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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.
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.
Transport Processes and Separation Process Principles by Christie John Geankoplis.
Copyright 2003 Pearson Education, Inc., Publishing as Prentice Hall PTR. All rights reserved.
Figure 10.2-1. Equilibrium plot for SO2-water system
at 293 K (20C) and p=1 atm.
AAA xxHep 6.29
Transport Processes and Separation Process Principles by Christie John Geankoplis.
Copyright 2003 Pearson Education, Inc., Publishing as Prentice Hall PTR. All rights reserved.
Single-stage equilibrium process
Figure 10.3-1. Single-stage equilibrium process.
1120 VLVL
11112200 yVxLyVxL
Total balance equation
Component balance equation
Transport Processes and Separation Process Principles by Christie John Geankoplis.
Copyright 2003 Pearson Education, Inc., Publishing as Prentice Hall PTR. All rights reserved.
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
Transport Processes and Separation Process Principles by Christie John Geankoplis.
Copyright 2003 Pearson Education, Inc., Publishing as Prentice Hall PTR. All rights reserved.
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
solutemabsorptivu kmol
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
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=mx
)()( 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:
Analytical determination of the
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
*02213 21 yAAAAAyy
Analytical determination of the
number of theoretical stages NNN AAAyAAAyy 2*0211 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
Transport Processes and Separation Process Principles by Christie John Geankoplis.
Copyright 2003 Pearson Education, Inc., Publishing as Prentice Hall PTR. All rights reserved.
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.
Transport Processes and Separation Process Principles by Christie John Geankoplis.
Copyright 2003 Pearson Education, Inc., Publishing as Prentice Hall PTR. All rights reserved.
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*=mx
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
solutemabsorptivu kmol
1 y
yY
yGG 1'
Modified component balance equation
avavy
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
Transport Processes and Separation Process Principles by Christie John Geankoplis.
Copyright 2003 Pearson Education, Inc., Publishing as Prentice Hall PTR. All rights reserved.
Figure 10.6-3. Packed tower flows and characteristics for absorption.
Transport Processes and Separation Process Principles by Christie John Geankoplis.
Copyright 2003 Pearson Education, Inc., Publishing as Prentice Hall PTR. All rights reserved.
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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