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