0
dor control devices or systems are-essentially the same as those controlling other types of air pollutionfume incineration, wet-scrubbing absorption, process change, or product elimination. Chemical oxidation and masking primarily are employed for odor control. Odors may be controlled by many commonly employed techniques. However; the low olfactory threshold of many compounds and the difficdty in measuring their residual concentrations make subjective techniques necessary to determine the effectiveness of odor correction systems. Odor measurement
Sensory judgments of community nuisance odors may be made by exposing a panel of judges to the odorous atmosphere in question. Such a procedure ofrten constitutes preliminary investigation of odor complaints and may be preparatory to more systematic odor measurement. Where outdoor odor in-
tensities are low enough to be tolerable, and where they vary from time to time in accordance with atmospheric turbulence, changes in wind direction, and changes in the odor source, it may be advantageous for the judges to expose themselves directly to the atmosphere rather than to a previously collected sample. There are some instances, however, where it is advantageous or necessary to present an odorant sample for sensory measurements. This is the case when the odorant must be diluted, concentrated, warmed, cooled, or otherwise modified before people can be exposed to it, or when it is necessary to have a uniform sample large enough to be presented to a number of judges at the same time. A number of factors are incidental to sensory odor evaluation and must be carefully considered in any odor measurement program under controlled conditions. For discriminatory judgment, such as appraising the degree of odor reduction resulting from a given
Amos Turk The City College of the City University of New York New York, N.Y. 10031
Robert C. Haring ChemTech Services, Inc. West Hagen, Conn. 06516
Robert W. Okey Consulting Engineer Westport, Conn. 06880
Odor control technology Industrial odors have been frequently ignored in the past, but environmental agencies are beginning to curb smelly emissions 602 Environmental Science & Technology
effective dilution, responses of trained judges are more precise than those of untrained persons. Judges for odor discrimination tests should be selected by sensory screening procedures. Identification of samples should be randomized and unbiased. Sequential effects such as presenting odors in a uniform series of concentrations (either increasing or decreasing) should be avoided. Each measurement should be restricted to a single variable (for example, comparing relative intensities, acceptabilities, and fishy qualities of two odors should be elicited in three separate exposures). Control technology
An almost standard method of controlling odorous pollutants is dispersing them to a concentration level at which they are no longer detected, or if detected, are no longer offensive. Elevated emission stacks are widely employed for reducing concentrations of organic emissions or other odorous chemicals t o values below the odor threshold. When
feature dispersion will not solve an odor problem, more modern and positive control methods must be applied. High-temperature air oxidation converts malodorous organic vapors or particulates to carbon dioxide and water. Inorganic materials, such as hydrogen sulfide, ammonia, or cyanides, may also be treated this way. Materials containing halogens are not usually suitably controlled by air oxidation. For vapors below the particular flammability limit, direct-flame oxidation or catalytic oxidation may be used. Where gases are present in concentrations above the lower flammability limit, direct flare burning can be employed. In general, heat recovery systems are recommended in any oxidation system to lower costs. Adsorption is extremely useful for concentrating odorants to facilitate recovery, for conversion to innocuous products, or for another ultimate disposal method. The most common adsorbent is activated carbon. Since it is
Curing of phenolic rescresoting operations, etc.-phenol, sols, xylenols, and carvacrol. Petroleum refinery emisns, pesticides mfg., as odorant leakmercaptans, sulgdes, and disul~
&-
3s.
