Process for producing ammonia synthesis gas with high temperature shift and low steam-to-carbon ratio

Abstract

A process for producing ammonia synthesis gas from a hydrocarbon-containing feedstock in a front-end, comprising the steps of steam reforming of said feedstock, obtaining a synthesis gas comprising hydrogen, carbon monoxide and carbon dioxide; a treatment of said synthesis gas including shift of carbon monoxide and subsequent removal of carbon dioxide, wherein the shift of the synthesis gas includes high-temperature shift with an iron-based catalyst and at a temperature greater than 300? C. and the global steam-to-carbon ratio of the front end is 2.6 or less; a corresponding plant and a method for revamping a front-end of an ammonia plant are also disclosed.

Claims

1. A process for producing ammonia synthesis gas from a hydrocarbon-containing feedstock in a front-end, the process comprising the steps of: steam reforming of said feedstock, obtaining a synthesis gas comprising hydrogen, carbon monoxide and carbon dioxide; a treatment of said synthesis gas including shift of carbon monoxide and subsequent removal of carbon dioxide, wherein: the shift of the synthesis gas includes a step of high-temperature shift at a temperature greater than 300? C. with an iron-based catalyst; the global steam-to-carbon molar ratio of the front end is 2.6 or less; and wherein said steam reforming comprises: a first reforming step including a primary steam reforming and optionally including a pre-reforming before said primary steam reforming, thus obtaining a first reformed gas; a second reforming step with a stream of an oxidant, thus obtaining a synthesis gas comprising hydrogen, carbon monoxide and carbon dioxide; said steps of first and second reforming being performed in series, said second reforming step being carried out by using oxygen or enriched air comprising at least 50% oxygen, as the oxidant stream.

2. The process according to claim 1, said global steam-to-carbon molar ratio being in the range 1.5 to 2.6.

3. The process according to claim 1, said first reforming step including a pre-reforming step, the feed of said pre-reforming step being the hydrocarbon feedstock mixed with steam, said feed having a steam-to-carbon molar ratio equal to said global molar ratio, and no further steam being added during the process.

4. The process according to claim 1, wherein the synthesis gas obtained after said second reforming step being then mixed with steam, prior to its feeding to the high-temperature shift, in order to raise the global steam-to-carbon molar ratio of the front end.

5. The process according to claim 4, wherein said first reforming step includes a pre-reforming step, and where the pre-reforming and the primary steam reforming are carried out with a low steam-to-carbon molar ratio which is lower than said global molar ratio.

6. The process according to claim 5, wherein said low molar ratio is less than 2.

7. The process according to claim 4, wherein the primary steam reforming is carried out with a steam-to-carbon molar ratio which is greater than said global molar ratio, and an amount of fresh hydrocarbon feedstock is mixed with the effluent of the primary steam reforming, prior to said second reforming step.

8. The process according to claim 7, the steam-to-carbon molar ratio of the primary steam reforming being in the range 2.7 to 3.

9. The process according to claim 4, wherein said first reforming step includes a pre-reforming and a primary steam reforming, the pre-reforming is carried out with a first steam-to-carbon molar ratio and the primary steam reforming is carried out with a second steam-to-carbon molar ratio which is equal to or greater than said first molar ratio, while both said first and second molar ratio are lower than the global molar ratio.

10. The process according to claim 9, said first molar ratio being in the range 0.5 to 2, and said second molar ratio being around 1.5-2.

11. The process according to claim 9, wherein the effluent of the primary steam reforming step is mixed with an amount of fresh hydrocarbon.

12. The process according to claim 9, wherein an amount of the pre-reformed gas leaving said pre-reforming step is mixed with the effluent of the primary steam reforming, thus bypassing said primary reforming.

13. The process according to claim 1, wherein said global steam-to-carbon molar ratio is in the range 2.2 to 2.4.

14. The process according to claim 1, including also a low-temperature shift which is carried out after the high-temperature shift and at around 200? C., a carbon dioxide removal section and optionally a methanation section.

15. The process according to claim 1, said hydrocarbon feedstock being desulphurized natural gas.

16. The process according to claim 1, said temperature of the high-temperature shift being in the range 320 to 500? C.

17. The process according to claim 5, wherein said low molar ratio is around 1.5.

18. The process according to claim 6, wherein said low molar ratio is around 1.5.

19. A method of revamping a front-end of an ammonia plant, said front-end producing ammonia synthesis gas by steam reforming of a hydrocarbon-containing feedstock, wherein said front-end comprises a primary reforming stage and an air-fired secondary reforming stage, and also comprises a high-temperature shift converter with an iron based catalyst, and wherein the original front-end operates with a global steam-to-carbon molar ratio of 2.6 or greater, wherein: the amount of the hydrocarbon feedstock and the amount of steam fed to the front-end are regulated in such a way that the global steam-to-carbon molar ratio of the revamped front-end is 2.6 or less; and said secondary reforming stage is modified to operate with oxygen or enriched air with at least 50% oxygen as oxidant stream, instead of air.

