10 Control of Volatile Organic Compounds(VOCs) Volatile organic compounds(VOCs)are liquids or solids that contain carbon(carbon bonded to carbon, hydrogen, nitrogen, or sulfur, but not carbonate carbon aCOs nor carbide carbon as in CaC2 or Co or CO2), which vaporize at significant rates are probably the second-most widespread and diverse class of emissions after particulates VOCs are a large family of compounds. Some(eg, benzene)are toxic and carcinogenic, and are regulated individually as hazardous pollutants. Most VOCs are believed not to be toxic (or not very toxic) to humans. Our principal concern with vOCs is that they participate in the"smog reaction and also in the formation of secondary particles in the atmosphere. These latter are mostly in the fine particle size range. Some VOCs are powerful infrared absorbers and thus contribute to the problem of global warning 10.1 VOCs We may now state, as an approximate rule, that VOCs are those organic liquids or solids whose room temperature vapor pressures are greater than about 0.01 psia(=0.0007 atm) and whose atmospheric boiling points are up to about 500 F( 260C), which means most organic compounds with less than about 12 carbon atoms. Materials with higher boiling points evaporate quite slowly into the atmosphere unless they are heated, and hence they are less likely to become art of our VOC problems. (If vaporized, they condense in the atmosphere, forming part of our fine particle problem. A lighted cigarette produces a gaseous mixture of high-boiling organic compounds; when this mixture is cooled on leaving the cigarette it forms a smoke of fine articulate droplets. They are part of our particulate problem, but not our VOC problem.) he legal definition used for regulatory purposes does not set a lower vapor pressure limitation and excludes a large variety of fetal bottle compounds Sealing washer photochemical reactivity, including methane, ethane, and most halogenated with boiling points above about 500-F will have negligible emission rates under normal circumstances. so the ab sence of a lower limitation causes little The terms VOC and hydrocarbon(HC) are not identical. but often are practically identical. Strictly speaking, a Sealing waher hydrocarbon contains only hydrogen and called a"hydrocarbon fuel"because it contains mostly hydrogen and carbon atoms, but also some oxygen, nitrogen sulfur atoms Acetone al ingredient of nail polish remover, is a voc but is not strictly speaking a hydrocarbon because it contains an oxygen atom. In common usage it would often be grouped with the hydrocarb ons Cuff is not attached Fig. 10-1 Three kinds of seals:(a) a static seal, as side cuff es exists between a carbonated beverage bottle and its Flexible bottle caps;(b)a packed seal, as exists between the valve stem and valve body of simple faucet and as alse trainNmIssIor exists on many simple pumps;(c)a rotary seal of the sone pumps
10－1 10 Control of Volatile Organic Compounds(VOCs) Volatile organic compounds (VOCs) are liquids or solids that contain organic carbon (carbon bonded to carbon, hydrogen, nitrogen, or sulfur, but not carbonate carbon as in CaCO3 nor carbide carbon as in CaC2 or CO or CO2), which vaporize at significant rates. VOCs are probably the second-most widespread and diverse class of emissions after particulates. VOCs are a large family of compounds. Some (e.g., benzene) are toxic and carcinogenic, and are regulated individually as hazardous pollutants. Most VOCs are believed not to be toxic (or not very toxic) to humans. Our principal concern with VOCs is that they participate in the "smog" reaction and also in the formation of secondary particles in the atmosphere. These latter are mostly in the fine particle size range. Some VOCs are powerful infrared absorbers and thus contribute to the problem of global warning. 10.1 VOCs We may now state, as an approximate rule, that VOCs are those organic liquids or solids whose room temperature vapor pressures are greater than about 0.01 psia (= 0.0007 atm) and whose atmospheric boiling points are up to about 500 。 F (= 260。C), which means most organic compounds with less than about 12 carbon atoms. Materials with higher boiling points evaporate quite slowly into the atmosphere unless they are heated, and hence they are less likely to become part of our VOC problems. (If vaporized, they condense in the atmosphere, forming part of our fine particle problem. A lighted cigarette produces a gaseous mixture of high-boiling organic compounds; when this mixture is cooled on leaving the cigarette it forms a smoke of fine particulate droplets. They are part of our particulate problem, but not our VOC problem.) The legal definition used for regulatory purposes does not set a lower vapor pressure limitation and excludes a large variety of compounds that have negligible photochemical reactivity, including methane, ethane, and most halogenated compounds. Compounds with boiling points above about 500~F will have negligible emission rates under normal circumstances, so the absence of a lower vapor pressure limitation causes little problem. The terms VOC and hydrocarbon (HC) are not identical, but often are practically identical. Strictly speaking, a hydrocarbon contains only hydrogen and carbon atoms. But gasoline is normally called a "hydrocarbon fuel" because it contains mostly hydrogen and carbon atoms, but also some oxygen, nitrogen, and sulfur atoms. Acetone, CH3-CO-CH3, the principal ingredient of nail polish remover, is a VOC but is not strictly speaking a hydrocarbon because it contains an oxygen atom. In common usage it would often be grouped with the hydrocarbons. Fig. 10-1 Three kinds of seals: (a) a static seal, as exists between a carbonated beverage bottle and its bottle caps; (b)a packed seal, as exists between the valve stem and valve body of simple faucet, and as also exists on many simple pumps; (c)a rotary seal of the type common on the drive shafts of automobiles and some pumps
