Oxygen isotope separation method

Abstract

An oxygen isotope separation method that separates 18O2 and 16O2, said method comprising: an adsorption step in which a gas mixture containing 18O2 and 16O2 is passed through an adsorbent that is -143°C or less, thereby causing the 18O2 to be adsorbed onto the adsorbent and storing unadsorbed gas in a first tank; and a desorption step in which the temperature of the adsorbent is increased, thereby desorbing the gas adsorbed onto the adsorbent and storing the gas in a second tank, wherein the average pore size of the adsorbent determined using nitrogen adsorption measurement is 0.35-0.7 nm.

Description

[Technical Field]

[0001] The present disclosure relates to a method for separating oxygen isotopes. [Background technology]

[0002] As a stable isotope of oxygen, 16 O, 17 O and 18 O is known. 18 O can be used in cancer drugs, environmental research, and as a radioactively labeled probe in isotopic analytical methods such as positron emission tomography (PET), advanced medical imaging, brain disease research, and other useful clinical radiopharmaceuticals. 18 The natural abundance of O2 is only 0.204 atomic percent.

[0003] Therefore, various methods for separating oxygen isotopes have been investigated. For example, in Patent Document 1, a method for separating oxygen isotopes using a distillation method is described. 18 A method for separating O has been described. However, the method described in Patent Document 1 has low exchange efficiency and requires a large-scale device. On the other hand, Patent Documents 2 and 3 disclose methods for selectively adsorbing specific isotopes using an adsorbent. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Japanese Patent Application Publication No. 7-14819 [Patent Document 2] Japanese Patent Application Laid-Open No. 2006-7004 [Patent Document 3] Japanese Patent Application Publication No. 2019-42644 Summary of the Invention [Problem to be solved by the invention]

[0005] However, the methods described in Patent Documents 2 and 3 have room for improvement in terms of separation factor.

[0006] The present disclosure has been made in view of the above, and aims to provide a simpler technique that improves the efficiency of oxygen isotope separation. [Means for solving the problem]

[0007] A method for separating oxygen isotopes according to an embodiment of the present disclosure includes: 18 O2 and 16 A method for separating oxygen isotopes by separating O2, 18 O2 and 16 By passing a mixed gas containing O2 through an adsorbent at temperatures below -161.2°C, 18 The method includes an adsorption step of adsorbing O2 and storing unadsorbed gas in a first tank, and a desorption step of desorbing the gas adsorbed by the adsorbent by increasing the temperature of the adsorbent and storing it in a second tank, where the adsorbent is a zeolite with an average pore width of 0.35 nm to 0.7 nm determined by nitrogen adsorption measurements. Note that zeolites with this average pore width range correspond to zeolites with pore diameters ranging from the collision diameter of the adsorbed molecule (oxygen) to twice the collision diameter.

[0008] According to the above-mentioned oxygen isotope separation method, a zeolite having an average pore width in the above-mentioned range is used as an adsorbent, and the separation is carried out under the above-mentioned low-temperature conditions. 18 O2 and 16 By passing a mixed gas containing O2 through the adsorbent, 18 It is possible to selectively adsorb O2. Furthermore, by increasing the temperature of the adsorbent, the adsorbed gas is desorbed and stored in a second tank. 18 The gas with an enriched O2 concentration can be collected. Therefore, according to the above oxygen isotope separation method, 18 The method for separating oxygen isotopes described above can separate O2 with high separation efficiency. Furthermore, the operation is not complicated, and it is simpler than conventional methods, and can increase the separation efficiency of oxygen isotopes.

[0009] Here, after the desorption step, the gas stored in the second tank is passed through the adsorbent at -161.2°C or lower, thereby 18 The method may further include performing one or more of the following steps: a repeated adsorption step of adsorbing O2 and storing unadsorbed gas in the first tank; and a repeated desorption step of desorbing the gas adsorbed by the adsorbent by increasing the temperature of the adsorbent and storing it in the second tank. As described above, the gas stored in the second tank is passed through the adsorbent again, and the same operation is repeated, thereby desorbing the gas adsorbed in the second tank. 18 The O2 concentration can be further increased.

[0010] In an embodiment, the number of times of the repeated adsorption step and the repeated desorption step may be three or more. In this case, the amount of the adsorbed gas contained in the gas stored in the second tank may be reduced. 18 The O2 concentration can be increased to more than 99.8% by volume, resulting in a higher purity 18 O2 gas can be obtained.

