JPH11270965A - Air liquefaction separation device and operation method thereof - Google Patents

Abstract

(57) [Summary] [PROBLEMS] To reduce or reduce the imbalance in the reflux ratio generated in a crude argon column during the operation of increasing and decreasing the amount of gas, thereby quickly performing the operation of increasing and decreasing the amount of purity of the product gas while minimizing the fluctuation. SOLUTION: A crude argon column 7 is provided with a liquid reservoir 6 for storing a reflux liquid generated by an argon condenser 5, and the liquid reservoir is
A reflux liquid introduction path 71 for extracting and guiding the reflux liquid of the crude argon tower 7 and a reflux liquid supply path 72 for supplying the reflux liquid stored in the liquid reservoir 6 to the crude argon tower 7 are provided. A control valve 21V for adjustment was provided.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an air liquefaction / separation apparatus and an operation method thereof, and more particularly, to air liquefaction in which at least argon (including crude argon) is collected as a product by liquefying and rectifying air at a low temperature. The present invention relates to an air liquefaction separation apparatus capable of quickly responding to a large fluctuation in product demand and a method of operating the same.

[0002]

2. Description of the Related Art In steel mills and chemical factories, etc., who are consumers of industrial gases such as oxygen, nitrogen, and argon, large demand fluctuations of the gas are frequently repeated. In order to cope with this, an air liquefaction / separation apparatus, which is a facility for producing these industrial gases, is required to have an ability to quickly respond to a large increase or decrease in the amount of products.

For example, a backup device such as a gas holder or a liquefied gas storage tank is attached to an air liquefaction / separation device to store product gas, and when demand increases, product gas is supplied from the backup device. Product gas is stored in a backup facility. However, large backup equipment is required to cope with large fluctuations in product gas demand, and if the storage capacity of the backup equipment is exceeded when demand decreases, equipment is turned down and product gas is blown off. I had to.

[0004] In the gas holder system,
In order to secure the supply pressure of the product gas even when the demand increases, the operation is performed by setting the discharge pressure of the product compressor to a pressure much higher than the pressure required for the product gas supply, so there is a large waste of power. There is a problem. On the other hand, the method of liquefying and storing the product gas has a problem that a large amount of power is required for liquefying the product gas. Furthermore, the blowing of product gas means excessive production, which obviously consumes wasted power.

[0005]

Therefore, the capability of the air liquefaction / separation apparatus itself is required to be able to rapidly increase or decrease the production amount of product gas in accordance with the increase or decrease in demand. If the amount is rapidly increased or decreased, there is a disadvantage that the purity of the product fluctuates.

That is, when the amount of increase or decrease of the distillation column is performed, the change in the flow rate of the ascending gas has almost no delay.
Since the flow rate of the reflux liquid changes with time delay,
When the load of the distillation column is changed, a control system is set so that the amount of the reflux liquid is changed earlier than the amount of the ascending gas.

However, in a crude argon column provided in an apparatus for producing argon as a product gas, the flow rate of the rising gas and the flow rate of the reflux liquid are uniquely controlled by a crude argon condenser provided at the top of the column. Therefore, it is not possible to provide a time difference between the change of the liquid amount and the change of the gas amount, for example,
If a rapid operation change such as 3% per minute in the raw material air ratio is performed, the purity of the product argon is impaired or the purity fluctuates greatly, so that the operation change of the crude argon column cannot be quickly performed. Was. In particular, a crude argon column using an ordered packing causes a remarkable fluctuation in purity or the like as compared with a tray-type column, and this is a great limitation for speeding up the operation of increasing or decreasing the amount.

[0008] Accordingly, the present invention reduces the unbalance of the reflux ratio generated in the crude argon column during the operation of increasing or decreasing the amount, thereby rapidly changing the purity of various product gases including the product argon while minimizing the fluctuation. An object of the present invention is to provide an air liquefaction / separation apparatus capable of performing a volume operation and a method of operating the same.