Hydrocarbons are only slightly soluble in water, so we can normally separate liquid HCs from liquid water by simple phase separation and decantation. However, the water left behind often contains enough dissolved hydrocarbon that it cannot be discharged to the sewer or natural body of water without additional treatment. Polar VOCs, which almost all contain an oxygen or nitrogen atom in addition to carbons and hydrogens(alcohols, ethers, aldehydes and ketones, carboxylic acids, esters, amines, nitriles) are much more soluble in water. This difference in solubilities makes the polar VOCs easier to remove from a gas stream by scrubbing with water, but harder to remove from water once they dissolve in it 10.2 Control By Prevention If possible, we prevent the formation of a vOc-containing air or gas stream, which we must treat by some kind of tailpipe control device. The ways of doing this for VOCs are substitution, process modification. and leakage control 10.2.1 Substitution Oil-based paints, coatings, and inks harden by the evaporation of vOC solvents such as paint thinner into the atmosphere. Water-based paints are concentrated oil-based paints, emulsified in water. After the water evaporates, the small amount of organic solvent in the remaining paint must also evaporate for the paint to harden. Switching from oil-to water-based paints, coatings, and inks greatly reduces but does not totally eliminate the emissions of VOCs from painting, coating, or printing. For many applications, e.g., house paint, the water-based paints seem just as good as oil-based paints. But water-based paints have not yet been developed that can produce auto body finishes as bright, smooth, and durable as the high-performance oil-based paints and coatings now There are numerous other examples where a less volatile or nonvolatile solvent can be substituted for the more volatile one. This replacement normally reduces but does not eliminate the emission of vocs In addition, a less toxic solvent can often be substituted for a more toxic one, although the more toxic solvents often have special solvent properties that are hard to replace Replacing gasoline as a motor fuel with compressed natural gas or propane is also a form of lbstitution that reduces the emissions of vOCs because those fuels can be handled. metered. and burned with fewer VOC emissions than can gasoline. The petroleum industry is working hard to improve the burning properties, handling, and use of gasoline, to make it as low-emission a fuel as compressed natural gas and propane, so that gasoline can keep its dominant position in the auto 10.2.2 Process modification Process modification to prevent or reduce the formation of the vOC stream may be more economical than applying the control options discussed below. Often substitution and process modification are indistinguishable. Changing fuels or solvents without changing their use is clearly substitution. Changing from standard solvent-based painting to fluidized-bed coating could be considered process modification or substitution. Replacing gasoline-powered vehicles with electric-powered vehicles is a form of modification that reduces the emissions of Ocs, as well as emission of carbon monox nitrogen oxides, in the place the vehicle is. On the other hand, it causes other emissions where the electricity is generated If we consider the process as"get workers from their homes to their place of employment, "then improved public transport, mandatory ride pools, etc, are modifications of the process that reduce emissions of VOCs(and of CO and NOx) Many coating, finishing, and decoration processes that at one time depended on evaporating olvents have been replaced by others that do not, e.g, fluidized-bed powder coating and ultraviolet lithography Finding alternatives to Voc solvents and fuels can be difficult, but it is often the most cost-effective way to reduce VOC emissions Seal leaks. Many small emissions of VOCs occur as leaks at seals. In recent years these have come under regulatory control because, as the larger sources are controlled, these become a more significant part of the remaining problem Fig 10. 1 shows three kinds of seals. Fig 10. 1a shows a static seal, as exists between the bottle cap and the top of a bottle of carbonated beverage. A thin washer of elastomeric material is compressed between the metal cap and the glass bottle top This compressed material forms a seal
10－2 Hydrocarbons are only slightly soluble in water, so we can normally separate liquid HCs from liquid water by simple phase separation and decantation. However, the water left behind often contains enough dissolved hydrocarbon that it cannot be discharged to the sewer or natural body of water without additional treatment. Polar VOCs, which almost all contain an oxygen or nitrogen atom in addition to carbons and hydrogens (alcohols, ethers, aldehydes and ketones, carboxylic acids, esters, amines, nitriles) are much more soluble in water. This difference in solubilities makes the polar VOCs easier to remove from a gas stream by scrubbing with water, but harder to remove from water once they dissolve in it. 10.2 Control By Prevention If possible, we prevent the formation of a VOC-containing air or gas stream, which we must treat by some kind of tailpipe control device. The ways of doing this for VOCs are substitution, process modification, and leakage control. 10.2.1 Substitution Oil-based paints, coatings, and inks harden by the evaporation of VOC solvents such as paint thinner into the atmosphere. Water-based paints are concentrated oil-based paints, emulsified in water. After the water evaporates, the small amount of organic solvent in the remaining paint must also evaporate for the paint to harden. Switching from oil- to water-based paints, coatings, and inks greatly reduces but does not totally eliminate the emissions of VOCs from painting, coating, or printing. For many applications, e.g., house paint, the water-based paints seem just as good as oil-based paints. But water-based paints have not yet been developed that can produce auto body finishes as bright, smooth, and durable as the high-performance oil-based paints and coatings now used. There are numerous other examples where a less volatile or nonvolatile solvent can be substituted for the more volatile one. This replacement normally reduces but does not eliminate the emission of VOCs. In addition, a less toxic solvent can often be substituted for a more toxic one, although the more toxic solvents often have special solvent properties that are hard to replace. Replacing gasoline as a motor fuel with compressed natural gas or propane is also a form of substitution that reduces the emissions of VOCs, because those fuels can be handled, metered, and burned with fewer VOC emissions than can gasoline. The petroleum industry is working hard to improve the burning properties, handling, and use of gasoline, to make it as low-emission a fuel as compressed natural gas and propane, so that gasoline can keep its dominant position in the auto fuel market. 