JPH1163812A - Manufacture and device for low-purity oxygen - Google Patents

Abstract

PROBLEM TO BE SOLVED: To recover reflux liquid required for high-pressure distillation at high recovery percentage, by supplying fluid abundant in liquefied nitrogen as the reflux liquid of a low-pressure tower, and discharging oxygen and nitrogen separated in a low-pressure tower, heating it by the heat exchange with material air, and recovering them as oxygen gas and nitrogen gas. SOLUTION: Nitrogen gas for boosting introduced into an expansion turbine 12 is lowered in temperature, depressurized by adiabatic expansion to generate cold required for process, and is led to a condensing evaporator 13 provided at a low-pressure middle part 7b through a path 45, evaporates the falling liquid flowing down within the low-pressure tower 7, also liquefies itself into liquefied nitrogen, and passes through a path 46. The nitrogen gas is depressurized into 3.7 kgf/cm<2> abs with a valve 47a, joins the path 30 for liquefied nitrogen returning from a high-pressure tower 5, and is led into the apex of the low-pressure tower 7. What is more, the pressure of the expansive nitrogen gas coming out of the expansion turbine 12 is set, so that the temperature difference between the boiling-point temperature of the falling liquid in the condensing evaporator 13 and the liquefaction temperature of the expansive nitrogen gas may come to the specified temperature.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method and an apparatus for producing low-purity oxygen, and more particularly, to a method in which compressed, purified, and cooled raw material air is introduced into a double distillation column and subjected to distillation and separation under a relatively high pressure. The present invention relates to a method and an apparatus for producing low-purity oxygen mainly for recovering low-purity oxygen (99% O ₂ or less) as a product.

[0002]

2. Description of the Related Art In recent years, low-purity oxygen has been used in integrated coal gasification combined cycle power generation equipment and heavy residue gasification power generation equipment, and further demand is expected in the future. I have. Since these facilities consume a large amount of oxygen, it is desired to reduce the production cost of oxygen in particular.

In order to reduce the production cost of low-purity oxygen in these power generation facilities, there is a method of supplying compressed air of the power generation facility as a part of raw air introduced into an air liquefaction / separation apparatus for producing low-purity oxygen. Proposed. Since the pressure of the compressed air supplied from the power generation equipment is higher than that of the raw material air used in a general air liquefaction / separation apparatus, it is necessary to effectively use this relatively high pressure.

[0004] As one method of effectively utilizing a high pressure, distillation separation is performed at a higher pressure than in a conventional air liquefaction separator, and the resulting product is recovered at a higher pressure. There is a method in which low-purity oxygen gas of a product is directly sent to a consuming facility at a pressure recovered from an air liquefaction / separation device without recompression.

In the method of distillation under high pressure (high pressure distillation), the value of the relative volatility of nitrogen to oxygen is smaller than that in the method of liquefaction and rectification under low pressure (low pressure distillation). The separation effect decreases. In order to compensate for this, it is necessary to increase the reflux ratio (the ratio between the descending liquid (reflux liquid) and the ascending gas), and the amount of reflux liquid at the top of the low-pressure column of the double distillation column must be larger than that in low-pressure distillation. Must.

On the other hand, in the process of recovering low-purity oxygen, it is not necessary to strictly separate argon and oxygen. The descending liquid amount and the rising gas amount in the lower part of the tower can be reduced.

[0007] For this purpose, US Pat. No. 3,210,951 discloses that a part of the raw air is heat-exchanged with liquefied oxygen at the bottom of the low-pressure column, and the liquefied oxygen is vaporized to generate a rising gas in the low-pressure column. Heat exchange between the nitrogen gas at the top of the high-pressure column and the descending liquid flowing down the middle of the low-pressure column, and liquefying the nitrogen gas by vaporizing the descending liquid, and using this liquefied nitrogen as the reflux liquid of the high-pressure tower and the low-pressure tower The process used is described.