[0009]

In order to achieve the above object, the present invention provides an air liquefaction / separation apparatus and a method for operating the same, comprising: a double rectification column having a lower tower, an upper tower and a main condensing evaporator; A crude argon column having an argon condenser as a liquefaction cold source at the top of the column, wherein the compressed, purified, and cooled raw material air is liquefied and rectified to obtain at least argon as a product in the air liquefaction separation device. The crude argon column is provided with a liquid reservoir for storing the reflux liquid of the crude argon column, and a reflux liquid introduction path for guiding the reflux liquid of the crude argon column to the liquid reservoir, and a reflux liquid stored in the liquid reservoir. A reflux liquid supply path for supplying the crude argon column is provided, and at least one of the reflux liquid introduction path and the reflux liquid supply path is provided with a flow control means for controlling a flow rate of the reflux liquid flowing through the path. It is characterized in that was.

Further, the air liquefaction / separation apparatus of the present invention is characterized in that a flow rate detecting means and a flow rate controlling means are provided in the reflux liquid supply path. Liquid level control means for controlling the liquid level of the liquid reservoir by a signal from the liquid level detection means in the liquid introduction path or the reflux liquid supply path, characterized in that the liquid reservoir, the argon condenser and It is characterized in that it is provided in the middle of a liquefied argon path connecting the top of the crude argon column.

The method for operating an air liquefaction / separation apparatus according to the present invention is the method for operating an air liquefaction / separation apparatus having the above structure, wherein at least a part of the reflux liquid of the crude argon column is stored in the liquid storage, The stored reflux liquid is supplied to the crude argon column at a flow rate according to the operation state of the apparatus. Further, in the method for operating the air liquefaction / separation apparatus of the present invention, the flow rate of the reflux liquid when the apparatus is operated in a constant state is such that the flow rate of the reflux liquid supplied to the crude argon column from the liquid reservoir is constant. Or so that the liquid level of the liquid reservoir is controlled to be constant, the rate of increase or decrease of the flow rate of the reflux liquid when the apparatus shifts to the increasing operation or the decreasing operation, It is characterized in that the flow rate of the liquefied air serving as a cold source of the argon condenser is increased or decreased more rapidly.

[0012]

FIG. 1 is a system diagram showing an embodiment of an air liquefaction / separation apparatus according to the present invention. This air liquefaction / separation apparatus produces crude argon gas (RAr), oxygen gas (GO ₂ ), and nitrogen gas (GN ₂ ) as products, and comprises a lower tower 1, an upper tower 2 and a main condensing evaporator 3 A double rectification column 4 having a gas turbine, a crude argon column 7 having an argon condenser 5 and a liquid reservoir 6, a main heat exchanger 8 and a supercooler 9 for exchanging heat of various gases and liquids, and a predetermined device And various control means for controlling the operation state.

Compressed by a compressor (not shown) to a predetermined pressure,
The raw material air (AIR) purified by the purification equipment flows into the main heat exchanger 8 through a control valve 11V controlled by a flow rate controller (FIC) 11, and is extracted from the double rectification column 4 and the like. After being cooled to a predetermined temperature by performing heat exchange with various low-temperature gases, it is introduced into the lower part of the lower tower 1 through a path 51.

The feed air introduced into the lower tower 1 is separated into a nitrogen gas at the top of the tower and an oxygen-enriched liquefied air at the bottom of the tower by a rectification operation in the tower. The nitrogen gas at the top of the tower
2 and is introduced into the main condensing evaporator 3 and liquefied by heat exchange with liquefied oxygen at the bottom of the upper column.
Is returned to the top of the lower tower 1 via a path 54 and becomes the reflux liquid of the lower tower 1. On the other hand, the liquefied nitrogen branched to the path 53 is cooled by the subcooler 9, passes through a control valve 12 V controlled by a flow controller (FC) 12, and is introduced into the top of the upper tower 2 from a path 55. To become the reflux liquid of the upper tower 2.

The oxygen-enriched liquefied air at the bottom of the lower column is
A control valve 13V controlled by a liquid level controller (LC) 13 for extracting the oxygen-enriched liquefied air amount (liquid level) at the bottom of the lower tower is extracted from the passage 56 and cooled by the supercooler 9. Street,
It is introduced into the middle stage of the upper tower 2 through a path 57. Further, from the lower stage of the lower tower 1, liquefied air, which is a part of the reflux liquid flowing down in the tower, is extracted to a passage 58 and cooled by the subcooler 9, and then the flow rate indicating controller (FIC) 14 Is introduced into the argon condenser 5 as a cold source through a control valve 14V controlled by The liquefied air (air) vaporized by condensing the argon in the argon condenser 5 is introduced into the middle part of the upper tower 2 via the path 59.

