Process for nitric acid production

Abstract

Integrated process for the synthesis of ammonia and nitric acid, comprising a synthesis of nitric acid including the following steps: a) subjecting a stream of ammonia (10) to catalytic oxidation, obtaining a gaseous stream containing nitrogen oxides (13); b) subjecting said gaseous stream to a process of absorption of nitrogen oxides, providing nitric acid (16) and a tail gas (17) containing nitrogen and residual nitrogen oxides; c) subjecting at least a portion of said first tail gas (17) to a process of removal of nitrogen oxides, providing a nitrogen oxides-depleted tail gas (18), and comprising a synthesis of ammonia by catalytic conversion of a make-up gas (126, 226) comprising hydrogen and nitrogen in an ammonia synthesis loop, wherein at least a portion (18b, 18d, 21) of said second tail gas is used as nitrogen source for obtaining said make-up gas (126, 226).

Claims

1. An integrated process for synthesis of ammonia and nitric acid, the integrated process including synthesizing the nitric acid by a method comprising: a) subjecting a stream of ammonia to oxidation, thereby obtaining a gaseous stream containing nitrogen oxides; b) subjecting said gaseous stream to a process of absorption of nitrogen oxides, thereby providing nitric acid and a first tail gas containing nitrogen and residual nitrogen oxides; c) subjecting at least a portion of said first tail gas to a process of removal of nitrogen oxides, thereby providing a second tail gas containing nitrogen and having a lower content of nitrogen oxides than said first tail gas; and d) synthesizing ammonia by catalytic conversion of a make-up gas including hydrogen and nitrogen in an ammonia synthesis loop; wherein at least a portion of said second tail gas is used as a nitrogen source for said make-up gas; and wherein the first tail gas provided by said step b) is entirely or partially subjected to said step c) and the resulting NO.sub.x-depleted tail gas splits into two portions, a first portion being used as nitrogen source to obtain said ammonia make-up gas and a second portion being work-expanded, or wherein the first tail gas provided by said step b) splits into two portions, a first portion being subjected to said step c) and a second portion being subjected to a further step for removal of NO.sub.x, thus providing two streams having a lower content of nitrogen oxides than said first tail gas, a first stream being a nitrogen source for said ammonia make-up gas and a second stream being work-expanded.

2. The integrated process according to claim 1, wherein said at least a portion of said second tail gas is added to a hydrogen-containing synthesis gas, thus providing said make-up gas.

3. The integrated process according to claim 2, wherein said hydrogen-containing synthesis gas is obtained by conversion of a hydrocarbon feedstock, said conversion including at least one of reforming or catalytic partial oxidation (CPOx).

4. The integrated process according to claim 2, wherein a pressure of said make-up gas is elevated to a pressure of the synthesis loop in a make-up gas compressor and said at least a portion of said second tail gas being supplied at a suction of said make-up gas compressor.

5. The integrated process according to claim 2, wherein said step b) is carried out substantially at a pressure of the hydrogen-containing synthesis gas, said pressure being is at least 15 bar.

6. The integrated process according to claim 2, wherein said second tail gas contains oxygen and said at least a portion of said tail gas includes a nitrogen source for said make-up gas is subjected to a process for oxygen removal, said process for oxygen removal includes a pressure-swing adsorption (PSA) process.

7. The integrated process according to claim 6, wherein said process for oxygen removal includes the PSA and including a start-up phase in which an air stream is subjected to said PSA process that provides a nitrogen-containing stream, said nitrogen-containing stream being used as nitrogen source for obtaining said make-up gas.

8. The integrated process according to claim 1, wherein said make-up gas is obtained by primary reforming of a hydrocarbon feedstock followed by secondary reforming or primary reforming of a hydrocarbon feedstock followed by catalytic partial oxidation (CPOx), the secondary reforming being carried out in a secondary reformer and the CPOx being carried out in a CPOx unit, and wherein said at least a portion of said second tail gas being a nitrogen source for said make-up gas is supplied at the inlet of said secondary reformer or at the inlet of said CPOx unit.

9. The integrated process according to claim 8, wherein the secondary reforming or the CPOx is carried out in a presence of an air stream, nitrogen required to obtain the make-up gas being partially provided by said second tail gas and a balance nitrogen being supplied by said air stream.

Description

BRIEF DESCRIPTION OF THE FIGURES

(1) FIG. 1 shows a simplified block scheme of an integrated plant for the synthesis of ammonia and nitric acid according to a first embodiment of the invention, wherein reforming is carried out in a steam reformer.

(2) FIGS. 2 and 3 are variants of FIG. 1.

