air separation unit

Abstract

An air separation unit capable of maintaining the amount and purity of nitrogen produced even when the oxygen concentration in the feed air increases. [Solution] Air separation unit A comprises a main heat exchanger 1, a high-pressure rectification column 2, a nitrogen condenser 3, a low-pressure rectification column 4, a crude argon rectification column 5, a crude argon condenser 6, a subcooler 7, an expansion turbine 8, a nitrogen compressor 9, an air purification unit 10, an external liquid nitrogen supply source 11, and a control unit 12. The recycled nitrogen flow rate is adjusted by adjusting recycle nitrogen flow control valve V2 based on the flow rate and oxygen concentration of the feed air, thereby controlling the production volume and purity of the product nitrogen gas. Specifically, a portion of the product nitrogen is recycled to a cold box, and the rectification process is improved to increase the nitrogen recovery rate, and the recycled nitrogen flow rate is controlled based on the oxygen concentration in the feed air, the oxygen concentration in the rectification section of the high-pressure rectification column, and / or the oxygen concentration in the rectification section of the low-pressure rectification column.

Description

[Technical Field]

[0001] The present invention relates to an air separation unit that can maintain the production volume and purity of product nitrogen gas even when the oxygen concentration in the feed air increases. [Background technology]

[0002] In a cryogenic air separation unit, feed air is compressed by a compressor, cooled in a cooler, and then adsorbed and removed of water and carbon dioxide in an adsorption tower (feed air purification unit), before being supplied to a separator called a cold box. A rectification tower is installed inside the cold box, and the feed air is rectified to obtain product gases of nitrogen, argon, and oxygen. Gas components that are not recovered as products are discharged from the cold box as waste gas, but because they are dry, they are used as gas (regeneration gas) to regenerate the adsorbent installed in the adsorption tower.

[0003] It is known that the composition of feed air varies depending on the operating conditions of the apparatus. For example, Patent Document 1 points out that the nitrogen concentration in the feed air increases when the nitrogen component in the regeneration gas discharged from the adsorption tower is re-introduced into the feed air compressor. To address this problem, it is disclosed that an oxygen analyzer is installed in the feed air supply path, and a liquid oxygen buffer (liquid oxygen storage tank) is installed, and the amount of feed air and the amount of oxygen or nitrogen recovered are controlled.

[0004] Patent Document 2 discloses a method for maintaining the purity of the product nitrogen by extracting waste nitrogen (low-purity nitrogen gas) from the intermediate stage of the low-pressure rectification column, but the nitrogen recovery rate may be adversely affected by the amount of waste nitrogen.

[0005] Patent Document 3 discloses a method for improving rectification by extracting a portion of the low-pressure product nitrogen as recycled nitrogen for refrigeration generation and supplying a portion of the recycled nitrogen used in the expansion turbine to the high-pressure rectification column. [Prior art documents] [Patent documents]

[0006] [Patent Document 1] Japanese Patent No. 7339929 [Patent Document 2] Japanese Utility Model No. 3249640 [Patent Document 3] Japanese Patent No. 7564517 Summary of the Invention [Problem to be solved by the invention]

[0007] However, air separation units designed for the semiconductor industry are designed to maximize nitrogen production, and the regeneration gas (waste gas) sent to the adsorption tower is often rich in oxygen. Therefore, if the regeneration gas discharged from the adsorption tower is taken in again as feed air, the increase in oxygen concentration will reduce the nitrogen concentration in the feed air, resulting in a decrease in nitrogen production or a decrease in purity. To address this issue, if a liquid nitrogen buffer is used instead of the liquid oxygen buffer used in Patent Document 1, the cost is high because of the energy required to liquefy nitrogen gas. In addition, since the demand for product oxygen in air separation units for the semiconductor industry is limited, there is no need to control the amount of feed air and the amount of oxygen recovered. The method of controlling the amount of feed air and the amount of recovered nitrogen in Patent Document 1 aims to deal with an increase in the nitrogen concentration in the feed air and an overshoot in the amount of recovered nitrogen, which is contrary to maximizing the amount of nitrogen produced.

