PROCESS FOR THE SYNTHESIS OF AMMONIA

Abstract

A dual pressure process for the synthesis of ammonia from a make-up gas, wherein the make-up gas is reacted in two steps in series, the second step operating at a greater pressure than the first step, and wherein a portion of the effluent of the first step is recycled back to the first step, said portion containing unreacted make-up gas.

Claims

1) A process for the synthesis of ammonia from a make-up gas (11), the process comprising a first reactive step at a first pressure (P.sub.1) and a second reactive step at a second pressure (P.sub.2) greater than the first pressure, wherein: the first reactive step provides a product stream (14) containing ammonia and a gaseous effluent (15) containing unreacted make-up gas; a first portion (16, 16a) of said gaseous effluent is subjected to said second reactive step, and a second portion (15b, 16b) of said gaseous effluent is recycled to said first reactive step.

2) A process according to claim 1, comprising a step of compression of the gaseous effluent of the first reactive step from said first pressure (P.sub.1) to said second pressure (P.sub.2), and the second portion (15b, 16b) of said gaseous effluent being taken before or after said step of compression.

3) A process according to claim 2, wherein said second portion (15b) is taken before said step of compression and is recycled to said first reactive step through an ejector (6).

4) A process according to claim 3, wherein said make-up gas (11) works as motive fluid inside said ejector (6).

5) A process according to claim 4, wherein said make-up gas (11) mixes with said second portion (15b) inside the ejector (6) forming the input stream (13) to said first reactive step.

6) A process according to claim 2, wherein said second portion (16b) is taken after said step of compression and is recycled directly to said first reactive step.

7) Process according to claim 6, wherein said second portion (16b) mixes with said make-up gas (11) forming the input stream (23) to said first reactive step.

8) Process according to any one of the previous claims, said second reactive step providing a purge gas stream (18) containing hydrogen and said purge gas stream (18) being directed to a purge gas recovery unit (5) wherein at least part of said hydrogen is separated as a hydrogen-containing stream (21).

9) Process according to claim 8, wherein said hydrogen-containing stream (21) mixes with said make-up gas (11).

10) A plant for the synthesis of ammonia from a make-up gas (11), comprising a first synthesis loop (2) operating at a first pressure (P.sub.1) and a second synthesis loop (4) operating at a second pressure (P.sub.2) greater than the first pressure, wherein: the first synthesis loop (2) provides a product stream (14) containing ammonia and a gaseous effluent (15) containing unreacted make-up gas; a first line (16, 16a) is arranged to supply a first portion of said gaseous effluent to said second synthesis loop (4), and a second line (15b, 16b) is arranged to recycle a second portion of said gaseous effluent to said first synthesis loop (2).

11)A plant according to claim 10, comprising a compressor (3) wherein said gaseous effluent is compressed from said first pressure (P.sub.1) to said second pressure (P.sub.2), and said second line (15b, 16b) being at the suction-side or delivery-side of said compressor (3).

12)A plant according to claim 11, said second line (15b) being at the suction-side of said compressor (3), the plant comprising an ejector (6) feeding the second portion of the gaseous effluent to the first synthesis loop (2).

13) A method for revamping a plant for the synthesis of ammonia from a make-up gas (11), the plant comprising: a first synthesis loop (2) operating at a first pressure (P.sub.1), wherein said make-up gas (11) reacts providing a first product stream (14) and a gaseous effluent (15) containing unreacted make-up gas; a second synthesis loop (4) operating at a second pressure (P.sub.2) greater than the first pressure, wherein said gaseous effluent reacts providing a second product stream (17); a compressor (3), wherein said gaseous effluent is compressed from the first pressure (P.sub.1) to the second pressure (P.sub.2), the method being characterized by: installation of a line (15b, 16b) for recycling a portion of said gaseous effluent to the first synthesis loop (2), said line being installed at the suction-side of the compressor (3) or at the delivery-side thereof.

14) A method according to claim 13, said line (15b) being installed at the suction-side of the compressor (3), the method comprising the installation of an ejector (6) at the inlet of the first synthesis loop (2).

15) A method according to claim 14, said ejector (6) being installed on the feeding line of the make-up gas to the first synthesis loop (2), so as the make-up gas acts as motive fluid in said ejector (6).

