JP6082898B2 - Method for separating carbon isotopes from graphite - Google Patents

Description

The present invention relates to a method for separating a carbon isotope from graphite.

One of the carbon isotopes, ¹³ C, is expanding its use in the medical field and the nuclear industry. Particularly in the medical field, it is used as a marker reagent for diagnosis by nuclear magnetic resonance or the like.
In addition, ¹⁴ C is produced in graphite used as a reflector in a graphite moderation reactor and has a radioactive nature, so its treatment method is being studied. In graphite-reduced nuclear reactors, thousands of tons of graphite are generated as waste along with decommissioning. The amount of radioactive carbon contained in this graphite is about 1-2%. Here, if 2% of carbon in 1000 tons of graphite is activated, 20 tons must be treated as radioactive waste, but the remaining 980 tons of graphite can be treated as general waste. Therefore, if the carbon isotope is processed without separation, 1000 tons of radioactive waste will be generated. The disposal of radioactive carbon from two nuclear reactors is a major issue in Japan, but there are many old nuclear reactors that emit a large amount of radioactive carbon in the United Kingdom and other countries, and their treatment is a major issue.
^The importance of ¹⁴ C separation is understood as a common recognition among countries, but there are few research papers in this field. ¹⁴ C Is a stable isotope for separation and enrichment of ¹³ C It is effective to apply this separation method.
Patent Documents 1 to 4 show a method for treating and disposing of used graphite.

Japanese Unexamined Patent Publication No. 3-179299 JP-A-6-94896 JP-A-6-182158 JP 2003-307594 A

In the method disclosed in Patent Document 1, activated graphite is gasified by hydrogenation or combustion, and ¹⁴ C is separated from the gas by isotope separation. This isotope separation method is performed by gas diffusion method, centrifugal separation method, distillation method, chemical exchange method, etc., but its specific method is not shown.
Patent Document 2 discloses that radioactive graphite waste is incinerated in an incinerator and the produced CO ₂ is recovered in a gas state by an absorption tower, but does not describe separation of isotope gas.
The method shown in Patent Document 3 is to separate carbon isotopes by laser irradiation and can be applied to ¹⁴ C separation, but it can be applied to separation of ¹⁴ C from a large amount of used graphite material. Needs to be considered.
The method disclosed in Patent Document 4 is such that graphite is burned and the generated CO ₂ gas is introduced into a sodium hydroxide aqueous solution and recovered. However, there is no description regarding the separation of isotope gas. Absent.
An object of this invention is to provide the separation method of the carbon isotope from the graphite which can isolate | separate a carbon isotope from graphite efficiently.

The method for separating carbon isotopes from graphite according to the present invention is a carbon isotope separation method for separating carbon isotopes from graphite containing ¹² C and ¹³ C or ¹⁴ C. A process in which a mixed gas is generated, a process in which hydrogen is reacted with the generated carbon dioxide isotope mixed gas to generate a methane isotope mixed gas, and a chamber in which the generated methane isotope mixed gas is filled with an adsorbent flushed copies two, the quantum molecular sieve effect, ¹² CH ₄ most adsorbed on the adsorbent than the big ¹³ CH ₄ or ¹⁴ CH ₄ and ¹² CH ₄ molecular weight than, from the adsorbent methane isotopes mixed gas adsorbed A step of concentrating ¹³ CH ₄ or ¹⁴ CH ₄ by repeating desorption and adsorption to an adsorbent, and a step of separating the concentrated ¹³ CH ₄ or ¹⁴ CH ₄ into ¹³ C or ¹⁴ C and hydrogen , The adsorbent is finely treated by heat treatment. Characterized by using the activated carbon fibers having pores having an adjusted size of the entrance diameter 0.5-1 nm.

In the concentration step, the methane isotope mixed gas is adsorbed on the adsorbent by setting the temperature in the chamber to a low temperature state where the quantum molecular sieving effect occurs, and the temperature inside the chamber is higher than the temperature at which the quantum molecular sieving effect occurs By setting the temperature, the methane isotope mixed gas adsorbed on the adsorbent is desorbed from the adsorbent.
Thus, the adsorption and desorption (concentration) of the methane isotope mixed gas can be efficiently performed only by adjusting the temperature.

The adsorption of the methane isotope mixed gas onto the adsorbent in the concentration step can be performed at −196 ° C., which is the boiling point of liquid nitrogen.
In the concentration step, the methane isotope mixed gas was allowed to flow through a plurality of chambers, and the methane isotope mixed gas adsorbed on the adsorbent was repeatedly desorbed and adsorbed and recovered, and was not adsorbed. The methane isotope mixed gas is collected in a gas reservoir, and the collected methane isotope mixed gas is passed through the chamber to re-separate the methane isotope mixed gas.
In this case, the carbon isotope can be completely separated by finally collecting only ¹² CH _{4 in the} gas reservoir.

Further, in the concentration step, the methane isotope mixed gas adsorbed and desorbed in the other chambers can be introduced alternately into the two chambers, and the adsorption and desorption can be repeated for concentration.
An adsorbent having pores smaller than 1 nm, preferably about 0.7 nm, can be used for the adsorbent.
Further, in the step of generating the methane isotope mixed gas, the unreacted carbon dioxide isotope mixed gas is collected as dry ice and gasified to generate a methane isotope mixed gas again. Thus, it is preferable to separate all the graphite.

