Nitrogen gas separation method and nitrogen gas separation device

Abstract

To provide a nitrogen gas separation method and a nitrogen gas separation device where nitrogen gas can be obtained while saving more energy.SOLUTION: In a nitrogen gas separation method, a raw material gas is pressurized and supplied to two or more adsorption towers 1A, 1B filled with an adsorbent; nitrogen gas is separated from the raw material gas as a product gas by repeating an adsorption step, a pressure equalization step, a desorption step and the pressure equalization step in each adsorption tower 1A, 1B and the product gas is introduced into a product tank 2; an absorption / desorption time during which the adsorption step and the desorption step are performed in each adsorption tower 1A,1B is extended with respect to a reference absorption / desorption time corresponding to the reduction in the flow rate of the product gas flowed out from the product tank 2 from a reference flow rate; and at this time, a first absorption / desorption time is longer than the reference absorption / desorption time when a specific temperature that may influence the temperature of the adsorbent is equal to a first temperature and on the other hand, the second absorption / desorption time is longer than the first absorption / desorption time at a second temperature where the specific temperature is lower than the first temperature.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to a nitrogen gas separation method and a nitrogen gas separation apparatus. [Background technology]

[0002] In recent years, nitrogen gas has been used in a wide variety of fields, from industrial gases used in the heat treatment of metals, the manufacture of semiconductors, and explosion-proof seals in chemical plants to filled gases for food preservation. Nitrogen gas is produced by a so-called pressure swing adsorption (PSA) method, in which high-pressure air, a raw material gas, is fed into an adsorption tower filled with an adsorbent, and oxygen gas is adsorbed onto the adsorbent, resulting in the separation of nitrogen gas as a product gas. For example, a device using this PSA-based nitrogen gas separation method is known, as described in Patent Document 1.

[0003] In the device described in Patent Document 1, the adsorption process is terminated when the total integrated value of the numerical value corresponding to the product nitrogen gas flow rate from the start of the adsorption process exceeds a predetermined fixed value. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Patent No. 5559755 Summary of the Invention [Problem to be solved by the invention]

[0005] In the technology of Patent Document 1, the adsorption / desorption time during which each adsorption tower performs the adsorption process and desorption process is extended as the product nitrogen gas flow rate decreases. When the adsorption / desorption time is extended, the number of exhausts per unit time in the desorption process decreases, thereby reducing the exhaust volume, thereby reducing the consumption of raw material gas and achieving energy savings. However, simply setting the adsorption / desorption time based solely on the product nitrogen gas flow rate does not necessarily result in sufficient energy savings.

[0006] The present invention has been made in view of the above circumstances, and has an object to provide a nitrogen gas separation method and a nitrogen gas separation apparatus that can obtain nitrogen gas with less energy consumption. [Means for solving the problem]

[0007] As a result of extensive research, the inventors have focused on the fact that the adsorption and desorption characteristics of the adsorbent packed in the adsorption tower change in response to changes in the temperature of the adsorbent, and have discovered that by changing the adsorption and desorption time in response to changes in specific temperatures that can affect the temperature of the adsorbent, it is possible to obtain nitrogen gas with greater energy savings, leading to the completion of the present invention.

[0008] Therefore, in order to solve the above problems, the present invention comprises the following method and apparatus.

[0009] The nitrogen gas separation method according to the present invention comprises: feeding a raw gas containing nitrogen gas and oxygen gas to two or more adsorption towers filled with an adsorbent; By compressor In this nitrogen gas separation method, nitrogen gas is supplied under pressure, and each adsorption tower repeatedly performs an adsorption step, a pressure equalization step, a desorption step, and a pressure equalization step to separate nitrogen gas from the raw material gas as a product gas, and the product gas is introduced into a product tank. In this nitrogen gas separation method, the adsorption and desorption time during which each adsorption tower performs the adsorption and desorption steps is extended relative to the standard adsorption and desorption time in response to a decrease in the flow rate of the product gas flowing out of the product tank below a standard flow rate. The temperature of the raw gas supplied by the compressor is indicated by When a specific temperature that can affect the temperature of the adsorbent is a first temperature, a first adsorption / desorption time is set to be longer than the reference adsorption / desorption time, and when the specific temperature is a second temperature that is lower than the first temperature, a second adsorption / desorption time is set to be longer than the first adsorption / desorption time. and reducing the amount of the raw material gas supplied from the compressor in response to a decrease in the specific temperature from the first temperature to the second temperature. .

[0010] According to this nitrogen gas separation method, the adsorption / desorption time during which each adsorption tower performs the adsorption process and the desorption process is extended relative to the standard adsorption / desorption time in response to a decrease in the flow rate of the product gas flowing out of the product tank below the standard flow rate. When the flow rate of the product gas flowing out of the product tank decreases below the standard flow rate, the purity of the nitrogen gas obtained in the adsorption tower becomes higher than necessary. Therefore, the adsorption / desorption time during which each adsorption tower performs the adsorption process and the desorption process is extended in response to a decrease in the flow rate of the product gas flowing out of the product tank. As a result, in the adsorption tower performing the adsorption process, a portion of the oxygen gas adsorbed by the adsorbent is desorbed, thereby reducing the purity of the nitrogen gas that was increased higher than necessary due to the decrease in the flow rate of the product gas flowing out of the product tank. This prevents an excessive increase in the purity of the nitrogen gas obtained in the adsorption tower performing the adsorption process. Furthermore, in the adsorption tower performing the desorption process, the number of exhausts per unit time decreases as the adsorption / desorption time is extended, thereby reducing the amount of exhaust, and therefore the supply amount of raw gas can be reduced. Therefore, nitrogen gas of the desired purity can be obtained with reduced energy.

[0011] The adsorption and desorption characteristics of the adsorbent filled in the adsorption tower change depending on the temperature of the adsorbent. The lower the specific temperature that can affect the adsorbent temperature, the slower the molecular motion of the adsorbent, making it difficult for oxygen gas adsorbed by the adsorbent to desorb. Focusing on these adsorption and desorption characteristics of the adsorbent, a first adsorption and desorption time longer than the reference adsorption and desorption time is set when the specific temperature is a first temperature, while a second adsorption and desorption time longer than the first adsorption and desorption time is set when the specific temperature is a second temperature lower than the first temperature. In this way, if the adsorbent exhibits characteristics that make it difficult to desorb oxygen gas as the specific temperature decreases, the adsorption and desorption time is extended in the adsorption tower performing the adsorption process, thereby increasing the opportunities for oxygen gas to desorb from the adsorbent and preventing an excessive increase in the purity of the resulting nitrogen gas. Furthermore, by extending the adsorption and desorption time in accordance with a decrease in the specific temperature, the number of evacuation operations per unit time in the adsorption tower performing the desorption process is further reduced, thereby reducing the amount of evacuation, thereby enabling a smaller supply of raw material gas. Therefore, when the flow rate of the product gas flowing out of the product tank is reduced, nitrogen gas of the desired purity can be obtained with less energy in response to the decrease in the specific temperature.