[0008] SUMMARY OF THE INVENTION An object of the present invention is to provide an air separation unit that can maintain the production volume and purity of product nitrogen even when the oxygen concentration in the feed air increases. [Means for solving the problem]

[0009] The air separation unit (A) of the present disclosure comprises: an air purification device (10) having an adsorption bed for purifying raw air; a raw air flow meter (FI1) for measuring the flow rate of the purified raw air; A first oxygen concentration meter (AIC1) that measures the oxygen concentration of the purified raw air; a main heat exchanger (1) into which purified raw air is introduced from a hot end and discharged from a cold end; a higher pressure rectification column (2) into which feed air discharged from the main heat exchanger is introduced; a nitrogen condenser (3) for condensing the vapor stream introduced from the higher pressure rectification column (2) and for drawing out a reflux liquid; a low-pressure rectification column (4) into which the oxygen-enriched liquid withdrawn from the bottom of the high-pressure rectification column (2) is introduced; an oxygen gas outlet pipe (L331) for extracting oxygen gas from the gas phase of the nitrogen condenser (3); a crude argon rectification column (5) into which argon feed gas derived from an intermediate portion of the rectification section of the low-pressure rectification column (4) is introduced; a crude argon condenser (6) for condensing the vapor stream introduced from the crude argon rectification column (5) and discharging the condensed vapor as reflux; a product nitrogen gas outlet pipe (L44) for discharging the product nitrogen gas from the top (44) of the low-pressure rectification column (4); a recycle nitrogen pipe (L441) branching from the product nitrogen gas outlet pipe (L44), for delivering a part of the product nitrogen gas as recycled nitrogen gas and returning it to the top of the high-pressure rectification column (2); a nitrogen compressor (9) provided in the recycled nitrogen piping (L441) for compressing recycled nitrogen gas; a recycle nitrogen flow meter (FI2) provided downstream of the nitrogen compressor (9); a recycle nitrogen flow control valve (V2) provided downstream of the recycle nitrogen flow meter (FI2); and a control unit (12) for controlling the recycle nitrogen flow control valve (V2) based on the oxygen concentration measured by the first oxygen concentration meter (AIC1), the feed air flow rate measured by the feed air flow meter (FI1), and the nitrogen flow rate measured by the recycle nitrogen flow meter (FI2), thereby adjusting the flow rate of the recycle nitrogen gas returned to the upper part (23) of the high-pressure rectification column (2).

[0010] According to the above configuration, the control unit (12) evaluates the oxygen content of the feed air supplied to the air separation unit (A) based on the oxygen concentration of the first oxygen concentration meter (AIC1) and the feed air flow meter (FI1), and controls the recycle nitrogen flow control valve (V2) to achieve the calculated optimum recycle nitrogen flow rate. This adjusts the amount of recycle nitrogen returned to the higher-pressure rectification column (2), optimizing the amount of reflux liquid in the higher-pressure rectification column (2), improving the rectification process and maintaining the production volume and purity of the product nitrogen gas. The oxygen concentration meter may be a general oxygen concentration meter, such as a zirconia type.

[0011] The recycled nitrogen pipe (L441) may be merged with the vapor flow inlet pipe (L23) that introduces the vapor flow from the higher pressure rectification column (2) into the nitrogen condenser (3), rather than with the upper part of the higher pressure rectification column (2). In this case, the number of piping nozzles connected to the higher pressure rectification column (2) can be reduced, thereby simplifying the piping. Depending on the substance state of the recycled nitrogen gas (gas, liquid, or gas-liquid mixture), it may be possible to select whether to return the recycled nitrogen gas to the upper part of the higher pressure rectification column (2) or to the vapor flow inlet pipe (L23).

[0012] The air separation unit (A) of the present disclosure may further include a second oxygen concentration meter (AIC2) for measuring the oxygen concentration in an intermediate portion of the rectifying section (20) of the high-pressure rectifier (2) and / or a third oxygen concentration meter (AIC3) for measuring the oxygen concentration in an intermediate portion of the rectifying section (42) of the low-pressure rectifier (4).