In this process, the rising gas and the descending liquid at the lower part of the low pressure column can be reduced, and the nitrogen gas at the top of the high pressure column is liquefied with the descending liquid flowing down the lower part of the lower pressure column. The operating pressure of the column can be set lower than before, the power of the feed air compressor is reduced, and low-purity oxygen can be produced at low cost. However, since part of the feed air is used for heat exchange with liquefied oxygen at the bottom of the low-pressure column, the amount of gas rising in the high-pressure column decreases, and the amount of liquefied nitrogen liquefied in the main condenser also decreases. Insufficient reflux liquefied nitrogen to the top, reducing product oxygen recovery.

In US Pat. No. 5,080,703, as a method of increasing the reflux liquid, a part of the low-pressure nitrogen extracted from the top of the low-pressure column is compressed by a circulating compressor forming a circulation cycle, and then the high-pressure nitrogen is compressed. A process is described in which liquefaction is performed by heat exchange with oxygen-enriched liquefied air at the bottom of the column, and liquefied nitrogen produced is introduced at the top of the low-pressure column to form a reflux liquid.

This process requires compression power for compressing and circulating low-pressure nitrogen, introduces most of the feed air into the high-pressure column, and liquefies all of the ascending gas in the high-pressure column at the bottom of the low-pressure column. Since the ascending gas is liquefied by exchanging heat with oxygen to vaporize the liquefied oxygen and introduced into the top of the low-pressure column, the amounts of the ascending gas and descending liquid at the lower portion of the low-pressure column cannot be reduced. Therefore, there is a problem in that the processing amount in the high-pressure column is relatively large and the production cost is high even though the process is a process for recovering low-purity oxygen.

In the present invention, therefore, in recovering low-purity oxygen at a high pressure by performing distillation at a relatively high pressure, the amount of ascending gas and the amount of descending liquid at the lower part of the low-pressure column are reduced to reduce the throughput in the high-pressure column. An object of the present invention is to provide a method and an apparatus for producing low-purity oxygen, which can reduce the amount of reflux liquid necessary for high-pressure distillation and recover the product in a high yield, thereby reducing the production cost.

[0012]

Means for Solving the Problems To achieve the above object, the method for producing low-purity oxygen of the present invention comprises at least low-purity oxygen by subjecting raw air to low-temperature distillation in a double distillation facility having a high-pressure column and a low-pressure column. In a method for producing low-purity oxygen in which oxygen is separated and recovered as a product, a step of compressing raw air, a step of pre-cooling the compressed raw air, and removing impurities such as moisture and carbon dioxide from the pre-cooled raw air. A step of purifying, a step of cooling the purified raw air by heat exchange with a fluid obtained by low-temperature distillation, and a nitrogen-enriched fluid by introducing the cooled raw air into the high-pressure column and performing low-temperature distillation. A step of separating the nitrogen-enriched fluid and the oxygen-enriched fluid separated in the high-pressure column into a low-pressure column to separate them into nitrogen and oxygen, Nitrogen A step of extracting a part of the pressurized fluid and heating it by heat exchange with the raw air, a step of pressurizing the heated nitrogen-enriched fluid, and a step of subjecting the pressurized nitrogen-enriched fluid to a fluid obtained by cryogenic distillation. Cooling by heat exchange;
Adiabatic expanding the cooled nitrogen-enriched fluid, liquefying the adiabatic expanded nitrogen-enriched fluid by heat exchange with a descending liquid flowing down the center of the low-pressure column, and liquefying the liquefied nitrogen-enriched fluid. Supplying as a reflux liquid of the low pressure column,
Extracting the oxygen and nitrogen separated in the low-pressure column, raising the temperature by heat exchange with raw material air, and recovering as oxygen gas and nitrogen gas.

Further, in the method for producing low-purity oxygen of the present invention, the pressure of at least a part of the recovered nitrogen gas is increased, and the increased pressure of the nitrogen gas is cooled by heat exchange with a fluid obtained by low-temperature distillation. And liquefying the cooled nitrogen gas by heat exchange with a descending liquid flowing down the middle of the low-pressure column, and introducing the liquefied liquefied nitrogen as a reflux liquid of the low-pressure column. In addition, at least a part of the pressurization of the nitrogen-enriched fluid is performed by using work by adiabatic expansion of the nitrogen-enriched fluid, and at least a part of the raw material air is supplied to an air compressor of a power generation facility. It is characterized by being compressed air supplied from the company.