(3) FIG. 4 shows a simplified block scheme of an integrated plant for the synthesis of ammonia and nitric acid according to a second embodiment of the invention, wherein reforming is carried out in a primary reformer and a secondary reformer.

DETAILED DESCRIPTION

(4) The plant of FIG. 1 comprises a section 1 for the synthesis of nitric acid and a section 100 for the synthesis of ammonia.

(5) Said section 1 essentially includes a reactor 2 for the catalytic oxidation of ammonia, an absorption tower 3, a heat exchanger 4, a nitrogen oxides removal unit 5, a gas expander 6 and an air compressor 7.

(6) The operation of said section 1 is as follows.

(7) An ammonia stream 10 and an air flow 11 are mixed to form the input stream 12 of the reactor 2, wherein ammonia is catalytically oxidized to nitrogen monoxide (NO) and in minor amounts to nitrous oxide (N.sub.2O), and at least a portion of the nitrogen monoxide is further oxidized to nitrogen dioxide (NO.sub.2) or dinitrogen tetroxide (N.sub.2O.sub.4), thus providing a gaseous stream 13.

(8) Said air flow 11 provides the amount of oxygen required for the catalytic oxidation of ammonia and oxidation of nitrogen monoxide. The air compressor 7 is used for compressing an air flow 14 from atmospheric pressure to a suitable pressure before its admission into the reactor 2.

(9) The term of “nitrogen oxides” or “NO.sub.x” will be used below to denote the following: nitrogen monoxide, nitrogen dioxide, dinitrogen tetroxide and nitrous oxides.

(10) The gaseous stream 13 is contacted with a stream of water 15 and admitted to the absorption tower 4, wherein NO.sub.x are at least partially absorbed in to yield nitric acid 16. Generally, said absorption tower 3 is a tray or packed column where NO.sub.x are absorbed in water to form nitric acid.

(11) The absorption tower 3 also provides a tail gas 17 as overhead product, which is mostly composed of nitrogen and contains smaller amounts of oxygen and NO.sub.x. Said tail gas 17 is pre-heated in the heat exchanger 4 and subsequently fed to the NO.sub.x removal unit 5, which provides a NO.sub.x-depleted product gas 18.

(12) According to the example of the figure, the NOx removal unit 5 carries out a non-selective catalytic reduction process (NSCR), thus providing a NOx-depleted product gas 18 essentially comprising nitrogen and substantially free of oxygen. Alternatively, a pressure swing adsorption (PSA) process can be used.

(13) The NO.sub.x-depleted product gas 18 from the NO.sub.x removal unit 5 splits into two portions, a first portion 18a is work-expanded in the expander 6 from the overhead pressure of the absorption tower 3 to the atmospheric pressure and a 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.

(14) Said expander 6 produces at least part of the power required by the compressors of the nitric acid plant, namely the air compressor 7 and, when a dual-pressure nitric acid process is carried out, a NO.sub.x compressor (not shown) of the feed stream to the absorption tower. The exhaust gas 19 is discharged into the atmosphere.

(15) The section 100 for the synthesis of ammonia essentially includes a front-end section 101, which provides a make-up gas 126, and a synthesis loop 102, which converts said make-up gas into ammonia 129. The pressure of said make-up gas 126 is elevated to the pressure of the synthesis loop 102 in a syngas compressor 103. Said front-end section 101 essentially comprises a desulphurizer 104, a steam reformer 105, a carbon monoxide shift conversion section 106 (which may comprise for example a high temperature shift converter and a low temperature shift converter), a purification section 107.

(16) The operation of said section 100 is as follows.

(17) A natural gas feedstock NG enters said desulphurizer 104, resulting in a stream 120 of desuplhurized natural gas. Said stream 120 is mixed with a steam current PS generating a stream 121 of process gas, which enters the steam reformer 105, wherein it is reformed to provide a reformed gas 122 mostly composed of hydrogen and containing minor amounts of other components including e.g. carbon monoxide, carbon dioxide, water, methane.

(18) Said reformed gas 122 is fed to the carbon monoxide shift conversion section 106, wherein carbon monoxide is converted into carbon dioxide to produce a shifted gas 123. Said shifted gas 123 is subjected to purification in the corresponding section 107. According to the example of the figure, said purification section 107 operates a pressure swing adsorption (PSA) process using molecular sieves, which provides a purified gas 124 essentially containing hydrogen and a CO2-containing tail gas stream 125.

(19) Said CO.sub.2-depleted gas 124 is mixed with the portion 18b from the NO.sub.x removal unit 5 of the nitric acid section 1 to provide the make-up gas 126 with the required H.sub.2:N.sub.2 molar ratio of around 3 for the ammonia synthesis reaction.

(20) The so obtained make-up gas 126 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 127.

(21) The integration between the section 1 and the section 100 is realized as follows.

