JPH04310509A - Removal of impurity in nitrogen gas - Google Patents

Abstract

PURPOSE:To easily and rapidly remove an impurity from nitrogen gas containing oxygen gas as the impurity to produce the nitrogen gas having high purity. CONSTITUTION:Adsorbing towers 6a, 6b filled with A type zeolite containing lithium ions and sodium ions in the crystals thereof as exchanging ions, the concentration of lithium ions being 55-75mol.%, preferably 65-70mol.%, based on the sum of both the kinds of the ions, are cooled to <=-50 deg.C, and nitrogen gas containing oxygen gas as an impurity is introduced into the adsorbing towers 6a, 6b to selectively adsorb and remove the oxygen gas.

Description

[Detailed description of the invention]

0001

[Industrial Application Field] The present invention is applicable to high-purity (99.999 volume %) used, for example, in advanced metal heat treatment atmospheres and inert atmospheres in the electronics and chemical industries.
The present invention relates to a method for removing impure oxygen from nitrogen in order to produce nitrogen gas of a certain degree or higher.

0002

2. Description of the Related Art Conventionally, a cryogenic air separation method is generally known as a method for producing the above-mentioned high-purity nitrogen gas. This method cools and liquefies raw air, and performs rectification using the difference in boiling point between nitrogen and oxygen to separate and recover nitrogen components.

[0003] As a method for producing nitrogen gas, a pressure swing adsorption (hereinafter simply referred to as PSA) method is known. This uses air as a raw material and separates and recovers nitrogen by removing nitrogen or components other than nitrogen from the raw air.

0004

[Problems to be Solved by the Invention] In the conventional cryogenic air separation method as a method for producing high-purity nitrogen gas,
The equipment for deep cooling becomes complicated, and manufacturing costs tend to increase, especially in the case of small- to medium-sized equipment.

In addition, in the PSA method, the nitrogen gas obtained by this method contains impurities such as hydrogen, carbon monoxide, oxygen,
Since the upper limit of nitrogen purity is usually in the range of 99.9 to 99.99% by volume, including methane, carbon dioxide, water, etc., in order to obtain higher purity nitrogen gas, the above nitrogen gas must be replaced by other means. need to be used for further purification. As a purification means, it is conceivable to attach a catalytic catalytic combustion reactor to the PSA apparatus to remove impurities (carbon monoxide, carbon dioxide, methane, etc.). However, in this case, a heating means is required for hydrogen addition, reaction, and regeneration, which causes problems such as increased manufacturing costs and complicated operations.

By the way, among the above impurities, hydrogen and carbon monoxide can be converted into carbon dioxide and water, which are easily adsorbable, by reacting with coexisting oxygen by bringing them into contact with a catalyst, but the remaining Since the difference in boiling point of oxygen and nitrogen is relatively small and the partial pressure of oxygen with respect to nitrogen is extremely small, it is difficult to separate them from each other.

On the other hand, as a means for separating oxygen and nitrogen from each other, a molecular sieve adsorbent having a predetermined effective pore diameter is cooled to a predetermined temperature, and a mixed gas of nitrogen and oxygen is supplied into the adsorbent. It is conceivable to take advantage of the property that nitrogen is not substantially adsorbed and only oxygen is adsorbed. However, a device for cooling the adsorbent to a predetermined temperature is required, and this cooling device causes a problem in that the manufacturing cost increases, similar to the cryogenic separation method.

In order to solve this problem, Japanese Patent Application Laid-open No. 2-30607 discloses an adsorption tower filled with an adsorbent having a molecular sieve effect that adsorbs only oxygen, which can be used to remove nitrogen gas containing oxygen as an impurity. proposed a method in which nitrogen gas is purified by selectively adsorbing and removing oxygen from nitrogen gas, and nitrogen gas vaporized by liquid nitrogen cooling an adsorption tower is mixed with this. However, the above method only uses a molecular sieve adsorbent that does not substantially adsorb nitrogen and only adsorbs oxygen at the adsorption temperature. and has an effective pore diameter of about 4 Å,
NaA type zeolite known as MS4A is exemplified, and although this adsorbent has the property that its effective pore diameter decreases as the temperature decreases, its effective pore diameter is smaller than the molecular diameters of nitrogen and oxygen. In other words, in order to prevent nitrogen from diffusing into the interior of the zeolite crystals and to allow only oxygen with a rather small molecular diameter to be diffused and adsorbed, the adsorption tower temperature must be raised to, for example, close to the liquid oxygen temperature (-183°C). (D.W. Breck, Zeo
lite Molecular Sieves, Jo
hn Wiley & Sons, New York,
pp. (pp. 638-640), the required amount of liquid nitrogen for cooling increases. Such low-temperature cooling is necessary because the effective pore diameter of NaA-type zeolite at room temperature is
This is because it is too large compared to the molecular diameter of nitrogen.

