reduce voc and hap emissions.pdf

30 www.cepmagazine.org June 2002 CEP

VOC Control

ver the past decade, tremendous scien-tific, political, social and economicchanges have impacted air quality andenvironmental regulations, prompting a

new look at the subject of controlling volatile organiccompound (VOC) and hazardous air pollutant (HAP)emissions. Some of the significant changes that influ-ence the selection of solutions for reducing emissionsof VOCs and HAPs are:

• The implementation of the Maximum AchievableControl Technology (MACT) provisions of the Clean AirAct. Many of the facilities affected by MACT standardsconsider the reporting and control of HAP emissions tobe the single largest driver toward VOC abatement.

• Increased public interest in the environmentthrough web-based access to right-to-know reports.This continues to prompt many companies to find inno-vative ways to reduce pollution.

• The emergence of regional air-quality initiativesrelated to ozone transport and market-based incen-tive programs such as emission-reduction credittrading programs.

• Highly political or newsworthy issues in airquality management, such as the recent legal battleover EPA’s ambient air-quality standard for ozone,the public dialogue over smog and urban sprawl, andthe efforts to balance the regulation of stationary andmobile sources.

• The refocusing of environmental policy from acommand-and-control format to a more collaborative,

incentive-based approach that promotes pollution pre-vention over end-of-pipe control.

• The emergence of international agreements to en-hance regional environmental cooperation, avoid po-tential environmental and trade disputes, promote ef-fective enforcement of environmental laws, and exploretransboundary air issues.

This article will help chemical engineers evaluateand select VOC and HAP control technologies. It is ex-cerpted from the book “Practical Solutions for Reduc-ing and Controlling Volatile Organic Compounds andHazardous Air Pollutants” (1), published by AIChE’sCenter for Waste Reduction Technologies (CWRT).The book includes information that the Du Pont Co.developed based on technical and economic evalua-tions of VOC and HAP abatement technologies. Thebook also includes information from EPA’s control-technology guidance documents. EPA’s National RiskManagement Laboratory, Sustainable Technology Divi-sion, provided funding for the book.

Thermal oxidation Thermal oxidation, or thermal incineration, is the

process of oxidizing combustible materials by raisingtheir temperature above the autoignition point in thepresence of oxygen and maintaining it at a high tem-perature for sufficient time to complete combustion tocarbon dioxide and water. Time, temperature, turbu-lence (for mixing) and the amount of oxygen affect therate and efficiency of the combustion process. These

Follow this roadmap to understand theabatement technologies available forcontrolling emissions of volatile organiccompounds and hazardous air pollutants andthe criteria for choosing among them.

Reduce VOCand HAPEmissions

OEdward C. Moretti,Baker Environmental, Inc.

CEP June 2002 www.cepmagazine.org 31

factors provide the basic design parameters for VOC andHAP oxidation systems. For safety considerations, themaximum concentration of the VOCs/HAPs in the wastegas must be substantially below the lower explosive limit(LEL) of the specific compound(s) being controlled. As arule, a safety factor of four (i.e., 25% of the LEL) is used,although some direct-flame oxidizers are able to operatesafely above this level. The waste gas may be diluted withambient air, if necessary, to lower the concentration.

The heart of the thermal incinerator (Figure 1) is a noz-zle-stabilized flame maintained by a combination of auxil-iary fuel, waste gas compounds, and supplemental air asrequired. When the waste gas passes through the flame, itis heated from the preheated inlet temperature to the igni-tion temperature. The ignition temperature varies for dif-ferent compounds and is usually determined empirically. Itis the temperature at which the combustion reaction rateexceeds the rate of heat loss, thereby raising the tempera-ture of the gases. Thus, any organic/air mixture will igniteif its temperature is raised high enough.

The required level of VOC/HAP control that must beachieved in the time that the waste gas spends in the ther-mal combustion chamber dictates the reactor temperature.The shorter the residence time, the higher the reactor tem-perature must be. Most thermal oxidizers are designed toprovide no more than one second of residence time to thewaste gas with typical temperatures of 650°C to 1,100°C(1,200–2,000°F). Once the unit is designed and built, theresidence time is not easily changed. The required reactiontemperature, therefore, becomes a function of the particulargaseous species and the desired level of control. To ensure98% destruction of non-halogenated organics, thermal oxi-dizers generally should be run at 870°C (1,600°F) with anominal residence time of 0.75 seconds. Destruction ofhalogenated organics may require an oxidation temperature

closer to 1,100°C (2,000°F) and, most likely, a post-oxida-tion water or caustic scrubber to remove highly corrosiveacid gases (e.g., HCl).

There are three types of thermal oxidation systems: di-rect flame, recuperative and regenerative. They are differ-entiated by the equipment used for heat recovery. A direct-flame thermal oxidizer, also known as an afterburner, con-sists of a combustion chamber and does not include anyheat recovery. A recuperative oxidizer includes a waste-

Pollution PreventionAn alternative to the end-of-pipe pollution control tech-

niques discussed here is pollution prevention. Pollution

prevention initiatives may be categorized as recovery, prod-

uct design changes, technology or process modifications,

input or material changes, in-process recycling and reuse,

and good operating practices (e.g., waste segregation, pre-

ventive maintenance, training and awareness programs,

and production scheduling).

More and more facilities are affirming the benefits of

pollution prevention and volume reduction: reduced capital

investment for end-of-pipe control, lower waste-disposal

costs, and lower annual operating and maintenance costs.