[0011] In an embodiment, the temperature of the adsorbent in the adsorption step may be −200° C. or higher. By setting the temperature of the adsorbent in the adsorption step within the above range, it is possible to achieve separation of oxygen isotopes with high separation efficiency while suppressing the cost required for cooling.

[0012] In an embodiment, the temperature of the adsorbent in the desorption step may be −50° C. or higher. By setting the temperature of the adsorbent in the desorption step within the above range, the amount of the carbon dioxide adsorbed by the adsorbent can be reduced. 18 The O2 can be appropriately desorbed and recovered in a second tank.

[0013] The aforementioned Detachment The desorption step may be an embodiment in which the temperature of the adsorbent is set to 200°C or higher and maintained for 10 minutes or longer. By including the above operation in the desorption step, the amount of the carbon dioxide adsorbed by the adsorbent can be reduced. 18 It is now possible to almost completely remove O2, 18The efficiency of O2 gas recovery can be improved.

[0014] The adsorbent may be configured as an adsorption cell packed in a column. By adopting the above configuration, it is possible to reliably bring the gas into contact with the adsorbent, 18 It can promote the adsorption of O2. [Effects of the Invention]

[0015] The present disclosure provides a technique that is simpler and has improved efficiency in separating oxygen isotopes. [Brief explanation of the drawings]

[0016] [Figure 1] 1(a) and 1(b) are diagrams showing N2 adsorption isotherms of zeolites. [Figure 2] FIG. 2 is a diagram illustrating the schematic configuration of the low-temperature adsorption device. [Figure 3] FIG. 3 is a diagram illustrating the results of adsorption separation. [Figure 4] 4(a) and 4(b) are diagrams illustrating the difference in adsorption and separation performance depending on the temperature. [Figure 5] FIG. 5 is a diagram showing an example of a low-temperature adsorption apparatus in which adsorption cells are arranged in multiple stages. [Figure 6] Figure 6 illustrates the effective size difference of oxygen isotopes, as described by the de Broglie wavelength. [Figure 7] FIG. 7 is a diagram illustrating the relationship between the pores of zeolite and the adsorption of isotopes. DETAILED DESCRIPTION OF THE INVENTION

[0017] In one embodiment, a method for separating oxygen isotopes is provided. 18This is a method for separating O2 using an adsorbent. The method described in this embodiment is facilitated by performing the separation by adsorption at low temperatures. Furthermore, the method described in this embodiment uses an adsorbent with nanopores, resulting in a small effective size described by the de Broglie wavelength. 18 It is thought that the selective entry of O2 into the nanopores improves separation performance.

[0018] (adsorbent) The adsorbent used in this embodiment is zeolite, and in particular, LTA zeolite, MFI zeolite, BEA zeolite, and FAU zeolite are used as the adsorbent. These zeolites have very small channels (tubular pores), and as described below, 18 It is believed to have high O2 adsorption performance. The zeolite framework structure is specified by the International Zeolite Association.

[0019] The pore shape of the zeolite was determined using the N adsorption isotherm at -196.2°C. Prior to the adsorption measurement, each sample was pretreated at 200°C for 6 hours under vacuum (1 mPa) to remove the adsorbed gas and moisture in the pores. Figure 1 shows the N adsorption isotherm of the zeolite. Figure 1(a) shows the N adsorption isotherm, and Figure 1(b) shows the isotherm on a semi-logarithmic scale.

[0020] In Figure 1, adsorption isotherms are shown for zeolites with framework codes K-LTA, Na-LTA, Ca-LTA, MFI, BEA, and FAU.

[0021] In addition, the specific surface area (S BET ), specific surface area (α s The specific surface area (S), pore volume, and average pore width (w) are shown in Table 1. BET ) is a value based on the Brunauer-Emmett-Teller theory (BET) and is the specific surface area (α s ) is α sFurthermore, the pore volume and average pore width (w) were also calculated based on the α s -plot analysis.

[0022] [Table 1]

[0023] From the results shown in Figure 1(a) and 1(b), it can be seen that the Ca-LTA zeolite, MFI zeolite, and BEA zeolite exhibited excellent thermal conductivity at very low relative pressures (P / P<10 -5 ) and the pore filling started. This result suggests that the pores were filled with N2 molecules in a very uniform manner.