[0012] The nitrogen gas separation method according to the present invention comprises: feeding a raw gas containing nitrogen gas and oxygen gas to two or more adsorption towers filled with an adsorbent; By compressor This nitrogen gas separation method separates nitrogen gas from a raw material gas as a product gas by repeatedly performing an adsorption process, a pressure equalization process, a desorption process, and a pressure equalization process in each adsorption tower. The temperature of the raw gas supplied by the compressor is indicated by When a specific temperature that can affect the temperature of the adsorbent is a first temperature, the adsorption / desorption time during which each adsorption tower performs the adsorption process and the desorption process is defined as a first adsorption / desorption time, and when the specific temperature is a second temperature that is lower than the first temperature, the adsorption / desorption time is defined as a second adsorption / desorption time that is longer than the first adsorption / desorption time. and reducing the amount of the raw material gas supplied from the compressor in response to a decrease in the specific temperature from the first temperature to the second temperature. .

[0013] This nitrogen gas separation method focuses on the fact that the adsorption and desorption characteristics of the adsorbent loaded in the adsorption tower change with changes in the adsorbent temperature. When a specific temperature that can affect the adsorbent temperature is a first temperature, the adsorption and desorption time for each adsorption tower to perform the adsorption and desorption processes is designated as a first adsorption / desorption time. When the specific temperature is a second temperature lower than the first temperature, the second adsorption / desorption time is set to a longer time. As a result, when the adsorbent exhibits characteristics that make it difficult to desorb oxygen gas as the specific temperature decreases, the adsorption / desorption time in the adsorption tower performing the adsorption process is extended to increase the opportunities for oxygen gas to desorb from the adsorbent, thereby preventing an excessive increase in the purity of the resulting nitrogen gas. Furthermore, by extending the adsorption / desorption time as the specific temperature decreases, the number of exhausts per unit time in the adsorption tower performing the desorption process is reduced, thereby reducing the amount of exhaust, and therefore the amount of raw gas supplied. Therefore, nitrogen gas of the desired purity can be obtained with reduced energy as the specific temperature decreases.

[0014] In the nitrogen gas separation method, the second adsorption / desorption time may be longer than the first adsorption / desorption time and may be three times or less the first adsorption / desorption time.

[0015] In this embodiment, when the specific temperature drops to a second temperature lower than the first temperature, each adsorption tower performs the adsorption step and the desorption step for a second adsorption / desorption time that is longer than the first adsorption / desorption time but not more than three times the first adsorption / desorption time. This makes it possible to prevent the adsorption / desorption time from becoming too long, while maintaining the effect of obtaining nitrogen gas of a desired purity in an energy-saving manner in response to a drop in the specific temperature.

[0016] In the above-described nitrogen gas separation method, when a portion of the product gas obtained in the adsorption tower undergoing the adsorption step is discharged as a cleaning gas to the adsorption tower undergoing the desorption step, the cleaning gas may be discharged at a first flow rate for a first cleaning time that is the same as the first adsorption / desorption time when the specific temperature is the first temperature, and the cleaning gas may be discharged at a second flow rate that is smaller than the first flow rate for a second cleaning time that is the same as the second adsorption / desorption time when the specific temperature is the second temperature.

[0017] In this embodiment, a portion of the product gas obtained in the adsorption tower undergoing the adsorption process is discharged as a cleaning gas to the adsorption tower undergoing the desorption process. The cleaning gas promotes the discharge of gas remaining in the adsorption tower undergoing the desorption process. If the cleaning gas is discharged at a constant flow rate for the same cleaning time as the adsorption / desorption time, there is a risk that more product gas than necessary will be consumed as cleaning gas. Furthermore, the effect of reducing the feed gas supply rate by extending the adsorption / desorption time in response to a decrease in the specific temperature will be reduced as the product gas is consumed as cleaning gas.

[0018] Therefore, when the specific temperature is the first temperature, the cleaning gas is discharged at a first flow rate for a first cleaning time that is the same as the first adsorption / desorption time, and when the specific temperature is the second temperature, the cleaning gas is discharged at a second flow rate that is smaller than the first flow rate for a second cleaning time that is the same as the second adsorption / desorption time. This prevents the product gas from being consumed as cleaning gas more than necessary while maintaining the effect of promoting the discharge of gas remaining in the adsorption tower by the discharge of the cleaning gas. Furthermore, the supply amount of raw material gas can be reduced by reducing the consumption of product gas as cleaning gas. Therefore, when a portion of the product gas is discharged as cleaning gas, nitrogen gas of the desired purity can be obtained with reduced energy.

[0019] In the above-described nitrogen gas separation method, when a portion of the product gas obtained in the adsorption tower undergoing the adsorption step is discharged as a cleaning gas to the adsorption tower undergoing the desorption step, the cleaning gas may be discharged for a first cleaning time that is shorter than the first adsorption / desorption time when the specific temperature is the first temperature, and the cleaning gas may be discharged for a second cleaning time that is shorter than the second adsorption / desorption time when the specific temperature is the second temperature.

[0020] In this embodiment, when a portion of the product gas is discharged as cleaning gas, the cleaning gas is discharged for a first cleaning time shorter than the first adsorption / desorption time when the specific temperature is a first temperature, and the cleaning gas is discharged for a second cleaning time shorter than the second adsorption / desorption time when the specific temperature is a second temperature. This prevents the product gas from being consumed as cleaning gas more than necessary while maintaining the effect of promoting the discharge of gas remaining in the adsorption tower by the discharge of cleaning gas. Furthermore, the supply amount of raw material gas can be reduced by reducing the consumption of product gas as cleaning gas. Therefore, when a portion of the product gas is discharged as cleaning gas, nitrogen gas of the desired purity can be obtained with reduced energy.

[0021] The nitrogen gas separation device according to the present invention is filled with an adsorbent and separates a raw material gas containing nitrogen gas and oxygen gas. By compressor two or more adsorption towers that separate nitrogen gas as a product gas from the raw material gas in response to supply under pressure; a product tank into which the product gas obtained in each of the adsorption towers is introduced; and a flow meter that measures the flow rate of the product gas flowing out of the product tank; The temperature of the raw gas supplied by the compressor is indicated by The system includes a thermometer that measures a specific temperature that can affect the temperature of the adsorbent, and a control unit that controls each of the adsorption towers to repeatedly perform an adsorption process, a pressure equalization process, a desorption process, and a pressure equalization process so as to obtain the product gas. The control unit performs adsorption / desorption control to extend the adsorption / desorption time, during which each of the adsorption towers performs the adsorption and desorption processes, relative to a reference adsorption / desorption time, in response to a decrease in the flow rate measured by the flow meter from a reference flow rate. In the adsorption / desorption control, the control unit sets a first adsorption / desorption time that is longer than the reference adsorption / desorption time when the temperature measured by the thermometer is a first temperature, and sets a second adsorption / desorption time that is longer than the first adsorption / desorption time when the measured temperature is a second temperature that is lower than the first temperature. and reducing the amount of the raw material gas supplied from the compressor in response to a decrease in the measured temperature from the first temperature to the second temperature. .