However, pollution prevention still faces some barriers to

wider use by engineers, designers and planners. Corporate

values about environmental protection influence the pro-

cess through which abatement strategies are selected and

the availability of capital for environmental projects. Also,

companies may be unwilling to accept the risks of innova-

tive strategies to prevent or abate pollution; these risks in-

clude unknown operating and maintenance costs, scale-up

problems, unacceptable process changes and unknown

long-term reliability. Finally, companies may be restricted

in applying pollution prevention strategies as a result of

traditional command-and-control regulations that impede

technological innovation and traditional regulators who

may be reluctant to approve these innovations.

Pollution prevention is the fundamental concept behind

nearly all emerging trends in environmental rulemaking

and regulatory compliance. International efforts to reduce

VOC and HAP emissions advocate pollution prevention, en-

ergy efficiency, integrated environmental-management sys-

tems, and sustainable development. Since there is a direct

link between a nation’s prosperity and its level of environ-

mental action, most countries with heavily urbanized areas

are attempting to allocate resources for programs to re-

duce VOCs, HAPs and other smog precursors. In addition,

socioeconomic conditions (e.g., energy costs and geogra-

phy) and cultural values may accelerate the transition to

pollution prevention and energy efficiency.

ThermalIncinerator

SupplementaryFuel

HeatExchanger(Optional)

Emission Source

Scrubber*

*Required for specific situations.

Stack

DilutionAir*

CombustionAir*

■ Figure 1. A thermal oxidizer may incorporate recuperative or regenerative energy recovery.

gas preheater and, if low-pressure steam or hot water canbe used on site, a secondary energy-recovery heat ex-changer; considerable fuel savings may be realized by re-covering the heat energy from the exhaust gas and pre-heating the incoming waste gas. A regenerative oxidizeruses a high-density media such as a ceramic-packed bedstill hot from a previous cycle to preheat an incomingVOC- or HAP-laden waste gas stream; such systems gen-erally have lower fuel requirements because of higher en-ergy recovery (up to 95%).

Thermal oxidation is a proven method for destroyingVOCs and HAPs, with destruction efficiencies up to99.9999% possible. Thermal oxidizers can be used to re-duce emissions from almost all VOC and HAP sources, in-cluding process vents, storage tanks, material transfer oper-ations, treatment, storage and disposal facilities, surfacecoating operations, and vessel cleaning operations. In gen-eral, thermal oxidizers are not well-suited to exhauststreams with highly variable flowrates, because the reducedresidence time and poor mixing resulting from highflowrates decrease the completeness of combustion, whichcauses the combustion chamber temperature to fall and thedestruction efficiency to drop.

Catalytic oxidation Catalytic oxidizers, or catalytic incinerators, operate

similarly to thermal oxidizers. The primary difference isthat the gas, after passing through the flame area, passesthrough a catalyst bed. The catalyst has the effect of in-creasing the oxidation reaction rate, allowing the reactionto occur at a lower temperature than is required for thermalignition. Waste gas is typically heated by auxiliary burnersto 320°C to 430°C (600–800°F) before entering the cata-lyst bed. The maximum design temperature of the catalystexhaust is 540°C to 675°C (1,000–1,250°F). Catalysts,therefore, also allow for smaller oxidizer size. Catalystsused for VOC/HAP abatement include metal oxides or pre-cious metals, such as platinum and palladium, which gen-erally have a longer service life and are more resistant topoisoning and fouling than less-expensive base-metal cata-lysts, such as manganese dioxide. The potential for poison-ing exists if the exhaust stream contains sulfur, silicon,phosphorus, arsenic or heavy metals.

In a catalytic oxidizer (Figure 2), the gas stream is intro-duced into a mixing chamber, where it is heated. The wastegas usually passes through a recuperative heat exchanger,where it is preheated by post-combustion gas. The heatedgas then passes through the catalyst bed. Oxygen andVOCs/HAPs migrate to the catalyst surface by diffusionfrom the gas stream and are adsorbed onto the active siteson the catalyst surface, where oxidation occurs. The oxida-tion reaction products are then desorbed from the activesites by the gas and diffuse back into the gas stream.

Over time, particulate matter can rapidly coat the cata-lyst and prevent active sites from aiding in the oxidation of

pollutants. This is known as blinding. Because the activesurface of the catalyst is contained essentially in relativelysmall pores, the amount of particulate matter need not belarge to blind the catalyst. There are no general guidelinesabout the concentration and size of particles that can betolerated by catalysts, because the pore size and volume ofcatalysts vary widely. This information is typically avail-able from catalyst manufacturers.

Catalytic oxidizers can be used to reduce emissionsfrom many VOC and HAP sources, including processvents, gasoline bulk-loading operations and solvent evapo-ration processes. They offer many advantages for the ap-propriate application: they operate at lower temperaturesand require less fuel than thermal oxidizers, they have asmaller footprint, and they need little or no insulation.However, selection should be considered carefully, sincethe sensitivity of catalytic oxidizers to inlet stream concen-trations, flow conditions and catalyst deactivation limitstheir applicability for many industrial processes.

AdsorptionThe design of an adsorber system depends on the chem-

ical characteristics of the VOC or HAP being recovered,the physical properties of the inlet stream (temperature,pressure and volumetric flowrate) and the physical proper-ties of the adsorbent. Physical adsorption is an exothermicprocess that is most efficient within a narrow range of tem-perature and pressure.

A typical adsorber is shown in Figure 3. Before the inletwaste gas enters an adsorber, it may be filtered to preventbed contamination by soot, resin droplets and large parti-cles entrained in the stream. The gas may be cooled tomaintain the bed at an optimum operating temperature andto prevent fires or polymerization of the hydrocarbons.When the bed is completely saturated (a condition called

VOC Control


CatalyticIncinerator

SupplementaryFuel

HeatExchanger(Optional)

Emission Source

*Required for sepcific situations.