[0024] Furthermore, as shown in Table 1, the average pore widths (openings) of K-LTA zeolite, Na-LTA zeolite, Ca-LTA zeolite, MFI zeolite, BEA zeolite, and FAU zeolite were 0.3 nm, 0.4 nm, 0.5 nm, 0.55 nm, 0.66 nm, and 0.74 nm, respectively. K-LTA zeolite and Na-LTA zeolite showed very small adsorption amounts compared to the other zeolites, suggesting the inaccessibility of narrow pores less than 0.4 nm.

[0025] On the other hand, FAU zeolite has a pore volume of 0.36 ml / g -1 The highest specific surface area (α s )854m 2 g -1 Furthermore, other zeolites have smaller specific surface areas compared to FAU zeolites, but have pore widths close to the kinetic diameter of oxygen, providing a kinetic diffusion barrier. This may lead to preferential adsorption of smaller oxygen isotopes.

[0026] (Low temperature adsorption device) Next, a cryogenic adsorption apparatus for separating oxygen isotopes using the above-mentioned adsorbent will be described with reference to FIG. 2. The cryogenic adsorption apparatus 1 includes: 16 O2 and 18The system has a tank T1 filled with a mixed gas of O2 and a tank T2 filled with He gas. Each of the tanks T1 and T2 is equipped with a mass flow controller (MFC).

[0027] Reservoir S1 (second tank) and reservoir S2 (first tank) are tanks for storing gas. While storing gas in reservoirs S1 and S2, the process of adsorption and desorption of isotopes onto the adsorbent is repeated, and analysis is performed using a quadrupole mass spectrometer (MS). Reservoir S1 is a tank for storing isotope-enriched gas, and reservoir S2 is a tank for storing unadsorbed mixed gas.

[0028] Additionally, a pressure gauge G, valves V1 to V11, and vacuum pumps P1 to P3 are provided on the flow path of the mixed gas. The flow path is made up of piping (diameter: 1 / 4 inch), and the gas moves inside the piping.

[0029] Furthermore, an adsorption cell was prepared by packing 50 mg of zeolite sample into a column (Swagelok: SS-4-WVCR-6-DF, 316 stainless steel welded VCR face seal joint, with porous metal gasket) with an inner diameter of 1 / 4 inch and a length of 2 inches. The adsorbent packed in the adsorption cell contains the above-mentioned zeolite at a content of 80% or more. The adsorbent may contain an inorganic binder such as alumina or silica as a secondary component other than zeolite. The density of the zeolite in the adsorption cell was adjusted to, for example, 0.15 g / mL or more. By keeping the zeolite density (packing degree) within the above range, the oxygen isotope separation performance of the zeolite can be maintained within the desired conditions.

[0030] To improve the separation performance of oxygen isotopes, the size (volume) of the adsorption cell is set to 0.15 mL or more. On the other hand, taking into consideration the passage speed of the mixed gas, the size (volume) of the adsorption cell is set to 10 mL or less.

[0031] In the cryogenic adsorption apparatus 1, tank T1 is connected to the upstream side (left side in the figure) of the adsorption cell via valves V1 and V4. Tank T2 is connected to the upstream side (left side in the figure) of the adsorption cell via valves V3 and V4. Gas passing through the adsorption cell can be transferred to reservoir S1 via valves V5 and V8, and to reservoir S2 via valves V5 and V6. Reservoir S1 is further connected to the upstream side (left side in the figure) of the adsorption cell via valves V9, V10, and V4. Gas movement within the flow path is controlled by vacuum pumps P1 and P2, which are connected via valve V2. A quadrupole mass spectrometer (MS) is also provided between valves V11 and V7. Gas analysis can be performed using these valves and vacuum pump P3.

[0032] The adsorption cell of the low-temperature adsorption apparatus 1 may be installed in a cryostat unit that enables pretreatment of the adsorbent under vacuum before an adsorption / separation experiment performed under the conditions described below (-196.2°C to -161.2°C).

[0033] Using the above-mentioned low-temperature adsorption apparatus 1, 16 O2 and 18 O2 can be separated. An example of the procedure will be explained below.

[0034] First, to make the composition ratio 1:1, 16 O2 and 18 The isotope mixed gas with O2 is stored in the tank T1. At this time, by performing an analysis in advance using this mixed gas in the MS, the MS can be calibrated based on the results. In addition, the mixed gas passing through the adsorption cell is kept at 120 Pa ( 16 O2+ 18 O2 = 1:1) flow rate is approximately 1 ml -1 The MFC is adjusted so that 16 O2+ 18The oxygen mixed gas with an O2 ratio of 1:1 is called the equimolar feed gas. The composition change of the mixed gas flow at the outlet of the adsorption cell is analyzed by MS. Furthermore, as a pretreatment, the adsorption cell is heated in vacuum at 200°C for 6 hours. This treatment corresponds to cleaning the adsorption cell.

