Air separation unit and air separation method

Abstract

An air separation unit that produces high-purity oxygen and argon and that can suppress a decrease in the amount of product argon recovered is provided. [Solution] An air separation unit (101) including a high-pressure column (4), a low-pressure column (5), an argon-enriched oxygen gas supply line (L8), and a high-purity oxygen column (12) that extracts a portion of the methane-free, argon-enriched liquefied oxygen produced in the argon column (6) from an intermediate portion of the argon column (6) and separates it into high-purity liquefied oxygen and methane-free, argon-enriched oxygen gas by cryogenic distillation, the air separation unit (101) further including a methane-free, argon-enriched oxygen gas return line (L16) that returns the methane-free, argon-enriched oxygen gas to either the argon column (6), the low-pressure column (5), or the argon-enriched oxygen gas supply line (L8), the return position of which is vertically lower than the extraction position of the methane-free, argon-enriched liquefied oxygen.

Description

[Technical Field]

[0001] The present invention relates to a hot gas generating device. [Background technology]

[0002] In recent years, there has been an increasing demand for high-purity oxygen, which is higher in purity than ordinary oxygen gas, in addition to nitrogen gas, oxygen gas, argon gas, and the like, in air separation units for semiconductor factories.

[0003] While ordinary oxygen gas contains argon (Ar) as an impurity at a concentration of about 0.2 to 0.5%, the high purity oxygen is required to have an argon impurity concentration of 1 ppm or less.

[0004] Furthermore, methane, which is less volatile than oxygen and is concentrated as an impurity in the product oxygen, must be kept below several tens of ppm, or even down to the ppb level in some cases. Therefore, when attempting to produce high-purity oxygen by further concentrating normal-purity product oxygen, the methane concentration in the concentrated product oxygen increases, and in some cases the product specifications cannot be met.

[0005] For this reason, methods for producing high-purity oxygen using a different method than further concentrating concentrated liquefied oxygen at the bottom of the low-pressure column of a typical air separation unit consisting of a high-pressure column, a low-pressure column, and an argon column have been studied. For example, a known method for producing high-purity oxygen is a process in which a mixture of argon and oxygen (methane-free argon-enriched liquefied oxygen) is extracted from the middle of the argon column and concentrated in a high-purity oxygen column to produce high-purity oxygen (see, for example, Patent Documents 1 to 8).

[0006] Patent Document 1 discloses a technique in which methane-free argon-enriched liquefied oxygen is extracted from the middle of argon column 10, concentrated in high-purity oxygen column 23, and the high-purity oxygen product is extracted from the bottom of high-purity oxygen column 23 through lines 25 and 26. It also describes that the methane-free argon-enriched oxygen gas extracted from the top of high-purity oxygen column 23 is returned to a position vertically above the position from which the methane-free argon-enriched liquefied oxygen is extracted from argon column 10, or to a position vertically above the position from which the argon-enriched liquefied oxygen is introduced into low-pressure column 6.

[0007] Patent Document 2 (for example, FIG. 11) discloses a technology in which methane-free argon-enriched liquefied oxygen is extracted from the middle of argon column 902, concentrated in high-purity oxygen column 102, and the high-purity oxygen product is extracted from the bottom of high-purity oxygen column 102 to line 114. There is no description as to the position to which the methane-free argon-enriched oxygen gas extracted from the top of high-purity oxygen column 102 is returned, but in the figure it is returned to the same position as the position from which the methane-free argon-enriched liquefied oxygen is extracted from argon column 902.

[0008] Patent Document 3 discloses a technique in which methane-free argon-enriched liquefied oxygen is extracted from the middle of an argon column 50, concentrated in a high-purity oxygen column 54, and the high-purity oxygen product is extracted from the bottom of the high-purity oxygen column 54 via line 124. There is no description as to the position to which the methane-free argon-enriched oxygen gas extracted from the top of the high-purity oxygen column 54 is returned, but in the drawing it is returned to the same position as the position from which the methane-free argon-enriched liquefied oxygen is extracted from the argon column 50.

[0009] Patent Document 4 (for example, FIG. 2) discloses a technique in which methane-free argon-enriched liquefied oxygen is extracted from the middle of argon column 26, concentrated in high-purity oxygen column 17A, and the high-purity oxygen product is extracted from the bottom of high-purity oxygen column 17A to line 25. It also discloses that the methane-free argon-enriched oxygen gas extracted from the top of high-purity oxygen column 17A is returned to the same level as the position in argon column 26 from which the methane-free argon-enriched liquefied oxygen was extracted.

[0010] Patent Document 5 (for example, FIG. 2) discloses a technique in which methane-free argon-enriched liquefied oxygen is extracted from the middle of argon column 12, concentrated in high-purity oxygen column 213, and the high-purity oxygen product is extracted from the bottom of high-purity oxygen column 213 via line 252. There is no description as to the position to which the methane-free argon-enriched oxygen gas extracted from the top of high-purity oxygen column 213 is returned, but in the drawing it is returned to the same position as the position from which the methane-free argon-enriched liquefied oxygen is extracted from argon column 12.

[0011] Patent Document 6 discloses a technology in which methane-free argon-enriched liquefied oxygen is extracted from the middle of argon column 5, concentrated in high-purity oxygen column 8, and the high-purity oxygen product is extracted from the bottom of high-purity oxygen column 8 to line L81. It also describes that the methane-free argon-enriched oxygen gas extracted from the top of high-purity oxygen column 8 is returned to a position vertically above the position from which the methane-free argon-enriched liquefied oxygen is extracted from argon column 5.

[0012] Patent Document 7 (for example, Figure 2) discloses a technology in which methane-free argon-enriched liquefied oxygen is extracted from the middle of argon column C4, concentrated in high-purity oxygen column C3, and the high-purity oxygen product is extracted from the middle of high-purity oxygen column C3 via line 64. It simply states that the methane-free argon-enriched oxygen gas extracted from the top of high-purity oxygen column C3 is returned to an appropriate position, and in the figure, it is returned to the same position as the position from which the methane-free argon-enriched liquefied oxygen was extracted from argon column C4.

[0013] Patent Document 8 (for example, FIG. 1) discloses a technique in which methane-free argon-enriched liquefied oxygen is extracted from between argon columns 17 and 18 via line 49, concentrated in high-purity oxygen column 16, and the high-purity oxygen product is extracted from the bottom of high-purity oxygen column 16 via line 50. The methane-free argon-enriched oxygen gas extracted from the top of high-purity oxygen column 16 via line 80 is returned to between argon columns 17 and 18 (the same position as the extraction position of line 49). [Prior art documents] [Patent documents]

[0014] [Patent Document 1] Patent No. 2696705 [Patent Document 2] U.S. Patent No. 5,049,173 [Patent Document 3] European Patent No. 2307835 [Patent Document 4] Special Publication No. 06-072740 [Patent Document 5] U.S. Patent No. 6,543,253 [Patent Document 6] Patent No. 7554407 [Patent Document 7] European Patent Application Publication No. 0682220 [Patent Document 8] European Patent No. 4214456 Summary of the Invention [Problem to be solved by the invention]

[0015] In the air separation units disclosed in Patent Documents 1 to 8, methane-free argon-enriched liquefied oxygen is extracted from the middle of the argon column and concentrated in a high-purity oxygen column, thereby recovering high-purity oxygen as a product. However, there is a problem in that the amount of product argon recovered is lower than when high-purity oxygen is not recovered.

[0016] An object of the present invention is to provide an air separation unit and an air separation method for producing high-purity oxygen and argon, which are capable of suppressing a decrease in the amount of product argon recovered. [Means for solving the problem]

[0017] The present inventors have conducted extensive research to solve the above-mentioned problems. As a result, as described above, in the conventional technology, the methane-free argon-enriched oxygen gas discharged from the top of the high-purity oxygen tower is returned to the same level as the position from which the methane-free argon-enriched liquefied oxygen was discharged from the argon tower, or to a position vertically higher than that. In contrast to this conventional technology, the inventors have found that the amount of product argon recovered can be increased by returning the methane-free argon-enriched oxygen gas to a position vertically lower than the position from which the methane-free argon-enriched liquefied oxygen was discharged from the argon tower, and have completed the present invention.

