PROCESS FOR THE PRODUCTION OF NITRIC ACID

Abstract

A process for producing nitric acid comprising: catalytic oxidation of ammonia in the presence of oxygen to form a nitrous gas containing NO, O2, N2O and water vapor; a catalytic abatement of N2O which is performed over a first catalyst; a catalytic conversion of NO into NO2 which is performed over a second catalyst; the so obtained nitrous gas is then subject to absorption in water to produce nitric acid.

Claims

1-25. (canceled)

26. A process for producing nitric acid, the process comprising: a) catalytic oxidation of ammonia in the presence of oxygen to form a nitrous gas containing NO, O.sub.2, N.sub.2O, and water vapor; b) processing said nitrous gas to reduce a content of N.sub.2O in said nitrous gas and convert NO into NO.sub.2; c) using the processed nitrous gas, obtained from step b), in an absorption step wherein NO.sub.2 is absorbed in water to produce nitric acid, wherein step b) includes: b1) a catalytic abatement of N.sub.2O that is performed by passing the nitrous gas over a first catalyst, at a temperature that is lower than a temperature of the catalytic ammonia oxidation at step a), b2) a catalytic conversion of NO into NO.sub.2 that is performed after the step b1), passing the nitrous gas over a second catalyst.

27. The process according to claim 26 wherein said first catalyst includes a transition metal-oxide or aluminum silicate.

28. The process according to claim 27 wherein said first catalyst includes an iron loaded ferrierite (Fe-FER) or a ferrierite that is not loaded with iron (FER).

29. The process according to claim 26 wherein said second catalyst includes a transition metal-oxide or an aluminum silicate.

30. The process according to claim 26 wherein said second catalyst includes iron loaded ferrierite (Fe-FER) or ferrierite that is not loaded with iron (FER).

31. The process according to claim 26 wherein each of the first catalyst and the second catalyst includes iron-loaded ferrierite (Fe-FER), the ferrierite of the first catalyst having a higher concentration of iron than the ferrierite of the second catalyst.

32. The process according to claim 26 wherein the step b1) is performed after at least one step of cooling the gas effluent from step a), and wherein the step b1) is performed at a temperature not greater than 700° C.

33. The process according to claim 26, wherein the step b1) is performed at a higher temperature than the step b2), the nitrous gas being cooled in at least one heat exchanger after step b1) and before step b2).

34. The process according to claim 33 wherein the catalytic abatement of N.sub.2O of step b1) is performed at 400° C. to 700° C., and the catalytic oxidation of NO of step b2) is performed at 150° C. to 500° C.

35. The process according to claim 26 wherein the first catalyst and/or the second catalyst are fitted in one or more equipment selected from a vessel, a reactor, a heat exchanger, or a pipe.

36. The process according to claim 35 wherein the first catalyst and/or the second catalyst are fitted in channels of a respective heat exchanger and/or in the pipe connecting two consecutive heat exchangers.

37. The process according to claim 26 wherein the first catalyst is fitted in a first equipment and the second catalyst is fitted in a second equipment, separate from the first equipment, and at least one heat exchanger is arranged to cool the gas effluent from the first equipment before it reaches the second equipment.

38. The process according to claim 26, wherein the first catalyst and the second catalyst are fitted in the same reactor or same pressure vessel and the reactor or pressure vessel includes cooling means arranged to cool the gas between the first catalyst and the second catalyst.

39. The process according to claim 26 wherein the first catalyst and/or the second catalyst is/are in any of the following forms: extrudate or 3D printed or pelletized or shaped as structured catalyst.

40. The process according to claim 26 wherein steps b1) and b2) are performed in a cooling train arranged to cool the nitrous gas effluent from the ammonia oxidation reactor and before it enters the absorber.

41. The process according to claim 26 wherein the second catalyst used in step b2) for the oxidation of NO contains iron-loaded ferrierite (Fe-FER) and is an aged catalyst previously used in the step b1) for decomposition of N2O.

