Method and apparatus for producing ammonia synthesis gas

Abstract

In a method for producing ammonia synthesis gas, a gas mixture containing hydrogen, argon, carbon monoxide, carbon dioxide and methane is derived from a reformer (R) in combination with a combustion unit (H), said mixture (1) being treated in order to remove its contained carbon dioxide, then cooled (3, 5) and separated in a cryogenic separation unit in an adiabatic chamber (CB), the mixture is separated in order to provide a liquid (21) which has been purified with carbon monoxide and / or methane, said liquid (21) being vaporized and fed as fuel to a combustion unit (H).

Description

Technical Field

[0001] The present invention relates to a method and apparatus for producing ammonia synthesis gas, which uses a method of purifying hydrogen by washing with nitrogen and recovering methane. Background Art

[0002] The unit for synthesizing ammonia is fed with a stoichiometric H2 / N2 mixture.

[0003] Some compounds, such as argon, methane, oxygen, CO, CO2, etc., are considered inert compounds or poisons for catalysts used in the synthesis of ammonia.

[0004] It is therefore necessary to purify the hydrogen which may be produced by steam reforming reactors, by autothermal reactors or by coal gasification reactors, to name only the most common ones.

[0005] One method of purifying this hydrogen operates at low temperatures and is known under the name of liquid nitrogen wash, in which the impure hydrogen is treated countercurrently in a column where it is sent to the bottom while a liquid nitrogen stream is fed to the column at the top.

[0006] The impure hydrogen produced by the steam reforming or coal gasification reactor is treated in a first step in a unit for removing acid gases of the CO 2 or H 2 S type in order to be sent to an adsorption purification unit to remove components that can solidify in the cold box.

[0007] After leaving the adsorption, the impure hydrogen is handled in a cold box.

[0008] The hydrogen is cooled to a temperature of around -182°C / -190°C to be fed into the bottom of a scrubber column, where liquid nitrogen is injected at the top.

[0009] This liquid nitrogen is likely to be produced in gaseous form by an air separation unit and liquefied in the main exchanger.

[0010] The liquid from the bottom of the scrubber column, which contains the impurities CO, Ar and CH4, is vaporized in the main exchanger before being used, for example, as fuel.

[0011] In the case of impure hydrogen with a CH 4 content of approximately a few mol %, it is known from WO 2012 / 097497 to separate the liquid phase formed before entry into the nitrogen scrubber and thereby produce a CH 4 -rich stream.

[0012] Thus, a pure H2+N2 mixture is produced at the top of the scrubber for the synthesis of NH3. The CH4-rich stream can be recycled upstream or fed to a unit for the production of H2, also producing a "waste" stream used as fuel, which contains a mixture of CO and Ar with residual contents of CH4, H2 and N2.

[0013] The waste stream is used as fuel and is burned, for example, in a combustion unit (e.g., a combustion unit in which combustion occurs in a closed chamber), the heat of combustion being transferred to the air by indirect heat transfer (indirectly fired heater). Heat can also be transferred by direct heat exchange. Such a combustion unit can generate heat for a reformer (e.g., an autothermal reformer) that produces a mixture of carbon monoxide and hydrogen. In fact, it generates flue gases containing CO , which are discharged in particular by the combustion of the CO contained in the waste stream. Summary of the Invention

[0014] One object of the present invention is to reduce the production of CO2 from the combustion unit by modifying the composition of the waste stream used as fuel so as to reduce its carbon monoxide and / or methane content.

