Chapter 2 (Adsorbtion).pdf

8/16/2019 Chapter 2 (Adsorbtion).pdf

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CHAPTER 2 :SUBTOPIC ADSORPTION


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OBJECTIVES

Students should be able to :

1. Explain adsorption process and process application

2. Identify type and characteristics of adsorbents


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Introduction

Definition: Adsorption is the adhesion of atoms, ions, or molecule

from a gas, liquid, or dissolved solid to a surface.

It is the separation of components in a fluid mixtures by thetransfer of one or more components (the adsorbates) tothe internal surface of a porous solid (the adsorbent) wherethey are held by intermolecular forces.

surface-based process


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Adsorbed solute: Adsorbate Solid material: Adsorbent


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Molecules/atoms/ions in a gas or liquid diffuse to tsurface of a solid, where they bond with the solid surfaor are held there by weak inter-molecular forces


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Desorption

• Adsorption-Saturated-Desorption• Regeneration process:• To recover Adsorbate and Adsorbent to be

reused• Use Desorbent or change in temperature or

pressure


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Application of adsorption

Liquid phase :

removal of organic compounds from water/organic solutions,

colored impurities from organics,

separations of paraffin from aromatics

Gas phase : Removal of waster from HC gases

Sulfur compound from NG

Solvents from air & other gases

Odors from air


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Contacting modes for adsorption

1. Stirred Tank

2. Cyclic fixed-bed, batch operation

3. Continuous countercurrent operation


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1. Stirred tank

Powdered adsorbent e.g. activated carbon – particle diameterless than 1 mm

Main application – wastewater treatment Spent adsorbent – removed by sedimentation or filtration


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2. Fixed bed batch

• Adsorbent particle size – 0.05 cm to 1.2 cm• Optimal particle size – bed pressure drop & solute transport rate• Main application – removal of organic compounds from water• Spent adsorbent – regenerated at high temperature


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3. Continuous Countercurrent

• Need to circulate solid adsorbent as moving bed to achieve steadystate operation

• Difficult in regenerating adsorbent when heavier HC presents

• Unfavourable economics compared to distillation


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Physical Properties of Adsorbents

Shape

i. Small pellets, beads, granules, cylindrical, powders

ii. Size ranging from 50 m to 1.2 cm Very porous structure (with many fine pores and pore volumes up to 50%

of total particle volume)

i. Macropore ( > 500Å) 50 nm

ii. Mesopore (20 - 500 Å)

iii. Micropore ( < 20 Å )

Based on International Union of Pure and Applied Chemistry (IUPAC) Specific surface area: 300 to 1,200 m2/g


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Range of adsorbents available Activated carbon - Made by thermal decomposition of wood

- Average pore diameter 10 to 60 A

- Usually organics adsorbed by activated carbon

- surface area 300 to 1200 m2/gSilica gel - Made by acid treatment of sodium silicate solution and then d

- Surface area 600 to 800 m2/g

- Average pore diameter 20 to 50 A

Activatedalumina

- Hydrated aluminum activated by heating to dry off the water

- Used mainly to dry gases and liquids

- Surface area 200 to 500 m2/g

- Average pore diameter 20 to 140 A Molecular sievezeolites

- Porous crystalline aluminosilicates

- Open crystal lattice containing precisely uniform pores; mdifferent from other types of adsorbents

- Different zeolites have pore sizes from 3 to 10A

- Used for drying, separation of HCs

Synthetic

polymers or resin

-Made by polymerising two major types of monomers

- eg. Styrene and divinylbenzene to adsorb nonpolar organicsaqueous solutions


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Activated Carbon

Coal-Based Activated Carbon for

Gas Purification


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Silica gel

Made by acid treatment of silicate solution and then d


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1. This product is made of the kind of γ activated alumina.

which is white granule with the big adsorptive capacity

for the polar material. Also, it enable to regeneration bychanging the pressure, temperature.

2. Mainly used in removing water, acetic acid

tetrabromoethane etc.

Activated alumina

http://www.21food.com/userImages/chemichael/chemichael$42134033.jpghttp://www.21food.com/userImages/chemichael/chemichael$42134033.jpg


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Molecular Sieve Zeolites


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Unique macro-porous synthetic polymer absorbent to

remove soluble and insoluble impurities from aquarium

water. For marine and freshwater use.