[0022] The nitrogen gas separation device according to the present invention is filled with an adsorbent and separates a raw material gas containing nitrogen gas and oxygen gas. By compressor two or more adsorption towers for separating nitrogen gas as a product gas from the feed gas in response to the feed gas being supplied under pressure; The temperature of the raw gas supplied by the compressor is indicated byThe system includes a thermometer that measures a specific temperature that can affect the temperature of the adsorbent, and a control unit that controls each of the adsorption towers to repeatedly perform the adsorption process, the pressure equalization process, the desorption process, and the pressure equalization process so that the product gas can be obtained. The control unit controls the adsorption and desorption time, during which each of the adsorption towers performs the adsorption and desorption processes, to be a first adsorption and desorption time when the temperature measured by the thermometer is a first temperature, and to be a second adsorption and desorption time that is longer than the first adsorption and desorption time when the measured temperature is a second temperature that is lower than the first temperature. and reducing the amount of the raw material gas supplied from the compressor in response to a decrease in the measured temperature from the first temperature to the second temperature. .

[0023] In the nitrogen gas separation apparatus, the control unit may perform control so that the second adsorption / desorption time is longer than the first adsorption / desorption time and is three times or less than the first adsorption / desorption time.

[0024] The nitrogen gas separation apparatus may further include a cleaning gas outlet line for discharging a portion of the product gas obtained in the adsorption tower undergoing the adsorption step as a cleaning gas to the adsorption tower undergoing the desorption step, and a flow rate adjustment unit for adjusting the flow rate of the cleaning gas flowing through the cleaning gas outlet line. In this case, the control unit controls the flow rate adjustment unit so that the cleaning gas flows through the cleaning gas outlet line at a first flow rate during a first cleaning time that is the same as the first adsorption / desorption time when the measured temperature is the first temperature, and so that the cleaning gas flows through the cleaning gas outlet line at a second flow rate that is smaller than the first flow rate during a second cleaning time that is the same as the second adsorption / desorption time when the measured temperature is the second temperature.

[0025] The nitrogen gas separation apparatus may further include a cleaning gas outlet line for causing a portion of the product gas obtained in the adsorption tower undergoing the adsorption step to flow as a cleaning gas to the adsorption tower undergoing the desorption step, and a flow path opening / closing unit for opening and closing the flow path of the cleaning gas outlet line. In this case, the control unit controls the flow path opening / closing unit so that the cleaning gas flows out of the cleaning gas outlet line in a first cleaning time that is shorter than the first adsorption / desorption time when the measured temperature is the first temperature, and so that the cleaning gas flows out of the cleaning gas outlet line in a second cleaning time that is shorter than the second adsorption / desorption time when the measured temperature is the second temperature. [Effects of the Invention]

[0026] As described above, according to the present invention, nitrogen gas can be obtained with less energy consumption. [Brief explanation of the drawings]

[0027] [Figure 1] 1 is a diagram showing the configuration of a nitrogen gas separation device according to one embodiment of the present invention. DETAILED DESCRIPTION OF THE INVENTION

[0028] Hereinafter, a nitrogen gas separation method and a nitrogen gas separation apparatus according to an embodiment of the present invention will be described with reference to the drawings.

[0029] The nitrogen gas separation apparatus 1 shown in Fig. 1 is an apparatus for separating nitrogen gas from a raw material gas containing nitrogen gas and oxygen gas to obtain a product gas containing nitrogen gas. The raw material gas may be, for example, air, but is not limited thereto, and may be any gas containing at least nitrogen gas and oxygen gas.

[0030] The nitrogen gas separation apparatus 1 includes a first adsorption tower 1A and a second adsorption tower 1B that constitute two or more adsorption towers, a product tank 2, a control unit 20, and a thermometer 23. The nitrogen gas separation apparatus 1 is also connected to a compressor 3.

[0031] The compressor 3 draws in the raw material gas through an inlet, pressurizes the drawn raw material gas, and discharges it from an outlet. The compressor 3 discharges the raw material gas so as to maintain a constant discharge pressure. A raw material gas supply line L1 is connected to the outlet of the compressor 3. The raw material gas supply line L1 is connected to the inlet of the first adsorption tower 1A via the first adsorption tower inlet line L1A and to the inlet of the second adsorption tower 1B via the second adsorption tower inlet line L1B. The compressor 3 supplies the raw material gas at a predetermined pressure to the first and second adsorption towers 1A and 1B via the raw material gas supply line L1 and the first and second adsorption tower inlet lines L1A and L1B. When air is used as the raw material gas, the compressor 3 draws air from the atmosphere. When a gas other than air is used as the raw material gas, the compressor 3 is connected to, for example, a raw material gas source containing the raw material gas in a container.

[0032] The first adsorption tower inlet line L1A is provided with a first intake valve CV1. The second adsorption tower inlet line L1B is provided with a second intake valve CV3. Furthermore, a first equalizing line L8 is provided connecting the first adsorption tower inlet line L1A and the second adsorption tower inlet line L1B. The first equalizing line L8 is provided with a first equalizing valve CV8. The first intake valve CV1, the second intake valve CV3, and the first equalizing valve CV8 are configured as valves that can be switched between open and closed states.

[0033] A gas discharge line L2 is connected to the first and second adsorption tower inlet lines L1A and L1B. The gas discharge line L2 includes a first discharge line L2A connected to the first adsorption tower inlet line L1A downstream of the first intake valve CV1, a second discharge line L2B connected to the second adsorption tower inlet line L1B downstream of the second intake valve CV3, and a discharge junction line L2C connected to the junction of the first discharge line L2A and the second discharge line L2B. A first discharge valve CV2 is provided in the first discharge line L2A. A second discharge valve CV4 is provided in the second discharge line L2B. The first discharge valve CV2 and the second discharge valve CV4 are configured as valves that can be switched between open and closed states.

[0034] The first adsorption tower 1A and the second adsorption tower 1B are filled with an adsorbent that adsorbs oxygen gas. When a raw material gas is supplied from the compressor 3, the first adsorption tower 1A and the second adsorption tower 1B separate nitrogen gas from the raw material gas by adsorbing oxygen gas in the raw material gas with the adsorbent, thereby producing a product gas containing a high concentration of nitrogen gas. The adsorbent filled in the first and second adsorption towers 1A and 1B may be any material that can adsorb oxygen gas, and for example, molecular sieve carbon may be used.

[0035] Molecular sieve carbon is a wood-, coal-, resin-, or pitch-based adsorbent made by carbonizing a pore-rich raw material, such as charcoal, coal, coke, coconut shell, resin, or pitch, at high temperatures to adjust the pore size to approximately 3 to 5 angstroms. Such molecular sieve carbon has the property of more easily adsorbing oxygen gas than nitrogen gas, and thus selectively adsorbs oxygen gas from a gas mixture containing nitrogen gas and oxygen gas, such as air. Furthermore, the oxygen gas adsorption capacity of molecular sieve carbon increases under high-pressure conditions. Therefore, molecular sieve carbon can adsorb a large amount of oxygen gas by pressurizing the first and second adsorption towers 1A and 1B, and then desorb the oxygen gas by depressurizing the first and second adsorption towers 1A and 1B.