Emissions of acids butyric, valeric, and phenylacetic ds. Burning operations, tarry ?
orobenzene odors. Varnish cooking, foundry core oven issions. Pungent, gassy odors acrylic materials, diesel exhaust, etc.
nearly neutral With respect to electrical charge, activated carbon is effective in adsorbing a wide variety of organic odorants, even in a humid atmosphere. Other adsorbents, simple or complex oxides such as silica gel, are more selective in the types of odorants which they will attract. Liquid scrubbing controls malodorous substances from many industrial and other sources. In this method, soluble constituents are absorbed into a liquid phase, or vapors are condensed by cold liquid streams. Hydrogen sulfide, organic sulfur gases, ammonia, organic nitrogen compounds such as amines, organic acids, chlorine, and other chlorine-containing compounds may he removed by scrubbing. The chemical in the scrubber system may be water, an aqueous solution or suspension of a reactive compound, or an organic solvent. For example, lime slurries absorb acidic gases such as hydrochloric acid (HCI) or sulfur dioxide (SO2), while dilute sulfuric acid absorbs
nitrogen bases. Liquid scrubbing devices include packed or tray-type towers; venturi, floating bed, or cyclonic scrubbers; and spray chambers. Removing odor-causing materials from a gas stream by condensation may be viewed as a special case of partial condensation. Such a case often occurs in the petrochemical industry where one or more gas stream components cannot be condensed at a given temperature and pressure. In partial condensation of gas streams, three situations can occur in the condensed liquid stream: the liquids are completely miscible, the liquids are partly miscible, or the liquids (Le., water/hydrocarbon) are immiscible. Odor control associated with particulate matter is a special problem. Odorants travel without dilution by adsorption on particulate matter. Small particles (less than 0.1 p in diam) can be transported downwind for great distances. Particulate matter can also contribute to odor problems by being self-
volatile; having an odor, or stimulating the sense of smell. One control method is improved combustion to burn organic particulate matter completely. Direct flame incineration is especially valuable where organic mists and droplets are involved. Many industrial particulate sources can be effectively controlled by electrostatic precipitators, fabric filters, and liquid scrubbers. Chemical reactions also may he used to destroy odors. With many organic substances, some particular functional group is associated with a particular type of odor, for example, the characteristic odors of aldehydes, sulfides, and phenols. The functional group, or the bond hy which it is attached to the molecule, is usually unsaturated, which makes the group readily susceptible to chemicaI oxidation. The products are often odorless or have a milder, less offensive odor than the original pollutant. Oxidizing agents used successfully in commercial applications are potassium permanganate, ozone, chlorine gas, or oxychlorine compounds. Potassium permanganate, the most powerful of the oxidizing agents available, can be used three ways: as a solution in a scrubbing tower, impregnating a water-absorbing inorganic suhstance with a solution, and spreading the permanganate as a dry mixture over the odor-producing area. Ozone is most effective for controlling organic odors, relatively dilute odors, and cases where particulate and condensable matter have
Fermentation. Odorous off-gasesfrom fermentation or processing wasre products are controlled here by direct flame oxidation; heat recouery helps offset costs
been previously removed. Chlorine also can be used in scrubber systems to oxidize odorants. It is especially useful in alkaline solutions to oxidize mercaptans, hydrogen sulfide, and organic sulfides. If odors cannot easily be oxidized, adsorbed, or scrubbed away, a convenient method is masking or counteracting. Commercially, most of the odor “modifiers” used to cancel out, or render pleasant, the malodor are proprietary mixtures customized to a specific problem. Paper t o fertilizer
For any particular industry, some combination of control methods is usually required. This stems from two major factors: most industries have a problem with more than one odorantowing to their chemical nature, these may require different treatments-and many odorants occur in relatively low concentration-frequently they are concentrated by one system and then processed for recovery or destruction by a second type of equipment. In terms of odor control, one of the most advanced industries is the paper industry, especially Kraft (alkaline pulping) mills. Sodium sulfide, one component of the alkaline liquor, generates a number of odorants which are detectable in minute quantities. Major odor control techniques include preventive measures, scrubbing towers, masking or counteracting, dilution to less than threshold levels, and even process modification. 604 Environmental Science 81 Technolog,