[0035] 16 O2 and 18 From a gas mixture with O2 18 The specific steps for separating O2 are as follows: (1) (Preliminary treatment) After filling the adsorption cell with a zeolite sample (50 mg to 100 mg), the sample is heat-treated in vacuum at 200°C for 6 hours to remove adsorbed gases such as moisture (the treatment described above). (2) Before introducing the mixed gas, all valves V1 to V11 are opened, and the vacuum pumps P1, P2, and P3 are operated to perform preliminary evacuation. All valves are closed once evacuation has been completed.

[0036] (3: Adsorption step) A certain amount of mixed gas is introduced from tank T1 through MFC into the adsorption cell maintained at a low temperature of -143°C or less. At this time, He in tank T2 acts as a carrier gas. As the mixed gas passes through the adsorbent maintained at a low temperature, smaller particles are formed. 18 The O2 isotope is adsorbed in large quantities into the pores of the microporous adsorbent. The valve is opened and closed so that the unadsorbed mixed gas is collected in reservoir S2. The molar concentration of the isotope in the mixed gas is monitored by MS for each adsorption-desorption cycle. Three conditions were set for acquiring the measurement data described below: -196.2°C, -173°C, and -161.2°C.

[0037] (4: Desorption step) After the mixed gas is passed through the adsorption cell, valves V4, V6, and V9 are closed, and valves V5 and V8 are opened. The temperature of the adsorption cell is increased by removing the liquid N2. As a result, the adsorbed gas (zeolite) in the cell is released. 18 O2 is desorbed by increasing the temperature of the cell. At this time, the gas desorbed from the adsorption cell, i.e. 18O2 is stored in reservoir S1. As the temperature of the zeolite adsorbent rises, the adsorbed gas gradually desorbs from the zeolite. More than 50% of the oxygen molecules adsorbed at -161.2°C are desorbed by lowering the temperature to -140°C, and more than 80% are desorbed at -100°C. Furthermore, most of the oxygen molecules are desorbed at -50°C. Therefore, by raising the temperature to -50°C or higher, the desorption of the adsorbed gas proceeds sufficiently. Note that above -120°C, which is the critical temperature of oxygen (the temperature above which oxygen does not liquefy), the adsorbed gas 18 O2 desorption can proceed smoothly.

[0038] (5: Desorption step) By heating the adsorption cell at 200°C for 6 hours, all gases adsorbed on the adsorbent (zeolite) are desorbed, and the state of the adsorbent is returned to its original state (purification process). The gases desorbed during this time are also stored in the reservoir S1. After that, the gas in the reservoir S1 is introduced again into the adsorption cell, which is maintained at a low temperature. 18 O2 adsorption and temperature-elevated adsorption cell 18 By repeating this process, the isotope concentration of the gas stored in the reservoir S1 can be increased. (6) In another approach, adsorption and concentration of a gas mixture can also be achieved by passing the gas mixture through a series of adsorption cells maintained at low temperatures. (7: Repeated adsorption process, repeated desorption process) As a result of concentration measurement by MS, 18 The above adsorption and desorption cycles were repeated until the O2 concentration reached 99.8% by volume. 18 Repeat the O2 gas enrichment process.

[0039] By the above procedure, highly concentrated HCl was obtained in the reservoir S1. 18 O2 gas can be collected and filled into a cylinder by using the open valve O.

[0040] 18 O2 and 16 The O2 adsorption ratio is determined from the observed mass intensity change using a pre-calibrated MS.16 against O2 18 Adsorption selectivity or separation factor S( 18 O2 / 16 O2) is determined from their adsorption ratios using the following equation (1):

[0041]

number

[0042] where ( 18 O2 / 16 O2) ads is the adsorbed phase 18 O2 and 16 is the mole fraction of O2, ( 18 O2 / 16 O2) g is in the bulk gas phase 18 O2 and 16 is the mole fraction of O2.

[0043] Adsorption and separation measurements were performed on a small number of zeolites at low temperature (-161.2°C). 16 O2 and 18 In the O2 mixed gas, 18 However, as time increased, O2 was preferentially adsorbed. 16 O2 and 18 The difference in adsorption amount with O2 depends greatly on the pore size of the zeolite.

