choice is some alkali, a source of OH that can remove the h by H++OH←H20 (11.5) Removing the H+ on the fight side of Eq.(11. 4)drives the equilibrium to the fight, greatly increasing the amount of H,S absorbed The Uses and Limitations of Absorbers and Strippers for Air Pollution Control Absorber-stripper combinations are widely used to remove HCs from exhaust gas streams. This example shows that the removal of H2S from natural gas and similar streams is simple and straightforward. The system also works extremely well for removing ammonia from a gas stream, because NHs is very soluble in water or in weak acids, forming a weak alkali by the following reaction NH3+H20→NH4+OH (11.7) It is possible to make practically complete removal of NH3 from gas streams with water or weak acids. The solubility of ammonia is so high that generally the simplest possible forms of this arrangement are satisfactory. To remove SOz from gas streams by this method is also relatively easy if there are no other acid gases present. For example, SO2 could be easily removed from N2 by the scheme using any weak alkali(for example, ammonium hydroxide), and the solution would be easily regenerated to produce pure SO2. The problem of removing sulfur dioxide from combustion gases is much more complex and difficult, as discussed NO and NOz are not readily removed from gas streams by the process. Although NOz is an acid gas that produces nitric acid by reaction with water 3NO2+H20÷2HNO3+NO (11.8) the reaction rate is slow. NO is not an acid gas, so that although we can remove NO2 from a gas stream with an alkaline solvent, we cannot remove No with the same solvent. For this reason, weak alkali solvents are not successful for the joint removal of NO and NO2 or for the rapid removal of NO2 alone. No other solvent is known that serves well for this task. (My generation has not found a suitable solvent to do this; fame and fortune await the person who finds a suitable solvent to remove NO, NO2, and SOz economically from combustion gases scheme is widely used in the chemical and petroleum industries to make separations not directly related to pollution control, e.g, the separation of COz from H. The absorption column can also be used without regenerating the absorbent solution if the amount of material to be collected is small and there is some acceptable way of disposing of the loaded absorbent Sulfur Removal from Hydrocarbons Once H2S has been separated from the other components of the gas, it is normally reacted with oxygen from the air in controlled amounts to oxidize it only as far as elemental sulfur, H2S+1/202→S+H2O and not as far as so H2S+3/202→SO2+H2O (11.10) The elemental sulfur is either sold for use in the production of sulfuric acid or land-filled if them is no nearby market for it Although the chemical reaction in Eq. (11. 9) for production of sulfur(the Claus process) is simple enough, there are a variety of Ways of carrying it out, and the details can be complex; see Kohl and Nielsen. Hundreds of such plants operate successfully throughout the world; every major petroleum refinery has at least one Because elemental sulfur is inert and harmless and because reduced sulfur in the form of hydrogen ulfide or related compounds can be easily oxidized to sulfur or sulfur oxides, the entire strategy of the petroleum and natural gas industries in dealing with reduced sulfur in petroleum, natural gas, and other process gases is to keep the sulfur in the form of elemental sulfur or reduced sulfur(for example, H2S). Oxygen from the air is virtually free, so we can always move in the oxidation direction at low cost. In contrast, hydrogen is an expensive raw material, so that moving in the reduction direction is expensive Sulfur in hydrocarbon fuels(natural gas, propane, gasoline, jet fuel, diesel fuel, furnace oil)is normally converted to So during combustion and then emitted to the atmosphere. Larg oil-burning facilities can have equipment to capture that SOz, but autos, trucks, and airplanes do not. The only way to limit the SO2 emissions from these sources is to limit the amount of sulfur in the fuel. For this reason the Clean air Act of 1990 lim its the amount of sulfur in diesel fuel to 0.05 percent by weight. Crude oils vary in their sulfur contents: low-sulfur crudes are called"sweet high-sulfur crudes, "sour. "If the fraction of the crude oil going to gasoline or diesel fuel has too l1-411－4 choice is some alkali, a source of