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 the10－3 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