Process and apparatus for producing ammonia synthesis gas

Abstract

Title of the invention: Method and apparatus for producing an ammonia synthesis gas In a method for producing an ammonia synthesis gas, a gaseous mixture containing hydrogen, argon, carbon monoxide, carbon dioxide and methane comes from a reformer (R) associated with a combustion unit (H), the mixture (1) being treated to remove the carbon dioxide it contains before being cooled (3,5) and separated in a cryogenic separation unit in a thermally insulated enclosure (CB), the mixture being separated to provide a liquid (21) which is vaporized and sent as fuel to the combustion unit (H) having been purified of carbon monoxide and / or methane. Figure 2

Description

Title of the invention: Method and apparatus for producing an ammonia synthesis gas

[0001] The present invention relates to a method and apparatus for producing an ammonia synthesis gas. It uses a process for purifying hydrogen by nitrogen washing with methane recovery.

[0002] The ammonia synthesis units are fed with a stoichiometric H2 / N2 mixture.

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

[0004] It is therefore necessary to purify the hydrogen which can be produced by a steam reforming reactor, by an autothermal reactor or by a coal gasification reactor to name only the most frequent.

[0005] One of the processes for purifying this hydrogen operates cryogenically and is known as liquid nitrogen washing where the impure hydrogen is treated counter-currently in a column where it is sent to the tank while the column is fed at the top with a flow of liquid nitrogen.

[0006] The impure hydrogen produced by a steam reforming or coal gasification reactor is first treated in a CO2, H2S type acid gas removal unit to be sent to an adsorption purification unit to remove components that can solidify in the cold box.

[0007] At the outlet of this adsorption, the impure hydrogen is treated in a cold box.

[0008] This hydrogen is cooled to a temperature around -182°C / - 190°C to feed the tank of a washing column where liquid nitrogen is injected at the top.

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

[0010] The liquid from the washing column tank containing the impurities CO, Ar and CH4 is vaporized in the main exchanger before being used, for example, as fuel.

[0011] In the case where the CH4 content in the impure hydrogen is of the order of several mol%, it is known from WO 2012 / 097497 to separate the liquid phase created before entering the nitrogen washing column and thus produce a fluid richer in CH4.

[0012] A pure H2+N2 mixture is thus produced at the top of the washing column for the synthesis of NH3. A fluid rich in CH4 can be recycled upstream or feed an H2 production unit, as well as a so-called residual fluid, used as fuel, containing a mixture of CO and Ar with residual contents of CH4, H2 and N2.

[0013] The residual fluid is used as fuel and for example is burned in a combustion unit, for example a combustion unit where combustion takes place in a closed enclosure, the heat of combustion being transferred to air by indirect heat transfer (in English "Indirect Fired Heater"). The heat can also be transferred by direct heat exchange. This combustion unit can generate heat for a reformer producing a mixture of carbon monoxide and hydrogen, for example an autothermal reformer. However, it generates fumes containing CO2 emitted in particular by the combustion of the CO contained in the residual fluid.

[0014] An aim of the invention is to reduce the production of CO2 from this combustion unit by modifying the composition of the residual fluid used as fuel to reduce its carbon monoxide and / or methane content.

[0015] According to an object of the invention, there is provided a method for producing an ammonia synthesis gas in which a gaseous mixture containing hydrogen, argon, carbon monoxide, carbon dioxide and methane comes from a reformer, the mixture being treated to remove the carbon dioxide it contains before being cooled and separated in a cryogenic separation unit in a thermally insulated enclosure, the mixture being separated in a first liquid nitrogen washing column to produce at the top of the column a first gas which is the ammonia synthesis gas and at the bottom a first liquid containing nitrogen, hydrogen, argon, carbon monoxide and methane, the liquid is separated in a second separation column to produce a second column top gas enriched in hydrogen and a second liquid depleted in hydrogen and enriched in methane and argon,the second hydrogen-depleted liquid is sent to a third separation column which produces in the bottom a third liquid enriched in methane and a third overhead gas which is sent to a fourth separation column which produces in the bottom a fourth liquid enriched in argon and a fourth overhead gas, the fourth gas being sent to a fifth column which produces a fifth liquid rich in carbon monoxide and a fifth gas rich in nitrogen, the fourth liquid being vaporized and sent as fuel to a combustion unit which produces fumes containing CO2 and heat which is used in the process of producing the gas mixture.

