ash from the coal.(In most such operations the scrubber is downstream of an electrostatic precipitator, but even so some particles pass through the precipitator and are caught in the scrubber. )Thus if we wished to sell the sodium sulfate, we would have to get it out of solution(by evaporation and crystallization) and then purify it. If we did we would find that the total amount produced in a few power plants would glut the current market, so that although a few power plants might sell their sodium sulfate, most could not. Because of its water solubility, it is not generally acceptable in landfills unless they are well protected from water infiltration ut the real difficulty is with carbon dioxide. Here we assumed that we could treat the exhaust gas with dilute alkaline solutions and remove the SOz, which is an acid gas. However, the exhaust gas from combustion sources contains another acid gas, CO2. Normally its concentration is about 12 percent, or 120 times that of the SO2. We are not generally concerned with the fate of COz, but if it gets into solution it will use up sodium hydroxide by the reaction 2 NaOH+ CO2>, CO3 +H,0 Any sodium hydroxide used up this way is not available to participate in Reaction(11. 14). The real problem is how to absorb one acid gas while not absorbing another acid gas that is present in much higher concentration! Fortunately, this is possible because SO2 forms a much stronger acid than does CO2. The reactions that occur in the liquid phase are these CO2(gas)<> CO2(dissolved); +H20<> H2 CO3 < H*+ HCO3(11.16) SO2(gas)<÷SO( dissolved);+H20<÷H2SO3<→H+HSO3(11.17 These show that each of the gases goes from the gas state to the dissolved state, then reacts with water to form the acid, which then dissociates to form hydrogen ion and the bisulfite or bicarbonate ion. If we find the right concentration of H in solution, it may be possible to drive the equilibrium in Eq (11. 16) to the left while driving the equilibrium in Eq.(11. 17) to the right. That is indeed possible if the concentration of hydrogen ions is between 10-and 10-6 mols per liter (pH=4 to 6). But this calculation shows that we cannot use an alkaline scrubbing solution at all alkaline solutions have pH values of 7 or more. To remove SO without absorbing COz, we must use a scrubbing solution that is a weak acid. Furthermore, we must be careful to control the ph of our solution so that it is acid enough m exclude cOz but not acid enough to exclude soz. As the solution absorbs SO2 it becomes more acid, and thus less able to absorb SO2. Controlling pH during the SOz absorption process is of crucial importance to the operation of these devices If the problem were to use Naoh to remove Soz from a gas stream that contained no other acid gases, this would be a simple problem for which ordinary chemical engineering techniques would be satisfactory. The real problem is different from this one for the following reasons 1. There is another acid gas, COz, present that will use up our alkali unless we keep he solution acid enough to exclude it 2. The amount of alkali needed is high, and the cost of sodium hydroxide is enough t we would prefer to use a cheaper alkali if possible 3. We have to do something with the waste product, either sell it or permanently of it 4. Because the volume of gas to be handled is very large, we must be very careful to keep the gas pressure drop in the scrubber low. The pressure drops that are normally used in the chemical and petroleum industry in gas absorbers are much oo large to be acceptable here Forced- Oxidation Limestone wet scrubbers he most widely used process to deal with these problems is forced-oxidation lime-stone wet scrubbing. There are a variety of flowsheets and of mechanical arrangements for this process; Figs 11.5 and 11.6, show one of the most commonly used varieties. In it we see that the flue gas, from which the solid fly ash particles have been removed, passes to a scrubber module where it passes countercurrent to a scrubbing slurry containing water and limestone particles(as well as particles of other calcium salts). In principle this is the same as the H2S scrubber in Examples 11. l and 11.2 Figure 11.6 shows the scrubber module as a vertical spray tower column with a single gas-liquid contacting tray, and with the bottom serving as a liquid storage and oxidation tank. At the top are two levels of entrainment separators. (These are often called Demisters, which is a brand name for one type. )The separators in the figure are chevron type. These devices cause the fine droplets carried with the gas to collect on their surfaces, coalesce, and fall back into the scrubber as drops large enough to fall counter to the upward-flowing gas Some other designs use a packing with a very high open area in the tower or specialized bubbler l1-711－7 ash from the coal. (In most