Stack

DilutionAir*

Preheater

Catalyst Bed

CombustionAir*

■ Figure 2. Catalytic oxidizers operate at lower temperatures than thermal oxidizers.

“breakthrough”) and needs to be regenerated, the incomingVOC- or HAP-laden stream is routed to an alternate bed.Bed regeneration is typically done by heating or applying avacuum to desorb the adsorbed gases; the bed is thencooled and dried, usually by blowing air through it with afan, and the vapors are routed to a recovery system such asa condenser, decanter or distillation tower. The regeneratedbed is returned to adsorber service, while the now-saturat-ed other bed is purged of VOCs/HAPs. The regenerationprocess may be repeated numerous times; however, the ad-sorbent must eventually be replaced because of a gradualdecrease in adsorptive capacity.

Common industrial adsorption systems often use acti-vated carbon, zeolite and polymeric adsorbents. Activatedcarbon, which is neither fully hydrophobic nor hydrophilic,has an affinity for both polar and non-polar molecules. Be-cause of this, humidity has a noticeable effect on activatedcarbon. Polymers and hydrophobic zeolite are generallymuch less sensitive to humidity, and much less susceptibleto fire, crumbling or powdering. Therefore, they do notneed to be replaced as often. However, activated carbonhas a lower initial cost.

There are three methods of bed regeneration: thermalswing, vacuum, and pressure swing. The traditional carbonadsorber system uses steam to desorb VOCs/HAPs fromthe activated carbon surface; other heat sources have beentried successfully, including microwaves, embeddedheaters and heated nitrogen. To enhance solvent recoveryand eliminate contamination of the VOC/HAP by steam,vacuum regeneration may be used. In pressure swing ad-sorption, twin adsorber beds alternately adsorb and desorbVOCs/HAPs; the heat of adsorption is conserved and sub-sequently used to desorb the VOCs/HAPs from the bed.

For adsorption systems that are well designed and oper-ated, continuous VOC and HAP removal efficiencies

greater than 95% can be achieved for a variety of solvents.Adsorption is applicable to continuous or intermittentstreams and is capable of handling a range of concentra-tions. Adsorbers are useful for recovering high-value sol-vents, but are not recommended for certain situations.First, high VOC/HAP concentrations may result in exces-sive temperature rise in the bed due to the large heat of ad-sorption. If flammable vapors are present, insurance re-quirements may limit inlet concentrations of VOCs andHAPs to less than 25% of the LEL. However, vent streamswith high VOC/HAP concentrations can be diluted with airor inert gases. Second, very-high-molecular-weight com-pounds (MW > 130) that are characterized by low volatili-ty (boiling point > 400°F or 204°C) are strongly adsorbedon carbon, making it difficult to remove them during re-generation. Conversely, low-molecular-weight compounds(MW < 45) do not adsorb readily on carbon. Third, proper-ly operated adsorption systems can be very effective whenhandling homogeneous waste gas streams, but are less sowith a stream containing a mixture of low- and high-molecular-weight hydrocarbons. The lighter organic com-pounds tend to be displaced on the adsorbent surface bythe heavier (higher boiling) components, greatly reducingsystem efficiency. Finally, gas stream humidity levelsabove 50% can affect working adsorber capacity at adsor-bate concentrations below 1,000 ppmv. Relative humiditymay be reduced by adding drier dilution air to the wastegas stream or by heating the gas with a heat exchanger.

Volume concentratorsVolume concentrators are designed specifically for the

control of low-concentration VOC or HAP gas streams.These devices raise the concentration of VOC/HAP vaporto allow more economical treatment of the concentratedcompound in the exhaust gas.

The most common volume concentrator is a rotarycarousel system. In this unit, one sector of the carousel isbeing used for adsorption while another sector is being re-generated (or desorbed) with hot gas. As the carouselturns, each section alternately adsorbs VOC and HAPfrom the waste gas and is then regenerated. The adsorbentcan be a zeolite, a mixture of zeolite and activated carbon,a mixture of zeolite and polymer adsorbents, or activatedcarbon or polymer adsorbent beds followed by zeolitebeds downstream.

A concentration ratio of well above 1,000:1 can often beobtained in a volume concentrator. (Permissible concentra-tion ratios may be limited by flammability considerations.)Of course, the flowrate of the regeneration gas is corre-spondingly lowered. This higher-concentration/lower-flow-rate regeneration gas can then be treated in various ways,including thermal oxidation, catalytic oxidation or fixed-bed adsorption.

Fluidized-bed concentrators have also been developed.They are designed for concurrent adsorption and desorp-


Decanter

Condenser

Emission Stream

Low-PressureSteam

Exhaust toAtmosphere

Adsorbers

■ Figure 3. In a dual-bed adsorption system, one bed is adsorbingVOCs/HAPs while the other bed is being regenerated.

tion in separate sections of the unit. VOC/HAP-laden gasenters one end and adsorbent enters the other end via a car-rier gas. The VOC/HAP is stripped off the adsorbent usingsteam, hot nitrogen or hot ambient air.

Volume concentrators can achieve 90% to 98% removalefficiency, depending on the number of rotors in series andthe inlet VOC/HAP concentration. Concentrators producesavings in the size and cost of downstream control equip-ment. When an existing end-of-pipe treatment system isoperating at its volumetric capacity, a concentrator can beused to extend the capacity of the system by pre-concen-trating the incoming waste gas stream. However, a volumeconcentrator coupled with another control device results ina more complex system, which may affect utility and main-tenance requirements. Also, other approaches, such assource reduction, may be more economical for reducingthe volume of waste gas.