10.2.2 Process Modification Process modification to prevent or reduce the formation of the VOC stream may be more economical than applying the control options discussed below. Often substitution and process modification are indistinguishable. (Changing fuels or solvents without changing their use is clearly substitution. Changing from standard solvent-based painting to fluidized-bed powder coating could be considered process modification or substitution.) Replacing gasoline-powered vehicles with electric-powered vehicles is a form of process modification that reduces the emissions of VOCs, as well as emission of carbon monoxide and nitrogen oxides, in the place the vehicle is. On the other hand, it causes other emissions where the electricity is generated. If we consider the process as "get workers from their homes to their place of employment," then improved public transport, mandatory ride pools, etc., are modifications of the process that reduce emissions of VOCs (and of CO and NOx). Many coating, finishing, and decoration processes that at one time depended on evaporating solvents have been replaced by others that do not, e.g., fluidized-bed powder coating and ultraviolet lithography. Finding alternatives to VOC solvents and fuels can be difficult, but it is often the most cost-effective way to reduce VOC emissions. Seal leaks. Many small emissions of VOCs occur as leaks at seals. In recent years these have come under regulatory control because, as the larger sources are controlled, these become a more significant part of the remaining problem. Fig 10.1 shows three kinds of seals. Fig 10.1a shows a static seal, as exists between the bottle cap and the top of a bottle of carbonated beverage. A thin washer of elastomeric material is compressed between the metal cap and the glass bottle top. This compressed material forms a seal
that prevents the escape of COz(carbonation), often for many years, Leaks through this kind of seal are generally unimportant. Sealing is more difficult when one of the two surfaces involved in the seal moves relative to the other Fig 10 1b shows a simple compression seal between a housing and a shaft. The example shown is a water faucet, in which a nut screws down over the body of the faucet to compress an elastomeric seal that is trapped between the body of the faucet and the stem of the valve. The compressed seal must be tight enough to prevent leakage of the high-pressure water inside the valve out along the edge of the stem, but not so tight that the valve cannot be easily rotated by hand. Students are probably aware from personal experience that this type of seal often leaks. If the leak is a small amount of water into the bathroom sink, that causes little problem; tightening the nut normally reduces the leak to a rate low enough that it becomes invisible(but does not become zero) Fig 10. lc shows in greatly simplified form the seal that surrounds the drive shaft of an automobile at the point where the shaft exits from the transmission. The inside of the transmission is filled with oil. the elastomeric seal is like a shirt cuff turned back on itself with the outside held solidly to the wall of the transmission and the inside held loosely against the rotating shaft by a garter spring. If we set that spring loosely, then there will be a great deal of leakage. If we set it tightly, then the friction and wear between the cuff and the shaft that rotates inside it will be excessive. Setting the tension on that spring requires a compromise between the desire for low leakage and the desire for low friction and wear. That compromise normally leads to a low, but not a zero, leakage rate, a small amount of oil is always dripping out, and accumulating on the floor of our garages. Valves and pumps also have shafts that must rotate, and hence they have the same kind of leakage problem All of the pumps and valves in facilities that process VOCs have this same kind of leakage problem. The seals regularly used are more complex versions of types b and c in Fig. 10.1. There is considerable regulatory pressure for the seals to be made more and more leak-tight. Mostly this goal will be accomplished by replacing simple, low-quality seals on pumps and valves with more complex and expensive, higher-quality seals a truly innovative example of voc leakage control occurred when the ARCO oil company sank and anchored a large, steel, inverted funnel over a natural gas seep at the bottom of the Santa Barbara Channel off the coast of southern California. The gas captured by the funnel is piped to shore and processed. The value of this gas is less than the cost of the equipment that captures it but the company thereby removed VOCs from the atmosphere and gained needed VOc pollution-control credit, at a lower cost per pound than it could have in any of its other southern California facilities 10.3 Control By Concentration And Recovery 10.3.1 Adsorption Adsorption means the attachment of molecules to the surface of a solid. In contrast, absorption means the dissolution of molecules within a collecting medium, which may be liquid or solid Generally, absorbed materials are dissolved into the absorbent, like sugar dissolved in water whereas adsorbed materials are attached onto the surface of a material. like dust on a wall Absorption mostly occurs into liquids, adsorption mostly onto solids. This section deals only with dsorption onto the surface of a solid adsorbent Adsorption is mostly used in air pollution control to concentrate a pollutant that is present in dilute form in an air or gas stream. The material collected is most often a VOc like gasoline or various paint thinners and solvents. The solid is most often some kind of activated carbon. The student is possibly familiar with cigarettes that have activated carbon filters to collect some of the harmful materials in the smoke. They are used once and thrown away. The student is probably less familiar with the activated carbon canisters used in industrial face masks. These are worn by workers exposed to solvents, as in paint spraying or solvent cleaning. The worker's lungs suck the air in through thin beds of activated carbon, contained in replaceable cartridges on the face mask, When the activated carbon is loaded (i.e, the solvent begins to come through into the worker's breathing space)the cartridge of activated carbon is discarded and a fresh one installed For large-scale air pollution applications, like collecting the solvent vapors coming off a large paint-drying oven or a large printing press, the normal procedure is to use several adsorption beds As shown in Fig 10.2, the contaminated air stream passes through two vessels in series. Inside each of the vessels is a bed of adsorbent that removes the vocs. from the second vessel the cleaned air, normally containing at most a few parts per million of VOCs, passes to the