[0018] In order to solve the above problems, the present invention provides the following means.

[0019] [1] A high-pressure column that compresses, pre-cools, purifies, and cools air containing oxygen, nitrogen, and argon to obtain low-temperature feed air, and then performs low-temperature distillation to separate the air into high-pressure nitrogen gas and high-pressure liquefied air; a low-pressure column for subjecting the low-pressure liquefied air obtained by reducing the pressure of the high-pressure liquefied air to cryogenic distillation and separating the low-pressure liquefied air into low-pressure nitrogen gas, low-pressure liquefied oxygen, and argon-enriched oxygen gas; an argon-enriched oxygen gas supply line for supplying the argon-enriched oxygen gas to an argon column described below; an argon column that performs cryogenic distillation of the argon-enriched oxygen gas supplied from the argon-enriched oxygen gas supply line to separate the argon gas, argon-enriched liquefied oxygen, and methane-free argon-enriched liquefied oxygen; a high-purity oxygen tower that extracts a portion of the methane-free argon-enriched liquefied oxygen produced in the argon tower from an intermediate portion of the argon tower and separates the resulting high-purity liquefied oxygen and methane-free argon-enriched oxygen gas by cryogenic distillation; a nitrogen condenser that performs indirect heat exchange between the high-pressure nitrogen gas and the low-pressure liquefied oxygen to liquefy the high-pressure nitrogen gas to produce high-pressure liquefied nitrogen and vaporize the low-pressure liquefied oxygen to produce low-pressure oxygen gas; an argon condenser that performs indirect heat exchange between the argon gas and low-pressure liquefied air obtained by reducing the pressure of the high-pressure liquefied air, thereby liquefying the argon gas to produce liquefied argon and vaporizing the low-pressure liquefied air to produce low-pressure air; a high-purity oxygen reboiler for vaporizing the high-purity liquefied oxygen to produce high-purity oxygen gas; a product argon recovery line for recovering at least one of a portion of the argon gas and a portion of the liquefied argon as a product; a high-purity oxygen product recovery line that recovers at least one of a portion of the high-purity liquefied oxygen and a portion of the high-purity oxygen gas as a product; an air separation apparatus comprising: a methane-free argon-enriched oxygen gas return line that returns the methane-free argon-enriched oxygen gas to any one of the argon column, the low-pressure column, and the argon-enriched oxygen gas supply line, the return position being vertically lower than the output position of the methane-free argon-enriched liquefied oxygen. [2] The air separation unit according to [1], wherein the high-purity liquefied oxygen and the high-pressure nitrogen gas are indirectly heat exchanged in the high-purity oxygen reboiler to vaporize the high-purity liquefied oxygen to produce high-purity oxygen gas, and the high-pressure nitrogen gas is liquefied to produce high-pressure liquefied nitrogen. [3] The air separation unit according to [1] or [2], wherein the methane-free argon-enriched oxygen gas return line is connected to the argon column. [4] The air separation unit according to any one of [1] and [2], wherein the methane-free argon-enriched oxygen gas return line is connected to the low-pressure column. [5] The air separation unit according to any one of [1] and [2], wherein the methane-free argon-enriched oxygen gas return line is connected to the argon-enriched oxygen gas supply line. [6] A high-pressure separation process in which low-temperature feed air obtained by compressing, pre-cooling, purifying, and cooling air containing oxygen, nitrogen, and argon is subjected to low-temperature distillation to separate it into high-pressure nitrogen gas and high-pressure liquefied air; a low-pressure separation step in which the low-pressure liquefied air obtained by reducing the pressure of the high-pressure liquefied air is subjected to low-temperature distillation to separate it into low-pressure nitrogen gas, low-pressure liquefied oxygen, and argon-enriched oxygen gas; an argon-enriched oxygen gas supply step of supplying the argon-enriched oxygen gas to an argon column; an argon separation step of cryogenically distilling the argon-enriched oxygen gas to separate it into argon gas, argon-enriched liquefied oxygen, and methane-free argon-enriched liquefied oxygen; a high-purity oxygen separation step in which a portion of the methane-free argon-enriched liquefied oxygen produced in the argon separation step is withdrawn from an intermediate portion of the argon column and separated into high-purity liquefied oxygen and methane-free argon-enriched oxygen gas by low-temperature distillation; a nitrogen condensation step in which the high-pressure nitrogen gas is indirectly heat exchanged with the low-pressure liquefied oxygen to liquefy the high-pressure nitrogen gas to produce high-pressure liquefied nitrogen and the low-pressure liquefied oxygen to vaporize the low-pressure liquefied oxygen to produce low-pressure oxygen gas; an argon condensation step in which the argon gas is indirectly heat exchanged with low-pressure liquefied air obtained by reducing the pressure of the high-pressure liquefied air to liquefy the argon gas and generate liquefied argon, and the low-pressure liquefied air is vaporized to generate low-pressure air; a high-purity oxygen vaporization step of vaporizing the high-purity liquefied oxygen to produce high-purity oxygen gas; a product argon recovery step of recovering at least one of a portion of the argon gas and a portion of the liquefied argon as a product; a high-purity oxygen product recovery step of recovering at least one of a portion of the high-purity liquefied oxygen and a portion of the high-purity oxygen gas as a product; the methane-free argon-enriched oxygen gas is used as part of the raw material for either the argon separation step or the low-pressure separation step, and the raw material is supplied from a position vertically lower than the position from which the methane-free argon-enriched liquefied oxygen is extracted. [7] The air separation method according to [6], wherein in the high-purity oxygen vaporization step, the high-purity liquefied oxygen is indirectly heat exchanged with the high-pressure nitrogen gas to vaporize the high-purity liquefied oxygen to produce high-purity oxygen gas, and the high-pressure nitrogen gas is liquefied to produce high-pressure liquefied nitrogen. [8] The air separation method according to [6] or [7], wherein the methane-free argon-enriched oxygen gas returned in the methane-free argon-enriched oxygen gas return step is used as part of the raw material for the argon separation step. [9] The air separation method according to [6] or [7], wherein the methane-free argon-enriched oxygen gas returned in the methane-free argon-enriched oxygen gas return step is used as part of the raw material for the low-pressure separation step. [Effects of the Invention]

[0020] According to the present invention, an air separation unit and an air separation method for producing high-purity oxygen and argon are provided that can suppress a decrease in the amount of product argon recovered. That is, according to the present invention, when product argon is recovered while high-purity oxygen product is recovered at the same time, the amount of product argon recovered can be increased. [Brief explanation of the drawings]

[0021] [Figure 1] 1 is a schematic diagram showing an air separation unit of a first embodiment. [Figure 2] FIG. 3 is a schematic diagram showing an air separation unit of a second embodiment. [Figure 3] FIG. 1 is a schematic diagram showing an air separation unit of Comparative Example 1. DETAILED DESCRIPTION OF THE INVENTION

[0022] The present invention will be described below based on preferred embodiments. The air separation unit and method of the present invention perform cryogenic distillation of air to separate oxygen, nitrogen, and argon. Note that the terms "high pressure" and "low pressure" used in the following description refer to relative pressures and temperatures in each embodiment, and do not specify pressure or temperature ranges.

[0023] [First embodiment] <Air separation unit> FIG. 1 is a schematic diagram showing an air separation unit of the first embodiment. 1, the air separation unit 101 of this embodiment includes a high-pressure column 4, a low-pressure column 5, an argon column 6, a high-purity oxygen column 12, a nitrogen condenser 7, an argon condenser 8, and a high-purity oxygen reboiler 13. In addition to the above-mentioned components, the air separation unit 101 also includes an air compressor 1, an air precooler 2, an air purifier 3, a main heat exchanger 9, a supercooler 10, and an expansion turbine 11. The air separation unit 101 further includes an argon-enriched oxygen gas supply line L8, a product argon recovery line L10, a product high-purity oxygen recovery line L17, a methane-free argon-enriched oxygen gas return line L16, and lines L1 to L7, L9, L11 to L15, and L18 to L21.

