Fiberbed Filters

Joe Mayo

Air-Clear, LLC, Frazer, Pennsylvania

Device Type

Fiberbed filters are specialized filtration devices that are primarily designed to coalesce and capture liquid contaminants such as acid mists and aerosols, the viscosity of which is low enough that they flow or can be made to flow from the fiberbed surface.

The design gets its name from the medium used. It consists of micron-size fibers 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 filament/mesh scrubbers in which they utilize target fibers in a wet environment. The fiberbed filter fibers, however, are in 5-15-gm diameter range, or a fraction of the diameter of the filament- or mesh-type scrubbers. The fiber spacing is therefore closer in a fiberbed filter and, in general, it can remove smaller diameter aerosols.

Figure 8.1 shows a cutaway view of a fiberbed filter unit. The individual filters (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 filter.

Typical Applications and Uses

The following are brief descriptions of common fiberbed filter 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-efficiency filter that can withstand a high-pressure drop is required, then fiberbeds are a potential control option (see Figure 8.1).

FIGURE 8.1

Cutaway of fiberbed filter (Air-Clear, LLC).

Acid Mist

Collecting acid mist was the first significant commercial use of fiberbed filters and is still the largest application for them. Most sulfuric acid manufacturing plants use fiberbed filters in the absorbing and drying towers to remove SO, 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 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 prefiltration or cooling.

If additional fiberbed surface area is required, a nesting or concentric- type filter can be built. In these designs, as shown in Figure 8.2, a fiberbed is mounted within another fiberbed, 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.

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 roofing manufacture often have solids that

FIGURE 8.2

Filter within a filter (Monsanto Enviro-Chem Systems, Inc.).

must be prefiltered before the fiberbeds. 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.

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 calendar emissions are often captured by canopy hoods that draw in ambient air that cools the exhaust. Prefilters are usually not required.

FIGURE 8.3

10,000 acfm system with prefilters (Air-Clear, LLC).

Coating/Laminations

Many coating and laminating processes, especially on fabric and vinyl, create visible emissions that fiberbed filters 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.

Electronics

Electronic component manufacturing, such as solder leveling, can create oil mist from the fluxes used. Fiberbeds can also be used as point source collection for acid mists, reducing the load on house scrubbers and reducing salt

FIGURE 8.4

With prefilters, water cooling coils on curing ovens (Air-Clear, LLC).

formation in the ductwork. Materials of construction must be carefully chosen because many of the materials are potentially corrosive.

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 prefiltration. The mineral oil-based emission from a tenter frame can be collected using a fiberbed as shown in Figure 8.5. Note the induced draft fan and exhaust duct located to the right of center.

Metal Working

Coolant and oil mists are often generated by the high temperatures at the tool working surface. Grinding operations usually require prefilters to protect the fiberbeds from swarf. Such a system is depicted in Figure 8.6. A water washdown system is sometimes used to flush the interior of the system free of the water-based coolant to avoid long-term growth of bacteria inside the system. In general, when insoluble particulate or fibers are present, a prefilter should be used.

FIGURE 8.5

30,000 acfm system on tenter frame (Air-Clear, LLC).

FIGURE 8.6

1000 acfm on five-station machining center (Air-Clear, LLC).

Lube Oil Vents and Reservoirs

Oil lubricating systems, such as used on gas and steam turbines, often emit oil mist due to the hot oil returning from the turbine. No cooling or prefiltration is usually required. The compact cylindrical design of the fiberbed shown in Figure 8.7 makes these easy to install on lube oil vents. These also serve to recover oil and thereby reduce maintenance expenses. A similar configuration is used on ocean-going naval vessels for crankcase ventilation systems (mentioned in Section 8.2.10).

Incinerator Emissions

Incinerators that burn toxic, hazardous, or radioactive materials may produce submicron particles that must be controlled. Typically located downstream of a wet scrubber, the fiberbeds can be made of polyester or other materials that can be completely incinerated to dispose of spent filter media.

FIGURE 8.7

300 cubic feet per minute (cfm) oil vent unit (Air-Clear, LLC).

Internal Combustion Engine Crankcase Vents

Internal combustion engines have crankcase oil mist emissions due to blow- by around the piston rings that are economically controlled by fiberbeds. This application is like lube oil reservoir vents (see Figure 8.8).