[0036] The product gas produced in the first and second adsorption towers 1A, 1B flows through the product gas withdrawal line L3 and is introduced into the product tank 2. The product gas withdrawal line L3 includes a first adsorption tower outlet line L3A connected to the outlet of the first adsorption tower 1A, a second adsorption tower outlet line L3B connected to the outlet of the second adsorption tower 1B, and a product gas confluence line L3C where the first adsorption tower outlet line L3A and the second adsorption tower outlet line L3B join and are connected to the product tank 2.

[0037] The first adsorption tower outlet line L3A is provided with a first extraction valve CV5. The second adsorption tower outlet line L3B is provided with a second extraction valve CV6. Furthermore, a second equalization line L7 is provided, connecting the upstream side of the first extraction valve CV5 in the first adsorption tower outlet line L3A to the upstream side of the second extraction valve CV6 in the second adsorption tower outlet line L3B. The second equalization line L7 is provided with a second equalization valve CV7. The first extraction valve CV5, the second extraction valve CV6, and the second equalization valve CV7 are each composed of valves that can be switched between open and closed states.

[0038] A cleaning gas outflow line L9 is connected to the second equalizing line L7 so as to bypass the second equalizing valve CV7 in the second equalizing line L7. The cleaning gas outflow line L9 is configured to have a smaller pipe diameter than the second equalizing line L7. In other words, because the cleaning gas outflow line L9 is a line for cleaning gas, the flow rate of the cleaning gas flowing as part of the product gas is configured to be smaller than the flow rate of the product gas flowing through the second equalizing line L7.

[0039] A cleaning gas flow rate control valve CV9 is provided in the cleaning gas outlet line L9. The cleaning gas flow rate control valve CV9 is provided to send a portion of the product gas as cleaning gas from the adsorption tower undergoing the adsorption process to the adsorption tower undergoing the desorption process, thereby facilitating the discharge of the raw material gas remaining in the adsorption tower undergoing the desorption process. The cleaning gas flow rate control valve CV9 is configured to be able to adjust the aperture of the cleaning gas outlet line L9 under the control of a control unit 20, which will be described later. The cleaning gas flow rate control valve CV9 functions as a flow rate control unit that adjusts the flow rate of the cleaning gas flowing through the cleaning gas outlet line L9 by adjusting the aperture of the cleaning gas outlet line L9. The cleaning gas flow rate control valve CV9 also functions as a flow path opening / closing unit that opens and closes the flow path of the cleaning gas outlet line L9.

[0040] The product gas obtained in the first and second adsorption towers 1A, 1B is introduced into the product tank 2 via the product gas merging line L3C. The product tank 2 is composed of a container having a temporary storage space for appropriately storing the introduced product gas, and the nitrogen gas concentration in the product gas stored in the product tank 2 is leveled. A product gas outlet line L10 is connected to the product tank 2. The product gas stored in the product tank 2 is discharged from the product tank 2 through the product gas outlet line L10. The product gas discharged from the product tank 2 is used by the user.

[0041] An oxygen concentration meter 21, a flow meter 22, and a product gas outlet valve 10 are provided on the product gas outlet line L10.

[0042] Oxygen concentration meter 21 measures the concentration of oxygen gas contained in the product gas flowing out from product tank 2. Control unit 20, which will be described later, calculates the concentration of nitrogen gas in the product gas based on the oxygen gas concentration measured by oxygen concentration meter 21, and calculates the nitrogen purity of the product gas based on the calculation result. Oxygen concentration meter 21 is located downstream of product tank 2 on product gas outflow line L10.

[0043] Flow meter 22 measures the flow rate of the product gas flowing out from product tank 2. Flow meter 22 is disposed on product gas outlet line L10 between oxygen concentration meter 21 and product gas outlet valve 10. Note that flow meter 22 may also be disposed on product gas outlet line L10 between product tank 2 and oxygen concentration meter 21.

[0044] The product gas outlet valve 10 is a valve for causing the product gas stored in the product tank 2 to flow out of the product tank 2. The product gas outlet valve 10 is configured as a valve that can be switched between open and closed states.

[0045] The thermometer 23 measures a specific temperature that may affect the temperature of the adsorbent filled in the first and second adsorption towers 1A and 1B. The specific temperature may be, for example, the ambient temperature of the nitrogen gas separation apparatus 1, the ambient temperature of the compressor 3, the temperature inside the first and second adsorption towers 1A and 1B, or the temperature of the raw gas supplied by the compressor 3. If the thermometer 23 is installed in the space where the nitrogen gas separation apparatus 1 is installed, the thermometer 23 measures the ambient temperature of the nitrogen gas separation apparatus 1 as the specific temperature. If the thermometer 23 is installed in the space where the compressor 3 is installed, the thermometer 23 measures the ambient temperature of the compressor 3 as the specific temperature. If the thermometer 23 is installed so that its temperature measuring portion is inserted into the first and second adsorption towers 1A and 1B, the thermometer 23 measures the temperature inside the first and second adsorption towers 1A and 1B as the specific temperature. When the thermometer 23 is provided near the intake or discharge port of the compressor 3, the thermometer 23 measures the temperature of the raw material gas supplied by the compressor 3 as the specific temperature.

[0046] The range of change in the specific temperature is assumed to be the environment in which the nitrogen gas separation device 1 is installed, ranging from the summer environmental temperature (e.g., approximately 40°C) to the winter environmental temperature in cold regions (e.g., temperatures below freezing).

[0047] The control unit 20 is electrically connected to the valves CV1 to CV9 and the product gas outlet valve 10. The control unit 20 controls the opening and closing of the valves CV1 to CV9 in accordance with the opening and closing of the product gas outlet valve 10 so that the adsorption process, pressure equalization process, desorption process, and pressure equalization process can be repeatedly performed in the first and second adsorption towers 1A and 1B.

[0048] Specifically, when the control unit 20 subjects the first adsorption tower 1A to the adsorption process and the second adsorption tower 1B to the desorption process, it opens the first intake valve CV1, the first extraction valve CV5, and the second discharge valve CV4, and closes the second intake valve CV3, the second extraction valve CV6, the first discharge valve CV2, the first equalizing valve CV8, and the second equalizing valve CV7.

[0049] When the control unit 20 subjects the first adsorption tower 1A to the desorption process and the second adsorption tower 1B to the adsorption process, it closes the first intake valve CV1, the first extraction valve CV5, the second discharge valve CV4, the first equalizing valve CV8, and the second equalizing valve CV7, and opens the second intake valve CV3, the second extraction valve CV6, and the first discharge valve CV2.