OH￾that can remove the H+ by H+ +OH- ↔ H20 (11.5) Removing the H+ on the fight side of Eq. (11.4) drives the equilibrium to the fight, greatly increasing the amount of H2S absorbed. The Uses and Limitations of Absorbers and Strippers for Air Pollution Control Absorber-stripper combinations are widely used to remove HCs from exhaust gas streams. This example shows that the removal of H2S from natural gas and similar streams is simple and straightforward. The system also works extremely well for removing ammonia from a gas stream, because NH3 is very soluble in water or in weak acids, forming a weak alkali by the following reaction: NH3 + H20 → NH4 + + OH- (11.7) It is possible to make practically complete removal of NH3 from gas streams with water or weak acids. The solubility of ammonia is so high that generally the simplest possible forms of this arrangement are satisfactory. To remove SO2 from gas streams by this method is also relatively easy if there are no other acid gases present. For example, SO2 could be easily removed from N2 by the scheme using any weak alkali (for example, ammonium hydroxide), and the solution would be easily regenerated to produce pure SO2. The problem of removing sulfur dioxide from combustion gases is much more complex and difficult, as discussed. NO and NO2 are not readily removed from gas streams by the process. Although NO2 is an acid gas that produces nitric acid by reaction with water, 3NO2 + H20 → 2HNO3 + NO (11.8) the reaction rate is slow. NO is not an acid gas, so that although we can remove NO2 from a gas stream with an alkaline solvent, we cannot remove NO with the same solvent. For this reason, weak alkali solvents are not successful for the joint removal of NO and NO2 or for the rapid removal of NO2 alone. No other solvent is known that serves well for this task. (My generation has not found a suitable solvent to do this; fame and fortune await the person who finds a suitable solvent to remove NO, NO2, and SO2 economically from combustion gases by the scheme) The scheme is widely used in the chemical and petroleum industries to make separations not directly related to pollution control, e.g., the separation of CO2 from H2. The absorption column can also be used without regenerating the absorbent solution if the amount of material to be collected is small and there is some acceptable way of disposing of the loaded absorbent. Sulfur Removal from Hydrocarbons Once H2S has been separated from the other components of the gas, it is normally reacted with oxygen from the air in controlled amounts to oxidize it only as far as elemental sulfur, H2S+ 1/2 O2 → S + H2O (11.9) and not as far as SO2, H2S + 3/2 O2 → SO2+H2O (11.10) The elemental sulfur is either sold for use in the production of sulfuric acid or land-filled if them is no nearby market for it. Although the chemical reaction in Eq. (11.9) for production of sulfur (the Claus process) is simple enough, there are a variety of Ways of carrying it out, and the details can be complex; see Kohl and Nielsen. Hundreds of such plants operate successfully throughout the world; every major petroleum refinery has at least one. Because elemental sulfur is inert and harmless and because reduced sulfur in the form of hydrogen sulfide or related compounds can be easily oxidized to sulfur or sulfur oxides, the entire strategy of the petroleum and natural gas industries in dealing with reduced sulfur in petroleum, natural gas, and other process gases is to keep the sulfur in the form of elemental sulfur or reduced sulfur (for example, H2S). Oxygen from the air is virtually free, so we can always move in the oxidation direction at low cost. In contrast, hydrogen is an expensive raw material, so that moving in the reduction direction is expensive. Sulfur in hydrocarbon fuels (natural gas, propane, gasoline, jet fuel, diesel fuel, furnace oil)is normally converted to SO2 during combustion and then emitted to the atmosphere. Large oil-burning facilities can have equipment to capture that SO2, but autos, trucks, and airplanes do not. The only way to limit the SO2 emissions from these sources is to limit the amount of sulfur in the fuel. For this reason the Clean Air Act of 1990 limits the amount of sulfur in diesel fuel to 0.05 percent by weight. Crude oils vary in their sulfur contents: low-sulfur crudes are called "sweet"; high-sulfur crudes, "sour." If the fraction of the crude oil going to gasoline or diesel fuel has too