Petroleum refineries are another major source of air pollutants. Mercaptans, sulfur-hydrocarbons, ammonia, hydrogen sulfide, and sulfur dioxide are among the malodorous compounds emitted from refineries. Ammonia (NHJ and hydrogen sulfide (H&) often occur together and are removed by water-injection scrubbing to form an ammonium sulfide wcter stream. This stream is then stripped in the presence of acid or base, and the HIS or NH8 given off is incinerated. If H S is present alone, scrubbing with an alkanolamine will remove it. After concentration by scrubbing, these compounds, together with waste hydrocarbons, may be destroyed by oxidation in flares, direct incineration, catalytic oxidation, or fluidized bed incineration. Phthalic anhydride pJants are notorious for odor control problems and other pollution troublys. Some 750 million Ib/year of phthalic anhydride are produced, mainly by air oxidation of naphthalene or o-xylene. The air volumes are high, and many side reactions occur to produce naphthoquinone, maleic and benzoic acids, cyclohexane, phthalide, and carbon monoxide. The most common form of control is catalytic oxidation equipment. The fertilizer industry has a number of sources of odorants-nitrogen oxides from nitric acid manufacture; ammonia, urea, and formaldehyde; and fluoride emissions from phosphate manufacture. Specific control methods are scrubbing with water or an absorbing medium,
catalytic oxidation of selected gas streams, and adsorption (with oxidation) on permanganate beds. Catalytic reduction of nitrogen oxides to nitrogen is useful in nitric acid plants. Roofing, adhesives, foundries
Asphalt saturators for manufacturing roofing materials operate at 400-500°F and vaporize lower boiling components. Also, moisture in the felt causes some steam distillation of higher-boiling materials. Both operations produce vaporous and particulate odorants. Odorous components are sulfurous and nitrogenous organic compounds and phenols. Emissions from asphalt saturators are generally collected by a hood system surrounding the saturator. High air volumes are required, rendering incineration impractical. Spray scrubbers (with or without permanganate oxidizer) and two-stage electrostatic precipitators have given the best results. In manufacturing synthetic detergents, odorants arise from raw material storage tanks, such as fuming sulfuric acid, and from the reaction kettles. These are generally controlled by a scrubbing unit. In the spray-drying unit large volumes of evaporated water are discharged from a stack and form a dense white plume. The odor associated with this is moderate and is usually handled by dispersion at high velocity from a high stack. Soap-making may generate some additional problems. An obnoxious fishy
1
odor is prevalent in tank vents and spray-drying towers. Control is obtained by incinerating vapors in steam boilers. Other types of successful control are adsorption on activated charcoal, chlorine oxidation, and acid mist scrubbing. Coke is produced by the high-temperature distillation of bituminous coal, in the absence of air. When a closed system is not used, gases escape during the charging and heating process. The most common method to minimize smoke escape is a collecting main over the row of furnaces. For new installations, improved emission collection can be obtained by sequential charging, closed pipeline charging, automatic door sealing, and complete hooding. The major odor problem of the founding or casting metal industry arises from manufacturing the cores used in the sand molds for casting. Drying oils (linseed oil) or resins (phenol/formaldehyde)-used in binders-aldehydes, and acids are given off. Odors from core ovens may be controlled by lowering
baking temperatures or by modifying binder composition. When this is not feasible, a catalytic or direct-fied afterburner must be used. In organic resin manufacture, finished plastics are made from mixtures of resins, fillers, and plasticizers by heating. In this process, vapors and mists (especially when dioctyl phthalate is used) are given off, creating odors in the exhaust system. Control methods normally used in this industry include electrostatic precipitators, scrubbers, filters, and incinerators. In general, electrostatic precipitators can be used only in thC absence of combustible vapors. Raw rubber is too plastic for many commercial uses, and thus must be cured (vulcanized) to lose plasticity and gain elasticity. Additives (organic chemicals with a substantial vapor pressure) increase tensile strength, abrasion resistance. resiliency, and other desired properties. Compounding rubber with the various additives generates heat, and, following compounding, vulcanization takes place by heating. These operations result in a very odorous exhaust gas. Direct flame incineration has proved to be the best method of destroying the odors. In one installation flame incineration reduced 350,000 odor units to 50 odor units/min. Adhesives are natural or synthetic substances used for joining or bonding materials by surface attachment. Three common types are protein, starch, and synthetic resins. Protein adhesives in-