[0050] When the control unit 20 subjects the first and second adsorption towers 1A, 1B to the pressure equalization process, it closes the first intake valve CV1, the second intake valve CV3, the first discharge valve CV2, the second discharge valve CV4, the first extraction valve CV5, and the second extraction valve CV6, and opens the first equalization valve CV8 and the second equalization valve CV7.

[0051] The control unit 20 controls the opening and closing of the valves CV1 to CV8 to control the adsorption and desorption times during which the first and second adsorption towers 1A, 1B perform the adsorption process and the desorption process, and also controls the pressure equalization time during which the first and second adsorption towers 1A, 1B perform the pressure equalization process. Furthermore, the control unit 20 controls the cleaning gas flow rate adjustment valve CV9 during the desorption process to adjust the outflow flow rate of the cleaning gas and to control the cleaning time according to the outflow of the cleaning gas.

[0052] The control unit 20 controls the adsorption / desorption time in the first and second adsorption towers 1A, 1B, and also controls the outflow flow rate of the cleaning gas or the cleaning time, based on the outflow flow rate of the product gas from the product tank 2 measured by the flow meter 22 in response to the opening and closing of the product gas outflow valve 10, and a specific temperature measured by the thermometer 23 that may affect the temperature of the adsorbent.

[0053] Next, a nitrogen gas separation method using the nitrogen gas separation apparatus 1 configured as above will be described.

[0054] When the adsorption step is performed in the first adsorption tower 1A, the desorption step is performed in the second adsorption tower 1B. The adsorption step is a step of separating nitrogen gas from a raw material gas to produce a product gas containing nitrogen gas. The desorption step is a step of regenerating the adsorbent by desorbing oxygen gas adsorbed on the adsorbent from the adsorbent.

[0055] The adsorption process in the first adsorption tower 1A and the desorption process in the second adsorption tower 1B are initiated by opening the first intake valve CV1, the first extraction valve CV5, the second discharge valve CV4, and the cleaning gas flow control valve CV9, and closing the first exhaust valve CV2, the second intake valve CV3, the second extraction valve CV6, and the first and second equalization valves CV8 and CV7.

[0056] In the adsorption step in the first adsorption tower 1A, first, a raw material gas is supplied from the compressor 3 to the first adsorption tower 1A via the raw material gas supply line L1 and the first adsorption tower inlet line L1A. The oxygen gas in the raw material gas supplied to the first adsorption tower 1A is adsorbed by the adsorbent, causing nitrogen gas to be separated from the raw material gas, producing a product gas containing nitrogen gas. The product gas produced in the first adsorption tower 1A is introduced into the product tank 2 via the first adsorption tower outlet line L3A and the product gas merging line L3C. The product gas introduced into the product tank 2 is discharged from the product tank 2 via the product gas outlet line L10 by opening the product gas outlet valve 10. A portion of the product gas produced in the first adsorption tower 1A is sent as cleaning gas to the second adsorption tower 1B via the cleaning gas outlet line L9.

[0057] Meanwhile, during the desorption process in the second adsorption tower 1B, the gas in the second adsorption tower 1B, together with the cleaning gas, is discharged via the second adsorption tower inlet line L1B, the second discharge line L2B, and the discharge junction line L2C to the outside at a pressure lower than the pressure inside the second adsorption tower 1B due to the pressure difference. This reduces the pressure inside the second adsorption tower 1B, and the oxygen gas adsorbed on the adsorbent is desorbed from the adsorbent. The desorbed oxygen gas is discharged from the second adsorption tower 1B together with the cleaning gas. This regenerates the adsorbent in the second adsorption tower 1B.

[0058] After the adsorption and desorption processes are completed, the pressure equalization process begins, in which the gas in the first adsorption tower 1A is transferred to the second adsorption tower 1B. The pressure equalization process begins by closing the first intake valve CV1, the first extraction valve CV5, and the second discharge valve CV4, and opening the first and second equalization valves CV8 and CV7.

[0059] In the pressure equalization step, the gas that filled the first adsorption tower 1A moves to the second adsorption tower 1B through the first equalization line L8 and the second equalization line L7.

[0060] After the pressure equalization step is completed, the desorption step in the first adsorption tower 1A and the adsorption step in the second adsorption tower 1B are performed. The desorption step in the first adsorption tower 1A and the adsorption step in the second adsorption tower 1B are initiated by opening the first exhaust valve CV2, the second intake valve CV3, the second extraction valve CV6, and the cleaning gas flow control valve CV9, and closing the first and second equalization valves CV8 and CV7.

[0061] During the adsorption process in the second adsorption tower 1B, the feed gas is supplied from the compressor 3 to the second adsorption tower 1B via the feed gas supply line L1 and the second adsorption tower inlet line L1B. At this time, oxygen gas in the feed gas is adsorbed by the adsorbent, and nitrogen gas is separated from the feed gas to produce a product gas containing nitrogen gas. The product gas produced in the second adsorption tower 1B is introduced into the product tank 2 via the second adsorption tower outlet line L3B and the product gas merging line L3C. The product gas introduced into the product tank 2 is discharged from the product tank 2 through the product gas outlet line L10 by opening the product gas outlet valve 10. A portion of the product gas produced in the second adsorption tower 1B is sent as cleaning gas to the first adsorption tower 1A via the cleaning gas outlet line L9.

[0062] Meanwhile, during the desorption process in the first adsorption tower 1A, the gas in the first adsorption tower 1A, together with the cleaning gas, is discharged to the outside at a pressure lower than the pressure inside the first adsorption tower 1A through the first adsorption tower inlet line L1A, the first discharge line L2A, and the discharge junction line L2C due to the pressure difference. This reduces the pressure inside the first adsorption tower 1A, and the oxygen gas adsorbed on the adsorbent is desorbed from the adsorbent. The desorbed oxygen gas is discharged from the first adsorption tower 1A together with the cleaning gas. This regenerates the adsorbent in the first adsorption tower 1A.

[0063] After the desorption step in the first adsorption tower 1A and the adsorption step in the second adsorption tower 1B are completed, a pressure equalization step is carried out to transfer the gas in the second adsorption tower 1B to the first adsorption tower 1A.

[0064] After the pressure equalization step is completed, the adsorption step is performed in the first adsorption tower 1A and the desorption step is performed in the second adsorption tower 1B. Thereafter, the above cycle is repeated in the first and second adsorption towers 1A and 1B.

[0065] Here, the compressor 3 has a characteristic that the efficiency of generating the raw material gas improves in response to a decrease in a specific temperature, including the ambient temperature of the compressor 3. Therefore, as the specific temperature decreases, the compressor 3 has a surplus capacity in generating the raw material gas, which makes it easier for the pressure in the first and second adsorption towers 1A, 1B, to which the raw material gas is supplied from the compressor 3, to increase. If the pressure in the first and second adsorption towers 1A, 1B increases, the purity of the nitrogen gas obtained in the first and second adsorption towers 1A, 1B may become higher than necessary.

