Absorption

5/25/2018 Absorption

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Absorption

Bla Simndi, Edit Szkely

Some of the slides are from Transport Processes

and Separation Process Principles by Christie John

Geankoplis.


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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.


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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 soluteAdiffusing through two phases.

yA*

AAGyA yyKN

, whereKyis the overall trasfer coefficient (mol/(m2s))

yA*would be equlibrium withxAL.


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Single-stage equilibrium process

Figure 10.3-1. Single-stage equilibrium process.

1120 VLVL

11112200 yVxLyVxL

Total balance equation

Component balance equation


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Figure 10.3-3. Number of stages

in a countercurrent multiple-stage contact

process.


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Balance equations

110 GLGL

NN

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

L and G are constant along the column.

yN+1>>y1

xN>>x0


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Operating line,

G=G1=G2=GN+1and

L=L0=L1=LN are constants

11 mNNm yGxLyGxL

Component balance equation of the control area:

11 NNmm yxG

LxG

Ly

This straight line is the operating line.

:G

11 mNNm yG

GxG

LyG

GxG

L

11 mNNm yxG

Lyx

G

L

1 NN yx

G

Lx

G

Ly


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

contact process.


10/35

LIQUID IN

LIQUID

OUT

GAS OUT

GAS IN

y 0.5

0.4

0.3

0.2

0.1

0.05

x

00 0.1 0.15 0.2 0.25

1

23

4

5

6

7

x0y1

y2 x1

y3 x2

y4 x3

y5 x4

y6 x5

y7 x6

y8 x7

x7

y8

x0

y1


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Figure 10.6-8. Theoretical number of

trays for absorption of SO2in Example

10.6-2.

Given:y1

yN+1y0

Result:N

LorxNcan be estimated

IfL/Gis large:NdecreasesxNdecreases

IfL/Gis small:Nincreases

xNincreases


12/35

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)


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Balance equations, simplified

solventkmol

solutemabsorptivukmol

1 x

xX

gasinertmabsorptivukmol


1 y

yY

X

X

Y

Y

1He'

1

1''

1'

0' YGXLYGXL NN

Form of Henrys law:

Form of total component balance equation:

xLL 1' solute-free solvent

yGG 1' solute-free gas


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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.


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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=mx

)()( 0112 xxLyyG

Gm

LA

0.*

0

*

00

1111

with xmequilibriuinion,concentratlhypoteticaiswhere,

and

y

m

yx

m

yxxmy

)(/ 0112 xxGLyy

Introducing the absorption coefficient:


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number of theoretical stages

*012 1 yAAyy

Component balance equation of the second thoretical stage:

)()( 1223 xxLyyG

12

12212

23

1

/

yAAy

yymG

Lymy

myGLyy

1

*

013 11 yAAyAAyy

.

.

.

might be continued

*02213 21 yAAAAAyy


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number of theoretical stagesNN

N AAAyAAAyy 2*

02

11 1

A

AAy

A

Ayy

NN

N

1

1

1

1 *0

1

11

A

AAxm

A

Ayy

NN

N

1

1

1

1

0

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=101

11

xmy

yyN N


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Typical locations of operating line

at absorption and at stripping

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

Vto Lstream; (b) for stripping ofAfrom Lto Vstream.


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

absorption; (b) stripping.


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Differential columns

dHAyyaKyGd y *

, where

Gis the molal flowrate of the gas (mol/s)yis the molar fraction

of the component of interest (-)

Kyis the overall masstransfer

coefficient (mol/m2s)

ais the relative surface area

of phase boundary (m2/m3)

y*=mx

A is the cross section of the column (m2)

dHis the differential height of column (m)


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y

dyG

y

dyG

y

ydG

y

yGdyGd

11

'1

'1'

2

gasinertmabsorptivukmol


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 *


22/35

1

0

y

y

*

0 1

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/G0,8is roughly independent from concentration.

1

0

y

y*1

1

11dy

yyy

y

AyaK

G

y

dyH av

avy


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Transfer Units

GG HTUNTUH

1

0

y

y*

1

1dy

yyAyaK

GH

avy

1

0

y

y*

1

1

dyyy

NTU

AyaK

G

HTU

G

avyG height of a transfer unit (m)

number of transfer units

11

1

y

y av


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Absorbers

falling film

absorber

packed column monotube absorber

liquid gas


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Absorbers

spray column bubble column plate-type absorbers

liquid gas


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


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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.


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Typical applications

separation of gases

production of HNO3


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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


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Typical applications waste


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Typical applications, waste

gas purification

removal of gaseous pollutants, such ashydrogen halides, SO2, ammonia, hydrogen

sulphide

or volatile organic solvents removal of CO2or H2S from natural gas

but also removal of dust with certain types

of scrubbers


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Typical absorbents in waste

gas purification

water, to remove solvents and gases such as

hydrogen halides or ammonia

alkaline solutions, to remove acid componentssuch as hydrogen halides, sulphur dioxide,

phenols, chlorine; also used for second-stage

scrubbing to remove residual hydrogen halides

after first-stage aqueous absorption; biogasdesulphurisation


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Typical absorbents in waste

gas purification

alkaline-oxidation solutions, i.e. alkaline solutionswith sodium hypochlorite, chlorine dioxide, ozone

or hydrogen peroxide

sodium hydrogensulphite solutions, to removeodour (e.g. aldehydes)

Na2S4solutions 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.


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THANK YOU!

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