such operations the scrubber is downstream of an electrostatic precipitator, but even so some particles pass through the precipitator and are caught in the scrubber.) Thus if we wished to sell the sodium sulfate, we would have to get it out of solution (by evaporation and crystallization) and then purify it. If we did, we would find that the total amount produced in a few power plants would glut the current market, so that although a few power plants might sell their sodium sulfate, most could not. Because of its water solubility, it is not generally acceptable in landfills unless they are well protected from water infiltration. But the real difficulty is with carbon dioxide. Here we assumed that we could treat the exhaust gas with dilute alkaline solutions and remove the SO2, which is an acid gas. However, the exhaust gas from combustion sources contains another acid gas, CO2. Normally its concentration is about 12 percent, or 120 times that of the SO2. We are not generally concerned with the fate of CO2, but if it gets into solution it will use up sodium hydroxide by the reaction 2 NaOH + CO2 → Na2CO3 + H20 (11.15) Any sodium hydroxide used up this way is not available to participate in Reaction (11.14). The real problem is how to absorb one acid gas while not absorbing another acid gas that is present in much higher concentration! Fortunately, this is possible because SO2 forms a much stronger acid than does CO2. The reactions that occur in the liquid phase are these: CO2(gas) <→ CO2(dissolved); +H20 <→ H2CO3 <→ H+ + HCO3 - (11.16) SO2(gas) <→ SO2(dissolved); +H20<→ H2SO3 <→ H++ HSO3 - (11.17) These show that each of the gases goes from the gas state to the dissolved state, then reacts with water to form the acid, which then dissociates to form hydrogen ion and the bisulfite or bicarbonate ion. If we find the right concentration of H+ in solution, it may be possible to drive the equilibrium in Eq. (11.16) to the left while driving the equilibrium in Eq. (11.17) to the right. That is indeed possible if the concentration of hydrogen ions is between 10-4 and 10-6 mols per liter (pH= 4 to 6). But this calculation shows that we cannot use an alkaline scrubbing solution at all; alkaline solutions have pH values of 7 or more. To remove SO2 without absorbing CO2, we must use a scrubbing solution that is a weak acid. Furthermore, we must be careful to control the pH of our solution so that it is acid enough m exclude CO2 but not acid enough to exclude SO2. As the solution absorbs SO2 it becomes more acid, and thus less able to absorb SO2. Controlling pH during the SO2 absorption process is of crucial importance to the operation of these devices. If the problem were to use NaOH to remove SO2 from a gas stream that contained no other acid gases, this would be a simple problem for which ordinary chemical engineering techniques would be satisfactory. The real problem is different from this one for the following reasons: 1. There is another acid gas, CO2, present that will use up our alkali unless we keep the solution acid enough to exclude it. 2. The amount of alkali needed is high, and the cost of sodium hydroxide is enough that we would prefer to use a cheaper alkali if possible. 3. We have to do something with the waste product, either sell it or permanently dispose of it. 4. . Because the volume of gas to be handled is very large, we must be very careful to keep the gas pressure drop in the scrubber low. The pressure drops that are normally used in the chemical and petroleum industry in gas absorbers are much too large to be acceptable here. Forced-Oxidation Limestone Wet Scrubbers The most widely used process to deal with these problems is forced-oxidation lime-stone wet scrubbing. There are a variety of flowsheets and of mechanical arrangements for this process; Figs. 11.5 and 11.6, show one of the most commonly used varieties. In it we see that the flue gas, from which the solid fly ash particles have been removed, passes to a scrubber module where it passes countercurrent to a scrubbing slurry containing water and limestone particles (as well as particles of other calcium salts). In principle this is the same as the H2S scrubber in Examples 11.1 and 11.2. Figure 11.6 shows the scrubber module as a vertical spray tower column with a single gas-liquid contacting tray, and with the bottom serving as a liquid storage and oxidation tank. At the top are two levels of entrainment separators. (These are often called Demisters, which is a brand name for one type.) The separators in the figure are chevron type. These devices cause the fine droplets carried with the gas to collect on their surfaces, coalesce, and fall back into the scrubber as drops large enough to fall counter to the upward-flowing gas. Some other designs use a packing with a very high open area in the tower or specialized bubbler