Double alkali process

Where most utility installations are the lime or limestone processes, it can be seen from Table IV that a very small percentage of industrial installations are of this type. Most of these installations are the once-through sodium carbonate, sodium hydroxide, and double alkali processes. Where the utility installations have been plagued with corrosion, erosion, scaling and fouling problems, the industrial installations, have to date performed much better. A number of systems showed a process reliability of greater than 85%. 

In the dual or double alkali process, an alkali salt that is considerably more soluble in water than limestone is used. The alkak salt is then regenerated using a second alkali, CaC03. There are several alkalies used in the absorber the most common are magnesium sulfite, sodium sulfite, and ammonium sulfite. A typical process using magnesium sulfite suAbsorption... 

Sulfur dioxide Double alkali process Sodium sulfite... 

Wet scmbber, venturi for SO2 application using the double alkali process. Venturi only made of Carpenter 20 in the throat, otherwise 6 mm neoprene on c/s, exdud-irig sump. FOB cost = 1 110 000 at inlet gas flow rate = 140 Nm /s with n = 0.62 for the range 8-280 Nm /s. L-rM = 2.0. L/M = 0.34. Factors excluding sump,... 

Formation of a chemical compound between absorbed component and a regeneration auxiliary substance (chemical regeneration). The absorbed component reacts with the auxiliary substance which is insoluble with the absorbent or solvent, and precipitates. Downstream to the regeneration unit is a thickener, and the solid is separated from the absorbent in the subsequent filter (e. g., the double alkali process to separate sulfur dioxide from flue gas. Sulfur dioxide forms a sulfite with an alkali solution used as an absorbent. When calcium hydroxide is added calcium sulfite is formed, which is insoluble in water and therefore precipitates. This is then separated from the alkali solution). 

Non-Regenerable, Alkali Metal-Based Processes, 544 Double Alkali Process, 546... 

The overall effects of the double alkali process are identical to those of die limestonedime slurry processes—SO2 is ranoved from die gas, lime or limestone is consumed, and a calcium sulfite or sulfate byproduct is produced. The int mediate st s, however, are quite different and result in a complete sqiaration of the SO2 absorption and byproduct precipitation reactions. This approach permits the gas to be contacted with a clear solution of highly soluble salts, thereby minimizing scaling, plugging, and oosion problems in the absorboit circuit. The use of a clear reactive solution instead of a slurry also offns the potential for a... 

Figure 7-12 shows the absorber section of a large double alkali plant during installation. A generalized flow sheet for the double alkali process is shown in Figure 7-13. The gas is contacted with a clear solution containing sodium sulfite, sodium sulfote, sodium bisulfite, and, in some cases, sodium hydroxide or caibonate. Sodium sulfite is the principal reactive component and is converted to bisulfite by the absorbed SO2. A sidestream of the recycling absorbent solution is removed and treated with lime (or limestone), which reacts with the...

The chemistry of the double alkali process has been described in considerable d lfa by Kaplan (1974,1976). The primary reactions involved are as follows ... 

Materials of construction for double alkali process plants are quite similar to those used for limestone/lime systems—stairdess steel (316L) venturis and scrubbers, rubbo -lined pumps and slurry lines, Hastelloy G tubes for direct steam tube gas rdieat, and fiberglass reinforced plastic or plastic-lined carbon steel tanks. 

Several processes have been developed based on the use of ammonium rather than sodium compounds as the active components of the absorbent solution of a double alkali process. As in the sodium-based double alkali process, absorbed sulfur dioxide is precipitated as an insoluble calcium salt to regenerate the absorbent The Kurabo process represents one form of the technology in which lime is used to remove sulfur from solution and the precipitate is calcium sulfate (gypsum). This process has reportedly been employed in five oil-fired industrial boilers in Japan. Another version of the technology, the SCRA process, uses limestone instead of lime to remove the sulfur compounds from solution. This process has been tested in a small pilot plant, but apparently has not been used commercially (Behrens et al., 1984). 

The ammonia-based double alkali process has the same advantage as the sodium-based system, compared to wet lime/limestone processes, of using a clear solution in the absorption step. However, both double alkali processes have the disadvantage of greater complexity... 

This process represents another alternative to the double alkali process. Sulfur dioxide is absorbed in dilute sulfuric acid, oxidized to sulfate by air blowing, then precipitated as gypsum by the addition of limestone. The oxidation rate is increased by the use of iron as a catalyst in the circulating acid and is also enhanced by the low pH of the solution. The process, which was developed by Chiyoda Chemical Engineering and Construction Company, Ltd., of Yokohama, has been used quite extensively in Japan. Fourteen plants were reportedly in operation at the end of 1977 ( do, 1977). 

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