20. The method according to claim 19, including the installation of a steam line (PS) for the addition a predetermined amount of steam to the synthesis gas leaving the secondary reforming stage prior to admission into the high-temperature shift converter.

21. The method according to claim 19, including the provision of a hydrocarbon feedstock bypass line, arranged in such a way that a portion of the feedstock bypasses said first reforming stage and is sent directly to the second reforming stage.

22. The method according to claim 19, wherein the primary reforming stage includes a pre-reformer, and the method including the provision of a bypass line of the primary reformer for a portion of the effluent of said pre-reformer, so that said portion is sent directly to the second reforming stage.

23. The method according to claim 19, comprising the installation of an autothermal reformer (ATR) in parallel with the existing primary and secondary reformers, said autothermal reformer being fed with a portion of the natural gas feed and steam, the method also comprising the routing of the effluent from said newly installed autothermal reformer to the high-temperature shift converter, and mixing of the effluent from said autothermal reformer with the effluent from the existing primary and secondary reformers.

Description

(1) The invention will be elucidated with reference to some preferred and non-limiting embodiments illustrated in FIGS. 1 to 8, wherein:

(2) FIGS. 1 to 5 are schemes of embodiments of the invention with primary and secondary reforming;

(3) FIGS. 6 to 8 are schemes of embodiments of the invention with autothermal reforming.

DESCRIPTION OF PREFERRED EMBODIMENTS

(4) Referring to FIGS. 1 to 8:

(5) PREREF denotes a pre-reformer,

(6) REF1 denotes a primary steam reformer, which is usually a tube reformer,

(7) REF2 denotes a secondary reformer,

(8) ATR denotes an autothermal reformer (ATR) if no upstream primary reformer (REF1) is installed

(9) HTS denotes a high-temperature shift converter,

(10) PS denotes a flow of steam,

(11) NG denotes a feedstock of natural gas,

(12) O2 denotes a current of oxygen or oxygen-rich air which is the oxidant stream fed to the secondary reformer REF2.

(13) FIG. 1 shows a first embodiment of the invention, where the front-end includes a pre-reformer PREREF upstream the primary reformer REF1. A natural gas feedstock NG is mixed with a first steam current PS and enters the pre-reformer PREREF. The pre-reformed gas leaving the pre-reformer is fed to the primary reformer REF1 and the gas leaving said primary reformer is fed to the oxygen-fired autothermal secondary reformer REF2. The reformed gas leaving said secondary reformer REF2 is mixed with a second amount of steam PS, and then enters the high-temperature shift converter HTS which operates at around 320-500? C. with an iron based catalyst, to convert CO into CO2. Then, the gas leaving said converter HTS is further treated according to known techniques, typically removal of carbon dioxide and (optionally) methanation.

(14) The removal of carbon dioxide may be carried out with any of the following techniques:

(15) In FIG. 1, the pre-reformer and the primary reformer operate at a low steam-to-carbon ratio, for example around 1.5, while the addition of the second amount of steam PS before the shift converter HTS raises the global steam-to-carbon ratio to 2.2-2.4.

(16) FIG. 2 shows a second embodiment where the feed of the pre-reformer PREREF, that is the natural gas NG mixed with steam PS, has a steam-to-carbon ratio equal to said global ratio, and no further steam is added during the process. In particular, no further steam is added before the shift converter HTS. In this embodiment, the steam-to-carbon ratio is preferably around 2.4.

(17) FIG. 3 shows a third embodiment, with no pre-reformer. A portion of the available feedstock bypasses the first steam reformer REF1. Accordingly, a first portion of the available feedstock NG is mixed with steam PS and enters the primary reformer REF1; a second remaining portion of said feedstock, on the other hand, is mixed the effluent of said primary reformer REF1. The resulting mixture is added with an oxygen stream O2 before it enters the secondary reformer REF2. A second amount of steam PS, as in FIG. 1, is mixed with the effluent of said secondary reformer REF2, before admission into the shift converter HTS.

(18) In this case, the steam reformer REF1 is run at a high steam-to-carbon ratio which is, for example, around 2.7-3, due to the portion of feedstock bypassing the reformer. Preferably, said second portion of the feedstock NG, which bypasses the primary reformer, is around 30% of the available feedstock.

(19) FIG. 4 shows a fourth embodiment which is similar to FIG. 3, but includes a pre-reformer PREREF. The bypass portion of feedstock, as shown, bypasses both the pre-reformer and the primary reformer REF1.

(20) FIG. 5 shows a fifth embodiment which is a variant of FIG. 4. The full amount of natural gas NG, mixed with steam PS, is fed to the pre-reformer PREREF. However, a portion of the effluent of said pre-reformer bypasses the subsequent primary reformer REF1, being mixed with the gas leaving said primary reformer. A second amount of steam PS, also in this case, is mixed with the effluent of the autothermal reformer REF2.

(21) FIG. 6 shows a sixth embodiment. The full amount of natural gas NG, mixed with steam PS, is fed to the pre-reformer PREREF. There is no primary reformer. The product of pre-reforming is routed to the autothermal reformer ATR fired with oxygen. A second amount of steam PS is mixed with the effluent of the autothermal reformer.

