JP7808301B2 - Oxygen isotope separation method - Google Patents

Description

The present disclosure relates to a method for separating oxygen isotopes.

The stable isotopes of oxygen are known as ^16O , ^17O , and ^18O . Of these, ^18O can be used as a radioactively labeled probe in isotope analysis methods such as positron emission tomography (PET), an advanced medical imaging technique, brain disease research, and other useful clinical radiopharmaceuticals _{, in addition to being used in cancer treatment drugs and environmental surveys. However, the natural abundance of 18O2} ⁱⁿ the atmosphere is only 0.204 atomic %.

Therefore, various methods for separating oxygen isotopes have been investigated. For example, Patent Document 1 describes a method for separating ^18O using a distillation method. 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.

Japanese Unexamined Patent Publication No. 7-14819 Japanese Patent Application Laid-Open No. 2006-7004 Japanese Patent Application Laid-Open No. 2019-42644

However, the methods described in Patent Documents 2 and 3 left room for improvement in terms of separation coefficient.

This disclosure has been made in consideration of the above and aims to provide a technology that is simpler and has improved efficiency in separating oxygen isotopes.

A method for separating oxygen isotopes according to one embodiment of _the present disclosure is a method for separating oxygen isotopes into ^18O2 _and ^16O2 , and includes an adsorption step of passing a mixed gas containing ^18O2 _{and 16O2} ^through an adsorbent at a temperature of -161.2 _° C or lower to adsorb ^18O2 onto the adsorbent and storing unadsorbed _gas in a first tank, and a desorption step of increasing the temperature of the adsorbent to desorb the gas adsorbed onto the adsorbent and store it in a second tank, wherein the adsorbent is a zeolite having an average pore width of 0.35 nm to 0.7 nm determined from nitrogen adsorption measurements. Note that zeolites in 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.

According to ^the above _- described oxygen isotope separation method, a zeolite having an average pore width within the above _- described range is used as an adsorbent, and a mixed gas containing ^18O2 and _16O2 is passed through the adsorbent under the above-described low-temperature conditions, thereby selectively adsorbing ^18O2 onto the adsorbent. Furthermore, by increasing the temperature of the adsorbent, the adsorbed gas is desorbed and stored in a second tank, allowing gas with _{an increased 18O2} ^{concentration} to be collected. Therefore, according to the above-described oxygen isotope separation method, ^18O2 can be separated with high separation efficiency. Furthermore, the above-described oxygen isotope separation method requires simple operations and is simpler than conventional methods _, while achieving higher oxygen isotope separation efficiency.

Here, the method may further include, after the desorption step, performing one or more of the following steps: a repeated adsorption step in which the gas stored in the second tank is passed through the adsorbent at a temperature of _{−161.2°C or lower to adsorb 18O2} ^onto the adsorbent and the unadsorbed gas is stored in the first tank; and a repeated desorption step in which the temperature of the adsorbent is increased to desorb the gas adsorbed onto the adsorbent and store it in the second tank. As described above, by passing the gas stored in the second tank again through the adsorbent and repeating the same operation, the ^18O2 concentration of _the gas stored in the second tank can be further increased.

The number of times the repeated adsorption step and the repeated desorption step are repeated may be three or more times, in _{which case the concentration of 18O2 contained in the gas stored in the second tank can be made greater than 99.8% by volume, thereby obtaining 18O2} ^gas _with ^higher purity.

The temperature of the adsorbent during the adsorption process may be -200°C or higher. By setting the temperature of the adsorbent during the adsorption process within the above range, it is possible to achieve separation of oxygen isotopes with high separation efficiency while suppressing the costs required for cooling.

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 18O2} ^adsorbed by the adsorbent can be appropriately desorbed and recovered in the second tank.

The desorption step may include maintaining the temperature of the adsorbent at 200° C. or higher for 10 minutes or longer. By including the above-described operation in the desorption step, it becomes possible to almost completely desorb the ¹⁸ O ₂ adsorbed by the adsorbent, thereby increasing the recovery efficiency of ^{the 18} O ₂ gas.