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0029] FIG. 1 is a scheme of a plant for the synthesis of ammonia from a make-up synthesis gas according to a first embodiment of the invention.

[0030] FIG. 2 is a variant of the plant of FIG. 1.

[0031] FIG. 3 shows in greater detail the first synthesis loop.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

[0032] The plant of FIG. 1 essentially comprises a first compressor 1 elevating the pressure of a make-up gas from the front-end pressure to a first pressure P.sub.1, a first ammonia synthesis loop 2, a second compressor 3 elevating the pressure of the effluent of the first loop 2 to a second pressure P.sub.2, a second ammonia synthesis loop 4, a purge gas recovery section 5 and an ejector 6. The first loop 2 operates at pressure P.sub.1 and the second loop 4 operates at pressure P.sub.2.

[0033] Said first pressure P.sub.1 is preferably in the range 60-130 bar and said second pressure P.sub.2 is preferably in the range 150-280 bar.

[0034] A make-up gas 10 is compressed in the first compressor 1 to the pressure P.sub.1. Said make-up gas 10 is obtained in a front-end section (not shown) of the plant, for example by reforming of a hydrocarbon feedstock at a pressure which is significantly lower than pressure P.sub.1, e.g. of around 15 to 30 bar.

[0035] The so compressed make-up gas 11 is mixed with a hydrogen-containing stream 21 extracted from said purge gas recovery section 5, thus obtaining a current 12.

[0036] Said current 12 and a gas stream 15b recirculated from the effluent 15 of the first synthesis loop 2 are supplied to the ejector 6, wherein they mix together thus forming the input stream 13 to said first synthesis loop 2. Said current 12 works as motive fluid to suck the gas stream 15b and supply it to the first loop 2.

[0037] The so obtained stream 13 enters the first synthesis loop 2, which provides a liquid ammonia product 14 and a gaseous stream 15 containing the unreacted make-up gas. As shown in FIG. 3, said stream 13 enters a converter 7 providing a partially reacted gaseous stream 24 containing ammonia and unreacted make-up gas. Said gaseous stream 24 is cooled inside a cooler-condenser 8, wherein ammonia partially condenses to provide a mixture 25. Said mixture 25 subsequently enters an ammonia separator 9, wherein the liquid ammonia product 14 is separated from the gaseous phase 15. Said gaseous phase 15 also contains ammonia not condensed in the cooler-condenser 8.

[0038] Said gaseous phase 15 splits into two portions 15a, 15b. The first portion 15a is supplied to the second compressor 3, wherein it is compressed to the pressure P.sub.2 providing a pressurized stream 16, and the second portion 15b is recirculated back to the delivery-side of the first compressor 1 in order to further react in the first synthesis loop 2.

[0039] Said pressurized stream 16 is fed to the second synthesis loop 4 wherein it further reacts to provide an ammonia product 17 and a purge gas 18.

[0040] Said purge gas 18 is continuously withdrawn from the synthesis loop 4 to limit the accumulation of inert gases and is sent to the recovery unit 5. Said purge gas 18 contains some ammonia, which is preferably recovered as an ammonia solution 19 in said recovery unit.

[0041] Said purge gas recovery unit 5 also separates the inert gases 20 from a hydrogen-containing stream 21, which is recycled to the delivery-side of the first compressor 1, mixing with the make-up gas 11 thus forming the current 12.

[0042] Alternatively, the hydrogen recovered from said purge recovery unit 5 can be recycled to the suction-side of the first compressor. Recycling the hydrogen 21 at the delivery-side or the suction-side of the first compressor 1 depends on the pressure at which the hydrogen-containing stream is discharged from the purge recovery unit 5.

[0043] FIG. 2 shows a variant of the plant of FIG. 1. Equipment and flow lines in common with the plant of FIG. 1 are indicated with the same reference numbers.

[0044] According to this embodiment, the gaseous phase 15 extracted from the first synthesis loop 2 is entirely supplied in the second compressor 3, thus providing the pressurized stream 16.

[0045] Said stream 16 splits into two portions 16a, 16b. The first portion 16a is fed to the second synthesis loop 4 and the second portion 16b is recirculated back to the delivery-side of the first compressor 1. Said second portion 16b mixes with the current 12 thus forming the input stream 23 to the first synthesis loop 2. In this case, the ejector 6 is not required thanks to the higher pressure of the stream 16b.