Absorption of Gases
Chapter 12
Coulson & Richardson’s
Chemical Engineering
Vol.2, Fifth Edition
Contents
Process description Equilibrium conditions Mechanism of absorption Wetted wall column Packed column Spray towers Plate towers Vessels with agitation Centrifugal absorbers Exothermic absorption
Process descriptionDefinition: Removal of one or more
selected components (SOLUTE) from a mixture of gases gases by absorption into a suitable liquid (SOLVENT)
2nd major operation of chemical engineering
Examples: removal of CO2 from Syn gas Removal of SOx and NOx from flue gases
General scheme
Absorption in a contactor-Absorber
Recovery in a contactor- Stripper
Feed gas
Pure gas
Solute gas
solvent
Theory
Conditions of equilibrium Solubility of gas is not substantially affected by
pressure Solubility decreases with temperature Absorption may involve evolution of heat Low temperatures favors absorption For low conc. Of gases and over wide range of
some gases, Henry’s law holds
AA HCP
Difference in partial pressure
makes the difference
Mechanism of absorption
C A
P A
Gas
film
bou
ndar
y Liqu
id fi
lm b
ound
ary
Gasfilm
liquidfilmPAG
PAi
CAi
CAL
interface
• proposed by WHITMAN
•Resistance to mass transfer lies within the gas and liquid films (Two films only)
• concentrations at the interface are in equilibrium
• there is instantaneous transfer at the interface
• controlling factor will be the change through the films on either side
Mechanism of Gas Absorption
Diffusion through a stagnant gas
Stefan’s law applies
Integration over whole thickness zG
dz
dC
C
CDN A
B
TVA '
A diffuses through stagnant gas film
A diffuses through stagnant liquid film
A enters the bulk of the liquid
1
2' lnB
B
GVA P
P
RTz
PDN
1
2' lnB
B
G
TVA C
C
z
CDN
Bm
BB
GVA P
PP
RTz
PDN 12'
Hence the rate of absorption of A per unit time per unit area is
OR
Where
Bm
AAGA P
PPPkN 21''
21'
AAGA PPkN
BmG
VG
G
VG PRTz
PDk
RTz
Dk ,'
In most cases the film thickness is not known, therefore, kG is used more frequently as a measure of rate of gas absorption per unit area per unit partial pressure diference
Diffusion in the Liquid phase
Diffusion in liquids is slow
No theoretical basis available for rate of diffusion comparable to kinetic theory of gases: the basic eq. Is
For unknown film thickness
L
AALA
ALA
z
CCDN
dz
dCDN
12'
'
21'
AALA CCkN
Rate of Absorption At steady stateRate of mass transfer through gas film
AiAGGA PPkN '
Rate of mass transfer through liquid film=
ALAiLA CCkN '
AiAG
ALAi
L
G
PP
CC
k
k
Par
tial P
ress
ure
(PA)
PAe
PAi
PAG
CAL CAi CAe
PA
G-P
Ai
CAi-CAL
AD
BE
F
Slope of the line=-kL/kG
Overall Mass Transfer Coefficients
It is normally difficult to have values of CAi or PAI, therefore, rate of mass transfer is related to over all transfer coefficients as;
When the equilibrium line is nearly straight, and Henreys law applies:
ALAeLA
AeAGGA
CCKN
PPKN
'
'
LGG kkK
11
GLL kkK 111
LG KK
1
Rate of Absorption in terms of mole fractions
For slope of the equilibrium line m
M=(yAi-yAe)/(xAi-xA)
The over all gas coefficient is
AAeLA
AeAGA
xxKN
yyKN
'''
'''
LGG k
m
kK ''''''
11
Factors affecting the mass transfer coefficient
Very soluble gas Almost insoluble gas Moderately soluble
gas
Par
tial P
ress
ure
(PA)
PAe
PAi
PAG
CAL CAi CAe
PA
G-P
Ai
CAi-CAL
AD
BE
F
Wetted Wall Column
44.083.0Re' ScBP
P
D
dh BM
V
D
44.083.0Re' ScBP
P
D