10－3 that prevents the escape of CO2 (carbonation), often for many years. Leaks through this kind of seal are generally unimportant. Sealing is more difficult when one of the two surfaces involved in the seal moves relative to the other. Fig 10.1b shows a simple compression seal between a housing and a shaft. The example shown is a water faucet, in which a nut screws down over the body of the faucet to compress an elastomeric seal that is trapped between the body of the faucet and the stem of the valve. The compressed seal must be tight enough to prevent leakage of the high-pressure water inside the valve out along the edge of the stem, but not so tight that the valve cannot be easily rotated by hand. Students are probably aware from personal experience that this type of seal often leaks. If the leak is a small amount of water into the bathroom sink, that causes little problem; tightening the nut normally reduces the leak to a rate low enough that it becomes invisible (but does not become zero). Fig 10.1c shows in greatly simplified form the seal that surrounds the drive shaft of an automobile at the point where the shaft exits from the transmission. The inside of the transmission is filled with oil. The elastomeric seal is like a shirt cuff turned back on itself with the outside held solidly to the wall of the transmission and the inside held loosely against the rotating shaft by a garter spring. If we set that spring loosely, then there will be a great deal of leakage. If we set it tightly, then the friction and wear between the cuff and the shaft that rotates inside it will be excessive. Setting the tension on that spring requires a compromise between the desire for low leakage and the desire for low friction and wear. That compromise normally leads to a low, but not a zero, leakage rate; a small amount of oil is always dripping out, and accumulating on the floor of our garages. Valves and pumps also have shafts that must rotate, and hence they have the same kind of leakage problem. All of the pumps and valves in facilities that process VOCs have this same kind of leakage problem. The seals regularly used are more complex versions of types b and c in Fig. 10.1. There is considerable regulatory pressure for the seals to be made more and more leak-tight. Mostly this goal will be accomplished by replacing simple, low-quality seals on pumps and valves with more complex and expensive, higher-quality seals. A truly innovative example of VOC leakage control occurred when the ARCO oil company sank and anchored a large, steel, inverted funnel over a natural gas seep at the bottom of the Santa Barbara Channel off the coast of southern California. The gas captured by the funnel is piped to shore and processed. The value of this gas is less than the cost of the equipment that captures it, but the company thereby removed VOCs from the atmosphere and gained needed VOC pollution-control credit, at a lower cost per pound than it could have in any of its other southern California facilities. 10.3 Control By Concentration And Recovery 10.3.1 Adsorption Adsorption means the attachment of molecules to the surface of a solid. In contrast, absorption means the dissolution of molecules within a collecting medium, which may be liquid or solid. Generally, absorbed materials are dissolved into the absorbent, like sugar dissolved in water, whereas adsorbed materials are attached onto the surface of a material, like dust on a wall. Absorption mostly occurs into liquids, adsorption mostly onto solids. This section deals only with adsorption onto the surface of a solid adsorbent. Adsorption is mostly used in air pollution control to concentrate a pollutant that is present in dilute form in an air or gas stream. The material collected is most often a VOC like gasoline or various paint thinners and solvents. The solid is most often some kind of activated carbon. The student is possibly familiar with cigarettes that have activated carbon filters to collect some of the harmful materials in the smoke. They are used once and thrown away. The student is probably less familiar with the activated carbon canisters used in industrial face masks. These are worn by workers exposed to solvents, as in paint spraying or solvent cleaning. The worker's lungs suck the air in through thin beds of activated carbon, contained in replaceable cartridges on the face mask. When the activated carbon is loaded (i.e., the solvent begins to come through into the worker's breathing space) the cartridge of activated carbon is discarded and a fresh one installed. For large-scale air pollution applications, like collecting the solvent vapors coming off a large paint-drying oven or a large printing press, the normal procedure is to use several adsorption beds. As shown in Fig. 10.2, the contaminated air stream passes through two vessels in series. Inside each of the vessels is a bed of adsorbent that removes the VOCs. From the second vessel the cleaned air, normally containing at most a few parts per million of VOCs, passes to the
