Recovery of sulfur dioxide from gas mixtures

Abstract

Ammonia is added to combined aqueous scrubbing solutions containing an organic base-sulfuric acid reaction product, preferably an aromatic amine-sulfuric acid reaction product, an organic base-sulfurous acid reaction product, preferably an aromatic amine-sulfurous acid reaction product, and water in one or more separating zones located exteriorly of an absorption tower. The combined aqueous scrubbing solutions are obtained from sulfurous acid and sulfuric acid solution scrubbing sections of the absorption tower wherein an organic base, preferably an aromatic amine, is utilized as absorbent in a lower absorber section of the tower to remove SO.sub.2 from a SO.sub.2 -containing gas mixture, followed by scrubbing the SO.sub.2 -depleted, aromatic amine-enriched gas from the absorber section first with sulfurous acid solution in the sulfurous acid scrubbing section of the tower to remove most of the gaseous aromatic amine therefrom and then with dilute sulfuric acid solution in the sulfuric acid scrubbing section of the tower, to remove a major portion of the residual gaseous aromatic amine from the gas. Liquid sulfuric acid solution containing a relatively high concentration of aromatic amine-sulfuric acid reaction product is withdrawn from the lowermost tray in a preferred tray-equipped tower scrubbing section, and a major portion, i.e. more than 50%, of this withdrawn sulfuric solution is passed back to and introduced preferably onto the fourth or third tray from the top of the tray-equipped scrubbing section. Liquid sulfuric acid solution containing a relatively low concentration of aromatic amine-sulfuric acid reaction product is also withdrawn from preferably the second tray or third tray from the top of this section when the introduction of the returned acid solution is onto respectively the third tray or fourth tray from the top of the section, and such withdrawn acid solution having the relatively low concentration of aromatic amine-sulfuric acid reaction product is returned and introduced onto the top tray of this sulfuric acid scrubbing section. The sulfur dioxide is stripped from the pregnant aromatic amine absorbent from the absorbing section in the stripping section of a stripping tower. The ammonia treatment of the combined aqueous scrubbing solutions in the external separating zone results in release of the aromatic amine from the aromatic amine-sulfuric acid reaction product by reaction of the ammonia with the aromatic amine-sulfuric acid reaction product and also in the formation of ammonium sulfate. Aqueous liquid solution containing the ammonium sulfate and aromatic aminesulfurous acid reaction product is withdrawn from a lower layer thereof, which is beneath an upper layer of released aromatic amine, in the separating zone and passed to the regenerator section of a stripping tower. Aromatic amine vapor and SO.sub.2 gas are liberated in the regenerator section and a considerable quantity of steam is generated therein. Aqueous solution containing ammonium sulfate is withdrawn from the lower portion of the regenerator section and, after bleeding off only a small portion of this aqueous solution, is passed, after cooling, into the upper portion of the sulfurous acid scrubbing section of the absorption tower and introduced therein. SO.sub.2 gas, obtained in the process, is supplied into the lower portion of this sulfurous acid scrubbing section and is dissolved in water of the introduced aqueous ammonium sulfate solution to form sulfurous acid. In another embodiment, the effluent gas from the sulfuric acid scrubbing section of the tower is scrubbed in alkali metal carbonate scrubbing section of the tower with a liquid alkali metal carbonate solution, e.g. a dilute aqueous sodium carbonate solution.