The adsorbent may be configured as an adsorption cell packed in a column. This configuration allows the gas to be reliably brought into contact with the adsorbent, _{thereby promoting the adsorption of 18O2} ^.

The present disclosure provides a technology that is simpler and has improved efficiency in separating oxygen isotopes.

1(a) and 1(b) are diagrams showing _N2 adsorption isotherms of zeolites. FIG. 2 is a diagram illustrating the schematic configuration of the low-temperature adsorption device. FIG. 3 is a diagram illustrating the results of adsorption separation. 4(a) and 4(b) are diagrams illustrating the difference in adsorption and separation performance depending on the temperature. FIG. 5 is a diagram showing an example of a low-temperature adsorption apparatus in which adsorption cells are arranged in multiple stages. FIG. 6 is a diagram illustrating the effective size difference of oxygen isotopes described by the de Broglie wavelength. FIG. 7 is a diagram illustrating the relationship between the pores of zeolite and the adsorption of isotopes.

One embodiment of a method for separating oxygen isotopes is a method for separating the isotope _gas ^18O2 from oxygen using an adsorbent. The method described in this embodiment promotes separation by performing separation by adsorption at low temperatures. Furthermore, the method described in this embodiment uses an adsorbent with nanopores, which is thought to improve separation performance by allowing ^18O2 _, which has a small effective size described by the de Broglie wavelength, to selectively enter the nanopores.

(Adsorbent)
The adsorbent used in this embodiment is zeolite, and in particular, LTA zeolite, MFI zeolite, BEA zeolite, and FAU zeolite are used as adsorbents. These zeolites have small channel (tubular pore) volumes and are presumed to have high ^18O2 adsorption performance, as described below. The zeolite framework structure is defined by _the International Zeolite Association.

The pore shape of zeolite was determined using the _N2 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 _N2 adsorption isotherm of zeolite. Figure 1(a) shows the _N2 adsorption isotherm, and Figure 1(b) shows the isotherm on a semi-logarithmic scale.

Figure 1 shows the adsorption isotherms for zeolites with framework codes K-LTA, Na-LTA, Ca-LTA, MFI, BEA, and FAU.

The specific surface area (S _BET ), specific surface area (α _s ), pore volume, and average pore width (w) of the above zeolite obtained from the measurement results shown in Figure 1 are shown in Table 1. The specific surface area (S _BET ) is a value based on the Brunauer-Emmett-Teller theory (BET), and the specific surface area (α _s ) is based on α _s -plot analysis. Furthermore, the pore volume and average pore width (w) were also calculated based on α _s -plot analysis.

The results shown in Figures 1(a) and 1(b) confirm that pore filling begins at very low relative pressures (P/P ₀ <10 ^-5 ) in Ca-LTA zeolite, MFI zeolite, and BEA zeolite. This result suggests very uniform micropore filling with N ₂ molecules.

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 other zeolites, suggesting the inaccessibility of narrow pores less than 0.4 nm.

On the other hand, FAU zeolite exhibits the highest specific surface area (α _s ) of 854 m ² g ^-1 with a pore volume of 0.36 ml/g ^-1 . Furthermore, the other zeolites have smaller specific surface areas compared to FAU zeolite, 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.

(Low temperature adsorption device)
Next, a cryogenic adsorption apparatus for separating oxygen isotopes using ^the above-mentioned adsorbent will be described with reference to Figure 2. The cryogenic adsorption apparatus 1 has a tank T1 filled with a mixed gas of ^16O2 and _18O2 , and a tank _T2 filled with He gas. A mass flow controller (MFC) is attached to each of the tanks T1 and T2.

Reservoir S1 (second tank) and reservoir S2 (first tank) are tanks for storing gas. While gas is stored 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.

In addition, a pressure gauge G, valves V1 to V11, and vacuum pumps P1 to P3 are installed on the mixed gas flow path. The flow path is made up of piping (diameter: 1/4 inch), and the gas moves through the piping.

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

To improve the separation performance of oxygen isotopes, the size (volume) of the adsorption cell should be 0.15 mL or more. On the other hand, taking into account the passage speed of the mixed gas, the size (volume) of the adsorption cell should be 10 mL or less.

