Transport Processes and Separation Process Principles by Christie John
Geankoplis. Copyright 2003 Pearson Education, Inc., Publishing as Prentice Hall
PTR.
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.
yA*
Concentration profile of a solute A diffusing through two phases.
N A K y y AG y A
, 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
.
p A He x A 29.6 x A
Figure 10.2-1. Equilibrium plot for SO2-water system at 293 K (20ºC) and p=1 atm. 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.
Total balance equation L0 V2 L1 V1
Component balance equation L0 x0 V2 y2 L1 x1 V1 y1
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. Transport Processes and Separation Process Principles by Christie John Geankoplis. Copyright 2003 Pearson Education, Inc., Publishing as Prentice Hall PTR. All rights reserved
.
Balance equations
L0 GN 1 LN G1 L0 xi 0 GN 1 yiN LN xiN G1 yi1
i 1,2,......I
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 Component balance equation of the control area: L xm G y N 1 L x N G ym 1 :G L G L G xm y N 1 x N ym 1 G G G G L L xm y N 1 x N ym 1 G G
L L xm x N y N 1 G G L L y x x N y N 1 G G
ym 1
This straight line is the operating line.
Figure 10.3-3. Number of stages in a countercurrent multiple-stage contact process. Transport Processes and Separation Process Principles by Christie John Geankoplis. Copyright 2003 Pearson Education, Inc., Publishing as Prentice Hall PTR. All rights reserved
.
GAS OUT
y 0.5
y1
x0
y2
x1
y3
x2
y4
x3
y5
x4
y6
x5
y7
x6
y8
x7
1 2 3 4 5
y8 LIQUID IN
0.4
0.3
0.2
0.1
6 7
GAS IN
y1
0 0.05
0 x0
LIQUID OUT
0.1
0.15
0.2
0.25 x7 x
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 Figure 10.6-8. Theoretical number of trays for absorption of SO2 in Example 10.6-2.
Minimum slope of the operation line (minimum liquid to gas ratio)
Transport Processes and Separation Process Principles by Christie John Geankoplis. Copyright 2003 Pearson Education, Inc., Publishing as Prentice Hall PTR. All rights reserved
.
Balance equations, simplified X Y
x kmol absorptivum solute 1 x kmol solvent
y kmol absorptivum solute 1 y kmol absorptivum inert gas
L' L 1 x
solute-free solvent
G ' G 1 y
solute-free gas
Form of Henry’s law: Y X He' 1 Y 1 X
Form of total component balance equation: L' X 0 G ' Y N 1 L' X N G ' Y1
Operating line Component balance equation of the control area: L' X m G ' Y N 1 L' X N G ' Ym1
L' L' Ym1 ' X m ' X N YN 1 G G
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 G ( y 2 y1 ) L ( x1 x0 ) y2 y1 L / G ( x1 x0)
y0* y1 y1 m x1 x1 and x0 m m , where y0* is hypotetical concentration, in equilibrium with x0.
Introducing the absorption coefficient: A
L mG
Analytical determination of the number of theoretical stages y2 y1 1 A A y0*
Component balance equation of the second thoretical stage: G ( y3 y2 ) L ( x2 x1 )
y L y y2 y1 y3 y 2 L / G 2 1 y 2 Gm m m y2 1 A A y1
y 1 2 A A
A A A y
y3 y1 1 A A y0* 1 A A y1 y3
2
1
2
. . . might be continued
* 0
Analytical determination of the number of theoretical stages
y N 1 y1 1 A A2 A N y0* A A2 A N 1 A N 1 1 AN * y N 1 y1 y0 A 1 A 1 A
1 A N 1 1 AN y N 1 y1 m x0 A 1 A 1 A
y N 1 y1 A N 1 A N 1 y N 1 m x0 A 1
Kremser (1930) Brown-Sauders (1932)
y m x0 1 1 log10 N 1 1 y1 m x0 A A N log10 A
when A=1
y N 1 y1 N y1 m x0
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. Transport Processes and Separation Process Principles by Christie John Geankoplis. Copyright 2003 Pearson Education, Inc., Publishing as Prentice Hall PTR. All rights reserved
.
Differential columns
d G y K y a y y* A dH
, 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)
d G y K y a y y* A dH y y dy dy G' d G' d G y d G' G 1 y 1 y 2 1 y 1 y
Y
y kmol absorptivum solute 1 y kmol absorptivum inert gas
G' G1 y
Modified component balance equation
K y a 1 y av y y * A dH dy G 1 y av 1 y
1 y av G dH dy * K y a1 y av A 1 y y y
1 y av G 0 dH y K y a1 y av A 1 y y y* dy 0 y1
H
Each parameters on the right side are dependent on concentration, thus numerical integration is needed. Assumptions: K y a1 y av Ky
is independent of concentration is proportional to G0,8
Thus: G/G0,8 is roughly independent from concentration. y1 1 y av dy G H dy * 1 y K y a1 y av A y0 1 y y y
Transfer Units H NTU G HTU G
1 y av 1 1 y y1 G 1 H dy * K y a1 y av A y0 y y
G HTU G K y a1 y av A NTU G
y1
y0
1 dy * yy
height of a transfer unit (m) number of transfer units
Absorbers liquid
falling film absorber
packed column
gas
monotube absorber
Absorbers liquid
spray column
bubble column
gas
plate-type absorbers
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.
Transport Processes and Separation Process Principles by Christie John Geankoplis. Copyright 2003 Pearson Education, Inc., Publishing as Prentice Hall PTR. All rights reserved
.
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.
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