PROCESS FOR NITRIC ACID PRODUCTION

Abstract

Integrated process for the synthesis of ammonia and nitric acid including: a) production of an ammonia make-up synthesis gas, comprising steam reforming of a hydrocarbon feedstock under provision of steam reforming heat; catalytic conversion of said make-up synthesis gas into ammonia; catalytic oxidation of a stream of ammonia obtaining a process gas; absorption of said process gas with water obtaining nitric acid, wherein at least a portion of the steam reforming heat is recovered from said hot process gas.

Claims

1-22. (canceled)

23. An integrated process for synthesis of ammonia and nitric acid, the integrated process comprising: a synthesis of ammonia including: a) production of an ammonia make-up synthesis gas, including steam reforming of a hydrocarbon feedstock (NG) under provision of steam reforming heat; and b) catalytic conversion of the ammonia make-up synthesis gas into ammonia; and a synthesis of nitric acid including: c) catalytic oxidation of a stream of ammonia, obtaining a hot process gas containing NO.sub.2; and d) absorption of the hot process gas in water, wherein NO.sub.2 react with water to produce a product stream containing nitric acid and a tail gas containing nitrogen; wherein the steam reforming heat is recovered from the hot process gas obtained from the catalytic oxidation of the stream of ammonia.

24. The integrated process according to claim 23, wherein the steam reforming heat includes: a first heat that is supplied to a feed stream including steam (PS) and the hydrocarbon feedstock (NG) before the steam reforming; and a second heat that is supplied to a reaction stream undergoing the steam reforming; wherein at least part of the second heat is recovered from the hot process gas obtained from the step c) of catalytic oxidation.

25. The integrated process according to claim 24, wherein the at least part of the second heat is recovered by indirect heat exchange between the reaction stream and the hot process gas leaving the step c) of catalytic oxidation, the hot process gas acting as hot medium and having a temperature greater than 700° C.

26. The integrated process according to claim 24, wherein at least 30% of the second heat is recovered from the hot process gas.

27. The integrated process according to claim 26, wherein the second heat is entirely recovered from the hot process gas.

28. The integrated process according to claim 24, wherein part of the second heat is provided by combustion of a fuel in a dedicated burner providing a hot flue gas, the fuel including hydrogen and/or hydrocarbons.

29. The integrated process according to claim 28, wherein the hot flue gas mixes with the hot process gas leaving the step c) of catalytic oxidation to form a mixed stream, and the second heat is recovered by indirect heat exchange between the reaction stream and the mixed stream, the mixed stream acting as hot medium and having a temperature greater than 850° C.

30. The integrated process according to claim 29, wherein the step a) includes performing steam reforming in a gas heated reformer, the gas heated reformer comprising a cold side traversed by the reaction stream and a hot side traversed by the hot medium.

31. The integrated process according to claim 30, wherein the gas heated reformer includes a shell-and-tube heat exchanger, a cold side being the tube-side and a hot side being the shell-side.

32. The integrated process according to claim 31, wherein at least part of the first heat is recovered from a stream of process gas after transferring the at least part of the second heat to the reaction stream undergoing the steam reforming, the stream having a temperature from 300° C. to 700° C.

33. The integrated process according to claim 23, wherein the ammonia (120) obtained from the step b) provides at least a portion of the stream of ammonia subjected to the step c).

34. The integrated process according to claim 23, wherein the tail gas obtained from the step d) containing NOx and N2O and being subjected to a process of removal of NOx and N2O, thus providing a treated tail gas impoverished of NOx and N2O, wherein at least a portion of the treated tail gas is a nitrogen source for the make-up synthesis gas.

35. The integrated process according to claim 34, wherein the at least a portion of the treated tail gas is added to a hydrogen-containing synthesis gas, thus providing the make-up synthesis gas.

36. The integrated process according to claim 35, wherein the hydrogen-containing synthesis gas is obtained by conversion of the hydrocarbon feedstock (NG) into a raw synthesis gas, which includes at least the steam reforming, and subsequent purification of the raw synthesis gas.