In the above-described cryogenic adsorption device 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, connected via valve V2. Furthermore, a quadrupole mass spectrometer (MS) is installed between valves V11 and V7. Gas analysis can be performed using these valves and vacuum pump P3.

In addition, the adsorption cell of the low-temperature adsorption device 1 may be installed in a cryostat unit that enables pretreatment of the adsorbent under vacuum before adsorption separation experiments conducted under the conditions described below (-196.2°C to -161.2°C).

¹⁶ O ₂ and ¹⁸ O ₂ can be separated using the above-described low-temperature adsorption device 1. An example of the procedure will be described below.

First, an isotopic mixed gas of ¹⁶ O ₂ and ¹⁸ O ₂ is stored in tank T1 so that the composition ratio is 1:1. At this time, this mixed gas is analyzed in advance using an MS, and the MS can be calibrated based on the results. The flow rate of the mixed gas (120 Pa, ¹⁶ O ₂ + ¹⁸ O ₂ = 1:1) passing through the adsorption cell is adjusted by an MFC to approximately 1 ml min ⁻¹ . The oxygen mixed gas with ¹⁶ O ₂ + ¹⁸ O ₂ = 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 preliminary treatment, the adsorption cell is heated in vacuum at 200°C for 6 hours. This treatment corresponds to cleaning the adsorption cell.

The specific procedure for separating ¹⁸ O ₂ from a mixed gas of ¹⁶ O ₂ and ¹⁸ O ₂ is as follows.
(1) (Preliminary treatment) A zeolite sample (50 mg to 100 mg) is filled into an adsorption cell, and then 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 is complete.

(3: Adsorption Step) A certain amount of mixed gas is introduced from tank T1 through an MFC into an adsorption cell maintained at a low temperature of -143 _° C or lower. At this time, He from tank T2 acts as a carrier gas. As the mixed gas passes through the adsorbent maintained at a low temperature, the smaller ^18O2 isotope is adsorbed in greater amounts into the pores of the microporous adsorbent. The valve is controlled to open and close so that 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.

(4: Desorption Step) After passing the mixed gas 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 ^18O2 adsorbed on the adsorbent (zeolite) in the cell is desorbed as the cell temperature increases. 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 increases, desorption of the adsorbed gas from the zeolite gradually progresses. 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, desorption of the adsorbed gas proceeds sufficiently. In addition, when the temperature is −120° C. or higher, which is the critical temperature of oxygen (the temperature above which oxygen does not liquefy), the adsorbed ¹⁸ O ₂ can be desorbed smoothly.

(5: Desorption step) The adsorption cell is heated at 200°C for 6 hours to desorb all gases adsorbed on the adsorbent (zeolite), returning the adsorbent to its original state (purification process). The gas desorbed during this process is also stored in reservoir S1. The gas in reservoir S1 is then introduced back into the adsorption cell ^, which is maintained at a low temperature. By repeating the adsorption of ^18O2 into the adsorption cell and desorption _{of 18O2 from} the adsorption cell by increasing the temperature, the isotope concentration of the gas stored in reservoir S1 can be increased.
(6) In another approach, adsorption and concentration of the gas mixture can also be achieved by flowing the gas mixture through a series of adsorption cells maintained at low temperatures.
(7: Repeated adsorption step, repeated desorption step) The above-described adsorption and desorption cycles for concentrating ¹⁸ O ₂ gas are repeated until the concentration of ¹⁸ O ₂ reaches 99.8% by volume as a result of concentration measurement by MS.

By the above procedure, highly enriched ¹⁸ O ₂ gas can be collected in the reservoir S1, which can also be filled into a cylinder by using the open valve O.

The adsorption ratio of ^18O2 _and ^16O2 is determined from _the observed change in mass intensity using ^a pre-calibrated MS. _The adsorption selectivity or ^separation factor S ₍ ^18O2 / _16O2 ) of ^18O2 over _16O2 is determined from their adsorption ratio using the following mathematical formula:

where ( ¹⁸ O ₂ / ¹⁶ O ₂ ) _{a ds} is the mole fraction of ¹⁸ O ₂ and ¹⁶ O ₂ in the adsorbed phase, and ( ¹⁸ O ₂ / ¹⁶ O ₂ ) _g is the mole fraction of ¹⁸ O ₂ and ¹⁶ O ₂ in the bulk gas phase.

