Process and plant for producing sulfuric acid from gases rich in sulfur dioxide

1. A process for producing sulfuric acid, oleum or liquid sulfur trioxide, in which a starting gas containing sulfur dioxide at least partly reacts with molecular oxygen in at least one contact with at least one contact stage to form sulfur trioxide, and in which the sulfur-trioxide-containing gas produced is introduced into an absorber and converted there to sulfuric acid, wherein a contact gas with a sulfur dioxide content of more than 80 vol-%, 0.01 to 10 vol-% oxygen, and with a volumetric ratio of sulfur dioxide to oxygen of more than 2.67:1 is introduced to a first contact stage.

2. The process as claimed in claim 1 , wherein the contact gas introduced to the first contact stage has a sulfur dioxide content of more than 90 vol-%.

3. The process as claimed in claim 1 , wherein the volumetric ratio of sulfur dioxide to oxygen of the contact gas introduced to the first contact stage is more than 8:1.

4. The process as claimed in claim 1 , wherein the contact gas introduced to the first contact stage consists of 80 to 99.99 vol-% sulfur dioxide, 0.01 to 10 vol-% oxygen, and 0 to maximally 10 vol-% nitrogen or another inert gas.

5. The process as claimed in claim 4 , wherein the contact gas is produced by combustion of elementary sulfur with tonnage oxygen.

6. The process as claimed claim 1 , wherein the first contact stage includes a catalyst comprising vanadium pentoxide.

7. The process as claimed in claim 6 , wherein the contact gas is introduced to the first contact stage, which includes a catalyst comprising vanadium pentoxide, with a temperature of at least 450° C.

8. The process as claimed in claim 1 , wherein the contact gas is introduced to the first contact stage with a pressure of 1 to 30 bar.

9. The process as claimed in claim 1 , wherein the sulfur dioxide content, the volumetric ratio SO 2 /O 2 , the inlet pressure and the inlet temperature of the contact gas introduced to the first contact stage are chosen such that in the contact stage a temperature is obtained, which is below the temperature that will lead to a damage of the catalyst, but above the operating temperature of the catalyst.

10. The process as claimed in claim 1 , wherein downstream of the first contact stage, two to four further contact stages are provided, which preferably are combined to one or two contacts.

11. The process as claimed in claim 10 , wherein the process gas leaving the first to penultimate contact stages is mixed with oxygen, is adjusted to an inlet temperature suitable for the next contact stage, and is supplied to the respectively next contact stage.

12. The process as claimed in claim 10 , wherein the process gas leaving the last contact stage is supplied to an absorber.

13. The process as claimed in claim 10 wherein from the process gas leaving the first contact stage and/or from one or more of the process gases leaving the second to last contact stages at least one partial stream is withdrawn, and this partial stream is mixed with the starting gas before the same enters the first contact stage and/or with the combustion gas used for the combustion of elementary sulfur for producing the sulfur-dioxide-containing starting gas and/or is supplied directly to the sulfur burner.

14. The process as claimed in claim 10 wherein from the process gas leaving the intermediate absorber and/or the final absorber at least one partial stream is withdrawn, and this partial stream is mixed with the starting gas before the same enters the first contact stage and/or with the combustion gas used for the combustion of elementary sulfur for producing the sulfur-dioxide-containing starting gas and/or is supplied directly to the sulfur burner.

15. The process as claimed in claim 1 , wherein only one contact with downstream absorber is provided, from the process gas leaving a contact stage a partial stream is withdrawn before and/or after the absorption stage, and this partial stream is mixed with the starting gas before the same enters the first contact stage and/or with the combustion gas used for the combustion of elementary sulfur for producing the sulfur-dioxide-containing starting gas and/or is supplied directly to the sulfur burner.

16. The process as claimed in claim 10 , wherein the contact gas introduced to the first contact stage consists of 80 to 99.99 vol-% sulfur dioxide, 0.01 to 10 vol-% oxygen, 0.01 to 20 vol-% sulfur trioxide, and 0 to maximally 10 vol-% nitrogen or another inert gas.

17. The process as claimed in claim 1 , wherein the volumetric ratio of sulfur dioxide to oxygen of the contact gas introduced to the first contact stage is more than 10:1.

18. The process as claimed in claim 1 , wherein the contact gas introduced to the first contact stage consists of 90 to 95 vol-% sulfur dioxide, 3 to 7 vol-% oxygen, and 0 to maximally 5 vol-% nitrogen or another inert gas.

19. The process as claimed in claim 6 , wherein the contact gas is introduced to the first contact stage, which includes a catalyst comprising vanadium pentoxide, with a temperature of at least 470° C.

20. The process as claimed in claim 1 , wherein the contact gas is introduced to the first contact stage with a pressure of 3 to 12 bar.

21. The process as claimed in claim 11 , wherein the process gas leaving the first to penultimate contact stages is mixed with oxygen upon passage through an intermediate absorber.

22. The process as claimed in claim 10 , wherein the contact gas introduced to the first contact stage consists of 90 to 95 vol-% sulfur dioxide, 3 to 7 vol-% oxygen, 0.01 to 5 vol-% sulfur trioxide, and 0 to maximally 5 vol-% nitrogen or another inert gas.