Process for the ammonia production

Abstract

Process and plant for the synthesis of ammonia from a hydrocarbon feedstock, comprising: primary reforming with steam and air-fired secondary reforming wherein primary reforming is performed at a temperature and pressure of at least 790 C. and 50 bar, and secondary reforming is carried out substantially in absence of excess air, the so obtained make-up synthesis gas having a H.sub.2 to N.sub.2 molar ratio in the range 2.5 to 3.

Claims

1. A process for the synthesis of ammonia from a hydrocarbon feedstock, the process comprising: a step of primary reforming of said hydrocarbon feedstock with steam obtaining a first reformed gas, a step of air-fired secondary reforming of said first reformed gas, obtaining a raw gas product, purification of said raw gas product obtaining a make-up synthesis gas, conversion of said make-up synthesis gas into ammonia in a synthesis loop, wherein: said primary reforming is performed at a temperature of at least 790 C. and pressure of at least 50 bar; said step of secondary reforming is carried out substantially in absence of excess air relative to the stoichiometric amount; said make-up synthesis gas has a H.sub.2 to N.sub.2 molar ratio of 2.5 or greater but less than 3 and the process includes extraction from said loop of a purge stream, separation of a hydrogen-containing stream from said purge stream and addition of said hydrogen-containing stream to said make-up gas in order to adjust said H.sub.2 to N.sub.2 ratio.

2. The process according to claim 1, wherein primary reforming is carried out in tubes filled with catalyst, and said tubes are made of an alloy chosen among the following: GX45NiCrSiNbTi3525 or GX40NiCrSiNb3525 according to EN 10027 classification, or HP alloys, HP mod alloys, HP mod Microalloy, HP Nb Microalloy, HP microalloy, HK microalloy according to ASTM A-608 and ASTM A-297 classification.

3. The process according to claim 1, said H.sub.2 to N.sub.2 molar ratio being in the range 2.6 to 2.8.

4. The process according to claim 1, said conversion of make-up synthesis gas into ammonia being carried out at a pressure which is 2.0 to 3.5 times the pressure of the primary reforming.

5. The process according to claim 4, said conversion of make-up synthesis gas into ammonia being carried out at a pressure in the range 100 to 200 bar.

6. The process according to claim 1, said separation being carried out with a membrane hydrogen recovery unit.

7. The process according to claim 1, comprising a step of compression of said make-up gas in a gas compressor, said synthesis loop includes a circulation compressor and delivery of said gas compressor is sent to the suction side of said circulation compressor of the loop.

8. The process according to claim 1, wherein an air feed for the secondary reforming is compressed in an air compressor which is powered by a steam turbine, said steam turbine being fed with a high-pressure steam and steam for said step of primary reforming is extracted from said turbine.

9. The process according to claim 8, wherein said steam turbine expands steam in excess with respect to the steam required for the air compressor, and drives a generator to produce electric power.

10. The process according to claim 1, wherein said purified synthesis gas is subjected to a drying treatment by means of ammonia washing.

11. The process according to claim 1, wherein the reforming process, which includes said steps of primary reforming and air-fired secondary reforming, is operated with a global steam-to-carbon ratio equal to or greater than 2.9.

12. The process according to claim 5, said conversion of make-up synthesis gas into ammonia being carried out at a pressure in the range 120 to 150 bar.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 is a scheme of a plant for the synthesis of ammonia according to an embodiment of the invention.

DETAILED DESCRIPTION

(2) FIG. 1 illustrates a block scheme of a plant 1 for the synthesis of ammonia comprising a front-end section 2 and an ammonia synthesis loop 3. The front-end 2 produces a make-up synthesis gas 21 which is compressed in a gas compressor 9 and is fed to the to the ammonia synthesis loop 3.

(3) The front-end section 2 comprises: a primary reformer 4; a secondary reformer 5; an air compressor 6; a purification section 7; a gas drying unit 8. The air compressor 6 and the synthesis gas main compressor 9 are directly driven by respective steam turbines 10 and 11. The air compressor 6 is preferably of the integrally geared type.

(4) The loop 3 comprises a block 12 comprising at least one catalytic reactor, a gas cooler and a liquid separator to produce liquid ammonia 23. Unreacted gas 24 is re-circulated in the loop 3 by a further compressor 14, also referred to as circulator.

(5) A hydrocarbon feedstock 15, such as natural gas, and steam 16 catalytically react in the primary reformer 4 at a temperature of at least 790 C. and a pressure of at least 50 bar.

(6) The partially reformed gas 17 leaving the primary reformer 1 further reacts in the secondary reformer 5 with the aid of an air supply 18 delivered by the air compressor 6.

(7) The turbine 10 driving the air compressor 6 is powered by a high pressure steam 30 which is preferably generated in the ammonia plant 1, e.g. by recovering heat from exhaust fumes of the convective section of the primary reformer. According to a preferred embodiment, the steam 16 for the primary reforming is extracted from said turbine 10.

(8) In some embodiments, the amount of steam 30 exceeds the amount which is necessary to power the compressor 6. Hence, the turbine 10 may be coupled also to a generator, to produce electric power.

(9) The fully reformed gas 19 leaving the secondary reformer 5 is treated in the purification section 7, for example by shift conversion, removal of carbon dioxide and methanation, resulting in a purified synthesis gas 20. Said gas 20 is further sent to the drying unit 8 for the removal of water contained therein, obtaining a substantially anhydrous stream 21. Said drying unit 8 is preferably an ammonia washing unit.

(10) Said stream 21 has a hydrogen/nitrogen molar ratio of 2.5 to 3 according to the invention.

(11) Said stream 21 is sent to the suction side of the synthesis gas main compressor 9 and the resulting high-pressure synthesis gas 22 is preferably fed to the circulator 14, as shown.

(12) A purge stream 27 containing unreacted hydrogen and nitrogen and inert gases (e.g. argon and methane) is extracted from the loop 3, for example form the delivery stream 26 of the circulator 14. Said purge stream 27 is sent to a hydrogen recovery unit 13 to separate a hydrogen-rich gaseous stream 25, which is returned to the suction of circulator 14, where it is mixed with the stream 24. This hydrogen-rich gaseous stream 25 serves to adjust the H.sub.2 to N.sub.2 ratio, in particular when the ratio of streams 21 and 22 (as produced by the front-end 2) is lower than 3. By adding hydrogen separated from the purge stream 27, said ratio is adjusted to 3, or close to 3, as required for the synthesis of ammonia.