Further, the apparatus for producing low-purity oxygen of the present invention comprises:
In a low-purity oxygen production apparatus that separates and recovers at least low-purity oxygen as a product by low-temperature distillation of the raw air, a raw air compressor that compresses the raw air, and a pre-cooling facility that pre-cools the compressed raw air, Purification equipment for purifying by removing impurities such as moisture and carbon dioxide from pre-cooled raw air, a main heat exchanger for exchanging the purified raw air with fluid obtained by low-temperature distillation, and cooling with the main heat exchanger High-pressure column that separates the separated feed air into nitrogen and oxygen by cryogenic distillation,
A double distillation column comprising a main condenser and a low-pressure column, a first booster that is withdrawn from the upper part of the high-pressure column and pressurizes the nitrogen gas that has been heated in the main heat exchanger, and is pressurized by the first pressurizer. An expansion turbine that adiabatically expands the nitrogen gas cooled by the main heat exchanger, and a condensing evaporator that liquefies the nitrogen gas adiabatically expanded by the expansion turbine with a descending liquid flowing down the middle of the low-pressure column. A path for supplying liquefied nitrogen liquefied by the condensing evaporator to the upper part of the low-pressure column as a reflux liquid, and a nitrogen gas recovery path for recovering the nitrogen and oxygen separated and generated in the low-pressure column by recovering the temperature in the main heat exchanger. And an oxygen gas recovery path.

Further, in the apparatus for producing low-purity oxygen of the present invention, the condensing evaporator is connected to the low-pressure column at least one theoretical stage from the introduction position of the oxygen-enriched liquefied air extracted from the bottom of the high-pressure column and introduced into the low-pressure column. It was arranged below, and a second booster that boosts a part of the nitrogen gas recovered in the nitrogen gas recovery path, and cooled the nitrogen gas pressurized by the second booster via the main heat exchanger And a path for introducing compressed air from the power generation equipment to a source air outlet path of the source air compressor. At least one of the column and the low-pressure column is a packed distillation column.

[0016]

FIG. 1 is a system diagram showing an example of an apparatus for producing low-purity oxygen according to the present invention. The low-purity oxygen production apparatus S includes, as main components, a raw air compressor 1 for compressing raw air, a precooling facility 2 for precooling raw air after compression, and a raw air after precooling. A purification facility 3 for purifying by removing impurities such as moisture and carbon dioxide;
A dual type comprising a main heat exchanger 4 for cooling purified raw material air with a fluid obtained by low-temperature distillation, a high-pressure column 5, a main condenser 6, and a low-pressure column 7 for separating the cooled raw material air into nitrogen and oxygen. A distillation column 8, a supercooler 9, 10 for cooling the reflux liquid introduced into the low-pressure column 7, a first booster 11 for raising the temperature of the nitrogen gas after the temperature is raised from the high-pressure column 5, and An expansion turbine 12 for adiabatically expanding the pressurized nitrogen gas, a condensing evaporator 13 for heat-exchanging the adiabatic expanded nitrogen gas with a descending liquid flowing down the center of the low-pressure column 7 to liquefy, and compressed air of the power generation equipment G And a compressed air introduction path 22 for introducing a part of the compressed air into a raw air outlet path 21 of the raw air compressor 1.

Next, a substantially equal amount of raw air is supplied from the raw air compressor 1 and the power generation equipment G to the low-purity oxygen producing apparatus S configured as described above, and low-purity oxygen as a product is produced. The method of the present invention will be described on the basis of a process in which most of the process is gaseous and some of the process is liquid.

12.533 kgf /
51000 Nm ³ / h of raw air compressed to ² cm ² abs and compressed air introduction path 2 from power generation equipment G at the same pressure
51500 Nm ³ / h compressed air introduced through the feed line 2 joins the raw air outlet path 21 and
³ / h of raw material air is introduced into the pre-cooling equipment 2. The raw material air cooled to a required temperature in the pre-cooling facility 2 is purified by adsorbing and removing impurities such as water and carbon dioxide contained in the refining facility 3, and then introduced into the main heat exchanger 4 through the passage 23. Then, the heat is exchanged with the low-temperature fluid derived from the double distillation column 8 to be cooled to substantially the dew point temperature, and then introduced into the lower part of the high-pressure column 5 through the path 24.

