Adsorption/ Desorption and Regeneration

1. ADSORPTION EQUILIBRIA
AND REGENERATION
VIVEK KUMAR

2. ADSORPTION
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.
d

3. ADSORPTION MECHANISM
Adsorption Mechanism

4. TYPES OF ADSORPTION
•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.

5. SOME GENERAL ISOTHERMS
IRREVERSIBLE
FAVORABLE
STRONGLY
FAVORABLE
UNFAVORABLE
FLUID PHASE CONCENTRATION, MASS/VOL

6. ADSORPTION ON SOLID SURFACES
 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
number of adsorption sites occupied
define θ = θ = 0~1
number of adsorption sites available

7. LANGMUIR ISOTHERM
Adsorption Isotherm: the mass of adsorbate per unit mass of adsorbent at equilibrium & at a
given temperature
Rate of adsorption ra = k a p (1 − f ) (f: fraction of surface
area covered)
Rate of desorption rd = k d f
ka p f
At equilibrium f =
ka p + kd 1-f
Mono-layer coverage m = ka ' f ( m: mass of adsorbate adsorbed
per unit mass of adsorbent)

8. For the Langmuir model linearization gives: Ce 1 C
= + e
qe K ⋅Q0
a Q0a
A plot of Ce/qe versus Ce should give a straight line with intercept :
1
K ⋅Q 0
a
1
and slope:
Q0
a
Or: 1 1 1 1
= 0+ ⋅
q e Qa K ⋅ Qa Ce
0

9. FREUNDLICH ISOTHERM
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.
1
q e =K FCe n
n and KF are system specific constants.
1
Here a plot of 1/qe versus 1/Ce should give a straight line with intercept 1/Q and slope
a
o
K ⋅Q0
a
1
For the Freundlich isotherm use the log-log version : log q e = log K F + log C
n
A log-log plot should yield an intercept of log KF and a slope of 1/n.

10. The BET isotherm
●
Theoretical development based on several
assumptions:
 multimolecular adsorption
 1st layer with fixed heat of adsorption H1
OT fig1.3  following layers with heat of adsorption
constant (= latent heat of condensation)
 constant surface (i.e. no capillary
condensation) gives
BET method useful, but has limitations
•microporous materials: mono - multilayer
p 1 C −1 p
adsorption cannot occur, (although BET surface = + ⋅
areas are reported routinely) va ( p0 − p) vm ⋅ C vm ⋅ C p0
•assumption about constant packing of N2
molecules not always correct?
•theoretical development dubious (recent or
molecular simulation studies, statistical
mechanics) - value of C is indication o f the p p
= I+ s⋅
shape of the isotherm, but not necessarily va ( p0 − p) p0
related to heat of adsorption

11. For the BET isotherm we can arrange the isotherm equation to get:
Ce K B −1 C e 1
= ⋅ +
(C S − C e ) ⋅ q e K B ⋅ Q 0 CS K B ⋅ Q 0
a a
1
Intercept = K B ⋅ Q0
a
KB − 1
0
Slope = K B ⋅ Q a ⋅ Cs

12. Simplified method
●
1-point method
 simplefied BET assuming value of C ≈ 100 (usually the case), gives
p 1 C −1 p p
= + ⋅ ≈ '
va ( p0 − p) vm ⋅ C vm ⋅ C p0 vm ⋅ p0
va ⋅ ( p0 − p)
v 'm =
p0
 usually choose p/p0 ≈ 0,15
 method underestimates the surface area by approx. 5%.

14. ADSORPTION ON SOLID SURFACE
Summary of adsorption isotherms
Name Isotherm equation Application
Cs BP Chemisorption and Useful in analysis of
Langmuir θ= = 0 reaction mechanism
C∞ 1 + B0 P physisorption
Temkin θ =c1ln(c2P) Chemisorption Chemisorption
Chemisorption and Easy to fit adsorption
Freundlich θ = c1 p 1 / C2
physisorption data
P / P0 1 c−1
BET = + ( P / P0 )
V ( 1 − P / P0 ) cVm cVm Multilayer physisorption Useful in surface area
determination

15. ADSORPTION ON SOLID SURFACE
Five types of physisorption isotherms are found over all solids
I
 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
II
 Type II for non-porous materials
amount adsorbed
 Type III porous materials with cohesive force between adsorbate molecules
III
greater than the adhesive force between adsorbate molecules and adsorbent
IV
 Type IV staged adsorption (first monolayer then build up of additional layers)
 Type V porous materials with cohesive force between adsorbate molecules
V and adsorbent being greater than that between adsorbate molecules
1.0
relative pres. P/P0

16. DETERMINATION OF APPROPRIATE MODEL
To determine which model to use to describe the
adsorption for a particular adsorbent/adsorbate
isotherms experiments are usually run. Data from
these isotherm experiments are then analyzed
using the following methods that are based on
linearization of the models.