Synthetic polymer / RESINS

ResinDA201-D macro-net non-polar adsorbentresin, used for discoloration of fruit juice


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Adsorption theory

Movement of an organic and/or inorgamolecule to a surface site requires four separaphenomena: Bulk fluid transport (external/ interphase mass transfe

Mass transfer of the solute from the bulk fluid by convection, through a thin film or boulayer , to the outer, solid surface of the adsorbent

Film transport (internal/ intraphase) Mass transfer of the solute by pore diffusion from the outer surface of the adsorbent

inner surface of the internal porous structure

Intraparticle (pore and/or surface diffusion)

Physical attachment


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Bulk Fluid transport

Film transport

Intraparticle

Physical Attachment

Adsorption theory


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Adsorption theory


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Adsorption theory

For chemisorption, which involves bond formation, the rate othe fourth kinetic step may be slow and even controlling

For physical adsorption, however step 4 is almosinstantaneous because it depends only on the collisionfrequency and orientation of the molecules with the porousurface. Thus, only three steps need to be considered here

During regeneration, the reverse of the four steps occurswhere the rate of physical desorption is instantaneous. Adsorption and desorption are accompanied by heat transfe

because of the exothermic heat of adsorption and the endothermicheat of desorption


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Method of adsorptionPhysical adsorption

Van der Waals adsorption

Low heat of adsorption

Non specific

Monolayer or multilayer

No dissociation of adsorbedspecies.

Only significant at relatively

low temperatures. Rapid, non-activated,

reversible.

No electron transfer.

Chemisorption

Activated adsorption

High heat of adsorption

Highly specific (to oneadsorbate)

Monolayer only

May involve dissociation

Possible over a wide range

of temperature Activated, may be slow and

irreversible

Electron transfer leading tobond formation betweensorbate and surface.


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Physical adsorption from agas occurs when the

intermolecular attractiveforces between molecules ofa solid and the gas aregreater than those betweenmolecules of the gas itself

Adsorption is likecondensation, which isexothermic and thus isaccompanied by a releaseof heat

Magnitude of the HOA canbe less or greater than theheat of vaporization, andchange with the extent ofadsorption

Commercial adsorbents relyon physical adsorption

Involves the formation ofchemical bonds between

the adsorbent andadsorbate in a monolayer Often with a release of heat

much larger than the heat ofvaporization

Chemisorption from a gasgenerally takes place only atT>200oC

Catalyst relies on

chemisorption

Physical adsorption Chemisorption

Method of adsorption


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Criteria for adsorbentselection

1. High selectivity to enable sharp separations.2. High capacity to minimize the amount adsorbent needed.3. Favorable kinetic and transport properties for rapid sorption.4. Chemical and thermal stability to preserve the amount and its properties5. Hardness and mechanical strength.6. High fouling resistance.

7. Capability of being regenerated relatively low cost.


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Equilibrium isotherm


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OBJECTIVES

Students should be able to

1. Identify and explain type of isotherm

2. Perform calculation for confirmation of adsorption isotherm


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Equilibrium relations foradsorbents

Concentration of a solute

in a fluid phase

Concentration of asolute in a solid phase

Data is plotted aadsorption isothermsT, P


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Equilibrium relations foradsorbents

The equilibrium isotherm places a limit on the extent to which a solute isadsorbed from a given on an adsorbent of given chemical compositionand geometry for a given set of conditions

Desirable/ favorable isotherm exhibit strong adsorption

Undesirable/ unfavorable isotherm exhibit low/ weak adsorption


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1. Linear Isotherm

Henry’s law is obeyed for dilute regionsolution:

q = Kc

C : concentration (fluid is liquid)

: kg adsorbate / m3 fluid

p : partial pressure (fluid is a gas)q : mass, moles or volumes of adsorbate (solutes) per unit mass or per unit

surface area of adsorbent

: kg adsorbate (solute) / kg adsorbent (solid)

K : an empirical, temperature-dependent constant (determined experiment


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q = Kcn

• Approximate data for many physical adsorptioParticularly useful for liquids

• K = Freundlich constant

• n = constant (n ≠ 1)• Both are determined experimentally.