The raw material air introduced into the high-pressure column 5 rises in the column and makes gas-liquid contact with a descending liquid flowing down in the column, so that nitrogen gas at the top of the column and oxygen-enriched liquefied air at the bottom of the column are brought into contact. Is separated into 7200 extracted from channel 25 at the bottom of the tower
The Nm ³ / h oxygen-enriched liquefied air is cooled to a supercooled state by exchanging heat with the low-temperature nitrogen in the subcooler 9, depressurized to 3.7 kgf / cm ² abs by the valve 26 a through the passage 26, It is introduced as a reflux into the middle part 7b of the low pressure column.

On the other hand, most of the nitrogen gas separated at the upper part of the high-pressure column 5 is led to the main condenser 6 from the path 27 and exchanges heat with liquefied oxygen at the bottom of the low-pressure column 7 to convert the liquefied oxygen. The gas is vaporized to generate ascending gas in the low-pressure column 7, liquefied and led out to the passage 28, and most of the gas is circulated as a reflux liquid.
Returned to the top. The remaining liquefied nitrogen 7200 Nm ³ / h branched from the path 28 and extracted to the path 29 is exchanged with low-temperature nitrogen in the subcooler 10 and cooled to a supercooled state. The pressure is reduced to 3.7 kgf / cm ² abs and introduced into the top of the low-pressure column 7 as a reflux liquid.

In the low-pressure column 7, the reflux liquid and the ascending gas are brought into gas-liquid contact for further distillation, and nitrogen is extracted from the top of the column and oxygen is extracted from the bottom of the column. The nitrogen gas 8 extracted from the top of the low-pressure column 7 to a path 31 constituting a nitrogen gas recovery path
0000 Nm ³ / h is to sequentially cool the refluxed liquefied nitrogen from the upper part of the high-pressure column 5 by the subcooler 10 and the oxygen-enriched liquefied air from the bottom of the high-pressure column 5 by the subcooler 9 through the passage 32. The temperature is raised by further cooling the raw material air in the main heat exchanger 4 through the path 33,
Path 3 at a pressure of 3.5 kgf / cm ² abs and a temperature of 12 ° C
4 is derived as product nitrogen gas GN.

On the other hand, of the oxygen separated at the lower part of the low-pressure column 7, 21550 Nm ³ / h of oxygen gas is extracted to a path 35 constituting an oxygen gas recovery path, and the raw air is cooled by the main heat exchanger 4. And the pressure is increased to 3.6 kgf
/ Cm ² abs, 95% purity from path 36 at 12 ° C
Of the product oxygen gas GO. At the same time, 8
Liquefied oxygen of 00 Nm ³ / h is led out of the passage 37 as product liquefied oxygen LO.

A portion of 33,000 Nm ³ / h of the nitrogen gas separated at the top of the high-pressure column 5 branches off from the path 27 and is withdrawn to the path 40, and is cooled by the main heat exchanger 4 to cool the raw air. The temperature is increased to 12 ° C., introduced into the first booster 11 through the path 41, and increased to 13.3 kgf / cm ² abs. The pressurized nitrogen gas is cooled by the cooler 11a through the passage 42 and further cooled by the main heat exchanger 4 with the low-temperature fluid through the passage 43 to an intermediate temperature. 12 is introduced.

The pressurized nitrogen gas introduced into the expansion turbine 12 is cooled down and cooled by adiabatic expansion to generate refrigeration necessary for the process, and passes through a path 45 to a condensing evaporator 13 provided in the low pressure column middle part 7b. While being guided and vaporizing the descending liquid flowing down in the low-pressure column 7, the liquid itself liquefies and becomes liquefied nitrogen, passes through the path 46, is cooled to a supercooled state by the supercooler 10, passes through the path 47, and passes through the valve 47a. 3.7kgf / cm
^The pressure is reduced to ² abs and merges with the reflux liquefied nitrogen path 30 from the high pressure column 5, and is introduced to the top of the low pressure column 7. The pressure of the expanded nitrogen gas flowing out of the expansion turbine 12 is set so that the temperature difference between the boiling point temperature of the descending liquid in the condensing evaporator 13 and the liquefaction temperature of the expanded nitrogen gas becomes a predetermined temperature. .

