adsorbent is of low cost, is very
difficult to regenerate, and the non-adsorbed component is the desired product of
very high value. In the majority of applications, the disposal of adsorbents as
waste is not an economic option and therefore regeneration is carried out either
in situ or external to the adsorption vessel to an extent that the adsorbents can be
reused.
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Practical methods of desorption and regeneration include one, or more usually a
combination, of the following:
Increase in temperature
Reduction in partial pressure
Reduction in concentration
Purging with an inert fluidDisplacement with a more strongly adsorbing species
Change of chemical conditions, e.g. pH
The final choice of regeneration method(s) depends on technical and economic
considerations.
PRACTICAL REGENERATION METHODS
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FIXED-BED ADSORPTION PROCESS AND THEIR
REGENERATION METHODS
A. Pressure Swing Adsorption (PSA)
Regeneration in a PSA process is achieved by reducing the partial pressure
of the adsorbate. There are 2 ways in which this can be achieved: (1) a
reduction in the system total pressure, and (2) introduction of an inert gas
while maintaining the total system pressure. In the majority of pressureswing separations a combination of the 2 methods is employed. Use of a
purge fluid alone is unusual.
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The Figure below shows the effect of partial pressure on equilibrium loading for
a Type I isotherm at a temperature of T1. Reducing the partial pressure from p1
to p2 causes the equilibrium loading to be reduced from q1 to q2.
PICTORIAL EXPLAINATION
Changes in pressure can be effected very
much more quickly than changes in
temperature, thus cycle time of pressure
swing adsorption (PSA) processes are
typically in the order of minutes or even
seconds.
PSA processes are often operated at low
adsorbent loadings because selectivity
between gaseous components is often
greatest in the Henry's Law region. It is
desirable to operate PSA processes close
to ambient temperature to take advantage
of the fact that for a given partial pressure
the loading is increased as the temperature
is decreased.
Typical PSA processes consist of 2-Bed
system, although other systems (e.g. 1-
Bed system or complex, multiple-beds
system) had also been developed.
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PSA processes is a popular process for performing bulk separations of gases.
Separations by PSA and VSA are controlled by adsorption equilibrium or
adsorption kinetics. 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. For air with about
21% oxygen and 79% nitrogen, a product of nearly 96% oxygen purity can be
obtained. When carbon molecular sieves are used, oxygen and nitrogen have
almost the same adsorption isotherms, but the effective diffusivity of oxygenis much larger than nitrogen. Hence more oxygen is adsorbed than nitrogen,
and a product of very high purity nitrogen ( 99%) can be obtained.
USES OF PSA PROCESSES
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B. TEMPERATURE SWING ADSORPTION (TSA)
Regeneration of adsorbent in a TSA process is ahieved by an increase in temperature.
The Figure below showed schematically the effect of temperature on the adsorption
equilibrium (Type I isotherm) of a single adsorbate.
For any given partial pressure of the
adsorbate in the gas phase (or
concentration in the liquid phase), an
increase in temperature leads to a decrease
in the quantity adsorbed. If the partial
pressure remains constant at p1,
increasing the temperature from T1 to T2
will decrease the equilibrium loading from
q1 to q2.
A relatively modest increase in
temperature can effect a relatively large
decrease in loading. It is therefore
generally possible to desorb any
components provided that the temperature
is high enough. However, it is important
to ensure that the regeneration
temperature does not cause degradation of
the adsorbents.
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A change in temperature alone is not used in commercial processes
because there is no mechanism for removing the adsorbate from the
adsorption unit once desorption from the adsorbents has occurred.
Passage of a hot purge gas or steam, through the bed to sweep out the
desorbed components is almost always used in conjunction with the
increase in temperature.
A very important characteristic of TSA processes is that they are usedvirtually exclusively for treating feeds with low concentrations of
adsorbates.
TSA.. CONTD........
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C. DISPLACEMENT PURGE ADSORPTION (DPA)
Adsorbates can be removed from the adsorbent surface by replacing them with a
more preferentially adsorbed species. This displacement fluid, which can be a
gas, a vapour or a liquid, should adsorb about as strongly as the componentswhich are to be desorbed. If the displacement fluid is adsorbed too strongly then
there may be subsequent difficulties in removing it from the adsorbent.
The mechanism for desorption of the original adsorbate involves 2 aspects:
(1) partial pressure (or concentration) of original adsorbate in the gas phase
surrounding the adsorbent is reduced
(2) there is competitive adsorption for the displacement fluid. The
displacement fluid is present on the adsorbent and thus will contaminate the
product.
One advantage of the displacement fluid method of regeneration is that the net
heat generated or consumed in the adsorbent will be close to zero because the
heat of adsorption of the displacement fluid is likely to be close to that of the
original adsorbate. Thus the temperature of the adsorbent should remain more or
less constant throughout the cycle.
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With neither pressure nor temperature changes from adsorption to desorption,
regeneration by displacement purge depend solely on the ability of the displacement
fluid to cleanse the bed in readiness for the next adsorption step. A typical
Displacement Purge Adsorption (DPA) process is shown in the Figure below.
PICTORIAL EXPLAINATION
A is the more strongly adsorbed
component in the binary feed mixture
of (A and B) while D is the
displacement purge gas. The feed
mixture of (A and B) is passed
through Bed 1 acting as the adsorber,which is preloaded with D from the
previous cycle (when Bed 1 was the
regenerator).
A is adsorbed and the product of a
mixture of (B and D) emerges from
the top of the column. (B and D) are
easily separated by distillation so that
B is collected in a relatively pure
state.
The displacement gas D then enters
Bed 2 acting as regenerator and from
which emerges a mixture of (A and
D). (A and D) can be separated
without difficulty in another
distillation column.
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In effect the original mixture of (A and B), which would have been difficult to separate by PSA or TSA, is separated
by the "intervention" of another strongly adsorbed component D. The ease of separation of A from D, and B from D,
in the additional distillation stages, is crucial in determining the economies of displacement purge cycle operation.
Examples of commercial processes include the separation of linear paraffins from mixtures containing branched
chain and cyclic isomers in the range of C10 - C18 hydrocarbons.
DPA.. CONTD........
Other Adsorption Cycles
Virtually all adsorption processes use changes in temperature, pressure, concentration of a competitvely
adsorbing component to effect adsorption and desorption. But presumably any other variables which could
effect changes in the shape of an adsorption isotherm could also be used.
One such variable is the pH. The bonding between some adsorbents and adsorbates such as amino acids in
water can be changed remarkably as the pH is changed from above the isoelectric point of the amino acid to
below its isoelectric point. The isoelectric point is the pH at which the amino acid molecule has zero charge.
The economic problem of using pH swing as a means to drive a cyclic process is the cost of the acid and base
required to change the pH, as well as the cost of disposal of the salt by-product.
Another means for changing the shape of the adsorption isotherm is the use of electric charge.
Electrosorption involves adsorption when the adsorbent is subjected to one voltage and the desorption when
the voltage is changed. Typically the voltage can be small, such as 1V or less. This process can only be
accomplished in cases in which both the adsorbent and the feed stream are highly conductive. An example is
EDA (ethylenediamine), which demonstrates different loading at different voltages.
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