42. The process according to claim 41, further comprising: using a Fe-FER catalyst in step b1) for the decomposition of N.sub.2O and for a predetermined service life; and after the above service life is completed, using aged catalyst taken from said step b1) in the step b2) as a catalyst for the oxidation of NO to NO.sub.2.

43. A plant for producing nitric acid, the plant comprising: an ammonia oxidation reactor configured for catalytic oxidation of ammonia in the presence of oxygen to form a nitrous gas containing NO, O.sub.2, N.sub.2O, and water vapor; an absorber where a NO.sub.2-containing gas is subjected to absorption in water to produce nitric acid; at least a gas cooler; a first bed or layer of a first catalyst for decomposition of N.sub.2O; and a second bed or layer of a second catalyst for oxidation of NO to NO.sub.2 that are arranged, in this order, between the ammonia oxidation reactor and the absorber, so that a nitrous gas produced in the oxidation reactor passes through the gas cooler, the first catalyst and then through the second catalyst before the nitrous gas enters the absorber.

44. The plant according to claim 43 wherein the first catalyst and/or the second catalyst includes a transition metal-oxide or aluminum silicate.

45. The plant according to claim 43 wherein the first catalytic bed or layer and a second catalytic bed or layer are part of a cooling train arranged between the ammonia oxidation reactor and the absorber.

46. The plant according to claim 43 wherein the first catalytic bed or layer and the second catalytic bed or layer are arranged in the same pressure vessel or arranged in two separate pressure vessels.

47. The plant according to claim 43, further comprising one or more of: a first heat exchanger arranged to cool the nitrous gas obtained from the oxidation of ammonia, before the nitrous gas enters the first catalytic bed or layer; a second heat exchanger arranged to remove heat from the gas effluent from the first catalytic bed or layer, before the gas effluent enters the second catalytic bed or layer; or a waste heat boiler arranged to recover heat from the effluent gas of the second catalyst, after the oxidation of NO to NO.sub.2.

48. The plant according to claim 43, wherein said first catalyst and second catalyst two catalysts are fitted in devices as vessel, reactors, heat exchanger.

49. A process for producing nitric acid, the process comprising: catalytic oxidation of ammonia in the presence of oxygen to form a nitrous gas containing NO, O2, N.sub.2O and water vapor; processing the so obtained nitrous gas and using the so obtained processed nitrous gas to produce nitric acid by absorption of NO.sub.2 in water; wherein the processing of nitrous gas comprises a step of oxidation of NO to NO.sub.2 that is performed over a Fe-FER catalyst.

50. In a process of production of nitric acid, using of aged Fe-FER catalyst, previously used for decomposition of N.sub.2O in a gas containing nitrogen, oxygen, N.sub.2O, NOx and water, as a catalyst for oxidation of NO to NO.sub.2, to increase the content of NO.sub.2 in a nitrous gas before contacting the gas with water for absorption of NO.sub.2 in water and production of nitric acid.

Description

DESCRIPTION OF FIGURES

[0070] FIG. 1 is a scheme of a preferred embodiment of the invention.

[0071] FIG. 2 is a plot of a temperature and oxidation profile of a preferred embodiment of the invention.

DETAILED DESCRIPTION

[0072] FIG. 1 discloses an example of the invention applied to a nitric acid dual pressure process. This term denotes a process where absorption is performed at a pressure greater than ammonia oxidation.

[0073] A mixture 1 of ammonia and air reacts in an ammonia oxidation reactor 2 over a suitable catalyst 3 to form a nitrous gas 4.

[0074] Ammonia oxidation with air is an exothermic reaction with the formation of NO (about 9% mol) and H2O (about 16%mol). Secondary reactions produce undesired components as N2 and N2O (typically about 1000 ppmv).