atmosphere. Meanwhile, a third vessel is being regenerated Steam passes through it, removing the adsorbed vocs from the Cleaned air with omly a adsorbent. The mixture of steam and vOCs coming from the top of water-cooled condenser that condenses both the VOCs and the steam. Both pass in liquid form to separator, where the VOCs, rmal dense than water and have little solubility in water, float on top and are decanted and sent to solvent recovery. COntiner a o I% hydrocarbon solvent Pig. 10-2 The typical arrangement automatic switching valves; and steam FIGURE 10.10 typical arrangement for adsorpoon of a voC from a gas stream, using three adsorbent beds automate gra separation After a suitable time period a set of automatically programmed valves changes the position of the containers in the flow sheet. (The containers do not move, their place in the piping arrangement changes )Container 1, which is most heavily loaded, goes to the regeneration position. Container 2, which is lightly loaded with VOCs, goes to the position where container I was; and container 3, which is now regenerated and very clean, goes to the position previously held by container 2, making the final cleanup on the air stream. Fig 10.2 shows the steam condensate leaving the phase separator, without specifying where it goes. As discussed above, this condensate will be saturated with dissolved VOC. The VOC concentration may be high enough to prevent its being sent back to the steam boiler, or for it to be discharged to a sewer. If there is no good way to deal with this stream, then the absorber solves a large air pollution problem but creates a small water pollution problem! Adsorbents. The most widely used adsorbent for VOCs is activated carbon. This somewhat fancier version of the charcoal used for barbecuing has an amazing amount of surface area In Example 7 15 we showed that catalyst supports typically have surface areas of 100 m-g, corresponding to internal wall thicknesses of 100 A. Adsorbents like activated carbon often have surface areas of 1000 m-g, corresponding to an internal wall thickness of 10 A,. This value is startlingly low, about four times the interatomic spacing in crystals! If adsorbents have this much surface area, then they must have internal walls only four atoms thick! Apparently they do. To make materials with this much surface area, one starts with a material, from which part can be removed on an atomic scale. In the case of activated carbon, one starts with wood (or peach pits, or coconut shells, or some other woody material) and heats it to a high enough temperature that the wood decomposes(pyrolyzes), producing a gas and leaving behind a solid carbon residue, in the form of these thin internal walls 10.3.2 Absorption(Scrubbing If we can find a liquid solvent in which the vOC is soluble and in which the remainder of the contaminated gas stream is insoluble, then we can use absorption to remove and concentrate the voC for recovery and re-use, or destruction. The standard chemical engineering method of removing any component from a gas stream--absorption and stripping--is sketched in Fig. 10.3. If we can find a liquid solvent in which the gaseous component we wish to selectively remove is much more soluble than are the other components in the gas stream, the procedure is quite traightforward. The feed gas enters the absorber, which is a vertical column in which the gas passes upward and the liquid solvent passes downward. Normally, bubble caps, sieve trays, or packing is used in the interior of the column to promote good countercurrent contact between the solvent and the gas. The stripped solvent enters the top of the column and flows countercurrent to the gas. By the time the gas has reached the top of the column, most of the component we wish to remove has been dissolved into the solvent; the cleaned gas passes on to the atmosphere or to its
10－4 atmosphere. Meanwhile, a third vessel is being regenerated. Steam passes through it, removing the adsorbed VOCs from the adsorbent. The mixture of steam and VOCs coming from the top of the vessel passes to a water-cooled condenser that condenses both the VOCs and the steam. Both pass in liquid form to a separator, where the VOCs, which are normally much less dense than water and have little solubility in water, float on top and are decanted and sent to solvent recovery. After a suitable time period a set of automatically programmed valves changes the position of the containers in the flow sheet. (The containers do not move; their place in the piping arrangement changes.) Container 1, which is most heavily loaded, goes to the regeneration position. Container 2, which is lightly loaded with VOCs, goes to the position where container I was; and container 3, which is now regenerated and very clean, goes to the position previously held by container 2, making the final cleanup on the air stream. Fig 10.2 shows the steam condensate leaving the zphase separator, without specifying where it goes. As discussed above, this condensate will be saturated with dissolved VOC. The VOC concentration may be high enough to prevent its being sent back to the steam boiler, or for it to be discharged to a sewer. If there is no good way to deal with this stream, then the absorber solves a large air pollution problem but creates a small water pollution problem! Adsorbents. The most widely used adsorbent for VOCs is activated carbon. This somewhat fancier version of the charcoal used for barbecuing has an amazing amount of surface area. In Example 7.15 we showed that catalyst supports typically have surface areas of 100 m2 /g, corresponding to internal wall thicknesses of 100 A. Adsorbents like activated carbon often have surface areas of 1000 m2 /g, corresponding to an internal wall thickness of 10 A,. This value is startlingly low, about four times the interatomic spacing in crystals! If adsorbents have this much surface area, then they must have internal walls only four atoms thick! Apparently they do. To make materials with this much surface area, one starts with a material, from which part can be removed on an atomic scale. In the case of activated carbon, one starts with wood (or peach pits, or coconut shells, or some other woody material) and heats it to a high enough temperature that the wood decomposes (pyrolyzes), producing a gas and leaving behind a solid carbon residue, in the form of these thin internal walls. 10.3.2 Absorption (Scrubbing) If we can find a liquid solvent in which the VOC is soluble and in which the remainder of the contaminated gas stream is insoluble, then we can use absorption to remove and concentrate the VOC for recovery and re-use, or destruction. The standard chemical engineering method of removing any component from a gas stream--absorption and stripping--is sketched in Fig. 10.3. If we can find a liquid solvent in which the gaseous component we wish to selectively remove is much more soluble than are the other components in the gas stream, the procedure is quite straightforward. The feed gas enters the absorber, which is a vertical column in which the gas passes upward and the liquid solvent passes downward. Normally, bubble caps, sieve trays, or packing is used in the interior of the column to promote good countercurrent contact between the solvent and the gas. The stripped solvent enters the top of the column and flows countercurrent to the gas. By the time the gas has reached the top of the column, most of the component we wish to remove has been dissolved into the solvent; the cleaned gas passes on to the atmosphere or to its Fig. 10-2 The typical arrangement for adsorption of a VOC from a gas stream, using three adsorbed beds; automatic switching valves; and steam desorption, condensation, and gravity separation
