Page 223 - Air Pollution Control Engineering

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202 Lawrence K. Wang et al.

packed bed of the tower. Packing may either be random dumped or structured, depending
on the given situation. Regardless of which type of packing is used, the purpose is to pro-
mote gas–liquid contact so that the pollutant(s) being removed from the gas stream is
absorbed into the liquid stream. The packed bed is held in place by a packing support
grid at the base of the packed bed. A bed limiter may be needed to hold down the top of
the packing.
In extremely large towers, intermediate packing support may also be required.
Excellent liquid distribution of the scrubbing liquor onto the packed bed is always
required in a scrubbing tower. Several types of distributor are available, as are full cone
spray nozzles, to ensure adequate wetting of the packing. Above the liquid inlet in the
tower is the mist eliminator. As with liquid distribution, various methods of forcing
droplets to coalesce from the gas stream exist. The most common types of mist elimi-
nator are mesh pads and chevron blades. Above the mist eliminator, the scrubber tower
narrows to the exit stack, where treated gas is released into the atmosphere. This type
of scrubber is used when extremely high removal efficiency of a pollutant(s) from a gas
stream is required, typically 99%.
2.2.4. Spray Tower Scrubbers
This is another option used to treat polluted gas streams. Three configurations of
spray towers are employed in industry:
1. Polluted gas flows upward as liquid spray flows downward (countercurrent flow pattern).
2. Polluted gas flow and liquid spray flow are both downward (cocurrent flow pattern).
3. Polluted gas flows laterally as liquid spray flows downward (perpendicular or cross-flow
pattern).
In flow pattern 1, if packing is added, the spray tower becomes a packed tower.
Packing is also sometimes used in both configurations 2 and 3 to enhance gas–liquid
contact. In these two situations, a hybrid of packed and spray towers is used.
When designing a strictly spray tower (no packing), the critical design parameters to
consider are tower height and diameter, gas and liquid flows, as well as liquid-to-gas
ratio, gas velocity, droplet size and liquid chemical composition. Spray towers often use
a higher liquid-to-gas ratio than for packed towers. The higher ratio is needed to achieve
high removal efficiency.
2.2.5. Tray Towers (Scrubbers)
This application uses a tower with numerous trays within (see Fig. 1a). As the scrubbing
liquor passes the tower, a certain amount of liquid is collected (or held) on each tray.
The trays have openings to give a specified open area, per the tower design, to allow for
gas to pass upward through the tower. As the gas passes and the liquid flows downward,
high gas–liquid contact occurs in the countercurrent flow scheme. Common types of
tray are bubble cap, perforated, and valve types. The number of trays within the tower
is a function of the needed removal efficiency of the tower.
2.2.6. Quenchers
As previously mentioned, quenchers are sometimes needed as a first step in con-
junction with another gas treatment step. Similar to a spray tower, the quencher is used
for temperature and humidification control of the polluted gas being treated. Often used