Precious Metal Recovery

Process catalysts such as palladium gauze in nitric acid manufacturing can be lost into the process stream. The high temperature stability and structural strength of fiberbeds make them ideal for recovering these valuable metals. This is the unusual case of fiberbeds being used to collect solid particulate.

FIGURE 8.8

Packaged fiberbed with prefilter (Air-Clear, LLO.

Vacuum Pumps

Vacuum pumps mechanically generate oil mist during their operation and, unless they are evacuating furnaces, are usually cool. Some applications such as silicon crystal growing contain solid particulate (silicon dioxide) and thus require prefiltration. The prefilter in the unit in Figure 8.9 removes the particles that could plug the main filter.

Another method of prefiltering involves encasing the main fiberbed candle with a removable outer filter. The man in Figure 8.9 has these prefilters draped over his shoulder. Note the retaining cage to the left.

You would not be well advised to use fiberbed designs to clean gas streams containing inert particulate or liquid aerosols that do not flow by gravity or

FIGURE 8.9

Removable filter medium (Monsanto Enviro-Chem Systems, Inc.).

resist water or solvent washing. Solid particulate can blind the filter. This problem is often solved using prefilters or prescrubbers.

Operating Principles

A fiberbed filter uses a densely packed bed of microfibers placed in the path of the contaminant gas stream. The fibers become obstacles that the gas and contaminants must traverse. The closely spaced arrangement of the fibers improves the probability that a contaminant, such as a liquid aerosol or acid mist, will adhere to and coalesce upon the fibers. As this procedure progresses, the liquid builds up to a point at which it can drain by gravity.

Primary Mechanisms Used

Fiberbed filters operate using three basic mechanisms: impaction, interception, and Brownian diffusion. Impaction and interception are popular mechanisms used in various gas cleaning devices. Brownian diffusion, however, is primarily found in use in fiberbed collectors.

As air containing particulate flows through a filter, the air flows around any obstacle (such as a filter fiber) that is in its path. But a particle with sufficient mass and momentum (such as a 5-pm particle) will not. Instead, the particle's inertia will cause it to continue along its original path until it strikes a filter fiber and is collected. This is termed as impaction.

Somewhat smaller particles, those in the range of 1-3 pm, are collected by interception. Because these smaller particles have less mass and therefore less momentum, they tend to follow the airstreams around a filter's fibers. However, they can stray a bit from the normal streamline and can graze the side of a fiber and be collected.

Small particles (less than 1 pm) have little mass and as a result follow the air as it winds its way through a filter. These particles have substantial random motion, called Brownian diffusion, due to collisions with nearby air molecules. This almost vibratory motion allows them to move independently of the motion of the bulk airstream. Like gases and chemical solutions, the particles tend to migrate or diffuse from areas of high-particle concentration to areas of low concentration. As the particles contact the filter's fibers and are collected, the concentration in the air near the fiber surface goes to zero. This cycle of diffusion and collection drives the removal of the submicron particles.

Because slower operating velocities increase the time available for the diffusion to occur, fiberbeds have infinite turndown capability. As the collected particles coalesce into larger droplets on the fiber surface, they drain from the filter by gravity.

One of the pioneering fiberbed designs was the Brinks mist eliminator. Manufactured by Monsanto Enviro-Chem, the fiberbeds are made from glass or polymer microfibers often in the form of candles. Figure 8.10 shows a Brinks fiberbed mist eliminator.

FIGURE 8.10

Brinks mist eliminator (Monsanto Enviro-Chem Systems, Inc.).

Design Basics

Fiberbed filters operate at inherently low vapor velocities both to maximize performance and to minimize pressure drop. Face velocities of 0.5 ft/s or less are common. In general, the higher the liquid loading, the slower the required gas velocity. This often results in a significant number of candles for even low gas volume applications.

An inner and outer cage usually supports each candle. The cage may be made from metallic or nonmetallic mesh of high open area. These cages retain the compressed fiber material that is captured between the cages. The outer cage is typically designed to be removed for repacking.

Because there is a time delay within which the captured aerosols or mists coalesce, a new candle can take several hours to wet out. The fiberbed achieves its best performance after the fibers are coated with a film of liquid (provided by either the contaminant itself, an irrigation system, or an administered fog or mist). It is not uncommon for a fiberbed to exhibit low efficiency when new.

