With the passage of clean air legislation and increased scrutiny of air emissions by federal, state and local regulatory agencies, well-designed and maintained air pollution control devices are no longer optional but mandatory for many finishing and chemical manufacturing processes.
Many states have implemented or plan to adopt the recently promulgated EPA standards for chromium emissions. Other states and municipalities have enacted even stricter requirements for chromium and other toxic air emissions.
In many states, compliance with hard chromium emission standards must be demonstrated by stack tests. EPA's revisions of national ambient air quality standards for particulate matter from 10 microns to 2.5 micrometers, presently referred to as PM10, forces scrubber technologies controlling emissions from plating operations and others to improve efficiency.
Scrubber technologies have progressed greatly. New scrubber packing designs provide for greatly increased mass transfer (transfer from the gas to liquid phase) and for more uniform distribution of the scrubbing solution throughout the packing section.
High-efficiency mesh pad mediums are designed into wet and dry scrubber systems. These mediums remove acid or alkaline mists. Some mesh pad configurations remove particles as small as one micron. High-efficiency pads now allow the use of HEPA arrestors that increase scrubber performance.
Custom-designed, high-efficiency scrubbers are now required. Most of the new units are designed for maximum corrosion resistance. However, if good operating procedures and regular maintenance are neglected, these units can become fouled and plugged, or the scrubbing solution can become too contaminated for absorption to take place. This results in excessive emissions, decreased air flow or both. As with any piece of equipment, the scrubber requires proper and regular maintenance.
Emission Requirements. The reason for developing high-efficiency scrubber technologies is the tightening emission standards for Toxic Air Compounds (TOC) across the country. Some states enforce emission rates at the top of the stack, as does California for chromium compounds. However, many states enforce these emissions at the company's property boundary, fence line or nearest critical receptor. The maximum ground level concentration (GLC) allowed at the company's property boundary is knows by several acronyms that vary from state to state. For many compounds, the GLC is established as one pct or 1/100 of the Threshold Limit Value (TLV), which is the maximum concentration of a compound that OSHA allows in the workplace.
However, for known carcinogens such as hexavalent chromium, the property line GLC is based on risk data and is much lower. For example, in North Carolina the Acceptable Ambient Level (AAL) for chromium compounds at the company's property boundary is 8.3 × 10-8 milligrams per cu meter. In Texas the GLC is 0.1 microgram per cu meter. In New York the Hazard Limiting Value (HLV) is 4.5 × 10-5 micrograms per cu meter. In new Jersey an individual risk assessment must be done for each stack, demonstrating that the maximum incremental cancer risk is at or below one in one million.
To ensure that these property line emission rates are met, most states require proof of the maximum stack concentration (MSC) through either a calculation using reliable data or a stack test. The resulting stack emission is then simulated to the company's property boundary through the use of computer modeling. Two methods of modeling are used: Screen modeling or the more accurate and expensive refined model. These meteorological simulations of emissions that cross the company fence line use variables that include MSC, stack height, stack velocity, distance to the property boundary, terrain, wind factors and downwash, among others.
Packed bed scrubber. One of the best available control technologies for reducing vaporous emissions of water soluble acids is the wet packed-bed scrubber. There are other technologies more suitable for emissions that evolve as mists, but for compounds that evolve as a gas or vapor (no measurable particles in the air stream) and are soluble in water the wet scrubber works best. The basic wet scrubber consists of a vessel with adequate cross-sectional area to provide the correct air velocity, a depth of packing media for distributing scrubbing solution, recirculation system to provide the correct volume of solution throughout the packing and a mist eliminator to prevent excess scrubbing solution from re-entraining into the exhaust stack. Most effective designs are either crossflow (horizontal) or countercurrent (vertical) flow. The principal behind the removal of gases in a wet scrubber is gas absorption.
A scrubber first reduces the velocity to 500 fpm or less. To accomplish this the scrubber vessel must have sufficient cross-sectional area relative to the volume of air exhausted. A small cross-sectional area will result in a velocity too fast for gas absorption.
Second, the gas stream passes through a bed of structured or randomly packed media. There are many types, sizes and configurations of random packings marketed today (spherical, tubular, saddle, tellerette, rings, as well as polyhedron designs). They range in size from one to five inches in diameter, in pressure drop and in sq ft of contact per cu ft. Therefore, they should be designed so that when randomly dumped into the scrubber vessel sufficient void spaces are available to permit uniform irrigation throughout the packed bed. The packing is not a filter, but a vehicle for uniformly distributing scrubbing solution, which enables the solution to be more efficient for gas absorption. Packing that channels solution to one area causes another area of the pack to remain dry. Gas cannot be absorbed in the dry area and will not be collected.