further uses. The loaded solvent, which now contains most of the component we are removing from the gas, passes to the stripper, which normally is operated at a higher temperature and/or lower pressure than the absorber. At this higher temperature and/or lower pressure, the solubility of the gas in the selective solvent is greatly reduced so the gas comes out of solution. In Fig. 10.3 the separated component is shown leaving as a gas for use, sale, or destruction. In some cases it is condensed and leaves as a liquid The stripped or lean solvent is sent back to the absorber column Very large absorption-stripping systems often use tray columns, of this section assumes that we air pollution control applications use internal packings. The rest are discussing packed absorber columns Functionally, this is the same as the adsorption process sketched in Fig. 10.2 The chosen component is selectively removed from the gas str adsorbent or into an absorbent in one vessel and is subsequently removed at much higher concentration(often practically pure)in another vessel at a higher Cleaned gas out Solute gas out temperature and/or lower Solvent Condenser essure absorption-stripping scheme separator in Fig. 10.3 is mechanically Liquid reflux move liquids with pumps and pipes. It is much harder to absorber move solids the same way. The adsorption equivalents 3 have been tried, bi the mechanical difficulties have been severe enough that Reboiler most adsorption is done with the solids remaining in place as shown in Fig. 10.2 Fig. 10-3 The flow diagram for the most co mmon method for removing semisteady-state operation one Component from a gas stream The absorption solvent must have the following properties 1. It must afford reasonable solubility for the material to be removed, and, if this material is to be recovered at reasonable purity, it must not dissolve and thus carry along any of the other components of the gas stream 2. In the absorber, the gas being treated will come to equilibrium with the stripped solvent. The vapor pressure of the solvent, at absorber temperature, must be low enough that if the cleaned gas to be discharged to the atmosphere, the emission of solvent is small enough to be permissible Some solvent is lost this way; the cost of replacing it must be acceptable. If the solvent is water this is not a problem(unless we need the gas to be dry for its next use), but for other solvents this 3. At the higher temperature (or lower pressure) of the stripping column, the absorbed material must come out of solution easily, and the vapor pressure of the solvent must be low enough that it does not contaminate the recovered VOC. If the solvent vapor pressure in the stripper is too large, one may replace the stripper by a standard distillation column(combination stripper and rectifier) to recover the transferred material at adequate purity 4. The solvent must be stable at the conditions in the absorber and stripper, and be usable for a considerable time before replacement 5. The solvent molecular weight should be as low as possible, to maximize its ability to absorb This requirement conflicts with the low solvent vapor pressure requirement, so that a compromise must be made 10.4 Control By Oxidation The final fate of vOCs is mostly to be oxidized to COz and H20, as a fuel either in our engines or urnaces, in an incinerator, in a biological treatment device, or in the atmosphere(forming ozone and fine particles). VOC-containing gas streams that are too concentrated to be discharged to the atmosphere but not large enough to be concentrated and recovered are oxidized before discharge, either at high temperatures m an incinerator or at low temperatures by biological oxidation Application to boilers, furnaces, flares, etc. One of the first major undertakings in the history of air pollution control was the control of emissions from coal-buri boilers. furnaces. etc Unburned coal or products of incomplete combustion formed a substantial part of these emissions
10－5 further uses. The loaded solvent, which now contains most of the component we are removing from the gas, passes to the stripper, which normally is operated at a higher temperature and/or a lower pressure than the absorber. At this higher temperature and/or lower pressure, the solubility of the gas in the selective solvent is greatly reduced so the gas comes out of solution. In Fig. 10.3 the separated component is shown leaving as a gas for use, sale, or destruction. In some cases it is condensed and leaves as a liquid. The stripped or lean solvent is sent back to the absorber column. Very large absorption-stripping systems often use tray columns, but the small ones used in most air pollution control applications use internal packings. The rest of this section assumes that we are discussing packed absorber columns. Functionally, this is the same as the adsorption process sketched in Fig. 10.2. The chosen component is selectively removed from the gas stream onto an adsorbent or into an absorbent in one vessel and is subsequently removed at much higher concentration (often practically pure) in another vessel at a higher temperature and/or lower pressure. The absorption-stripping scheme in Fig. 10.3 is mechanically simpler because it is easy to move liquids with pumps and pipes. It is much harder to move solids the same way. The adsorption equivalents of Fig. 10.3 have been tried, but the mechanical difficulties have been severe enough that most adsorption is done with the solids remaining in place as shown in Fig. 10.2, using a semisteady-state operation. The absorption solvent must have the following properties: 1. It must afford reasonable solubility for the material to be removed, and, if this material is to be recovered at reasonable purity, it must not dissolve and thus carry along any of the other components of the gas stream. 