environmental controls

AIR POLLUTION CONTROL IN THE

FINISHING INDUSTRY

BY GORDON HARBISON

DÜRR ENVIRONMENTAL INC., PLYMOUTH, MICH.

Being responsible for reducing volatile organic compound (VOC) emissions inpaint and coating operations seems to be akin to a quest to circumnavigate theglobe. At the end of your quest, you are right back where you started. If customcoaters are not able to convert to “environmentally friendly” coating alternatives,such as waterbornes, UV-cure or powder coatings, they must deal with everincreasingemission regulations through some kind of VOC control technology.Choosing the right equipment for VOC control applications depends primarilyon the exhaust air volume and the average concentration of VOCs.

VOC CONTROL PRIMER

VOC Destruction

Thermal oxidation is a process whereby most of the VOCs are broken downand recombined with oxygen to produce water vapor and carbon dioxide. Thewater vapor and carbon dioxide are naturally occurring and environmentallyfriendly, therefore safe for venting into the atmosphere. Thermal oxidationoccurs by heating the polluted air to an elevated temperature (typically 1,300°Fto 1,800°F). At such temperatures, the pollutant molecules spontaneouslydisassociate and recombine with available oxygen to create the carbon dioxideand water vapor. The efficiency of oxidation and the design of most oxidizers isgoverned by the residence time, the combustion chamber temperature and theamount of turbulence the air stream sees.

Catalyst Improves Fuel Efficiency

A Catalyst is a substance that promotes oxidation without being consumed bythe process. VOC catalyst can be added to the combustion chamber of almostany oxidizer to promote VOC destruction at lower operating temperatures (typically600°F to 900°F), lowering fuel usage.

Note: Catalytic oxidizers are only suitable for processes whose constituents willnot adversely affect the life of the catalyst.

VOC Capture

Concentrators take advantage of a chemical surface phenomenon and the tendencyof VOCs and other pollutants to adhere to certain types of materials suchas activated carbon and zeolites. Adsorbent media are selected for their tendencyto attract pollutants as well as their high surface area — qualities that allow themto trap and hold more pollutants. When emission gases pass through the adsorbentmedia in a concentrator the pollutants stay behind, trapped in the media.The pollutants can then be removed from the media by desorption — passinga much smaller quantity of very hot air through the media. The smaller volumeof desorption air contains a very high concentration of pollutants that can bedestroyed efficiently by oxidation.

CONCENTRATOR/OXIDIZER SYSTEMS

Combining technologies creates “Capture & Control” systems that use an integratedconcentrator and final treatment system to process large volumes ofprocess air, concentrate VOCs in a smaller volume of air, destroy the pollutantsin the air and use the heat from the destruction process as part of the concentrationprocess.

OXIDATION TECHNOLOGY

The most reliable and acceptable means of destroying VOCs, HAPs, and odorsavailable today is thermal oxidation. Oxidation, typically, is an energy intensivetechnology wherein a polluted air stream is heated to a high temperature setpointthat is predetermined by the nature of the pollutant. The simplest form ofan oxidizer is a direct-fired burner that elevates the air temperature from incominglevels to combustion levels. Because of the high cost of heating the processexhaust stream to the required oxidation temperature most thermal oxidizersincorporate sometype of primary heat recovery. Primary heat recovery transfersenergy from the hot clean gas stream exiting the oxidizer into the incomingpolluted gas stream. This reduces the amount of additional energy requiredto achieve the oxidation temperatures. There are two widely used methods ofrecovering this thermal energy, recuperative and regenerative.

Oxidizer Selection Criteria

In order to select which type of oxidizer is most advantageous for a specificapplication, the following information must be known:

• Process exhaust flow rate: If the process exhaust stream flow rate is belowabout 3,000 scfm, regenerative systems are generally not practical. This isbecause the fuel savings gained by the highly efficiency regenerative heatrecovery is generally not sufficient to offset the increased capital costand maintenance of the RTO when compared to a recuperative or directflame system. At flow rates above approximately 25,000 scfm, direct flameoxidizers are at a severe economic disadvantage because of their very highfuel cost. However, it is not unheard of for direct flame systems of thissize or larger to be installed where secondary heat recovery boilers can beused to offset the high fuel cost. Another case where direct flame systemsare favored for large air volumes is for emergency or stand-by systemswhich operate very few hours per year.