Scrubber spray chambers and spray nozzles should be designed to thoroughly saturate the media. Most crossflow scrubber designs incorporate sprays in front of the pack. Depending on the depth of the pack, top-mounted sprays irrigate the packing from overhead. Countercurrent scrubbers incorporate sprays above the packed bed, enabling the distributed scrubbing solution to trickle down through he packing, effectively contacting the contaminated gas stream moving upward.
Solid-cone spray nozzles are effective distributors, but have greater potential for plugging if solid particles are present in the recirculated solution. Hollow cone nozzles are a lower maintenance alternative when the potential for plugging is present. However, it is critical to use a sufficient number of nozzles for packing irrigation to ensure that no portions of the packed bed remain dry.
Regulatory changes such as PM2.5 have forced closer scrutiny and higher efficiency control of high concentrations of acid or base vapor evolving from pickling operations, bright dip operations, chemical manufacturing, odor control or municipal waste systems. Scrubbers must be custom designed to properly treat concentration and solubility characteristics of the contaminant.
Packing depths of five to eight gpm/sq ft are common for controlling high concentrations of HCL and HNO3. They use versatile monitoring instrumentation that permits precise injection of a base solution into the recirculation system to help neutralize the acid vapor and increase the scrubbers capability for absorption. Multiple stage systems using two or three scrubber units in series with six to 12 ft of pack each and an irrigation rate of six to 12 gpm/sq ft each, are often used in control of NO and NO2 from bright dip operations, aluminum or titanium etching processes, and also to control SO2 and H2S gages from municipal waste treatment systems. Each stage could incorporate a specific chemical additive into the recirculation solutions to neutralize, oxidize or deoxidize the contaminant, depending on the nature of that contaminant.
The final stage of the scrubber must be an effective mist eliminator to prevent re-entrained scrubbing solution from going to atmosphere. The two most common mist eliminator designs are chevron baffle angles and mesh pads. Chevron baffles are designed to create abrupt directional changes in the air stream, thereby impinging wet particles onto the baffle's surface or catchment. Chevron baffles are typically effective on larger particles (12 to 100 microns). Because plugging potential is minimal, they are considered low maintenance mist eliminators.
Mesh pad eliminators used in wet scrubbers usually consist of woven or knitted polypropylene monofilament in either a random or specific configuration. They work by mechanical impingement and are velocity dependent. The particle to be collected must be traveling with sufficient velocity to impact and adhere to the fiber. These are designed to remove specific particle sizes.
Smaller monofilaments and tighter voids are obviously more efficient in removing smaller particle sizes (one to three microns). But the downside is they have a greater potential for plugging and require more care and maintenance. The most effective mesh pad has a filament diameter and void space large enough to minimize plugging potential and have a functional liquid drainage capacity. This mesh pad configuration is usually effective on particles five to 10 microns in diameter.
Wet Scrubber Maintenance Considerations. Following are some maintenance tips to keep scrubbers working properly. They are relatively simple and should be followed closely.
Scrubber spray chambers and nozzles should be inspected regularly (one to two times a week) to ensure they are not plugged. Poor irrigation can also cause the packing section or mist eliminator to become plugged or fouled. Therefore, it is imperative that each nozzle sprays at its designed flow rate.
The packing section should be inspected often to ensure against solids buildup that would plug portions of the pack. This could result in increased static pressure, reducing ventilation and preventing irrigation of other portions of the packed section. This can result in excess emissions. If contaminant buildup on the spaced section occurs, the scrubber recirculation tank should be drained and replenished with fresh water, and perhaps an applicable solvent added. The packing section should then be flushed to break down trapped solids. The process may have to be repeated, depending on the degree of fouling.
If maintenance has been neglected and the severity of the plugging is too great, the tedious job of removing, cleaning and re-packing must be done. Most scrubber designs incorporate an access door for this purpose. However, if maintenance inspections are done on a regular basis this should not occur.
The scrubber mist eliminator should also be inspected on a regular basis. The catchment on a chevron baffle can become filled with solids, rendering it ineffective. The mesh pad eliminator is even more prone to plugging. Although most scrubber designs do not employ a washdown spray chamber for the mist eliminator, many do provide access areas for spraying the eliminator manually or removing the mesh pad eliminator for a more thorough cleaning.
Additionally, the scrubber recirculation system should be keep reasonably clean to ensure the solution is capable of gas absorption; to minimize buildup of solids in packed and mist eliminator sections; and to prevent plugging of spray chambers and nozzles.