[0044] The results of the above separation are shown in Figure 3. In Figure 3, the separation factor S( 18 O2 / 16The graph shows the time-dependent change in the adsorption capacity of K-LTA zeolite (O2) over a 60-minute period. Note that the average pore diameter of K-LTA zeolite, determined from nitrogen adsorption measurements, is smaller than the diameter of an oxygen molecule (0.35 nm), meaning that most oxygen molecules cannot penetrate the pores and are unable to be adsorbed, resulting in no separation selectivity. Furthermore, when the average pore diameter determined from nitrogen adsorption measurements exceeds 1.5 times the diameter of an oxygen molecule, the adsorption performance and selectivity decrease, and when it exceeds twice the diameter of an oxygen molecule, almost no separation selectivity is observed. Therefore, FAU, with an average pore diameter of 0.74 nm, does not exhibit the ability to separate oxygen isotopes.

[0045] Table 2 also shows the initial separation factor at -161.2°C and the separation factors after 5 and 10 minutes.

[0046] [Table 2]

[0047] According to Figure 3 and Table 2, the difference between the first 10 minutes and after 10 minutes is as follows: 16 against O2 18 It can be seen that there is a large difference in the selectivity of O2. The separation factor S( 18 O2 / 16 O2) was 37±5.6 in the early stage, and S( 18 O2 / 16 O2) = 2.3 ± 0.2 and remained at 1.5 ± 0.1 after 10 minutes.

[0048] On the other hand, MFI zeolite has a high separation factor S( 18 O2 / 16 O2) = 38 ± 3.4, and the separation factor S( 18 O2 / 16 O2) = 1.8 ± 0.2 was maintained. The other zeolites showed slightly inferior selectivity.

[0049] On the other hand, K-LTA zeolite with a pore width of 0.3 nm and FAU zeolite with a pore width of 0.74 nm were unable to selectively adsorb oxygen isotopes under the same experimental conditions as above. However, FAU zeolite was confirmed to have selectivity at temperatures below -183°C. Furthermore, the boiling point of methane, -161.2°C, can be easily achieved using low-temperature energy from the liquefied natural gas industry, which makes it possible to save energy used for separating oxygen isotopes.

[0050] Next, temperature-dependent selectivity will be explained. Figures 4(a) and 4(b) show the results of evaluating temperature-dependent selectivity for Ca-LTA zeolite. Figure 4(a) shows the S( 18 O2 / 16 4(b) shows the temperature dependence of the oxidative stress (O2) over time, and Fig. 4(b) shows the initial stage. As shown in Fig. 4, Ca-LTA zeolite exhibited excellent initial properties even at two temperatures lower than -161.2°C, namely, -196.2°C and -173°C. 18 Shows O2 separation performance.

[0051] As explained above, the low-temperature adsorption separation method has high selectivity. Also, as shown in Figure 5, when a mixed gas is passed through an adsorption cell using zeolite as an adsorbent, the selectivity is determined by the adsorbent, and therefore, 18 It preferentially adsorbs O2. Then, when the adsorbed gas is collected, 18 A gas with an increased concentration of O2 is obtained. 18 When the gas with an increased O2 concentration is passed through the adsorption cell again, it is adsorbed by the cell. 18 By repeating this process, the concentration of O2 becomes 99.8% by volume or more, as shown in Figure 5. 18 Since all zeolites that can be used as adsorbents have high separation factors, O2 gas can be obtained at concentrations higher than 99.8% by volume. 18 The number of adsorption-desorption steps to obtain O2 is estimated to be four.

[0052] Based on the above results, a separation device can be designed in which a series of zeolite-filled adsorption cells are connected and maintained at temperatures ranging from -161.2°C to -196.2°C. In this device, by repeating the concentration process by adsorption-desorption four times, 18 The O2 concentration can be increased from 2000 ppm to over 99.8% by volume. This method is similar to other activated carbons that have been used in industrial applications. 18 Compared to O2 adsorption separation methods, this method is much simpler, faster, more energy-efficient, and can be realized as a compact device.

[0053] The method described in this embodiment provides a highly efficient adsorbent for rapid low-temperature adsorption, and the fast adsorption-desorption process allows for a shorter time. 18 It is possible to concentrate O2. In addition, a completely new principle must be applied to obtain the separation method in this embodiment.