• Process exhaust stream temperature: If the polluted waste gas streamtemperature is above approximately 600°F, regenerative systems aredisfavored because the high temperatures can reduce the reliability andlongevity of the valve system. In addition, at these temperatures, thereis less difference in fuel consumption to justify the additional cost andcomplexity. If the exhaust temperature is significantly above 1000°F,recuperative systems are disfavored versus direct flame systems againbecause the difference in fuel consumption becomes too small to justifythe added first cost.

• Pollutant concentration levels: The concentration of pollutants in thewaste gas stream can have a major impact on the selection of the type ofthermal oxidizer system. Direct flame oxidizers are capable of handlingthe broadest range of hydrocarbon concentrations, from parts per billionlevels to pure hydrocarbon vapors. For waste gas streams with concentrationsover 25% LEL, special considerations are routinely taken to preventflashback from the oxidizer to the waste generating source.The cost ofthis flexibility is the high fuel cost for this type of oxidizer. Recuperativeand regenerative oxidizers are limited gas streams with less than approximately25% LEL but for different reasons. For a regenerative system, thisrestriction is primarily due to the danger a thermal run-away situation.In a thermal run-away, the oxidation of the excessive hydrocarbon concentrationcauses the combustion chamber outlet temperature to rise.This additional heat is recovered by the heat exchange system, whichincreases the combustion chamber inlet temperature, causing a furtherincrease in the combustion chamber outlet temperature and so on untilan excessive temperature is reached. A regenerative system is vulnerableto thermal run-away because they are capable of auto-thermal operation.This is a situation where the heat produced by oxidation of the pollutantsis enough to operate the system with no additional input from theburner. In auto-thermal operation, the burner can be shut down and theoxidizer will sustain operation as long as the hydrocarbon loading is highenough. A recuperative thermal oxidizer on the other hand is not capableof self-sustaining operation. In fact, they are purposely designed to avoida self-sustaining situation because this type of operation will overheatand damage the heat exchanger. The burner must always operate toprovide the additional heat tobring the pre-heated waste gas to the fulloxidation temperature. As the pollutant loading increases the burner willthrottle back by an amount equal to the heat of oxidation. However, if theburner throttles back too far, the oxidation reaction will not be properlyinitiated and the combustion chamber temperature will crash.

• Type of Pollutant Process: Exhaust streams that contain high levels ofacid or compounds that convert into acids (Chlorine, Fluorine, Bromine,Sulfur, etc.) must be treated with special care. Any of these elements, whichare present in many important industrial solvents and cleaning agents willattack metal alloys at high temperatures and can form highly corrosiveacids in the presence of water at low temperatures. With special materialsof construction and design techniques all types of thermal oxidizers can bemade to resist low levels of these elements. However, if the levels of acid arehigh or unpredictable, a direct flame type oxidizer is most preferred. Thisis because this type of oxidizer has no heat transfer system to be corrodedby the acids.

