Process for the production of concentrated sulfuric acid and/or oleum from a wet sulfur dioxide feed

In a process for the production of concentrated sulfuric acid and/or oleum by the catalytic oxidation of SO.sub.2 to SO.sub.3 using a gas which contains water vapor in addition to inert components, SO.sub.2 and oxygen, and absorption of the SO.sub.2 is sulfuric acid, the improvement which consists essentially of the steps of

• a. passing a wet gas containing at least 5 g/Nm.sup.3 and up to 35 g/Nm.sup.3 water vapor in a first stage over a primary catalyst so as to oxidize the SO.sub.2 to SO.sub.3,
• b. contacting the reaction gas emerging from step (a) with cold 96 to 99% sulfuric acid so as to cool said gas from an inlet temperature of about 180.degree. to 200.degree. C to an exit temperature of about 40.degree. to 60.degree. C,
• c. absorbing the water vapor content of the gas emerging from step (b) with cold 96 to 99% sulfuric acid in a separate unit,
• d. reheating the gas emerging from step (c),
• e. contacting the gas emerging from step (d) in an intermediate absorption unit with 98 to 99% sulfuric acid so as to absorb substantially all SO.sub.3 formed in step (a),
• f. oxidizing the unreacted SO.sub.2 in the gas emerging from step (e) in a second stage on a secondary catalyst substantially in the absence of water vapor,
• g. absorbing the SO.sub.3 from the gas emerging from step (f) in sulfuric acid.