Air Pollution Control Equipment Selection Guide, 2nd Edition

79

8

Fiberbed Filters

Joe Mayo

Air-Clear, LLC, Frazer, Pennsylvania

8.1 DEVICE TYPE

Fiberbed lters are specialized ltration devices that are primarily designed to

coalesce and capture liquid contaminants such as acid mists and aerosols, the vis-

cosity of which is low enough that they ow or can be made to ow from the ber-

bed surface.

The design gets its name from the medium used. It consists of micron-size bers

that are compressed tightly in a mat or bed, which provides the surface area and gas

path thickness needed to capture the pollutant.

These designs are somewhat related to lament/mesh scrubbers in that they uti-

lize target bers in a wet environment. The berbed lter bers, however, are in the

5 to 15 μm diameter range, or a fraction of the diameter of the lament or mesh type

scrubbers. The ber spacing is therefore closer in a berbed lter and, in general, it

can remove smaller diameter aerosols.

Figure8.1 shows a cutaway view of a berbed lter unit. The individual lters

(sometimes called candles, given their shape) are mounted on a tube sheet in either

a hanging or sitting position. The unit shown shows them hanging from a tube sheet.

The small J-shaped pieces under each candle are liquid traps that allow the liquid to

drain but prevent gases from bypassing the lter.

8.2 TYPICAL APPLICATIONS AND USES

The following are brief descriptions of common berbed lter applications. With

one exception, they all involve the collection of liquid droplets. In general, if the

exhaust stream is wet or the particles in the exhaust are liquids, or if a high-efciency

lter that can withstand a high pressure drop is required, then berbeds are a poten-

tial control option (see Figure8.1).

8.2.1 aCiD mist

Collecting acid mist was the rst signicant commercial use of berbed lters and

is still the largest application for them. Most sulfuric acid manufacturing plants

use berbed lters in the absorbing and drying towers to remove SO

3

and liquid

acid mist from the air. Fiberbeds are also used to remove residual mist in the

exhaust of wet scrubbers, particularly hydrochloric acid scrubbers, because the

80 Air Pollution Control Equipment Selection Guide

reaction with the scrubbing liquid can be violent and creates a visible emission

from the scrubber. These are typically cool and clean applications, requiring no

preltration or cooling.

If additional berbed surface area is required, a nesting or concentric type lter

can be built. In these designs, as shown in Figure8.2, a berbed is mounted within

another berbed, thus increasing the face area of the medium and slowing the gas

velocity. The reduced gas velocity is said to improve the capture of aerosols and

mists.

8.2.2 asphalt proCessinG

Equipment used in asphalt processing include coaters, saturators, converters (blow

stills), storage tanks, and truck loading and unloading facilities. The coaters and

saturators used in roong manufacture often have solids that must be preltered

before the berbeds. Saturator exhaust may also require cooling. Tanks and loading

racks usually achieve adequate cooling through radiant losses in the ductwork and

have little solid particulate. Asphalt converters are also relatively free of solids but

may require cooling. Such a unit is shown in Figure8.3.

8.2.3 plastiCizer/Vinyl/pVC proCessinG

Vinyl and PVC processing, such as calendaring, coating, and curing operations, emit

oily plasticizers and other materials that can cause a substantial exhaust stack plume.

While oven exhaust must usually be cooled to condense the vapors, coater and calen-

dar emissions are often captured by canopy hoods that draw in ambient air that cools

the exhaust. Prelters are usually not required.

FIGURE8.1 Cutaway of berbed lter (Air-Clear, LLC).

81Fiberbed Filters

8.2.4 CoatinG/laminatinG

Many coating and laminating processes, especially on fabric and vinyl, create visible

emissions that berbed lters can effectively control. The emissions are typically

generated during the drying and curing phase of the operation, so the exhaust is hot

and usually requires cooling to condense the vapors. The cooling coil housing is on

the right side in Figure8.4.

8.2.5 eleCtroniCs

Electronic component manufacturing, such as solder leveling, can create oil mist

from the uxes used. Fiberbeds can also be used as point source collection for acid

mists, reducing the load on house scrubbers and reducing salt formation in the duct-

work. Materials of construction must be carefully chosen because many of the mate-

rials are potentially corrosive.

8.2.6 textile proCessinG

Textile tenter frame ovens and dryers can emit a mixture of pollutants including oils,

resins, waxes, tars, and various solids, producing a prodigious stack plume. This hot,

dirty exhaust requires both cooling and preltration. The mineral oil–based emission

FIGURE8.2 Filter within a lter (Monsanto Enviro-Chem Systems, Inc.).