17. ADSORPTION-DESORPTION HYSTERESIS
●
Hysteresis is classified by IUPAC (see
fig.)
●
Traditionally desorption branch used
for calculation
●
H1: narrow distribution of mesopores
●
H2: complex pore structure, network
effects, analysis of desorption loop
Handbook misleading
fig 2 s 431  H2: typical for activated carbons
●
H3 & 4: no plateau, hence no well-
defined mesopore structure, analysis
difficult
 H3: typical for clays

18. PORES AND POROUS SOLIDS
 Pore sizes
 micro pores dp <20-50 nm
 meso-pores 20nm <dp<200nm
 macro pores dp >200 nm
 Pores can be uniform (e.g. polymers) or non-uniform (most metal oxides)
 Pore size distribution
 Typical curves to characterise pore size:
 Cumulative curve
 Frequency curve wt dw
dd
 Uniform size distribution (a) &
∆wt a
non-uniform size distribution (b) a b
b
∆d d d
Cumulative curve Frequency curve

19. 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

20. Hg PENETRATION
●
Based on measuring the volume of a non-wetting liquid forced into the pores by
pressure (typically mercury)
●
Surface tension will hinder the filling of the pores, at a given pressure an
equilibrium between the force due to pressure and the surface tension is
established:
P ⋅ π ⋅ r 2 = −2π ⋅ r ⋅ γ ⋅ cos α
where P = pressure of Hg, γ is surface tension and α is the angle of wetting
●
Common values used: γ = 480 dyn/cm and α= 140° give average pore
radius
r=
75000
2
[ Å]
P[kp / cm ]
valid in the range 50 - 50000Å

21. PORE SIZE DISTRIBUTION
●
If the Hg-volume is recorded as a function of pressure and this curve is
differentiated we can find the pore size distribution function V(r)=dV/dr
OT fig 2.3.

22. FACTORS WHICH AFFECT ADSORPTION EXTENT (AND
THEREFORE AFFECT ISOTHERM) ARE:
Adsorbate:
Solubility
In 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).
pH
pH 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.,  H rxn is generally negative. Here heat is
given off by the reaction therefore as T increases extent of adsorption decreases.
Presence of other solutes
In general, get competition for a limited number of sites therefore get reduced extent of
adsorption or a specific material.

23. DEFINITION
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 q e.
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 dissociated adsorption 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.

24. ADSORPTION EQUILIBRIA
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.
Heats of Adsorption
Gas adsorption to a solid is exothermic.
The magnitude and variation as a function of coverage may reveal information
concerning the bonding to the surface.
Q
Calorimetric methods determine heat, Q evolved. qi =  
 n V
qi = integral heat of adsorption
 δQ
qD =   qD = differential heat of adsorption
 δ n  V ,T

25. HEAT OF ADSORPTION .. CONTINUED
●
Since ∆G=∆H-T∆S, it is clear that for ∆G to be negative,
∆H of adsorption process must be negative. That is, the
adsorption is an exothermic process.
●
the amount of gas adsorbed will decrease as the
temperature is increased.
● The molar enthalpy, ∆adsHm, of adsorption in reversible
system will adhere to the Clausius-Clapeyron equation
 ∂ ln p  ∆ ads H m
 ÷ =−
08/23/12  ∂T  n RT 2
● The subscript n represents an isosteric adsorption. ∆adsHm
is called the molar isosteric enthalpy of adsorption.

26. ENTHALPY OF ADSORPTION
Heats of adsorption change as a function of surface coverage
M g + S surface ⇔ M − S surface
∆G 0
AD = − RT ln K = ∆H
0 0
AD − T∆ S 0
AD
∆H ∆S0 0
ln K = −
0
+ AD AD
RT R
 δ 0 ∆H AD
0
 δT ln K  = RT 2
differentiate
 θ
Van’t Hoff equation

27. 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.

28. PRACTICAL REGENERATION METHODS
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 fluid
Displacement 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.

29. 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
pressure swing separations a combination of the 2 methods is employed.
Use of a purge fluid alone is unusual.

30. PICTORIAL EXPLAINATION
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.
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.

31. USES OF PSA PROCESSES
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
oxygen is much larger than nitrogen. Hence more oxygen is adsorbed than
nitrogen, and a product of very high purity nitrogen ( 99%) can be obtained.

32. 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.

33. TSA.. CONTD........
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 used
virtually exclusively for treating feeds with low concentrations of
adsorbates.

34. 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 components
which 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.

35. PICTORIAL
EXPLAINATION
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.
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.

36. DPA.. CONTD........
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.
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.

37. THANK
YOU