Page 312 - Air Pollution Control Engineering

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Wet and Dry Scrubbing 291

a certain open area per the tower’s design. As polluted gas being treated enters the
tower, the gas passes upward through the tower. The slurry liquid (the absorbent) is
introduced into the top of the tower and flows downward. Therefore, the slurry
flows across each tray and the liquid finds its way to the tray openings. As a result,
the two opposing flows are forced to interact, with resultant gas–liquid surface con-
tacts that allow for the pollutant present in the gas to absorb into the liquid, as seen
in Fig. 20.
Tray towers known as sieve towers utilize a design gas velocity sufficient to force the
gas to form bubbles as the gas passes through the tray openings. Figure 20 illustrates
this method of forcing gas–liquid contacts. An alternate design for tray towers is the
valve tray tower. These towers use a “bubble cap” on each tray opening. Each bubble
cap is also surrounded with a cage intended to constrain the flow of liquid (see Fig.
20). As the polluted gas flows upward through the tray openings, these caps keep the
downward flowing slurry in an agitated condition. This forces the gas to exit each cap
at near Venturi scrubber velocity. Tray towers typically operate at a pressure drop
below that of a Venturi scrubber but well above the pressure drop of a spray tower. A
typical pressure loss for a tray tower is about 20 in. of water. The power consumption
of such towers is therefore significant.
5. Most packed towers are of vertical design so as to utilize countercurrent flow
between the gas and liquid (see Fig. 21). Inside the tower is a packed bed. The pack-
ing that comprises the packed bed is in the tower to force increased gas–liquid con-
tact to improve absorption efficiency. Packings of a wide variety of shapes, sizes,
and material of construction are available. Additionally, packings can form several
structures. A fixed structure such as the honeycomb packing seen in Fig. 21a is pos-
sible. Also, a random yet fixed structure such as the glass spheres presented in Fig.
21b may be used. The packing may also be mobile, as illustrated in Fig. 21c. The
glass spheres become fluidized with sufficient gas velocity through the tower. In
normal operation of the fluidized-bed scrubber, the packing actually passes out of
the tower, where it is normally collected for reuse. Finally, rods, decks, vanes, or
some other fixed structure may be used inside the tower as in Fig. 21e,f. As such,
in this last choice, there is actually no “packing” per se; the rods are used to force
gas–liquid contact.
When properly designed, packed towers do not need high power. Packed towers are
normally designed for pressure losses that overlap or are slightly higher than with tray
towers. A packed (wet scrubber) will normally operate with pressure drop in the range
of 2–8 in. of water.

A combination tower, as implied by the name, is the use of two or possibly more of these
absorption techniques in a single tower. As such, the combination tower will allow for tar-
geted pollutant removal (absorption) and/or operational flexibilities not possible with a
tower that utilizes only one absorption technique. In the combination tower, discrete chem-
ical and physical conditions are possible in different sections of the tower. Thus, one unit
installation may be used to accomplish multiple goals. A combination tower will obviously
be larger than a single absorption technique tower. Therefore, initial capital costs will be
greater for a combination tower versus the single absorption technique tower. However,
the costs of the single tower may be favorable when compared to the costs of two indi-
vidual absorption towers. Combination towers that have been successfully used in indus-
try are spray/Venturi and spray/packed tower combinations.