[0075] Hot nitrous gas 4 produced in ammonia oxidation reactor, is cooled up to about 500° C. passing through a superheater 6 and an evaporator 7. The item 5 denotes a support of the ammonia oxidation catalyst. The ammonia oxidation catalyst is possible to be supported on heat-resistant inert material in the form of beds, packings or honeycombs, which, viewed in the flow direction, have a depth of at least 5 cm, preferably at least 10 cm, in particular at least 20 cm and very particularly preferably from 20 to 50 cm. The inert material is contained in a basket and it is possible to cool the basket with a cooling medium.

[0076] A high-temperature de-N2O catalyst 8 is positioned between the evaporator 7 and a tail gas heater 8. Said catalyst 8 may be installed below the evaporator 7 and performs a N2O abatement, preferably to a residual N2O of not more than 20 ppm.

[0077] Particularly, when the nitrous gas passes through the catalyst 8 the N2O is decomposed into N2 and O2.

[0078] The passage through the catalyst 8 also increases the temperature due to NO oxidation up to about 530° C. The NO2/NOx ratio at the outlet of the catalyst 8 is about 0.25.

[0079] The nitrous gas leaving the catalyst 8, now with a reduced content of N2O, is cooled in the tail gas heater 9 and then passes through a low-temperature catalyst 10 where NO is oxidized to NO2.

[0080] The passage through the catalyst 10 increases the temperature up to about 370° C. and the NO2/NOx ratio to about 0.7.

[0081] This effluent gas from the low-temperature catalyst 10 traverses a waste heat boiler 11 and then goes via line 20 to an economizer 12 and a condenser 13.

[0082] The economizer 12 removes heat from the nitrous gas, decreasing the nitrous gas temperature close to dew point of the nitric acid. Nitric acid condensation is performed in the condenser 13 with cooling water.

[0083] Nitric acid condensed (weak acid) is recovered at line 14 and sent to an absorption tower. Nitrous gas 15 separated from weak acid is mixed with exhaust air 16 coming from a bleacher; the so obtained mixture 17 is sent to a nitrous gas compressor 18. In the nitrous gas compressor 18, the pressure is increased to about 12 bar abs and temperature rise up to 160° C. due to gas compression and further NO oxidation.

[0084] The delivery line 19 of the compressor 18 goes to an absorber where the gas is contacted with water for the production of nitric acid.

[0085] The high-temperature de-N2O catalyst 8 reduces the N2O concentration in nitrous gas to a proper level (N2O reduction preferably up to 98%), and boosts the NO oxidation. The low-temperature catalyst 10 performs NO oxidation reaction at about 300° C., increasing considerably the oxidation (NO2 / NOx ratio 0.7), and the temperature level up to about 370° C.

[0086] In the state of art, the temperature downstream the pipe at the outlet of the tail gas heater 9 is about 260° C., with a NO2/NOx ratio of about 0.6.

[0087] The higher temperature level reached downstream the low-temperature catalyst, allows to recover heat at higher temperature and produce more steam.

[0088] The low-temperature catalyst allows to reach higher level of NO2/NOx ratio at the inlet of the condenser 13 (about 80% compared to 73% in state of art), and that promotes the acid condensation. Since the weak acid 14 quantity produced in the process is higher (+3%) than state of art, nitrous gas at the inlet of nitric compressor is slightly lower and the required power decreases at the nitric compressor 18 (-1%). This leads to an additional power saving for the plant: the superheated steam generated is 2% higher than state of art, and the steam exported, considering steam turbine consumption and internal plant steam requirements, is 3% higher.

[0089] FIG. 2 illustrates a temperature and oxidation profile in a preferred embodiment of the invention.

[0090] The lines C1 and C2 show the oxidation level which is defined as NO2/NOx in molar base. The line C1 shows the oxidation level for the low pressure section in a typical nitric acid process of the prior art. The line C2 shows the oxidation level in an embodiment of the invention as illustrated in FIG. 1. Relevant points of the process are marked with letters A to K.

[0091] The line EQ represents the thermodynamic equilibrium for the oxidation reaction which sets an upper limit for the oxidation process, i.e. for the oxidation and temperature that can be reached in the process.