[0066] Furthermore, the adsorption / desorption characteristics of the adsorbent filled in the first and second adsorption towers 1A and 1B change depending on the temperature of the adsorbent. The lower the specific temperature that can affect the temperature of the adsorbent, the slower the molecular motion of the adsorbent, making it more difficult for oxygen gas adsorbed by the adsorbent to be desorbed. In the nitrogen gas separation apparatus 1 according to this embodiment, the adsorption / desorption time in the first and second adsorption towers 1A and 1B is changed depending on the specific temperature measured by the thermometer 23, focusing on the adsorption / desorption characteristics of the adsorbent. In this case, the adsorption / desorption time in the first and second adsorption towers 1A and 1B is changed depending on the specific temperature measured by the thermometer 23, regardless of the flow rate of the product gas outflowing from the product tank 2 measured by the flowmeter 22. Therefore, the installation of the flowmeter 22 may be omitted.

[0067] Specifically, the control unit 20 controls the adsorption / desorption times in the first and second adsorption towers 1A and 1B to be a first adsorption / desorption time when the specific temperature measured by the thermometer 23 is a first temperature, and to be a second adsorption / desorption time longer than the first adsorption / desorption time when the specific temperature is a second temperature lower than the first temperature. As a result, if the adsorbent exhibits characteristics that make it difficult to desorb oxygen gas as the specific temperature decreases, the adsorption / desorption time in the adsorption towers performing the adsorption process is extended to increase the opportunities for oxygen gas to be desorbed from the adsorbent, thereby preventing an excessive increase in the purity of the resulting nitrogen gas. Furthermore, by extending the adsorption / desorption time as the specific temperature decreases, the number of exhausts per unit time in the adsorption towers performing the desorption process is reduced, thereby reducing the amount of exhaust gas, and therefore the amount of raw gas supplied from the compressor 3. Therefore, nitrogen gas of a desired purity can be obtained with reduced energy as the specific temperature decreases.

[0068] Furthermore, in the nitrogen gas separation apparatus 1, if the flow rate of the product gas flowing out from the product tank 2 decreases below the reference flow rate, the purity of the nitrogen gas obtained in the first and second adsorption towers 1A and 1B will become higher than necessary.

[0069] Therefore, in the nitrogen gas separation device 1 of this embodiment, the adsorption and desorption times in the first and second adsorption towers 1A and 1B are controlled based on the outflow flow rate of the product gas from the product tank 2 measured by the flow meter 22 and a specific temperature that may affect the temperature of the adsorbent measured by the thermometer 23.

[0070] Specifically, the control unit 20 performs adsorption / desorption control to extend the adsorption / desorption time in the first and second adsorption towers 1A and 1B relative to the reference adsorption / desorption time in response to a decrease in the product gas outflow rate from the product tank 2 measured by the flowmeter 22 compared to the reference flow rate. As a result, in the adsorption towers performing the adsorption process, a portion of the oxygen gas adsorbed by the adsorbent is desorbed, thereby reducing the purity of the nitrogen gas, which was increased more than necessary due to the decrease in the product gas outflow rate from the product tank 2. This prevents an excessive increase in the purity of the nitrogen gas obtained in the adsorption towers performing the adsorption process. Furthermore, in the adsorption towers performing the desorption process, the number of exhausts per unit time decreases as the adsorption / desorption time increases, thereby reducing the exhaust volume, thereby reducing the amount of raw gas supplied by the compressor 3. Therefore, nitrogen gas of the desired purity can be obtained with reduced energy.

[0071] Furthermore, in the adsorption / desorption control, the control unit 20 sets a first adsorption / desorption time longer than the reference adsorption / desorption time when the specific temperature measured by the thermometer 23 is a first temperature, and sets a second adsorption / desorption time longer than the first adsorption / desorption time when the specific temperature is a second temperature lower than the first temperature. As a result, if the adsorbent exhibits characteristics that make it difficult to desorb oxygen gas as the specific temperature decreases, the adsorption / desorption time in the adsorption tower performing the adsorption process is lengthened to increase the opportunities for oxygen gas to be desorbed from the adsorbent, thereby preventing an excessive increase in the purity of the resulting nitrogen gas. Furthermore, by lengthening the adsorption / desorption time as the specific temperature decreases, the number of exhausts per unit time in the adsorption tower performing the desorption process is further reduced, thereby reducing the amount of exhaust gas, and thereby reducing the amount of raw gas supplied by the compressor 3. Therefore, when the flow rate of the product gas flowing out of the product tank 2 decreases, nitrogen gas of the desired purity can be obtained with more energy savings as the specific temperature decreases.

[0072] Furthermore, in the adsorption / desorption control, the control unit 20 controls the second adsorption / desorption time to be longer than the first adsorption / desorption time but not more than three times the first adsorption / desorption time. In this case, when the specific temperature measured by the thermometer 23 drops to a second temperature lower than the first temperature, the first and second adsorption towers 1A, 1B perform the adsorption process and the desorption process for the second adsorption / desorption time, which is longer than the first adsorption / desorption time but not more than three times the first adsorption / desorption time. This makes it possible to prevent the adsorption / desorption time from becoming too long, while maintaining the effect of obtaining nitrogen gas of a desired purity in an energy-saving manner in response to a drop in the specific temperature.

[0073] In the nitrogen gas separation apparatus 1 according to this embodiment, a portion of the product gas obtained in the adsorption tower undergoing the adsorption process is discharged as a cleaning gas through the cleaning gas outlet line L9 to the adsorption tower undergoing the desorption process. The cleaning gas promotes the discharge of gas remaining in the adsorption tower undergoing the desorption process.

[0074] If the cleaning gas is discharged at a constant flow rate for the cleaning time that is the same as the adsorption / desorption time, there is a risk that more product gas than necessary will be consumed as cleaning gas. Furthermore, the effect of reducing the amount of raw material gas supplied by lengthening the adsorption / desorption time in response to a decrease in the specific temperature measured by the thermometer 23 will be reduced as the product gas is consumed as cleaning gas.

[0075] Therefore, the control unit 20 controls the cleaning gas flow rate control valve CV9 so that the cleaning gas flows out through the cleaning gas outlet line L9 at a first flow rate during a first cleaning time, which is the same as the first adsorption / desorption time, when the specific temperature measured by the thermometer 23 is the first temperature. On the other hand, the control unit 20 controls the cleaning gas flow rate control valve CV9 so that the cleaning gas flows out through the cleaning gas outlet line L9 at a second flow rate, which is smaller than the first flow rate, during a second cleaning time, which is the same as the second adsorption / desorption time, when the specific temperature is the second temperature. This prevents the product gas from being consumed as cleaning gas more than necessary while maintaining the effect of promoting the discharge of gas remaining in the adsorption tower by the flow of cleaning gas into the adsorption tower undergoing the desorption step. Furthermore, the reduction in the consumption of product gas as cleaning gas reduces the amount of raw gas supplied by the compressor 3. Therefore, when a portion of the product gas is discharged as cleaning gas, nitrogen gas of a desired purity can be obtained with reduced energy consumption.

