Process for separating SO.sub.2 from a current of gas containing the same

A process for producing H.sub.2 SO.sub.4 from a cold, moist current of gas containing up to about 8% by volume of SO.sub.2 by the nitrogen oxide process in a system of reaction zones, which process produces a strong sulfuric acid having a H.sub.2 SO.sub.4 -content of at least 70%, by weight, in which process the current of SO.sub.2 -containing gas having a stoichiometric oxygen content for carrying out this process in practice is caused to flow, at substantially atmospheric pressure, successively

• (a) through an acid denitration zone;
• (b) through an SO.sub.2 -processing zone in which said current is brought into intimate contact with dilute sulfuric acid having a concentration of less than 70% H.sub.2 SO.sub.4 and being substantially free from nitrose, said dilute acid flowing in a cycle through the SO.sub.2 -processing zone;
• and said SO.sub.2 -processing zone being of sufficient length to have SO.sub.2 present therein substantially to the end thereof, while substantially avoiding dissolution of nitrogen oxides in said cycle of dilute acid;
• the dilute acid cycling through the SO.sub.2 -processing zone being intermittently introduced into said denitration zone to regulate the concentration of the acid therein;
• (c) through a nitrogen oxides-absorption zone;
• all of the nitrose-containing acid exiting from said absorption zone being indirectly heated to a temperature above 60.degree. C. and then introduced into said denitration zone by-passing said SO.sub.2 -processing zone and having nitrogen oxides withdrawn in said denitration zone from said nitrose-containing acid into the gas current;
• thereby obtaining a denitrated sulfuric acid exiting from the denitration zone which acid has an H.sub.2 SO.sub.4 --concentration of between 70 and 85% by weight, is substantially free from nitrose and is withdrawable from said system as product acid;
• a portion of said denitrated sulfuric acid being conveyed through said nitrogen oxides absorption zone and absorbing nitrogen oxides from the gas current in said absorption zone;
• said process further comprising the steps of
• (.alpha.) removing a portion of the said dilute sulfuric acid, substantially free from nitrose, from said SO.sub.2 -processing zone and bringing the last-mentioned portion into contact with a cold moist current of said SO.sub.2 -containing gas in a pretreatment zone upstream of said denitration zone in said system;
• (.beta.) reintroducing a portion of the acid formed in (.alpha.) which is more diluted by moisture from said gas and which is free from nitrose into the sulfuric acid cycling in the SO.sub.2 -processing zone, this portion of the acid carrying the portion of the moisture content of the SO.sub.2 -containing gas thereby diluting the sulfuric acid and thus bypassing the gas flowpath leading to the denitration zone;
• and
• (.gamma.) at least when the water-content of the more diluted sulfuric acid exiting from the pretreatment zone increases to the extent that the H.sub.2 SO.sub.4 -content of the denitrated sulfuric acid leaving the denitration zone drops below 70% by weight of H.sub.2 SO.sub.4, removing a portion of the dilute sulfuric acid from the pre-treatment tower before it is added to the acid cycling in the SO.sub.2 processing zone and conveying the last-mentioned portion through an acid-dehydration zone, wherein it is heated to evaporate water therefrom, and then returning the resulting more concentrated acid to the SO.sub.2 processing zone.