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Absorption

Béla Simándi, Edit Székely

Some of the slides are from Transport Processes

and Separation Process Principles by Christie John

Geankoplis.

Absorption

• In absorption a gas mixture is contacted

with a liquid solvent to remove one or more

components from the gas phase.

• The opposite of absorption is stripping,

where in a liquid mixture is contacted with

a gas to remove components from the liquid

to the gas phase.

• Distinction should be made between

physical absorption and chemical

absorption.

Transport Processes and Separation Process Principles by Christie John Geankoplis.

Copyright 2003 Pearson Education, Inc., Publishing as Prentice Hall PTR. All rights reserved.

Concentration profile of a solute A diffusing through two phases.

yA*

AAGyA yyKN

, where Ky is the overall trasfer coefficient (mol/(m2s))

yA* would be equlibrium with xAL.



Figure 10.2-1. Equilibrium plot for SO2-water system

at 293 K (20ºC) and p=1 atm.

AAA xxHep 6.29



Single-stage equilibrium process

Figure 10.3-1. Single-stage equilibrium process.

1120 VLVL

11112200 yVxLyVxL

Total balance equation

Component balance equation



Figure 10.3-3. Number of stages

in a countercurrent multiple-stage contact

process.

Balance equations

110 GLGL NN

I,iyGxLyGxL iiNNiNNi ......2,111100

L and G are constant along the column.

yN+1>>y1

xN>>x0

Operating line,

G=G1=G2=GN+1 and

L=L0=L1=LN are constants

11 mNNm yGxLyGxL

Component balance equation of the control area:

11 NNmm yxG

Lx

G

Ly

This straight line is the operating line.

:G

11 mNNm yG

Gx

G

Ly

G

Gx

G

L

11 mNNm yxG

Lyx

G

L

1 NN yxG

Lx

G

Ly



Figure 10.3-3. Number of stages in a countercurrent multiple-stage

contact process.

LIQUID IN

LIQUID

OUT

GAS OUT

GAS IN

y 0.5

0.4

0.3

0.2

0.1

0.05

x

0

0 0.1 0.15 0.2 0.25

1

2

3

4

5

6

7

x0 y1

y2 x1

y3 x2

y4 x3

y5 x4

y6 x5

y7 x6

y8 x7

x7

y8

x0

y1

Figure 10.6-8. Theoretical number of

trays for absorption of SO2 in Example

10.6-2.

Given: y1

yN+1

y0

Result: N

L or xN can be estimated

If L/G is large: N decreases

xN decreases

If L/G is small: N increases

xN increases

Transport Processes and Separation Process Principles by

Christie John Geankoplis.

Copyright 2003 Pearson Education, Inc., Publishing as

Prentice Hall PTR. All rights reserved.

Minimum slope of the operation

line (minimum liquid to gas ratio)

Balance equations, simplified

solvent kmol

solutemabsorptivu kmol

1 x

xX

gasinert mabsorptivu kmol


1 y

yY

X

X

Y

Y

1He'

1

1''

1'

0' YGXLYGXL NN

Form of Henry’s law:

Form of total component balance equation:

xLL 1' solute-free solvent

yGG 1' solute-free gas

Operating line

1''

1''

mNNm YGXLYGXL

Component balance equation of the control area:

1'

'

'

'

1 NNmm YXG

LX

G

LY

This straight line is the operating line.

Analytical determination of the

number of theoretical stages

(L and G are constants)

• If both the operationg line and

the equilibrium curve are linear:

– L/G is constant

– y=m∙x

)()( 0112 xxLyyG

Gm

LA

0.*

0

*

00

1111

with xmequilibriuin ion,concentrat lhypotetica is where,

and

y

m

yx

m

yxxmy

)(/ 0112 xxGLyy

Introducing the absorption coefficient:


number of theoretical stages *

012 1 yAAyy

Component balance equation of the second thoretical stage:

)()( 1223 xxLyyG

12

12212

23

1

/

yAAy

yymG

Ly

m

y

m

yGLyy

1*013 11 yAAyAAyy

.

.