AbsorptionAbsorption, or scrubbing, is often used to separate

gaseous streams containing high concentrations of organ-ics, especially water-soluble compounds such as methanol,ethanol, isopropanol, butanol, acetone and formaldehyde. Itis widely used to abate VOC/HAP emissions during naturalgas purification and coke byproduct recovery. However, itis more commonly used for controlling inorganic gases,such as HCl, than for VOCs/HAPs.

The use of absorption as the primary control techniquefor organic vapors is subject to several limiting factors.One factor is the availability of a suitable solvent. TheVOC/HAP must be soluble in the absorbing liquid. Somecommon solvents that may be useful for VOCs/HAPs in-clude water, mineral oils or other nonvolatile petroleumoils. Another factor is the availability of vapor/liquid equi-librium data for the specific organic/solvent system inquestion. Such data are necessary for the design of ab-sorber systems; however, they are not readily available foruncommon organic compounds.

Another consideration in the use of absorption is thetreatment or disposal of the material removed from the ab-sorber. In most cases, the scrubbing liquid containing theVOC/HAP is regenerated by stripping, where the VOC/HAPis desorbed from the absorbent liquid, typically at elevatedtemperatures and/or under vacuum. The VOC/HAP is thenrecovered as a liquid in a condenser. The stripping processmay create water disposal problems that may require awastewater treatment system to handle the contaminants.

CondensationSeparation via condensation can be achieved by increas-

ing the system pressure at a given temperature (compres-sion condensation) or by lowering the temperature at aconstant pressure (refrigerated condensation). Most com-mercial condensers are refrigerated condensers.

In a two-component system where one of the compo-

nents is non-condensible (e.g., air), condensation occurs atthe dew point (saturation) when the partial pressure of thevolatile compound is equal to its vapor pressure. For more-volatile compounds (i.e., compounds with lower boilingpoints), a larger amount of the compound remains as vaporat a given temperature; hence, to remove or recover thecompound, a lower temperature would be required for sat-uration and condensation. In such cases, refrigeration canbe used to obtain the lower temperatures needed to achieveacceptable removal efficiencies.

The basic equipment in a refrigerated condenser systemincludes a condenser, refrigeration unit(s) and auxiliaryequipment (e.g., precooler, recovery/storage tank,pump/blower and piping). For applications requiring lowtemperatures (below about –30°F or –34°C), multistage re-frigeration systems are frequently used. Alternatively, cryo-genic condensation using liquid nitrogen is emerging as asafe and effective control system.

Condensers are widely used as raw-material and/orproduct-recovery devices. They may be used to recoverVOCs/HAPs upstream of other control devices, or theymay be used alone for controlling vent streams containinghigh VOC/HAP concentrations. They can handle both in-termittent and continuous flowrates. They are especiallywell-suited for low-flow (< 100 scfm), high-concentration(> 2,500 ppmv) vents from storage tanks, reactors and sol-vent unloading operations.

Condensers can be used to remove non-halogenated andhalogenated VOCs/HAPs without the need for expensiveauxiliary equipment. If the vent stream contains watervapor or if the VOC/HAP has a high freezing point (e.g.,benzene), ice or frozen hydrocarbons may form on the con-denser tubes or plates. This will reduce the heat transfer ef-ficiency and thus reduce the removal efficiency, as well asincrease the pressure drop across the condenser. In suchcases, a precooler may be used to remove the moisture be-fore the vent stream enters the condenser.

Depending on the type of condenser used, disposal ofthe spent coolant can be a problem.

FlaresA flare system begins with a collection header and

knock-out drum to remove water or condensed hydrocar-bons. The knock-out drum is typically either a horizontalor vertical vessel located at or close to the base of the flareor a vertical unit inside the base of the flare stack. Liquidsmust be removed from the vent stream because they canextinguish the flame or cause irregular combustion andsmoking; the flaring of liquids can also generate a spray ofburning chemicals that could reach the ground and create asafety hazard.

Vent streams are also typically routed through a flamearrestor to prevent possible flame flashback, which occurswhen the vent-stream flowrate to the flare is too low andthe flame front drops down into the flare stack. Purge gases

VOC Control


(e.g., nitrogen, carbon dioxide or natural gas) also help toprevent flashback.

The vent stream enters the base of the flame, where it isheated by fuel and pilot burners at the flare tip. Pilot burn-ers positioned around the outer perimeter of the flare tipensure reliable ignition of the vent stream. The vent streamflows into the combustion zone, where it is oxidized.

Flares are generally categorized in two ways: by theheight of the flare tip (i.e., ground-level or elevated), and bythe method of enhancing mixing at the flare tip (i.e., steam-assisted, air-assisted, pressure-assisted or unassisted).

Flares can be used to control almost any VOC/HAPstream and can typically handle large fluctuations inVOC/HAP concentration, flowrate, heating value and inertspecies content. Flaring is appropriate for continuous,batch and variable-flow vent-stream applications, but flaresare primarily used as safety devices. Most chemical plantsand petroleum refineries have flare systems designed to re-lieve emergency process upsets that require the release oflarge volumes of gas. These large-diameter flares are de-signed to handle emergency releases but also can be usedto control vent streams from various process operations.Gases flared from refineries, petroleum production andchemical plants are composed largely of low-molecular-weight VOCs/HAPs and have high heating value.

Flares have few controls, resulting in lower maintenancecosts than other VOC/HAP control devices. On the otherhand, they can produce undesirable noise, smoke, heat ra-diation and light. Furthermore, they cannot be used to treatwaste streams containing halogenated compounds.

