induced engineers to develop dry throwaway processes that would have fewer corrosion and scaling difficulties and would produce a waste product much easier to handle and dispose of. All of these systems inject dry alkaline particles into the gas stream, where they react with the gas to remove SOz. The SO2-containing particles are then captured in the particle collection device that the plant must have to collect fly ash(most often a baghouse, sometimes an ESP). If successful this approach eliminates the problems with disposal of wet scrubber sludge and all the difficulties involved with the wet limestone process. It increases the volume of dry solids to be disposed of, but that is considered a less difficult problem. The flow diagrams for such systems are sketched in Fig.11.7. he first two call for the injection of powdered limestone or lime into the boiler. In the high-temperature part of the furnace the limestone would convert to lime, so that either way the active reagent would be CaO. The desired reaction is CasOs would then oxidize to Caso4. In principle this should work, but most tests have shown that to get high SO collection efficiencies one must put a large excess of lime or limestone into the system, thus increasing reagent costs, increasing the load on the particle collector, and increasing the volume of solid wastes to be disposed of. However, if one uses more reactive(and much more expensive) NaHCO3 or Na2 CO3, the collection efficiency is much better, mostly because of the much higher chemical reactivity of these sodium salts The design of such devices is, in principle, done the same way as in Examples 11. I and 11.2 However. here we have co-flow which is much less efficient than counterflow. Mass transfer between gases and solids is much less well understood than that between gases and liquids, so that the design of these devices is much more heavily dependent on test and empiricism than is the design of systems like that Wet-dry systems. Wet-dry systems combine some features of the preceding two kinds of systems Spray dryers are widely used in the process industries. Masters presents a five-page list of products that are commercially spray dried, e. g, powdered milk, instant coffee, laundry detergents, tc. In all such spray dryers a liquid (almost always water)containing dissolved or suspended solids is dispersed as droplets into a hot gas stream. The dispersion can be done by a high-pressure gas-atomizing nozzle or a rapidly rotating(about 10 000 rpm)atomizing wheel. The hot gas is well above the boiling temperature of water, so that the water in the droplets evaporates rapidly The particles formed from the evaporating drops are dry before they reach the wall or bottom bin of the dryer, so they form a free- flowing powder that is easily removed In industry a spray dryer is most often used when the product is heat sensitive. The drying is done very quickly, and the powder can be cooled quickly alter it leaves the dryer. In addition, by controlling the solids concentration in the feed and the size of the droplets, one may control the size of the particles produced, often producing a particle size distribution not easily obtained any other way. With soluble solids one can often produce particles that are hollow spheres. The student should study some powdered coffee(not freeze-dried coffee)or laundry detergent as an example of products made this way In treating SO2-containing flue gases, the hot gas enters the spray dryer chamber, usually from the side and/or top and flows out most often at the bottom or side or through an outlet tube that dips down into the dryer vessel. The reagent slurry, is dispersed as 10-to 50-u drops, containing about 30 weight percent solids. The resulting dry particles are small enough that most are carried along with the gas stream; this feature is different from most spray dryers for consumer products, in which the particles are large enough(or the air velocity small enough) that most of the particles settle to the bottom of the dryer. This is called a wet-dry system because part of it is like a wet lime scrubber and part is like dry sorbent injection. The freshly formed drops behave very much like the drops in a wet lime scrubber. The SOz dissolves in the water and reacts there with the dissolved Ca(OH)z. As the water evaporates from the drops, the individual fine particles in them coalesce to form a single porous panicle from each drop. This particle then behaves like the dry sorbent particles injected as shown in Fig. 11.7 Comparing this device to the wet limestone scrubber, we see that the drops are much smaller(20 73 mm =1/150). The time the gas spends in the scrubbing environment is roughly twice as large However, once the particles become dry, their reactive capacity is greatly reduced compared to the drops in wet scrubbers. In addition, the co-flow pattern is much less efficient The amount of. water one can introduce in these devices is limited by the amount that the hot gas can evaporate. If more water is introduced than is needed to cool the gas to the adiabatic saturation l1-1111－11 induced engineers to