[0013] According to the above configuration, the control unit (12) adjusts the aperture of the recycle nitrogen flow control valve (V2) based on the oxygen concentration in the intermediate portion of the rectifying section (20) of the high-pressure rectifying column (2) and / or the oxygen concentration in the intermediate portion of the rectifying section (42) of the low-pressure rectifying column (4), thereby optimizing the flow rate of the recycle nitrogen gas. The control unit (12) can respond quickly by knowing the oxygen concentration inside the rectifying column as well as the oxygen concentration of the feed air.

[0014] The air separation unit (A) of the present disclosure may further include an external liquid nitrogen supply source (11) that supplies liquid nitrogen (LIN) to the low-pressure rectification column (4), and the control unit (12) may control the supply of the liquid nitrogen based on the oxygen concentration measured by the first oxygen concentration meter (AIC1).

[0015] According to the above-described configuration, liquid nitrogen is supplied from the external liquid nitrogen supply source (11) to the low-pressure rectification column (4), thereby improving the rectification process of the low-pressure rectification column (4) and supplying the necessary refrigeration to the air separation unit (A), thereby maintaining the production amount and purity of the product nitrogen gas. A liquid level indicator (LIC) may be provided at the bottom (21) of the higher pressure rectification column (2) to monitor the supply of liquid nitrogen so as not to be excessively supplied.

[0016] The control unit (12) of the air separation unit (A) of the present disclosure may adjust the amount of feed air introduced based on an increase or decrease in the amount of product nitrogen gas produced. Note that the feed air line (L1) may be provided with a flow control valve (not shown) between the feed air purifier (10) and the main heat exchanger (1) for adjusting the flow rate of the feed air.

[0017] According to the above configuration, the control unit (12) adjusts both the recycled nitrogen flow rate and the flow rate of the introduced feed air, thereby enabling precise control of the purity of the product nitrogen gas and the production amount.

[0018] The air separation unit (A) Various measuring instruments such as flow rate measuring instruments, pressure measuring instruments, temperature measuring instruments, and liquid level measuring instruments, Various valves such as control valves and gate valves, Piping that connects each element; may have

[0019] (effect) (1) The control unit evaluates the amount of oxygen in the feed air supplied to the air separation unit based on the oxygen concentration measured by the first oxygen concentration meter and the flow rate measured by the feed air flow meter, and controls the recycle nitrogen flow control valve to achieve the calculated optimal recycle nitrogen flow rate. This adjusts the amount of recycled nitrogen gas returned to the high-pressure rectification column, optimizing the amount of reflux liquid in the high-pressure rectification column, improving the rectification process and maintaining the production volume and purity of the product nitrogen gas. Since the oxygen concentration in nitrogen gas produced for the semiconductor industry is at the sub-ppb level, analyzing the oxygen concentration in the nitrogen gas product tends to be expensive. However, the present invention, which measures the oxygen concentration in the feed air, is relatively inexpensive because it can use a commonly used oxygen concentration meter. (2) By returning the recycled nitrogen to the vapor flow inlet pipe connected to the nitrogen condenser instead of to the top of the high-pressure rectification column, the number of nitrogen piping nozzles connected to the high-pressure rectification column can be reduced, thereby simplifying the piping. (3) By installing an oxygen concentration meter in the middle of the rectification section of the high-pressure rectification column and / or the low-pressure rectification column and measuring the oxygen concentration not only in the feed air but also in the rectification environment inside the rectification column, the control unit can quickly respond to changes in the rectification environment inside the rectification column. (4) By supplying liquid nitrogen from an external liquid nitrogen supply source to the top of the low-pressure rectification column, the rectification of the low-pressure rectification column can be improved, and the refrigeration required for the air separation unit can be supplied to maintain the production volume and purity of the product nitrogen gas. Furthermore, by installing a liquid level gauge (LIC) at the bottom of the high-pressure rectification column, excessive supply of liquid nitrogen can be prevented. (5) The control unit adjusts both the recycled nitrogen flow rate and the amount of raw air introduced, allowing for precise control of the production amount and purity of the product nitrogen gas. [Brief explanation of the drawings]