Installed cost of wet scrubbing equipment
Adsorption equipment costs
150
1600
125 1200
Tanneries and rendering plants
0
E92
100
1
800
0
U
s
-
75
C
Coffee roasting plants Fertilizer industry (mild steel scrubber)
400 50
0
25 2000
5000
20,000 Air rate (SCFM)
Solvent rate 100 Gph 1000 SCFV
50000
80.000
20
40
60
80
Gas rate (1000 cfrn) :lnc udes scrubbers, f a n s , rnctcrs, pumps. duct u s r k erecticn. electrical. and rnechanica costs)
Volume 6 , Number 7, July 1972 605
clude many fish and animal products and are the most likely to cause odor emissions. The animal by-products are degreased with a petroleum naphtha, hydrolyzed by lime treatment, and dried. Gases emitted during these treatments include many degradation products, chiefly complex nitrogen and sulfur compounds. Odor control seems best achieved by sodium hypochlorite scrubbing. Other systems in use include incineration, adsorption, masking, and other types of chemical oxidation. Pesticides to textiles
Annual pesticide production is estimated at 1 billion lb/year. While some problems may occur in processing or grinding natural pesticide products, the main odor source is the manufacture of synthetic organic chemicals used as pesticides. In the manufacture of DDT, for example, vapors of alcohol, chlorine, chloral, chlorobenzene, SOn, and HC1 may be emitted from various parts of the operation. The acidic gases can usually be 9 5 % controlled by a water scrubber. In washing and steaming the finished DDT, scrubbing with water or caustic soda solution will provide adequate control. The manufacture of 2,4-D (an organic herbicide) generates similar problems, including vapors of phenol, chlorine, chloroacetic acid, dichlorophenol,
and HCl. As with DDT, the best system is an alkaline scrubber to remove the acidic gases (including the phenols). In the manufacture of other pesticides, air oxidation, dispersion, adsorption, and masking are used in one system or another. Pharmaceuticals include a long list of synthetic chemicals. In addition, two types of pharmaceutical production offer especially difficult problems-the fermentation industry, and processing biological material, especially glands, urine, and blood. In fermentation, principal odor emissions are off-gases produced in fermentation or in processing waste products such as spent mash and micella. One of the best odor control systems is scrubbing in an evaporator-cooler equipped with a n entrainment separator, followed by a carbon-adsorption system. In biological processing, treatment of beef lung, for example, to produce a blood anticoagulant is particularly odorous. This problem is solved by a water scrubber followed by feeding chlorine with the stack gas from the scrubber. Some 2000 paint and varnish manufacturing establishments are widely distributed throughout the U S . The principal operations are cooking organic resins and mixing resins with organic solvents. Many of the vaporized compounds are intensely odorous, especially solvents lost in grinding and thin-
ning operations. Odor emissions can be controlled by condensation and absorption by scrubbing; scrubbing and adsorption by charcoal; combustion; dispersal from high stacks; and substitution of water-based for solvent-based coatings. Essential oils are natural or synthetic chemicals used for perfumes and for flavoring foods. Although these odors are pleasant, they may be obnoxious in the vicinity of manufacturing operations, owing to high concentrations or simply to continual presence. The common practice for controlling such emissions is running exhaust air over activated carbon filters to adsorb the odorants. The textile industry converts natural fibers and synthetic organic polymers into finished textiles. Most of the odorant problems arise in the spinning operations of synthetic polymers. These include : Fiber
Solvent emitted
Cellulose acetate
Acetone and/or methylene chloride Carbon disulfide None Dimethyl formamide Methanol
Rayon Nylon Acrylics Polyesters
In these operations, solvents are recovered by large carbon-bed adsorbers with recycle of the exhaust air. Direct flame combustion is then used as a