Adsorption and separation measurements showed that in a mixture of ¹⁶ O ₂ and ¹⁸ O ₂ gases, ¹⁸ O ₂ was preferentially adsorbed onto a few zeolites at a low temperature of −161.2°C. However, the difference in the adsorption amounts of ¹⁶ O ₂ and ¹⁸ O ₂ with increasing time was highly dependent on the pore size of the zeolite.

The results of the above separation are shown in Figure 3. Figure 3 shows the change in separation factor S( ¹⁸ O ₂ / ¹⁶ O ₂ ) over 60 minutes at -161.2°C for each of Na-LTA zeolite, Ca-LTA zeolite, MFI zeolite, and BEA zeolite. Note that K-LTA zeolite exhibits no separation selectivity because its average pore diameter, 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 not adsorbed. Furthermore, adsorption performance and selectivity decrease when the average pore diameter, determined from nitrogen adsorption measurements, exceeds 1.5 times the diameter of an oxygen molecule, and almost no separation selectivity is observed when the average pore diameter exceeds twice the diameter of an oxygen molecule. Therefore, FAU, with an average pore diameter of 0.74 nm, does not exhibit oxygen isotope separation.

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

From Figure 3 and Table 2, it can be observed that there is a large difference in the selectivity of ¹⁸ O ₂ over ¹⁶ O ₂ between the first 10 minutes and those after 10 minutes. The separation factor S( ¹⁸ O ₂ / ¹⁶ O ₂ ) of Ca-LTA zeolite was 37±5.6 at the initial stage, S( ¹⁸ O ₂ / ¹⁶ O ₂ )=2.3±0.2 after 5 minutes, and maintained at 1.5±0.1 after 10 minutes.

On the other ^hand , the MFI zeolite showed a high separation factor S( ^18O2 / _16O2 ) ₌ 38 ± _3.4 at the initial stage and maintained a separation factor S( ^18O2 / ^16O2 ) ₌ 1.8 ± 0.2 after 5 min. The other zeolites showed slightly inferior selectivity.

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 be selective 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, making it possible to save energy used in separating oxygen isotopes.

Next, temperature-dependent selectivity will be explained. Figures 4(a) and 4(b) show the results of evaluating the temperature-dependent selectivity for Ca-LTA zeolite. Figure 4(a) shows the time-dependent change in the temperature dependence of S( ¹⁸ O ₂ / ¹⁶ O ₂ ) for Ca-LTA zeolite over 60 minutes, and Figure 4(b) shows an excerpt from the initial stage. As shown in Figure 4, Ca-LTA zeolite exhibits excellent ¹⁸ O ₂ separation performance even at two temperatures lower than -161.2°C, namely, -196.2°C and -173°C.

As explained above, the cryogenic adsorption separation technique has high selectivity. Furthermore, as shown in Figure 5, when a mixed gas is passed through an adsorption cell using zeolite as an adsorbent, ¹⁸ O ₂ is preferentially adsorbed according to the selectivity determined by the adsorbent. The adsorbed gas is then recovered, yielding a gas with an increased ¹⁸ O ₂ concentration. Furthermore, when this gas with an increased ¹⁸ O ₂ concentration is passed through the adsorption cell again, the proportion of ¹⁸ O ₂ adsorbed by the cell increases. By repeating this process, as shown in Figure 5, ¹⁸ O ₂ gas with a concentration of 99.8% or more by volume can be obtained. Because all zeolites that can be used as adsorbents have high separation factors, it is estimated that the number of adsorption-desorption steps required to obtain ¹⁸ O ₂ with a concentration of greater than 99.8% by volume is four.

Based on these results, we can design a separation device consisting of a series of connected zeolite-filled adsorption cells maintained at temperatures ranging from -161.2°C to -196.2°C. Furthermore, by repeating four cycles of adsorption-desorption concentration in this device, _{the 18O2} ^{concentration} can be increased from 2000 ppm to over 99.8% by volume. This method is much simpler, faster, more energy-efficient, and more compact than other ^18O2 adsorptive separation methods using activated _carbon that have been confirmed for industrial use.

