Process for producing ammonia synthesis gas

Abstract

A process and a related equipment for producing ammonia synthesis gas from a hydrocarbon-containing feedstock (20), the process comprising the steps of: primary reforming with steam (21), secondary reforming with an oxidant stream (23), and purification of the effluent of said secondary reforming, said purification comprising a step of shift conversion (13) of carbon monoxide, wherein the synthesis gas (25) produced by said secondary reforming is subject to a medium-temperature shift over a copper-based catalyst, and the global steam to carbon ratio of the process is not greater than 2.

Claims

1. A process for producing ammonia synthesis gas from a hydrocarbon-containing feedstock, the process comprising the steps of: primary reforming with steam, secondary reforming with an oxidant stream, and purification of the effluent of said secondary reforming, said purification comprising a step of shift conversion of carbon monoxide, wherein: the synthesis gas produced by said secondary reforming is subject to a medium-temperature shift over a copper-based catalyst, and a fraction of the feedstock bypasses the primary reforming, the steam to carbon ratio in the primary reforming being 2.5 to 3, and/or steam is added in multiple stages of the process, wherein the global steam to carbon ratio of the process that is the total amount of the moles of steam over the moles of carbon introduced with the feedstock is not greater than 2.

2. The process according to claim 1, wherein the synthesis gas produced by said secondary reforming is added with steam, and the so obtained flow of synthesis gas added with steam is subject to said medium-temperature shift.

3. The process according to claim 1, said step of medium-temperature shift being carried out in an isothermal condition.

4. The process according to claim 3, said step of medium-temperature shift being carried out at a temperature in the range of 200 to 300? C.

5. The process according to claim 1, said oxidant stream being any of air, O.sub.2-enriched air or pure oxygen.

6. The process according to claim 1, said oxidant stream being oxygen at a purity of 95% or greater.

7. The process according to claim 1, said primary reforming comprising a pre-reforming stage.

8. The process according to claim 7, and said primary reforming with pre-reforming stage being operated at a steam to carbon ratio between 1 and 1.5.

9. The process according to claim 7, wherein the feedstock is divided into a first fraction and a second fraction, said first fraction being directed to the pre-reforming and primary reforming, and said second fraction by-passes the primary reforming step and is rejoined with the effluent of the primary reforming, before admission to the secondary reforming.

10. The process according to claim 7, wherein the effluent of the pre-reforming stage is divided into two fractions, a first fraction is directed to the primary reforming step and a second fraction bypasses said primary reforming step and is rejoined with the effluent from said primary reforming step.

11. The process according to claim 10, wherein an amount of steam is added to said first fraction.

12. The process according to claim 10, said pre-reforming stage being operated at a steam to carbon ratio of 0.5 to 1.5.

13. The process according to claim 1, said primary reforming being carried out with no pre-reforming stage.

14. The process according to claim 13, wherein a portion of the feedstock bypasses said primary reforming, said portion being preferably 40% of more of the feedstock.

15. The process according to claim 14, wherein said primary reforming is carried out at a steam to carbon ratio of 2.7 to 3.

16. The process according to claim 1, further comprising the step of purification of the synthesis gas after the medium-temperature shift, including one or more of the following: a low-temperature shift, carbon dioxide removal, cryogenic separation, or a treatment step of adsorption, preferably pressure swing adsorption.

Description

BRIEF DESCRIPTION OF THE FIGURES

(1) FIG. 1 is a scheme of a first embodiment of the invention.

(2) FIG. 2 is a scheme of a second embodiment of the invention.

(3) FIG. 3 is a scheme of a third embodiment of the invention.

(4) FIG. 4 is a scheme of a fourth embodiment of the invention.

(5) FIG. 5 is a scheme of a fifth embodiment.

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT

(6) FIG. 1 discloses a scheme of a front-end for generation of ammonia synthesis gas, comprising: a primary reforming section which includes a pre-reformer 10 and a primary reformer 11; a secondary reformer 12; a medium-temperature shift (MTS) converter 13. The pre-reformer, primary reformer and secondary reformer are also denoted by symbols PRE, REF_1 and REF_2.