• Particulate Emission Levels Process: Exhaust streams containing particulatemust be given special consideration. There are a great number ofwaste gas sources that contain both gaseous hydrocarbon pollutants andparticulate pollutants. In most cases, the particulate can be filtered outupstream of the thermal oxidizer. However, in many cases, it is possibleto avoid the additional complexity and cost of a filtration system throughproper selection of the thermal oxidizer and its operation. Particulatecan be broken down into two basic categories, organic and inorganic.An example of an organic particulate is an oil mist from machiningoperation. This type of pollutant will either accumulate in the ductworkand cooler parts of the thermal oxidizer or penetrate to the combustionchamber. Any particulate that accumulates in the cooler parts may needto be periodically cleaned out. Obviously, provisions must be made inthe oxidizer design to allow cleaning. In general, any type of thermaloxidizer is capable of handling purely organic particulate. However, asthe total loading increases, increasing amounts of maintenance will berequired. One feature of regenerative type systems for these applicationsis that the can be programmed to perform a thermal self-cleaning or bakeout. This process brings heat from the combustion chamber into thelower portions of the heat exchange media and valves and can burn offaccumulated organic material. With this feature, regenerative systems arefavored in high organic particulate applications because the manpowerand disruption to operation is minimal for a bake out compared tocleaning of other types of systems. Any organic particulate that enters thecombustion chamber will be oxidized as any other hydrocarbon would.Oxidation of a particle takes longer than a gas because the particle mustfirst be broken down and volatilized before the thermal oxidation reactioncan take place. This takes time and therefore, a thermal oxidizerwith sufficient residence time to oxidize gaseous compounds, may beinadequate for particulate. In this case, the oxidizer would have elevatedhydrocarbons in the exhaust from the partially oxidized particulate andwould also show elevated levels of carbon monoxide. If the particulateis fine, less than about 10 micrometers, and of low concentration, lessthan about 10 grain/standard cubic foot, adequate performance can beachieved with an oxidizer of normal design. It may be necessary to raisethe operating temperature by 100°F or so to achieve required emissionperformance. For significantly higher levels or sizes, some pre-filtrationis usually favored. Inorganic particulate presents different challenges.Inorganic particulate can be any of a wide variety of substances rangingfrom common dust, to soil, metals, paint pigments or salts. Each typehas specific characteristics and therefore requires special considerationsin oxidizer design. Inorganic compounds can react with oxidizer components,fuse and foul certain parts, accelerate corrosion or cause erosiondamage. Because there are such a wide range of possibilities, no generalguideline can be given that would cover all inorganic particulate.

• Required Pollutant Control Efficiency: Many federal, state and local VOCand HAP emission limits for surface coating operations are expressed interms of one or more of the following:lb per gallon minus waterlb per gallon coating solids as applied (e.g. as sprayed)lb per gallon of applied coating solids (e.g. auto & light truck)These limits may be met either by applying coatings meeting these emissionlimits without add-on controls or achieving an equivalent limit with add-oncontrols. For auto and light truck surface coating operations, the paint solidstransfer efficiency (TE) is part of the calculation. Some state and local regulationsrequire a minimum TE for certain coating operation inaddition to a VOCor HAP content limit.If a catalytic or thermal oxidizer is used to control VOC or HAP emissions, 95%minimum destruction efficiency is generally required. An overall 90% minimumVOC or HAP destruction efficiency is generally required if a carbon or zeoliteadsorber is used to concentrate emissions prior to destruction in an oxidizer.However, 80% combined system destruction efficiencies have been allowed for plasticparts spray booths employing a carbon adsorber in series with a thermal oxidizer.At least one permitting agency requires a minimum VOC control efficiencyfor major sources and others allow it as an alternative to laborious record keepingrequired to demonstrate compliance with individual coating emission limits.

• Ohio Administrative Code (OAC) 3745-21-07 (G) Operations UsingLiquid Organic Material requires discharge of organic materials (i.e.VOC) be reduced by at least 85% from applying, evaporating or dryingany photochemically reactive material and any liquid organic materialthat is baked, heat-cured or heat polymerized.

• Section 215.205 22 Illinois Regulation 11427 allows operators of coatinglines alternative emission limitations to individual coating emissionlimits for emissions controlled by an afterburner (thermal oxidizer):81% (75% for can coating) reduction in the overall emissions of volatileorganic material from the coating line, andOxidation to carbon dioxide and water of 90% of nonmethane volatileorganic material (measured as total combustible carbon) which entersthe afterburner.

• Under South Coast Air Quality Management District (SCAQMD) ofCalifornia (Los Angeles) Rules 1107 Coating of Metal Parts & Productsand 1145 Plastic, Rubber and Glass Coating, lines may comply with theseregulations using pollution control equipment provided VOC emissionsare reduced as follows:

• The control device shall reduce VOC emissions from an emission collectionsystem by at least 95% by weight or the output of the air pollutioncontrol device is 50 PPM by volume calculated as carbon with no dilution.