BiofiltrationThe key component of a biofiltration system (Figure 4)

is a biofilter, which consists of a bed made of natural ma-terials (e.g., compost, soil or bark) that is kept wet bysprayers that continuously supply water to maintain highhumidity. An adequate moisture level is very important tothe proper functioning and efficiency of a biofilter becausethe degradation processes are exothermic and tend to dry

the filter beds. Typically, the organicpollutants in the waste gas dissolvein water droplets and then are con-verted by microorganisms in the bio-layer. The type of microorganismused depends on the type ofVOC/HAP. Most microorganisms re-quire neutral pH conditions.

The natural materials used as thefilter substrate supply the nutrientsnecessary to support the growth andsurvival of the microorganisms. De-composition of cells also leads to therelease of nutrients. The substrateparticle size is selected to provide alarge absorbing surface and minimum

flow resistance. Additives such as bark, polystyrene or acti-vated carbon may be included to enhance performance andminimize pressure drop.

The most suitable type of biofilter for chemical plantemissions is a closed, fixed-bed design (similar to a carbonadsorber, where the organic media takes the place of theactivated carbon). Water spray nozzles are used to supplymoisture. Some designs include pre-humidification of thewaste gas stream. Temperature and humidity can be moreeffectively controlled in this closed design than in an opensystem, and effluent monitoring is easier.

Compounds of low molecular weight that are water-sol-uble and contain oxygen atoms are good candidates forbiofiltration. Aldehydes, ketones, alcohols, ethers, estersand organic acids degrade rapidly in biofilters, but halo-genated hydrocarbons and polyaromatic hydrocarbons donot. Removal efficiencies greater than 90% have beendemonstrated for easily degradable compounds.

Degradation of sulfur-, nitrogen- or halogen-containingcompounds leads to formation of acids that lower the pHof the aqueous layer in biofilters. Alkaline compounds suchas lime are added to the filter material to control pH; how-ever, after a period of time there is an accumulation of saltsthat clog the bed. Bed lifetime is limited to two to fiveyears. Occasional washing of the bed can extend its life insome cases.

Because of the uncertainties involved in the biologicalprocess, pilot testing is necessary for most potential appli-cations. Pilot testing provides scale-up parameters requiredto obtain the desired overall removal efficiency and the fil-ter-bed volume needed to achieve the desired level ofVOC/HAP destruction.

Membrane technologyIn a typical membrane separator, the waste gas stream is

fed to an array of membrane modules, where organic sol-vents preferentially permeate the membrane. The organicsin the permeate stream are then condensed and removed asa liquid for recycle or recovery. The purified gas stream is


BiofilterHumidifierBlower

Ducting

Raw Gas

Filter Material

Clean Gas

Air Distribution System

■ Figure 4. Adequate moisture is critical to the proper functioning of a biofiltration system.

removed as the residue. Transport through the membrane isinduced by maintaining the vapor pressure on the permeate(downstream) side of the membrane lower than the vaporpressure on the feed (upstream) side. In some cases, a vac-uum pump is required on the permeate side to maintain thisdriving force.

A compound permeates the membrane at a rate deter-mined by its permeability in the membrane material andpartial-pressure driving force. In some systems, the feedstream is compressed on the feed side of the membrane toprovide the pressure drop for the membrane and/or toallow operation of the solvent condenserat a higher temperature. Generally, thepermeate is five to 20 times more concen-trated in VOC/HAP than the feed stream.

Membrane modules can be of eitherhollow-fiber or spiral-wound construc-tion. Membrane separation systems mayhave either one stage or multiple stagesas necessary to achieve desired recoveryefficiencies. Most membranes are madefrom synthetic polymers; however, somevendors are considering inorganic materi-als, such as ceramics, to deal with morerigorous applications. The membranesare thin, multilayer films made by coat-ing a microporous support membranewith a very thin, dense film. The supportmembrane provides mechanical strength,while the thin film performs the separa-tion. Thinner films promote higher per-meation rates. These membranes are then

incorporated into modules that can withstand tempera-tures up to 60°C (140°F). Membrane life can be as longas three years.

Membrane separations should be considered for low-flow, high-concentration waste gas streams, where conden-sation or adsorption prove to be either uneconomical or un-able to achieve the desired level of recovery efficiency.

Emerging technologiesResearchers and equipment manufacturers currently are

evaluating two other VOC/HAP abatement technologies:ultraviolet oxidation (UV) technology and plasma technol-ogy. However, these technologies have encountered site-specific constraints (e.g., emission stream characteristics,regulatory concerns, economic considerations) that havelimited their commercial availability.

UV oxidation uses oxygen-based oxidants like ozone(O3), peroxide (H2O2), hydroxyl free radicals (OH–), and O–

radicals to convert VOC/HAP into carbon dioxide and waterin the presence of UV light. The UV radiation excites theoxidants, which destroy the VOC/HAP through direct oxi-dation. In certain applications, a wet scrubber may be usedto remove HCl and Cl2 generated during the oxidation ofchlorinated solvents. A carbon adsorber may be used to re-move unreacted VOC/HAP from the oxidation process.

When the UV frequency range is chosen to match theabsorption characteristics of the compound, maximumremoval can be achieved. This process is highly energy-efficient for very dilute streams, and no pollutantbyproducts are created. However, the use of a waterscrubber could require wastewater treatment. UV oxida-tion has been applied commercially to surface coatingoperations with waste-gas flowrates between 20,000 and90,000 scfm.

VOC Control


Table 1. Typical pretreatment considerations.