37. The integrated process according to claim 36, wherein the pressure of the make-up synthesis gas being elevated to the pressure of the step b) in a suitable make-up gas compressor and the at least a portion of the treated tail gas being supplied at the suction of the make-up gas compressor.

38. The integrated process according to claim 36, wherein the at least a portion of the treated tail gas is added to the raw synthesis gas before purification, providing a raw make-up synthesis gas.

39. The integrated process according to claim 38, wherein the treated tail gas includes oxygen and the raw make-up synthesis gas being subjected to a process wherein oxygen reacts with the synthesis gas, thus providing a raw make-up gas impoverished of oxygen.

40. The integrated process according to claim 23, wherein the step a) includes a first stage of steam reforming and a second stage of steam reforming under provision of steam reforming heat, the second stage receiving the effluent of the first stage, and wherein the tail gas obtained from the d) is subjected to a combustion process providing at least part of the steam reforming heat to one of the first stage and second stage, preferably to the second stage of steam reforming.

41. The integrated process according to claim 40, wherein the combustion process is a non-selective catalytic reduction (NSCR) process.

42. A plant, comprising: a section for synthesis of ammonia and a section for synthesis of nitric acid, wherein the section for the synthesis of ammonia includes: a front-end section for synthesis of an ammonia make-up synthesis gas, including a gas heated reformer receiving steam reforming heat and a feed stream including steam and hydrocarbons; and a synthesis loop, wherein the ammonia make-up synthesis gas is converted into ammonia; wherein the section for the synthesis of nitric acid includes: an oxidation reactor, wherein a stream of ammonia is oxidized to provide a hot process gas containing NO2; and an absorption tower, wherein the process gas is absorbed in water and the water reacts with NO2 to provide a product stream containing nitric acid and a tail gas containing nitrogen, wherein the gas heated reformer is traversed by the hot process gas as hot medium, thus providing at least part of the steam reforming heat.

43. The plant according to claim 42, further comprising a heat exchanger for pre-heating the feed stream of the gas heated reformer, wherein a stream of process gas leaving the gas heated reforming acts as hot medium.

44. The plant according to claim 42, further comprising: wherein the front-end section includes a further gas heated reformer receiving steam reforming heat and the effluent of a preceding gas heated reformer; a combustion section wherein the tail gas from the absorption tower is subjected to combustion to provide a flue gas, and the further gas heated reformer is traversed by the flue gas as hot medium, thus providing at least part of the steam reforming heat.

Description

BRIEF DESCRIPTION OF THE FIGURES

[0059] FIG. 1 shows a simplified block scheme of an integrated plant for the synthesis of ammonia and nitric acid according to the invention.

[0060] FIG. 2 shows an embodiment of the invention, wherein part of the reforming heat is provided by combustion of a fuel.

[0061] FIG. 3 shows another embodiment of the invention, wherein reforming is carried out in a first steam reformer and a second steam reformer.

[0062] FIGS. 4 to 6 show further embodiments, wherein the tail gas of the nitric acid section is a nitrogen source for the make-up synthesis gas.

DETAILED DESCRIPTION

[0063] The plant of FIG. 1 comprises a section 1 for the synthesis of nitric acid and a section 100 for the synthesis of ammonia.

[0064] The nitric acid section 1 essentially includes a reactor 2 for the catalytic oxidation of ammonia, an absorption tower 3, a first heat exchanger 4, a second heat exchanger 5, a NOx and N2O removal unit 6 and a gas expander 7.

[0065] The ammonia section 100 essentially includes a front-end section 101, which provides a make-up gas 119, and a synthesis loop 102, which converts said make-up gas 119 into ammonia 120. The pressure of said make-up gas 119 is elevated to the pressure of the synthesis loop 102 in a syngas compressor 103. The block 104 in the figure represents a steam reformer, the block 105 comprises a shift converter and a subsequent heat exchanger, and the block 106 represents a PSA unit.