2. Freundlich isotherm


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3. Langmuir isotherm

q = (qo c )/ (K + c) (12.1-3)• For gases

• Assumptions:

• Monolayer coverage on adsorbent

• No interactions between adsorbent molecules

• All adsorbate molecule/adsorbent interactionsare the same

• Only a fixed number of active sites available

• Adsorption is reversible and reached anequilibrium condition


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Example: Adsorption Isotherms

Batch tests were performed in the laboratory using solutions of phenol in wparticles of granular activated carbon. The equilibrium data at room temare shown in the table below. Determine the isotherm that fits the data.

c

(kg phenol/m3 solution)

q

(kg phenol/kg carbon)

0.322 0.150

0.117 0.122

0.039 0.094

0.0061 0.059

0.0011 0.045

Example 12.1-1


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Example 12.1-1 Linear: q = Kc

q vs c

straight line with slope K

Freundlich: log q = log K + n log c

log q vs log c slope: n y-axis intercept: log K

Langmuir: 1/q = (K/qo) (1/c) + 1/qo

1/q vs 1/c

slope: K/qo y-axis intercept: 1/qo


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Example 12.1-1

Linear Law

0

0.02

0.04

0.06

0.080.1

0.12

0.14

0.16

0 0.1 0.2 0.3 0.4

c

q


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Example 12.1-1 Langmuir Isotherm

0

5

10

15

20

25

0 200 400 600 800 1000

1/c

1 / q


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Example: Adsorption IsothermsExample 12.1-1

log K = - 0.7183

K = 0.199

n = 0. 229

Freundlich Isotherm

y = 0.229x - 0.701

-1.6

-1.4

-1.2

-1

-0.8

-0.6

-0.4

-0.2

0

-4 -3 -2 -1 0

log c

log

q

229.0199.0 cq

Gives a straight line, hence follows the

Freundelich isotherm.


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Batch Adsorption• When quantities to be treated are of small amount.• Isotherms and material balance are needed.

• Material balance on the adsorbate:

q F M + c F S = q M + cS

where:q F = initial concentration of solute adsorbed on the sol

q = final concentration at equilibrium M = amount of adsorbent, kgS = volume of feed solution, m3

c F = initial concentration of solute in the fluid phasec = final concentration at equilibrium in the fluid phas


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Batch Adsorption

• q F M + c F S = q M + cS

• When variable q is plotted versus c , the result is a straigline.

• If equilibrium isotherm is also plotted on the same graph,the intersection of both line gives the final equilibriumvalues of q and c.


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Example: Batch Adsorption

Example 12.2-1:

A wastewater solution having a volume of 1.0 m3 contai0.21 kg phenol/m3 of solution . A total of 1.40 kg of fre

granular activated carbon is added to the solution , whicis then mixed thoroughly to reach equilibrium. Using thisotherm from Example 12.1-1, what are the finequilibrium values, and what percent of phenol extracted


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Example: Batch Adsorption Example 12.2-1:

0(1.40) + 0.21(1.0) = q (1.40) + c (1.0)

q = 0.15- 4.17 c (a)

From the isotherm

q = 0.199 c 0.229 (b)

q F M + c F S = q M + cS

E ample: Batch Adsorption


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Example: Batch Adsorption

Example 12.2-1:

At intersection q = 0.106 kg phenol/kg carbon

c = 0.062 kg phenol/m3

% extracted = (cF - c)(100)/cF = (0.21-0.062)(100)/0.21

= 70.5 %

0

0.05

0.1

0.15

0 0.05 0.1 0.15 0.2

c, kg phenol/m3 solution

q,kg

phenol/kg

adsorbent


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Design of Adsorption

Column


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Coverage

Fixed bed adsorption design

Regeneration of adsorbents

Students should be able to :1. Design a fixed bed adsorption column

2. Explain regeneration process of adsorbents

Fixed Bed Adsorption Design


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• Introduction and concentration profiles

- Usually employ fixed bed of granular particles- The fluid to be treated is usually passes down through the pac

at a constant flow rate

- Mass transfer resistances are important in the fixed-bed procethe process is unsteady state.- The overall dynamic of the system determine the efficiency of

process, rather than just the equilibrium considerations



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Fixed Bed Adsorption Design• Introduction and concentration profiles (contd )

- The concentration of the solute in the fluid phase and of the solid adsphase change with TIME and POSITION in the fixed bed as the adsor

proceeds- Inlet: solid is assumed to contain no solute at the start of the process- As the fluid first come into contact with the inlet, most of the MASS T

and ADSORPTION takes place here- As fluid passes thru the bed, the concentration in this fluid DROPS VE

RAPIDLY with distance in bed and REACHES ZERO well before the enreached


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• After a short time, solid near entrance almost SATURATED and most of thetransfer and adsorption now takes place at a point slightly farther from the • The major part of the adsorption at any time takes place in a relatively naadsorption or mass transfer zone• As the solution continues to flow, this mass-transfer zone (S-shaped), movthe column.•This outlet concerntration remains near zero until the mass transfer zone sreach the tower outlet at t4.•Then the outlet concentration starts to rise.