[0024] An air compressor 1, an air precooler 2, an air purifier 3, and a main heat exchanger 9 are provided on a line L1, and are devices that compress, precool, purify, and cool air AIR containing oxygen, nitrogen, and argon.

[0025] High-pressure column 4 is a rectification column that compresses, pre-cools, purifies, and cools air (AIR) containing oxygen, nitrogen, and argon, and then performs low-temperature distillation on the obtained low-temperature feed air to separate it into high-pressure nitrogen gas and high-pressure liquefied air. The low-temperature feed air is obtained by compressing, pre-cooling, purifying, and cooling air (AIR) using the air compressor 1, air pre-cooler 2, air purifier 3, and main heat exchanger 9 described above.

[0026] The low-pressure column 5 is a distillation column that performs low-pressure distillation on the low-pressure liquefied air obtained by reducing the pressure of the high-pressure liquefied air separated in the high-pressure column 4, and separates the low-pressure liquefied air into low-pressure nitrogen gas, low-pressure liquefied oxygen, and argon-enriched oxygen gas.

[0027] The argon column 6 is a fractionation column that performs low-temperature distillation on the argon-enriched oxygen gas supplied from the argon-enriched oxygen gas supply line L8 to separate it into argon gas, argon-enriched liquefied oxygen, and methane-free argon-enriched liquefied oxygen.

[0028] The high-purity oxygen tower 12 is a fractionation tower that extracts a portion of the methane-free, argon-enriched liquefied oxygen separated in the argon tower 6 from the middle of the argon tower 6 and separates it into high-purity liquefied oxygen and methane-free, argon-enriched oxygen gas by low-temperature distillation.

[0029] The nitrogen condenser 7 is housed in the bottom of the low-pressure column 5 and is a heat exchanger that performs indirect heat exchange between high-pressure nitrogen gas and low-pressure liquefied oxygen. As described above, the nitrogen condenser 7 performs indirect heat exchange between the high-pressure nitrogen gas and the low-pressure liquefied oxygen, liquefying the high-pressure nitrogen gas to produce high-pressure liquefied nitrogen, and vaporizing the low-pressure liquefied oxygen to produce low-pressure oxygen gas.

[0030] The argon condenser 8 is a heat exchanger that performs indirect heat exchange between argon gas and low-pressure liquefied air obtained by reducing the pressure of high-pressure liquefied air. As described above, the argon condenser 8 performs indirect heat exchange between argon gas and low-pressure liquefied air obtained by reducing the pressure of high-pressure liquefied air, liquefying the argon gas to produce liquefied argon, and vaporizing the low-pressure liquefied air to produce low-pressure air.

[0031] The high-purity oxygen reboiler 13 is housed in the bottom of the high-purity oxygen column 12 and vaporizes the high-purity liquefied oxygen to produce high-purity oxygen gas. An example of the high-purity oxygen reboiler 13 is a heat exchanger that indirectly exchanges heat between the high-purity liquefied oxygen and high-pressure nitrogen gas. For example, the high-purity oxygen reboiler 13 may indirectly exchange heat between the high-purity liquefied oxygen and high-pressure nitrogen gas to vaporize the high-purity liquefied oxygen to produce high-purity oxygen gas, and liquefy the high-pressure nitrogen gas to produce high-pressure liquefied nitrogen.

[0032] The argon-enriched oxygen gas supply line L8 is a line for supplying argon-enriched oxygen gas to the argon column 6. One end of the argon-enriched oxygen gas supply line L8 is connected to the low-pressure column 5, and the other end is connected to the argon column 6.

[0033] The product argon recovery line L10 is a line that recovers at least a portion of the argon gas and a portion of the liquefied argon as a product. In the example shown in Figure 1, the product argon recovery line L10 branches off from line L9, which discharges argon from the top or upper part of the argon column 6. A portion of the argon discharged from the top or upper part of the argon column 6 branches off from line L9 to product argon recovery line L10, where it is heated to room temperature in the main heat exchanger 9 and recovered as product argon gas GAR.

[0034] The high-purity oxygen product recovery line L17 is a line that recovers at least one of a portion of the high-purity liquefied oxygen and a portion of the high-purity oxygen gas as a product. In the example shown in Figure 1, the high-purity liquefied oxygen that was not vaporized in the high-purity oxygen reboiler 13 is led to the high-purity oxygen product recovery line L17 and recovered as the high-purity liquefied oxygen product PLO.

[0035] The methane-free argon-enriched oxygen gas return line L16 is a line for returning the methane-free argon-enriched oxygen gas to any one of the argon column 6, the low-pressure column 5, and the argon-enriched oxygen gas supply line L8. The methane-free argon-enriched oxygen gas return line L16 returns the methane-free argon-enriched oxygen gas to any one of the argon column 6, the low-pressure column 5, and the argon-enriched oxygen gas supply line L8 at a return position that is vertically lower than the position at which the methane-free argon-enriched liquefied oxygen is discharged.

[0036] The main heat exchanger 9 and the subcooler 10 are devices that exchange heat between the fluids introduced therein and discharge the fluids after the heat exchange. The expansion turbine 11 is a device that adiabatically expands a portion of the low-temperature feed air cooled in the main heat exchanger 9 to generate the refrigeration necessary for the operation of the device and also generates low-pressure feed air.

[0037] The air separation unit 101 configured as described above can suppress a decrease in the amount of product argon gas (GAR) recovered. That is, when the air separation unit 101 recovers product argon gas (GAR) while simultaneously recovering product high-purity oxygen (PLO), it can increase the amount of product argon gas (GAR) recovered compared to conventional air separation units. Below, an example in which product high-purity oxygen (PLO) and product argon gas (GAR) are produced by the air separation unit 101 of this embodiment will be described in more detail.

[0038] First, in the air separation unit 101, air AIR containing oxygen, nitrogen, and argon is sequentially supplied via line L1 to an air compressor 1, an air precooler 2, an air purifier 3, and a main heat exchanger 9. The air AIR is compressed, precooled, purified, and cooled in the air compressor 1, the air precooler 2, the air purifier 3, and the main heat exchanger 9 to become low-temperature feed air. A portion of the obtained low-temperature feed air is sent to an expansion turbine 11 via a line L2 branching from line L1. The low-temperature feed air sent to the expansion turbine 11 is adiabatically expanded, becomes low-pressure feed air, and is supplied to an intermediate section of the low-pressure column 5 via line L2.

[0039] The low-temperature feed air cooled in main heat exchanger 9 is supplied to the bottom of high-pressure column 4 via line L1, and as it ascends within high-pressure column 4, nitrogen, a low-boiling point component, is concentrated due to gas-liquid contact with the reflux liquid flowing down within high-pressure column 4. As a result, high-pressure nitrogen gas is produced at the top of high-pressure column 4. In addition, as the reflux liquid flows down within high-pressure column 4, oxygen, a high-boiling point component, is enriched. As a result, high-pressure liquefied air is produced at the bottom of high-pressure column 4. The high-pressure liquefied air taken out from the bottom of high-pressure column 4 is cooled in subcooler 10 via line L6, reduced in pressure by pressure reducing valve V2, and then supplied to low-pressure column 5 via line L6 as low-pressure liquefied air.