[0015] According to the subject matter of the present invention, there is provided a method for producing ammonia synthesis gas, wherein a gas mixture containing hydrogen, argon, carbon monoxide, carbon dioxide and methane originates from a reformer, is treated to remove the carbon dioxide contained therein, is then cooled and separated in a cryogenic separation unit in an adiabatic chamber, the mixture is separated in a first column washed with liquid nitrogen to produce a first gas as ammonia synthesis gas at the top of the column and a first liquid containing nitrogen, hydrogen, argon, carbon monoxide and methane at the bottom of the column, the liquid being separated in a second separation column to produce a second overhead gas enriched in hydrogen and a second overhead gas depleted in hydrogen. The method comprises the following steps: sending the hydrogen-poor second liquid to a third separation tower, which produces a third liquid enriched in methane at the bottom of the tower and a third tower top gas, sending the third tower top gas to a fourth separation tower, which produces a fourth liquid enriched in argon at the bottom of the tower and a fourth tower top gas, sending the fourth gas to a fifth tower, which produces a fifth liquid enriched in carbon monoxide and a fifth gas enriched in nitrogen, and vaporizing the fourth liquid and sending it as fuel to a combustion unit, which produces flue gas containing CO2 and heat, and the heat is used in the process for producing the gas mixture.

[0016] According to other optional features:

[0017] vaporizing the third and / or fifth liquid and sending it upstream of the reformer,

[0018] a fuel (for example natural gas) optionally treated in a desulfurization unit and subsequently sent to a prereformer and said reformer to produce a gas which is treated by at least one shift operation to produce said mixture containing hydrogen, argon, carbon monoxide, carbon dioxide and methane,

[0019] sending the third and / or fifth liquid upstream of the pre-reformer or upstream of the desulfurization unit,

[0020] not sending any portion of the fourth liquid upstream of the reformer,

[0021] recirculating at least 50%, even at least 70%, of the carbon monoxide present in the gas mixture at the inlet of the thermally insulated chamber upstream of the reformer,

[0022] recycling at least 50%, even at least 70%, of the methane present in the gas mixture at the inlet of the adiabatic chamber upstream of the reformer,

[0023] sending less than 50%, or even less than 30%, of the carbon monoxide present in the gas mixture at the inlet of the insulating chamber to the combustion unit,

[0024] sending less than 50%, even less than 30%, of the methane present in the gas mixture at the inlet of the insulating chamber to the combustion unit,

[0025] • The first, second, third, fourth and fifth towers are contained within a single adiabatic chamber.

[0026] According to another subject of the present invention, there is provided an apparatus for producing ammonia synthesis gas, comprising a cryogenic separation unit, the cryogenic separation unit comprising a first tower for washing with liquid nitrogen, a second separation tower, a third separation tower, a fourth separation tower, and a fifth tower; an adiabatic chamber; an apparatus for treating a gas mixture containing hydrogen, argon, carbon monoxide, carbon dioxide, and methane originating from a reformer to remove the carbon dioxide contained therein; an apparatus for cooling the mixture purified from carbon dioxide; an apparatus for sending the cooled and purified mixture to be separated to the cryogenic separation unit in the adiabatic chamber, washing with liquid nitrogen in the first tower to produce a first gas as ammonia synthesis gas at the tower top, and producing a first liquid containing nitrogen, hydrogen, argon, carbon monoxide, and methane at the tower bottom; means for sending the first liquid to be separated to a second separation tower to produce a second tower top gas enriched in hydrogen and a second liquid depleted in hydrogen and enriched in methane and argon; means for sending the second hydrogen-depleted liquid to a third separation tower for producing a third liquid enriched in methane at the bottom of the tower and producing a third tower top gas; means for sending the third tower top gas to a fourth separation tower for producing a fourth liquid enriched in argon at the bottom of the tower and producing a fourth tower top gas; means for sending the fourth gas to a fifth tower for producing a fifth liquid enriched in carbon monoxide and a fifth gas enriched in nitrogen; means for vaporizing the fourth liquid and means for sending the vaporized fourth liquid as fuel to a combustion unit, wherein the combustion unit produces flue gas containing CO2 and heat for producing the gas mixture.

