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Gas absorption
within packed towers takes place in one of two ways, either:
1. By physical contact and dissolution of gas in water
or by
2. A scrubbing reaction between the gas and a scrubbing reagent.
The performance
and efficiency of a packed tower scrubber is generally dependent
upon the following factors:
1. The packing surface area over which gas/liquid transfer will
occur
2. The even distribution of the scrubbing liquor throughout the
packing
3. Gas velocity through the packed bed
4. The liquor flow rate through the packed bed 
A packed tower consists of the following main sections:
BASE SECTION
OR SUMP
Acts as a liquid reservoir for the column and is connected to the
recirculation pump inlet. It also has connections for the adding
of water, dosing of chemicals and the drainage of spent liquor.
PACKED
BED SECTION,
Throughout which gas absorption occurs. the bed compromises many
pieces of loose random packing, the size and shape of which is selected
to give optimum performance. The bed is supported on a purpose built
packing support plate.
UPPER SECTION
Made up of the liquid distributor to ensure even liquor distribution
across the packing. The distributor will be either a spray system,
pan and riser or strough type depending on the application. Above
the distributor, a droplet eliminator prevents liquid carry over.
The eliminator could be of a vertical of horizontal design depending
on the outlet or discharge requirement.
Typical
examples of gases removed by these types of scrubber include:
1. Hydrogen Chloride
2. Ammonia
3. Oxides of Nitrogen
4. Sulphur Dioxide
5. Phosgene
6. Chlorine
7. Hydrogen Sulphide
8. Amines
9. Bromine
10. Nitric acid
11. Methanol
12. Formaldehydeain
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