As a result, in the method of high purification using nitrogen obtained by pressure swing adsorption, a large amount of liquid nitrogen is required to be used as an auxiliary, resulting in poor economic efficiency. Furthermore, as the temperature decreases, the rate of gas diffusion into the adsorbent decreases significantly, and as a result, the adsorption zone becomes longer, resulting in an increase in the amount of adsorbent required and a longer time required for temperature rise and desorption after adsorption. Problems also remain unresolved.

[0010] To solve this problem, it is sufficient to use a zeolite with a slightly smaller effective pore diameter than the NaA type zeolite, which at room temperature already has an effective pore diameter located between the molecular diameters of nitrogen and oxygen. It would be best if there was one, but no such adsorbent is known among the industrially available adsorbents. It may also exhibit similar effects at a relatively higher temperature than the NaA type zeolite. When such an adsorbent is used at the same temperature as NaA-type zeolite, the diffusion of nitrogen into the zeolite crystal is relatively more inhibited in the former, so the oxygen adsorption capacity increases and the oxygen removal effect also decreases. It is thought that this will increase.

[0011] Furthermore, in regenerating the adsorbent, if the temperature is raised to regenerate the adsorbent so as to desorb oxygen in a short time, the effective pore diameter of the adsorbent becomes larger, so that nitrogen can also be adsorbed. Zeolite basically adsorbs nitrogen preferentially over oxygen, so if you simply raise the temperature and regenerate using a raw material gas mainly composed of nitrogen, there will be a lot of nitrogen in the zeolite crystal (adsorption temperature Since the temperature is higher than that of the In addition, since there is a large amount of nitrogen in the adsorption tower internal space, some of it will be re-adsorbed in the temperature range where the pore diameter is greater than the molecular diameter of nitrogen during the temperature cooling process in preparation for the next adsorption operation. . When the adsorption temperature is reached, the nitrogen present in the zeolite crystal cannot be desorbed, which inhibits oxygen adsorption and causes a decrease in adsorption capacity.

[0012] The present invention was made to solve such conventional problems, and it is possible to easily and quickly remove impurities from nitrogen gas containing oxygen as an impurity, and to produce highly pure nitrogen gas. The object of the present invention is to provide a method for removing impure oxygen from nitrogen that can be obtained.

[0013]

[Means for Solving the Problems] This invention provides that within a crystal,
Contains lithium ions and sodium ions as exchangeable ions, and lithium ions account for 55 to 75% of both ions.
The temperature inside the adsorption tower filled with A-type zeolite, which preferably accounts for 65 to 70 mol%, is cooled to -50°C or lower, and nitrogen gas containing oxygen as an impurity is introduced into the adsorption tower to remove oxygen. It is designed to selectively adsorb and remove.

It is preferable to use purified liquid nitrogen as the cooling medium for the adsorption tower, and to mix the nitrogen gas vaporized by cooling the adsorption tower with the purified gas from the adsorption tower.

[0015] In the above operation, the adsorption operation is terminated before the oxygen breakthrough, the pressure inside the adsorption tower is reduced to below the adsorption pressure, and the adsorbed oxygen and nitrogen present in the adsorption tower internal space are removed, and then It is preferable to repeat the operation of introducing nitrogen containing impure oxygen into the adsorption column again. In addition, the adsorption operation is terminated before the oxygen breakthrough, and the temperature inside the adsorption tower is raised above the adsorption temperature, and the pressure inside the adsorption tower is reduced to below the adsorption pressure, and the adsorbed oxygen and the space inside the adsorption tower are reduced. After nitrogen is removed and the temperature inside the adsorption tower is lowered to the adsorption temperature while maintaining the reduced pressure state, an operation may be performed in which the nitrogen is introduced into the adsorption tower containing impure oxygen again.

