Davy Powergas Gmbh

Norman E Nicholson

John Scarlett

John F Flintoff

Edgar E Bailey

Sodium sulfate is purged from a sulfur dioxide removal system involving contact of a sulfur dioxide-containing gas with a solution containing sodium sulfite to absorb sulfur dioxide from the gas. The spent absorbing solution is regenerated by desorbing sulfur dioxide and recycled for further use. To avoid an unduly large build-up of sulfate in the system, at least a portion of the absorbing-desorbing medium, e.g. spent absorbing solution, containing sodium sulfate and a relatively large amount of sodium bisulfite is treated to reduce the amount of water in the medium so that there is precipitated therefrom up to about 10 weight percent undissolved solids containing sodium sulfate in greater concentration than would otherwise be obtained in the absorption-desorption cycle. The insolubles containing sodium sulfate are removed from the liquid, and the liquid can be returned to the sulfur dioxide removal system. In one preferred aspect of the invention, up to about 75 weight percent of the entire stream of spent absorbing solution is treated to precipitate therefrom up to about 10 weight percent undissolved solids which are relatively rich in sodium sulfate content. In the invention the sodium sulfate-containing solids can be separated from the liquid which is then subjected to a desorption operation to produce sulfur dioxide, and the latter operation can be conducted while maintaining at least about 25 weight percent undissolved solids in the desorption zone.

Sulfur dioxide removal process

Sodium sulfate is purged from a sulfur dioxide removal system involving contact of a sulfur dioxide-containing gas with a solution containing sodium sulfite to absorb sulfur dioxide from the gas. The spent absorbing solution is regenerated by desorbing sulfur dioxide and recycled for further use. To avoid an unduly large build-up of sulfate in the system, at least a portion of the absorbing-desorbing medium containing sodium sulfate and a relatively large amount of sodium bisulfite, e.g. spent absorbing solution, is contacted with a treating gas containing at least a small amount of sulfur dioxide while reducing the amount of water in the medium so that a slurry is obtained having up to about 10, or even up to about 20, weight percent precipitated solids containing sodium sulfate in greater concentration than would otherwise be obtained in the absorption-desorption cycle. Preferably, the sulfur dioxide-containing treating gas is unsaturated with respect to water. The insolubles containing sodium sulfate are removed from the liquid, and the liquid can be returned to the sulfur dioxide removal system. In one preferred aspect of the invention, up to about 75 weight percent of the entire stream of spent absorbing solution is treated to form up to about 10 weight percent undissolved solids which are relatively rich in sodium sulfate content. In the invention the sodium sulfate-containing solids can be separated from the liquid which is subjected to a desorption operation to produce sulfur dioxide, and the latter operation can be conducted while maintaining at least about 25 weight percent undissolved solids in the desorption zone.

Process for purging sodium sulfate from a sulfur dioxide removal system

Sodium thiosulfate is purged from a sulfur dioxide removal system involving contact of a sulfur dioxide-containing gas with a solution containing sodium sulfite to absorb sulfur dioxide from the gas. A portion of the liquid from a desorption zone containing a minor amount of sodium thiosulfate and a relatively large amount of sodium bisulfite, is treated to reduce the amount of water in the medium so that solids are precipitated from the liquid phase. The insolubles containing sodium sulfites are removed from the liquid. The liquid separated from the solids can be discarded and thereby purge thiosulfate more selectively with respect to other sodium salts. Also, the sodium values of the sodium thiosulfate-containing purge liquid can be recovered in active form by chemical reduction, and, preferably, carbonation. A sodium sulfate purge material from the absorption-desorption system can also be subjected to the chemical reduction system for further recovery of active sodium values.

There is disclosed a process for reducing the loss of sodium values in a system for removing sulfur dioxide from a gas by the use of an absorption-desorption cycle employing aqueous sodium sulfite as the essential absorption solution. Sodium sulfate and/or sodium thiosulfate build-up in the system is avoided and the loss of sodium values reduced by subjecting sodium sulfate and/or sodium thiosulfate-containing material to chemical reduction to form a product containing a substantial amount of sodium carbonate. Preferably, the reduced product is subjected to carbonation followed by reaction of the bicarbonate and bisulfide components of the mixture to give a resulting liquid which is more suitable for use in the absorption-desorption process. By-product gases can be incinerated to produce sulfur dioxide-containing gases which can be charged to the sulfur dioxide absorption zone. Sodium thiosulfate can be purged from the absorption-desorption system by treating a purge stream from the desorption zone of the process.

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Stockton-on-Tees, United Kingdom

Norman E Nicholson