Process for the production of nitric acid with tertiary abatement of N2O and NOx

Abstract

A nitric acid production process, comprising tertiary abatement of N2O and NOx on a tail gas withdrawn from an absorption stage, said abatement including passing the tail gas over a sequence of a deN2O stage comprising a Fe-z catalyst and a deNOx stage comprising a V2O5-TiO2 catalyst in the presence of gaseous ammonia, wherein the tail gas at the inlet of deN2O stage and the tail gas at the inlet of deNOx stage have a temperature greater than 400° C.

Claims

1. A nitric acid production process, comprising a step of catalytic oxidation of ammonia, producing a process gas comprising nitrogen oxides NOx and N2O, and a step of absorption of said process gas with water, producing a product stream containing nitric acid, and a stream of a tail gas containing NOx and N2O, wherein the content of NOx and N2O of said gas stream is reduced by the steps of: passing said gas stream, without any previous stage of removal of NOx, over a deN2O first stage comprising an iron-loaded zeolite catalyst for decomposition of N2O, obtaining an effluent gas stream with a reduced content of N2O, passing said effluent of the deN2O stage over a deNOx second stage comprising a V2O5-TiO2 catalyst, in the presence of gaseous ammonia as a reducing agent, wherein: a) the N2O and NOx containing gas streams at the inlet of said first stage and at the inlet of said second stage have a temperature greater than 400° C., and at least one of the following conditions is met: b1) the N2O and NOx containing gas stream before admission into the first stage has a NOx molar content of less than 1000 ppm; b2) the N2O and NOx containing gas stream before admission into the first stage has O2 molar content of less than 4%; b3) the molar ratio of ammonia over NOx at the inlet of the second stage is 0.9 to 1.1.

2. The process according to claim 1, wherein condition b1) further provides that said gas stream containing NOx and N2O, before admission into the first stage, has a NOx molar content of less than 750 ppm.

3. The process according to claim 2, wherein condition b 1) further provides that said gas stream containing NOx and N2O, before admission into the first stage, has a NOx molar content of less than 500 ppm.

4. The process according to claim 1, wherein condition b2) further provides that said gas stream containing NOx and N2O, before admission into the first stage, has O2 molar content of less than 3%.

5. The process according to claim 1, wherein condition b3) further provides that the molar ratio of ammonia over NOx at the inlet of the second stage is 0.95 to 1.05.

6. The process according to claim 5, wherein condition b3) further provides that the molar ratio of ammonia over NOx at the inlet of the second stage is equal to 1 or approximately 1.

7. The process according to claim 1, wherein at least two of the conditions b1), b2) and b3) are met.

8. The process according to claim 7, wherein all of the conditions b1), b2) and b3) are met.

9. The process according to claim 1, wherein said condition a) provides that both gas streams at the inlet of the first stage and of the second stage have a temperature equal to or greater than 415° C.

10. The process according to claim 9, wherein said condition a) provides that both gas streams at the inlet of the first stage and of the second stage have a temperature equal to or greater than 430° C.

11. The process according to claim 1, wherein the effluent of the first stage is not subject to intermediate cooling through an intermediate cooling stage before the admission into the second stage.

12. The process according to claim 1, comprising the addition of ammonia to the effluent of the first stage before the admission into the second stage.

13. The process according to claim 1, wherein the byproduct formation of N2O in the second stage is less than 30 ppm.

14. The process according to claim 1, wherein the first stage and/or the second stage comprise one or more radial-flow catalytic beds.

15. The process according to claim 14, wherein one or more catalytic beds of the first stage and one or more catalytic beds of the second stage are either contained in the same vessel or in two separate vessels for the first stage and second stage respectively.

16. The process according to claim 1, wherein said step of absorption is performed at an absorption pressure equal to or greater than 6 bar.

17. The process according to claim 16, wherein said step of absorption is performed at an absorption pressure equal to or greater than 9 bar.

18. The process according to claim 16, wherein said step of absorption is performed at an absorption pressure equal to or greater than 11 bar.

19. The process according to claim 1, further comprising a step of reducing the content of N2O of said process gas obtained from the catalytic oxidation of ammonia, before the absorption step.