[0016] According to other optional features: • the third and / or fifth liquid is vaporized and sent upstream of the reformer. • a fuel, for example natural gas, is optionally treated in a desulfurization unit, then sent to a pre-reformer and the reformer producing a gas which is processed by at least one shift operation producing the mixture containing hydrogen, argon, carbon monoxide, carbon dioxide and methane. • the third and / or fifth liquid is sent upstream of the pre-reformer or upstream of the desulfurization unit. • no part of the fourth liquid is sent upstream of the reformer. • at least 50%, or even at least 70%, of the carbon monoxide present in the gas mixture at the inlet of the thermally insulated enclosure is recycled upstream of the reformer. • at least 50%, or even at least 70% of the methane present in the gas mixture at the inlet of the thermally insulated enclosure is recycled upstream of the reformer. • less than 50% or even less than 30% of the carbon monoxide present in the gas mixture at the inlet of the thermally insulated enclosure is sent to the combustion unit. • less than 50% or even less than 30% of the methane present in the gas mixture at the inlet of the thermally insulated enclosure is sent to the combustion unit. • the first, second, third, fourth and fifth columns are located inside a single thermally insulated enclosure.

[0017] According to another object of the invention, there is provided an apparatus for producing an ammonia synthesis gas comprising a cryogenic separation unit comprising a first liquid nitrogen washing column, a second separation column, a third separation column, a fourth separation column and a fifth column, a thermally insulated enclosure, means for treating a gas mixture containing hydrogen, argon, carbon monoxide, carbon dioxide and methane coming from a reformer, to remove the carbon dioxide it contains, means for cooling the purified mixture to carbon dioxide, means for sending the cooled and purified mixture to separate in the cryogenic separation unit in the thermally insulated enclosure,in the first liquid nitrogen washing column to produce at the top of the column a first gas which is the ammonia synthesis gas and in the bottom a first liquid containing nitrogen, hydrogen, argon, carbon monoxide and methane, means for sending the first liquid to separate in the second separation column to produce a second top of column gas enriched in hydrogen and a second liquid depleted in hydrogen and enriched in methane and argon, means for sending the second liquid depleted in hydrogen to the third separation column which produces in the bottom a third liquid enriched in methane and a third, head gas, means for sending the third head gas to the fourth separation column which produces in the tank a fourth liquid enriched in argon and a fourth gas at the head, means for sending the fourth gas to a fifth column which produces a fifth liquid rich in carbon monoxide and a fifth gas rich in nitrogen, means for vaporizing the fourth liquid and means for sending the vaporized fourth liquid as fuel to a combustion unit which produces fumes containing CO2 and heat which is used to produce the gaseous mixture.

[0018] The invention will be described in more detail with reference to the figures in which:

[0019] [Fig. 1] schematically represents a process for producing ammonia synthesis gas.

[0020] [Fig.2] represents the cryogenic separation of the gas mixture used to produce ammonia synthesis gas.

[0021] [Fig. 1] represents a process for producing ammonia synthesis gas NH3 in which a stream of hydrocarbons NG, for example natural gas, is sent to an optional desulfurization unit HD. The gas is then sent to a prereformer PR and to a reformer R, for example of the autothermal type, heated by the heat of a combustion unit H fueled by hydrocarbons NG, for example natural gas. The reformer produces a gas which mainly comprises 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 partly removed in a scrubbing unit M, for example by a Rectisol ® type process using methanol scrubbing.Then the acid gas purified gas is sent to an adsorption purification unit (not shown) to remove components that may solidify during cryogenic separation.

[0022] The mixture 1 entering the cryogenic enclosure CB still contains hydrogen, carbon monoxide, carbon dioxide, methane and argon. The mixture is separated by cryogenic separation comprising washing with liquid nitrogen in a separation unit CB which produces the ammonia synthesis gas 2 containing nitrogen and hydrogen. The gas 2 is sent to an ammonia production unit S.

