Method for revamping a front-end of an ammonia plant

Abstract

A method for revamping a front-end of an ammonia plant, said front-end comprising a reforming section (1, 2) with air-fired secondary reformer or autothermal reformer (2), a treatment section (3) of the effluent from said reforming section, and an air feed compressor (6), wherein an O.sub.2-containing stream (8) is directed to said reforming section (2) for use as oxidant, at least one nitrogen stream (9) is introduced at a suitable location of the front-end, to provide a desired molar ratio between hydrogen and nitrogen in the product gas, and at least part of said nitrogen stream (9) is compressed via said feed compressor (6).

Claims

1. A method for revamping a front-end of an ammonia plant, said front-end delivering a product gas for the synthesis of ammonia and comprising: a reforming section, comprising an air-fired secondary reformer or autothermal reformer, operating at a front-end pressure; a treatment section treating the effluent from said reforming section; an air compressor suitable for feeding air to said reforming section for use as an oxidant; the method comprising the steps of: feeding an O2-containing stream to said reforming section for use as oxidant; introducing at least one nitrogen stream at a location of the front-end, to provide a molar ratio between hydrogen and nitrogen in the product gas, and compressing at least part of said nitrogen stream via said feed compressor.

2. The method according to claim 1, wherein said O2-containing stream and said nitrogen stream are generated by an air separation unit.

3. The method according to claim 2, wherein said revamping comprises the installation of an air separation unit.

4. The method according to any claim 1, wherein all of said nitrogen stream is compressed by said air compressor.

5. The method according to claim 1, wherein said nitrogen stream is delivered by an air separation unit and enters said air compressor at near-ambient pressure.

6. The method according to claim 1, wherein: a first nitrogen current is compressed by said air compressor and a second nitrogen current, at a pressure higher than said first current, is fed directly to the front-end by-passing said compressor.

7. The method according to claim 6, wherein said first current and said second current are produced by an air separation unit, said first current being delivered by said unit at a near-ambient pressure and said second current being delivered by said unit at around said frontend pressure.

8. The method according to claim 1, wherein said O2-containing stream has a purity of at least 90% molar.

9. The method according to claim 1, wherein said O2-containing stream comprises a suitable amount of nitrogen, which, added with said nitrogen stream, provides the required H2:N2 molar ratio in the product gas.

10. The method according to claim 9, wherein said O2-containing stream has a purity of at least 50% molar.

11. The method according to claim 1, wherein said nitrogen stream is delivered by an air separation unit and enters said air compressor in the range 1 to 5 bar.

12. The method according to claim 10, wherein said O2-containing stream has a purity of 50 to 90% molar.

13. The method according to claim 12, wherein said O2-containing stream has a purity of 50 to 90% molar.

Description

BRIEF DESCRIPTION OF THE FIGURES

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

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

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

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

(4) FIG. 1 shows a first embodiment of the invention.

(5) A front end for the generation of ammonia synthesis gas comprises a primary reformer 1, a secondary reformer 2 and a treatment section 3. Said treatment section 3 may include shift converters and purification units, i.e. CO.sub.2 removal and methanation. The purified gas leaving said section 3 feeds ammonia synthesis section 4.

(6) The secondary reformer 2 is originally fired by air 7 delivered by a compressor 6.

(7) The method of the invention makes use of an oxygen current 8 and a nitrogen current 9 furnished by an air separation unit 5. Installation of said unit 5 may be part of the method of the invention, in some embodiments.

(8) Said air separation unit 5 delivers the nitrogen current 9 at near-ambient pressure, for example 1 to 5 bar. Said current 9 is compressed via the air compressor 6, and the compressed nitrogen 10 is delivered to a suitable location of the front-end, preferably the treatment section 3. For example, the nitrogen is introduced after a CO2 removal section or after a methanation section. The air line 7 may be discontinued.

(9) Since the nitrogen current 9 is compressed by the air compressor 6, one of the internal compressor of the ASU 5 can be avoided, i.e. the ASU is not required to export the nitrogen current 9 at the higher front-end pressure.

(10) The plant of FIG. 1 operates as follows: a natural gas feedstock NG is mixed with a steam current PS and enters the primary reformer 1. The reformed gas leaving the primary reformer is fed to the auto-thermal secondary reformer (2), now operating as oxygen-fired reformer due to oxygen input 8 from the ASU 5. The ASU 5 also furnishes nitrogen 9 to be added to the treatment section 3 in order to provide the required H.sub.2:N.sub.2 molar ratio of around 3 for the ammonia synthesis reaction. The compressor 6 is used for compressing all the nitrogen separated from air and leaving the ASU 5.

(11) The purity of the current 9 is preferably above 90%. For example, the flow rate of the oxidant is 1428 kmol/h and the oxygen contained is 1365 kmol/h, while the nitrogen is 30 kmol/h and other components (e.g. argon) are 33 kmol/h.

(12) FIG. 2 shows a second embodiment of the invention, where the ASU 5 delivers a first nitrogen current 9 at low pressure and a second current 9 at high pressure. Typically, said second current 9 is cryogenically pumped at the frontend pressure, e.g. 20 to 50 bar.

(13) The first current 9 is compressed with the air compressor 6, while the second current 9 is fed directly to the treatment section 3, possibly joined with the delivery 10 of said compressor.

(14) The ratio between these two currents 9 and 9 is selected in order to match the capacity of the existing air compressor 6 thereby minimizing or avoiding the modifications.

(15) FIG. 3 shows a third embodiment of the invention, where the ASU 5 delivers all the nitrogen 9 at a low pressure and the compressor 6 is not revamped compared to the original conditions. In this case, the amount of nitrogen in the current 9 may not be enough to reach the required 3:1 ratio, and it can be preferred to use an oxygen current 8 of a lower purity, to introduce the missing nitrogen with said current 8.

(16) The purity of said current 8 may be for example around 70-90%. For example, the flow rate of the oxidant is 1754 kmol/h and the oxygen contained is 1365 kmol/h, while the nitrogen is 356 kmol/h and other components (e.g. argon) are 33 kmol/h.

(17) The relative balance of oxygen and nitrogen depends on the process, particularly on the methods for purification of the syngas (e.g. by PSA, N.sub.2 sweeping PSA, or liquid nitrogen wash, or simple methanation) and on the purge stream in the synthesis loop. PSA (pressure swing adsorption) may be used to remove carbon dioxide and other impurities such as CO, CH4, Ar. For instance, if the purification section includes a PSA unit, a slight excess of oxygen to be fed to the process is required. In this process traces of hydrogen are also removed, hence the consumption of oxygen slightly increases. On the other hand, if the purification section includes a liquid NW (nitrogen washing) unit for the removal of impurities like CO, Ar and CH.sub.4 from a crude hydrogen stream, a slight excess of nitrogen to be fed to the process is required. In fact in this process a certain amount of nitrogen (e.g. 10-15%) is lost in the tail gas, thus requiring an increased consumption.

(18) The effect of these modifications, however, does not modify substantially the method exposed, since the variation of the total oxygen or nitrogen flow is small if compared with the total requirement for the ammonia process.

(19) Similarly, an air separation unit could also deliver other nitrogen streams, depending on the specific process and plant requirements, for continuous or discontinuous flows, such as nitrogen for regeneration of molecular sieves, or sweeping of a PSA unit, or liquid nitrogen for cooling of a liquid nitrogen wash, or for filling a liquid nitrogen tank. Also the effect of these modifications does not modify substantially the method exposed.