The method described in this embodiment provides a highly efficient adsorbent for rapid low _{-temperature adsorption, enabling the rapid adsorption-desorption process to concentrate 18O2} ⁱⁿ a shorter time. Furthermore, a completely new principle must be applied to achieve the separation method in this embodiment.

Recently, it has been shown that the uncertainty due to the effective size difference described by the de Broglie wavelength (λ _T = (h ² /2πmkT) ^1/2 ) is generated in nanopores due to the quantum molecular sieving effect (QMS), which is pronounced at low temperatures, resulting in the rapid separation of light gas isotopes such as hydrogen and helium. Figure 6 shows a schematic diagram of a molecule exhibiting quantum fluctuations that change the molecular size of oxygen isotopes at low temperatures. However, as shown in Table 3 below, the size difference between ¹⁸ O ₂ and ¹⁶ O ₂ from the de Broglie wavelength changes from 0.002 nm to 0.0017 nm as the temperature increases from -196.2 °C to -161.2 °C.

These subtle differences mask _the difference in QMS between the two isotopes, and as a result, ^the current translational QMS mechanism alone cannot be expected to efficiently separate large isotopic gas molecules such as ^18O2 and _16O2 in nanoporous materials.

In addition to translational motion, _O2 molecules possess other vibrational and rotational degrees of freedom, the magnitude of which may depend on molecular weight. However, when adsorbed _O2 molecules are isolated from one another, the vibrational states remain essentially intact in the gas and adsorbed phases. In contrast, however, the rotational degrees of freedom confined to nanopores are severely restricted below −143 °C.

As a result, the rotational energy difference between ^18O2 _and ^16O2 _, combined with translational motion, may lead to the selective adsorption of ^18O2 in zeolite nanopores. When _O2 molecules form a partially ordered structure in nanopores at low temperatures, the collective quantum motions associated with vibrational and rotational modes may result in an explicit energy difference between ^18O2 _{and 16O2} . Therefore, this collective nuclear quantum effect (NQE) of the molecular weight _- dependent adsorbed _oxygen ensemble can enable the selective adsorption ^{of 18O2} _{from 16O2} ^on nanoporous materials.

In zeolites with nanopores whose pore width is twice the molecular diameter of the isotope gas (2 × σ(O ₂ )), the interconnected cylindrical pores are presumed to have higher confinement capacity than the large nanopores in BEA or FAU zeolites. Therefore, these zeolites are likely to form an ordered O ₂ adsorption phase, resulting in high selectivity and achieving highly selective adsorption of ¹⁸ O _2. Figure 7 shows a schematic diagram of how the oxygen isotopes ¹⁸ O ₂ and ¹⁶ O ₂ are selectively adsorbed when a zeolite with channels (tubular pores) and cages is used as an adsorbent. As shown in Figure 7, the small pores of zeolites presumably allow only the smaller ¹⁸ O ₂ molecules to enter the pores and reject the larger ¹⁶ O ₂ , resulting in the preferential adsorption of ¹⁸ O ₂ .

(Effects of the embodiment)
The oxygen isotope separation method described in the above embodiment is ^a method for separating oxygen isotopes into ^18O2 and _16O2 , and includes an adsorption process and a desorption process. In the adsorption process _, a mixed gas containing _18O2 and ^16O2 is passed through an adsorbent ^at -143° _C or lower, thereby adsorbing ^18O2 to the adsorbent, and _the unadsorbed gas is stored in a first tank (reservoir S2). In the desorption process, the temperature of the adsorbent is increased, thereby desorbing the gas adsorbed by the adsorbent and storing it in a second tank (reservoir S1). In addition, the adsorbent is a zeolite with an average pore width of 0.35 nm to 0.7 nm as measured by nitrogen adsorption.