Costs for catalyzed air oxidation equipment 6.0 2
2 50
Operational costs
.c M
5
. 0 7
v)
e
30
D 0
n 2
5
10 Gas rate (SCFM 10-I)
606 Environmental Science & Technology
15
final control on the draw-off air that maintains an oxygenated atmosphere. Phenolic and burnt-phenolic odors produced during glass fiber spinning are controlled by alkaline scrubbing, catalytic and flame combustion, and ozonization. Finishing operations-flame-proofing, rot-proofing, water repellency, and crease resistance-applied to textile fabrics also cause many problems. Almost all of these processes involve organic solvents or reactants such as formaldehyde. Valuable solvents can be recovered by carbon-bed adsorbers. Combustion methods or other oxidative operations such as permanganate beds complete the treatment. Food, tanneries, mobile sources
Food and agriculture industries are a source of a wide variety of odors. One of the major problems is the large physical area in which the odors are emitted, particularly in animal husbandry, where 500,000 to 1,000,000 chickens or 15,000 to 50,000 cattle are confined in a single area. The odorous gases which arise from these areas include.ammonia, hydrogen sulfide, aliphatic alcohols, many aldehydes, and various organic amines, mercaptans, and sulfides. Chemical treatment methods are based on oxidation or neutralization of acidic materials. Simple air exposure or treatment with permanganate can provide oxidation; lime is used for neutralization. Improved handling methods, or “good housekeeping,” can contribute to, but not fully achieve, odor control. These include: location away from residential areas, keeping feeding areas and animal pens dry, better handling and quick removal of manure, orderly runoff collection and manure disposal, and prompt disposal of dead animals. In slaughterhouses and fish-processing plants, air may be exhausted through scrubbers, oxidative systems, or combustion mechanisms to reduce odors. Canneries for fruits and vegetables also can he sources of odors. The major problem arises from waste products such as hulls, leaves, rinds, pods, cuttings, etc. Cannery waste is commonly digested in either aerobic or anaerobic lagoons, and can cause odors when upsets occur in the operation. Leaves, stalks, and cuttings may he incinerated or handled by sanitary landfill practices. Tannery processes can cause odors due to the proteinaceous nature of hides, hair, and waste flesh. As in slaughterhouses, good housekeeping is a neces-
sity. Ozone and masking agents are used to control the remaining odorants. Municipal sewage treatment plants emit many organic gases, including hydrogen sulfide, mercaptans, sulfides, ammonia, amines, indoles, and other organic substances. Odor control techniques at any sewage plant consist of careful plant design, proper operation, and diligent maintenance of operating equipment. Odors emitted from the plants or ponds and lagoons can be handled in several ways-chlorine and ozone can be used as oxidants, or lime slurry can be added to control hydrogen sulfide. Adding specific biological organisms suppresses hydrogen sulfide production. Air blowers, ozone generation, or other gas-oxidizing systems may be installed also. Some incinerators consume solid wastes and difficult-to-treat industrial wastes and may release odorants themselves if improper design or operation takes place. Diesel-powered and gasoline-powered land vehicles and airplanes are classified as mobile odorant sources. No single method has proved to provide consistent and universal reduction of odorants from diesel-powered vehicles. Broadly speaking, the problem is caused hy incomplete combustion, and any change in design or operating conditions which will improve combustion will decrease odor. Little is known about odors from automobiles and airplanes. Formaldehyde and other low-molecular-weight aldehydes have been isolated from gasoline exhaust. As with diesels, proper maintenance and good driver operation will reduce odors.
Amos Turk is professor of chemistry at The City College of the City University of New York. Dr. Turk has studied various problems related to atmospheric gases and vapors, including odor measure. menf, gas tracing, and odor control b y adsorption and oxidation systems.
Robert C . Haring helped form and became a partner in ChemTech Services, Inc. His industrial positions include research chemist, laboratory director, production manager, and seueral administrative posts. He retired from Olin Corp. before forming ChemTech Services, Inc. Address inquiries to Dr. Haring.
Additional reading Cain. W. S., “Odor intensity: Differences
in the Exponent of the Psychophysical Function,” Perception and Prychophysic