The liquefied nitrogen introduced into the upper tower 2 from the passage 55, the oxygen-enriched liquefied air from the passage 57, and the air (vaporized liquefied air) from the passage 59 are rectified by the upper tower 2. This separates into nitrogen gas at the top of the column and liquefied oxygen at the bottom of the column.

The nitrogen gas at the top of the tower is extracted to a passage 60 and becomes a cold source of the subcooler 9. Then, the temperature of the raw material air is increased by cooling the raw air in the main heat exchanger 8, and a flow controller ( FC)
The gas is collected as product nitrogen gas (GN ₂ ) from the path 61 through a control valve 15V controlled by the control valve 15.

The liquefied oxygen at the bottom of the tower is evaporated by performing heat exchange with the nitrogen gas in the main condensing evaporator 3 to become oxygen gas. Gas. After the oxygen gas extracted to the passage 62 is heated in the main heat exchanger 8, the flow rate indicating controller (FI)
C) Path 6 through a control valve 16V controlled by 16
3 is collected as product oxygen gas (GO ₂ ). A part of the liquefied oxygen at the bottom of the tower is withdrawn to the passage 64 and
, A control valve 1 controlled by a liquid level controller (LC) 17 for detecting the amount of liquid oxygen (liquid level) at the bottom of the upper tower.
After passing through 7 V, it is withdrawn from the passage 65 as product liquefied oxygen (LO ₂ ) or as safe liquefied oxygen to prevent the concentration of hydrocarbons in the liquefied oxygen.

Further, from the upper middle part of the upper tower 2, the impure nitrogen gas is extracted to the path 66, passes through the subcooler 9 and the main heat exchanger 8, and detects the pressure at the top of the upper tower 2. It is discharged as waste nitrogen gas (RN ₂ ) from a passage 67 through a control valve 18V controlled by a pressure controller (PC) 18.

Then, from the middle part of the upper tower 1, feed argon as a raw material gas for collecting argon is extracted to a passage 68, and the flow rate is detected by a flow meter (FI) 19 while the crude argon tower 7 is detected. And becomes a rising gas of the crude argon column 7. The crude argon gas generated at the top of the column by the rectification in the crude argon column 7 is introduced into the argon condenser 5 through a path 69, and is mostly liquefied by heat exchange with the liquefied air. The liquid is introduced into the top of the crude argon column 7 through a reflux liquid introduction path 71 forming a liquefied argon path 70 connecting the top of the crude argon tower 7, the liquid reservoir 6, the reflux liquid supply path 72, and the control valve 21 V. , And becomes a reflux liquid of the crude argon tower 7. The control valve 21V, which is a flow rate adjusting means for adjusting the amount of the reflux liquid, is connected to a liquid level controller (LC) 20, which is a liquid level detecting means for detecting the liquid level in the liquid reservoir 6, and the reflux liquid supply path 72. It is controlled by a flow rate controller (FIC) 21 which is a flow rate detecting means for detecting the flow rate of the flowing reflux liquid. Further, the reflux liquid (oxygen-enriched liquid) flowing down to the bottom of the crude argon column 7 is supplied to the passage 7.
It is returned to the upper tower 2 by 3. The sampled crude argon gas (RAr) is extracted from the upper part of the liquid reservoir 6 to the path 74, and after the temperature is raised in the main heat exchanger 8, the flow rate indicating controller (FI)
C) Path 7 through control valve 22V controlled by 22
Taken out of 5.

As described above, the liquid reservoir 6 is provided in the middle of the liquefied argon path 70, and the flow rate indicating controller 21 and the control valve 21V for controlling the amount of reflux liquid supplied from the liquid reservoir 6 to the crude argon column 7 are provided. Accordingly, the amount of the reflux liquid in the crude argon column 7 can be arbitrarily set according to the operation state of the apparatus. That is, since the amount of change in the reflux liquid in the operation of increasing or decreasing the amount of the apparatus can be changed prior to the amount of change in the rising gas, the rectification operation in the column can be maintained in an appropriate state, and Variations in argon purity can be reduced. Furthermore, since the upper tower 2 and the like, which are affected by the operation state of the crude argon column 7, can also be kept in an appropriate operation state, fluctuations in purity of product oxygen gas and the like can be reduced, and the purity of each product gas is set. It does not decrease below the purity.