Thermal Oxidation DilutionPreheating

Catalytic Oxidation DilutionParticulate RemovalPreheating

Adsorption, CoolingVolume Concentrators Dehumidification

DilutionParticulate Removal

Absorption CoolingParticulate Removal

Condensation Dehumidification

Flaring Liquid Knock-out

Biofiltration HumidficationCoolingParticulate Removal

Membrane Separation Particulate Removal

VOC/HAP Abatement Technology

Typical PretreatmentConsiderations

Table 2. Applicability of VOC/HAP abatement systems.

AbatementTechnology

Waste Gas Flowrate(scfm)

Thermal Oxidation < 10,000 (Thermal Afterburner) 60% of LEL (Thermal Afterburner)*250–100,000 (Recuperative) 25% of LEL (Recuperative)2,000–500,000 (Regenerative) 10% of LEL (Regenerative)

Catalytic Oxidation < 75,000 25% of LEL

Adsorption No practical limit 100–5,000

Volume > 10,000 < 100Concentrators

Absorption < 100,000 > 200

Condensation < 3,000 > 1,000

Flare No practical limit No practical limit

Biofiltration < 100,000 < 1,000

Membrane < 500 > 5,000Separation

* Special safety considerations apply when the waste stream concentration is 25% of the lower explosive limit (LEL) or higher.

VOC/HAP Concentration(ppmv)

Plasma technology can be divided into corona dischargereactors and electron beam reactors. Plasma is a high-temper-ature ionized gas that is very reactive. The “hot” plasma caninitiate dissociation reactions in VOC/HAP molecules. Thedecomposition reaction is initiated by free-radical mecha-nisms. Therefore, it is important to establish optimum condi-tions (e.g., electron density and residence time) to generateenough free radicals for the reactions to reach completion.

Plasma reactors can be highly selective for the decom-position of halogenated VOC/HAP, since halogen free radi-cals are highly reactive.

Comparison of abatement technologiesWhich VOC/HAP control device is best for your needs?

Fortunately, there are only a few main criteria for selectingan appropriate control device. One of those criteria may bea regulation that mandates a specific type of control equip-ment. However, most regulations impose a level of control(e.g., MACT or BACT) but do not prescribe a specific con-trol device.

The main selection criteria for VOC/HAP abatementtechnologies are costs, VOC/HAP concentration, vent-gasflowrate and the required control level. In addition, pre-treatment of the vent gas may be required for some controldevices and could affect project costs.

Pretreatment considerationsPretreatment refers to the methods and practices used to

condition a VOC/HAP-laden stream prior to its entry intoan abatement device. While pretreatment considerationsrarely are barriers to equipment selection, they are crucialto a thorough evaluation of the environmental and econom-ic impacts and the ultimate operational success of a partic-ular technology.

Table 1 summarizes typical pretreatment considerationsfor common VOC/HAP abatement technologies. The neces-sity of pretreatment is often a function of emission streamcharacteristics; therefore, pretreatment considerations iden-tified in the table are not relevant for all applications.


≤100 1,000

Catalytic Oxidation RegenerativeThermal

Oxidation

RecuperativeThermal Oxidation

Recuperative Thermal OxidationRegenerative Thermal OxidationBiofiltration Condensation

Rotor Concentrator+

Catalytic Oxidationor Carbon Adsorption

BiofiltrationThermal Afterburner

Condensation

10,000 100,000

35–90

10–25

5–10

1

50–200

Waste Gas Flowrate, scfm

Was

te G

as In

let V

OC

Co

nce

ntr

atio

n, p

pm

v

Was

te G

as E

xit V

OC

Co

nce

ntr

atio

n, p

pm

v

4,500

1,000

100

10

Basis: 4,500 ppmv = 25% of Lower Explosive LimitContains no halogenated organicsContains no catalyst poisonsContains no particulates

Note: Consider carbon adsorption for recovery when- Recovered solvent value > $0.15/lb- Concentration > 500 ppmv- Gas flowrate > 1,000 scfm > 500 ppmv

Catalytic OxidationThermal AfterburnerRotor Concentrator + Catalytic Oxidation or Carbon Adsorption

■ Figure 5. Use this selection map (which is based on lowest net present costs) to choose an appropriate VOC/HAP-abatement technology.

Flowrate and concentrationAir flow and pollutant loading are the two most impor-

tant design criteria for VOC/HAP control devices. Table 2summarizes the full range of flowrates and VOC/HAP con-centrations for the control devices.

Figure 5 is an applicability chart for the most popularcontrol technologies. The figure shows the range offlowrate and concentration in which the devices operatemost efficiently and cost-effectively. Many of the controldevices are technically capable of operating outside the in-

dicated ranges, but operation outsidethese ranges may not be cost-effective.

Other key properties of controlequipment design may also drive theselection of a control device for an ap-plication outside its indicated cost-ef-fective range. These properties includethe heat content, oxygen content,moisture content, halogen or metalscontent, temperature and pressure ofthe vent gas stream, and the molecularweight, vapor pressure, solubility andadsorptive properties of the specificVOC/HAP in the vent gas stream.

As shown in Figure 5, thermal andcatalytic oxidizers can be used over afairly wide range of organic vapor con-centrations, provided adequate safetyprecautions are implemented forVOC/HAP loadings greater than 25%of the LEL. Thermal afterburners canbe used for higher-fuel-value streamsthan other thermal oxidizer systems (upto 60% of the LEL in some cases). Re-generative and recuperative oxidizersperform best at medium to high concen-trations, because the heat of combustionof organic gases is sufficient to sustainthe temperatures required without theaddition of expensive fuel. Catalytic ox-idizers can and have been used effec-tively at low to high VOC/HAP concen-trations and low to high flowrates. Ther-mal afterburners are best suited for low-flow applications, where heat recoverydoes not pay for itself. Waste streamsthat have steady and high flowrates overlong periods of time can be treated eco-nomically with recuperative or regener-ative oxidizers.