(22) According to the example of the figure, the absorption tower 3 of the nitric acid 1 is operated at a lower pressure than the front-end section 101 of the ammonia section 100. For example, the absorption tower 3 is operated at a pressure of 5 bar and the front-end section 101 at a higher pressure of 15-20 bar.

(23) As a consequence, said second portion 18b of the NO.sub.x-depleted gas needs to be compressed to the front-end pressure. To this purpose, said portion 18b is sent to a nitrogen booster 300 before being introduced to the section 100. According to this example, said nitrogen booster 300 has a compression ratio higher than 3, compressing to 15-20 bar an inlet stream 18b at 5 bar.

(24) The effluent of the nitrogen booster 300 is subsequently mixed with the hydrogen-containing gas 124 leaving the purification section 107, thus providing the make-up synthesis gas 126.

(25) FIG. 2 shows a variant of the plant illustrated in FIG. 1. According to the example of this figure, the NOx removal unit 5 carries out a selective catalytic reduction process (SCR), thus providing a NOx-depleted product gas 18 essentially comprising nitrogen and also containing oxygen (<5% mol), which is detrimental for the ammonia synthesis catalyst and needs be removed. To this purpose, the second portion 18b of the NOx-depleted product gas is subjected to an oxygen removal treatment before being fed to the ammonia section 1.

(26) According to the example of FIG. 2, said treatment is carried out in a pressure swing adsorption (PSA) unit 301 after cooling of said portion 18b in a heat exchanger 302. Said unit 301 provides an oxygen stream 20 and an oxygen-depleted stream 21, which is used as process nitrogen for the ammonia synthesis.

(27) The nitrogen booster 300 shown in the figure is located downstream of the PSA unit 301 to elevate the pressure of said oxygen-depleted stream 21 to the front-end pressure. Alternatively, the nitrogen booster can be located upstream said PSA unit 301.

(28) FIG. 3 shows a further variant of the plant illustrated in FIG. 1.

(29) The tail gas 17 provided by the absorption tower 3 splits into a first portion 17a and a second portion 17b. Said first portion 17a is fed to a NO.sub.x removal unit 5 and said second portion 17b is fed to the NO.sub.x removal unit 50.

(30) The nitric acid section is indicated, for this embodiment, with the reference number 1a.

(31) According to the example of the figure, the NOx removal unit 5 carries out a selective catalytic reduction process (SCR), providing a NOx-depleted product gas 18c mainly comprising nitrogen and also containing some oxygen.

(32) On the other hand, the NOx removal unit 50 is based on a pressure swing adsorption (PSA) process, which removes both NO.sub.x and O.sub.2 into stream 22, thus providing a NOx-depleted product gas 18d essentially comprising nitrogen and substantially free of oxygen.

(33) Said NOx-depleted product gas 18c is work-expanded in the expander 6, while said gas 18d is exported from the nitric acid section 1a and fed to the ammonia section 100 as process nitrogen for the synthesis of ammonia.

(34) According to this example, the absorption tower 3 of the nitric acid 1a is operated substantially at the same pressure of the front-end section 101, e.g. at about 15 bar. Hence said NOx-depleted product gas 18d is mixed with the purified gas 124 to provide the make-up gas 126 without being previously compressed in a nitrogen booster as in FIGS. 1, 2.

(35) FIG. 4 shows an integrated plant according to another embodiment of the invention. Said plant comprises the section 1 for the synthesis of nitric acid and a section 200 for the synthesis of ammonia.

(36) The section 200 for the synthesis of ammonia includes a front-end section 201, a synthesis loop 202 and a syngas compressor 203. Said front-end section 201 essentially comprises a desulphurizer 204, a primary reformer 205, a secondary reformer 206, an air compressor 207, a carbon monoxide shift conversion section 208, a carbon dioxide removal section 209 and a methanator 210.

(37) The operation of said section 200 is as follows.

(38) A natural gas feedstock NG enters said desulphurizer 204, resulting in a stream 220 of desuplhurized natural gas. Said outlet stream 220 is mixed with a steam current PS generating a stream 221 of process gas, which enters the primary reformer 205, wherein it is converted in a mixture of carbon monoxide (CO), carbon dioxide (CO.sub.2) and hydrogen by passage over a suitable catalyst. The reformed gas 222 delivered by the primary reformer 205 is then introduced into the secondary reformer 206, wherein reforming is achieved with the internal combustion of part of the reaction gas with an oxidant.

(39) Said oxidant is provided by a stream 230, which is obtained by mixing the portion 18b of NOx-depleted tail gas with an air flow 223. Hence, said stream 230 also represents the nitrogen source to obtain the make-up gas.