[0027] According to another aspect of the present invention, the device comprises:

[0028] vaporizing means, means for sending the third and / or fifth liquid to be vaporized in the vaporizing means and means for sending the vaporized liquid upstream of said reformer,

[0029] a desulfurization unit and a pre-reformer upstream of the reformer, a shift unit downstream of the reformer, means for conveying fuel for treatment in the desulfurization unit, means for conveying the treated fuel to the pre-reformer and the reformer, and means for conveying the gas produced by the reformer to the shift unit to produce the mixture containing hydrogen, argon, carbon monoxide, carbon dioxide and methane. BRIEF DESCRIPTION OF THE DRAWINGS

[0030] The present invention will now be described in more detail with reference to the accompanying drawings, in which:

[0031] [ Figure 1 ] The diagram represents a method for producing ammonia synthesis gas.

[0032] [ Figure 2 ] stands for cryogenic separation of gas mixtures for the production of ammonia synthesis gas. DETAILED DESCRIPTION

[0033] [ Figure 1 ] represents a process for producing ammonia synthesis gas NH3, in which a hydrocarbon stream NG (for example natural gas) is fed to an optional desulfurization unit HD. This gas is subsequently fed to a pre-reformer PR and a reformer R, for example an autothermal reformer, which is heated by the heat of a combustion unit H fed with hydrocarbon NG, for example natural gas. The reformer produces a gas comprising mainly hydrogen, carbon monoxide, carbon dioxide, methane and argon. This gas is heated in a unit B and then part of the carbon monoxide it contains is converted into carbon dioxide in two shift units SH1, SH2. Acid gases, such as carbon dioxide, are partially removed in a scrubbing unit M, for example by scrubbing with methanol. The gas purified of acid gases is then sent to a unit for purification by adsorption (not shown) in order to remove components that may solidify during cryogenic separation.

[0034] The mixture 1 entering the cryogenic chamber CB still contains hydrogen, carbon monoxide, carbon dioxide, methane and argon. This mixture is separated by cryogenic separation, which includes washing with liquid nitrogen in the separation unit CB to produce an ammonia synthesis gas 2 containing nitrogen and hydrogen. This gas 2 is sent to the ammonia production unit S.

[0035] The air separation unit ASU produces nitrogen N for nitrogen scrubbing in the unit CB and oxygen O for the reformer.

[0036] The argon-containing gas 15 is sent to the combustion unit H and the carbon monoxide and / or methane-containing gas 21 is sent upstream of the reformer R (or upstream of the pre-reformer PR or upstream of the desulfurization device HD). The combustion unit H generates heat for the reformer R and contains a burner.

[0037] [ Figure 2 ] represents an adiabatic chamber CB, which contains two heat exchangers 3, 5, a first separation tower K1 (which is a tower for washing with liquid nitrogen), a second separation tower K2, a third separation tower K3, a fourth separation tower K4 and a fifth separation tower K5.

[0038] The mixture 1 at the adsorption outlet, containing hydrogen, carbon monoxide, carbon dioxide, methane, and argon, is cooled in heat exchangers 3 and 5 to a temperature of approximately -182°C / -190°C. Nitrogen N, originating from air separation, is also liquefied in these exchangers. The cooled mixture 1 is fed to the bottom of a scrubber column K1, and the liquefied nitrogen is fed to the top to produce ammonia synthesis gas 2 at the top of column K1. The bottom liquid, containing nitrogen, hydrogen, carbon monoxide, methane, and argon, is fed to a second column K2 to reduce its hydrogen content. A hydrogen-enriched stream 8 exits the top of column K2, while a liquid 9, containing proportionally less hydrogen than the liquid 7, exits the bottom of column K2. The liquid 9 is fed to a third column, which produces a liquid 13 at the bottom that is enriched in methane relative to the liquid 7 and an overhead gas 11 that is depleted in methane relative to the liquid 7. The gas 11 is sent to a fourth column which produces at the bottom a liquid 15 enriched in argon relative to the gas 11 and an overhead gas 17 depleted in argon and methane relative to the gas 11 .

