Process for argon and nitrogen production

Abstract

A process comprising: subjecting a process gas containing NOx to a stage for absorption of NOx in a suitable absorption means, obtaining nitric acid and a tail gas containing nitrogen, argon and residual NOx; subjecting said tail gas to a treatment which comprises at least one NOx removal stage, obtaining a conditioned tail gas; subjecting at least a portion of said conditioned tail gas to a separation treatment, obtaining a product stream containing argon and a product stream containing nitrogen.

Claims

1. A process, comprising: subjecting a process gas containing NO.sub.x to a NO.sub.x absorption stage in an absorption means, thereby obtaining nitric acid and a tail gas containing nitrogen, argon and residual NO.sub.x; subjecting said tail gas to a treatment comprising at least one NO.sub.x removal stage, thereby obtaining a conditioned tail gas; and subjecting at least a portion of said conditioned tail gas to a separation treatment, thereby obtaining a first product stream containing argon and a second product stream containing nitrogen.

2. The process of claim 1, wherein said first product stream has an argon content of at least 99.5% (vol).

3. The process of claim 2, wherein said argon content is at least 99.95% (vol).

4. The process of claim 2, wherein said argon content is at least 99.995% (vol).

5. The process of claim 1, wherein said second product stream has a nitrogen content of at least 99.5% (vol).

6. The process of claim 5, wherein said nitrogen content is at least 99.95% (vol).

7. The process of claim 5, wherein said nitrogen content is at least 99.995% (vol).

8. The process of claim 1, wherein said conditioned tail gas contains an amount of argon of at least 0.9% (vol).

9. The process of claim 1, wherein said conditioned tail gas contains an amount of NOx not greater than 200 ppm.

10. The process of claim 9, wherein the amount of NO.sub.x is not greater than 30 ppm.

11. The process of claim 9, wherein the amount of NO.sub.x is not greater than 5 ppm.

12. The process of claim 1, wherein said conditioned tail gas contains an amount of N.sub.2O not greater than 1000 ppm.

13. The process of claim 12, wherein said amount of N.sub.2O is not greater than 100 ppm.

14. The process of claim 12, wherein said amount of N.sub.2O is not greater than 30 ppm.

15. The process of claim 12, wherein said amount of N.sub.2O is not greater than 10 ppm.

16. The process of claim 1, wherein said conditioned tail gas contains an amount of oxygen not greater than 5% (vol).

17. The process of claim 16, wherein said amount of oxygen is from 2 to 3% (vol).

18. The process of claim 1, wherein said conditioned tail gas has a pressure greater than 4 bar.

19. The process of claim 18, wherein said pressure is from 4 bar to 15 bar.

20. The process of claim 1, wherein the process gas containing NO.sub.x is obtained by oxidation of a stream of ammonia in the presence of air or enriched air.

21. The process of claim 1, wherein the conditioned tail gas contains no more than 800 ppm of CO.sub.2.

22. The process of claim 1, wherein the separation treatment includes a cryogenic treatment.

23. The process of 38, wherein said separation treatment comprises: cooling and subsequently expanding the conditioned tail gas, thereby obtaining a partial liquefaction, and subjecting the liquefied fraction to fractional distillation.

24. The process of claim 22, further comprising removing CO.sub.2 before the cryogenic treatment.

25. The process of claim 24, wherein removing CO.sub.2 before the cryogenic treatment is effected with a molecular sieve.

26. A plant, comprising: an absorption tower, fed with a process gas containing NO and configured to absorb NOx in an absorption means, for providing nitric acid and a tail gas containing nitrogen, argon and residual Nox; a treatment unit for said tail gas, configured to remove NO and give a conditioned tail gas; and a separation section, configured to separate a first product stream containing argon and a second product stream containing nitrogen, said separation section being fed with at least a portion of said conditioned tail gas.

27. The plant of claim 26, wherein the separation section comprises: a heat exchanger configured to refrigerate said at least one portion of the conditioned tail gas, obtaining a refrigerated gas; an expander for said refrigerated gas, obtaining a partially liquefied gas; a separator, wherein the liquefied fraction of said partially liquefied gas is separated from the non-liquefied fraction; and a distillation apparatus, which receives said liquefied fraction and which separates said stream containing argon and said stream containing nitrogen.

28. The plant of claim 26, further comprising a reactor for oxidation of a stream of ammonia in the presence of air or enriched air, obtaining said process gas containing NO.sub.x.

Description

BRIEF DESCRIPTION OF THE FIGURES

(1) FIG. 1 shows a simplified diagram of the plant according to the invention.

(2) FIG. 2 shows a diagram of a plant for the combined production of nitric acid, argon and nitrogen according to a preferred embodiment of the invention.

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT

(3) The plant according to FIG. 1 comprises essentially an absorption tower 4, a treatment unit 6 for the tail gas leaving said tower, an expander 7 and a section 2 for the separation of a stream of argon and a stream of nitrogen.

(4) Said plant operates as follows.

(5) A process gas 22 containing NOx and, in a smaller amount, N.sub.2O and a stream of water 23 are fed to an absorption tower 4. Inside said tower 4, the NOx are partly absorbed in water to give a stream 24 containing nitric acid and a tail gas 25 mainly containing nitrogen and smaller amounts of oxygen, argon, water, N.sub.2O and residual NOx.

