Sodium dual alkali process

Several power plants have been equipped with dual alkali processes. These are throw-away processes with two liquid loops. In one common process, the scrubbing liquid is a clear solution of sodium sulfite. The absorption of sulfur dioxide converts the sodium sulfite to sodium bisulfite. In the regeneration loop, an alkali such as lime slurry is added the sodium sulfite solution is regenerated and a mixture of calcium sulfite and calcium sulfate is precipitated. The slurry is... 

Application of the Sodium Dual Alkali Scrubbing Process to High Chloride Gas Streams... 

In the sodium-based dual alkali process, the acid gases are absorbed by a solution of sodium salts at a pH range of 5-8. The solution is regenerated outside the scrubber with lime or limestone to produce a solid waste containing calcium sulfate and calcium sulfite. Some sodium salts are lost with the waste and must be made up by the addition of NaOH or Na2C03. The principal chemical reactions are as follows ... 

Application of the sodium-based dual alkali process to high chloride gas streams is of interest not only to the Army but also to other operators of combustion systems using conventional fuels such as coal where toxic chloride-containing wastes are co-fired for destruction purposes. 

Thyssen-CEA Environmental Systems, Inc. (initially Combustion Equipment Associates, Inc.) and Arthur D. Little, Inc., have developed, over the past few years, a dual alkali process for removing SO2 from flue gas generated in coal-fired utility boilers This process is based on the absorption of SO2 in an alkaline sodium solution, followed by regeneration of the absorbing solution by reaction with a second alkali, calcium. These reactions generate insoluble calcium-sulfur salts which are discharged from the system as a moist cake. 

In the dual-alkali process, a recycled alkaline solution of sodium salts is the scrubbing liquid. The scrubber effluent is treated with slaked lime to precipitate insoluble calcium sulfite and calcium sulfate, while regenerating the alkalinity of the solution. Then, the calcium salts are thickened and filtered from the recycled solution. This system requires only a small make-up of sodium alkali and produces a smaller amount of solids for disposal than a simple lime scrubbing system. 

In the double alkali (or dual alkali) process for flue gas desulfurization, die gas is contacted with a solution of soluble alkali, such as sodium sulfite or sodium hydroxide, which absorbs the SO2. The resulting solution is then reacted with a second alkaline material (normally lime or limestone) to precipitate the absorbed SO2 as insoluble calcium sulfite and regenerate the absorbent solution. Several alkali combinations are possible however, this discussion is limited to the sodium/calcium case. 

The Dowa process is claimed to offer sevoal advantages in comparison with both conventional limestone/lime and sodium dual alkali systems (Nolan and Seaward, 1983). These include... 

Dual Alkali A flue-gas desulfurization process. The sulfur dioxide is absorbed in aqueous sodium hydroxide and partially oxidized, and this liquor is then treated with calcium hydroxide to regenerate the scrubbing solution and precipitate calcium sulfate. Developed by Combustion Equipment Associates and Arthur D. Little. 

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... 

Dual alkali systems have been extensively applied to scrubbing SO2 from boiler flue gas. In most of these applications, the objective in using a dual alkali system as compared to a single alkali system, is to minimize the use of the more expensive sodium as compared to the calcium in lime. However, the most cost-effective operation for the system on the MPF is one that will eliminate the expensive drying process. The design shown in Figure 1 may actually require considerably more alkali than... 

Fig. 5 Flow diagram of the dual-alkali scrubber process, using lime to regenerate the sodium alkali. The clarified liquid from the thickener contains dissolved calcium sulfate, which would produce calcium carbonate scale in the scrubber when it contacts the carbon dioxide in the flue gas. It is therefore precipitated in the softening reactor by a combination of carbon dioxide and sodium carbonate, and the resulting calcium carbonate precipitate is removed by the hydrocyclone.

The presence of sodium sulfate and sodium chloride is principally the result of secondary absorption reactions. Sodium sulfate is formed by the oxidation of sodium sulfite via reaction with oxygen absorbed from the flue gas. Oxidation also occurs in other parts of the system where process solutions are exposed to air however, the amount of oxidation is small relative to the oxidation which occurs in the absorber. At steady state, the sulfate must leave the system either as calcium sulfate or as a purge of sodium sulfate at the rate at which it is being formed in the system. Although a practical limit for the level of oxidation that can be tolerated by the limestone dual alkali system has not yet been established, it appears that oxidation rates equivalent to 15 to 20% of the S02 removed might be accommodated without intentional purges of sodium sulfate. 

The process applied in a conventional visbreaking unit in which a proprietary additive and water are added to the feedstock/recycled bottoms prior to the soaker reduce the inventory of heavier feedstocks and products. The presence of the oil-soluble dual catalyst (Carrazza et al., 1997a, 1997b) and water prevents the buildup of coke precursors and deposition of sediment that often occurs during visbreaking. The catalyst may be supported on support material or mixed directly with the feedstock. The first metal is chosen from the nonnoble Group VIII metals and the second is an alkali metal such as potassium or sodium. 

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