[0039] The gas 17 is separated in a fifth column K5 to produce at the top a stream 19 enriched in nitrogen and depleted in carbon monoxide relative to the gas 17 and at the bottom a liquid 21 enriched in carbon monoxide and depleted in nitrogen relative to the stream 17 .

[0040] The argon-enriched liquid 15 is sent as fuel to the combustion unit H. Since a portion of the methane has been removed in the column K3 , the liquid 15 contains less methane than it would if the column K3 were not present.

[0041] The liquid 13 enriched in methane and / or the liquid 21 enriched in carbon monoxide is vaporized and sent between the reformer R and the pre-reformer PR just upstream of the reformer R, or just before the pre-reformer PR, or just before the desulfurization unit HD.

[0042] Thus, by successive separations with towers K1 to K5 containing internals enabling mass transfer between the liquid and gaseous phases, the adiabatic chamber CB containing the tower K1 for washing with nitrogen is able to produce an H2+N2 mixture for the synthesis of NH3, a CO-rich gas, CH4 containing H2 and N2 (which can be recycled upstream of the unit R generating an H2-rich fluid treated in the adiabatic chamber CB) and an offgas containing a large part of the argon that cannot be recycled (resulting from the vaporization of the liquid 15).

[0043] Most of the CO and / or CH4 are then separated and recycled upstream, which reduces their amounts in the argon-rich fluid and thus significantly reduces the amount of CO2 emitted by combustion of the argon-rich fluid.

[0044] At least 50%, even at least 70%, of the carbon monoxide present in the gas mixture 1 at the inlet of the adiabatic chamber is recirculated upstream of the reformer R.

[0045] At least 50%, even at least 70%, of the methane present in the gas mixture 1 at the inlet of the adiabatic chamber CB is recycled upstream of the reformer R.

[0046] Less than 50%, even less than 30%, of the carbon monoxide present in the gas mixture at the inlet of the adiabatic chamber CB is sent to the combustion unit H combined with the reformer R.

[0047] Less than 50%, even less than 30%, of the methane present in the gas mixture at the inlet of the adiabatic chamber CB is sent to the combustion unit H combined with the reformer R.

Claims

1. A method for producing ammonia synthesis gas, wherein a gas mixture containing hydrogen, argon, carbon monoxide, carbon dioxide and methane originates from a reformer (R), said mixture (1) is treated to remove the carbon dioxide contained therein, then cooled (3, 5) and separated in a cryogenic separation unit in an adiabatic chamber (CB), said mixture being separated in a first column (1) for washing with liquid nitrogen to produce a first gas (2) as ammonia synthesis gas at the top of the column and a first liquid (7) containing nitrogen, hydrogen, argon, carbon monoxide and methane at the bottom of the column, said liquid being separated in a second separation column (K2) to produce a second top gas (8) enriched in hydrogen and a gas depleted in hydrogen and enriched in methane. The method comprises the following steps: producing a second liquid (9) rich in methane and argon, sending the second liquid poor in hydrogen to a third separation tower (K3), which produces a third liquid (13) rich in methane at the bottom of the tower and produces a third overhead gas (11), sending the third overhead gas to a fourth separation tower (K4), which produces a fourth liquid (15) rich in argon at the bottom of the tower and produces a fourth overhead gas (17), sending the fourth gas to a fifth tower (K5), which produces a fifth liquid (21) rich in carbon monoxide and a fifth gas (19) rich in nitrogen, vaporizing the fourth liquid and sending it as fuel to a combustion unit (H), which produces flue gas containing CO2 and heat, the heat being used for the method of producing the gas mixture.

2. The method according to claim 1, wherein the third and / or fifth liquid (13, 21) is vaporized and sent upstream of the reformer (5).