[0066] The operation of section 1 is as follows.

[0067] An ammonia stream 10 and an air flow 11 are fed to the reactor 2, wherein ammonia is catalytically oxidized, e.g. over platinum gauzes, to nitrogen monoxide (NO) and in minor amounts to nitrous oxide (N2O). As the gas cools, a portion of the nitrogen monoxide is further oxidized to nitrogen dioxide (NO2) or dinitrogen tetroxide (N2O4) depending on the residence time, temperature and pressure, and the composition relative to chemical equilibrium.

[0068] Said ammonia stream 10 is provided by the synthesis loop 102 of the ammonia section 100. Said air flow 11 provides the amount of oxygen required for the catalytic oxidation of ammonia and oxidation of nitrogen monoxide. The term of NOx is used below to collectively denote NO, NO2 and N2O4.

[0069] A process gas 12 containing NOx and N2O leaves the reactor 2. Said process gas 12 has a temperature of about 800-1000° C., for example of about 850-950° C. It is subsequently cooled to a temperature of about 300-700° C., for example of about 450-600° C., by indirect heat exchange with a reaction stream undergoing the steam reforming reaction in the steam reformer 104. Said heat exchange takes place in the steam reformer 104.

[0070] After heat exchange, the cooled process gas leaves the steam reformer 104 as stream 13 and enters the first heat exchanger 4, wherein is used as hot medium for pre-heating a tail gas 18 effluent of the absorption tower 3.

[0071] The process gas exits the first heat exchanger 4 as stream 14 and is subjected to thermal recovery in the second heat exchanger 5, with production of steam 16.

[0072] The process gas leaves the second heat exchanger as stream 15 and is admitted to the absorption tower 3, wherein NO2 are absorbed in water to yield nitric acid 17 and said tail gas 18. Said tail gas 18 is mostly composed of nitrogen and contains N2O and residual amounts of NOx.

[0073] The tail gas 18 is pre-heated in the first heat exchanger 4, as already mentioned above, and subsequently fed to the NOx and N2O removal unit 6, thus providing a treated tail gas 19 impoverished of NOx and N2O. The latter is work-expanded in the expander 7 for energy recovery and the exhaust gas 20 is discharged into the atmosphere.

[0074] The section 100 for the synthesis of ammonia operates as follows.

[0075] A desulphurized natural gas feedstock NG mixes with a steam current PS, generating a stream 110. Said stream 110, after being pre-heated in a suitable heat exchanger (not shown), enters the steam reformer 104, wherein it is reformed into a reformed gas 111 mostly composed of hydrogen, carbon monoxide, carbon dioxide, water and containing amounts of other components including e.g. residual methane.

[0076] The steam reforming heat is provided by the hot process gas 12 leaving the oxidation reactor 2 of the nitric acid. For example, said steam reaformer 104 is a shell-and-tube reforming exchanger, wherein the cold side (e.g. the tubes) contains a reforming catalyst and is traversed by the reaction stream undergoing the steam reforming reaction and the hot side (e.g. the shell) is traversed by the hot process gas 12.

[0077] The reformed gas 111 has a temperature of about 600-900° C., for example of about 750-850° C., and is subjected to thermal recovery in a heat exchanger 107, with production of steam 112.

[0078] The reformed gas 111 leaves said heat exchanger 107 as stream 113 and enters the block 105, wherein carbon monoxide is converted into carbon dioxide to produce a shifted gas and said shifted gas is subjected to thermal recovery producing steam 115. After thermal recovery, the shifted gas exits the block 105 as stream 114 and is subjected to purification in the corresponding section 106. According to the example of the figure, said purification section 106 operates a pressure swing adsorption (PSA) process, which provides a purified gas 116 essentially containing hydrogen and a CO2-containing tail gas stream 117.

