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14
Spray Towers/Scrubbers
14.1 DEVICE TYPE
Spray tower scrubbers use spray nozzles to extend the surface area of the scrubbing
liquid to enhance mass transfer of contaminant gas(es) into the liquid. They are pri-
marily used for gas absorption.
Spray scrubbers include designs that use spray nozzles (hydraulically or air or
steam atomized) to absorb gases and control larger diameter (+10 μ) particulate.
14.2 TYPICAL APPLICATIONS AND USES
Spray tower scrubbers are often used on wet ue gas desulfurization (FGD) systems
at public and industrial power generation facilities. These FGD systems use lime
or limestone slurries as the scrubbing liquid. Their open vessel design is an advan-
tage where plugging or scaling may occur. The simplicity of the design makes them
a lower cost alternative for high gas volume scrubbing applications (over 100,000
actual cubic feet per minute [acfm]).
They are also used as part of quenching and gas conditioning systems wherein the
gas must be brought to saturation or near saturation with water.
Most spray towers are countercurrent in design wherein the gas ows vertically
upward and the liquid falls downward through the ascending gases. Some units, used
for odor control, are horizontally oriented using a multiplicity of concurrent spray
sections in series. Often, a series of bafes is incorporated into the scrubber vessel
to change the direction of the gas stream and to provide intimate contact of the spray
and the contaminant gas.
Some spray tower designs have been modied to act as direct contact condensers
in applications wherein packed devices may plug with solids. Though not as mechan-
ically efcient in that application as pure countercurrent designs (such as a packed
tower), the ability to resist plugging and therefore provide greater on-line availability
can make the spray tower condenser attractive.
Spray scrubbers cover a wider variety of designs. These vary from devices as
simple as a spray header in a duct to cyclonic type devices (often called preformed
spray scrubbers).
14.3 OPERATING PRINCIPLES
A common characteristic of this type of scrubber is the use of spray nozzles to extend
the liquid surface and produce target droplets.
At least one spray zone is produced in a spray tower using at least one spray
nozzle in a containing vessel. In practice, however, most spray towers use multiple
spray zones to achieve the required gas cleaning efciency. Figure 14.1 shows a
128 Air Pollution Control Equipment Selection Guide
sectional view of a spray tower. The gas inlet is typically horizontally oriented into
the containing vessel. A multiplicity of spray zones is used, each containing an array
of nozzles. In FGD applications, these nozzles are wear-resistant designs (such as
silicon carbide) since the scrubbing liquid is an abrasive slurry of lime or limestone.
The hydraulic pressure applied to the liquid acts as stored energy. When this
pressurized liquid ashes from the spray nozzle, the energy stored is expended in
producing a spray. The high relative velocity between the liquid and surrounding
gas causes a shearing action that breaks the liquid into tiny droplets. The net effect
is that the liquid surface area increases so that the contaminant gas or gases can be
more readily absorbed.
After the spray is produced, the contaminant gas is absorbed through the liquid
lm. If a reactive chemical is contained in the droplet, the contaminant will react,
forming a by-product (usually a salt) of lower vapor pressure. Therefore, the con-
taminant remains in the droplet.
Most droplets fall by gravity in counterow designs to the sump. Quite often,
the scrubber is mounted directly over the sump to facilitate this separation. A small
portion of the spray goes overhead with the gas. This droplet dispersion is controlled
using chevron type droplet eliminator(s) in the case of a gas stream containing
particulate, or mesh pads if the gas stream is low in or devoid of particulate. The
Chevron
s
Chevron
Sprays
Spray
Zones
Retention
Sump
Gas
Inlet
Pump
Spray Tower
FIGURE14.1 Spray tower sectional view.

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