[0054] Recently, the de Broglie wavelength (λ T =(h 2 / 2πmkT) 1 / 2 ), uncertainty based on the effective size difference described by the quantum molecular sieving effect (QMS) is generated in nanopores at low temperatures, resulting in rapid separation of light gas isotopes such as hydrogen and helium. Figure 6 shows a schematic of a molecule exhibiting quantum fluctuations that change the molecular size of oxygen isotopes at low temperatures. However, as shown in Table 3 below, when the temperature is increased from -196.2 °C to -161.2 °C, the difference from the de Broglie wavelength 18 O2 and 16 The size difference of O2 varies from 0.002 nm to 0.0017 nm.

[0055] [Table 3]

[0056] These subtle differences obscure the difference in QMS between the two isotopes. As a result, the current translation-based QMS mechanism alone cannot 18 O2 and 16 Large isotopic gas molecules such as O2 cannot be expected to be efficiently separated using nanoporous materials.

[0057] In addition to translational motion, O2 molecules possess other vibrational and rotational degrees of freedom, the magnitude of which may depend on the molecular weight. However, when adsorbed O2 molecules are isolated from each other, the vibrational states remain essentially intact in the gas and adsorbed phases. In contrast, however, the rotational degrees of freedom confined in nanopores are severely restricted below -143 °C.

[0058] the result, 18 O2 and 16 The difference in rotational energy of O2 and the translational motion combine to form the nanopores of zeolite. 18 This may lead to selective adsorption of O2. When O2 molecules form a partially ordered structure in the nanopores at low temperatures, collective quantum motions associated with vibrational and rotational modes can be observed. 18 O2 and 16 This collective nuclear quantum effect (NQE) of the adsorbed oxygen ensemble, which depends on the molecular weight, can lead to an explicit energy difference between the adsorbed oxygen and the nanoporous material. 16 From O2 18 O2 can be selectively adsorbed.

[0059] In zeolites with nanopores whose pore width is twice the molecular diameter of the isotope gas (2 × σ(O2)), the interconnected cylindrical pores are estimated to have higher confinement capacity than the large nanopores in BEA or FAU zeolites. Therefore, these zeolites are likely to form an ordered O2 adsorption phase, leading to high selectivity. 18 It is estimated that this will realize highly selective adsorption of oxygen isotopes. Figure 7 shows the adsorption of oxygen isotopes when zeolite with channels (tubular pores) and cages is used as an adsorbent. 18 O2 and16 As shown in Figure 7, the small pores of zeolite selectively adsorb O2. 18 Only O2 molecules preferentially enter the pores, 16 As a result, O2 is rejected. 18 It is speculated that a state in which O2 is preferentially adsorbed is achieved.

[0060] (Effects of the embodiment) The oxygen isotope separation method described in the above embodiment includes the steps of: 18 O2 and 16 The method for separating oxygen isotopes from O2 includes an adsorption step and a desorption step. 18 O2 and 16 By passing a mixed gas containing O2 through an adsorbent at temperatures below -143°C, 18 The O2 is adsorbed, and the unadsorbed gas is stored in a first tank (reservoir S2). In the desorption process, the temperature of the adsorbent is increased to desorb the gas adsorbed in the adsorbent and store it in a second tank (reservoir S1). The adsorbent is a zeolite with an average pore width of 0.35 nm to 0.7 nm as measured by nitrogen adsorption.

[0061] According to the above method, a zeolite having an average pore width in the above range is used as an adsorbent, and the adsorbent is heated under the above low temperature conditions. 18 O2 and 16 By passing a mixed gas containing O2 through the adsorbent, 18 It is possible to selectively adsorb O2. Furthermore, by increasing the temperature of the adsorbent, the adsorbed gas is desorbed and stored in a second tank. 18 The gas with an enriched O2 concentration can be collected. Therefore, according to the above oxygen isotope separation method, 18 The method for separating oxygen isotopes described above can separate O2 with high separation efficiency. Furthermore, the operation is not complicated, and it is simpler than conventional methods, and can increase the separation efficiency of oxygen isotopes.

[0062] After the desorption step, the gas stored in the second tank (reservoir S1) is passed through the adsorbent at a temperature of -161.2°C or lower, thereby 18 The method may further include performing a repeated adsorption step of adsorbing O2 and storing unadsorbed gas in a first tank (reservoir S2) and a repeated desorption step of desorbing the gas adsorbed by the adsorbent by increasing the temperature of the adsorbent and storing it in a second tank (reservoir S1) one or more times. As described above, by repeating the operation of passing the gas stored in the second tank through the adsorbent again, the amount of the gas stored in the second tank can be reduced. 18 The O2 concentration can be further increased.