[0076] In this case, the control unit 20 may control the cleaning gas flow rate control valve CV9 so that the cleaning gas flows out through the cleaning gas outlet line L9 for a first cleaning time that is shorter than the first adsorption / desorption time when the specific temperature measured by the thermometer 23 is the first temperature, and so that the cleaning gas flows out through the cleaning gas outlet line L9 for a second cleaning time that is shorter than the second adsorption / desorption time when the specific temperature is the second temperature. In this case, the outflow rate of the cleaning gas in the cleaning gas outlet line L9 is maintained constant at a reference cleaning gas flow rate that is set corresponding to the reference adsorption / desorption time. The first cleaning time and the second cleaning time may be the same, or the second cleaning time may be shorter than the first cleaning time. By shortening the cleaning time compared to the adsorption / desorption time while maintaining a constant outflow rate of the cleaning gas, it is possible to suppress excessive consumption of the product gas as cleaning gas while maintaining the effect of promoting the discharge of gas remaining in the adsorption tower by the flow of the cleaning gas into the adsorption tower undergoing the desorption step. Moreover, by suppressing consumption of the product gas as cleaning gas, it is possible to reduce the amount of raw material gas supplied by the compressor 3. Therefore, when part of the product gas is discharged as cleaning gas, nitrogen gas of a desired purity can be obtained with reduced energy consumption. [Example]

[0077] A specific example carried out using the nitrogen gas separation apparatus 1 shown in FIG. 1 will be described below.

[0078] Example 1 In Example 1, as shown in Table 1 below, the flow rate of the product gas flowing out of the product tank 2 is 20 Nm 3 / h, the adsorption / desorption time in the first and second adsorption towers 1A and 1B is set to the standard adsorption / desorption time of 40 seconds, and the flow rate of the cleaning gas is set to the standard cleaning gas flow rate of 4.0 Nm 3 / h, and the cleaning time with the cleaning gas was set to the standard cleaning time of 40 seconds. The flow rate of the product gas flowing out of the product tank 2 was set to 20 Nm 3 / h 15Nm 3 / h, 10Nm 3 / h, 7Nm 3 / h, the adsorption / desorption time in the first and second adsorption towers 1A and 1B was extended from the standard adsorption / desorption time of 40 seconds, the flow rate of the cleaning gas was maintained constant at the standard flow rate, and the cleaning time was set to the same as the extended adsorption / desorption time. Furthermore, when the flow rate of the product gas flowing out of the product tank 2 decreased from the standard flow rate, the adsorption / desorption time in the first and second adsorption towers 1A and 1B was lengthened in response to the decrease in the specific temperature measured by the thermometer 23 to 35°C, 20°C, and 5°C.

[0079] [Table 1]

[0080] (Comparative Example 1) Comparative Example 1 differs from Example 1 in that, as shown in Table 2 below, when the flow rate of the product gas flowing out of product tank 2 decreased relative to the reference flow rate, the adsorption and desorption times in first and second adsorption towers 1A and 1B remained constant even when the specific temperature measured by thermometer 23 decreased to 35°C, 20°C, and 5°C. Other than this, the same procedures were followed as in Example 1.

[0081] [Table 2]

[0082] Tables 1 and 2 show the raw material gas amount, which indicates the amount of raw material gas supplied by compressor 3, and the product gas nitrogen concentration, which indicates the nitrogen purity of the product gas, calculated from the oxygen gas concentration measured by oxygen concentration meter 21. As is clear from a comparison between Tables 1 and 2, in Example 1, compared to Comparative Example 1, when the flow rate of the product gas flowing out from product tank 2 decreases, the amount of raw material gas supplied by compressor 3 is smaller while maintaining the nitrogen purity of the product gas at the desired purity (99.99%). This shows that in Example 1, when the flow rate of the product gas flowing out from product tank 2 decreases, nitrogen gas of the desired purity can be obtained with less energy in accordance with a decrease in the specific temperature.

[0083] Example 2 In Example 2, as shown in Table 3 below, the flow rate of the product gas flowing out of the product tank 2 is 20 Nm 3 / h 15Nm 3 / h, the adsorption / desorption times in the first and second adsorption towers 1A and 1B were increased in response to the specific temperature measured by the thermometer 23 decreasing to 35°C, 20°C, and 5°C, and the flow rate of the cleaning gas was decreased while the cleaning time with the cleaning gas was kept the same as the adsorption / desorption time.

[0084] [Table 3]

[0085] Similar to Table 1, Table 3 shows the raw material gas amount indicating the amount of raw material gas supplied by compressor 3, and the product gas nitrogen concentration indicating the nitrogen purity of the product gas calculated from the oxygen gas concentration measured by oxygen concentration meter 21. As is clear from a comparison of Tables 1 and 3, in Example 2, when a portion of the product gas is discharged as cleaning gas, if the flow rate of the product gas discharged from product tank 2 decreases, the flow rate of the cleaning gas is reduced in accordance with the decrease in the specific temperature, thereby making it possible to reduce the amount of raw material gas supplied by compressor 3 while maintaining the nitrogen purity of the product gas at the desired purity (99.99%). This shows that in Example 2, when a portion of the product gas is discharged as cleaning gas, nitrogen gas of the desired purity can be obtained with reduced energy.

[0086] Example 3 In Example 3, as shown in Table 4 below, the flow rate of the product gas flowing out of the product tank 2 is 20 Nm 3 / h 15Nm 3 / h, the specific temperature measured by the thermometer 23 changes to 35°C, 20°C, and 5°C, and accordingly, the adsorption / desorption times in the first and second adsorption towers 1A and 1B are changed to be longer, and the flow rate of the cleaning gas is changed to 4.0 Nm3, which is the standard flow rate of the cleaning gas. 3 The washing time with the washing gas was set to be shorter than the adsorption / desorption time while maintaining the flow rate constant at 1 / h.

[0087] [Table 4]

[0088] Similar to Table 1, Table 4 shows the feed gas amount indicating the amount of feed gas supplied by compressor 3, and the product gas nitrogen concentration indicating the nitrogen purity of the product gas calculated from the oxygen gas concentration measured by oxygen concentration meter 21. As is clear from a comparison of Tables 1 and 4, in Example 3, when a portion of the product gas is discharged as a cleaning gas, if the flow rate of the product gas discharged from product tank 2 decreases, the amount of feed gas supplied by compressor 3 can be reduced while maintaining the nitrogen purity of the product gas at the desired purity (99.99%) by making the cleaning time with the cleaning gas shorter than the adsorption / desorption time. This shows that in Example 3, when a portion of the product gas is discharged as a cleaning gas, nitrogen gas of the desired purity can be obtained with reduced energy. [Explanation of symbols]

[0089] 1. Nitrogen gas separator 1A 1st adsorption tower 1B 2nd adsorption tower 2 Product tank 20 Control Unit 22 Flow meter 23 Thermometer CV9 Cleaning gas flow control valve (flow control part, flow path opening and closing part) L9 Cleaning gas outflow line

Claims

1. A nitrogen gas separation method comprising: supplying a feed gas containing nitrogen gas and oxygen gas under pressure by a compressor to two or more adsorption towers filled with an adsorbent; separating nitrogen gas from the feed gas as a product gas by repeatedly performing an adsorption step, a pressure equalization step, a desorption step, and a pressure equalization step in each adsorption tower; and introducing the product gas into a product tank, an adsorption / desorption time during which each of the adsorption towers performs an adsorption step and a desorption step is extended relative to the standard adsorption / desorption time in response to a decrease in the flow rate of the product gas flowing out of the product tank compared to a standard flow rate; a first adsorption / desorption time longer than the reference adsorption / desorption time when a specific temperature, which is indicated by the temperature of the raw material gas supplied by the compressor and which may affect the temperature of the adsorbent, is a first temperature; a second adsorption / desorption time longer than the first adsorption / desorption time when the specific temperature is a second temperature lower than the first temperature; and an amount of the raw material gas supplied from the compressor is reduced as the specific temperature decreases from the first temperature to the second temperature.

