Process for producing ammonia synthesis gas and a method for revamping a front-end of an ammonia plant

Abstract

A process for producing ammonia make-up synthesis gas and a procedure for revamping a front-end of an ammonia plant for producing ammonia make-up synthesis gas are disclosed, wherein the make-up synthesis gas is produced by means of steam reforming of a hydrocarbon gaseous feedstock; said front-end includes a primary reformer, a secondary reformer, a shift conversion section, a CO2 removal section and optionally a methanation section; a shell-and-tube gas-heated reformer is installed after said secondary reformer, and a portion of the available feedstock is reformed in the tubes of said gas-heated reformer, and heat is provided to the shell side of said gas-heated reformer by at least a portion of product gas leaving the secondary reformer, possibly mixed with product gas leaving the tubes of said gas-heated reformer.

Claims

1. A procedure for revamping a front-end of an ammonia plant, said front-end being arranged to produce ammonia synthesis gas containing hydrogen and nitrogen by steam reforming of a hydrocarbon gaseous feedstock, said front-end including a primary reformer, a secondary reformer, a shift conversion section, and a CO.sub.2 removal section, said procedure including at least: installation of a gas-heated reformer after said secondary reformer, said gas-heated reformer being a shell-and-tube heat exchanger having a tube side and a shell side, and providing a catalytic reforming of a first gas current passing in the tube side and indirect heating of said first gas current by a second current traversing the shell side, said first current including a portion of the available hydrocarbon feedstock, the remaining portion of said feedstock being directed to said primary reformer, and said second current comprising at least a portion of a product gas effluent from said secondary reformer, said secondary reformer being an air-fired secondary reformer, and the procedure including the step of modifying said secondary reformer to operate with O.sub.2-enriched air, and wherein said front-end includes a methanation section and said procedure provides for the addition of nitrogen to a product gas flowing in said methanation section or to a product gas effluent from said methanation section, or wherein the procedure provides for the installation of a final purification section after said CO.sub.2 removal section, for the removal of inert gases from CO.sub.2-depleted product gas effluent from said CO.sub.2 removal section, and nitrogen is added to a product gas flowing in said final purification section or to a product gas effluent from said final purification section.

2. The procedure of claim 1, wherein said first current is a portion of a mixed flow comprising steam and the available hydrocarbon feedstock, which is redirected to said gas-heated reactor while the remaining portion is directed to said primary reformer.

3. The procedure of claim 2, said mixed flow having a steam-to-carbon ratio of between 2 and 3.5.

4. The procedure of claim 1, said second current comprising product gas effluent from said secondary reformer or autothermal reformer, and also comprising product gas leaving said tube side of said gas-heated reformer.

5. The procedure of claim 1, said 02-enriched air being obtained by adding an oxygen flow to ambient air.

6. The procedure of claim 5, said oxygen flow being in an amount to provide a molar concentration of oxygen in the enriched air between 25% and 70%.

7. The procedure of claim 5, said oxygen flow being delivered by an air-separation unit.

8. The procedure of claim 7, further comprising the provision and the installation of said air-separation unit.

9. The procedure of claim 1, further including the revamping of said shift conversion section and/or the revamping of said CO.sub.2 removal section.

10. The procedure of claim 9, including the revamping of said shift conversion section by means of one or more of the following: the conversion of one or more existing axial-flow shift converters into axial-radial shift converters; adding one or more shift converters in parallel to the existing ones; replacing one or more existing adiabatic high-temperature shift converters with one or more isothermal medium-temperature shift converters.

11. The procedure of claim 10, including the provision of one or more isothermal medium-temperature shift converters or the modification of one or more existing shift converters to operate as medium shift converters, wherein said medium-temperature shift converters include a copper-based catalyst, and comprise a heat exchanger immersed in the catalyst, to remove the heat produced by the exothermic shift conversion.

12. The procedure of claim 11, said medium temperature being in the range of 200-300 C.

13. The procedure of claim 1, said purification section including a methanation section.

14. The procedure of claim 1, said purification section including a nitrogen wash section or a cryogenic condensation section for condensation of nitrogen and inerts, or a PSA unit.

15. The procedure of claim 14, said purification section including a nitrogen wash section or a cryogenic condensation section, said procedure including the provision of a nitrogen line for addition of nitrogen before or into said nitrogen wash section or said cryogenic condensation section, and said nitrogen being in an amount suitable to obtain a purified synthesis gas containing hydrogen and nitrogen in a molar ratio around 3 to 1.

16. A process for producing ammonia synthesis gas containing hydrogen and nitrogen by steam reforming of a hydrocarbon gaseous feedstock, including: mixing said hydrocarbon gaseous feedstock with steam, reforming a first portion of the so obtained mixed flow of gaseous feedstock and steam in a primary reformer and then in a secondary reformer or in an autothermal reformer, obtaining a first product gas, reforming a second portion of said mixed flow in a gas-heated reactor, obtaining a second product gas, said gas-heated reactor being heated by a current of product gas comprising at least a portion said first product gas, wherein: said secondary reformer operates with O.sub.2-enriched air, and nitrogen is added to a product gas flowing in a final purification step or to a product gas effluent from said final purification step.

17. The process of claim 16, said secondary reformer or autothermal reformer operating with O.sub.2-enriched air having a concentration of oxygen between 25% and 70% molar.

