Dual alkali process scrubbers

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 ... 

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. 

Dowa process Similar to dual alkali except J.A12(S04)3 solution used in scrubber. 

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.

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