• The owner/operator demonstrates that the system collects at least 90% byweight of the emissions generated by the sources of the emissions.Other examples of minimum required or allowable VOC and HAP collectionand destruction efficiencies can be found in various federal, state, and localregulations.In many cases the most advantageous type of oxidizer can be selected based onthe following general guidelines. In other cases two or more oxidizer types maybe practical and a detailed economic analysis based upon your specific costs offuel and electricity will be required to determine the best selection.

Recuperative Oxidizers

A recuperative oxidizer is a direct-fired unit that employs integral primary heatrecovery. To minimize the energy consumption of the oxidizer, the hot air exitingthe combustion chamber is passed over an air-to-air heat exchanger. The heatrecovered is used to preheat the incoming pollutant laden air. The primary heatexchangers are usually supplied as either a plate-type or a shell and tube typeheat exchanger. These heat exchangers can be designed for various heat transferefficiencies, but the nominal maximum is 70%. Thus by the addition of a heatexchanger, the net heat load on the burner can be reduced by up to 70% of thatrequired in a DFTO. The addition of the heat exchanger, because it is made ofheat corrosion resistant alloy, substantially increases the cost of the oxidizersystem. Also, the fan for moving the polluted gas through the oxidizer must bemore powerful to overcome the additional pressure drop of the heat exchanger.In most cases, the savings in fuel will more than offset the additional up-frontcost within the first two years of operation, however, even with 70% heat recovery,recuperative oxidizers can be expensive to operate, especially if the airflowis large and has dilute concentration levels, unless additional secondary heatrecovery can be applied to the customer’s process.

Regenerative Thermal Oxidizers (RTOS)

A regenerative oxidizer is also a direct-fired oxidizer that employs integral primaryheat recovery. However, the RTO operates is periodic, repetitive cyclerather than a steady state mode. Instead of conventional heat exchangers whichindirectly transfer heat from hot side to cold side across the exchanger walls,RTOs use a store and release mechanism. The hot gases exiting the combustionchamber of an RTO are made to pass over a bed of inert and temperature tolerantmedia with a high heat capacity. The temperature difference between thegas and the media causes heat transfer to occur between the gases and the bed.The heat storage media is either a granular or structured form of heat resistantceramic. Once the bed has been saturated with heat, the air flow is reversedand redirected by a valve mechanism. Reversed flow allows the cooler processair to pass over the hot bed, and hence become preheated before entering thecombustion chamber where the remaining heat is provided by a burner. Thehot gas is redirected to a cold bed (one that just completed being an inlet bed)and “regenerates the bed, making it hot and ready for the next pre-heat cycle.In other words, one bed (or chamber) is used as a heat source and one is used asa heat sink. The flow through an RTO must be frequently reversed in order tomaximize heat recovery and media regeneration.The nature of an RTOs heat recovery process requires it to have at least twobeds of appropriate heat recovery media. In many applications, the additionalstep of purging a bed before reversing the flow through it from inlet to exhaustis necessary to maintain very high destruction efficiencies. This purge stepcreates the requirement for an additional (or odd number) chamber makingthe RTO more complicated and more expensive than a recuperative oxidizer.RTO systems can utilize more than two beds (operating in parallel) in order to becapable of handling larger air volumes. The primary advantage of an RTO is loweroperating costs due to high heat recovery and low fuel consumption. Depending onthe mass of media included in an RTO, heat recoveries of up to 95% are common.Because of their capability for high heat recovery, RTOs are often operated in an“auto-thermal” or self-sustaining mode, where the heat content of the VOCs beingoxidized is enough to sustain the combustion chamber temperature at setpoint,requiring no external fuel input.RTOs are a well-proven technology, but are being called on to become moreefficient than ever, to reduce operating costs to even lower levels than havetraditionally been seen. That challenge has been met by developing improvementsin heat transfer media, alternative oxidation technology and fuel usageoptimization techniques.