The candles themselves typically use a mounting flange that is bolted to the tube sheet. The tube sheet must be designed for the laden weight (wet weight) of the fiberbed candle, not just its dry weight. Given that the tube sheet is weakened by the openings required for the candles, special care must be taken in stiffening the tube sheet sufficiently.

The accumulated liquid must be given a path through which it can drain, otherwise the candle retains the liquid, and its effective open area decreases. Small J-shaped traps are often used on each individual candle to allow the liquid to drain, while preventing liquid from bypassing the candle and reducing efficiency. These traps must be filled with liquid before operation. They must also be of sufficient depth to seal at the maximum anticipated pressure drop. This usually results in a seal leg of 12-18 inches overall length.

Operating/Application Suggestions

Fiberbed filters can provide reliable service on applications where the contaminants flow from the filter media rather than being retained on the media. It is not unusual for candles to be used for many years without replacement in acid recovery service, for example.

Following are some measures that can be taken to maximize the useful life of a fiberbed system.

Filter Cleaning

Fiberbed filters cannot be cleaned in the traditional sense, as their structure is delicate and easily damaged. Accumulations of soluble materials such as salts can be removed by irrigating or flushing the filter with water or another suitable liquid. Waxes and tars can often be removed by heating the filters indirectly through injection of low-pressure steam into the filter vessel. Several hours of heating (with the system shut down) can liquefy waxes and other materials, enabling them to drain from the filters. Detergent sprays can sometimes also be used to flush insoluble materials from the filters, but this procedure usually must be done daily to remove the insoluble materials before they accumulate.

Fiberbed Filter Life

Fiberbed life in any given application is determined by four major factors. These are the concentration of foulants (materials not draining from the filters), fiberbed surface area, starting pressure drop of the filters, and the pressure available from the exhaust blower. As foulants build up on the filters, the pressure drop across the filters increases. When the limit of the fan static pressure capacity is reached, the filters must be replaced.

While the foulant concentration cannot be changed, the other three items can. Increasing the number of filters both increases the surface area and decreases the pressure drop. Increasing the pressure capability of the fan further increases fiberbed's life, because this allows the pressure drop to increase further before reaching the fan's limit.

Because all the pressure capability of the fan is not needed when the filters are clean, a damper or variable frequency drive (VFD) is used to control exhaust flow. A damper would be mostly closed at startup, and a VFD would be running the fan at a low rpm. As the pressure drop increases, the damper is opened or the VFD speeds the fan up to maintain flow. When the damper is fully open or the fan is running at maximum speed, the limit of the system has been reached and the filters should be replaced.

With all these variables it is difficult to generalize, but in fiberbed systems professionally designed for the application, filter life is usually anywhere from 2 to 6 years.

Fire Protection If the Contaminant Is Combustible

Fiberbeds are often used to collect combustible contaminants. This can be accomplished safely if a few precautions are taken.

Fire protection is an important part of any system collecting combustible materials. Fires usually begin upstream of the fiberbed system, for example, in a direct-fired oven. If the fire spreads to the oil-saturated fiberbed filters, they may catch fire. Burning fiberbeds is difficult to extinguish because their thick walls act as an insulator.

Water sprinklers are the best choice for fire protection, because they can be used to flood the fiberbeds. Water not only extinguishes the fire but also carries away heat, reducing the possibility of reignition. Isolating the fiberbed chamber and smothering the fire with steam or carbon dioxide can also be used. In any case, the filters should be removed from the vessel as soon as possible after a fire and monitored to ensure they do not reignite.

Fire detectors are quite useful in minimizing fire damage. They should be located on the inlet and the outlet to the system and should be tied into the control system to shut down the system fan (to reduce the available oxygen), sound an alarm, and activate diversion dampers if used. They are available in a variety of temperature ranges and should be selected based on the maximum temperatures expected in the application to avoid unnecessary shutdowns.

Fire dampers can also be used to minimize the spread of a fire. The damper is located on the inlet to the fiberbed system and closes when temperatures indicative of a fire is detected. This stops the flow of air through the filter vessel, which can occur even if the exhaust fan is shut down, due to chimney effect.