20. The process according to claim 19, wherein said step of reducing the content of N2O of said process gas includes a catalytic decomposition of N2O.

21. A nitric acid production plant, including at least a reactor for catalytic oxidation of ammonia and an absorber for production of nitric acid, said absorber producing a nitric-acid containing product stream and a tail gas containing N2O and NOx, the plant comprising a tertiary abatement system for abatement of N2O and NOx of said tail gas with a process according to claim 1, the system comprising: a sequence of at least one first catalytic bed for decomposition of N2O and at least one second catalytic bed for selective reduction of NOx, said at least one first catalytic bed comprising an iron-loaded zeolites catalyst and said at least one second catalytic bed comprising a V2O5-TiO2 catalyst wherein said at least one catalytic bed for decomposition of N2O receives the tail gas effluent from the absorber without any previous removal of NOx; at least one device to introduce ammonia between said at least one first catalytic bed and said least one second catalytic bed, so that the selective reduction of NOx is performed in the presence of ammonia as reducing agent, a connection arranged to feed the effluent gas of the at least one first catalytic bed to the at least one second catalytic bed including no heat exchanger to cool the gas.

22. The plant according to claim 21, wherein the at least one first catalytic bed and the at least one second catalytic bed are contained in a single pressure vessel.

23. The plant according to claim 21, wherein the at least one first catalytic bed and/or the at least one second catalytic bed are axial-flow and comprises a structured catalyst, wherein the catalyst is deposited or impregnated on a monolithic support.

24. The plant according to claim 23, wherein the at least one first catalytic bed and/or the at least one second catalytic bed are axial-flow and comprises a structured catalyst, wherein the catalyst is deposited or impregnated on a honeycomb support.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 shows a simplified block scheme of a process for the synthesis of nitric acid plant according to an embodiment of the invention.

DETAILED DESCRIPTION

(2) FIG. 1 illustrates a process/plant 1 for the synthesis of nitric acid which makes use of the invention for tertiary abatement of N2O and NOx in the tail gas withdrawn from the absorber.

(3) The main equipment of the plant 1 comprise: a reactor 2 for the catalytic oxidation of ammonia, a first deN2O stage 3, a cooler 4, an absorption tower 5, a heat exchanger 6, a second deN2O stage 7, a deNOx stage 8 and a gas expander 9. The reactor 2 and the first deN2O stage 3 are typically included in the same equipment (the burner).

(4) The first deN2O stage 3 is in a secondary position according to the nomenclature used in the field of nitric acid production, since it is located after the ammonia oxidizer 2 but before the absorber 5. The second deN2O stage 7 is in a tertiary position, being after the absorber 5 and before the expander 9.

(5) The first deN2O stage 3 comprises a catalytic bed made of a suitable secondary catalyst.

(6) The second deN2O stage 7 comprises a catalytic bed made of iron-exchanged zeolites catalyst. Said deNOx stage 8 comprises a catalytic bed made of V2O5-TiO2-based catalyst. The catalytic bed of the second deN2O stage 7 and the catalytic bed of the deNOx stage 8 can be installed in the same pressure vessel.

(7) Said catalysts can be structured catalysts (e.g. a catalyst deposited or impregnated on a monolithic support such as a honeycomb support) or pellet catalysts according to different embodiments. Either or both of the deN2O and deNOx catalysts can be structured catalysts.

(8) The advantage of the structured catalyst is that it has more available cross area to the passage of the gas than pellet catalyst, hence it entails a low pressure drop even in case of axial flow. Axial flow means flow in the direction of the main axis of the catalyst bed, for example the vertical axis in case of catalyst bed arranged in a vertical reactor.

(9) An ammonia stream 10 and an air flow 11 are mixed to form the input stream 12 of the reactor/ammonia oxidizer 2, wherein ammonia is catalytically oxidized to nitrogen monoxide (NO) over platinum catalytic gauzes. Minor amounts of nitrous oxide (N2O) are formed as byproduct of the ammonia oxidation to NO. A portion of the nitrogen monoxide is further oxidized to nitrogen dioxide (NO2) or dinitrogen tetroxide (N2O4) in the presence of oxygen from the air. The reactor 2 therefore produces a gaseous stream 13 comprising N2O and NOx. Here, the term NOx collectively denotes NO, NO2 and N2O4.