• Heat Transfer Media: Traditionally, the heat transfer beds of an RTOare composed of ceramic saddles, randomly packed into an insulatedchamber. The airflow through the saddles is forced to make manychanges in direction and velocity. Due to the turbulent nature of theairflow, the pressure drop across the bed increases with the square ofthe airflow. Dürr’s investigations into the fundamental principles ofRTO operation led to the development and application of a structuredheat transfer media. These investigations indicated that a heat transfermedia having straight airflow passages of constant cross-section offersignificantly improved performance over traditional saddles by providingmore laminar airflow characteristics. The improved performancecan be seen in a lower pressure drop across the packed beds of an RTO.Structured packing is a ceramic monolithic block, composed of silicaalumna ceramic. Each block is approximately 12” tall, 6” wide and 6”long, and has hundreds of parallel passages, each approximately 1/8”square, extending from top to bottom. It’s physical and performancecharacteristics allow for a higher airflow velocity through a packed bed,resulting in a more compact RTO which is attractive to land-lockedplants that may not have the normal space required for an RTO. Thishigher bed velocity also allows for a unique solution to plants thathave existing RTO equipment that may require additional airstreamtreatment capacity. Increased flow in a traditional saddle packed bedrequires an exponential increase in pressure drop and motor horsepower,quickly overloading existing handling capacity. Replacementof an existing saddle bed with ceramic monolith can not only reducethe pressure drop for existing capacity, but also provide almost a 40%increase in incoming airflow capacity with the existing motor and fan,while providing better thermal performance, lowering the natural gasconsumption of the RTO.

• Regenerative Catalytic Oxidation (RCO): RCO’s are a recent hybridVOC abatement technology that is gaining acceptance in plants whereenergy cost are high and the hours of operation are long. An RCOcombines the benefits of an RTO with the benefits of catalysis. Byaddinga precious metal catalyst to the combustion chamber of an RTOsystem, the catalyst provides hydrocarbon conversion at a much loweroperating temperature than an RTO, typically 600°F to 1000°F, whichthereby reduces the auxiliary fuel requirements. The precious metalcatalyst, like all catalysts, is a substance which accelerates the rate of achemical reaction, i.e. oxidation, without the catalyst or the substancebeing consumed. Another benefit of a precious metal catalyst is its abilityto eliminate not only VOCs, but also secondary products, notablyCO and NOx. In addition, a precious metal-based catalyst is much moreresistant to poisoning and fouling than base metal catalysts. Like structuredpacking, converting an existing RTO to an RCO is possible, andoften beneficial depending on the operating and energy consumptionconditions in the plant. Adding a layer of proprietary precious metalcatalyst on top of the ceramic media in the RTO’s combustion chamberwill allow the combustion chamber operating temperature to be loweredto roughly 800°F. In large air volume systems, this fuel savings canbe significant. The proprietary catalyst in Durr systems is impregnatedin the ceramic media of choice, either saddles or structured packing. Insome instances, an RCO system may not be a beneficial choice. Theseexceptions result from either the presence of a stream that containsorganometallic or inhibiting compounds that will cause degradationof catalyst performance. Each VOC stream needs to be examined toensure there are no catalyst poisons such as silicon, phosphorus, arsenicor other heavy metals. In addition, the catalyst performance couldbe masked or fouled by particulate in the air stream. However, thecatalyst can be recharged relatively easily. It is important to discuss theproperties of individual air streams before making any decisions onthe applicability of catalyst in an RCO, but for many, the potential foroperating cost savings is large.