[0040] The low-pressure liquefied air supplied to the low-pressure column 5 is concentrated with oxygen, a high-boiling component, as it flows downward due to gas-liquid contact with the ascending gas in the low-pressure column 5. As a result, low-pressure liquefied oxygen is produced at the bottom of the low-pressure column 5. In addition, the low-pressure feed air produced in the expansion turbine 11 and supplied to the low-pressure column 5 and the low-pressure oxygen gas produced in the nitrogen condenser 7, which will be described later, are concentrated with nitrogen, a low-boiling component, as they rise due to gas-liquid contact with the reflux liquid in the low-pressure column 5. As a result, low-pressure nitrogen gas is produced at the top of the low-pressure column 5. The low-pressure nitrogen gas withdrawn from the top of the low-pressure column 5 is heated in the subcooler 10 and the main heat exchanger 9 via line L20 and then recovered as product nitrogen gas GN. In addition, the low-pressure liquefied oxygen accumulated at the bottom of the low-pressure column 5 is recovered as liquefied oxygen LO via line L14. Furthermore, low-pressure nitrogen-enriched air taken out from the middle part of the low-pressure column 5 is passed through a line L21, heated in the subcooler 10 and the main heat exchanger 9, and then recovered as waste gas WG.

[0041] The high-pressure nitrogen gas concentrated at the top of the high-pressure column 4 is discharged via line L3 and supplied to a nitrogen condenser 7 housed in the bottom of the low-pressure column 5.

[0042] The nitrogen condenser 7 performs indirect heat exchange between the high-pressure nitrogen gas supplied via line L3 and the low-pressure liquefied oxygen accumulated at the bottom of the low-pressure column 5, liquefying the high-pressure nitrogen gas to produce high-pressure liquefied nitrogen and vaporizing the low-pressure liquefied oxygen to produce low-pressure oxygen gas. The high-pressure liquefied nitrogen produced in the nitrogen condenser 7 is returned to the high-pressure column 4 via line L4, and a portion of the high-pressure liquefied nitrogen is cooled in the subcooler 10 via line L5 branching off from line L4, and is then depressurized by pressure reducing valve V1 and supplied to the low-pressure column 5 as low-pressure liquefied nitrogen.

[0043] Furthermore, a portion of the low-pressure oxygen gas produced in the nitrogen condenser 7 is heated in the main heat exchanger 9 via line L13 and then recovered as product oxygen gas GO.

[0044] Furthermore, argon-enriched oxygen is extracted from the middle of the low-pressure column 5 and supplied to the argon column 6 via an argon-enriched oxygen gas supply line L8. The composition of the argon-enriched oxygen is, for example, 0.05% or less nitrogen, 5 to 15% argon, and 1 to 10 ppm methane.

[0045] As the argon-enriched oxygen supplied to the argon column 6 ascends within the argon column 6, argon, a low boiling point component, is concentrated by gas-liquid contact with the reflux liquid flowing down within the argon column 6. As a result, argon gas is produced at the top of the argon column 6. The argon gas is argon from which oxygen has been removed, and its composition can be, for example, 1% or less nitrogen and 0.1 ppm to 1.5% oxygen.

[0046] Furthermore, while the reflux liquid flows down through the argon column 6, oxygen, which is a high-boiling point component, is concentrated. As a result, argon-enriched liquefied oxygen is produced at the bottom of the argon column 6. The argon-enriched liquefied oxygen produced at the bottom of the argon column 6 may contain methane as an impurity. The argon-enriched liquefied oxygen is returned from the bottom of the argon column 6 via line L12 to the position from which the argon-enriched oxygen was extracted from the low-pressure column 5. Meanwhile, methane-free argon-enriched liquefied oxygen, which does not contain methane as an impurity, is produced in the argon column 6 as a mixture of argon and oxygen. The composition of the methane-free argon-enriched liquefied oxygen can be, for example, 0.5 ppm or less of methane, 0.02% or less of nitrogen, and 3 to 90% of argon.

[0047] As described above, in the air separation unit 101 of this embodiment, the argon column 6 separates the argon-enriched oxygen gas separated in the low-pressure column 5 into argon gas, argon-enriched liquefied oxygen, and methane-free argon-enriched liquefied oxygen.

[0048] The argon gas concentrated at the top or upper part of the argon column 6 is discharged through line L9 and supplied to an argon condenser 8. A portion of the argon gas concentrated at the top or upper part of the argon column 6 is heated in a main heat exchanger 9 via a product argon recovery line L10 branching off from line L9, and then recovered as product argon gas GAR.

[0049] The argon condenser 8 performs indirect heat exchange between argon gas supplied via line L9 and low-pressure liquefied air obtained by reducing the pressure of high-pressure liquefied air, liquefying the argon gas to produce liquefied argon and vaporizing the low-pressure liquefied air to produce low-pressure air. In the illustrated example, high-pressure liquefied air taken out from an intermediate section of the high-pressure column 4 is cooled in a subcooler 10 via line L7, reduced in pressure by a pressure reducing valve V3, and then supplied to the argon condenser 8 as low-pressure liquefied air.

[0050] The liquefied argon produced in the argon condenser 8 is returned to the argon column 6 via a line L11. In addition, the low-pressure air produced in the argon condenser 8 is supplied to the low-pressure column 5.

[0051] A portion of the methane-free, argon-enriched liquefied oxygen separated in the argon column 6 is extracted from an intermediate portion of the argon column 6 via line L15 and supplied to the top or upper portion of the high-purity oxygen column 12 via line L15. In this case, the extraction position of the methane-free, argon-enriched liquefied oxygen (the connection position between the argon column 6 and line L15) is preferably set at a position three or more theoretical plates, more preferably five or more theoretical plates, upward from the bottom of the argon column 6.

[0052] The methane-free, argon-enriched liquefied oxygen, which is extracted from the middle of the argon column 6 via line L15 and supplied to the high-purity oxygen column 12, is separated by low-temperature distillation into high-purity liquefied oxygen and methane-free, argon-enriched oxygen gas. That is, while the reflux liquid flows down in the high-purity oxygen column 12, oxygen, which is a high-boiling point component, is concentrated. As a result, high-purity liquefied oxygen is produced at the bottom of the high-purity oxygen column 12.

[0053] The methane-free argon-enriched oxygen gas that has been supplied to the high-purity oxygen tower 12 and separated from the high-purity liquefied oxygen is returned to the argon tower 6 via the methane-free argon-enriched oxygen gas return line L16. In this case, the return position of the methane-free argon-enriched oxygen gas to the argon tower 6 is set vertically lower than the output position of the methane-free argon-enriched liquefied oxygen described above. Here, the argon tower 6 is a tower that is arranged so that its axial direction is vertically above and below. Therefore, "the return position of the methane-free argon-enriched oxygen gas to the argon tower 6 is set vertically lower than the output position of the methane-free argon-enriched liquefied oxygen" means that the connection position of the argon tower 6 and the methane-free argon-enriched oxygen gas return line L16 is vertically lower (i.e., downward) than the connection position of the argon tower 6 and the line L15. The position where the methane-free argon-enriched oxygen gas is returned to the argon tower 6 (i.e., the connection position between the argon tower 6 and the methane-free argon-enriched oxygen gas return line L16) is preferably at least three theoretical plates, more preferably at least five theoretical plates, downstream from the point where the methane-free argon-enriched liquefied oxygen is extracted (the connection position between the argon tower 6 and line L15).

[0054] By setting the return position of the methane-free argon-enriched oxygen gas vertically lower than the outlet position of the methane-free argon-enriched liquefied oxygen, the amount of ascending gas increases in a region (section) of the argon column 6 vertically lower than the outlet position of the outlet line (line L15) for the methane-free argon-enriched liquefied oxygen, thereby promoting rectification. As a result, the amount of product argon recovered from the top of the argon column 6 can be increased.

[0055] Although not shown in the figures, the same effect as that described above can be obtained by merging the methane-free argon-enriched oxygen gas return line L16 (see FIG. 1) with the argon-enriched oxygen gas supply line L8 (see FIG. 1) that supplies argon-enriched oxygen from the low-pressure column 5 to the argon column 6, instead of connecting the methane-free argon-enriched oxygen gas return line L16 (see FIG. 1) with the argon-enriched oxygen gas supply line L8 (see FIG. 1). As described above, when the methane-free argon-enriched oxygen gas return line L16 (see Figure 1) is joined to the argon-enriched oxygen gas supply line L8 (see Figure 1), the return position of the methane-free argon-enriched oxygen gas return line L16 (see Figure 1) is the connection position between the argon-enriched oxygen gas supply line L8 (see Figure 1) and the argon tower 6, and is inevitably vertically lower than the output position of the methane-free argon-enriched liquefied oxygen. As described above, when the methane-free argon-enriched oxygen gas return line L16 is joined to the argon-enriched oxygen gas supply line L8, the return position of the methane-free argon-enriched oxygen gas does not mean the point where the two lines join, but rather the position where the argon-enriched oxygen gas supply line L8 after joining is connected to the argon tower 6.

