1. 2022/11/26 Aerosol & Particulate Research Lab 1
Gas Absorption
Definition: transfer of a gaseous
component (absorbate) from the
gas phase to a liquid (absorbent)
phase through a gas-liquid
interface.
Q: What are the key parameters that affect the effectiveness?
Q: How can we improve absorption efficiency?
Mass transfer rate:
gas phase controlled absorption
liquid phase controlled absorption
Reading: Chap 13
Q: Does it matter if it’s gas phase or liquid phase controlled?
2. 2022/11/26 Aerosol & Particulate Research Lab 2
Gas Absorption Equipment
Mist
Eliminator
Liquid
Spray
Packing
Liquid outlet
Dirty gas in
Spray
nozzle
Clean gas out
Countercurrent
packed tower
Spray tower
Mycock et al., 1995
Redistributor
Q: Limitations of a
spray tower? Q: Why redistributor?
Clean gas out
Dirty gas in
3. 2022/11/26 Aerosol & Particulate Research Lab 3
Berl
Saddle
Intalox
Saddle
Raschig
Ring
Lessing
Ring
Pall
Ring
Tellerette
Three-bed cross
flow packed tower
Liquid spray Dry Cell
Packing
Mycock et al., 1995
Q: Criteria for good packing materials?
5. 2022/11/26 Aerosol & Particulate Research Lab 5
Mass Balance
In = Out
1
2
2
1 m
m
m
m L
G
L
G
2
1
2
1 x
x
L
y
y
G m
m
Slope of Operating
Line = Lm/Gm
Lm: molar liquid flow rate
Gm: molar gas flow rate
x: mole fraction of solute in pure liquid
y: mole fraction of solute in inert gas
Known: ??
Unknown: ??
(for a dilute system)
Gas in
Liquid out
Gas out
Liquid in
7. 2022/11/26 Aerosol & Particulate Research Lab 7
Generally, actual liquid
flow rates are specified
at 25 to 100% greater
than the required
minimum.
• G = 84.9 m3/min (= 3538 mole/min). Pure water is used
to remove SO2 gas. The inlet gas contains 3% SO2 by
volume. Henry’s law constant is 42.7 (mole fraction of SO2 in
air/mole fraction of SO2 in water). Determine the minimum water
flow rate (in kg/min) to achieve 90% removal efficiency.
Q: How much is X2 if fresh
water is used? What if a
fraction of water is recycled?
8. 2022/11/26 Aerosol & Particulate Research Lab 8
• Channeling: the gas or liquid flow is much greater at some
points than at others
• Loading: the liquid flow is reduced due to the increased gas
flow; liquid is held in the void space between packing
• Flooding: the liquid stops flowing altogether and collects in
the top of the column due to very high gas flow
Problems with high gas flow
• Gas flow rate is 3538 mole/min and the minimum liquid flow rate
is 2448 kg/min to remove SO2 gas. The operating liquid rate is
50% more than the minimum. The packing material selected is 2”
ceramic Intalox Saddles. Find the tower diameter and pressure
drop based on 75% of flooding velocity for the gas velocity.
Properties of air:: molecular weight: 29 g/mole; density: 1.17×10-3
g/cm3. Properties of water:: density: 1 g/cm3; viscosity: 0.8 cp.
9. 2022/11/26 Aerosol & Particulate Research Lab 9
L
G
G
L
g
F
G
L
G
L
2
.
0
2
)
'
(
L: mass flow rate
of liquid
G: mass flow rate
of gas
G’: mass flux of gas
per cross sectional
area of column
F: Packing factor
: specific gravity
of the scrubbing
liquid
L: liquid viscosity
(in cP; 0.8 for water)
(dimensionless)
10. 2022/11/26 Aerosol & Particulate Research Lab 10
Mass Transfer
difference
ion
concentrat
area
l
interfacia
/
d
transferre
mass
of
rate
k
Flux
Two-Film Theory (microscopic view)
I
G
G
p
p
k
J
L
I
L
C
C
k
J
I
I HC
p
L
G
L
G
HC
p
k
H
k
J
/
/
1
1
Cussler, “Diffusion”, Cambridge U. Press, 1991.