In the air liquefaction / separation apparatus thus formed, when the production amount of the product gas is increased or decreased, each controller controls the opening of each control valve according to a preset program, and controls each path. This is performed by increasing or decreasing the flow rate of flowing gas-liquid.

For example, from an operation state of 100% load, 70
When the amount is reduced to the% load state, the control valve 11V is throttled by the flow rate indicator controller 11 of the raw air supply line, the raw air supply amount is reduced to 70% at a predetermined deceleration, and at a predetermined timing. Each controller and control valve are operated, and the flow rate of gas and liquid in each section is finally changed to a flow rate corresponding to a 70% load condition. In addition, the flow rate of each part at the time of a 70% load is not simply 70%, but is adjusted to an appropriate flow rate so that the purity of each product gas can maintain a predetermined purity.

Table 1 shows that the raw material air amount at 100% load is 100,000 Nm ³ / h, and the purity guarantee value is 99.6 vo.
1% or more of product oxygen gas is collected at 21,000 Nm ³ /
h, the collection amount of crude argon gas having a purity of 97 vol% or more is 870 Nm ³ / h, the raw material air amount at 70% load is 70000 Nm ³ / h, the collection amount of product oxygen gas is 14680 Nm ³ / h, Gas collection volume is 5
To 20 nm ³ / h, in the apparatus set respectively, shows a flow of a main part ^[Nm 3 / h] and a purity (gas composition) [vol%] of when operating in a stable state in each load is there.

[Table 1]

In the apparatus in such a setting, the case where the liquid reservoir 6, the control valve 21V and the like are provided (this embodiment) and the case where no liquid reservoir and the like are provided (conventional example) were compared. In the present embodiment, the amount of change in the flow rate of each gas and liquid during the increase operation and the decrease operation is set to 2% per minute only for the change amount of the reflux liquid supplied from the liquid reservoir 6 to the crude argon column 7. Other than the above, all were changed at 1% per minute (the pressure of the waste nitrogen gas was controlled to be constant). On the other hand, the amount of reflux liquid in the crude argon column 7 in the conventional example having no liquid reservoir or control valve depends on the amount of liquefied air introduced into the argon condenser 5, and the amount of reflux liquid is directly controlled. No (all others 1% per minute).

First, when the increasing operation is performed from the 70% load to the 100% load, the argon concentration in the feed argon supplied from the upper tower 1 to the crude argon tower 7 via the path 68 is determined in the present embodiment. After about 40 minutes, the minimum value is about 6v
ol%, and thereafter gradually increased to a predetermined 9.43 vol%, whereas in the conventional example, the minimum value was about 5 vol% after about 40 minutes.

In the present embodiment, the oxygen concentration in the crude argon collected from the passage 75 reaches a maximum value of about 2.2 vol% in about 6 hours after the start of the increase, and then decreases very slowly. On the other hand, in the conventional example, the maximum value was about 5 vol% after about 4 hours. In the present embodiment, the nitrogen concentration in the crude argon reaches a maximum value of about 0.3 in about 30 minutes after the start of the increase.
It became 6 vol% and returned immediately, but in the conventional example,
It slowly changed from about 2 hours after the start of the dose increase.

In the present embodiment, the purity of the product oxygen gas collected from the passage 63 reaches a maximum value of about 9 at about 2 hours after the start of the increase.
In the conventional example, the maximum value was about 99.85 vol% after about 2 hours, and both gradually decreased thereafter.

Next, when performing a weight reduction operation from a 100% load to a 70% load, in this embodiment, the argon concentration in the feed argon is set to a minimum value of about 6 minutes in about 30 minutes after the start of the weight reduction.
vol.%, and hardly fluctuated as a whole, whereas the conventional example had a maximum value of about 20 after about 40 minutes.
vol%, and returned to a predetermined value in about 2 hours thereafter.

The oxygen concentration in the crude argon hardly fluctuated in this embodiment, whereas in the conventional example, it reached a minimum value of about 0.7 vol% after about 80 minutes, and gradually increased thereafter.

In the present embodiment, the purity of the product oxygen gas reaches a maximum value of about 99.8 vol% in about 60 minutes after the start of the weight reduction, but it hardly fluctuates as a whole. After about 100 minutes, the minimum value was below the guaranteed value, about 99 vol%, and gradually increased thereafter.