2. In the absorber, the gas being treated will come to equilibrium with the stripped solvent. The vapor pressure of the solvent, at absorber temperature, must be low enough that if the cleaned gas is to be discharged to the atmosphere, the emission of solvent is small enough to be permissible. Some solvent is lost this way; the cost of replacing it must be acceptable. If the solvent is water this is not a problem (unless we need the gas to be dry for its next use), but for other solvents this can be a problem. 3. At the higher temperature (or lower pressure) of the stripping column, the absorbed material must come out of solution easily, and the vapor pressure of the solvent must be low enough that it does not contaminate the recovered VOC. If the solvent vapor pressure in the stripper is too large, one may replace the stripper by a standard distillation column (combination stripper and rectifier) to recover the transferred material at adequate purity. 4. The solvent must be stable at the conditions in the absorber and stripper, and be usable for a considerable time before replacement. 5. The solvent molecular weight should be as low as possible, to maximize its ability to absorb. This requirement conflicts with the low solvent vapor pressure requirement, so that a compromise must be made. 10.4 Control By Oxidation The final fate of VOCs is mostly to be oxidized to CO2 and H20, as a fuel either in our engines or furnaces, in an incinerator, in a biological treatment device, or in the atmosphere (forming ozone and fine particles). VOC-containing gas streams that are too concentrated to be discharged to the atmosphere but not large enough to be concentrated and recovered are oxidized before discharge, either at high temperatures m an incinerator or at low temperatures by biological oxidation. Application to boilers, furnaces, flares, etc. One of the first major undertakings in the history of air pollution control was the control of emissions from coal-burning boilers, furnaces, etc. Unburned coal or products of incomplete combustion formed a substantial part of these emissions. Fig. 10-3 The flow diagram for the most common method for removing one Component from a gas stream
These were one of the easiest pollutants to control; all that is required for good control is sufficient excess air and adequate mixing between the burning coal, its decomposition products, and the air. To get complete combustion with imperfect m nixing, one must supply excess air in addition to that ded for stoichiometric combustion. The amount of excess air to be used is determined by economics. At zero excess air, some valuable fuel escapes unburned to pollute the atmosphere Large amounts of excess air lower the combustion temperature by diluting the combustion (fracticts, and carry away more heat in the exhaust gas. This lowers the furnace's efficiency furnaces operate with 5 to 30 percent excess air. Autos have variable excess air naing on load. The optimum amount of excess air for voc destruction is generally optimum for fuel efficiency; air pollution control officers try to induce furnace operators to use the timum amount for voc destruction mixing problem is especially difficult in flares. These are safety devices used in oil refineries and many other processing plants. All vessels containing fluids under pressure have high-pressure elief valves that open if the internal pressure of the vessel exceeds its safe operating value. Al ousehold water heaters have such a valve to prevent tank rapture in some unlikely but not impossible circumstances. With a hot water heater, if the valve opens, hot water drops onto the floor. In the case of a large petroleum-processing vessel(distillation column, cracker, isomerize tc. )the material released is an inflammable VOC, which cannot safely be dropped on the floo The outlets of a refinery's relief valves are piped to a flare(or"flare stack"), which is an elevated mix air into the gas being released. These flares handle significant amounts of vocs only during process upsets and emergencies at the facilities they serve. When there is a small release, the steam jets can often mix the gas and air well enough that there is practically complete combustion For a large release the mixing is inadequate, and the large, bright orange, smoky flame from the flare indicates a significant release of unburned or partly burned voc In the coal combustion process one difficulty, even in well-designed modem furnaces, is that some articles of coal and some hydrocarbons pass out of the flame zone before they can be combusted These are called soot. In modem steam boilers this soot will collect in parts of the furnace where it is too cold for soot to burn, typically on the tubes in which the water is boiled or the steam rheated. If soot is allowed to collect there it will impede heat transfer and make the boiler less efficient. The cure for this problem is a soot blower, which is typically a fixed or moving steam jet that blows high-pressure steam onto the surface of the tubes to remove this soot normally, soot blowing is required only a few minutes per day. Soot dislodged in this way exits the furnace as short-period emissions of black smoke. Most public relations officers ask plant engineers to do all soot blowing at nigl Biological Oxidation(Biofiltration) as discussed above. the ultimate fate of vocs is to be oxidized to co, and H,o. either in our engines or furnaces, or incinerators, or in the environment. Many microorganisms will carry out these reactions fairly quickly at room temperature. They form the basis of most sewage treatment plants(oxidizing more complex organic materials than the simple voCs of air pollution interest Microorganisms can also oxidize the vOCs contained in gas or air streams. The typical biofilter (not truly a filter but commonly called one; better called a highly porous biochemical reactor) consists of the equivalent of a swimming pool, with a set of gas distributor pipes at the bottom covered with several feet of soil or compost or loam in which the microorganisms live. The tonta mn aed vas nters through the watrer ton aipes an he ssi sland y hep to eg b oizeal bw ithe microorganisms that live there these devices have soil depths of 3 to 4 ft void volumes of 50%, upward gas velocities of0.005 to 0.5 ft/s, and gas residence times of 15 to 60 s. They work much better with polar VOCs, which are fairly soluble in water than with HCs whose solubility is much less. The microorganisms must be kept moist, protected from conditions that could injure them, and in some cases given nutrients. Because of the long time the gases must spend in them, these devices are much larger and take up much more ground surface than any of the other devices discussed in this chapter. In spite of these drawbacks, there are some applications for which they are economica 10.5 Summary 1. VOCs are emitted from e variety of sources and have a wide variety of individual components, each with its own properties. We use VOCs mostly as petroleum-based fuels and solvents. The emissions come from fuel and solvent usage, transportation, and storage 2. The control alternatives are prevention, concentration and recovery, or oxidation 3. Some of these control options can also be used for non-VOC emissions, e.g., incineration for odor control of HS, adsorption for SOz or mercury vapor and leakage control for any process source 10-6