[0016]

[Function] With the above configuration, the temperature inside the adsorption tower filled with type A zeolite, which contains lithium ions and sodium ions as exchangeable ions in the crystal, and in which lithium ions account for 55 to 75% of both ions, is controlled. The nitrogen gas containing oxygen as an impurity is cooled to -50°C or lower and introduced into this adsorption tower to selectively adsorb and remove oxygen. It can be purified to high purity nitrogen gas with a purity of 99.999% by volume or higher. Moreover, the cooling temperature of the adsorption tower is -50 to -
The temperature can be as high as about 70° C., which has the advantage of not requiring a large amount of cooling medium. Further, when purified liquid nitrogen is used as a cooling medium, after its use, the refrigerant can be effectively used by combining it with purified nitrogen gas as product nitrogen.

[0017]

[Example] As shown in FIG. 1, an apparatus for carrying out this invention includes a normal PSA apparatus A that separates and recovers 99.9 to 99.99 volume % nitrogen gas from a raw material gas, and It consists of a purification device B that removes impurities contained in the obtained primary nitrogen gas and refines it into high-purity nitrogen gas with a nitrogen purity of 99.999% by volume or more.

The raw material air pressurized by the raw material air compressor 11 is sent to a catalytic reactor 2 filled with a precious metal catalyst 21 such as palladium, and a trace amount of the raw material air is added to the raw material air by the catalytic reaction in this catalytic reactor 2. The hydrogen and carbon monoxide present in the water are oxidized and changed into water and carbon dioxide. In addition,
This reaction occurs at room temperature, so no special heating equipment is required.

The raw material gas exiting the catalytic reactor 2 passes through an aftercooler 31 and a drain separator 32 to remove condensed water, and the raw material gas in this state is sent to either the adsorption tower 4a or 4b of the PSA device A. One side is fed through valve 41.

This PSA apparatus A has two adsorption towers 4a and 4.
b and a first vacuum pump 12, and this adsorption tower 4
A and 4b are filled with molecular sieve activated carbon as an adsorbent 40. This adsorbent 40 preferentially adsorbs oxygen and the like in the raw material gas, increasing the nitrogen ratio (concentrating),
Primary nitrogen gas (nitrogen gas containing oxygen as an impurity) containing impurities (oxygen, water, carbon dioxide, methane, etc.) is recovered through valves 42 , 43 and valve 44 . Note that the oxygen adsorbed in the adsorption towers 4a, 4b is desorbed by the first vacuum pump 12 through the valve 45, thereby regenerating the adsorption towers 4a, 4b.

[0021] The above-mentioned primary nitrogen gas is sent to the purifier B through the connecting pipe 51. This purification apparatus B basically consists of two low-temperature adsorption towers 6a, 6b and a second vacuum pump 13. These low-temperature adsorption towers 6a, 6b are connected to the product storage tank 7 by a recovery pipe 52, and the low-temperature adsorption tower 6a, 6b
The highly purified nitrogen gas obtained is stored in a product nitrogen storage tank 7.

The low-temperature adsorption towers 6a and 6b are connected to a liquid nitrogen storage tank 8 via a valve 65.
The inside of the low-temperature adsorption towers 6a and 6b is -5 due to liquid nitrogen from
It is designed to be cooled to about 0 to -70°C. The high-purity nitrogen gas vaporized by cooling the low-temperature adsorption towers 6a and 6b passes through the valve 66 to the recovery pipe 52.
The nitrogen gas flows into the purified nitrogen gas, joins with the purified nitrogen gas, and is stored in the product nitrogen storage tank 7 as product nitrogen. The liquid nitrogen storage tank 8 and the low-temperature adsorption towers 6a and 6b are connected to each other by another pipe line via a liquid nitrogen evaporator 9 and a valve 67, and the vaporized nitrogen gas vaporized by the liquid nitrogen evaporator 9 is transferred to the valve 67.
7 to the low-temperature adsorption towers 6a, 6b, and this vaporized nitrogen gas raises the temperature of the low-temperature adsorption towers 6a, 6b to room temperature. The vaporized nitrogen gas used to raise the temperature also flows into the recovery pipe 52 through the valve 66 and is stored in the product nitrogen storage tank 7 as product nitrogen.

