Process for producing sulfuric acid

In the contact process for producing sulfuric acid from undried feed gases where a SO.sub.3 -containing humid gas is produced, said gas is directly cooled with aqueous sulfuric acid, sulfuric acid is condensed in the gases cooled below the dew point of the sulfuric acid, and the water not required to form sulfuric acid is discharged as water vapor with the end gases, the improvement which comprises

• i. adding sulfuric acid of 70-95% by weight to cocurrently flowing, SO.sub.3 -containing humid gases so that the latter are directly cooled to 120.degree.-230.degree. C;
• ii. collecting a major part of the sulfuric acid in a sump;
• iii. indirectly cooling a partial stream of the collected sulfuric acid and then recycling to the SO.sub.3 -containing gases;
• iv. discharging the other part of the sulfuric acid as product acid;
• v. admixing cold air to the cooled gases, subsequently treating the gas-air mixture with dilute sulfuric acid of 5-70% by weight and collecting and diluting further the dilute sulfuric acid by an addition of water;
• vi. recycling a partial stream of the further diluted sulfuric acid to the gas-air mixture while adding the other part of further diluted sulfuric acid to the recycled acid of 70-95% by weight;
• vii. controlling the rates at which air and water are added so that the exhaust gas which has been treated with the dilute sulfuric acid has a temperature which is harmless for the succeeding mist collecting units, the treatment with the dilute sulfuric acid resulting in an evaporation of water substantially at the rate at which it is added;
• and
• viii. collecting the acid mists from the exhaust gas in a mist collector.