[0092] The dotted line “HNO3 cond” is the condition in which nitric acid condenses.

[0093] The oxidation NO+½ O2 ➔ NO2 is an exothermic reaction and the reaction heat causes the gas temperature to increase along the pipes.

[0094] The oxidation heat is recovered to obtain the maximum energy recovery without over-complicating the process and without the risk of working in corrosive areas.

[0095] It should be noted that: in heat exchangers, the temperature of the nitrous gas may decrease and the NO2/NOx ratio may increase due to the volume of the equipment. In pipeline connecting heat exchangers, oxidation and temperature increase due to NO oxidation. Oxidation in pipeline depends on volume of the pipes, so temperature and oxidation level is basically defined by plant layout.

[0096] The operating line C2 of the invention is now described.

[0097] Point A denotes the nitrous gas effluent from the ammonia oxidation catalyst at a temperature of about 900° C.

[0098] The segment A to B denotes cooling of the nitrous gas from 900° C. to a temperature slightly above 600° C. due to heat removed by the catalyst support 5 (e.g. internally cooled) and the superheater 6. At this high temperature range, no oxidation of NO occurs.

[0099] The segment B to C denotes the subsequent cooling in the evaporator 7 to about 500° C. At this temperature range oxidation of NO begins reaching about 5% at the outlet of the evaporator 7 (point C).

[0100] The segment C to D denotes the passage through the high-temperature deN2O catalyst 8. The high-temperature catalyst 8 reduces the N2O concentration in nitrous gas to a proper level, preferably N2O reduction up to 98%, and boosts the NO oxidation, reaching the thermodynamic value. It can be appreciated that point D lies practically on the equilibrium curve EQ.

[0101] The segment D to E denotes cooling of the nitrous gas through the tail gas heater 9.

[0102] The segment E to F denotes the strong oxidation of NO through the low-temperature catalyst 10. Said catalyst 10 performs NO oxidation reaction at about 300° C., increasing considerably the oxidation NO2/NOx ratio up to 0.7 and the temperature level up to about 370° C.

[0103] The subsequent segment F to G denotes cooling in the waste heat boiler 11. The segment G to H denotes a slight heating and oxidation occurring through the pipe 20. The segment H to J denotes cooling in the economizer 12 and the segment J to K relates to the piping from the economizer 12 to the condenser 13. The oxidation ratio at the inlet of the condenser (point K) is about 80%.

[0104] The curve C1 denotes a prior art process wherein the nitrous gas starting from the same point A at 900° C. is cooled up to about 420° C. in a superheater and evaporator; the nitrous gas coming out from the evaporator flows through the bottom of the ammonia oxidation reactor and a line connecting to a tail has heater, increasing the NO2/NOx ratio to about 0.4 and the temperature to about 460° C.; in a pipe connecting the tail gas heater to the economizer the NO2/NOx ratio further increases to about 0.6 and the temperature rises to about 260° C. An economizer recovers heat from nitrous gas decreasing the nitrous gas temperature close to the nitric acid dew point. At the end of the curve C1 (inlet of nitric acid condenser) the oxidation ratio is about 73%.

[0105] The advantages of the invention can be appreciated by comparing the curve C2 of the invention with the curve C1 of the prior art.

[0106] It can be seen that the curve C2 of the invention better approaches the ideal curve EQ, which is reached at points D and F. The final oxidation reached by the invention at the inlet of the condenser 13 is around 80% at point K, compared with 73% reached by the reference prior art. This higher ratio promotes the acid condensation.

[0107] Particularly, the invention reaches a higher temperature and oxidation thanks to the low-temperature oxidation catalyst. The reference prior art, in absence of such catalyst, reaches a temperature of about 260° C. and a NO2/NOx ratio of about 0.6 in the connecting pipe between the tail gas heater and the economizer. The higher temperature reached by the invention allows to recover heat at higher temperature and produce more steam.