The low-temperature adsorption towers 6a and 6b are each filled with an activated carbon adsorbent 60 on the upstream side and a molecular sieve adsorbent 61 such as LiNaA type zeolite on the downstream side. This L
iNaA type zeolite contains lithium ions and sodium ions as exchangeable ions in the crystal, and lithium ions account for 55 to 75% of both ions, preferably 65 to 75%.
It consists of A-type zeolite, which accounts for 70 mol%. The primary nitrogen gas from the PSA device A is introduced into either one of the low-temperature adsorption towers 6a and 6b through the valve 62,
Hydrocarbons, mainly water, carbon dioxide, and methane, are adsorbed and removed from this nitrogen gas by the activated carbon adsorbent 60, and the remaining nitrogen gas, which consists of substantially most nitrogen components and a trace amount of oxygen components, is adsorbed by molecular sieves. agent 61.

When this molecular sieve adsorbent 61 is cooled to about -50 to -70°C by liquid nitrogen gas, its effective pore diameter, which was approximately 4 Å at room temperature, is reduced to approximately 3.5 to 3.5 Å. As a result, nitrogen (molecular diameter: 3.64 Å) of the nitrogen gas is not substantially adsorbed, and only oxygen (molecular diameter: 3.46 Å) is adsorbed by the molecular sieve adsorbent 61. Refined nitrogen gas, which has become highly purified by removing impurities such as water, carbon dioxide, and oxygen from the primary nitrogen gas, is recovered into the product nitrogen storage tank 7 through valves 63 and 64 as product nitrogen.

The primary nitrogen gas from the PSA device A is transferred to the two low-temperature adsorption towers 6a and 6 by switching the valve 62.
Switching is performed automatically so that the supply is alternately supplied to
While one adsorption tower is in the adsorption step, the other adsorption tower is regenerated. The switching of the valve 62 is performed before the oxygen adsorbed by one of the molecular sieve adsorbents 61 in the adsorption process reaches a breakthrough state. Regeneration of one of the adsorption towers is carried out after closing the valve 65 to stop the supply of liquid nitrogen, opening the valve 67 and supplying vaporized nitrogen gas to raise the temperature of the adsorption tower to around room temperature, or after increasing the temperature. This is done by operating the second vacuum pump 13 while This reduces the pressure inside the adsorption tower and removes oxygen.
Water, carbon dioxide, etc. are desorbed and discharged through valve 68.

[0026] By raising the temperature and reducing the pressure inside the adsorption tower to regenerate it, and proceeding to the next adsorption operation while maintaining this reduced pressure state, the inhibition of oxygen adsorption due to residual nitrogen is greatly reduced. In addition to using vaporized nitrogen gas obtained by evaporating liquid nitrogen, the temperature of the adsorption towers 6a and 6b may be raised by, for example, refluxing a portion of the purified nitrogen gas to the adsorption towers 6a and 6b.

According to the above method, the primary nitrogen gas (nitrogen purity 99.9 to 99.9
9% by volume) can be purified into high-purity nitrogen gas that is equivalent to or higher than that obtained by cryogenic separation (nitrogen purity: 99.999% by volume). In addition, liquid nitrogen for cooling the low-temperature adsorption towers can be used effectively by combining it with purified nitrogen gas as product nitrogen after its use, and the adsorption towers 6a and 6b are cooled at high temperatures (-50 to -70°C), which has the advantage of not requiring a large amount of liquid nitrogen.

Generally, the PSA device A is provided with a liquid nitrogen storage tank for backup in case of failure or inspection, and an evaporator for evaporating the liquid nitrogen.
An apparatus for carrying out the above method can be easily configured by simply adding a low temperature adsorption tower to apparatus A and connecting the low temperature adsorption tower and the liquid nitrogen storage tank to each other. In addition, the operation is easy, and nitrogen gas can be reliably purified to high purity.

[0029] In the above embodiment, the PSA device A is configured to adsorb and remove oxygen components from raw air, but the present invention is not limited to this. It may be configured as follows,
Further, the number of adsorption towers may be increased as appropriate.

Further, in the above embodiment, two low-temperature adsorption towers are provided, but for example, three or more towers may be provided to continuously purify nitrogen gas. Alternatively, a system may be adopted in which only one low-temperature adsorption tower is used and only gas obtained by vaporizing liquid nitrogen is used temporarily during regeneration. Further, in the above embodiment, the nitrogen gas produced in the PSA device A is purified into high-purity nitrogen gas by the purification device B, but the present invention is not limited to this. For example, nitrogen gas separated by a membrane separation method can be purified. It may also be configured to be supplied to purifier B for purification.