[0023] An air separation device ASU produces nitrogen N for nitrogen washing in the CB unit and oxygen O for the reformer.

[0024] A gas 15 containing argon is sent to a combustion unit H and a gas 21 containing carbon monoxide and / or methane is sent upstream of the reformer R (or upstream of the pre-reformer PR or upstream of the desulfurization unit HD). The combustion unit H produces

[0025] [Fig.2] represents a thermally insulated enclosure CB containing two exchangers heat 3.5, a first separation column Kl which is a liquid nitrogen washing column, a second separation column K2, a third separation column K3, a fourth separation column K4 and a fifth separation column K5.

[0026] The mixture 1, at the outlet of the adsorption, containing hydrogen, carbon monoxide, carbon dioxide, methane and argon is cooled in the heat exchangers 3, 5 to a temperature around -182°C / -190°C. In these exchangers also liquefies nitrogen N coming from the air separation. The cooled mixture 1 is sent to the bottom of the washing column K1 and the liquefied nitrogen is sent to the top, so that an ammonia synthesis gas 2 is produced at the top of the column KL. The bottom liquid containing nitrogen, hydrogen, carbon monoxide, methane and argon is sent to the second column K2 to reduce its hydrogen content, a flow 8 enriched in hydrogen exits the top of the column K2 and a liquid 9 containing proportionally less hydrogen than the liquid 7 exits the bottom of the column K2.The liquid 9 is sent to a third column which produces in the tank a liquid enriched in methane 13 compared to the liquid 7 and a head gas 11 depleted in methane compared to the liquid 7. The gas 11 is sent to a fourth column which produces in the tank a liquid enriched in argon 15 compared to the gas 11 and a head gas 17 depleted in argon and methane compared to the gas 11.

[0027] The gas 17 is separated in a fifth column K5 to produce at the top a flow 19 enriched in nitrogen and depleted in carbon monoxide compared to the gas 17 and at the bottom a liquid 21 enriched in carbon monoxide and depleted in nitrogen compared to the fluid 17.

[0028] The argon-enriched liquid 15 is sent as fuel to the combustion unit H. Since some of the methane has already been removed in column K3, the liquid 15 contains less methane than it would contain if column K3 were not present.

[0029] The liquid 13 enriched in methane and / or the liquid 21 enriched in carbon monoxide are vaporized and sent either just upstream of the reformer R between the reformer R and the prereformer PR, or just before the preformer PR or just before the desulfurization unit HD.

[0030] Thus, by successive separation by columns K1 to K5 containing internals allowing the mass transfer between the liquid and gas phases, the thermally insulated enclosure CB containing the nitrogen washing column K1 is capable of producing a mixture H2+N2 for the synthesis of NH3, a gas rich in CO, CH4 containing H2 and N2 which can be recycled upstream of the unit R which generates the fluid rich in H2 which is treated in the insulated enclosure CB as well as a waste gas (produced by vaporizing the liquid 15) which contains the majority of the argon which cannot be recycled.

[0031] The majority of the CO and / or CH4 is then separated and recycled upstream, which reduces their quantity in the argon-rich fluid and thus considerably reduces the quantity of CO2 emitted by combustion of the argon-rich fluid.

[0032] At least 50%, or even at least 70%, of the carbon monoxide present in the gas mixture 1 at the inlet of the thermally insulated enclosure is recycled upstream of the reformer R.

[0033] At least 50%, or even at least 70% of the methane present in the gas mixture 1 at the inlet of the thermally insulated enclosure CB is recycled upstream of the reformer R.

[0034] Less than 50%, or even less than 30% of the carbon monoxide present in the gas mixture at the inlet of the thermally insulated enclosure CB is sent to the combustion unit H associated with the reformer R.

[0035] Less than 50%, or even less than 30% of the methane present in the gas mixture at the inlet of the thermally insulated enclosure CB is sent to the combustion unit H associated with the reformer R.