Adsorption systems operate best atconcentrations ranging from 20 ppmvto 10,000 ppmv and flowrates from1,000 scfm to 50,000 scfm. Concentra-tions above 10,000 ppmv may lead to

excessive bed temperatures. At higher flowrates, the pres-sure drop across the bed becomes too high for standardblowers, while at lower flowrates, the required bed volumeis large and its cost becomes prohibitive.

Absorption is usually considered only when theVOC/HAP concentration is above 200 to 300 ppmv. Belowthese levels, the rate of mass transfer of the VOC/HAP tothe solvent is low enough to make reasonable designs im-practical. There are, however, no practical limits on the gasflowrate that an absorber can handle.

VOC Control


Table 3. Estimating investment costs for VOC/HAP abatement.

Cost Element Basis

Engineered Equipment Vendor estimate

Miscellaneous Engineered Equipment 0%–10% of Engineered Equipment

Equipment Installation 10%–40% of (Engineered Equipment + Miscellaneous Engineered Equipment)

Field-Erected Equipment Vendor estimate

Equipment Foundations, Supports, Platforms 5%–15% of (Engineered Equipment + Miscellaneous Engineered Equipment + Equipment Installation + Field-erected Equipment)

Installed Equipment Sum of first five cost elements

Process and Service Piping, Instrumentation, Usually based on a combination of historical and Electrical values from previous projects and vendor

estimates

Architectural and Civil Items (e.g., pipe Contractor estimatesupports, dikes, trenches, pilings, sanitary

sewers, concrete pads)

Buildings and Structures (e.g., process 5%–15% of (Installed Equipment + buildings, silo supports, control rooms, Piping/Instrumentation/Electrical)offices, laboratories, maintenance shops)

Dismantlement and Rearrangement 0%–10% of (Installed Equipment + (e.g., existing equipment, buildings, piping, Piping/Instrumentation/Electrical + asbestos insulation) Architectural/Civil + Buildings/Structures)

Freight/Quality Assurance/Procurement 7% of Engineered Equipment

Sales Tax Varies by state. Some states exempt pollution abatement equipment from sales taxes.

Construction Contractor (e.g., payroll, taxes, 20% of (Installed Equipment + benefits, tools and equipment, rentals, Piping/Instrumentation/Electrical + overhead, profit) Architectural/Civil + Buildings/Structures +

Dismantlement/Rearrangement)

Other Identifiable Costs (e.g., fire protection, Project-specificrailcar/track loading spots, waste treatment allowances, substations, cooling towers, demineralized water, site preparation such as roads and walkways)

Direct Costs Sum of all previous costs

Project Management 15%–22% of Direct Costs

Contracts Administration 2%–5% of Direct Costs

Contingencies 10%–30% of (Direct Costs + Project Management Costs + Contracts Administration)

Total Investment Sum of all costs

Condensers can process waste gasstreams containing high VOC/HAP con-centrations (usually above 1,000 ppmv)but relatively low flows (usually below3,000 scfm). Flowrates above 3,000 scfmmay require significantly larger heat-transfer areas.

Flares can be used to control almostany VOC/HAP stream and can handlefluctuations in VOC/HAP concentra-tion, flowrate, heat content and inertgas content.

Biofilters are cost-competitive forflowrates above 1,000 scfm andVOC/HAP concentrations below ap-proximately 300 to 500 ppmv.

Membrane separation systems aresuitable for low-flow, high-concentra-tion waste gas streams. As a rule ofthumb, membranes are applicable forflowrates below 500 scfm andVOC/HAP concentrations above5,000 ppmv.

CostsThere are three main economic mea-

sures for selecting a cost-effectiveVOC/HAP control device: net presentvalue (NPV), total investment and totalannual operating costs. The total invest-ment in a specific control device in-cludes the equipment costs, installationcosts, site preparation and constructioncosts, and costs for land, working capital and offsite facil-ities. Total annual operating costs include direct costs(which are proportional to the quantity of exhaust gasprocessed by the control system), indirect costs (whichare independent of the exhaust gas flowrate) and creditsfor recovered chemicals.

There are many models for estimating NPV, invest-ment and annual operating costs for VOC/HAP abatementsystems. They range from simple computer-based spread-sheet functions (e.g., Excel) to EPA cost programs (e.g.,CO$T-AIR) to sophisticated financial-management soft-ware. The EPA’s computer-based cost programs are pub-licly available on its website and are simple to use. How-ever, publicly available cost models may underestimatecosts because they are based on an average of multipledata sets. Facilities that install sophisticated process con-trol equipment and safety equipment and that use moreexpensive materials of construction may encounter costshigher than those estimated by the EPA’s and other pub-licly available cost models.

Table 3 presents a model for estimating the total in-vestment costs for VOC/HAP control devices. It is based


Rules-of-Thumb for VOC and HAP Abatement

• The minimum capital investment into end-of-pipe

equipment is $40 per scfm.

• The minimum net present cost of end-of-pipe equipment

is $60 per scfm.

• The minimum incentive for volume reduction is $30

per scfm.

• VOC concentration has little effect on the economics of

end-of-pipe treatments for dilute streams.

• The cost to treat streams containing halogenated

organics, particulates, or both will be at least double

the cost for simple nonhalogenated streams.

• Reducing diluent flow at the source and standardization

of equipment design will have the largest impact on

minimizing investment in end-of-pipe abatement.

Table 4. Annual operating cost factors.