dh BM
V
D
44.0ScP
P
D
dh BM
V
D
44.083.0Re' ScBP
P
D
dh BM
V
D
1000 40000
8
200
water
Other solvent
s
Re
44.0ScP
P
D
dh BM
V
D
dV
BMD jBDP
P
u
h
17.0
56.0
Re'
Chilton & Colburn
Coefficients in Packed Towers
A: Gas Film Controlled Processes
G
p
G
pD
ca
DSc
dG
D
dhSh
where
ScSh
'Re
Re
Coefficients in Packed Towers
B: liquid Film Controlled Processes
50.0'
LL
L
LL
L
D
L
D
ak
L’
kLa
Fig. 12.7
Equipment for Gas Absorption
Wetted wall column we have discussed it already
Packed TowersPlate TowersVessels with AgitatorsCentrifugal AbsorbersSpray TowersCentrifugal Spray Towers
Wetted wall column
we have discussed it already
Equipment for Gas Absorption
Packed Towers 25 mm-4.5 m diameter 30 m or more height see more details on pages 213-232 of ref-1
Liquid out
Pac
ked
bed
Gas in
Gas out
Liquid in
Distributor
Hold-down plate
Packing supportx x
Y Y ABCD
Dry packing
Wet drained packing
Low liq
uid
rate
High
liqui
d ra
te
Pre
ssur
e dr
op
Gas velocity
Height of packed tower
Gm: moles of inert gas/(unit time)(unit cross-sectional area)
Lm: moles of solute free liquor/(unit time)(unit cross-sectional area)
Y: moles of solute gas A/mole of inert gas B in gas phase,
X: moles of solute A/mole of inert solvent in liquid phase,
2
1
Y
YiG
m
YY
dY
aPk
GZ
2
1
Y
YiTL
m
XX
dX
aCk
LZ
Plate Towers for gas absorption
Bubble caps or sieve tray columns are some times used when: Loads can not be handled in packed column of
1m diameter When deposition of solids may take place When involved heat effects are high
Plate efficiency ranges 20-80 %
Vessels with agitators
A gas is bubbled through a liquid along with agitation to saturate the liquid with the gas
Absorption coefficient varied directly with Pv for vaned-disc or flat-paddle agitator
KGa us0.67
baffles
agitator
Gas vanes
The centrifugal absorber
Repeated spray formation for increased surface area
A set of concentric rings intermeshes with a second set of rings attached to a rotating plate
Liquid fed to the centeris carried up the 1st ring, splashes over to the baffle and falls into the gap between the next rings.
It runs in a similar way passing from ring to ring through the unit.
The gas can be introduced at the top ( for co-current flow) or at the bottom ( counter-current flow).
Depth fo the ring is not important and mass transfer takes place when liquid spray is mixed with the gas
See Fig 12.32 and Table 12.7 for some experimental data on CO2 absorption.
Liquid gas
liquid
gas
Rotating plate
Stationary rings
Spray Towers
Gas enters at the bottom and liquid is introduced at the top as series of sprays
Performance is poor because the droplets tend to coalesce after they have fallen through a few meters
Thus reducing interfacial area Resistance of equivalent liquid film
is high Therefore, these units are useful
when major resistance lies within the gas film
Used for absorbing Ammonia in Water with moderate success
Also used as humidifiersgas
gas
liquid
liquid
Centrifugal Spray Towers
Gas enters tangentially at the bottom
Liquid is sprayed into the gas stream from a spray manifold
Liquid drops are subjected to centrifugal force before they are taken out of the gas
Centrifugal force tends to reduce the resistance in the liquid film
One illustration of the principle is shown here Liquid
inlet
Gas inlet Liquid
spray manifold
Core buster disc
Anti-spin valves
damper