Adsorption Vivek Neeri

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

Qq

qi = integral heat of adsorption

V

D

n

Qq

,

qD = differential heat of adsorption


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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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Adsorption Vivek Neeri