[0020] [Figure 1] 1 is a diagram showing an air separation unit of a first embodiment. DETAILED DESCRIPTION OF THE INVENTION

[0021] Several embodiments of the present disclosure will be described below. The embodiments described below are examples of the present disclosure. The present disclosure is not limited to the following embodiments and includes various modified forms implemented within the scope of the present disclosure. Note that not all of the configurations described below are necessarily essential configurations of the present disclosure. Upstream and downstream are based on the flow direction of the fluid (liquid, gas).

[0022] (Embodiment 1) An air separation unit A of the first embodiment will be described with reference to FIG. Air separation unit A includes a main heat exchanger 1, a high-pressure rectification column 2, a nitrogen condenser 3, a low-pressure rectification column 4, a crude argon rectification column 5, a crude argon condenser 6, a subcooler 7, an expansion turbine 8, a nitrogen compressor 9, an air purification unit 10, a control unit 12, a first oxygen concentration meter AIC1, a feed air flow meter FI1, and a recycle nitrogen flow meter FI2.

[0023] Feed air is pressurized to a predetermined pressure by a feed air compressor (not shown), and moisture and carbon dioxide are removed in an air purification unit 10. The main heat exchanger 1 cools the purified feed air introduced from its hot end and discharges it from its cold end. The cooled feed air is introduced into the higher-pressure rectification column 2 via a feed air line L1. A feed air flow meter FI1 and a first oxygen concentration meter AIC1 are provided in the feed air line L1 upstream of the main heat exchanger 1, and measure the flow rate and oxygen concentration of the feed air, respectively.

[0024] The high-pressure rectification column 2 comprises a bottom 21, a rectifying section 20, and an upper section 23. The feed air line L1 is connected to the bottom 21. The oxygen-enriched liquid stored in the bottom 21 is heat-exchanged and cooled in the subcooler 7 via the oxygen-enriched liquid line L21, and then sent to an intermediate section of the rectifying section 42 of the low-pressure rectification column 4. The oxygen-enriched liquid line L21 is equipped with a pressure-reducing valve (not shown). The rectifying section 42 is composed of a first rectifying section 42a, a second rectifying section 42b, a third rectifying section 42c, and a fourth rectifying section 42d. The cooled and depressurized oxygen-enriched liquid is preferably sent between 42b and 42c.

[0025] A portion of the oxygen-enriched liquid that has been heat exchanged and cooled in the subcooler 7 is introduced into the refrigerant reservoir (liquid phase) 62 of the crude argon condenser 6 via the oxygen-enriched liquid branch line L211.

[0026] The nitrogen condenser 3 is provided above the higher-pressure rectification column 2 and comprises a refrigerant reservoir 32 and a gas phase portion 33. A vapor stream of nitrogen gas extracted from the upper portion 23 of the higher-pressure rectification column 2 is introduced into the nitrogen condenser 3 via vapor stream inlet pipe L23, where it is cooled and liquefied by heat exchange with liquid oxygen in the refrigerant reservoir 32, to form liquid nitrogen. A portion of the liquid nitrogen is sent via a liquid nitrogen line L231 branching off from L23 to the subcooler 7, where it is cooled and cooled, and then sent to the upper portion 43 of the lower-pressure rectification column 4. The remainder of the liquid nitrogen returns to the top 23 of the higher-pressure rectification column 2 as reflux.

[0027] The low-pressure rectification column 4 comprises a bottom 41, a rectification section 42, an upper section 43, and a top section 44. Oxygen gas discharged from the gas phase section 33 of the nitrogen condenser 3 is introduced into the bottom 41 of the low-pressure rectification column 4 via an evaporated gas line L33 and returned to the nitrogen condenser 3. The bottom 41 may also serve as the refrigerant reservoir section 32 of the nitrogen condenser 3.