[0063] The number of times the repeated adsorption step and the repeated desorption step are repeated may be three or more. In this case, the adsorption-desorption process is carried out four or more times. Therefore, the amount of the HCl contained in the gas stored in the second tank is reduced. 18 The O2 concentration can be increased to more than 99.8% by volume, resulting in a higher purity 18 O2 gas can be obtained.

[0064] The temperature of the adsorbent in the adsorption step may be -200°C or higher. By setting the temperature of the adsorbent in the adsorption step within the above range, it is possible to achieve separation of oxygen isotopes with high separation efficiency while suppressing the cost required for cooling. Note that a similar temperature can also be set in the repeated adsorption steps. In particular, an example of the temperature of the adsorbent is assumed to be -196.15°C to -161.15°C (77K to 112K), in which case the isotope separation performance can be improved.

[0065] In an embodiment, the temperature of the adsorbent in the desorption step may be −50° C. or higher. By setting the temperature of the adsorbent in the desorption step within the above range, the amount of the carbon dioxide adsorbed by the adsorbent can be reduced. 18 The O2 can be appropriately desorbed and recovered in a second tank.

[0066] DetachmentThe desorption step may be an embodiment in which the temperature of the adsorbent is set to 200°C or higher and maintained for 10 minutes or longer. By including the above operation in the desorption step, the amount of the carbon dioxide adsorbed by the adsorbent can be reduced. 18 It is now possible to almost completely remove O2, 18 This can improve the O2 gas recovery efficiency. In the first desorption step of the process, moisture adsorbed on the zeolite is removed, so it may be necessary to secure a certain length of time for the desorption step (for example, 6 hours). However, since the adsorbent is not released into the atmosphere while the process is repeated, 18 The desorption of O2 will be completed in a very short time, so even if you set it to 10 minutes or more as above, 18 O2 desorption can be performed properly.

[0067] The adsorbent may be configured as an adsorption cell packed in a column. By adopting the above configuration, it is possible to reliably bring the gas into contact with the adsorbent, 18 It can promote the adsorption of O2.

[0068] Although several embodiments have been described above, the present disclosure is not limited to the above embodiments. Furthermore, the descriptions of the above embodiments can be applied to each other. [Explanation of symbols]

[0069] 1...low-temperature adsorption device, S1, S2...reservoirs, T1, T2...tanks, V1~V11...valves.

Claims

1. １８ O ２ and １６ O ２ A method for separating oxygen isotopes, comprising: １８ O ２ and １６ O ２ By passing a mixed gas containing １８ O ２ an adsorption step of adsorbing the gas and storing unadsorbed gas in a first tank; a desorption step of increasing the temperature of the adsorbent to desorb the gas adsorbed by the adsorbent and storing the gas in a second tank; The method for separating oxygen isotopes, wherein the adsorbent is a zeolite having an average pore width of 0.35 nm to 0.7 nm as determined by nitrogen adsorption measurement.

2. After the desorption step, The gas stored in the second tank is passed through the adsorbent at a temperature of −143° C. or less, thereby １８ O ２ 2. The method for separating oxygen isotopes according to claim 1, further comprising: performing, one or more times: a repeated adsorption step of adsorbing the gas onto the adsorbent and storing unadsorbed gas in the first tank; and a repeated desorption step of increasing the temperature of the adsorbent to desorb the gas adsorbed onto the adsorbent and storing the gas in the second tank.

3. 3. The method for separating oxygen isotopes according to claim 2, wherein the repeated adsorption step and the repeated desorption step are repeated three or more times.

4. 4. The method for separating oxygen isotopes according to claim 1, wherein the temperature of the adsorbent in the adsorption step is −200° C. or higher.

5. 5. The method for separating oxygen isotopes according to claim 1, wherein the temperature of the adsorbent in the desorption step is −50° C. or higher.

6. 5. The method for separating oxygen isotopes according to claim 1, wherein the desorption step comprises maintaining the temperature of the adsorbent at 200° C. or higher for 10 minutes or longer.

7. 7. The method for separating oxygen isotopes according to claim 1, wherein the adsorbent is configured as an adsorption cell packed in a column.

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