develop dry throwaway processes that would have fewer corrosion and scaling difficulties and would produce a waste product much easier to handle and dispose of. All of these systems inject dry alkaline particles into the gas stream, where they react with the gas to remove SO2. The SO2-containing particles are then captured in the particle collection device that the plant must have to collect fly ash (most often a baghouse, sometimes an ESP). If successful, this approach eliminates the problems with disposal of wet scrubber sludge and all the difficulties involved with the wet limestone process. It increases the volume of dry solids to be disposed of, but that is considered a less difficult problem. The flow diagrams for such systems are sketched in Fig. 11.7. The first two call for the injection of powdered limestone or lime into the boiler. In the high-temperature part of the furnace the limestone would convert to lime, so that either way the active reagent would be CaO. The desired reaction is CaO + SO2 ---> CaSO3 (11.20) CaSO3 would then oxidize to CaSO4. In principle this should work, but most tests have shown that to get high SO2 collection efficiencies one must put a large excess of lime or limestone into the system, thus increasing reagent costs, increasing the load on the particle collector, and increasing the volume of solid wastes to be disposed of. However, if one uses more reactive (and much more expensive) NaHCO3 or Na2CO3, the collection efficiency is much better, mostly because of the much higher chemical reactivity of these sodium salts. The design of such devices is, in principle, done the same way as in Examples 11.1 and 11.2. However, here we have co-flow, which is much less efficient than counterflow. Mass transfer between gases and solids is much less well understood than that between gases and liquids, so that the design of these devices is much more heavily dependent on test and empiricism than is the design of systems like that. Wet-dry systems. Wet-dry systems combine some features of the preceding two kinds of systems. Spray dryers are widely used in the process industries. Masters presents a five-page list of products that are commercially spray dried, e.g., powdered milk, instant coffee, laundry detergents, etc. In all such spray dryers a liquid (almost always water) containing dissolved or suspended solids is dispersed as droplets into a hot gas stream. The dispersion can be done by a high-pressure gas-atomizing nozzle or a rapidly rotating (about 10 000 rpm) atomizing wheel. The hot gas is well above the boiling temperature of water, so that the water in the droplets evaporates rapidly. The particles formed from the evaporating drops are dry before they reach the wall or bottom bin of the dryer, so they form a free-flowing powder that is easily removed. In industry a spray dryer is most often used when the product is heat sensitive. The drying is done very quickly, and the powder can be cooled quickly alter it leaves the dryer. In addition, by controlling the solids concentration in the feed and the size of the droplets, one may control the size of the particles produced, often producing a particle size distribution not easily obtained any other way. With soluble solids one can often produce particles that are hollow spheres. The student should study some powdered coffee (not freeze-dried coffee) or laundry detergent as an example of products made this way. In treating SO2-containing flue gases, the hot gas enters the spray dryer chamber, usually from the side and/or top and flows out most often at the bottom or side or through an outlet tube that dips down into the dryer vessel. The reagent slurry, is dispersed as 10- to 50-μ drops, containing about 30 weight percent solids. The resulting dry particles are small enough that most are carried along with the gas stream; this feature is different from most spray dryers for consumer products, in which the particles are large enough (or the air velocity small enough) that most of the particles settle to the bottom of the dryer. This is called a wet-dry system because part of it is like a wet lime scrubber and part is like dry sorbent injection. The freshly formed drops behave very much like the drops in a wet lime scrubber. The SO2 dissolves in the water and reacts there with the dissolved Ca(OH)2. As the water evaporates from the drops, the individual fine particles in them coalesce to form a single porous panicle from each drop. This particle then behaves like the dry sorbent particles injected as shown in Fig. 11.7. Comparing this device to the wet limestone scrubber, we see that the drops are much smaller (20 μ/3 mm = 1/150). The time the gas spends in the scrubbing environment is roughly twice as large. However, once the particles become dry, their reactive capacity is greatly reduced compared to the drops in wet scrubbers. In addition, the co-flow pattern is much less efficient. The amount of .water one can introduce in these devices is limited by the amount that the hot gas can evaporate. If more water is introduced than is needed to cool the gas to the adiabatic saturation