high-pressure treatment of H2S, either plate or packed towers are used, with little problem. For the SO2 problem, three plausible arrangements are sketched in Fig. 11. 4. The first of these is a simple bubbler, in which the gas is forced under pressure through perforated pipes submerged in the th through the liquid the Liquid chemical equilibrium Liquid with it. If the liquid p enough and the bubbles are small enough, this kind of m96::08。° device will bring the gas close to chemical equilibrium with th liquid. However, it has a high pressure pressure must at least FIGURE 1L4 equal the hydrostatic hree plausible arrangements for scrubbing a gas with a liquid: (a)bubbler. (b) spray chamber, and (c) packed head of the liquid. If for exam liquid is a foot deep, then the hydrostatic head will be 12 inches of liquid, which is large enough to be quite expensive(see Example 7.3). Plate-type distillation and absorption columns are, in effect, a series of such bubblers, stacked one above the other, with the gas flowing up from one to the next and the liquid flowing down from one to the next through pipes called downcomers. At high pressures, where pressure drops are unimportant, they are the most widely used device The second arrangement is a spray chamber. In it the gas flows up through an open chamber while the scrubbing liquid falls from spray nozzles, much like the heads in bathroom showers, through the gas. In this arrangement the gas pressure drop is small, but it is difficult to approach equilibrium because the gas does not contact the liquid as well as it does in the bubbler Nonetheless, it is widely used because of its simplicity, low pressure drop, and resistance to scale deposition and plugging The third arrangement is a packed column, which is similar to the spray chamber except that the open space is filled with some kind of solid material that allows the liquid to coat its surface and run down over it in a thin film. The gas passes between pieces of solid material and comes in good contact with the liquid films, in the most primitive of these, the solid materials were gravel or crushed rocks. More advanced ones use special shapes of ceramic, plastic, or metal that are fabricated to provide the optimum distribution of liquid surface for contact with the gas. This third kind of contactor can be designed to have a better mass transfer per unit of gas pressure drop than ither of the other two kinds. All three of these arrangements, plus combinations of them, plus some other arrangements are in current use for removal of SO2 from power plant stack gases The gas velocities in such devices range from about I ft/s in a packed tower to 10 ft/s for a spray hamber. If we assume we are going to treat the gas in Example 11. 5 in a spray chamber at a gas velocity of 10 Its, the cross-sectional area perpendicular to the gas flow will QA=1667ft2=155m Such devices are almost al ways cylindrical, because that shape is easier and cheaper to fabricate than, for example, a rectangular vessel of equal cross-sectional area. For this example the diameter would be(4. 1667 ft/)o5=46 ft=14 m. A typical length in the flow direction would be 50 ft. That is a very large diameter for any piece of chemical plant equipment, but not for a power plant Often the flow will be divided into several smaller scrubbers in parallel. This choice avoids having to ship or fabricate too large a vessel and ensures that one of the vessels can be taken out of service for maintenance while the rest are in operation. Thus the power plant can continue to operate while one part of the scrubber is out of service What problems might power plant operators encounter? First, there is the question of what to do with the sodium sulfate produced. Sodium sulfate(also called"salt cake") is used in detergent manufacture and in paper making, as well as in some miscellaneous uses. However, for those it must be quite pure. The sodium sulfate produced in this process would be contaminated with fly l1-611－6 high-pressure treatment of H2S, either plate or packed towers are used, with little problem. For the SO2 problem, three plausible arrangements are sketched in Fig. 11.4. The first of these is a simple bubbler, in which the gas is forced under pressure through perforated pipes submerged in the scrubbing liquid. As the bubbles rise through the liquid, they approach chemical equilibrium with it. If the liquid is deep enough and the bubbles are small enough, this kind of device will bring the gas close to chemical equilibrium with the liquid. However, it has a high pressure drop. The gas pressure must at least equal the hydrostatic head of the liquid. If, for example, the liquid is a foot deep, then the hydrostatic head will be 12 inches of liquid, which is large enough to be quite expensive (see Example 7.3). Plate-type distillation and absorption columns are, in effect, a series of such bubblers, stacked one above the other, with the gas flowing up from one to the next and the liquid flowing down from one to the next through pipes called downcomers. At high pressures, where pressure drops are unimportant, they are the most widely used device. The second arrangement is a spray chamber. In it the gas flows up through an open chamber while the scrubbing liquid falls from spray nozzles, much like the heads in bathroom showers, through the gas. In this arrangement the gas pressure drop is small, but it is difficult to approach equilibrium because the gas does not contact the liquid as well as it does in the bubbler. Nonetheless, it is widely used because of its simplicity, low pressure drop, and resistance to scale deposition and plugging. The third arrangement is a packed column, which is similar to the spray chamber except that the open space is filled with some kind of solid material that allows the liquid to coat its surface and run down over it in a thin film. The gas passes between pieces of solid material and comes in good contact with the liquid films, in the most primitive of these, the solid materials were gravel or crushed rocks. More advanced ones use special shapes of ceramic, plastic, or metal that are fabricated to provide the optimum distribution of liquid surface for contact with the gas. This third kind of contactor can be designed to have a better mass transfer per unit of gas pressure drop than either of the other two kinds. All three of these arrangements, plus combinations of them, plus some other arrangements are in current use for removal of SO2 from power plant stack gases. The gas velocities in such devices range from about 1 ft/s in a packed tower to 10 ft/s for a spray chamber. If we assume we are going to treat the gas in Example 11.5 in a spray chamber at a gas velocity of 10 ft/s, the cross-sectional area perpendicular to the gas flow will be A= Q/A=1667ft2=155m2 Such devices are almost always cylindrical, because that shape is easier and cheaper to fabricate than, for example, a rectangular vessel of equal cross-sectional area. For this example the diameter would be (4 .1667 ft2 /π)0.5 = 46 ft = 14 m. A typical length in the flow direction would be 50 ft. That is a very large diameter for any piece of chemical plant equipment, but not for a power plant. Often the flow will be divided into several smaller scrubbers in parallel. This choice avoids having to ship or fabricate too large a vessel and ensures that one of the vessels can be taken out of service for maintenance while the rest are in operation. Thus the power plant can continue to operate while one part of the scrubber is out of service. What problems might power plant operators encounter? First, there is the question of what to do with the sodium sulfate produced. Sodium sulfate (also called "salt cake") is used in detergent manufacture and in paper making, as well as in some miscellaneous uses. However, for those uses it must be quite pure. The sodium sulfate produced in this process would be contaminated with fly