As shown in the present embodiment, the nitrogen gas in the upper part of the high-pressure column 5 is led out, pressurized and expanded, and the expanded nitrogen gas and the descending liquid flowing down in the low-pressure column 7 are heated by the condensation evaporator 13. It is exchanged, the descending liquid is vaporized, and the expanded nitrogen gas is liquefied and introduced as a reflux liquid at the top of the low-pressure column. Therefore, the load on the main condenser 6 is reduced, and the low-pressure column which vaporizes and rises in the main condenser 6 The amount of rising gas in the lower part 7a can be reduced, and the amount of descending liquid in the lower part 7a of the low-pressure tower can be reduced by the amount of the descending liquid flowing down the low-pressure tower 7 vaporized by the condensing evaporator 13. Power consumption can be reduced corresponding to the process. Further, when the product is used at a higher pressure, the product is compressed from a higher pressure than before, so that the compression ratio can be reduced, and the cost of compressing power for compressing the product can be reduced.

The condensing evaporator 13 vaporizes the descending liquid in the low-pressure column 7 to generate a rising gas, thereby increasing the rising gas in the low-pressure column middle portion 7b and the low-pressure column upper portion 7c. The liquefied nitrogen liquefied in the high pressure column 5
By introducing to the top of the low-pressure column in addition to the liquefied nitrogen from the above, the descending liquid in the upper portion 7c of the low-pressure column can also be increased, so that even in the high-pressure distillation, the effect of separating nitrogen and oxygen is reduced, and the yield is high. Products can be obtained, and production costs can be reduced.

The first booster 11 and the expansion turbine 1
2 is connected by a shaft (not shown), and the power consumption can be reduced by utilizing the work by the expansion of the pressurized nitrogen gas in the expansion turbine 12 as the pressurizing power of the nitrogen in the first pressurizer 11. .

Further, when high-pressure compressed air is supplied from the power generation equipment G as a part of the raw air as in the present embodiment, the power cost for compressing the raw air can be reduced and the high pressure of the compressed air can be reduced. Is effectively used to perform high-pressure distillation,
Since the product can be taken out at a high pressure and supplied directly to the use destination, there is no need for a compression means for delivering the product, and the equipment cost and the compression power cost can be reduced. Further, if the amount of compressed air supplied from the power generation equipment G is supplied according to the load of the power generation equipment, for example, the excess compressed air at night is supplied according to the surplus, the compressed air can be further effectively used. Therefore, the power can be leveled and the production cost of low-purity oxygen can be further reduced.

Then, the condensing evaporator 13 is disposed at least one theoretical stage below the introduction position of the liquefied air introduced from the high-pressure column 5 through the path 26 into the low-pressure column central portion 7b, thereby lowering the low-pressure column upper portion 7c. In addition to the descending liquid flowing down, the high pressure tower 5
The oxygen-enriched liquefied air introduced from the bottom is
Can exchange heat with the expanded nitrogen gas, evaporating the descending liquid to increase the ascending gas in the low-pressure column middle part 7b and the upper part 7c, decrease the descending liquid in the low-pressure tower lower part 7a, and reduce The liquefied liquefied nitrogen is introduced into the top of the low-pressure column 7 as a reflux liquid, and the function of the condensing evaporator 13 to increase the descending liquid in the upper portion 7c of the low-pressure column 7 can be sufficiently exhibited.

Further, the high pressure column 5 and / or the low pressure column 7 may be a packed distillation column using a packing having a smaller pressure loss than the sieve tray, so that the sieve tray is used when the feed pressure of the raw material air is the same. The product can be recovered at a higher pressure than if it had been.