Fi d B d Ad ti D i


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Fixed Bed Adsorption Design• Breakthrough Concentration Curve

- Then, the outlet conc starts to rise, and at t 5 the outlet conc has risen to c bwhich is called the break point

- After the break-point time is reached, the concentration c rises very rapidlyto point c d , which is the end of the breakthrough curve, where the bed is

judged ineffective.- The break-point concentration represents the maximum that can be discard

and often taken as 0.01 to 0.05 for cb/co.- For a narrow MTZ, the breakthrough curve is very steep and most of the b

capacity is used at the break point (this makes efficient use of the adsorbeand lowers energy costs for regeneration)

breakthrough concentration profile in the fluid at outlet of bed


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Fixed Bed Adsorption Design• Capacity of Column and Scale- Up Design Method

- Mass Transfer Zone (MTZ) width and shape depends on:- the adsorption isotherm- flowrate- mass transfer rate to the particles- diffusion in the pores.

- For systems with a favorable isotherm, similar to Freundlich and Laacquires the typical S shape. MTZ is constant in height as it moves thr

- For unfavorable isotherm i.e. Isotherm is linear; MTZ width increalength

- A favorable isotherm for adsorption is unfavorable for effective regene


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• Capacity of Column and Scale- Up Design Method

- A number of theoritical methods have been publishedwhich predict the Mass Transfer Zone (MTZ) andconcentration profiles in the bed.

- Hence, experiments in laboratory scale are needed inorder to scale up the results.



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Shaded area =Total or stoichiometric capacity of the packed tower

dt c

ct t )1(

00

(12.3-1)Time equivalent to the totalor stoichiometric capacity


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Fixed Bed Adsorption DesignCrosshatched area = Usable capacity of bed up to the break-point tim


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• t u

: time equivalent to the usable capacity or tim

at which the effluent concentration reachesmaximum permissible level.

(12.3-2)dt

c

ct

bt

u )1(0

0

• t u very close to t b

• t u/t t is the fraction of the total bed capacity orlength utilized up to the break point


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H B : length of bed used up to the break point

( H T : Total bed length)

(12.3-3)T

t

u B H t

t

H


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H UNB : Length of unused bed (mass transfer zone)

(12.3-4)

H T = H UNB + H B (12.3-5)

T

t

u

UNB H t

t H )1(


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Design Steps1. Determine the the length of bed needed to achieve the

required usable capacity, H B2. Determine H UNB3. Calculate H T


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Scale-up principle

1. If a system is tested with different bedlength, it gives breakthrough curve of thesame shape.

2. The amount of length of unused bed (HUNB)does not change with the total bed length.

3. Hence, tb is proportional to HB.


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Example: Fixed Bed Adsorption Design

Example 12.3-1 A waste stream of alcohol vapour in air from a process was adsorbed byactivated carbon particles in a packed bed having a diameter of 4 cm and lengthof 14 cm containing 79.2 g of carbon. The inlet gas stream having aconcentration c

oof 600 ppm and a density of 0.00115 g/cm3 entered the bed at

a flow rate of 754 cm3 /s. Data in Table 12.3-1 give the concentrations of thebreakthrough curve. The breakpoint concentration is set at c /c

o= 0.01.

Determine :

1. Break point time

2. Fraction of total capacity used up to the break point time

3. Length of the unused bed

4. Total bed length


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Example 12.3-1

Table 12.3-1

Time,h c/c o Time, h c/c o

0 0 5.5 0.658

3 0 6.0 0.9033.5 0.002 6.2 0.933

4 0.030 6.5 0.975

4.5 0.155 6.8 0.993

5 0.396


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Example 12.3-1

The plotted data fromTable 12.3-1



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Example 12.3-1

Based on Figure 12.3-3

At break point conc. 0.01: t b

= 3.65 h; t d

= 6.95 h

dt c

ct t )1(

00

= A 1 + A 2 = 3.65 + 1.51 = 5.16 h

dt c

ct

bt

u )1(65.3

00

= A 1 = 3.65 h

t u / tt = 3.65/5.16 = 0.707

T

t

u B H

t

t H = 0.707(14) = 9.9 cm

T

t

u

UNB H t

t H )1( = (1 - 0.707)14 = 4.1 cm

Example: Fixed Bed Adsorption Desig


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Example: Fixed Bed Adsorption DesigExample 12.3-1

b) If the breakpoint time required for a new column is 6.0 h, what the new total length

t u is proportional to H B

t u = 3.65 H B = 9.9 cm

t b’ = 6 h

dt c

ct

bt

u )1('6

00

= A 1’ = 6 h

B

u

u B H t

t H

'' = (6 /3.65 )(9.9) = 16.3 cm

H T ’= H UNB + H B = 16.3 + 4.1 = 20.4 cm

799.04.20

3.16

'