.

might be continued

*0

2213 21 yAAAAAyy


number of theoretical stages NN

N AAAyAAAyy

2*

0

2

11 1

A

AAy

A

Ayy

NN

N

1

1

1

1 *0

1

11

A

AAxm

A

Ayy

NN

N

1

1

1

10

1

11

11

1

01

11

N

N

N

N

A

AA

xmy

yy Kremser (1930)

Brown-Sauders (1932)

A

AAxmy

xmy

N

N

10

01

0110

log

111log

when A=1 01

11

xmy

yyN N



Typical locations of operating line

at absorption and at stripping

Figure 10.6-10. Location of operating lines: (a) for absorption of A from

V to L stream; (b) for stripping of A from L to V stream.



Figure 10.6-11. Operating line for limiting conditions: (a)

absorption; (b) stripping.

Differential columns

dHAyyaKyGd y *

, where

G is the molal flowrate of the gas (mol/s)

y is the molar fraction

of the component of interest (-)

Ky is the overall masstransfer

coefficient (mol/m2s)

a is the relative surface area

of phase boundary (m2/m3)

y*=m∙x

A is the cross section of the column (m2)

dH is the differential height of column (m)

y

dyG

y

dyG

y

ydG

y

yGdyGd

11'

1'

1'

2

gasinert mabsorptivu kmol


1 y

yY

yGG 1'

Modified component balance equation

av

avy

y

HAyyyaK

y

dyG

1

d1

1

*

y

yyy

y

AyaK

GH av

avy

d1

1

1d

*

dHAyyaKyGd y *

1

0

y

y

*

01

1

1d dy

yyy

y

AyaK

GH av

avy

H

Each parameters on the right side are

dependent on concentration, thus numerical

integration is needed.

Assumptions:

avy yaK 1 is independent of concentration

yK is proportional to G0,8

Thus: G/G0,8 is roughly independent from concentration.

1

0

y

y*1

1

11dy

yyy

y

AyaK

G

y

dyH av

avy

Transfer Units

GG HTUNTUH

1

0

y

y*

1

1dy

yyAyaK

GH

avy

1

0

y

y*

1

1

dyyy

NTU

AyaK

GHTU

G

avy

G height of a transfer unit (m)

number of transfer units

11

1

y

y av

Absorbers

falling film

absorber

packed column monotube absorber

liquid gas

Absorbers

spray column bubble column plate-type absorbers

liquid gas



Figure 10.6-3. Packed tower flows and characteristics for absorption.



Random packings

Figure 10.6-4. Typical random or dumped tower packings: (a) Raschig

ring; (b) Berl saddle; (c) Pall ring; (d) Intalox metal, IMTP; (e) Jaeger

Metal Tri-Pack.

Typical applications

• separation of gases

– production of HNO3

Typical applications

• separation of gases

– production of HNO3

– separation of produced gases

– fractionation of hydrocarbons

– sweetening of natural gases (acid gas removal)

• waste gas purification

Typical applications, waste

gas purification

• removal of gaseous pollutants, such as

hydrogen halides, SO2, ammonia, hydrogen

sulphide

• or volatile organic solvents

• removal of CO2 or H2S from natural gas

• but also removal of dust with certain types

of scrubbers

Typical absorbents in waste

gas purification

• water, to remove solvents and gases such as

hydrogen halides or ammonia

• alkaline solutions, to remove acid components

such as hydrogen halides, sulphur dioxide,

• phenols, chlorine; also used for second-stage

scrubbing to remove residual hydrogen halides

after first-stage aqueous absorption; biogas

desulphurisation

Typical absorbents in waste

gas purification • alkaline-oxidation solutions, i.e. alkaline solutions

with sodium hypochlorite, chlorine dioxide, ozone

or hydrogen peroxide

• sodium hydrogensulphite solutions, to remove

odour (e.g. aldehydes)

• Na2S4 solutions to remove mercury from waste

gas

• acidic solutions, to remove ammonia and amines

• monoethanolamine and diethanolamine solutions,

suitable for the absorption and recovery of

hydrogen sulphide.

THANK YOU!

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