Transcript Absorption

Reading: Chap 13
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
Q: Does it matter if it’s gas phase or liquid phase controlled?
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Gas Absorption Equipment
Spray tower
Clean gas out Countercurrent
Clean gas out
Spray
nozzle
Dirty gas in
Redistributor
Q: Limitations of a
spray tower?
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Q: Why redistributor?
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packed tower
Mist
Eliminator
Liquid
Spray
Packing
Dirty gas in
Liquid outlet
Mycock et al., 1995
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http://www.tri-mer.com/Printable/vertical_flow_cross_section.html
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Three-bed cross
flow packed tower
Liquid spray
Dry Cell
Packing
Berl
Saddle
Intalox
Saddle
Raschig
Ring
Lessing
Ring
Q: Criteria for good packing materials?
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Pall
Ring
Tellerette
Mycock et al., 1995
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Known: ??
Gas out
Unknown: ??
Liquid in
Gas in
Mass Balance
In = Out
Liquid out
Gm1  Lm2  Gm2  Lm1
Gm  y1  y2   Lm x1  x2 
(for a dilute system)
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
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Slope of Operating
Line = Lm/Gm
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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.
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Problems with high gas flow
• 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
• 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.
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(G' ) 2 F L0.2
G  L g
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)
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L
G
G
(dimensionless)
L
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Mass Transfer
rateof mass   interfacial 
 concentration 
Flux  
/

k
  area 
 difference 
transferre
d

 



J  M / A  k Ci  C 
mass
J: flux ( area  time )
k: mass transfer coefficient
CI
Two-Film Theory (microscopic view)
J k G  pG  pI 
CL
(gas phase flux)
J k L CI  CL 
pG
(liquid phase flux)
pI  HCI
pI
1
 pG  HCL 
J
1 / kG  H / k L
Cussler, “Diffusion”, Cambridge U. Press, 1991.
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(overall flux)
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1
pG
K OL 
C* 
J  K OL C*  CL 
1 / k L  1 / kG H (equivalent concentration
H
(overall liquid phase MT coefficient)
1
to the bulk gas pressure)
K



 K OG pG  p*
OG
p*  HCL
1
/
k

H
/
k
(overall gas phase MT coefficient)
G
L (equivalent pressure to the
bulk concentration in liquid)
2
Macroscopic analysis of a packed tower
Mole balance on the solute over the
differential volume of tower
 accumulation    flow of solutein 
 of solute   minus flow out 

 

dy
dx
0  G 'm
 L 'm
dz
dz
1
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G 'm
 x  x1 
( y  y1 )
L'm
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L’m: molar flux
of liquid
G’m: molar flux
of gas
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Mole balance on the solute in the gas only
solute   solute flow in   solute lost 

 accumulation    minus flow out    by absorption

 
 

dy
0  G 'm
 K OG aP ( y  y*)
dz
Z
y1
G 'm
dy
 Z   dz  

0
y Z  y  y *
K
aP
OG
(tower height)
a: packing area per volume
y*  Hx
 y1  Hx1 
1

Z 
ln
K OG aP 1 / G 'm  H / L'm   yZ  HxZ 
 y1  Hx1 
G 'm
1



ln
K OG aP 1  HG'm / L'm   yZ  HxZ 
1
HTU?
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NTU?
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Mass balance
x1 , y1
L' m
x  x1 
y  y1 
G 'm
Equilibrium
y*  Hx
x1 , y1 *
y1
G 'm
dy
Z
KOG aP yZ  y  y *
x Z, y Z
x Z, y Z*
Alternative solution:
G 'm
y1  y z
Z

;
KOG aP yLM
yLM

y  y   y

*

y
1
z
z
 y1  y1* 

ln
* 
 yz  yz 
*
1

Assumptions for dilute/soluble systems?
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Pure amine
Lm = 0.46 gmole/s
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.
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0.04% CO2
1.27% CO2
Gm = 2.31
gmole/s
C* = 7.3%
CO2 in amine
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Absorption of concentrated vapor
Mole balance on the controlled volume
x 1 , y1
d
d
0   (G 'm y )  ( L'm x)
dz
dz
Gas flux
 1 

G'm  G'm0 
1 y 
x1, y1*
Liquid flux
 1 
L'm  L'm 0 

1 x 
xZ, yZ
 y1  L'm 0  x
x1 

 



1  y1  G 'm 0  1  x 1  x1 

y
 y1  L'm 0  x
x1 
 


1  

 1  y1  G 'm 0  1  x 1  x1 
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xZ, yZ*
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Mole balance on the gas in a differential tower volume
G'm 0 dy
0
 K OG aP( y  y*)
2
1  y  dz
Z 
Z
0
G'm0 y1
dy
dz 
 HTU  NTU
2

y
KOG aP Z (1  y)  y  y *
G 'm0
HTU 
K OG aP
NTU  
y1
yZ
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dy
2
(1  y ) ( y  y*)
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HTU
For a given packing material and
pollutant, HTU does not change much.
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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
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