Process and plant for the production of ammonia make-up gas including production of a hydrogen gas by steam reforming

Abstract

A process and plant for producing a makeup synthesis gas (20) for the synthesis of ammonia, comprising: reforming of a hydrocarbon feedstock (10) in a steam reformer (SMR), purification of said gas product obtaining a hydrogen synthesis gas, addition of a suitable amount of nitrogen (19) to said hydrogen synthesis gas, wherein said purification includes separation of hydrogen in a main separation unit (S1), which produces a hydrogen current (14) and a tail gas (15), and wherein further hydrogen is recovered from the said tail gas (15) in an additional separation unit (S2); some embodiments provides that an amount of gas product (24) is also fed to said additional separation unit; a corresponding method for the revamping of a front-end of an ammonia plant is also disclosed.

Claims

1. A process for producing a makeup synthesis gas for the synthesis of ammonia, comprising: reforming of a hydrocarbon feedstock into a gas product comprising hydrogen, purification of said gas product, obtaining a hydrogen synthesis gas, adding nitrogen to said hydrogen synthesis gas, wherein said purification includes a first step of separation of hydrogen which produces a hydrogen current and a tail gas, wherein: at least a portion of said tail gas is subject to a further step of separation of hydrogen, to obtain a further hydrogen current which contains hydrogen recovered from said tail gas, and wherein said further hydrogen current is added to said hydrogen synthesis gas.

2. The process according to claim 1, wherein said first step of separation of hydrogen and/or said further step of separation of hydrogen is carried out by means of pressure swing adsorption.

3. The process according to claim 1, wherein the pressure of said tail gas is raised prior to the feeding to said further step of hydrogen separation.

4. The process according to claim 3, the discharge pressure of tail gas from said first step of hydrogen separation being less than 0.3 bar.

5. The process according to claim 1, wherein: a stream of said gas product is subjected to said further step of hydrogen separation.

6. The process according to claim 5, wherein said purification of the gas product includes steps of shift conversion of carbon monoxide to carbon dioxide, and removal of carbon dioxide, prior to said first step of hydrogen separation, and at least a portion of said gas product subjected to the further step of hydrogen separation is taken after the shift conversion and prior to the carbon dioxide removal, bypassing said carbon dioxide removal.

7. A plant for the production of makeup synthesis gas for the synthesis of ammonia, comprising a reforming section (SMR) for the reforming of a hydrocarbon feedstock into a gas product comprising hydrogen, a purification section suitable for the removal of components other than hydrogen from said gas product and for obtaining a hydrogen synthesis gas, and means to add nitrogen to said hydrogen synthesis gas, wherein said purification section includes a first hydrogen separation section which produces a first hydrogen current and a tail gas, wherein said purification section comprises a second hydrogen separation section, which is arranged to treat at least a portion of said tail gas, producing a second hydrogen current containing hydrogen recovered from said tail gas; wherein means are provided to add said second hydrogen current to said hydrogen synthesis gas.

8. The plant according to claim 7, said first hydrogen separation section and/or said second hydrogen separation section operating according to a pressure swing adsorption process.

9. The plant according to claim 7, said purification section comprising a compression section for feeding tail gas from the first hydrogen separation section to the second hydrogen separation section.

10. The plant according to claim 7, comprising also a flow line arranged to feed a stream of said gas product to said second hydrogen separation section.

11. The plant according to claim 10, said purification section comprising a carbon dioxide removal section (CDR), wherein at least a portion of said stream of gas product directed to the second hydrogen separation section is taken prior to the carbon dioxide removal section, bypassing said carbon dioxide removal section.

12. A method for revamping a plant for producing a makeup synthesis gas for the synthesis of ammonia, wherein said plant comprises a reforming section (SMR) for the reforming of a hydrocarbon feedstock into a gas product comprising hydrogen, a purification section for the removal of components other than hydrogen from said gas product and for obtainment of a hydrogen synthesis gas, and means to add a nitrogen current to said hydrogen synthesis gas, said purification section including a first hydrogen separation section producing a first hydrogen current and a tail gas, wherein the method comprises: installation of a second hydrogen separation section, redirection of at least a portion of said tail gas to said second hydrogen separation section, said second separation section being arranged to provide a second hydrogen current containing hydrogen recovered from said tail gas; and installation of means to add said second hydrogen current to said hydrogen synthesis gas.

13. The method according to claim 12, further comprising the step of lowering the original tail gas discharge pressure of said first hydrogen separation section.

14. The method according to claim 12, further comprising: a revamping of said reforming section (SMR), increasing the production rate of said reforming section and thus providing an increase of the flow rate of reformed gas product leaving said reforming section, and arranging means to direct a portion of said gas product to said second separation section.

15. The method according to claim 14 wherein: the purification section of the plant comprises a carbon dioxide removal section (CDR), and the method comprising the installation of a bypass line arranged to feed a portion of product gas to the second separation section bypassing said carbon dioxide removal section (CDR).