[0056] A high-purity oxygen reboiler 13 is installed at the bottom of the high-purity oxygen column 12. Here, the high-pressure nitrogen gas concentrated at the top or upper part of the high-pressure column 4 is discharged via line L18 and supplied to the condensation passage of the high-purity oxygen reboiler 13.

[0057] The high-purity oxygen reboiler 13 performs indirect heat exchange between the high-purity liquefied oxygen accumulated at the bottom of the high-purity oxygen column 12 and the high-pressure nitrogen gas supplied via line L18, vaporizing the high-purity liquefied oxygen to produce high-purity oxygen gas, and liquefying the high-pressure nitrogen gas to produce high-pressure liquefied nitrogen.

[0058] The high-purity oxygen gas produced by the high-purity oxygen reboiler 13 becomes the ascending gas in the high-purity oxygen column 12 .

[0059] The high-purity liquefied oxygen that is not vaporized in the high-purity oxygen reboiler 13 is recovered as product high-purity liquefied oxygen PLO via product high-purity oxygen recovery line L17.

[0060] In addition, the high-pressure liquefied nitrogen produced in the high-purity oxygen reboiler 13 is cooled in the subcooler 10 via line L19, depressurized by the pressure reducing valve V4, and then supplied to the low-pressure column 5 via line L19 as low-pressure liquefied nitrogen.

[0061] <Air separation method> Next, an air separation method using air separation unit 101 of this embodiment (hereinafter also referred to as the air separation method of this embodiment) will be described. The air separation method of this embodiment includes a high-pressure separation step, a low-pressure separation step, an argon-enriched oxygen gas supply step, an argon separation step, a high-purity oxygen separation step, a nitrogen condensation step, an argon condensation step, a high-purity oxygen vaporization step, a product argon recovery step, a product high-purity oxygen recovery step, and a methane-free argon-enriched oxygen gas return step.

[0062] [High-pressure separation process] In the high-pressure separation process, air containing oxygen, nitrogen, and argon is compressed, pre-cooled, purified, and cooled to obtain low-temperature feed air, which is then subjected to low-temperature distillation to separate it into high-pressure nitrogen gas and high-pressure liquefied air.

[0063] For example, in the high-pressure separation step, as shown in FIG. 1, air AIR containing oxygen, nitrogen, and argon introduced from the atmosphere into line L1 is compressed by air compressor 1, precooled by air precooler 2, purified by air purifier 3, and cooled by main heat exchanger 9 to obtain low-temperature feed air. The low-temperature feed air obtained in this manner is supplied to the bottom of high-pressure column 4 via line L1. The supplied low-temperature feed air is then subjected to low-temperature distillation in high-pressure column 4 to be separated into high-pressure nitrogen gas and high-pressure liquefied air. For example, high-pressure nitrogen gas is produced at the top of high-pressure column 4, and high-pressure liquefied air is produced at the bottom of high-pressure column 4.

[0064] [Low pressure separation process] In the low-pressure separation step, the low-pressure liquefied air obtained by reducing the pressure of the high-pressure liquefied air is subjected to low-pressure distillation to separate it into low-pressure nitrogen gas, low-pressure liquefied oxygen, and argon-enriched oxygen gas.

[0065] For example, in the low-pressure separation step, as shown in Fig. 1, high-pressure liquefied air is taken out from the bottom of the high-pressure column 4, reduced in pressure by a pressure reducing valve V2 provided in the middle of line L6, and supplied as low-pressure liquefied air via line L6 to the low-pressure column 5. The supplied low-pressure liquefied air is then cryogenically distilled in the low-pressure column 5 and separated into low-pressure nitrogen gas, low-pressure liquefied oxygen, and argon-enriched oxygen gas.

[0066] [Argon-enriched oxygen gas supply process] In the argon-enriched oxygen gas supply step, the argon-enriched oxygen gas obtained in the low-pressure separation step is supplied to the argon column 6. For example, the argon-enriched oxygen gas extracted from the low-pressure column 5 via the argon-enriched oxygen gas supply line L8 is supplied to the argon column 6.

[0067] [Argon separation process] In the argon separation step, the argon-enriched oxygen gas is cryogenically distilled to separate it into argon gas, argon-enriched liquefied oxygen, and methane-free argon-enriched liquefied oxygen.

[0068] For example, in the argon separation step, as shown in FIG. 1, argon-enriched oxygen gas supplied to an argon column 6 is cryogenically distilled to separate the gas into argon gas, argon-enriched liquefied oxygen, and methane-free argon-enriched liquefied oxygen. Specifically, while the argon gas ascends within the argon column 6, argon, a low-boiling-point component, is concentrated by gas-liquid contact with the reflux liquid flowing down within the argon column 6, and argon gas is produced at the top of the argon column 6. Furthermore, while the reflux liquid flows down within the argon column 6, oxygen, a high-boiling-point component, is concentrated, and argon-enriched liquefied oxygen is produced at the bottom of the argon column 6. Furthermore, methane-free argon-enriched liquefied oxygen, which does not contain methane as an impurity, is produced within the argon column 6 as a mixture of argon and oxygen.

[0069] [High-purity oxygen separation process] In the high-purity oxygen separation step, a portion of the methane-free, argon-enriched liquefied oxygen produced in the argon separation step is extracted from the middle of the argon column 6 and separated into high-purity liquefied oxygen and methane-free, argon-enriched oxygen gas by low-temperature distillation.

[0070] For example, in the high-purity oxygen separation step, as shown in FIG. 1 , methane-free argon-enriched liquefied oxygen extracted from the middle of the argon column 6 via line L15 is supplied to the high-purity oxygen column 12. The methane-free argon-enriched liquefied oxygen supplied to the high-purity oxygen column 12 is then cryogenically distilled to separate it into high-purity liquefied oxygen and methane-free argon-enriched oxygen gas. Specifically, by supplying the methane-free argon-enriched liquefied oxygen to the high-purity oxygen column 12, oxygen, which is a high-boiling point component, is concentrated while the reflux liquid flows down within the high-purity oxygen column 12. As a result, high-purity liquefied oxygen is produced at the bottom of the high-purity oxygen column 12.

[0071] [Nitrogen condensation process] In the nitrogen condensation step, high-pressure nitrogen gas is indirectly heat exchanged with low-pressure liquefied oxygen to liquefy the high-pressure nitrogen gas to produce high-pressure liquefied nitrogen, and the low-pressure liquefied oxygen is vaporized to produce low-pressure oxygen gas.

[0072] For example, the nitrogen condensation step is performed in a nitrogen condenser 7 housed in the bottom of the low-pressure column 5, as shown in Fig. 1. The nitrogen condenser 7 performs indirect heat exchange between high-pressure nitrogen gas supplied via line L3 and low-pressure liquefied oxygen accumulated in the bottom of the low-pressure column 5, liquefying the high-pressure nitrogen gas to produce high-pressure liquefied nitrogen and vaporizing the low-pressure liquefied oxygen to produce low-pressure oxygen gas. Note that the high-pressure liquefied nitrogen produced in the nitrogen condenser 7 is returned to the high-pressure column 4 via line L4, and a portion of the high-pressure liquefied nitrogen is cooled in a subcooler 10 via line L5 branching from line L4, depressurized by a pressure reducing valve V1, and then supplied to the low-pressure column 5 as low-pressure liquefied nitrogen.

[0073] [Argon condensation process] In the argon condensation process, argon gas is indirectly heat exchanged with low-pressure liquefied air obtained by reducing the pressure of high-pressure liquefied air, thereby liquefying the argon gas to produce liquefied argon, and vaporizing the low-pressure liquefied air to produce low-pressure air.