2. A nitrogen gas separation method comprising: supplying a feed gas containing nitrogen gas and oxygen gas under pressure by a compressor to two or more adsorption towers filled with an adsorbent; and separating nitrogen gas from the feed gas as a product gas by repeatedly performing an adsorption step, a pressure equalization step, a desorption step, and a pressure equalization step in each adsorption tower; a first adsorption / desorption time is an adsorption / desorption time during which each adsorption tower performs an adsorption process and a desorption process when a specific temperature, which is indicated by the temperature of the raw material gas supplied by the compressor and which may affect the temperature of the adsorbent, is a first temperature; and a second adsorption / desorption time is a second adsorption / desorption time that is longer than the first adsorption / desorption time when the specific temperature is a second temperature that is lower than the first temperature, and the amount of the raw material gas supplied from the compressor is reduced as the specific temperature decreases from the first temperature to the second temperature.

3. 3. The nitrogen gas separation method according to claim 1, wherein the second adsorption / desorption time is longer than the first adsorption / desorption time and is not more than three times the first adsorption / desorption time.

4. 4. The nitrogen gas separation method according to claim 1, wherein, when a portion of the product gas obtained in an adsorption tower subjected to an adsorption step is discharged as a cleaning gas to an adsorption tower subjected to a desorption step, the cleaning gas is discharged at a first flow rate for a first cleaning time that is the same as the first adsorption / desorption time when the specific temperature is the first temperature, and the cleaning gas is discharged at a second flow rate that is smaller than the first flow rate for a second cleaning time that is the same as the second adsorption / desorption time when the specific temperature is the second temperature.

5. 4. The nitrogen gas separation method according to claim 1, wherein, when a portion of the product gas obtained in an adsorption tower subjected to an adsorption step is discharged as a cleaning gas to an adsorption tower subjected to a desorption step, the cleaning gas is discharged for a first cleaning time that is shorter than the first adsorption / desorption time when the specific temperature is the first temperature, and the cleaning gas is discharged for a second cleaning time that is shorter than the second adsorption / desorption time when the specific temperature is the second temperature.

6. two or more adsorption towers filled with an adsorbent, each of which separates nitrogen gas as a product gas from a raw material gas containing nitrogen gas and oxygen gas in response to the raw material gas being supplied under pressure by a compressor; a product tank into which the product gas obtained in each of the adsorption towers is introduced; a flow meter that measures the flow rate of the product gas flowing out of the product tank; a thermometer for measuring a specific temperature indicated by the temperature of the raw material gas supplied by the compressor and which may affect the temperature of the adsorbent; a control unit that controls the adsorption step, the pressure equalization step, the desorption step, and the pressure equalization step to be repeatedly performed in each of the adsorption towers so as to obtain the product gas, The control unit In response to the flow rate measured by the flow meter decreasing below the reference flow rate, an adsorption / desorption control is performed to extend the adsorption / desorption time during which each of the adsorption towers performs the adsorption step and the desorption step relative to the reference adsorption / desorption time; The adsorption / desorption control is performed by setting a first adsorption / desorption time that is longer than the reference adsorption / desorption time when the temperature measured by the thermometer is a first temperature, and setting a second adsorption / desorption time that is longer than the first adsorption / desorption time when the measured temperature is a second temperature that is lower than the first temperature, and by reducing the amount of the raw material gas supplied from the compressor as the measured temperature decreases from the first temperature to the second temperature.

7. two or more adsorption towers filled with an adsorbent, each of which separates nitrogen gas as a product gas from a raw material gas containing nitrogen gas and oxygen gas in response to the raw material gas being supplied under pressure by a compressor; a thermometer for measuring a specific temperature indicated by the temperature of the raw material gas supplied by the compressor and which may affect the temperature of the adsorbent; a control unit that controls the adsorption step, the pressure equalization step, the desorption step, and the pressure equalization step to be repeatedly performed in each of the adsorption towers so as to obtain the product gas, The control unit controls the adsorption / desorption time, during which each adsorption tower performs the adsorption process and the desorption process, to be a first adsorption / desorption time when the temperature measured by the thermometer is a first temperature, and to be a second adsorption / desorption time that is longer than the first adsorption / desorption time when the measured temperature is a second temperature that is lower than the first temperature, and reduces the amount of the raw material gas supplied from the compressor as the measured temperature decreases from the first temperature to the second temperature.

8. 8. The nitrogen gas separation apparatus according to claim 6, wherein the control unit controls the second adsorption / desorption time to be longer than the first adsorption / desorption time and to be three times or less the first adsorption / desorption time.

9. a cleaning gas outflow line for allowing a portion of the product gas obtained in the adsorption tower subjected to the adsorption step to flow as cleaning gas to the adsorption tower subjected to the desorption step; a flow rate adjusting unit that adjusts the flow rate of the cleaning gas flowing through the cleaning gas outflow line, The nitrogen gas separation apparatus according to any one of claims 6 to 8, wherein the control unit controls the flow rate adjustment unit so that the cleaning gas flows out of the cleaning gas outflow line at a first flow rate during a first cleaning time that is the same as the first adsorption / desorption time when the measured temperature is the first temperature, and so that the cleaning gas flows out of the cleaning gas outflow line at a second flow rate that is smaller than the first flow rate during a second cleaning time that is the same as the second adsorption / desorption time when the measured temperature is the second temperature.

10. a cleaning gas outflow line for allowing a portion of the product gas obtained in the adsorption tower subjected to the adsorption step to flow as cleaning gas to the adsorption tower subjected to the desorption step; a flow path opening / closing unit that opens and closes a flow path of the cleaning gas outflow line, The nitrogen gas separation apparatus according to any one of claims 6 to 8, wherein the control unit controls the flow path opening and closing unit so that the cleaning gas flows out of the cleaning gas outflow line in a first cleaning time that is shorter than the first adsorption / desorption time when the measured temperature is the first temperature, and so that the cleaning gas flows out of the cleaning gas outflow line in a second cleaning time that is shorter than the second adsorption / desorption time when the measured temperature is the second temperature.

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