(10) Said gaseous stream 13 is supplied to the first de-N2O stage 3, wherein an amount of N2O is catalytically decomposed over the iron-exchanged zeolites catalyst, providing a gaseous stream 14 with a reduced content of N2O. The amount of N2O decomposed in said stage 3 is preferably not greater than 90%, more preferably not greater than 80% of the N2O contained in the stream 13.

(11) The secondary deN2O stage 3 is an optional feature of the invention. In some embodiments the stream 13 effluent from the reactor 2, possibly after cooling, is directly fed to the absorber 5.

(12) The stream 14 is cooled in the heat exchanger 4 to become stream 15 and subsequently admitted to the absorption tower 5. Inside the absorption tower 5, NOx are at least partially absorbed in water to form a nitric acid containing product 16. Generally, said absorption tower 5 is a tray or packed column.

(13) The absorption tower 5 also provides a tail gas 17, which is mostly composed of nitrogen and contains smaller amounts of oxygen, N2O and residual NOx.

(14) Said tail gas 17 is pre-heated in the heat exchanger 6 to a temperature of about 430° C., and subsequently fed to the second de-N2O stage 7 through the flow line 18. Here, N2O is catalytically decomposed over iron-exchanged zeolites, providing a N2O-depleted effluent 19.

(15) Said N2O-depleted effluent 19 is added with ammonia 20 as reducing agent, thus forming the input stream 21 of the de-NOx stage 8. Inside said stage 8, NOx are catalytically reduced providing a purified gas 22 with a low content of NOx and N2O.

(16) The purified gas 22 is work-expanded in the expander 9 to the atmospheric pressure. The power produced by the expander 9 can be used e.g. to drive compressors of the nitric acid plant (not shown).

(17) The exhaust gas 23 is discharged into the atmosphere. Said exhaust gas 23 typically contains less than 50 ppmv of N2O and less than 50 ppmv of NOx.

Example 1

(18) The following data (table 1) were obtained for tertiary abatement of N2O and NOx in a nitric acid plant, from a tail gas withdrawn from the absorber, using a deN2O stage based on Fe-zeolite followed by a deNOx stage based on V2O5-TiO2. The gas was preheated before the deN2O stage. NH3 was added as reductant before the deNOx stage.

(19) TABLE-US-00001 TABLE 1 Tail gas flow rate 100 000 Nm.sup.3/hour Tail gas inlet temperature (to DeN2O stage) 430° C. Tail gas pressure 10 bar absolute Tail gas inlet molar composition N2O 1 300 ppmv NOx 1 000 ppmv Degree of oxidation 0.2 O2 3% N2 Balance Operation Space velocity of DeN2O catalyst bed 10 000 1/h NH3/NOx molar ratio 1   Space velocity of DeNOx catalyst bed 20 000 1/h

(20) The resulting tail gas effluent, after the deNOx stage, has N2O concentration of <50 ppmv (>96% abatement) and a NOx concentration of <50 ppmv (>95% abatement).

Example 2

(21) (comparative example for preferred embodiment of invention with secondary abatement).

(22) A prior art abatement system includes secondary DeN2O (based on known secondary catalyst) and tertiary DeNOx based on V2O5-TiO2 catalyst.

(23) The secondary abatement designed for 95% abatement of N2O and having an inlet N2O of 1200 ppm would theoretically result in 60 ppm residual N2O if there were no bypass around it. A bypass stream of secondary catalyst of at least about 5% of the inlet flow rate is typically to be expected. The N2O in the bypass is unabated. As a consequence, the residual N2O concentration in the gas from the secondary abatement system (including the bypass) is 120 ppm. The stream is treated in the further sections of the prior art nitric acid plant, and it leaves the absorber as tail gas which is treated in the DeNOx. The DeNOx does not abate N2O, which is emitted to atmosphere.

(24) In the case of the invention, the gas from absorber is subjected to further tertiary deN2O upstream deNOx, hence it reaches a lower level of N2O.