According to ^the above method, zeolite having an average pore width within the above range is used as an adsorbent, and a mixed gas containing ^18O2 and _16O2 is passed through the adsorbent under _the above low-temperature conditions, thereby selectively adsorbing ^18O2 onto _the adsorbent. Furthermore, by increasing the temperature of the adsorbent, the adsorbed gas is desorbed and stored in a second tank, and gas with an increased ^18O2 concentration can be collected. Therefore, according to the above oxygen isotope separation method, _18O2 can be separated with high separation efficiency. Furthermore, the above oxygen isotope separation method does not require complicated operations, _is simpler than conventional methods ^, and can increase the separation efficiency of oxygen isotopes.

The method may further include performing ^{one or more of} the following steps after the desorption step: a repeated adsorption step in which the gas stored in the second tank (reservoir S1) is passed through the adsorbent at a temperature of -161.2°C or lower to adsorb _18O2 onto the adsorbent, and the unadsorbed gas is stored in the first tank (reservoir S2); and a repeated desorption step in which the temperature of the adsorbent is increased to desorb the gas adsorbed on the adsorbent and store it in the second tank (reservoir S1). As described above, by repeating the process of passing the gas stored in the second tank through the adsorbent again, the ^18O2 concentration of _the gas stored in the second tank can be further increased.

The number of times the repeated adsorption step and the repeated desorption step may be three or more. In this case, the adsorption-desorption process is performed four or more times. Therefore, the concentration of ^18O2 contained in the gas stored in the second tank can be _{made greater than 99.8% by volume, and 18O2} ^gas _with higher purity can be obtained.

The temperature of the adsorbent during the adsorption process may be -200°C or higher. By setting the adsorbent temperature during the adsorption process within this range, it is possible to achieve high separation efficiency of oxygen isotopes while suppressing the cost required for cooling. A similar temperature may also be set during repeated adsorption processes. In particular, an example of the adsorbent temperature is -196.15°C to -161.15°C (77K to 112K), which may improve isotope separation performance.

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 18O2} ^adsorbed by the adsorbent can be appropriately desorbed and recovered in the second tank.

The desorption step may include maintaining the temperature of the adsorbent at 200°C or higher for 10 minutes or longer. By including the above-described operation in the desorption step, it is possible to almost completely desorb _{the 18O2} ^adsorbed on the adsorbent, thereby improving the recovery efficiency of ^18O2 gas _. In the initial desorption step of the process, a relatively long desorption time (e.g., 6 hours) may be required to remove moisture and other substances adsorbed on the zeolite. However, since the adsorbent is not exposed to the atmosphere during repetition of the process, desorption of ^18O2 is completed in a very short time, and even if _{the time is set to 10 minutes or longer as described above, 18O2 can} ^be appropriately desorbed.

The adsorbent may be configured as an adsorption cell packed in a column. This configuration allows the gas to be reliably brought into contact with the adsorbent, thereby promoting the adsorption _{of 18O2} ^.

Although several embodiments have been described above, the present disclosure is in no way limited to the above-described embodiments. Furthermore, the descriptions of the above-described embodiments can be applied to each other.

1...low-temperature adsorption device, S1, S2...reservoirs, T1, T2...tanks, V1 to V11...valves.

Claims (7)

A method for separating oxygen isotopes, which separates ^18O2 _and ^16O2 _, comprising:
an adsorption step of passing a mixed gas containing ¹⁸ O ₂ and ¹⁶ O ₂ through an adsorbent at a temperature of −143° C. or lower to adsorb ¹⁸ O ₂ onto the adsorbent, 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. After the desorption step,
2. The oxygen isotope separation method according to claim 1, further comprising _{: a repeated adsorption step of passing the gas stored in the second tank through the adsorbent at −143° C. or lower to adsorb 18O2} ^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 by the adsorbent and store it in the second tank, one or more times. The oxygen isotope separation method described in claim 2, wherein the repeated adsorption process and the repeated desorption process are repeated three or more times. The oxygen isotope separation method according to any one of claims 1 to 3, wherein the temperature of the adsorbent during the adsorption step is -200°C or higher. The oxygen isotope separation method according to any one of claims 1 to 4, wherein the temperature of the adsorbent during the desorption step is -50°C or higher. 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. The oxygen isotope separation method described in any one of claims 1 to 6, wherein the adsorbent is configured as an adsorption cell packed in a column.