3. A method according to any one of the preceding claims, wherein a fuel (NG), such as natural gas, is optionally treated in a desulfurization unit (HD) and subsequently sent to a pre-reformer (PR) and to the reformer (R) to produce a gas which is treated by at least one shift operation (SH1, SH2) to produce the mixture (1) containing hydrogen, argon, carbon monoxide, carbon dioxide and methane.

4. The method according to claims 2 and 3, wherein a third and / or fifth liquid (13, 21) is sent upstream of the pre-reformer (PR) or upstream of the desulfurization unit (HD).

5. The method according to claim 1, wherein no portion of the fourth liquid (15) is sent upstream of the reformer (R).

6. The method according to claim 1 , wherein at least 50%, or even at least 70%, of the carbon monoxide present in the gas mixture (1) at the inlet of the adiabatic chamber (CB) is recycled upstream of the reformer (R).

7. The method according to claim 1, wherein at least 50%, or even at least 70%, of the methane present in the gas mixture (1) at the inlet of the adiabatic chamber (CB) is recycled upstream of the reformer (R).

8. The method according to claim 1, wherein less than 50%, or even less than 30%, of the carbon monoxide present in the gas mixture (1) at the inlet of the insulating chamber (CB) is sent to the combustion unit (H).

9. The method according to claim 1, wherein less than 50%, or even less than 30%, of the methane present in the gas mixture (1) at the inlet of the insulating chamber (CB) is sent to the combustion unit (H).

10. An apparatus for producing ammonia synthesis gas, comprising a cryogenic separation unit, the cryogenic separation unit comprising a first column (K1) for washing with liquid nitrogen, a second separation column (K2), a third separation column (K3), a fourth separation column (K4) and a fifth column (K5); an adiabatic chamber (CB); an apparatus (M) for treating a gas mixture (1) containing hydrogen, argon, carbon monoxide, carbon dioxide and methane originating from a reformer (R) to remove the carbon dioxide contained therein; an apparatus (3, 5) for cooling the mixture purified from carbon dioxide; an apparatus for sending the cooled and purified mixture to be separated to the cryogenic separation unit in the adiabatic chamber, washing with liquid nitrogen in the first column to produce a first gas (2) as ammonia synthesis gas at the column top, and producing a first liquid (7) containing nitrogen, hydrogen, argon, carbon monoxide and methane at the column bottom; and an apparatus for separating the first liquid to be separated. means for feeding the second hydrogen-depleted liquid to a second separation tower to produce a second hydrogen-enriched overhead gas (8) and a second liquid (9) depleted in hydrogen and enriched in methane and argon; means for feeding the hydrogen-depleted second liquid to a third separation tower, the third separation tower producing a methane-enriched third liquid (13) at the bottom of the tower and producing a third overhead gas (11); means for feeding the third overhead gas to a fourth separation tower, the fourth separation tower producing an argon-enriched fourth liquid (15) at the bottom of the tower and producing a fourth overhead gas (17); means for feeding the fourth gas to a fifth tower, the fifth tower producing a fifth liquid (21) enriched in carbon monoxide and a fifth gas (19) enriched in nitrogen; means for vaporizing the fourth liquid and means for feeding the vaporized fourth liquid as fuel to a combustion unit (H), the combustion unit (H) producing flue gas containing CO2 and heat for producing the mixture.

11. The device according to claim 10, comprising vaporizing means, means for sending the third and / or fifth liquid (13, 21) to be vaporized in the vaporizing means and means for sending the vaporized liquid upstream of the reformer (5).

12. An apparatus comprising a desulfurization unit (HD) and a pre-reformer (PR) upstream of the reformer, a shift unit (SH1, SH2) downstream of the reformer, a device for conveying fuel (NG) for treatment in the desulfurization unit (HD), a device for conveying the treated fuel to the pre-reformer (PR) and the reformer (R), and a device for conveying the gas produced by the reformer to the shift unit to produce the mixture (1) containing hydrogen, argon, carbon monoxide, carbon dioxide and methane.

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