[0074] For example, as shown in Fig. 1, the argon condensation step is performed in an argon condenser 8 provided above the argon column 6. The argon condenser 8 performs indirect heat exchange between argon gas supplied via line L9 and low-pressure liquefied air obtained by reducing the pressure of high-pressure liquefied air, thereby liquefying the argon gas to produce liquefied argon, and vaporizing the low-pressure liquefied air to produce low-pressure air. In the illustrated example, high-pressure liquefied air taken out from an intermediate portion of the high-pressure column 4 is cooled in a subcooler 10 via line L7, reduced in pressure by a pressure reducing valve V3, and then supplied to the argon condenser 8 as low-pressure liquefied air.

[0075] The liquefied argon produced in the argon condenser 8 is returned to the argon column 6 via a line L11. In addition, the low-pressure air produced in the argon condenser 8 is supplied to the low-pressure column 5.

[0076] [High-purity oxygen vaporization process] In the high-purity oxygen vaporization step, high-purity liquefied oxygen is vaporized to produce high-purity oxygen gas.

[0077] For example, the high-purity oxygen vaporization step can be performed in a high-purity oxygen reboiler 13, as shown in Fig. 1. The high-purity oxygen reboiler 13 is housed, for example, in the bottom of the high-purity oxygen tower 12, and vaporizes high-purity liquefied oxygen to produce high-purity oxygen gas. An example of the high-purity oxygen reboiler 13 is a heat exchanger that performs indirect heat exchange between high-purity liquefied oxygen and high-pressure nitrogen gas.

[0078] For example, in the high-purity oxygen reboiler 13, high-purity liquefied oxygen may be indirectly heat exchanged with high-pressure nitrogen gas to vaporize the high-purity liquefied oxygen to produce high-purity oxygen gas and to liquefy the high-pressure nitrogen gas to produce high-pressure liquefied nitrogen. For example, the high-purity oxygen reboiler 13 indirectly heat exchanges the high-purity liquefied oxygen accumulated at the bottom of the high-purity oxygen column 12 with high-pressure nitrogen gas supplied via line L18 to vaporize the high-purity liquefied oxygen to produce high-purity oxygen gas and to liquefy the high-pressure nitrogen gas to produce high-pressure liquefied nitrogen. Here, the high-pressure nitrogen gas concentrated at the top or upper part of the high-pressure column 4 is extracted via line L18 and supplied to the condensation passage of the high-purity oxygen reboiler 13.

[0079] The high-pressure liquefied nitrogen produced in the high-purity oxygen reboiler 13 is cooled in the subcooler 10 via line L19, reduced in pressure by the pressure reducing valve V4, and then supplied to the low-pressure column 5 via line L19 as low-pressure liquefied nitrogen.

[0080] [Product argon recovery process] In the product argon recovery step, at least one of a portion of the argon gas and a portion of the liquefied argon is recovered as a product.

[0081] For example, in the product argon recovery step, as shown in FIG. 1, concentrated argon gas can be discharged to the top or upper part of the argon column 6 and the discharged argon gas can be recovered. For example, argon gas is discharged from the argon column 6 via line L9 and passed through product argon recovery line L10 branching off from line L9. The discharged argon gas is heated in a main heat exchanger 9 and then recovered as product argon gas GAR. Although not shown, in the argon condensation step described above, a portion of the liquefied argon obtained by liquefying argon gas in an argon condenser 8 may be recovered as a product. The liquefied argon may be vaporized in a main heat exchanger 9 as shown in FIG. 1, heated to room temperature, and then recovered as product argon gas GAR.

[0082] [High-purity oxygen recovery process] In the high-purity oxygen product recovery step, at least one of a portion of the high-purity liquefied oxygen and a portion of the high-purity oxygen gas is recovered as a product.

[0083] For example, in the product high-purity oxygen recovery step, as shown in FIG. 1, high-purity liquefied oxygen that is not vaporized in the high-purity oxygen reboiler 13 can be recovered as product high-purity liquefied oxygen PLO via the product high-purity oxygen recovery line L17.

[0084] [Methane-free argon-enriched oxygen gas return process] The methane-free argon-enriched oxygen gas return step is a step in which the methane-free argon-enriched oxygen gas obtained in the high-purity oxygen separation step is returned as part of the feedstock to either the argon separation step or the low-pressure separation step. The air separation unit 101 shown in Figure 1 returns the methane-free argon-enriched oxygen gas as part of the feedstock to the argon separation step.

[0085] In the air separation method of this embodiment, the methane-free argon-enriched oxygen gas return step sets the feedstock supply position of the methane-free argon-enriched oxygen gas returned as part of the feedstock (in other words, the return position of the methane-free argon-enriched oxygen gas) vertically lower than the outlet position of the methane-free argon-enriched liquefied oxygen. This configuration increases the amount of ascending gas in the region (section) of the argon column 6 below the outlet position of the methane-free argon-enriched liquefied oxygen outlet line (line L15), thereby promoting rectification. As a result, the amount of product argon recovered from the top of the argon column 6 can be increased.

[0086] In the methane-free argon-enriched oxygen gas return step in the air separation method of this embodiment, for example, the methane-free argon-enriched oxygen gas obtained in the high-purity oxygen separation step is returned from the high-purity oxygen tower 12 to the argon tower 6 via the methane-free argon-enriched oxygen gas return line L16. Although not particularly limited, the feed position of the methane-free argon-enriched oxygen gas to the argon tower 6 (i.e., the connection position of the argon tower 6 and the methane-free argon-enriched oxygen gas return line L16) is preferably at least three theoretical plates, more preferably at least five theoretical plates, below the output position of the methane-free argon-enriched liquefied oxygen (the connection position of the argon tower 6 and the line L15).

[0087] Furthermore, although not shown in the drawings, in the methane-free argon-enriched oxygen gas return step, instead of connecting the methane-free argon-enriched oxygen gas return line L16 (see FIG. 1) to the argon column 6, the methane-free argon-enriched oxygen gas return line L16 (see FIG. 1) can be merged with the argon-enriched oxygen gas supply line L8 (see FIG. 1) that supplies argon-enriched oxygen from the low-pressure column 5 to the argon column 6, thereby achieving substantially the same effect as in the case described above. As described above, when the methane-free argon-enriched oxygen gas return line L16 (see Figure 1) is joined to the argon-enriched oxygen gas supply line L8 (see Figure 1), the raw material supply position for the methane-free argon-enriched oxygen gas is the connection position between the argon-enriched oxygen gas supply line L8 (see Figure 1) and the argon column 6, which is necessarily vertically lower than the output position of the methane-free argon-enriched liquefied oxygen. As described above, when the methane-free argon-enriched oxygen gas return line L16 is joined to the argon-enriched oxygen gas supply line L8, the raw material supply position of the methane-free argon-enriched oxygen gas does not mean the point where the two lines join, but rather the position where the argon-enriched oxygen gas supply line L8 after joining is connected to the argon column 6.

[0088] [Second embodiment] <Air separation unit> 2 is a schematic diagram showing an air separation unit of a second embodiment. In the second embodiment described below, components common to those of the first embodiment are given the same reference numerals in the drawing and will not be described again.

[0089] 2, the air separation unit 102 of this embodiment includes a high-pressure column 4, a low-pressure column 5, an argon column 6, a high-purity oxygen column 12, a nitrogen condenser 7, an argon condenser 8, and a high-purity oxygen reboiler 13. In addition to the above-mentioned components, the air separation unit 102 also includes an air compressor 1, an air precooler 2, an air purifier 3, a main heat exchanger 9, a supercooler 10, and an expansion turbine 11. The air separation unit 101 further includes an argon-enriched oxygen gas supply line L8, a product argon recovery line L10, a product high-purity oxygen recovery line L17, a methane-free argon-enriched oxygen gas return line L16, and lines L1 to L7, L9, L11 to L15, L18, and L19.