[0079] Said purified gas 116 is added with nitrogen 118 to provide the make-up gas 119 with the required H2:N2 molar ratio for the ammonia synthesis reaction.

[0080] The so obtained make-up gas 119 is fed to the syngas compressor 103, wherein its pressure is elevated to the pressure of the synthesis loop 102. The make-up gas is then fed to the loop 102, wherein it is converted into ammonia 120. Further steam 121 is recovered in the synthesis loop 102.

[0081] The integration between the section 1 and the section 100 is realized by: a) feeding the hot process gas 12 obtained in the oxidation reactor 2 of the nitric acid section 1 to the steam reformer 104 of the ammonia section 100 for thermal recovery thus obtaining the steam reforming heat, and b) by feeding a portion 10 of the ammonia 120 obtained in the synthesis loop 102 of the ammonia section 100 to the oxidation reactor 2 of the nitric acid section 1.

[0082] FIG. 2 shows another embodiment of the invention. According to the example of this figure, the nitric acid section 1 further comprises a burner 8 fed with hydrogen (or natural gas) 21 as fuel and an air flow 22 as oxidant. A hot flue gas 22 leaves the burner 8 and mixes with the hot process gas 12 from the oxidation reactor 2 thus forming a stream 23. Said stream 23 is subjected to indirect heat exchange with the reaction mixture undergoing the reaction of steam reforming within the steam reformer 104, thus providing the steam reforming heat.

[0083] According to the embodiment of FIG. 2, the hot process gas 12 leaving the oxidation reactor 2 provides a first portion of the steam reforming heat and the flue gas 22 produced by combustion of the fuel 21 inside the burner 8 provides the remaining portion thereof.

[0084] FIG. 3 shows another embodiment, according to which the front-end section 101 comprises a first steam reformer 104 and a second steam reformer 204. According to the example of said figure, the reforming heat of said second steam reformer 204 is provided by combustion of a fuel F within the removal unit 6, wherein NOx and N2O are removed by a process of non-selective catalytic reduction (NSCR).

[0085] The input stream 110 is partially reformed in the first steam reformer 104, providing a partially reformed gas 111 with a temperature of about 700-800° C. The latter is further reacted in the second steam reformer 204, providing a reformed gas 211 which is then subjected to thermal recovery and subsequent purification.

[0086] The tail gas 18 from the absorption tower 3 contains oxygen, preferably in an amount above 4% by volume. Said tail gas 18 is pre-heated to a temperature higher than 500° C. in the heat-exchanger 4 and is subjected to NSCR in the NOx and N2O removal unit 6. Said removal unit 6 is also supplied with fuel F, which undergoes combustion in the presence of the oxygen contained in the tail gas 18.

[0087] The treated tail gas 19 impoverished of NOx and N2O leaves the removal unit 6 with a temperature higher than 900° C. Said tail gas 19 is subjected to indirect heat exchange with the reformed gas 111 inside the second steam reformer 204, wherein it is cooled thus providing the reforming heat.

[0088] The cooled tail gas leaves the second steam reformer 204 as stream 21 and is work-expanded in the expander 7. The exhaust gas 22 is discharged into the atmosphere.

[0089] FIGS. 4 and 5 show particular embodiments of the invention, wherein part of the tail gas from the nitric acid section 1 is used as nitrogen source for the synthesis of ammonia.

[0090] The tail gas 18 leaving the absorption tower 3 splits into a first portion 18a and a second portion 18b. Said first portion 18a is pre-heated in the heat exchanger 4, fed to the NOx and N2O removal unit 6 and then work-expanded in the expander 7. Said second portion 18b is exported from the nitric acid section 1 and fed to the ammonia section 100 to act as process nitrogen for the synthesis of ammonia. Said second portion 18b is initially subjected to NOx and N2O removal in a further unit 60 thus providing a nitrogen-containing gas 61 impoverished of NOx and N2O.