(40) The air compressor 207 is used for compressing the air 224 to a suitable pressure before its admission into the secondary reformer 206.

(41) The reformed gas 225 leaving the secondary reformer is then purified in the carbon monoxide shift conversion section 208, carbon dioxide removal section 209 and methanator 212 to provide a make-up gas 226 with the required H.sub.2:N.sub.2 molar ratio of around 3 for the ammonia synthesis reaction. Said synthesis gas 226 is fed to the syngas compressor 203 and subsequently to the synthesis loop 207, wherein it is converted into ammonia 227.

(42) According to the example of the figure, the absorption tower 3 and the front-end section 201 are operated at substantially the same pressure and no nitrogen booster is required on the flowline of the second portion 18b of the NO.sub.x-depleted product gas.

(43) On the other hand, when the absorption tower 3 is operated at a lower pressure than the front-end section, the portion 18b of the NO.sub.x-depleted product gas is sent to a nitrogen compressor before being mixed with the air flow 223 and fed to the secondary reformer 206, or alternatively it is injected at an appropriate stage of the air compressor 207.

Example

(44) With reference to FIG. 2, the advantage of the invention will be better elucidated by way of the example below.

(45) The process for the synthesis of nitric acid is of the mono-pressure type, i.e. the reactor 2 and the absorption tower 3 are operated at substantially the same pressure of 6 bar, and the NO.sub.x removal unit is based on a SCR process.

(46) The nitric production rate is 1,100 MTD (as 100% acid) and the ammonia production rate is of 630 MTD. The so obtained nitric acid will be neutralized with ammonia thereby producing ammonium nitrate, and ammonia is essentially produced in the amount needed to make the nitric acid and the ammonium nitrate.

(47) According to this example, the total flow rate of the tail gas 18 from the SCR is 6,620 kmol/h. Said tail gas 18 contains about 97% of N.sub.2, about 3% of O.sub.2 and very small residual amounts of NO.sub.x and NH.sub.3 (ppm level).

(48) The nitrogen required for the ammonia production is 770 kmol/h. The PSA unit 301 has a nitrogen recovery of 85%. Hence, about 940 kmol/h of the tail gas 18, namely only about 14% of the whole tail gas flow rate, is routed to the PSA unit 301 as stream 18b at the nitric acid absorption pressure.

(49) Since the feed stream of the PSA unit 301 is a nearly pure nitrogen stream, the amount of oxygen to be adsorbed is relatively small, which simplifies the PSA, requires a relatively small amount of adsorbent, and enables high recoveries of nitrogen. A high nitrogen recovery is desirable to minimize the flow rate of the tail gas slipstream, hence minimizing the loss of power recovery from the tail gas expander 6 compared to the prior art process.

(50) The adsorbent materials of the PSA unit 301 are for example activated carbon (the so-called “carbon molecular sieves”, CMS, also used for separating nitrogen from air), or zeolites.

(51) The advantages in terms of performances of the process according to the invention over the prior art will become apparent from the comparison of the power balance for the most relevant machines, as shown in table 1.

(52) In the process of the prior art, power is produced mainly by the tail gas expander and steam turbines in the nitric acid plant, while is consumed by the air separation unit and nitrogen compressor in the ammonia plant and the process air compressor in the nitric acid plant.

(53) In the process according to FIG. 2, power is similarly produced by the tail gas expander 6 and steam turbines in the nitric acid section, while is consumed by the nitrogen booster 300 in the ammonia section and the process air compressor 7 in the nitric acid plant.

(54) The other compressors in the ammonia process (i.e. syngas and ammonia refrigerant) have the same power consumption in the prior art and in the new process, hence they do not alter the result of the power balance comparison and are therefore herein neglected for sake of simplicity.

(55) TABLE-US-00001 TABLE 1 Power balance, comparison Prior art New process (FIG. 2) [kW] [kW] Air separation unit −2 910    0 Nitrogen compressor −2 340    0 Nitrogen Booster    0 −1 100 Subtotal, ammonia plant −5 250 −1 100 Process air compressor −17 910  −17 910  Tail Gas Expander 10 400  8 950 Steam turbine 12 320 12 320 Subtotal, nitric acid plant  4 810  3 360 Total, ammonia and nitric acid plant  −440  2 260

(56) As clear from the table above, the loss of power on the tail gas expander of the new process (of about 14%, or 1,450 kW) is surprisingly well compensated by the power saved in the air separation unit and in the nitrogen compressor.

(57) As a result, while the power balance is negative for the prior art process, with an overall consumption of 440 kW, it is positive for the new process, with a power surplus of 2,260 kW.

(58) Hence, the new process is not only less expensive, but also consumes less energy (or leaves more surplus for export).