[0028] Argon feed gas discharged from the rectifying section 42 of the low-pressure rectifying column 4, preferably between the first rectifying section 42a and the second rectifying section 42b, is introduced into the bottom 51 of the crude argon rectifying column 5 via the argon feed gas line L42. Nitrogen gas discharged from the top 44 of the low-pressure rectifying column 4 passes through the subcooler 7 and the main heat exchanger 1 via the product nitrogen gas discharge pipe L44, and is taken out as low-pressure product nitrogen gas (GAN).

[0029] The recycled nitrogen pipe L441 is a pipe that branches off from the product nitrogen gas outlet pipe L44 after passing through the main heat exchanger 1. The recycled nitrogen pipe L441 discharges a portion of the low-pressure product nitrogen gas (GAN) as recycled nitrogen gas. The recycled nitrogen gas is compressed by the nitrogen compressor 9, cooled by the main heat exchanger 1, and then introduced into the upper portion 23 of the higher-pressure rectification column 2. Upstream of the main heat exchanger 1 on the recycled nitrogen pipe L441, a nitrogen compressor 9, a recycled nitrogen flow meter FI2, and a recycled nitrogen flow control valve V2 are provided.

[0030] The crude argon rectification column 5 comprises a bottom 51, a rectification section 52, and a top 53. The bottom fluid line L51 of the crude argon rectification column 5 is a line through which a bottom fluid (oxygen-enriched liquid) is drawn from the bottom 51 of the crude argon rectification column 5 and is introduced below the outlet position of the argon feed gas line L42 of the rectification section 42 of the low-pressure rectification column 4. The vapor stream of the crude argon rectification column 5 is introduced into the crude argon condenser 6 via L53, where it is condensed and returned to the crude argon rectification column 5 as reflux liquid. The product argon outlet line L531 is a line through which a portion of the reflux liquid liquid argon (LAR) that is returned to the top 53 of the crude argon rectification column 5 is drawn.

[0031] The crude argon condenser line L63 is a line for extracting gas from the gas phase section 63 of the crude argon condenser 6 and introducing it into the rectification section 42 of the low-pressure rectification column 4 above the introduction position of the oxygen-enriched liquid line L21. The gas from the crude argon condenser 6 is preferably introduced between the second rectification section 42b and the third rectification section 42c of the low-pressure rectification column 4, and above the position where the oxygen-enriched liquid is introduced.

[0032] The oxygen-enriched liquid discharged from the bottom 21 of the higher-pressure rectification column 2 is introduced into the hot end of the subcooler 7 and discharged from the cold end. A portion of the liquid nitrogen discharged from the nitrogen condenser 3 is introduced into the hot end of the subcooler 7 via L231, discharged from the cold end, and introduced into the upper portion 43 of the lower-pressure rectification column 4. The product nitrogen gas discharged from the top 44 of the lower-pressure rectification column 4 is introduced into the cold end of the subcooler 7, discharged from the hot end, and heated in the subcooler 7.

[0033] The oxygen gas outlet pipe L331 is a line branched off from the evaporation gas line L33. A portion of the oxygen gas discharged from the nitrogen condenser 3 is introduced into the main heat exchanger 1 via L331. A first control valve V1 is provided in L331 and is used to adjust the flow rate of the oxygen gas.

[0034] The expansion turbine 8 introduces the oxygen gas from the oxygen gas outlet pipe L331 into the cold end of the main heat exchanger 1, discharges it from the intermediate section, and then expands and cools it. The expanded and cooled gas is introduced back into the cold end of the main heat exchanger 1, discharged from the warm end, and discharged as waste gas. The cooled oxygen gas may be used as a regeneration gas for the adsorbent in the air purification device 10 before being discharged as waste gas.