Next, the results of a comparison of impurity adsorption effects using NaA type zeolite and LiNaA type zeolite will be shown.

A LiNaA type zeolite having a molar ratio of lithium ions to sodium ions of 67:33 was prepared, and the adsorption equilibrium of nitrogen and oxygen between this and NaA type zeolite was measured at each temperature. The measurement was carried out using an Omnisorp 100 pore distribution analyzer (manufactured by Omicron Technologies). Approximately 1 g was placed in a bath kept at a constant temperature with various cryogens.
This was done by adding a zeolite sample, supplying oxygen or nitrogen gas, and drawing an adsorption isotherm using the constant volume method. Adsorption capacity (unit: Nc) at a specific pressure at each temperature
m3-gas/g-adsorbent) in Table 1 (Li
(in the case of NaA type zeolite) and Table 2 (in the case of NaA type zeolite). The above LiNaA type zeolite is produced, for example, as follows. M.S.
0.1 mol/l of 4A particles (product manufactured by Union Showa)
immersed in a lithium chloride aqueous solution with a concentration of
By holding and stirring the MS4A, the sodium ions in the MS4A are exchanged with lithium ions, and by adjusting the immersion time, a predetermined exchange rate is achieved.

[0033]

[Table 1]

[0034]

[Table 2]

In the above table, the extremely large amount of high partial pressure oxygen adsorbed at -186°C is due to condensation on the surface or in the pores.

Generally, the adsorption capacity should increase as the temperature decreases, but in the case of zeolite, it may decrease if the molecular sieve effect comes into play. At this time, nitrogen has a larger molecular diameter than oxygen, so even if a larger amount of nitrogen is adsorbed in the high temperature range due to the difference in affinity for zeolite, when the temperature drops to a certain temperature range, the amount of nitrogen adsorbed suddenly decreases. However, a phenomenon appears in which the amount of oxygen adsorption continues to increase due to the effect of lowering the temperature. Even if diffusion adsorption within the zeolite crystal becomes impossible due to the molecular sieve effect, a small amount of adsorption on the outer surface of the crystal is possible, and the amount increases as the temperature decreases, so it begins to gradually increase again.

Since the target is a trace amount of oxygen in nitrogen, the partial pressure of the former is overwhelmingly larger, and the adsorption of nitrogen by co-adsorption also occurs in proportion to the partial pressure, but the adsorption position of nitrogen is This is the outer surface of the zeolite crystal, while the adsorption position for oxygen is within the crystal, so oxygen adsorption is not inhibited even under conditions where a large amount of nitrogen exists.

[0038] In the above LiNaA type zeolite,
The amount of nitrogen adsorbed at -47°C has already become slightly lower than that at -19°C, and an adsorption inhibition effect has begun to appear. At -72°C, the temperature drops even further, and it is thought that nitrogen cannot be adsorbed within the zeolite crystals. On the other hand, the oxygen adsorption isotherm at -72℃ is N
Since it is almost the same as that of aA type zeolite, no inhibition of oxygen adsorption is observed. This adsorbed nitrogen is on the outer surface of the crystal, -18
At 6°C, it increases again. Therefore, the maximum temperature at which nitrogen is excluded from the zeolite crystal and selective adsorption of oxygen occurs is above -70°C, and since diffusion inhibition has already begun at -47°C, between -50 and -70°C. It is believed that there is.

On the other hand, in NaA type zeolite, -
Such a phenomenon was first observed at 186°C. In reality, such a phenomenon probably begins at temperatures below -72°C and above -186°C, but the above cited document (
D. W. Breck, Zeolite Molecule
According to Ar Sieves, p. 639), the amount of nitrogen adsorbed continues to increase monotonically as the temperature decreases up to at least -100°C, and the amount of nitrogen adsorbed is about the same as the amount of oxygen adsorbed at -150°C. , it can be seen that the temperature at which nitrogen begins to be eliminated from the crystal is clearly lower than in the case of LiNaA type zeolite.

Based on the above experimental results, nitrogen gas containing a low concentration of oxygen was flowed through an adsorbent layer filled in a stainless steel coil (serpentine tube) immersed in an ethanol/dry ice mixed refrigerant, and the outlet The oxygen concentration of the adsorbents was measured using a galvanic cell analyzer, and the differences between the two adsorbents were compared. The measurement conditions are as follows.