Cost Element Unit Cost Factor (U.S. Dollars)

Raw MaterialsActivated Carbon (5-yr life) $2.80/lbPlatinum Catalyst (5-yr life) $4,200/ft3Membrane Module (3-yr life) $450/m2

Polymeric Adsorbent (5-yr life) $3.00/lbZeolite Honeycomb (5-yr life) $1,000/unitCarbon Fiber Wheel (5-yr life) $1,000/unitCompost Bed (3-yr life) $10/yd3

UtilitiesElectricity $0.04/kWhNatural Gas $2.50/MMBtuCooling Water $0.05/MgalFiltered Water $0.15/MgalSteam $3.50/MlbNitrogen $0.045/lb

OtherWastewater Treatment $0.35/lb organic

Cost FactorsTechnical Support $160,000/yrOperations Labor $430,000/yr for 3-shift coverageMaintenance 4% of total investment/yrTaxes and Insurance 0.75% of total investment/yrGeneral Plant Overhead — Investment-related 0.5% of total investment/yrGeneral Plant Overhead — Labor-related 24% of (Operating + Maintenance Labor)/yrProject Liaison Costs (1 yr only) 2% of total investmentStart-up Costs (1 yr only) 10% of total investmentCreep Investment 1.5% of total investment/yrWorking Capital 60 days of cash costs

Cash-Flow BasesDepreciation (6-yr accelerated) 20%, 32%, 19%, 12%, 12%, 5%Income Tax Rate 40%Discount Rate for Net Present Cost 12%Capital Recovery 10% borrowing costs, 10-yr life

on Du Pont’s extensive work in evaluating control tech-nologies. The table is intended as a guide to preparingcost estimates for installing new or replacement equip-ment. In many situations, it can be used along withsound engineering judgment to evaluate project eco-nomics. However, investment costs can be highly pro-ject-specific, so Table 3 may not accurately predict in-vestment costs for all projects.

Table 4 presents cost factors that may be used to esti-mate annualized costs or annual cash outflows (for an NPVcalculation). They also are derived from Du Pont’s re-search. The same warnings apply to the use of these costfactors — annual costs can be highly project-specific andmay vary from year to year, so use good engineering judg-ment in applying them.

Numerous cost comparisons can be made using the cost

models and cost factors from Tables 3and 4. Figure 6 presents an overviewof investment and net present costs forend-of-pipe treatment of VOC andHAP emissions (based on a 12% dis-count rate and a 10-yr equipment life).The costs shown in Figure 6 assumethere are no halogenated organics andno particulates in the stream to betreated; the costs to treat streams con-taining these compounds would bedouble the costs in Figure 6.

It is important to note that thecosts in Figure 6 do not include the“hidden costs” of installing and op-erating a VOC/HAP control device,such as permitting costs and compli-ance costs. Frequently, the installa-tion of a VOC/HAP control devicemay be a “physical or operational”change that triggers state and/or fed-eral air permitting rules. Preparation

of an air permit application may include a thorough eval-uation of alternative control devices, applicable regula-tions and air dispersion impacts. Furthermore, compli-ance demonstrations (e.g., stack testing, continuous orparametric monitoring, recordkeeping, reporting andwork practices) may be required to ensure that theVOC/HAP control devices are operating correctly.

Reference 1 includes over 30 figures detailing the in-vestment, annual and net present costs for each of theVOC/HAP control devices described in this article overa range of VOC/HAP concentrations and exhaust streamflowrates. CEP

VOC Control


EDWARD C. MORETTI is the manager of environmental compliance at

Baker Environmental, Inc. (Airport Office Park, Bldg. 3, 420 Rouser Rd.,

Coraopolis, PA 15108; Phone: (412) 269-6055; Fax: (412) 269-6097;

E-mail: [email protected]). He holds a BSChE from the Univ.

of Pittsburgh and a Master of Public Management from Carnegie

Mellon Univ.’s Heinz School of Public Policy and Management. He is a

consultant speaker on Clean Air Act and environmental policy issues

for several professional organizations. He is the author of the 1993

and 2001 CWRT publications on VOC and HAP control, as well as

several other encyclopedia and journal articles on VOC and HAP

abatement. He is a former chair of the Pittsburgh Local Section of

AIChE and serves on the board of directors for several professional

and community organizations.

For Additional Details

This article is based on the book “Practical Solutions for Reduc-

ing Volatile Organic Compounds and Hazardous Air Pollutants,”

published by the American Institute of Chemical Engineers’ Cen-

ter for Waste Reduction Technologies (CWRT). The book includes

additional information on VOCs and HAPs, current regulations,

regulatory trends, pollution prevention, and modifying and de-

signing plants for VOC/HAP abatement. To order the book (soft-

cover, 150 pages, $65.00, Publication C-14, ISBN 0-8169-0831-1),

call 1-800-AIChemE (1-800-242-4363) or go to AIChE’s online pub-

lications catalog at www.aiche.org/pubcat. For information about

AIChE’s Center for Waste Reduction Technologies, contact Jo

Rogers, CWRT Director, at [email protected] or

(212) 591-7727.

Literature Cited1. American Institute of Chemical Engineers, “Practical Solutions

for Reducing and Controlling Volatile Organic Compounds and Haz-ardous Air Pollutants,” AIChE, Center for Waste Reduction Tech-nologies, New York, NY (2001).

100 1,000 5,000 10,000

Investment, $/scfm

Net Present Cost, $/scfm

25,000 50,000 100,000

$/sc

fm

Waste Gas Flowrate, scfm

10,000

1,000

100

10

1

■ Figure 6. Overview of investment and net present costs for end-of-pipe treatment technologies.

reduce voc and hap emissions.pdf

Documents