• Natural Gas Injection (NGI): Typically a natural gas burner system isused to provide the energy required to make-up the heat that is notrecovered by a regenerative oxidizer (around 5% of the energy requiredto reach setpoint). An incoming airstream with a high enough concentrationof hydrocarbons, would provide enough energy from autoignitionof the hydrocarbons for the oxidation process to be self-sustaining,i.e. require no burner operation for make-up energy. NaturalGas Injection (NGI) is a means of artificially creating a self-sustainingcondition in an airstream with a low concentration of hydrocarbons. Anatural gas burner system is provided and utilized for system pre-heat.Once the heat exchange media is saturated and hot enough to elevatethe airstream above autoignition levels, the burner and combustionblower is turned off, and natural gas or methane is safely injected intothe incoming airstream, enriching it to the concentration levels necessaryfor self-sustaining operation. NGI actually improves the thermalefficiency of an RTO because it eliminates the requirement for combustionair being introduced, and thereby mitigates the mass imbalance inairflow between the regenerator bed that is on inlet and the bed that ison outlet. In commercial application, NGI improves an RTO’s thermalefficiency by approximately 1% or more overall. Another advantage toNGI is an improvement in NOx emissions from an RTO. The burner isthe single biggest contributor of NOx to the exhaust stream of an RTO,due to the high flame temperatures. Eliminating the burner from operatingsignificantly decreases the NOx levels seen in operatingRTOs.Due to the lower combustion temperatures of an RCO, NGI is not atool that is utilized in conjunction with catalyst. However, many existingsystems could see a decrease in operating fuel usage, by a simple,low cost retrofit that would install a Natural Gas Injection system tothe RTO, especially those airstreams not conducive to catalyst usage.

ADSORPTION TECHNOLOGY

Concentrators

Rotary concentrators are a continuous adsorption technology commonlyapplied to very dilute airstreams with relatively low hydrocarbon concentrations.Classified as a capture device, Rotary adsorbers can be used to concentratethe emissions into smaller airstreams with much higher concentrations(typically by a factor of 10 or higher) that can be handled by a smaller oxidationor destruction device much more economically. Continuous adsorption isachieved through the use of rotating media, a section of which is simultaneouslydesorbed. This design eliminates the need for dual running and stand-by fixedadsorption beds.The hydrocarbon-laden air passes through the rotary adsorption unit wherethe hydrocarbons are adsorbed onto an adsorbent media such as activated carbonor hydrophobic zeolite. The large volume of incoming air, now purified bythe adsorption process, is exhausted to atmosphere. The hydrocarbons whichwere adsorbed are then continuously removed from the media by desorptionwith a higher-temperature, low-volume airstream. This high concentrationdesorption air is delivered to an oxidation device for destruction.Concentration of hydrocarbons into a smaller airstream is a significant benefitto operating costs to a destruction device. By decreasing the airflow, thedevice is inherently smaller and less costly to purchase. By increasing the concentration,the auxiliary fuel benefit of the hydrocarbons is increased, in many cases,almost to the level of self-sustaining operation, where the customer’s natural gasrequirements are virtually eliminated. Traditionally, concentrators were appliedand justified on very large airstream volumes, but recent commercial applicationshave been on airstreams of 30,000 scfm and smaller.

Media Choices

The key to effective adsorption is the medium that is used. The most widely usedmedium is activated carbon because it is very effective, readily available and longlasting. Zeolite has also found a niche due to higher removal efficiencies for lowmolecular weight, polar, solvents.

Activated Carbon

Being relatively inexpensive and lightweight, with pores ranging from 1 to 50Ångstroms (Å), carbon can adsorb most paint solvents and even semiVOCs(SVOCs) such as plasticizers. Though widely used and preferred, activated carbonis not without disadvantages. The three primary drawbacks are:

1. Its combustibility, with the potential to promote a fire when heatedabove 600°F.

2. Its hydrophobic structure, which requires relative humidity control.Carbon’s adsorption capacity drops significantly at 50 to 60% relativehumidity. Reheat coils are often required, especially when controlling awet venture paint spray booth.

3. Impurities that naturally occur in carbon. These impurities can act ascatalysts and promote polymerization or oxidation of solvents such asmethyl ethyl ketone (MEK) and cyclohexanone, resulting in byproductsthat cannot be desorbed or that might be hazardous.In certain applications, a granular activated carbon (GAC) pre-filter isinstalled upstream of the carbon adsorption media. A GAC prefilter,often termed a sacrificial bed, adsorbs high boiling VOCs or SVOCs.GAC protects the activated carbon media from being saturated withcompounds that can not be completely desorbed by the limited desorptiontemperature (250°F) typically used with carbon media. A GAC bedalso dampens fluctuations in VOC content, typical of paint spray boothapplications, providing a relatively steady VOC concentration to thedownstream media.