When the ratio of the liquid product is to be increased, the first booster 11
Assuming that all of the compression power is supplied, the processing amount of the expansion turbine 12 becomes an amount corresponding to the increase in the product liquid, excessive nitrogen is introduced into the condensing evaporator 13, and the recovery rate of the product oxygen decreases. That is, in order to increase the ratio of liquid products,
It is necessary to increase the amount of cold generation, and there are two means for increasing the amount of processing of the expansion turbine 12 or increasing the pressure before expansion by keeping the amount constant. When the throughput is increased, the flow rate of nitrogen introduced into the condensing evaporator 13 is increased, so that the descending liquid and the rising gas in the lower part 7a of the low pressure column are further reduced.
The distillation effect in a is deteriorated, and the recovery rate is reduced.

Therefore, when increasing the ratio of the liquid product, the expansion work in the expansion turbine 12 is performed by the first booster 11.
It is possible to use a part of the compression power and increase the boost pressure by supplying the remaining compression power from the outside and increase the amount of cooling while maintaining the processing amount of the expansion turbine 12 at a desired amount. The liquid product collection ratio can be increased without lowering the recovery rate.

When the raw material air is supplied at a pressure higher than the pressure shown in the embodiment, as shown by a broken line in FIG. This can be dealt with by providing a path for circulating a part of the product nitrogen gas led to the path 34 and reusing it as the reflux liquid of the low-pressure column 7.

That is, it is extracted from the top of the low-pressure tower 7 to the path 31, and the subcooler 10, the path 32, the subcooler 9, and the path 3
3. At least a portion of the nitrogen gas, which has passed through the main heat exchanger 4 and whose cold has been collected and is led out to the path 34, branches off to the path 50, is introduced into the second booster 51, and is pressurized to a predetermined pressure. . The pressurized nitrogen gas is cooled by the cooler 51a, introduced into the main heat exchanger 4 from the path 52, cooled to near the boiling point by the low-temperature fluid, passed through the path 53, and expanded by the expansion turbine 12 in the expansion turbine 12. To the route 45.
The combined nitrogen gas is led to the condensing evaporator 13 provided in the low-pressure column middle part 7b, and vaporizes the descending liquid flowing down in the low-pressure column 7 and liquefies itself as liquefied nitrogen, and passes through the path 46. It is cooled to a supercooled state by the subcooler 10, depressurized by the valve 47 a through the path 47, merges with the reflux liquefied nitrogen path 30 from the high-pressure column 5, and is introduced into the top of the low-pressure column 7. Thereby, the amount of reflux liquefied nitrogen introduced to the top of the low-pressure column 7 can be increased.

As described above, even in the case of high-pressure distillation at a higher pressure, the insufficient reflux liquid can be replenished with the decrease in the specific volatility in the high-pressure distillation, so that the product can be produced without lowering the yield. Obtainable. Further, the process is particularly effective when recovering nitrogen or low purity oxygen as a liquid product.

[0036]

As described above, according to the method and apparatus for producing low-purity oxygen of the present invention, the effect of separating nitrogen and oxygen in the upper to middle parts of the low-pressure column is impaired even in a high-pressure distillation operation. As a result, a product with high pressure can be obtained without lowering the yield.

[Brief description of the drawings]

FIG. 1 is a system diagram showing one embodiment of an apparatus for producing low-purity oxygen of the present invention.

[Explanation of symbols]

1 ... raw material air compressor, 2 ... pre-cooling equipment, 3 ... refining equipment, 4
... Main heat exchanger, 5 ... High pressure tower, 6 ... Main condenser, 7 ... Low pressure tower, 8 ... Double distillation tower, 9,10 ... Supercooler, 11 ... First pressurizer, 12 ... Expansion turbine, 13 ... Condensation evaporator, 51 ...
Second booster

Claims (9)

