Method for the removal of No
A process is described for reducing the NO x concentration and N 2 O concentration from the residual gas from nitric acid production. The process encompasses the passing of the residual gas leaving the absorption column, prior to entry into the residual gas turbine, through a combination of two stages. The first stage here reduces the NO x content and the second stage the N 2 O content of the gas, the NO x /N 2 O ratio prior to entry of the gas into the second stage being in the range from 0.001 to 0.5, and this gas being brought into contact in the second stage with a catalyst which is substantially composed of one or more iron-loaded zeolites.
1. A process for reducing the NO x concentration and N 2 O concentration from the tail gas from nitric acid production using an absorption column and a tail gas turbine, where the tail gas leaving the absorption column is passed, prior to entry into the tail gas turbine, through a combination of two stages, the first stage reducing the NO x content by catalytic reduction, and the second stage reducing the N 2 O content of the gas by decomposition into nitrogen and oxygen, and where the molar NO x /N 2 O ratio prior to entry of the gas into the second stage is in the range from 0.01 to 0.5, and in the second stage this gas is brought into contact with a catalyst which comprises one or more iron-loaded zeolites, the operating pressure in the second stage being from 4 to 12 bar, with the proviso that said one or more iron-loaded zeolites does not comprise FER type iron-loaded zeolite.
2. The process as claimed in claim 1 , characterized in that the iron-loaded zeolite(s) present in the catalyst are of MFI, BEA, MOR and/or MEL type.
3. The process as claimed in claim 2 , characterized in that the iron-loaded zeolite (s) are of MFI type.
4. The process as claimed in claim 3 , characterized in that the zeolite is an Fe-ZSM-5.
5. The process as claimed in claim 1 , characterized in that the temperature of the first stage is <400° C.
6. The process as claimed in claim 1 , characterized in that the temperature of the second stage is in the range of 300 to 550° C.
7. The process as claimed in claim 1 , characterized in that both stages are operated at a pressure in the range of from 4 to 12 bar.
8. The process as claimed in claim 1 , characterized in that the first stage is operated using the SCR process.
9. The process as claimed in claim 1 , characterized in that, after leaving the absorption column and prior to entry into the first or second stage, use is made of the tail gas whose water content is in the range from 0.05 to 1% by volume.
10. The process as claimed in claim 1 , characterized in that, prior to entry into the second stage, NO x content of the gas is in the range from 1 to 200 ppm and the N 2 O content of the gas is in the range from 200 to 2000 ppm.
11. The process as claimed in claim 5 , characterized in that the temperature of the first stage is <350° C.
12. The process as claimed in claim 6 , wherein the temperature of the second stage is in the range of 350 to 500° C.
13. The process as claimed in claim 9 , wherein the water content of the tail gas is in the range from 0.1 to 0.8% by volume.
14. The process of claim 1 , wherein the catalyst comprises greater than 50% by weight of said iron-loaded zeolites.
15. The process of claim 1 , wherein the catalyst comprises greater than 70% by weight of said iron-loaded zeolites.
16. The process of claim 14 , wherein the iron-loaded zeolite(s) are of the MFI type.
17. The process of claim 15 , wherein the iron-loaded zeolite(s) are of the MFI type.
18. The process of claim 16 , wherein the iron-loaded zeolite is an Fe-ZSM-5.
19. The process of claim 17 , wherein the iron-loaded zeolite is an Fe-ZSM-5.
20. The process as claimed in claim 1 , wherein the molar NO x /NO 2 ratio is less than 0.1.
21. The process as claimed in claim 1 , wherein the molar NO x /NO 2 ratio is from 0.01 to 0.1.
22. The process as claimed in claim 1 , wherein the first stage catalytic reduction reduces the NO content to less than 100 ppm.