Supply gas type Oxygen: 107p
pm/N2 (standard gas) flow rate
2.5cm/s (superficial linear velocity) Adsorption temperature -72℃ Adsorption pressure 5.0atm Adsorbent Type LiNaA type zeolite (Li:Na=67:33)
NaA type zeolite
Packing density 0.63g/c
m3 Filling height 20
0 cm Note that the adsorbent used was one that had been heated to 250° C. while being evacuated to 1 Torr or less, immersed in the above refrigerant while being evacuated, and pressurized to atmospheric pressure with pure nitrogen. As a result of the above measurement, in the case of NaA type zeolite,
Almost simultaneously with the start of adsorption, oxygen was observed at the outlet, confirming that almost no adsorption occurred. This is because, as can be seen from the static adsorption experiment, oxygen cannot be adsorbed by substitution while nitrogen, which has a higher affinity for zeolite, is adsorbed in the pretreatment operation.

In the case of LiNaA type zeolite, the breakthrough start time (time for the outlet oxygen concentration to reach 10% of the inlet oxygen concentration) was 1.6 hours. Furthermore, the outlet oxygen concentration up to that point was at least 3 ppm, clearly demonstrating the effect of selective adsorption and removal of oxygen.

[0043]

[Effects of the Invention] As explained above, the present invention provides Li
The temperature inside the adsorption tower filled with NaA type zeolite is cooled to below -50°C, and normal P is introduced into the adsorption tower.
Primary nitrogen gas produced by the SA method (nitrogen purity 99.
This method introduces nitrogen gas containing oxygen as an impurity (e.g., 9 to 99.99 volume %), and selectively adsorbs and removes oxygen. It can be purified to high purity nitrogen gas with a nitrogen purity of 99.999% by volume or higher. Moreover, the cooling temperature of the adsorption tower can be set to a high temperature of about -50 to -70°C, which makes it possible to reduce the amount of adsorbent and cooling medium used, and to increase the temperature after adsorption and desorption. It has the advantage of not requiring much time. Further, when purified liquid nitrogen is used as a cooling medium, after its use, it can be effectively used by combining it with purified nitrogen gas from an adsorption tower. Furthermore, by raising the temperature and reducing the pressure inside the adsorption tower for regeneration, and proceeding to the next adsorption operation while maintaining this reduced pressure state, inhibition of oxygen adsorption due to residual nitrogen can be significantly reduced.

[Brief explanation of drawings]

FIG. 1 is an explanatory diagram of the configuration of an apparatus for implementing the present invention.

[Explanation of symbols]

A PSA device B Purification equipment 6a, 6b Low temperature adsorption tower 8 Liquid nitrogen storage tank 61 Molecular sieve adsorbent

Claims (4)

[Claims] Claim 1: The crystal is filled with type A zeolite that contains lithium ions and sodium ions as exchangeable ions, and the lithium ions account for 55 to 75%, preferably 65 to 70 mol% of both ions. Removal of impure oxygen in nitrogen, characterized by cooling the temperature inside the adsorption tower to -50°C or lower, introducing nitrogen gas containing oxygen as an impurity into the adsorption tower, and selectively adsorbing and removing oxygen. Method. 2. A claim characterized in that purified liquid nitrogen is used as a cooling medium for the adsorption tower, and the nitrogen gas vaporized by cooling the adsorption tower is mixed with the purified gas from the adsorption tower. 1. The method for removing impure oxygen in nitrogen as described in 1. 3. The adsorption operation is stopped before the oxygen breakthrough, the pressure inside the adsorption tower is reduced to below the adsorption pressure, and the adsorbed oxygen and nitrogen present in the space inside the adsorption tower are removed, and then the impurity is removed again. The method for removing impure oxygen from nitrogen according to claim 1 or 2, characterized in that the operation of introducing nitrogen containing oxygen into an adsorption column is repeated. 4. The adsorption operation is terminated before the oxygen breakthrough, and the temperature inside the adsorption tower is raised to above the adsorption temperature, and the pressure inside the adsorption tower is reduced to below the adsorption pressure, thereby removing the adsorbed oxygen and the space inside the adsorption tower. A claim characterized in that the nitrogen present in the adsorption tower is removed, the temperature inside the adsorption tower is lowered to the adsorption temperature while maintaining the reduced pressure state, and then the adsorption tower containing impure oxygen is introduced again. Item 2. The method for removing impure oxygen in nitrogen according to item 1 or 2.