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6. ABSORPTION
Absorption is a process where transfer of a gaseous component from gas phase to
liquid phase takes place. More specifically in air pollution control, absorption
involves the removal of objectionable gaseous contaminant from a process stream
by dissolving them in liquid
1. Absorbent: the liquid, usually water, into which contaminant is absorbed
2. Absorbate or solute :The gaseous contaminant being absorbed, such as
SO2, H2S, etc.
3. Carrier gas: the inert portion of gas stream, usually air, from which the
contaminant is to be removed [1].
4. Interface: the area where the gas phase and the absorbent contact each
other.
5. Solubility: the capability of the gas to be dissolved in a liquid.

7. Step1: The pollutant diffuses from bulk area of the gas phases to the gas
liquid interface.
Step2: gaseous pollutant transfer across the interface to the liquid phase.
This second step is extremely rapid.
Step3: The pollutant diffuses bulk area of the liquid, making room for
additional gas molecule to absorb
MECHANISM OF ABSORPTION
SOLUBILITY
Solubility of contaminant affects the amount of contaminant that can be
adsorbed. It is a function of both the temperature and, to a lesser extent
pressure of a system. As we increase the temperature of the system the
amount of gas that can be absorbed by liquid decreases, while as with
increasing the pressure generally absorption increases

8. Spray towers are useful for large volume
handling with relatively low pressure drop and
high efficiency. In general smaller the droplet
size the greater the turbulence, the more
chance for absorption of the gas
Moisture eliminator reduces the amount of
moisture in the gases being released. Spray
tower has less gas liquid interfacial area so
they are less effective in removal of gaseous
contaminant.

9. This type of tray contains horizontal trays or
plate that provides large gas liquid
interfacial areas. The polluted air is introduced
from one side of the bottom of the column,
rise up through the opening in each tray, and
the rising gas prevents the liquid from draining
through the opening.
Due to repeated contact of gas and liquid the
contaminant are removed and the clean air
emerges from the top.
In bubble cap tray column, the
contaminated gas moves upward until they
strike the cap, at which point they are
diverted downward and discharged as small
bubbles from slots at the bottom of the caps

11. In packed tower the contact time
between vapour and liquid is
increased by introducing packing.
The packing material has a large
surface to volume ratio and a large
void ratio that offers minimum
resistance to gas flow

12. In adsorption process the contaminant removal is done by passing a
stream of effluent gas through a porous solid material (adsorbent)
contained in adsorption bed. The surface of porous solid material
attracts and holds the gas (the adsorbate) by either by physical or
chemical adsorption.

14. STEPS IN ADSORPTION PROCESS Adsorption occurs in three steps
Step 1: The contaminant diffuses from the bulk gas stream to the external surface of the
adsorbent material
Step 2: The contaminant molecule migrate external surface to the macropores,
transitional pores, and micropores within each adsorbent.
Step 3: The contaminant molecule adheres to the surface in the pore. Following figure
illustrates this overall diffusion and adsorption process.

15. SALIENT FEATURE OF ADSORPTION PROCESS
(1) Adsorption processes are used extensively on large-scale applications having
solvent vapour concentrations in the range of 10 to 10,000 ppm.
(2) Prior to becoming saturated with the solvents, the adsorbent is isolated from
the gas stream and treated to drive the solvent compounds out of the solid
adsorbent and into a small volume, high concentration gas stream.
(3) The desorbed gas stream is then treated to recover and reuse the solvents.
(4) The adsorbent is cooled (if necessary) and returned to adsorption service.
(5) Because the adsorbent is treated and placed back in service, these adsorption
processes are termed regenerative.
(6) Adsorption processes usually operate at efficiencies of 90% to 98% over long
time periods.