[Claims] 1. A method for producing low-purity oxygen, comprising separating and recovering at least low-purity oxygen as a product by subjecting raw air to low-temperature distillation in a double distillation apparatus having a high-pressure tower and a low-pressure tower, wherein the raw air is compressed. Pre-cooling the compressed raw air, purifying the pre-cooled raw air by removing impurities such as moisture and carbon dioxide, and heat-exchanging the purified raw air with a fluid obtained by low-temperature distillation. A step of cooling, a step of introducing the cooled raw material air into the high-pressure column and performing low-temperature distillation to separate it into a nitrogen-enriched fluid and an oxygen-enriched fluid, and the nitrogen-enriched fluid separated in the high-pressure column. Introducing oxygen and an oxygen-enriched fluid into a low-pressure column to separate them into nitrogen and oxygen; and extracting a portion of the nitrogen-enriched fluid separated by the high-pressure column and heating by exchanging heat with raw air. And is heated Pressurizing the nitrogen-enriched fluid,
Cooling the pressurized nitrogen-enriched fluid by heat exchange with the fluid obtained by low-temperature distillation; adiabatic expansion of the cooled nitrogen-enriched fluid; A step of liquefying by heat exchange with a descending liquid flowing down the center of the column, a step of supplying a liquefied nitrogen-enriched fluid as a reflux liquid of the low-pressure column, and extracting oxygen and nitrogen separated in the low-pressure column to obtain a raw material Recovering oxygen gas and nitrogen gas by raising the temperature by heat exchange with air. 2. A step of increasing the pressure of at least a part of the recovered nitrogen gas; a step of cooling the increased pressure of the nitrogen gas by heat exchange with a fluid obtained by low-temperature distillation; The method according to claim 1, further comprising the steps of: liquefying by heat exchange with a descending liquid flowing down the center of the low pressure column; and introducing the liquefied liquefied nitrogen as a reflux liquid of the low pressure column. Production method. 3. The production of low-purity oxygen according to claim 1, wherein the pressurization of the nitrogen-enriched fluid is performed at least in part by utilizing the work of adiabatic expansion of the nitrogen-enriched fluid. Method. 4. The method for producing low-purity oxygen according to claim 1, wherein at least a part of the raw air is compressed air supplied from an air compressor of a power generation facility. 5. A low-purity oxygen producing apparatus for separating and recovering at least low-purity oxygen as a product by low-temperature distillation of raw air, wherein a raw air compressor for compressing the raw air and a pre-cooled compressed air for the raw air are provided. A pre-cooling facility, a purification facility for removing impurities such as moisture and carbon dioxide from the pre-cooled feed air, and a main heat exchanger for heat-exchanging the purified feed air with a fluid obtained by low-temperature distillation. A double distillation column comprising a high-pressure column, a main condenser and a low-pressure column for low-temperature distillation of the raw material air cooled by the heat exchanger to separate it into nitrogen and oxygen; A first booster that pressurizes the nitrogen gas heated by the heat exchanger, an expansion turbine that adiabatically expands the nitrogen gas that is pressurized by the first booster and cooled by the main heat exchanger, and nitrogen that is adiabatically expanded by the expansion turbine Gas A condensation evaporator that liquefies by exchanging heat with the descending liquid flowing down the center of the low pressure column, a path for supplying liquefied nitrogen liquefied by the condensation evaporator to the upper part of the low pressure column as a reflux liquid, and a separation and generation by the low pressure column An apparatus for producing low-purity oxygen, comprising: a nitrogen gas recovery path and an oxygen gas recovery path for recovering nitrogen and oxygen by recovering the temperature in the main heat exchanger. 6. The condensing evaporator is disposed at least one theoretical stage below an introduction position of oxygen-enriched liquefied air that is extracted from the bottom of the high-pressure column and introduced into the low-pressure column. The apparatus for producing low-purity oxygen according to claim 5. 7. A second pressurizer for pressurizing a part of the nitrogen gas recovered in the nitrogen gas recovery path, and after cooling the nitrogen gas pressurized by the second booster through the main heat exchanger, 6. The apparatus for producing low-purity oxygen according to claim 5, further comprising a path for introducing into the condensing evaporator. 8. The apparatus for producing low-purity oxygen according to claim 5, wherein a compressed air introduction path for guiding compressed air from a power generation facility is connected to a raw air outlet path of the raw air compressor. . 9. The apparatus for producing low-purity oxygen according to claim 5, wherein at least one of the high-pressure column and the low-pressure column is a packed distillation column.