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 seal10－2 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