Process for producing concentrated sulfuric acid

In a process for producing concentrated sulfuric acid from hot starting H.sub.2 O- and SO.sub.2 -containing gases having an H.sub.2 O/SO.sub.2 molar ratio above 1.25, the gases being formed by the combustion of H.sub.2 S-containing gases or by the thermal cracking of aqueous waste sulfuric acid in a combustion or thermal cracking zone, and the process comprising:

• cooling the hot starting gases to a temperature of from 400.degree. to 480.degree. C. and lowering their water content;
• oxidizing the SO.sub.2 in the gases to SO.sub.3 in a plurality of contact stages, including a first stage, a last stage and at least one intermediate stage;
• cooling the gases between the contact stages;
• and
• absorbing the sulfur trioxide and water vapor to form sulfuric acid;
• the improvement which comprises effecting the cooling of the hot starting gases produced in the combustion or thermal cracking zone and the lowering of the water content thereof by:
• splitting the H.sub.2 O- and SO.sub.2 -containing gas stream emanating from the combustion or thermal cracking zone into two gas streams;
• feeding one of said two split gas streams into the first contact stage;
• cooling the other of said two split gas streams by contacting the gases thereof with an amount of cooling water effective to condense a portion of the water vapor contained in the gases;
• separating the condensed water from said gases;
• recycling the thus cooled gas stream having lowered water vapor content back to the gas stream emanating from the combustion or thermal cracking zone, such that the recycled cooled gases are mixed with the starting hot gases produced in the zone prior to the point at which said starting hot gases are split into two streams;
• the portion of the water vapor condensed in the cooling step and the proportion of gases split into said two split streams being such that the H.sub.2 O/SO.sub.2 molar ratio and the temperature of the gases fed into the first contact stage are 1.0 to 1.25 and 400.degree.-480.degree. C., respectively.