[0035] When the oxygen concentration in the raw air increases or decreases, the control unit 12 controls the opening of the recycled nitrogen flow control valve V2 to adjust the flow rate of the recycled nitrogen gas so that the oxygen concentration of the product nitrogen gas falls within a predetermined concentration range, thereby maintaining the purity of the product nitrogen gas. When the oxygen concentration in the feed air measured by the first oxygen concentration meter AIC1 increases, the opening of the recycle nitrogen flow control valve V2 is increased to increase the amount of recycled nitrogen gas returning to the high-pressure rectification column 2, thereby improving the rectification process. The control unit 12 also monitors the flow rate of the recycled nitrogen gas using a recycled nitrogen flow meter FI2 so that the production amount of the product nitrogen gas falls within a preset range. The preset purity and production amount of the product nitrogen gas may be determined arbitrarily by the supply destination.

[0036] The second oxygen concentration meter AIC2 measures the oxygen concentration in an intermediate portion of the rectification section 20 of the high-pressure rectification column 2. The third oxygen concentration meter AIC3 measures the oxygen concentration in an intermediate portion of the rectification section 42 of the low-pressure rectification column 4. By further providing the second oxygen concentration meter AIC2 and / or the third oxygen concentration meter AIC3, the air separation unit A inputs not only the oxygen concentration of the feed air but also the oxygen concentration inside the high-pressure rectification column 2 and / or the low-pressure rectification column 3 into the control unit 12, and quickly adjusts the recycle nitrogen gas flow rate in response to changes in the rectification environment.

[0037] An external liquid nitrogen supply source 11 supplies liquid nitrogen (LIN) to the low-pressure rectification column 4. When the reflux liquid in the rectification column decreases, the impurity concentration, i.e., the oxygen concentration, of the product nitrogen gas increases. Therefore, the control unit 12 adjusts the recycled nitrogen gas returned to the high-pressure rectification column 2 based on the oxygen concentration of the feed air measured by the first oxygen concentration meter AIC1, and also supplies liquid nitrogen (LIN) from the external liquid nitrogen supply source 11 to the low-pressure rectification column 4, thereby controlling the oxygen concentration of the product nitrogen gas. To avoid an oversupply of liquid nitrogen, the control unit 12 monitors the liquid level at the bottom 21 of the high-pressure rectification column 2 using a level gauge LIC, and adjusts the third valve V3 provided on the liquid nitrogen supply line L11 to increase or decrease the amount of liquid nitrogen supplied.

[0038] The control unit 12 adjusts the amount of feed air introduced based on an increase or decrease in the amount of product nitrogen gas produced. The flow rate of the feed air is controlled using a flow control valve (not shown) provided in the feed air line L1 between the feed air purification device 10 and the main heat exchanger 1.

[0039] (Another embodiment) (1) Although not specifically stated, pressure regulators, flow rate controllers, etc. may be installed in each piping line to adjust the pressure or flow rate. (2) Although not specifically stated, control valves, gate valves, etc. may be installed on each piping line. (3) Although not specifically stated, each tower may be equipped with a pressure regulator, a temperature measuring device, etc., for pressure or temperature regulation.