[0090] The air separation apparatus 102 of this embodiment differs from the air separation apparatus 101 of the first embodiment shown in Figure 1 described above in the location where the methane-free argon-enriched oxygen gas separated in the high-purity oxygen column 12 is returned via the methane-free argon-enriched oxygen gas return line L16. The air separation apparatus 102 of this embodiment is configured almost the same as the air separation apparatus 101 of the first embodiment shown in Figure 1, except for the location where the methane-free argon-enriched oxygen gas is returned via the methane-free argon-enriched oxygen gas return line L16.

[0091] 2, the air separation apparatus 102 of this embodiment is configured to return the methane-free argon-enriched oxygen gas separated in the high-purity oxygen column 12 to the low-pressure column 5 via the methane-free argon-enriched oxygen gas return line L16. In the air separation apparatus 102 of this embodiment, the methane-free argon-enriched oxygen gas return position in the low-pressure column 5 (the connection position between the low-pressure column 5 and the methane-free argon-enriched oxygen gas return line L16) is vertically lower than the discharge position of the methane-free argon-enriched liquefied oxygen (the connection position between the argon column 6 and line L15).

[0092] The position at which the methane-free argon-enriched oxygen gas is returned to the low-pressure column 5 (i.e., the connection position between the low-pressure column 5 and the methane-free argon-enriched oxygen gas return line L16) is preferably vertically lower than the point at which the argon-enriched oxygen gas is discharged (the connection position between the low-pressure column 5 and the argon-enriched oxygen gas supply line L8), and more preferably at a position that is three or more theoretical plates below the point at which the argon-enriched oxygen gas is discharged (the connection position between the low-pressure column 5 and the argon-enriched oxygen gas supply line L8). This increases the amount of gas rising in the region (section) below the discharge position of the methane-free argon-enriched liquefied oxygen discharge line (line L15) from the argon column 6, and in the region (section) between the connection position of the argon-enriched oxygen gas supply line L8 and the connection position of the methane-free argon-enriched oxygen gas return line L16 from the low-pressure column 5, thereby promoting rectification, and as a result, the amount of product argon recovered from the top of the argon column 6 can be increased.

[0093] <Air separation method> Next, an air separation method using the air separation unit 102 of this embodiment (hereinafter also referred to as the air separation method of this embodiment) will be described. The air separation method of this embodiment includes a high-pressure separation step, a low-pressure separation step, an argon-enriched oxygen gas supply step, an argon separation step, a high-purity oxygen separation step, a nitrogen condensation step, an argon condensation step, a high-purity oxygen vaporization step, a product argon recovery step, a product high-purity oxygen recovery step, and a methane-free argon-enriched oxygen gas return step.

[0094] The air separation method of this embodiment is configured almost the same as the air separation method of the first embodiment, except that the methane-free argon-enriched oxygen gas return step is a step in which the methane-free argon-enriched oxygen gas obtained in the high-purity oxygen separation step is returned as part of the feedstock for the low-pressure separation step. The methane-free argon-enriched oxygen gas return step in the air separation method of this embodiment can be realized by returning the methane-free argon-enriched oxygen gas separated in the high-purity oxygen column 12 to the low-pressure column 5 via the methane-free argon-enriched oxygen gas return line L16, as in the air separation apparatus 102 shown in Figure 2 described above.

[0095] <Other embodiments> Next, other embodiments of the air separation unit and air separation method will be described. Detailed explanations of the air separation unit and air separation method of the other embodiments will be omitted using drawings, and the following description will be given with reference to the air separation unit 101 shown in FIG.

[0096] The high-purity liquefied oxygen delivered to the high-purity oxygen product recovery line L17 is pressurized by a pump (not shown), vaporized in the main heat exchanger 9, and heated to room temperature to obtain high-pressure high-purity oxygen gas product. This makes it possible to produce high-pressure high-purity oxygen gas product without using an oxygen gas compressor.

[0097] A portion of the high-purity oxygen gas vaporized in the high-purity oxygen reboiler 13 is led to a line (not shown) and heated to room temperature in the main heat exchanger 9, thereby obtaining a low-pressure high-purity oxygen gas product.

[0098] Instead of the high-pressure nitrogen gas supplied to the high-purity oxygen reboiler 13, a portion of the low-temperature feed air flowing through line L1 and cooled in the main heat exchanger 9 can be used. In this case, the low-temperature feed air is liquefied by indirect heat exchange in the high-purity oxygen reboiler 13 to become high-pressure liquefied air, which is supplied to the high-pressure column 4, or is reduced in pressure by a pressure reducing valve and then supplied to the low-pressure column 5 or the argon condenser 8. This increases the temperature of the fluid liquefied in the high-purity oxygen reboiler 13, allowing for a larger temperature difference between the fluids and enabling a smaller heat transfer area.

[0099] Furthermore, instead of the high-pressure nitrogen gas supplied to the high-purity oxygen reboiler 13, high-pressure liquefied air delivered from the high-pressure column 4 to line L6 or line L7 can be used. In this case, the high-pressure liquefied air is cooled by indirect heat exchange in the high-purity oxygen reboiler 13 to a subcooled state (a state in which its temperature is lower than its boiling point), and is then reduced in pressure by a pressure reducing valve before being supplied to the low-pressure column 5 or the argon condenser 8. This eliminates the need for gas to be supplied to the high-purity oxygen reboiler 13, increasing the amount of rising gas in the high-pressure column 4 and the high-pressure nitrogen gas supplied to the nitrogen condenser 7, increasing the amount of heat exchanged in the nitrogen condenser 7, increasing the amount of rising gas in the low-pressure column 5, improving the rectification conditions, and enabling the flow rate of the product argon to be increased.

[0100] The high-pressure nitrogen gas in line L18 can be heated to room temperature in the main heat exchanger 9, pressurized by a blower, and cooled in the main heat exchanger 9 before being supplied to the high-purity oxygen reboiler. The blower can also be driven by an expansion turbine 11. This increases the liquefaction temperature of the high-pressure nitrogen gas supplied from line L18, allowing for a larger temperature difference between the fluids in the high-purity oxygen reboiler 13 and a smaller heat transfer area. [Example]

[0101] The present invention will be described in detail below using examples and comparative examples. Example 1 A simulation of the air separation unit 101 of the present invention shown in FIG. 1 was carried out using a simulator used in the design of an actual unit. In carrying out the simulation, the maximum amount of recoverable product argon gas GAR was calculated when the air AIR flow rate was set to 100 and 0.42 of the product high-purity liquefied oxygen PLO was recovered. The impurities in the product high-purity liquefied oxygen PLO were set to an argon concentration of 10 ppb or less and a methane concentration of 30 ppm or less. The results are shown in Table 1.

[0102] [Table 1]

[0103] <Comparative Example 1>

[0104] A simulation was carried out on an air separation unit 103 of Comparative Example 1 shown in FIG. 3 as a conventional air separation unit using a simulator used in the design of an actual unit. Here, FIG. 3 is a schematic diagram showing the air separation unit of Comparative Example 1. The air separation unit 103 of Comparative Example 1 shown in FIG. 3 differs from the air separation unit 101 of the first embodiment shown in FIG. 1 described above in that the methane-free argon-enriched oxygen gas separated in the high-purity oxygen column 12 is returned via the methane-free argon-enriched oxygen gas return line L16. Specifically, the air separation unit 10 3 is configured to return the methane-free argon-enriched oxygen gas separated in the high-purity oxygen tower 12 to the same vertical position as the output position of the methane-free argon-enriched liquefied oxygen. Note that in Figure 3, for reasons of drawing, the return position of the methane-free argon-enriched oxygen gas to the argon tower 6 (i.e., the connection position of the argon tower 6 and the methane-free argon-enriched oxygen gas return line L16) is drawn to be slightly lower than the output position of the methane-free argon-enriched liquefied oxygen (the connection position of the argon tower 6 and the line L15), but in reality, the return position of the methane-free argon-enriched oxygen gas and the output position of the methane-free argon-enriched liquefied oxygen are at the same vertical position.