Looking at the situation of the above-mentioned increase operation and decrease operation, in the conventional example, the oxygen concentration in the crude argon greatly increases when increasing the amount, and the purity of the product oxygen gas decreases greatly when decreasing the amount. It is understood that none of these numerical values can satisfy the permissible value or the guaranteed value, and it is necessary to set the fluctuation rate of the flow rate to be much lower than 1% per minute. On the other hand, in the present embodiment, these density fluctuations are suppressed, and the numerical values sufficiently satisfy the allowable value and the guaranteed value.

By setting the rate of change of the reflux liquid in the crude argon column 7 faster than the other rates of change, in particular, faster than the rate of change of the liquefied air introduced as a cold source into the argon condenser 5, Since the change in the liquid amount can be made to correspond to the change in the rising gas amount, not only the above-described effect of suppressing the concentration fluctuation can be obtained, but also the overall flow rate increasing / decreasing speed can be set higher than in the past. In addition, the operation of increasing or decreasing the amount can be performed quickly.

The crude argon column 7 during steady operation
The amount of the reflux liquid may be controlled such that the flow rate of the reflux liquid supply path 72 is kept constant by the flow rate controller 21, and the liquid level in the liquid reservoir 6 is kept constant by the liquid level controller 20. You may.

As shown by the imaginary line in FIG. 1, the reflux liquid introduction path 71 and the reflux liquid supply path 72 are connected to
A similar effect can be obtained by providing a path 76 and a valve 76V for bypassing the flow path, allowing a fixed amount of reflux liquid to flow through this path 76, and passing the remainder through the liquid reservoir 6 for flow rate adjustment. In this case, the size of the liquid reservoir 6 and the like can be reduced.

FIG. 2 is a system diagram showing another embodiment of the air liquefaction / separation apparatus of the present invention. This air liquefaction separation device
The crude argon tower 30 has an upper and lower two-layer structure, and an upper rectification stage 3
A reflux liquid introduction path 33 for extracting at least a part of the reflux liquid flowing down in the column and leading the liquid to the liquid reservoir 32 is connected below 1
A reflux liquid supply path 35 for supplying the reflux liquid in the liquid reservoir 32 into the tower is connected above the lower rectification stage 34, and the liquid level in the liquid reservoir 33 is connected to the reflux liquid supply path 35. Level controller (LC) 36 for detecting the flow rate and a flow rate indicating controller (FIC) for detecting the flow rate of the reflux liquid flowing through the reflux liquid supply path 35.
A control valve 37V controlled by 37 is provided.

In this embodiment, part or all of the reflux liquid flowing down the crude argon column 30 is drawn out of the tower through a reflux liquid introduction path 33 and stored in a liquid reservoir 32, and a predetermined amount of the reflux liquid is refluxed. The liquid supply path 35 separates the crude argon tower 30 from the liquid reservoir 32.
And the lower rectification stage 34
Is formed so that the amount of the reflux liquid in can be arbitrarily set.

Therefore, in this embodiment as well, although the amount of reflux liquid in the entire crude argon column cannot be adjusted, the amount of reflux liquid flowing down the lower rectification stage 34 is appropriately adjusted according to the operation state of the apparatus. Therefore, it is possible to suppress the concentration fluctuation and to speed up the increase / decrease amount driving operation in substantially the same manner as in the embodiment.

In this embodiment, the reflux liquid introduction path 33
And the control valve 3 is connected to the reflux liquid supply path 35.
7 V was provided to adjust the supply amount of the reflux liquid to the crude argon column 30. However, a control valve was not provided in the reflux liquid supply path 35, and a control valve was provided in the reflux liquid introduction path 33. Even if it is formed so as to adjust the amount of reflux liquid withdrawn from the middle of the above, the amount of reflux liquid in the lower rectification stage 34 can be arbitrarily adjusted.

In this embodiment, the configuration of the crude argon column 30 and its surroundings, and the route 74 for collecting the crude argon are described.
However, except that it is branched from the gas path 38 connecting the top of the crude argon column 30 and the argon condenser 5, it is formed in the same manner as the embodiment shown in FIG. The same components as those of the first embodiment are denoted by the same reference numerals, and the detailed description is omitted.