'

'

'

T

B

t

u

H

H

t

t (Fraction of the new bedused up to the break point)


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Example: Fixed Bed Adsorption Design Example 12.3-1

c) Determine the saturation loading capacity of the carbon. Air flow rate= (754 cm3 /s)(3600s)(0.0115g/cm3) = 3122 g air/h

600 ppm = 600 g alcohol in 1 million g of air

Total alcohol adsorbed =

= 9.67 g alcohol

Saturation capacity =

16.5)(3122(10

6006

h

air g

air g

adsorbed alcohol g

carbon g

alcohol g

carbon g

alcohol g 1220.0

2.79

67.9


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Processing Variables and Adsorption Cycles

• Large scale adsorption:1) cyclic batch system -alternately saturated & thenregenerated2) continuous flow system- continuous flow of adsorbentcountercurrent to a flow of feed

• Bed regeneration method• Temperature-swing cycle• Pressure-swing cycle• Inert-purge gas stripping cycle• Displacement-purge cycle.

Temperature Swing Adsorption (Thermal


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Temperature Swing Adsorption (Thermalmeans)

• Regeneration process – increase in temperature• Increase in temperature leads to a decrease in the

quantity adsorbed

• Important note - regeneration temperature does notcause degradation of the adsorbents

• Mechanism – passage of a hot purge gas or steam

• Important characteristic – to treat feeds with lowconcentrations of adsorbates

Temperature Swing Adsorption (Purasiv process)


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p g p ( p )-Fluidized bed for adsorption-Moving bed for desorption Purasiv process

1. Adsorbent particles are attrition-r2. In the adsorption section, sieve tr

the raw gas passing up through tfluidizing the adsorbent particles

3. The fluidized solids flow like a liquinto the downcomer, and onto the

4. From the adsorption section, thedesorption section, where, as moflow down through preheating tubthrough desorption tubes.

5. Steam is used for indirect heatin

tubes and for stripping in the des6. At the bottom of the unit, the regpicked up by a carrier gas, which gas-lift line to the top, where the onto the top tray to repeat the adcycle.


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Temperature Swing Adsorption

• Effect of temperature on the

adsorption equilibrium of asingle adsorbate

• If the partial pressureremains constant at p1,increasing the temperature

from T1 to T2 will decreasethe equilibrium loading fromq1 to q2.


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Bed TSA System

Cycle Steps1- adsorption at T1 to breakthrough2- heating of the bed to T2 (T2 > T1)3- desorption at T24- cooling of the bed to T1


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Pressure Swing Adsorption (Mechanical wo

• Regeneration process – reducing the partial pressure of theadsorbate

• 2 ways:

• Introduction of an inert gas while maintaining the total systempressure

• Cycle time very quick (minutes or second)

• Operate PSA close to ambient temperature – at a given partial pressurethe loading is increased as temperature decreased

• Popular for performing bulk separation of gases – controlled byadsorption isotherm or adsorption kinetics

• Use only with gases (liquid has little or no effect with change in pressu

P S i Ad ti


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Pressure Swing Adsorption

• Effect of partial pressure onequilibrium loading attemperature T1

• Reducing the partial pressure

from p1 to p2 causes theequilibrium loading to bereduced from q1 to q2

Bed PSA System


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Bed PSA System• Each bed operates alternately :

• Pressurisation followed byadsorption

• Desorption by depressurisation(blowdown) followed by a purge

• Adsorption – pressure greater thanatmospheric

• Desorption – pressure being

atmospheric

• Pressurisation – feed gas

• Purging – effluent (non-adsorbed)product gas

I t t i i l


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Inert-purge gas stripping cycle

• Adsorbate is removed by passing a non-adsorbing or inertgas through the bed.

• Mechanism for desorption:

• Partial pressure (or concentration) of original adsorbatein the gas phase surrounding the adsorbent is reduced

Di l t P l


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Displacement-Purge cycle• Removal of adsorbates by replacing them with a more

preferentially adsorbed species

• Mechanism for desorption:

• Partial pressure (or concentration) of original adsorbategas phase surrounding the adsorbent is reduced

• There is competitive adsorption for the displacement flu

• One advantage – net heat generated or consumed will be zero because heat of adsorption of the displacement fluid to the original adsorbate – adsorbent temperature constanthroughout the cycle

Di l t P Ad ti


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Displacement Purge Adsorption


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THANK YOU.

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Chapter 2 (Adsorbtion).pdf