[0105] In carrying out the simulation of the air separation unit 103 of Comparative Example 1, the maximum amount of recoverable product argon gas GAR was calculated when the flow rate of AIR was set to 100 and 0.42 of product high-purity liquefied oxygen PLO was recovered, as in Example 1. The impurities in the product high-purity liquefied oxygen PLO were set to an argon concentration of 10 ppb or less and a methane concentration of 30 ppm or less.

[0106] <Result> As shown in Table 1 above, the simulation results show that the conventional air separation unit, air separation unit 103 of Comparative Example 1, had a product argon gas flow rate of 0.75. On the other hand, the air separation unit 101 of Example 1 improved the product argon gas flow rate to 0.77, and was able to recover more product argon gas than the air separation unit 103 of Comparative Example 1. [Explanation of symbols]

[0107] 1...Air compressor 2...Air precooler 3...Air purifier 4...High-pressure tower 5...Low pressure tower 6...Argon tower 7...Nitrogen condenser 8...Argon condenser 9…Main heat exchanger 10…supercooler 11...Expansion turbine 12...High-purity oxygen tower 13...High purity oxygen reboiler 101...Air separation unit of the first embodiment 102...Air separation unit according to a second embodiment 103...Conventional air separation unit L1~L7, L9, L11~L15, L18~L21...lines L8: Argon-enriched oxygen gas supply line L10: Product argon recovery line L16: Methane-free argon-enriched oxygen gas return line L17...High-purity oxygen recovery line V1~V4...Reducing valve

Claims

1. a high-pressure column that compresses, pre-cools, purifies, and cools air containing oxygen, nitrogen, and argon to obtain low-temperature feed air, and then performs low-temperature distillation to separate the air into high-pressure nitrogen gas and high-pressure liquefied air; a low-pressure column for subjecting the low-pressure liquefied air obtained by reducing the pressure of the high-pressure liquefied air to cryogenic distillation and separating the low-pressure liquefied air into low-pressure nitrogen gas, low-pressure liquefied oxygen, and argon-enriched oxygen gas; an argon-enriched oxygen gas supply line for supplying the argon-enriched oxygen gas to an argon column described below; an argon column that performs cryogenic distillation of the argon-enriched oxygen gas supplied from the argon-enriched oxygen gas supply line to separate the argon gas, argon-enriched liquefied oxygen, and methane-free argon-enriched liquefied oxygen; a high-purity oxygen tower that extracts a portion of the methane-free argon-enriched liquefied oxygen produced in the argon tower from an intermediate portion of the argon tower and separates the portion into high-purity liquefied oxygen and methane-free argon-enriched oxygen gas by cryogenic distillation; a nitrogen condenser that performs indirect heat exchange between the high-pressure nitrogen gas and the low-pressure liquefied oxygen to liquefy the high-pressure nitrogen gas to produce high-pressure liquefied nitrogen and vaporize the low-pressure liquefied oxygen to produce low-pressure oxygen gas; an argon condenser that performs indirect heat exchange between the argon gas and low-pressure liquefied air obtained by reducing the pressure of the high-pressure liquefied air, thereby liquefying the argon gas to produce liquefied argon and vaporizing the low-pressure liquefied air to produce low-pressure air; a high-purity oxygen reboiler for vaporizing the high-purity liquefied oxygen to produce high-purity oxygen gas; a product argon recovery line for recovering at least one of a portion of the argon gas and a portion of the liquefied argon as a product; a high-purity oxygen product recovery line that recovers at least one of a portion of the high-purity liquefied oxygen and a portion of the high-purity oxygen gas as a product; an air separation apparatus comprising: a methane-free argon-enriched oxygen gas return line for returning the methane-free argon-enriched oxygen gas to any one of the argon column, the low-pressure column, and the argon-enriched oxygen gas supply line; an air separation apparatus, wherein when the methane-free argon-enriched oxygen gas return line returns the methane-free argon-enriched oxygen gas to either the argon column or the low-pressure column, the return position is vertically lower than the output position of the methane-free argon-enriched liquefied oxygen; and when the methane-free argon-enriched oxygen gas is returned to the argon-enriched oxygen gas supply line, the return position is the position where the argon-enriched oxygen gas supply line connects to the argon column and is vertically lower than the output position of the methane-free argon-enriched liquefied oxygen.

2. 2. The air separation unit according to claim 1, wherein the high-purity liquefied oxygen and the high-pressure nitrogen gas are indirectly heat exchanged in the high-purity oxygen reboiler to vaporize the high-purity liquefied oxygen to produce high-purity oxygen gas and to liquefy the high-pressure nitrogen gas to produce high-pressure liquefied nitrogen.

3. 3. The air separation unit of claim 1, wherein the methane-free argon-enriched oxygen gas return line is connected to the argon column.

4. 3. The air separation unit of claim 1, wherein the methane-free argon-enriched oxygen gas return line is connected to the low-pressure column.

5. 3. The air separation unit according to claim 1, wherein the methane-free argon-enriched oxygen gas return line is connected to the argon-enriched oxygen gas supply line.

6. a high-pressure separation step in which low-temperature feed air obtained by compressing, pre-cooling, purifying, and cooling air containing oxygen, nitrogen, and argon is subjected to low-temperature distillation to separate the air into high-pressure nitrogen gas and high-pressure liquefied air; a low-pressure separation step in which the low-pressure liquefied air obtained by reducing the pressure of the high-pressure liquefied air is subjected to low-temperature distillation to separate it into low-pressure nitrogen gas, low-pressure liquefied oxygen, and argon-enriched oxygen gas; an argon-enriched oxygen gas supply step of supplying the argon-enriched oxygen gas to an argon column; an argon separation step of cryogenically distilling the argon-enriched oxygen gas to separate it into argon gas, argon-enriched liquefied oxygen, and methane-free argon-enriched liquefied oxygen; a high-purity oxygen separation step in which a portion of the methane-free argon-enriched liquefied oxygen produced in the argon separation step is withdrawn from an intermediate portion of the argon column and separated into high-purity liquefied oxygen and methane-free argon-enriched oxygen gas by low-temperature distillation; a nitrogen condensation step in which the high-pressure nitrogen gas is indirectly heat exchanged with the low-pressure liquefied oxygen to liquefy the high-pressure nitrogen gas to produce high-pressure liquefied nitrogen and the low-pressure liquefied oxygen to vaporize the low-pressure liquefied oxygen to produce low-pressure oxygen gas; an argon condensation step in which the argon gas is indirectly heat exchanged with low-pressure liquefied air obtained by reducing the pressure of the high-pressure liquefied air to liquefy the argon gas and generate liquefied argon, and the low-pressure liquefied air is vaporized to generate low-pressure air; a high-purity oxygen vaporization step of vaporizing the high-purity liquefied oxygen to produce high-purity oxygen gas; a product argon recovery step of recovering at least one of a portion of the argon gas and a portion of the liquefied argon as a product; a high-purity oxygen product recovery step of recovering at least one of a portion of the high-purity liquefied oxygen and a portion of the high-purity oxygen gas as a product; the methane-free argon-enriched oxygen gas is used as part of the raw material for either the argon separation step or the low-pressure separation step, and the raw material is supplied from a position vertically lower than the position from which the methane-free argon-enriched liquefied oxygen is extracted.

7. 7. The air separation method according to claim 6, wherein in the high-purity oxygen vaporization step, the high-purity liquefied oxygen and the high-pressure nitrogen gas are indirectly heat exchanged to vaporize the high-purity liquefied oxygen to produce high-purity oxygen gas, and the high-pressure nitrogen gas is liquefied to produce high-pressure liquefied nitrogen.

8. 8. The air separation method according to claim 6, wherein the methane-free argon-enriched oxygen gas returned in the methane-free argon-enriched oxygen gas returning step is used as part of the raw material for the argon separation step.

9. 8. The air separation method according to claim 6, wherein the methane-free argon-enriched oxygen gas returned in the methane-free argon-enriched oxygen gas returning step is used as part of the feedstock for the low-pressure separation step.

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