Claims

Claims

1. A process for producing an ammonia synthesis gas in which a gaseous mixture containing hydrogen, argon, carbon monoxide, carbon dioxide and methane is supplied from a reformer (R), the mixture (1) being treated to remove the carbon dioxide it contains before being cooled (3, 5) and separated in a cryogenic separation unit in a thermally insulated enclosure (CB), the mixture being separated in a first liquid nitrogen washing column (1) to produce at the top of the column a first gas (2) which is the ammonia synthesis gas and at the bottom a first liquid (7) containing nitrogen, hydrogen, argon, carbon monoxide and methane, the liquid is separated in a second separation column (K2) to produce a second hydrogen-enriched column top gas (8) and a second hydrogen-depleted liquid (9) enriched in methane and argon,the second hydrogen-depleted liquid is sent to a third separation column (K3) which produces in the bottom a third liquid (13) enriched in methane and a third overhead gas (11) which is sent to a fourth separation column (K4) which produces in the bottom a fourth liquid (15) enriched in argon and a fourth gas (17) at the top, the fourth gas being sent to a fifth column (K5) which produces a fifth liquid (21) rich in carbon monoxide and a fifth gas (19) rich in nitrogen, the fourth liquid being vaporized and sent as fuel to a combustion unit (H) which produces fumes containing CO2 and heat which is used in the process of producing the gas mixture.,

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

3. Method according to one of the preceding claims in which a fuel (NG), for example natural gas, is optionally treated in a desulfurization unit (HD), then sent to a pre-reformer (PR) and the reformer (R) producing a gas which is treated by at least one shift operation (SH1, SH2) producing the mixture containing hydrogen, argon, carbon monoxide, carbon dioxide and methane (1).

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

5. Method according to one of the preceding claims in which no part of the fourth liquid (15) is sent upstream of the reformer (R).

6. Method according to one of the preceding claims in which at least 50%, or even at least 70%, of the carbon monoxide present in the gas mixture (1) at the inlet of the thermally insulated enclosure (CB) is recycled upstream of the reformer (R).

7. Method according to one of the preceding claims in which at least 50%, or even at least 70% of the methane present in the gas mixture (1) at the inlet of the thermally insulated enclosure (CB) is recycled upstream of the reformer (R).

8. Method according to one of the preceding claims in which less than 50% or even less than 30% of the carbon monoxide present in the gas mixture (1) at the inlet of the thermally insulated enclosure (CB) is sent to the combustion unit (H).

9. Method according to one of the preceding claims in which less than 50% or even less than 30% of the methane present in the gas mixture (1) at the inlet of the thermally insulated enclosure (CB) is sent to the combustion unit (H).

10. Apparatus for producing an ammonia synthesis gas comprising a cryogenic separation unit comprising a first liquid nitrogen washing column (K1), a second separation column (K2), a third separation column (K3), a fourth separation column (K4) and a fifth column (K5), a thermally insulated enclosure (CB), means (M) for treating a gas mixture (1) containing hydrogen, argon, carbon monoxide, carbon dioxide and methane from a reformer (R), to remove the carbon dioxide it contains, means (3, 5) for cooling the purified mixture to carbon dioxide, means for sending the cooled and purified mixture to separate in the cryogenic separation unit in the thermally insulated enclosure,in the first liquid nitrogen washing column to produce at the top of the column a first gas (2) which is the ammonia synthesis gas and in the bottom a first liquid (7) containing nitrogen, hydrogen, argon, carbon monoxide and methane, means for sending the first liquid to separate in the second se- column, preparation for producing a second hydrogen-enriched column overhead gas (8) and a second hydrogen-depleted liquid (9) enriched in methane and argon, means for sending the second hydrogen-depleted liquid to the third separation column which produces in the tank a third methane-enriched liquid (13) and a third overhead gas (11), means for sending the third overhead gas to the fourth separation column which produces in the tank a fourth argon-enriched liquid (15) and a fourth overhead gas (17), means for sending the fourth gas to a fifth column which produces a fifth carbon monoxide-rich liquid (21) and a fifth nitrogen-rich gas (19), means for vaporizing the fourth liquid and means for sending the vaporized fourth liquid as fuel to a combustion unit (H) which produces fumes containing CO2 and heat which is used to produce the mixture.

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