[0040] (Example) A simulation was performed using the configuration of embodiment 1 (FIG. 1). Feed air temperature: 20℃, 9.4barA, 1000Nm 3 Assume that the oxygen concentration in the feed air increased from 21.0% to 21.2% (oxygen ratio 1%, feed air ratio 0.2%) at / h. 426 Nm from the top of the high-pressure rectification column 2 3 / h of liquid nitrogen, 574 Nm from the bottom 21 of the high-pressure rectification column 2 3 / h of oxygen-enriched liquid is discharged, the oxygen concentration in the oxygen-enriched liquid before the increase in oxygen concentration is 36.5%, but after the increase in oxygen concentration, it increases to 36.9%. The discharged oxygen-enriched liquid is depressurized by a pressure reducing valve (not shown) in the oxygen-enriched liquid line L21 and then introduced into the rectification section 42 of the low-pressure rectification column 4. When the oxygen concentration increases, the amount of oxygen components that migrate to the gas phase during depressurization increases, and oxygen components are more likely to be mixed into the product nitrogen gas (GAN) discharged from the top 44 of the low-pressure rectification column 4. In this invention, in order to maintain the production volume of product nitrogen gas, recycled nitrogen gas of 5.7 Nm is used to offset the increase in oxygen concentration in the oxygen-enriched liquid. 3 / h is introduced into the higher-pressure rectification column 2. By doing so, the amount of liquid nitrogen (reflux liquid) obtained in the higher-pressure rectification column 2 is increased, and the oxygen concentration in the product nitrogen gas can be reduced by diluting the oxygen component of the oxygen-enriched liquid inside the higher-pressure rectification column 2 or by increasing the reflux liquid at the top 44 of the lower-pressure rectification column 4. The increase in liquid nitrogen (reflux liquid) contributes to an improvement in the recovery rate of the product nitrogen gas, and can therefore compensate for the decrease in the product nitrogen gas resulting from the recycling of nitrogen. [Explanation of symbols]

[0041] 1 Main heat exchanger 2 High-pressure rectification column 3 Nitrogen condenser 4 Low-pressure rectification column 5. Crude argon rectification column 6. Crude Argon Condenser 7 Subcooler 8. Expansion turbine 9 Nitrogen Compressor 10 Air Purification Device 11 External liquid nitrogen supply 12 Control Unit

Claims

1. an air purification device including an adsorption bed for purifying the feed air; a feed air flow meter for measuring the flow rate of the purified feed air; a first oxygen concentration meter for measuring the oxygen concentration of the purified feed air; a main heat exchanger into which purified raw air is introduced from a hot end and discharged from a cold end; a higher pressure rectification column into which feed air discharged from the main heat exchanger is introduced; a nitrogen condenser for condensing a vapor stream introduced from the higher pressure rectification column and for delivering a reflux liquid; a low-pressure rectification column into which the oxygen-enriched liquid withdrawn from the bottom of the high-pressure rectification column is introduced; an oxygen gas outlet pipe for outletting oxygen gas from the gas phase of the nitrogen condenser; a crude argon rectification column into which argon feed gas derived from an intermediate portion of the rectification section of the low-pressure rectification column is introduced; a crude argon condenser for condensing the vapor stream introduced from the crude argon rectification column and delivering the condensed vapor as reflux; a product nitrogen gas outlet pipe for discharging product nitrogen gas from the top of the low-pressure rectification column; a recycle nitrogen pipe branching from the product nitrogen gas outlet pipe, for delivering a portion of the product nitrogen gas as recycled nitrogen gas and returning the product nitrogen gas to an upper portion of the high-pressure rectification column; a nitrogen compressor provided in the recycled nitrogen piping and compressing the recycled nitrogen gas; a recycle nitrogen flow meter provided downstream of the nitrogen compressor; a recycle nitrogen flow control valve provided downstream of the recycle nitrogen flow meter; a control unit that controls the recycled nitrogen flow control valve based on the oxygen concentration measured by the first oxygen concentration meter, the feed air flow rate measured by the feed air flow meter, and the nitrogen flow rate measured by the recycled nitrogen flow meter to adjust the flow rate of the recycled nitrogen gas returned to an upper part of the high-pressure rectification column; An air separation unit comprising:

2. 2. The air separation unit according to claim 1, further comprising a second oxygen concentration meter for measuring the oxygen concentration in an intermediate portion of the rectification section of the higher-pressure rectification column and / or a third oxygen concentration meter for measuring the oxygen concentration in an intermediate portion of the rectification section of the lower-pressure rectification column.

3. an external liquid nitrogen source for supplying liquid nitrogen to the lower pressure rectification column; 3. The air separation unit according to claim 1, wherein the control unit controls the supply of the liquid nitrogen based on the oxygen concentration measured by the first oxygen concentration meter.

4. 3. The air separation unit according to claim 1, wherein the control unit adjusts the amount of feed air introduced based on an increase or decrease in the amount of nitrogen gas produced.

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