(22) FIG. 7 shows a seventh embodiment, identical to 6 except for the autothermal reformer ATR being fired with air.

(23) FIG. 8 shows an eighth embodiment which is a preferred embodiment for revamping an existing line comprising a primary reformer REF1 and a secondary reformer REF2. Said line is revamped by adding a new line with an autothermal reformer ATR. The natural gas feed NG is split between the line comprising a primary reformer REF1 and a secondary reformer REF2, and the newly added line comprising a prereformer PREREF and an autothermal reformer ATR. Steam PS is added to the feed NG both at the inlet of primary reformer REF1 and of pre-reformer PREREF. The product of the secondary reformer REF2 and the product of autothermal ATR are joined upstream of the shift converter HTS, and mixed with steam PS

(24) In all the above embodiments, the primary reformer REF1 operates preferably at a pressure around 30 bar, inlet temperature around 500? C. and outlet temperature around 750-800? C. The outlet temperature of the secondary reformer REF2 is around 1000? C. The outlet temperature of the autothermal reformer ATR is around 1000? C.

(25) In all the above embodiments, a low-temperature shift converter may be installed downstream the converter HTS. After the shift, a carbon dioxide removal section is normally provided. It should be noted that the synthesis gas does not contain nitrogen and hence the invention allows using a PSA (pressure swing absorption) or LNW (liquefied nitrogen wash).

EXAMPLES

(26) The following table 1 compares a prior art front end with a primary steam reformer and a secondary reformer, and global steam-to-carbon ratio of 2.6, with five examples which relate, respectively, to embodiments of FIG. 1, FIG. 2, FIG. 3, FIG. 6 and FIG. 7. The examples relate to a production of 3275 kmol/h of hydrogen. In the table, SMR denotes the steam methane reforming; RP denotes the reducing potential RP and S/DG denotes the steam/dry gas ratio (see above definitions).

(27) TABLE-US-00001 TABLE 1 Prior Ex. 1 Ex. 2 Ex. 3 Ex. 4 Ex. 5 art (FIG. 1) (FIG. 2) (FIG. 3) (FIG. 6) (FIG. 7) S/C ratio, 2.6 2.2 2.37 2.15 2.1 1.9 overall S/C inlet 2.6 1.5 2.37 2.7 N/A N/A SMR S/C inlet 1.5 2.37 1.0 0.5 Prereformer S/DG, inlet 0.40 0.51 0.48 0.48 0.68 0.43 HTS RP, inlet 1.47 1.64 1.64 1.65 1.2 1.2 HTS Natural Gas 1008 1054 998 1048 1208 1436 [kmol/h] Steam 2722 2318 2366 2254 2537 2729 [kmol/h] Oxygen (process 377 337 420 733 626 .sup.(1) [kmol/h] air) Hydrogen 3275 3275 3275 3275 3275 3275 [kmol/h] SMR 44.6 32.8 39.3 33.1 0 0 cat.tubes duty [MW] 100% 74% 88% 74% 0% 0% Syngas flow 106 71 69 71 74 168 .sup.(2) rate [t/h] 100% 67% 66% 67% 70% 94% .sup.(3) .sup.(1) (in the air stream) .sup.(2) (99 excluding N2) .sup.(3) (based on flow excluding N2)

(28) It can be noted that the duty of the steam methane reformer (primary reformer) is lower by 26% (ex. 1), 12% (ex. 2) and 26% (ex. 3), despite the same production of hydrogen. Furthermore, the syngas flow rate is considerably lower, being around 66-67% of the prior-art, also due to firing of the secondary reformer with oxygen instead of air. The size of a new plant can be reduced accordingly or, in a revamping, a larger capacity for a given size can be obtained.

(29) The table indicates also the values of the reducing potential RP and steam/dry gas S/DG, as above defined, which are such to allow the use of the iron based catalyst, despite the low steam-to-carbon ratio between 2 and 2.37 in the examples. It can be observed that the values of S/DG are highest for the cases with ATR only, and that the values of RP are lowest for the cases with ATR only, this suggests that even lower global S/C values can be used for the cases with ATR only.

(30) The following table 2 shows the reduced duty of the primary reformer, compared to the prior art.

(31) Values for examples 4 and 5 are not indicated (the steam reformer duty is zero).

(32) TABLE-US-00002 TABLE 2 Ex. 1 Ex. 2 Ex. 3 Prior art (FIG. 1) (FIG. 2) (FIG. 3) NH3 production based on 37 37 37 37 3275 kmol/h H2 flow [t/h] SMR duty [MW] 45 33 39 33 Specific SMR duty 1.2 0.9 1.1 0.9 [MWh/t NH3] Fuel burnt in ATR (assumed 145 189 169 210 100% CH4) [kmol/h] ATR duty [MW] 32 42 38 47 Specific ATR duty 0.9 1.1 1.0 1.3 [MWh/t NH3] Specific SMR + ATR duty 2.1 2.0 2.1 2.2 [MWh/t NH3] SMR/ATR duty 138% 78% 104% 71%