C
C
k
A
M
J i
/
pG
CI
pI
CL
)
(
time
area
mass
J: flux
k: mass transfer coefficient
(gas phase flux)
(liquid phase flux)
(overall flux)
11. 2022/11/26 Aerosol & Particulate Research Lab 11
*
*
p
p
K
C
C
K
J
G
OG
L
OL
L
G
OG
G
L
OL
k
H
k
K
H
k
k
K
/
/
1
1
/
1
/
1
1
L
G
HC
p
H
p
C
*
*
Macroscopic analysis of a packed tower
out
flow
minus
in
solute
of
flow
solute
of
on
accumulati
dz
dx
L
dz
dy
G m
m '
'
0
Mole balance on the solute over the
differential volume of tower
)
(
'
'
1
1 y
y
L
G
x
x
m
m
(equivalent concentration
to the bulk gas pressure)
(equivalent pressure to the
bulk concentration in liquid)
(overall liquid phase MT coefficient)
(overall gas phase MT coefficient)
1
2
L’m: molar flux
of liquid
G’m: molar flux
of gas
12. 2022/11/26 Aerosol & Particulate Research Lab 12
Mole balance on the solute in the gas only
absorption
by
lost
solute
out
flow
minus
in
flow
solute
on
accumulati
solute
*)
(
'
0 y
y
aP
K
dz
dy
G OG
m
1
*
'
0
y
y
OG
m
Z
Z y
y
dy
aP
K
G
dz
Z Hx
y
*
Z
Z
m
m
OG
m
Z
Z
m
m
OG
Hx
y
Hx
y
L
HG
aP
K
G
Hx
y
Hx
y
L
H
G
aP
K
Z
1
1
1
1
ln
'
/
'
1
1
'
ln
'
/
'
/
1
1
1
NTU?
HTU?
a: packing area per volume
(tower height)
13. 2022/11/26 Aerosol & Particulate Research Lab 13
1
1
'
'
x
x
G
L
y
y
m
m
Mass balance
Equilibrium
Hx
y
*
1
*
' y
y
OG
m
Z y
y
dy
aP
K
G
Z
x1, y1*
x1, y1
xZ, yZ*
xZ, yZ
Alternative solution:
*
*
1
1
*
*
1
1
1
ln
;
'
z
z
z
z
LM
LM
z
OG
m
y
y
y
y
y
y
y
y
y
y
y
y
aP
K
G
Z
Assumptions for dilute/soluble systems?
14. 2022/11/26 Aerosol & Particulate Research Lab 14
Pure amine
Lm = 0.46 gmole/s
0.04% CO2
1.27% CO2
Gm = 2.31
gmole/s
C* = 7.3%
CO2 in amine
Q: A Packed tower using organic amine at 14 oC
to absorb CO2. The entering gas contains 1.27%
CO2 and is in equilibrium with a solution of
amine containing 7.3% mole CO2. The gas
leaves containing 0.04% CO2. The amine,
flowing counter-currently, enters pure. Gas
flow rate is 2.31 gmole/s and liquid flow rate is
0.46 gmole/s. The tower’s cross-sectional area
is 0.84 m2. KOGa = 9.34×10-6 s-1atm-1cm-3. The
pressure is 1 atm. Determine the tower height
that can achieve this goal.
15. 2022/11/26 Aerosol & Particulate Research Lab 15
Absorption of concentrated vapor
Mole balance on the controlled volume
)
'
(
)
'
(
0 x
L
dz
d
y
G
dz
d
m
m
Gas flux
y
G
G m
m
1
1
'
' 0
Liquid flux
x
L
L m
m
1
1
'
' 0
1
1
0
0
1
1
1
1
0
0
1
1
1
1
'
'
1
1
1
1
'
'
1
x
x
x
x
G
L
y
y
x
x
x
x
G
L
y
y
y
m
m
m
m
x1, y1
x1, y1*
xZ, yZ*
xZ, yZ
16. 2022/11/26 Aerosol & Particulate Research Lab 16
Mole balance on the gas in a differential tower volume
*)
(
1
'
0 2
0
y
y
aP
K
dz
dy
y
G
OG
m
NTU
HTU
y
y
y
dy
aP
K
G
dz
Z
y
y
OG
m
Z
Z
1
*
)
1
(
'
2
0
0
aP
K
G
HTU
OG
m0
'
1
*)
(
)
1
( 2
y
yZ y
y
y
dy
NTU
17. 2022/11/26 Aerosol & Particulate Research Lab 17
HTU
For a given packing material and
pollutant, HTU does not change much.
18. 2022/11/26 Aerosol & Particulate Research Lab 18
Summary
• Transfer from gas phase to liquid phase; Gas
phase or liquid phase controlled mass transfer.
• Equipment: spray tower and packed tower.
• Equilibrium line (Henry’s law) and operating line
(mass balance).
• Design: (a) liquid flow rate by mass balance; (b)
tower diameter by flooding condition; (c) tower
height by mass transfer rate
• Dilute and concentrated system