Most scrubber recirculation designs recommend a continuous addition of fresh water. Up to five pct of total recirculation rate should be added to the recirculation tank and simultaneously overflowed to waste treatment. This makes up for evaporation of the recirculated solution occurring in the scrubber and helps keep the solution from becoming too contaminated. The recirculation tank should also be kept clean of sediment. These solids are easily stirred up and will inevitably contribute to plugging of spray nozzles, packing sections and the mist eliminator section.
Preventive maintenance components. Inspection and maintenance of the wet scrubber system can be made easier with the addition of accessories to monitor scrubber operation and ease the maintenance burden.
Pump flow meter. Any scrubber design has a specification for the required recirculation rate based on scrubber capacity, inlet concentration and packing depth. Monitoring this recirculation rate gives the assurance that the designed flow rate is maintained for proper irrigation of the packed section at all times. The pump flow meter reads out the number of gallons being sent to the scrubber from the pump discharge. If the reading is lower than the designed recirculation rate, it is evidence that either the spray chamber or some spray nozzles are plugged.
Wye strainer. The wye strainer is a simple, inexpensive filter installed in the piping between the pump discharge and the scrubber spray chamber. It collects particles from the recirculation flow that could potentially plug the spray system and perhaps the packing section. The advantage of the wye strainer is that it is much easier to clean than the spray chamber, nozzles or packing.
Magnahelic Gauge. This is an analog gauge that gives a constant reading of the pressure differential or pressure drop across a given medium. It is typically used to monitor the pressure drop across the scrubber packing media or mist eliminator. Certain types of random or structured packing and mesh configurations have specifically designed pressure drops based on density, void fractions, irrigation rate and gas velocity. If the magnahelic gauge readings begin to increase above designed pressure, it indicates that contaminants are building up on the media and corrective action should be taken.
High-efficiency mist eliminator. In the past it was believed that the best available control technology for all emissions, vapors and mists from typical plating and finishing processes was the wet scrubber. As emission standards for many of these chemistries, particularly chromium compounds, became more stringent, the high-efficiency mist eliminator became an increasingly viable option with regard to removal efficiency, water saving and economics. Many chemistries evolve in mist form, including but not limited to, sulfuric acid, nickel compounds, sodium hydroxide, nitric acid and chromium compounds.
Chromium compounds are best controlled by the mist eliminator. Multi-stage mist eliminators for controlling chromium emissions have been tested by EPA and private testing firms with excellent results. Documented test reports show emission rates from some units exceeded the California standard by 60 pct and surpassed the expected Federal MACT standard by a factor of as much as 100.
Most multi-stage designs incorporate a principal of variable stage removal. The rationale is to collect most of the particle in the gas stream in the first stage, thereby protecting the subsequent stages that are designed to handle the smaller remaining particles in the gas stream. A good way to accomplish this has been a three or four-stage unit, using a larger monofilament and voids configurations in the first stages and gradually decreasing monofilament size and voids downstream from stage to stage. Each stage is flushed with fresh water. Individual drain chambers should be used to prevent the wash water with the highest concentration of contaminant in stage one from flowing into stage two.
Mist eliminator maintenance considerations. Because of the plugging potential of high-efficiency mesh pads and the thick consistency of many of the chemistries that it controls, mist eliminators must be washed down on a regular schedule. If wash down is neglected, the contaminants can solidify deep inside the pads, and flushing the pad is ineffective. At this point, manually cleaning the pad is the only option.
This can be difficult, especially on larger units. It can be prevented if a proper wash-down schedule is maintained. A magnahelic or photohelic gauge should be used to monitor the pressure drop of each individual stage of the eliminator. If gauge readings climb, that stage should be washed down immediately for the duration specified by the manufacturer.
A control panel for automatically timed wash down of the mesh pads remembers to wash down the pads in the event maintenance personnel forget. This is an excellent method of fool-proofing your system so that flushing the mesh pads will not be neglected.
Most mist eliminator designs incorporate access doors to facilitate the cleaning process. Removal and thorough cleaning of the media should be completed on an annual basis. However, special care should be taken when reinstalling the media into the vessel to ensure proper fit, and to make certain that no gaps remain between the media and vessel wall. Because of the high pressure drop across the mesh pad, the smallest gap could provide a path for bypass. A gap could cause non-compliance.
Technology for controlling acid or base emissions has never been better and maintenance responsibilities have never been greater. Technology enables the latest emission standards to be met, but knowledgeable maintenance personnel and a well-designed maintenance schedule ensure continued compliance.