• Hydrophobic Zeolite: Zeolites are sometimes called molecular sievesbecause of their crystalline framework with pores and interconnectingvoids. The resulting homogeneous pore size prevents molecules largerthan a certain size from entering the lattice. By varying the structureand pore size, the selectivity for various size solvent molecules can beachieved. Synthetic zeolite has a much greater adsorption capacity thancarbon at low solvent concentrations, but carbon has a higher capacityat high concentrations. Hydrophobic zeolite, a synthetic porous silicate,is non combustible and capable of withstanding temperatures as highas 1,100°F when coated on a ceramic, honeycomb structure. It can bedesorbed at 400° F, the working limit of the desorption section seals. Ahigher operating temperature allows the removal of solvents with boilingpoints above 175°C (350° F). Often, versatility is sacrificed for selectivity.Synthetic zeolite has a lower capacity for some common solvents (e.g.,xylene and high flash aromatic naphtha 100). Because activated carbonhas a wide range of pore sizes it does not exhibit this type of selectivity.The two absorbents can be viewed as complimentary rather than competingtechnologies. One can take advantage of their different adsorption characteristicsand use carbon and zeolite together, both as separate phase and mix media,to control complex VOC streams at coating and other manufacturing facilities.In many cases the most advantageous type of media can be selected based ongeneral guidelines; however specific performance guarantees must be developedfrom laboratory analysis of individual process conditions. In many cases, one ormore concentrator types may be practical and a detailed economic analysis basedupon your specific costs of fuel and electricity will be required to determine thebest selection.

ALTERNATIVE STRATEGIES

Alternative technologies have been developed to oxidize solvents without theuse of high temperatures.

Ultraviolet Light, Ozone Oxidation (UV/OX) Systems

This technology has been used in a limited number of paint finishing applications.Solvent-laden air is fed into a chamber and exposed to high-intensityultraviolet (UV) light. High-intensity UV light prepares the solvent moleculefor oxidation. The air is then scrubbed with a high-intensity water-wash scrubber.Much of the solvent is transferred to the scrubber water. The water containsa strong oxidant (ozone), which converts the solvent to carbon dioxideand water. Solvent that is not removed in the scrubber passes through a twobedcarbon system. One bed adsorbs solvent while the other bed is in a solventdestruction mode. Ozone is injected into this bed and the solvent is oxidizedright on the carbon. No nitrogen oxides or carbon monoxide are formed inthis process, and high destruction efficiencies are possible. Wet scrubbingcan remove particulate as well as VOC. UV/OX systems are complicated, withmany dampers, valves, and motors. Systems are large, and operating costs toproduce ozone can be high. These systems have not been proven on very largeairstreams. Another disadvantage is that a wastewater stream is produced.

Biofiltration

Biofiltration units have been successful in abating odors and some VOC streams.Large chambers charged with bacteria are used to convert VOC to carbon dioxideand other compounds. No nitrogen oxides are created with biofiltration andenergy consumption is very low. However, bacteria need a relatively constantsupply of solvents to remain active. Very large amounts of space are requiredand very little past experience in paint applications is available.

CONCLUSION

Applying the Right Solution

It is quite clear that no one solution can be applied universally to all VOC abatementscenarios. The ideology of “One Size Fits All” is false and potentially costly.In choosing the right technology, it is important to examine both the process andthe airstream constituents to be abated. A careful review of current and futureregulations, along with local site considerations, i.e. utility costs, space constraintsand local regulationsshould be used to select the appropriate solution to the enduser’s problem.For paint/coating operations, effectively meeting today’s stricter VOC regulationsis an ongoing challenge. For larger operations, meeting the challengebecomes a matter of improving overall system efficiencies and economics whileretaining enough flexibility to adapt to new coating formulations. For smaller,previously unaffected operations, the challenge involves incorporating a newsystem into the overall operation and investing in new equipment. Careful considerationmust be given to future growth and flexibility while working withinthe constraints of economic resource limitations. A well-planned environmentalsystem can save many thousands of dollars, which can make a big difference to finishers trying to operate in a competitive industry.