10－6 These were one of the easiest pollutants to control; all that is required for good control is sufficient excess air and adequate mixing between the burning coal, its decomposition products, and the air. To get complete combustion with imperfect mixing, one must supply excess air in addition to that needed for stoichiometric combustion. The amount of excess air to be used is determined by economics. At zero excess air, some valuable fuel escapes unburned to pollute the atmosphere. Large amounts of excess air lower the combustion temperature by diluting the combustion products, and carry away more heat in the exhaust gas. This lowers the furnace's efficiency (fraction of heating value of the fuel transferred to whatever is being heated). Large industrial furnaces operate with 5 to 30 percent excess air. Autos have variable excess air, depending on engine load. The optimum amount of excess air for VOC destruction is generally higher than the optimum for fuel efficiency; air pollution control officers try to induce furnace operators to use the optimum amount for VOC destruction. The mixing problem is especially difficult in flares. These are safety devices used in oil refineries and many other processing plants. All vessels containing fluids under pressure have high-pressure relief valves that open if the internal pressure of the vessel exceeds its safe operating value. All household water heaters have such a valve to prevent tank rapture in some unlikely but not impossible circumstances. With a hot water heater, if the valve opens, hot water drops onto the floor. In the case of a large petroleum-processing vessel (distillation column, cracker, isomerizer, etc.) the material released is an inflammable VOC, which cannot safely be dropped on the floor. The outlets of a refinery's relief valves are piped to a flare (or "flare stack"), which is an elevated pipe with pilot lights to ignite any released VOCs. Many have steam jets running constantly to mix air into the gas being released. These flares handle significant amounts of VOCs only during process upsets and emergencies at the facilities they serve. When there is a small release, the steam jets can often mix the gas and air well enough that there is practically complete combustion. For a large release the mixing is inadequate, and the large, bright orange, smoky flame from the flare indicates a significant release of unburned or partly burned VOC. In the coal combustion process one difficulty, even in well-designed modem furnaces, is that some particles of coal and some hydrocarbons pass out of the flame zone before they can be combusted. These are called soot. In modem steam boilers this soot will collect in parts of the furnace where it is too cold for soot to burn, typically on the tubes in which the water is boiled or the steam superheated. If soot is allowed to collect there, it will impede heat transfer and make the boiler less efficient. The cure for this problem is a soot blower, which is typically a fixed or moving steam jet that blows high-pressure steam onto the surface of the tubes to remove this soot. Normally, soot blowing is required only a few minutes per day. Soot dislodged in this way exits the furnace as short-period emissions of black smoke. Most public relations officers ask plant engineers to do all soot blowing at night. Biological Oxidation (Biofiltration) As discussed above, the ultimate fate of VOCs is to be oxidized to CO2 and H20, either in our engines or furnaces, or incinerators, or in the environment. Many microorganisms will carry out these reactions fairly quickly at room temperature. They form the basis of most sewage treatment plants (oxidizing more complex organic materials than the simple VOCs of air pollution interest). Microorganisms can also oxidize the VOCs contained in gas or air streams. The typical biofilter (not truly a filter but commonly called one; better called a highly porous biochemical reactor) consists of the equivalent of a swimming pool, with a set of gas distributor pipes at the bottom, covered with several feet of soil or compost or loam in which the microorganisms live. The contaminated gas enters through the distributor pipes and flows slowly up through soil, allowing time for the VOC to dissolve in the water contained in the soil, and then to be oxidized by the microorganisms that live there. Typically these devices have soil depths of 3 to 4 ft, void volumes of 50%, upward gas velocities of 0.005 to 0.5 ft/s, and gas residence times of 15 to 60 s. They work much better with polar VOCs, which are fairly soluble in water than with HCs whose solubility is much less. The microorganisms must be kept moist, protected from conditions that could injure them, and in some cases given nutrients. Because of the long time the gases must spend in them, these devices are much larger and take up much more ground surface than any of the other devices discussed in this chapter. In spite of these drawbacks, there are some applications for which they are economical, and for which they are used industrially . 10.5 Summary 1. VOCs are emitted from a wide variety of sources and have a wide variety of individual components, each with its own properties. We use VOCs mostly as petroleum-based fuels and solvents. The majority of our VOC emissions come from fuel and solvent usage, transportation, and storage. 2. The control alternatives are prevention, concentration and recovery, or oxidation. 3. Some of these control options can also be used for non-VOC emissions, e.g., incineration for odor control of H2S, adsorption for SO2 or mercury vapor, and leakage control for any process source