18. The process of claim 16, further comprising the treatment of product gas comprising: shift conversion, removal of carbon dioxide, and purification of CO.sub.2-depleted product gas after said removal of carbon dioxide, and said purification including at least one of the following: a methanation process; nitrogen wash; cryogenic condensation; pressure-swing adsorption (PSA).

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 is a scheme of a front-end of an ammonia plant according to the invention.

(2) FIGS. 2 to 5 illustrate some embodiments of the invention concerning purification of the raw synthesis gas.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

(3) FIG. 1 illustrates a front-end of an ammonia plant including a tube primary reformer 1, a secondary reformer 2, a shift conversion section 3, a CO2 removal section 4, and a gas-heated reactor (GHR) 5 after the secondary reformer 2, and before the shift conversion section 3.

(4) Said gas-heated reactor 5 is basically a shell-and-tube equipment for indirect heat exchange between a first current in the tube side and a second current in the shell side. Said first current contains a gaseous hydrocarbon and steam. Said tubes of reactor 5 are filled or coated with a suitable catalyst for steam reforming.

(5) A gaseous hydrocarbon feedstock, for example desulphurized natural gas 10, is added with steam 11 forming a mixed flow 12. A first part 13 of said mixed flow 12 is directed to the tubes of the primary reformer 1, and the remaining part 14 of said mixed flow 12 is fed to the tube side of the gas-heated reactor 5. Further steam can be added to stream 14, according to some embodiments.

(6) The first part 13 of mixed steam and methane flow 12 is reformed in the primary reformer 1, obtaining a partial conversion of methane contained therein, and the effluent 15 is further converted in the secondary reformer 2 with oxygen-enriched air 18. Said oxygen-enriched air 18 is obtained by adding a suitable amount of oxygen 17 to ambient air 16. The oxygen 17 may be provided for example by an air-separation unit 25. Preferably said oxygen-enriched air 18 contains 25% to 50% of oxygen. Said air-separation unit 25 may also deliver a current of nitrogen of a high purity for a further use in the process, as illustrated for example in the FIGS. 2 to 5.

(7) Referring again to FIG. 1, the second part 14 of said flow 12 is reformed in the tubes of said gas-heated reactor 5. Here, the heat input to the reforming process is provided by the current 21 of hot product gas which traverses the shell side of the gas-heated reactor 5. Said current 21 comprises the product gas 19 from the secondary reformer 2 and also the product gas 20 leaving the tube-side of gas-heated reactor 5, which is joined with said product gas 19 as illustrated.

(8) Hence it can be said that the gas-heated reactor 5 operates in parallel to the train of primary reformer 1 and secondary reformer 2. Part of the available mixed flow 12 is converted through the reformers 1 and 2 to the first product gas 19, while another part is converted through the gas-heated reactor 5 to the second product gas 20.

(9) Preferably the first part 13 is more than 50% of the flow 12. In a preferred embodiment, the first part 13 is around 70% and the second part 14 is around 30% of the total amount of mixed flow 12. This ratio however may vary.

(10) After a passage in the shell side of the gas-heated reactor 5, said hot current 21, now cooled to 22, is fed to the shift conversion section 3.

(11) The effluent 23 of said shift conversion section 3 is treated in the CO2-removal section 4.

(12) The CO2-depleted stream 24 is preferably purified for example by removing residual methane and other inert gaseous components (e.g. Argon) before it is fed to an ammonia synthesis loop.

(13) FIGS. 2 to 5 illustrate some of the possible embodiments for the purification of said stream 24.

(14) According to FIG. 2, the CO2-depleted gas 24 is purified in a PSA section 26 and in a methanation section 27. Then the purified gas 28 is compressed in a compression section 29 and sent to a synthesis loop 30. The PSA may also be installed downstream the methanation section in a variant embodiment.

(15) A suitable amount of nitrogen is added via line 31 to the stream effluent from the PSA section 26. Said nitrogen 31 may come from the same ASU 25 which generates the oxygen 17 (FIG. 1).

(16) FIG. 3 illustrates a variant where said CO2-depleted gas 24 is purified in a nitrogen wash section 33, optionally after a methanation section 27. The necessary nitrogen 34 may be provided by the ASU 25 as above.

(17) FIG. 4 illustrates an embodiment where purification of said CO2-depleted gas 24 includes a cryogenic condensation in a suitable unit 35, after a first step of purification in a methanation section 27. A suitable amount of nitrogen 36 is added to the gas before it enters said cryogenic condensation unit 35.

(18) FIG. 5 illustrates a variant of FIG. 4 which includes a first compression section 29A before the cryogenic condensation unit 35, and a second compression section 29B after said unit 35. The first compression section 29A provides an initial compression and the second compression section 29B provides final compression after the purification in the cryogenic unit 35. The nitrogen 36 is preferably added to the gas stream after the initial compression and before it enters the cryogenic condensation unit 35.

(19) In the above embodiments, the amount of nitrogen via lines 31 or 34 or 36 is regulated in such a way that the purified product gas 30 contains the desired concentration of nitrogen for ammonia synthesis.

(20) Thanks to the reforming in parallel through the reformer 1 and gas-heated reactor 5, a front-end as illustrated in FIG. 1 is able to convert a greater amount of natural gas 10, i.e. it has a greater capacity, compared to a conventional front-end.

(21) According to some embodiments, the gas-heated reactor 5 is added during a revamping procedure of the front-end originally comprising the reformers 1, 2 and sections 3, 4. The other equipment, in particular the shift conversion section 3 and CO2 removal section 4, can also be revamped.