[0091] In the example of FIG. 4, said gas 61 is compressed in a booster 62 and subjected to further purification in a dedicated unit 63 to provide nitrogen 64 and a tail gas 65 containing oxygen and other impurities. Nitrogen 64 is then added to the purified gas 116 leaving the PSA unit 106, thus providing the make-up synthesis gas 119.

[0092] In the example of FIG. 5, said gas 61, after compression in said booster 62, is pre-heated in a heat exchanger 31 to a temperature higher than 500° C. to provide a pre-heated gas 66. The latter mixes with the effluent 111 of the steam reformer 104 in a reactor 30, wherein the oxygen contained in said gas 66 reacts with unconverted natural gas contained in the effluent 111, thus providing a stream 67 of raw make-up synthesis gas. The latter is subjected to thermal recovery in the heat exchanger 112 and to subsent purification.

[0093] FIG. 6 shows a variant of the embodiment of FIG. 3, wherein the tail gas 19 from the NSCR removal unit 6 splits into a first portion 19a and a second portion 19b. Said first portion 19a is subjected to indirect heat exchange inside the second steam reformer 204, and said second portion 19b mixes with the effluent 211 of the second reformer in a reactor 30, wherein the oxygen contained in the tail gas 19b reacts with unconverted natural gas contained in the effluent 211, thus providing a raw make-up gas 68 containing hydrogen and nitrogen. The latter is subjected to thermal recovery in the heat exchanger 112 and to subsent purification.

[0094] In the embodiments shown in FIGS. 5 and 6, the block 106 comprises a carbon dioxide removal section and a methanator.

Examples

[0095] With reference to FIG. 1, the advantages of the invention will be better elucidated by way of the example below.

[0096] Reference is made to a nitric acid section 1 with a nitric acid production rate of 1′500 t/d, wherein the process gas 12 leaving the oxidation reactor 2 has a temperature of 880° C. and is cooled to a temperature of 500° C. The steam reformer 104 of the ammonia section 100 is fed with a stream 110 at 450° C. and 25 bar, which has a steam to carbon (S/C) ratio equal to 3. The ammonia consumption for the nitric acid is 425 t/d.

[0097] The nitric acid section 1 provides the steam reformer 104 with a duty of 35 Gcal/h and allows to obtain a reformed gas 111 with a temperature of 800° C. In these conditions, the ammonia production rate is 425 t/d. It follows that the plant of FIG. 1 produces ammonia in a sufficient amount to make nitric acid.

[0098] With respect to the prior art, wherein the reforming heat is provided by combustion of a fuel, the transfer of a duty of 35 Gcal/h from the oxidation reactor 2 to the steam reformer 104 according to FIG. 1 allows to reduce the fuel consumption for the synthesis of ammonia (of an amount equivalent to 35 Gcal/h) and to reduce the excess steam produced in the nitric acid process (of a quantity equivalent to 35 Gcal/h, i.e. 45 t/h). Therefore, the energy consumption of a plant according to the invention is much lower than the energy consumption of a plant of the prior art plant with an ammonia production rate of 425 t/d, which consumes 250 Gcal/h of natural gas, i.e. 200 Gcal/h as process gas and 50 Gcal/h as fuel. The plant of the invention consumes at least 35 Gcal/h less of fuel, which would result in almost 15% lower overall gas consumption.

[0099] With reference to FIG. 2, the advantages of the invention will be better elucidated by way of the example below.

[0100] In a nitric acid section 1 with a production rate of 1′500 t/d, same as above, the flowrates of the fuel 21 and the oxidant 22 are properly chosen so as to increase the temperature of the process gas 12 from 880° C. to 1100° C. and the mass flowrate of 25%. By cooling the process gas 12 to a temperature of 500° C., a duty of 70 Gcal/h may be transferred to the steam reformer 104.

[0101] The above duty is two times greater than that transferred in the process of FIG. 1. This allows to produce double the amount ammonia, in particular in the amount needed to make nitric acid and also ammonium nitrate.