Air Pollution Control Equipment Selection Guide, 2nd Edition

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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.