(7) The synthesis gas 14 leaving said MTS converter 13 is normally treated in a carbon dioxide removal section. In some embodiments, said gas 14 can be further treated in an optional low-temperature shift (LTS) section, before the step of carbon dioxide removal, to maximize the conversion of the carbon monoxide into CO.sub.2. After carbon dioxide removal, the synthesis gas can be further purified by methanation or a cryogenic process. These steps are not described since they can be carried out with known techniques.

(8) A feed 20 of a gaseous hydrocarbon feedstock such as desulphurized natural gas is mixed with a first amount of steam 21 and admitted to the pre-reformer 10. The effluent of said pre-reformer 10 passes to the primary reformer 11 and then the effluent 22 of said primary reformer 11 is added with oxygen-containing stream 23 to form the input stream 24 of the secondary reformer 12.

(9) The effluent 25 of said secondary reformer 12 is added with a second amount of steam 26 to form the input stream 27 of the MTS converter 13.

(10) The oxygen-containing stream 23 may be air, enriched air or substantially pure oxygen, according to various embodiments of the invention. In a preferred embodiment said stream 23 is oxygen with a purity of 95% or greater.

(11) The effluent from the secondary reformer 12, which is usually at a temperature around 1000? C., is cooled in a recuperative heat exchanger (not shown) before admission to the MTS converter 13.

(12) The MTS converter 13 may comprise one or more isothermal catalytic reactors, comprising a copper-based catalytic bed and a plate heat exchanger immersed in the catalytic bed.

(13) In a preferred embodiment according to FIG. 1, the amount of feedstock 20 and of the first steam 21 is such to operate the pre-reformer 10 and the primary reformer 11 at a steam to carbon ratio of 1 to 1.5. The further addition of steam 26 brings the global steam to carbon ratio to a higher value which is however not greater than 2 according to the invention.

(14) FIG. 2 shows an embodiment where the incoming feedstock 20 is divided into a first portion 28 and a second portion 29. Said second portion 29 by-passes the primary reforming section, that is the pre-reformer 10 and the primary reformer 11. Said second portion 29 of the feedstock is then re-joined with the effluent 22 of the primary reformer 11, before the admission to the secondary reformer 12.

(15) The amount of additional steam 26, in this embodiment, depends on the amount of natural gas 29 bypassed around the primary reforming section: the larger the bypass stream 29, the larger the amount of steam 26.

(16) FIG. 3 shows another embodiment with pre-reforming where the full amount of fresh feedstock 20 and steam 21 is fed to a pre-reformer 10; the effluent 30 of the pre-reforming is divided into two fractions 31 and 32. Only the first fraction 31 is fed to the primary reformer 11, while the second fraction 32 by-passes the primary reformer and is re-joined with the effluent thereof.

(17) The first fraction 31 may be optionally added with an amount of steam 33 before admission to the primary reformer 11, to adjust the steam-to-carbon ratio. Hence, in this embodiment the pre-reformer 10 can run at a very low steam to carbon ratio. The steam to carbon ratio of the primary reformer 11 is regulated by the amount of bypass 32 and steam 33, while the global ratio is further adjusted by the steam 26.

(18) FIG. 4 shows an embodiment without pre-reforming. A portion 29 of feedstock 20 by-passes the primary reformer 11 and is joined with its effluent 22. In some embodiments, the portion 29 is a relevant portion of the total amount of feedstock 20, such as 40% or more. Accordingly, the primary reformer 11 is operated at a high steam to carbon ratio (e.g. around 2.7) while the global ratio computing also the fraction 29 and the other steam input(s), such as the steam line 26, is not greater than 2.

(19) FIG. 5 shows an embodiment without additional input of steam 26. Accordingly, all the steam enters via line 21 mixed with the feedstock 20. A portion of the feedstock 20 by-passes the primary reformer 11.

(20) It should be noted that steam line 26 is an optional feature also in the embodiments of FIGS. 2, 3 and 4. The global steam-to-carbon ratio is not greater than 2, whilst the steam-to-carbon ratio in the primary reformer (possibly with a pre-reformer) may be higher due to the carbon by-pass of line 29 or 32.