(6) Said tail gas 25 is sent to the treatment unit 6, where the NOx and optionally also the N.sub.2O are at least partly removed, providing a conditioned tail gas 26. The gas 26 leaving said treatment unit 6 has preferably a pressure ranging from 4 to 15 bar.

(7) Said conditioned gas 26 is advantageously split into two portions: a first portion 26a is expanded inside the expander 7 and discharged into atmosphere as stream 27, and a second portion 26b is fed to the section 2 and subjected to a separation treatment, obtaining a stream 40 containing argon and a stream 37 containing nitrogen.

(8) FIG. 2 shows in greater detail the plant of FIG. 1. It comprises in particular a section 1 for the synthesis of nitric acid and a section 2 for argon and nitrogen production.

(9) The section 1 essentially comprises a reactor 3 for catalytic oxidation of ammonia, an absorption tower 4, a heat exchanger 5, a unit 6 for NOx removal and optionally N.sub.2O removal, and an expander 7. Especially in the case of high-capacity plants, said section 1 also comprises a compressor between the reactor 3 and the absorption tower 4.

(10) Said section 1 operates as follows.

(11) A stream of ammonia 20 and an air flow 21 are fed the reactor 3. Inside the reactor 3 the ammonia is catalytically oxidized to give nitrogen monoxide NO and—in a smaller amount—dinitrogen monoxide N.sub.2O, and at least a portion of NO is further oxidized to give nitrogen dioxide NO.sub.2 or dinitrogen tetroxide N.sub.2O.sub.4, producing a gaseous stream 22.

(12) Said gaseous stream 22 and a stream of water 23 are introduced into the absorption tower 4, where the NOx are at least partly absorbed to give nitric acid 24.

(13) The absorption tower 4 also provides a tail gas 25 as head product, mainly containing nitrogen and smaller quantities of oxygen, water, argon, N.sub.2O and residual NOx.

(14) Said tail gas 25 is pre-heated in the exchanger 5 and subsequently fed to the unit 6. According to the example shown in FIG. 2, said unit 6 comprises a DeNOx section inside which the NOx are at least partially removed by means of selective catalytic reduction (SCR).

(15) The unit 6 operates at a pressure ranging from 4 to 15 bar and provides a gas 26 mainly containing nitrogen, 2-3% oxygen, 0.2-0.3% water, NOx<30 ppm and N.sub.2O<30 ppm.

(16) Said gas 26 is split into two portions: a first portion 26a is expanded inside the expander 7 and a second portion 26b is exported from the section 1 for the synthesis of nitric acid and is fed to the section 2 for argon and nitrogen production.

(17) The expander 7 produces at least part of the power required by the compressors (not shown) of the nitric acid section 1. The expanded gas 27 is discharged into atmosphere.

(18) The section 2 for argon and nitrogen production essentially comprises a heat exchanger 8, an expander 9, a separator 10 and a distillation apparatus 11.

(19) According to the example of FIG. 2, said apparatus 11 comprises: a first distillation column 12 which operates at a pressure of about 4-5 bar, a second distillation column 13 which operates at atmospheric pressure and a third distillation column 14 which separates argon.

(20) Said section 2 operates as follows.

(21) The portion 26b of the gas coming from the section 1 is mixed with a recycling stream 32 and is fed to the heat exchanger 8 where it is cooled releasing heat to the stream 31 coming from the separator 10 and obtaining a refrigerated gas 28.

(22) The refrigerated gas 28 is then sent to the expander 9, where it is partly liquefied. The expander 9 is represented by a valve or by a turbine depending on the embodiments.

(23) The partially liquefied gas 29 is fed to the separator 10. The separator 10 separates a liquid phase 30 and a gaseous phase 31. The liquid phase 30 is sent to the distillation apparatus 11, while the gaseous phase 31 is sent to the heat exchanger 8 in order to refrigerate the incoming gas 26b and is then reintroduced into the cycle as stream 32.

(24) In greater detail, the liquid phase 30 feeds the first column 12, which separates gaseous nitrogen 33 from the top and a liquid fraction 34 containing nitrogen, oxygen and argon from the bottom.

(25) The liquid fraction 34 is sent to the second column 13, while the nitrogen 33 feeds a condenser 15, wherein it condenses exchanging heat with a tail fraction 35 of the column 13.

(26) According to the example of FIG. 1, the stream of condensed nitrogen 36 leaving the condenser 15 is split into two portions: a first portion 36a is sent to the second column 13 and a second portion 36b is sent to the first column 12 as reflux stream.

(27) Said second column 13 separates nitrogen 37 and oxygen 38.

(28) A fraction 39 containing argon and oxygen is collected in an intermediate point of the second column 13 and is sent to the third column 14 which separates substantially pure argon 40 and oxygen 41.

Example

(29) In a plant which produces 500 MTD (metric tons per day) of nitric acid, a process gas containing 5-6% of NOx is obtained at the inlet of the absorption tower. At the outlet of the aforementioned tower the tail gas contains about 300-500 ppm of NOx and at the outlet of the treatment section (SCR) said gas contains about 0-22 ppm. Subjecting this gas to a separation section, about 77,000 kg/h of nitrogen and about 1′300 kg/h of argon are obtained.