When a packed column using the structured packing is used for the crude argon column 30, the liquid collecting section in the liquid collecting and distributing device provided between the upper and lower packings is used as a liquid reservoir. Utilization may be provided between the liquid collecting part and the liquid distributing part, or in the liquid distributing part, with an appropriate flow rate control means, or the reflux liquid collected by the liquid collecting part may be stored in an external liquid reservoir. Withdrawing and supplying the reflux liquid from the external liquid reservoir to the liquid distribution unit may be performed, and the flow rate may be controlled at an appropriate position of the extraction unit or the supply unit. Further, the configuration of FIG. 1 and the configuration of FIG. 2 may be used together.

[0042]

As described above, according to the present invention, a raw gas (feed argon) is supplied from below, and a liquid reservoir or the like is attached to a crude argon column provided with a condenser for generating a reflux liquid on the upper part. Since the flow rate of the reflux liquid can be adjusted, the amount of the rising gas in the crude argon column and the amount of the reflux liquid can be separately and independently operated, and the optimum reflux liquid amount can be adjusted according to the operation state of the apparatus. Can be set. For example, the amount of the reflux liquid supplied to the crude argon column is reduced during the volume reduction operation, and the flow rate of the reflux liquid supplied to the crude argon column can be rapidly increased during the volume increase operation. The imbalance in the rate of increase and decrease with the reflux liquid can be relaxed, the fluctuation range of the product purity can be reduced, and the operation of increasing and decreasing can be performed quickly.

[Brief description of the drawings]

FIG. 1 is a system diagram showing one embodiment of an air liquefaction / separation apparatus of the present invention.

FIG. 2 is a system diagram showing another embodiment.

[Explanation of symbols]

DESCRIPTION OF SYMBOLS 1 ... Lower tower, 2 ... Upper tower, 3 ... Main condensation evaporator, 4 ... Double rectification tower, 5 ... Argon condenser, 6 ... Liquid reservoir, 7 ... Crude argon tower, 8 ... Main heat exchanger, 9 ... Subcooler, 11 V to 22 V: control valve, 70: liquefied argon path, 71: reflux liquid introduction path, 72: reflux liquid supply path, FIC: flow rate indicating controller,
LC: Liquid level gauge

Claims (7)

[Claims] 1. A double rectification column having a lower column, an upper column, and a main condensation evaporator, and a crude argon column having an argon condenser having a liquefied air as a liquefaction cold source at the top of the column. In the air liquefaction / separation apparatus for collecting at least argon as a product by liquefying and cooling the cooled raw material air, the crude argon column is provided with a liquid reservoir for storing a reflux liquid of the crude argon column. A reflux liquid introduction path for guiding the reflux liquid of the crude argon column and a reflux liquid supply path for supplying the reflux liquid stored in the liquid reservoir to the crude argon tower, and the reflux liquid introduction path and the reflux liquid. An air liquefaction / separation apparatus characterized in that at least one of the supply paths is provided with a flow rate adjusting means for adjusting the flow rate of the reflux liquid flowing through the path. 2. The air liquefaction / separation apparatus according to claim 1, wherein a flow rate detecting means and a flow rate controlling means are provided in the reflux liquid supply path. 3. A liquid level detecting means is provided in the liquid reservoir, and the liquid supply path or the liquid supply path is
2. A liquid level control means for controlling a liquid level in a liquid reservoir in accordance with a signal from said liquid level detection means.
An air liquefaction separation device as described. 4. The air liquefaction / separation apparatus according to claim 1, wherein the liquid reservoir is provided in the middle of a liquefied argon path connecting the argon condenser and the top of the crude argon column. 5. The method for operating an air liquefaction / separation apparatus according to claim 1, wherein at least a part of the reflux liquid of the crude argon column is stored in the liquid reservoir. A method for operating an air liquefaction / separation apparatus, wherein a reflux liquid is supplied to the crude argon column at a flow rate according to the operation state of the apparatus. 6. The flow rate of the reflux liquid when the apparatus is operated in a constant state is such that the flow rate of the reflux liquid supplied from the liquid reservoir to the crude argon column is constant, or The method according to claim 5, wherein the liquid level of the liquid reservoir is controlled to be constant. 7. The method according to claim 1, wherein the rate of change of the flow rate of the reflux liquid when the apparatus shifts to the increase operation or the decrease operation is faster than the change rate of the flow rate of the liquefied air serving as a cold source of the argon condenser. The method for operating an air liquefaction / separation apparatus according to claim 5, wherein