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Znd. Eng. Chem. Res. 1991, 30,435-440
KINETICS AND CATALYSIS SO2 Removal from Hot Flue Gases by Lime Suspension Spray in a Tube
Reactor Aharon Sahar and Ephraim Kehat* Department of Chemical Engineering, Technion-Israel Institute of Technology, Haifa 32000,Israel
A laboratory-scale "dry" scrubbing tube reactor was constructed to handle synthetic flue gas with flow rates up to 7 m3/h. Hydrated lime suspended in fine droplets of water was sprayed countercurrently to the hot flue gases. Parametric tests of the effects of the stoichiometric ratio, reactor inlet and outlet temperatures, gas composition, and gas flow rate on the SO2and Ca(OH)2conversions were made. The SO2 conversion was generally in the range of 75-85%. A substantial part of the SO2 conversion takes place in the spray region due to the high density of droplets and particles, high turbulence, high droplet surface area, and droplet flow pattern in this region. Lowering the reactor temperature profile results in competing effects which lead to an optimal temperature for high conversion. The presence of oxygen in the gas mixture does not affect the SO2conversion. The C02 in the gas mixture competes with SO2 for reaction with Ca(OH)2and results in the reduction of the SO2 conversion by about 5 %
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The spray "dry" scrubbing process is a commercial process for controlling sulfur dioxide emissions from large combustion sources based on spray-drying technology (Klingepor, 1987). In this process, an alkaline suspension, usually lime, is sprayed radially cocurrent to the hot flue gases. The spray provides an extensive surface area for absorption, reaction, and drying. Solid particles carried over with the gas stream are collected in an electrostatic precipitator or in bag filters (Kyte, 1981). A model of spray-dry scrubbing of SO2 by Karlsson and Klingspor (1987) describes the flow regime in a spray dryer as well mixed. The reaction between SO2and Ca(OH9, is a by-product of the spray-drying process. Focusing on the spray-drying process prevents operating problems in the plant including the bag house but does not necessarily provide optimal conditions for SO2 conversion. In the present study, parameters of a variant of the "dry" scrubbing process were examined. Hydrated lime suspension is sprayed countercurrently into the hot flue gases, in order to increase the removal of SO2 and to minimize the reactor dimensions. A laboratory-scale dry scrubbing apparatus was built to handle flue gas with flow rates up to 7 m3/h. Tests of the effects of several parameters on the SO2and Ca(OH)2conversions were made. Some of the results, such as the effects of the stoichiometric ratio, the relative humidity, and the temperature of the reactor on the SO2conversion are consistent qualitatively with earlier studies (Klingspor, 1987; Karlsson and Klingspor, 1987). Additional parameters that have not been studied earlier, Le., the gas residence time in the reactor and the effect of C02 presence in the gas on the SO2 conversion, were examined.
Experimental Section A laboratory-scale dry scrubbing apparatus was constructed to handle flue gas at flow rates up to 7 m3/h. A schematic drawing of the apparatus is shown in Figure 1.
Flue gas was generated by mixing cylinder gases in appropriate amounts, measured by calibrated rotameters. Water vapor was added by bubbling part of the nitrogen through a thermostated humidifier. The gas was preheated to the desired temperature and was injected into the reactor through an annular distributor. The reactor consisted of a vacuum-insulated double glass tube 1.3 m long and 0.07 m in inside diameter. A thermal heater coil served as an additional active insulation. Three thermocouples, 0.03 m above the gas distributor, 0.1 m above the lime spray nozzle, and 0.1 m below the gas exit, measured the gas temperature in the reactor. The lime suspension was sprayed at the middle of the reactor countercurrent to the gas flow. At the top exit of the reactor, part of the gas enters an analyzer line via an annular porous filter heated to 200 "C in order to stop the reaction (Karlsson and Klingspor, 1987). The rest of the gas passed through a cyclone. The cyclone was heated to prevent condensation. Adhered particles on the cyclone wall were scraped off by a movable stainless steel strip operated by a magnet. Most of the solids were collected by the cyclone, and a minor part of the solids was collected at the bottom of the reactor. The lime system (Figure 2) was designed to prevent precipitation of particles and to provide a constant head for suction. Two syringes were connected to a glass tube with an inside diameter of 0.8 mm, which served as a bubble flowmeter. The first syringe was used to inject water and acid for cleaning, and the second syringe was used to inject a bubble of air to the bubble flowmeter. Nitrogen flow provided the energy for atomization. A special nozzle was built in order to obtain a spray of fine droplets at low suspension flow rates. The critical dimensions of the nozzle (in millimeters) and the nozzle system are shown in Figure 3. The nozzle was designed to prevent accumulation of particles at the edges. The droplet size distribution was measured at ambient pressure and temperature, using a 2600 D Malvern particle Q 1991 American Chemical
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436 Ind, Eng. Chem. Res., Vol. 30,No. 3, 1991
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Figure 1. Schematic diagram of the laboratory-scale spray dry scrubbing apparatus. Table I. Experimental Conditions reactor inlet temperature reactor inlet saturation temperature reactor outlet temperature reactor outlet saturation temperature gas flow rate (STP) gas av residence time in reactor gas av velocity in reactor feed suspension solids content SO2 inlet concentration
150 "C 35.2-49.1 O C 31-98 "C 46.2-54.4 "C 1.9-7.1 m3/h 2.1-9.4 s 0.14-0.50 m/s 11.29% 1030-1570 ppm
sizer. The droplet size 0(3,2)(Sauter mean diameter) in the experiments was in the range of 12-15 pm (Figure 4). The average droplet size increased with increased suspension flow rate and with decreased nitrogen flow rate. Particle size was measured by an image analyzer after dilution of the samples with a saturated Ca(OH), solution. The average particle size of the lime was 1.46 pm, of the cyclone particles was 1.02 pm, and of the particles at the bottom of the reactor was 0.090 mm. The Ca(OHI2conversion of the particles collected at the bottom of the reactor was low. A SO2 monitor, NEOTRONICS So-103,was used for rough continuous analysis of the gas. The quantitative analysis was made by passing the gas through an H202 solution, at a rate of 400 cm3/min, for 3 min. The SO2 concentration in the gas was calculated by back-titration of the solution with Na2C03 to the original pH of 4.5. Ca(OH)2conversion was measured by titrating cyclone samples with HCl in the presence of sucrose.
Table 11. Typical Gas Mixture Feed Composition for the Experimental Series series 1 2 3 4 5 V O ~% N2 90.4 85.7 77.9 77.9 73.0 vol % 02 4.7 4.7 vol % cog 12.5 12.5 12.7 V O ~% H2O 9.5 9.5 9.5 9.5 9.5 ppmSO2 1400 1400 1400 1400 no. of runs 38 2 2 2 5
A total of 49 runs was performed. The experimental conditions are detailed in Table I. Results The synthetic flue gas composition for the five experimental series are detailed in Table 11. The nitrogen flow rate includes the nitrogen injected with the lime suspension. The SO2concentration was kept approximately the same, at about 1400 ppm for all runs. Series 1 (38 of the 49 Runs). The gas mixture was composed of N2,H20,and SOz. The effect of changing the suspension flow rate at a constant gas flow rate (0.215 kmol/ h) on the SO2conversion is shown in Figure 5. (The numbers on the points in Figures 5-12 are run numbers.) Increasing the suspension flow rate increased the SO2 conversion linearly. Increasing the suspension flow rate at a constant gas flow rate increases the stoichiometric ratio (of Ca(OH)2to SO2)and also increases the reactor outlet
Ind. Eng. Chem. Res., Vol. 30, No. 3, 1991 437 LimeiWaier
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