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 emissions10－5 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