16. The method according to claim 12, where the existing hydrogen separation section and/or the newly installed second hydrogen separation section operates according to the pressure swing adsorption process.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 is a block diagram of an ammonia plant, including a front-end section according to an embodiment of the invention;

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

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT

(3) Referring to FIG. 1, a front-end for the synthesis of ammonia make-up gas comprises a steam methane reformer SMR, and a purification section comprising a shift converter SC, a carbon dioxide removal section CDR, a first pressure swing adsorption unit S1 for separation of hydrogen from the gas leaving said carbon dioxide removal section CDR.

(4) According to the embodiment of FIG. 1, the purification section comprises also a second pressure swing adsorption unit S2, arranged to process the tail gas discharged by the first unit S1 and containing come unrecovered hydrogen.

(5) More in detail, a hydrocarbon source 10, such as natural gas, is fed to the reformer SMR, producing a gas product 11 which comprises H.sub.2, CO, CO.sub.2, CH.sub.43 other hydrocarbons (HC) and other impurities.

(6) Since the desired output is hydrogen (H.sub.2) with a high purity, said gas product 11 is purified by means of conversion of CO into CO.sub.2 in the converter SC, obtaining a gas product 12, and removal of carbon dioxide in the section CDR.

(7) The CO.sub.2-depleted gas 13 is sent to the first PSA unit S1, which can be seen as the main hydrogen separation unit. Here, said gas 13 is separated into a current 14 of substantially pure hydrogen (e.g. containing more than 99.9% hydrogen), and a tail gas 15 containing methane and some non-separated hydrogen.

(8) At least a portion of said tail gas 15 is sent to the second PSA unit S2, via a booster compressor 16. Said compressor 16 delivers a flow 17 at a suitable pressure for feeding said second unit S2. Preferably the full amount of tail gas 15 is directed to said second unit S2, as shown.

(9) Said second unit S2 produces a second hydrogen current 18 which is mixed with the main hydrogen current 14 from the main PSA unit S1, and added with a nitrogen stream 19, to obtain a make-up gas 20 suitable for the synthesis of ammonia. The nitrogen stream 19 comes, for example, from an air separation unit ASU.

(10) Said makeup gas 20 feeds an ammonia synthesis loop denoted with AS.

(11) The second separation unit S2 produces also a residual gas 21 with a relatively high content of methane, which is for example recycled to fuel the reformer SMR.

(12) It is to be noted that the carbon dioxide removal section is an optional feature. In some embodiments, the gas product 12 leaving the shift converter SC may be sent directly to the main separation unit S1.

(13) The scheme of FIG. 1 is applicable to a new plant and to a revamped plant as well.

(14) An example of a revamping according to the invention is as follows.

(15) The original plant comprises the steam methane reformer SMR, the shift converter SC, the carbon dioxide removal section CDR, and the first PSA hydrogen separation unit S1. The tail gas 15 from said unit S1 may be originally recycled to the reformer SMR or discharged.

(16) Revamping of said plant provides the installation of the new PSA hydrogen separation unit S2 and related booster compressor 16. At least a portion of the tail gas 15 (from the first unit S1) is fed to said new hydrogen separation unit S2 via the compressor 16, and the so obtained current 18 containing additionally recovered hydrogen is merged with the output current 14 of the original first separation unit S1.

(17) In some embodiments, the pressure of tail gas 15, that is the discharge pressure of the first separation unit S1, is decreased after the installation of the new unit S2 and compressor 16. This is possible because the tail gas 15 is no longer required to feed the stem reformer SMR, and a suitable input pressure for the unit S2 is provided by the newly installed booster compressor 16. A lower discharge pressure has a positive effect on the efficiency of the separation process carried out in the pre-existing first unit S1, which means that also the amount of hydrogen in the current 14 is increased by means of the revamping process.

(18) Then, the invention provides more output of hydrogen, thanks to the additional hydrogen 18 recovered from tail gas 15, and thanks to the more hydrogen in the current 14 due to the lower tail gas discharge pressure of the original unit S1.

(19) The flow rate of nitrogen 19 is increased as a consequence, to provide the necessary amount of nitrogen. To this purpose, an existing ASU may need a proper revamping.

(20) The residual gas 21 leaving the new hydrogen separation unit S2 is preferably directed to the steam reformer SMR. It can be noted that said gas 21 contains a higher concentration of methane and has a higher heat value, compared to the tail gas 15 originally recycled to the steam reformer. Hence, also the use of tail gas as fuel takes advantage from the invention.

(21) It has to be noted that the revamping has no impact on the duty of steam reformer, shift converter and carbon dioxide removal section. Despite the additional output of hydrogen, the flow rate of streams 11, 12, 13 remains unchanged.

(22) Revamping of other items such as pumps, compressors, etc. might be provided according to the needs.

(23) FIG. 2 shows another embodiment of the invention where the second hydrogen separation unit S2 receives also a current of product gas 24. Said product gas 24 can be merged with the delivery stream 17 of the booster compressor 16.

(24) Said product gas 24 may be taken from any location in the purification section downstream the reformer SMR and prior to the first separation unit S1. In the example of FIG. 2, said product gas 24 is taken partly from the stream 12 leaving the shift converter SC (flow line 22), and partly from the de-carbonated stream 13 (flow line 23).

(25) The product gas of stream 22 bypasses the CO.sub.2 removal section CDR. Said bypass may be preferred when the capacity of said section CDR is not sufficient to process the full amount of gas 12 leaving the shift converter SC.