1) Absorption involves the transfer of a solute from the gas phase into a liquid phase, while desorption/stripping involves the transfer of a solute from the liquid phase into a gas phase. 2) Gas absorption is commonly carried out in vertical countercurrent columns and absorbers are often used with strippers to allow regeneration and recycling of the absorbent. 3) Common applications of absorption and stripping include removing H2S from sour natural gas or crude oil using solvents like monoethanolamine, and recovering gases for various industrial purposes.

1. Mass Transfer Operations
Department of Chemical Engineering Technology
Government College University Faisalabad
Mass Transfer Operations - CET, GCUF 1

2. Absorption (Scrubbing / Gas Washing) and Desorption (Stripping): Introduction [1]
Gas absorption is a mass transfer
operation in which one or more species
(solute) is removed from a gaseous
mixture by dissolution in a liquid
(solvent).
The insoluble component(s) present in
the gas, which is not absorbed is called
the carrier gas
Example: Removal of H2S and CO2
from sour natural gas
The opposite of absorption is stripping
(also called desorption), wherein a
liquid mixture is contacted with a gas
to selectively remove components by
mass transfer from the liquid to the gas
phase
Example: Removal of H2S from sour
crude oil Mass Transfer Operations - CET, GCUF 2

3. Absorption (Scrubbing / Gas Washing) and Desorption (Stripping): Introduction [2]
Mass Transfer Operations - CET, GCUF 3
 Absorption: The transfer of a solute from the gas phase into a liquid phase
 Desorption/Stripping: The transfer of a solute from the liquid phase into a gas phase
 Gas absorption is usually carried out in vertical countercurrent columns.
 Absorbers are normally used with strippers to permit regeneration (or recovery) and recycling of the absorbent

4. Why Absorption?
Absorption is a physical operation which is used to separate a specific gas from the gas mixture by using
an appropriate solvent.
The purpose of such scrubbing operations may be gas purification, e.g., removal of air pollutants from
exhaust gases; product recovery; or production of solutions of gases for various purposes.
For Example, H2S is removed from the sour natural gas using MEA (Monoethanolamine), because exposure
to hydrogen sulfide may cause irritation to the eyes and respiratory system. It can also cause apnea, coma,
convulsions; dizziness, headache, weakness, irritability, insomnia; stomach upset, and if liquid: frostbite.
Workers may be harmed from exposure to hydrogen sulfide. It is also highly corrosive gas and may cause
acidic rain.
H2S/CO2 is the solute in above example, natural gas is the carrier and MEA (Monoethanolamine) is the
solvent
Mass Transfer Operations - CET, GCUF 4

5. Absorption and Stripping: Applications
Gas absorption is an integral part of many chemical processes. Some examples are tabulated below:
Mass Transfer Operations - CET, GCUF 5

6. Important definitions of some useful terms dealing Absorption/Stripping
Absorption Column – a plate column where gas is
absorbed while a gas mixture bubbles through the liquid
which passes down from plate to plate
Downcomer – a tube or duct leading liquid from one plate
in column to the next plate
Downflow Flooding – in plate columns, excessive build up
of liquid on the plates caused by insufficient drainage
through the downcomers or excessive vapour/gas velocity
Entrainment flooding – in distillation and absorption,
excessive carry-over of suspended droplets following the
vapour or gas upwards through the column
Absorption Tower – a tower where liquid flows down over
a packing material in countercurrent or co-current flow with
a gaseous mixture while one or more of the gaseous
components are absorbed in the liquid
Channeling – the condition in which packing surface is not
wetted by the flowing liquid due to low liquid rates. It is the
chief reason for the poor performance of large packed
towers. Mass Transfer Operations - CET, GCUF 6

7. Important definitions of some useful terms dealing Absorption/Stripping
Lean Gas – outgoing gas mixture in air from the gas
absorption column/tower
Rich Gas – gas mixture entering the gas absorption
column/tower
Weak Liquor – the inlet Solvent
Strong Liquor – the outlet solution, which is enriched in
solute
Scrubbing – the process in which a liquid solvent is used to
remove dispersed particles from the gas
Packing – small objects used to provide a large contact area
between rising fluid and descending liquid in packed
columns, such as saddles, rings, grates and pieces of coke
Random Packing/Dumped Packing – in packed columns
and towers, packing units lying randomly as they happened
to fall when dumped down
Mass Transfer Operations - CET, GCUF 7

8. Factors affecting the selection of Solvent and Stripping medium
Following considerations/factors are taken into account while selecting the solvent/stripping
medium for absorption/desorption

9. Types of Equipment and Methods of Operation
 Proper selection of equipment and method of operation are important
since both have considerable effects on absorption
 Absorption and stripping are conducted mainly in trayed towers
(plate columns) and packed columns and less often in spray towers
and bubble columns
 A trayed tower is a vertical, cylindrical pressure vessel in which
vapor and liquid, which flow countercunrently, are contacted on a
series of trays or plates
 A packed column, shown in Figure, is a vertical, cylindrical
pressure vessel containing one or more sections of a packing
material over whose surface the liquid flows downward by gravity,
as a film or as droplets between packing elements. Vapor flows
upward through the wetted packing, contacting the liquid.
 If only one or two theoretical stages are required, only a very low
pressure drop is allowed, and the solute is very soluble in the liquid
phase, the use of a spray tower may be advantageous. As shown in
Figure, a spray tower consists of a vertical, cylindrical vessel filled
with gas into which liquid is sprayed.
 A bubble column, also shown in Figure, consists of a vertical,
cylindrical vessel partially filled with liquid into which the vapor is
bubbled. Vapor pressure drop is high, and only one or two theoretical
stages can be achieved. Such a device has a low vapor throughput
and should not be considered unless the solute has a very low
solubility in the liquid, thus requiring an appreciable residence time.
Figure: Industrial equipment for absorption and stripping:
(a) trayed tower; (b) packed column; (c) spray tower; (d)
bubble column;

10. Tray Openings for Vapor Passage
Mass Transfer Operations - CET, GCUF 10

11. Packed Columns
Mass Transfer Operations - CET, GCUF 11