ADVANTAGE OF THE INVENTION According to this invention, the separation method of the carbon isotope from graphite which can isolate | separate a carbon isotope from graphite efficiently can be provided.

It is a graph which shows the heat processing temperature dependence with respect to the external surface area of adsorption agent ACF-2 (activated carbon fiber). It is a graph which shows the heat processing temperature dependence with respect to the total surface area of ACF-2. It is a graph which shows the heating temperature dependence of the micropore capacity of ACF-2. It is a graph which shows the heating temperature dependence of the average pore diameter of ACF-2. It is an adsorption isotherm of ¹² CH ₄ and ¹² CD ₄ at the boiling point of methane (112 K) of ACF-1 (activated carbon fiber) having a pore diameter of 0.7 nm. It is explanatory drawing which shows the outline | summary of an adsorption | suction apparatus. Nanoporous body is an explanatory view showing an adsorption principle of a gas mixture of ¹² CH ₄ and ¹⁴ CH ₄ using (adsorbent). It is explanatory drawing which shows release | release of the adsorbed gas. It is a schematic explanatory drawing of the process of isolate | separating isotope carbon from the graphite in this Embodiment. It is a schematic diagram of the processing apparatus which isolate | separates ¹³ C or ¹⁴ C from the graphite containing a carbon isotope of ¹² C and ¹³ C or ¹⁴ C.

Preferred embodiments of the present invention will be described below in detail with reference to the accompanying drawings.
^In order to process ¹⁴ C without damaging the future human beings and the earth as much as possible, it is necessary to separate and concentrate ¹⁴ C to reduce the volume and isolate it. One of the effective means for reducing the amount of waste is to separate ¹⁴ CO ₂ and ¹² CO ₂ after combustion and CO ₂ conversion of the graphite part containing ¹⁴ C, and solidify only ¹⁴ CO ₂ It is. However, the separation of ¹⁴ CO ₂ and ¹² CO ₂ is very difficult.

  The separation method in this embodiment is to apply a completely new principle. Recently, uncertainties based on the quantum properties of light molecules are important in nano (nm) pores, and the amount of light isotope molecules adsorbed differs by more than 10%. In the present embodiment, the carbon isotope is separated using the uncertainty based on the quantum nature of the molecule. This isotope separation effect is said to be the quantum molecular sieve effect, and it is clear that at the boiling point of liquid nitrogen (77 K) between hydrogen and deuterium, a difference in storability of 10% or more appears when the nanopore size is selected. It has become.

In this embodiment, this quantum molecular sieving effect is applied to ¹² CH ₄ (molecular weight = 16), ¹³ CH ₄ (molecular weight = 17) and ¹⁴ CH ₄ (molecular weight = 18), and ¹² C, ¹³ C or ¹⁴ Efficient separation from C.
The effective size of the molecule due to the quantum action is estimated at the de Broglie wavelength, but the lower the temperature, the larger the size difference between the ¹² CH ₄ due to the quantum effect and ¹³ CH ₄ or ¹⁴ CH ₄ and the better the separation. To do. That is, ¹³ CH ₄ or ¹⁴ CH _{4 with} a large molecular weight has a smaller quantum effect than ¹² CH _{4 with} a small molecular weight, so the molecular size is small and the interaction is strong, so it adsorbs more in the pores than ¹² CH ₄ (Quantum molecular sieve effect).
Since it can be easily converted from CO ₂ to CH ₄ using a catalytic reaction, it is separated as CH ₄ which is a light and safe molecule. In addition, since methane after isotope separation can be easily separated into hydrogen and carbon by catalytic reaction, only ¹³ C or ¹⁴ C can be solidified and separated while separating clean energy H ₂ .

^«12 CH ₄ - ¹² CD ₄ search and structural analysis of the separation nanoporous body»
Prior to the separation of methane gas mixture of carbon isotope, as an experimental study, molecular weight differences are hydrogen isotopes of methane gas mixture is _{^{4 (12 CH 4 - 12 CD}} 4) optimal nano fine that can be used for the separation The candidates for pores were narrowed down.
(1) from the evaluation of the differences and molecular size of interest de Broglie wavelength, ¹² CH ₄ - ¹² CD ₄ pore structure and the pore wall structure analysis of the activated carbon fibers found to be suitable for separation (ACF), or pore entrance control It aims at surface modification.

(2) Test method The surface-modified activated carbon fibers were pitch type activated carbon fibers and two types of samples having different pore sizes (hereinafter referred to as ACF-1 and ACF-2). ACF-1 and ACF-2 were heat-treated at 1123, 1223, 1473, 1573, 1673 K for 1 hour, 2 hours, or 4 hours under Ar gas flow (200 ml / min). In addition, the influence of the pore structure on the temperature rise was also examined.
For the above sample, vacuum heating pretreatment was performed at 1 mPa, 423 K, and after removing the impurity gas adsorbed in the pores, using an automatic N ₂ adsorption measurement device (Autosorb-1, Cantachrome), The adsorption isotherm of N ₂ at 77 K was obtained, and the pore structure analysis was performed based on it. Specifically, the outer surface area, the total surface area, the micropore volume, and the average pore diameter were evaluated, and changes in the pore structure due to heat treatment were examined.

(3) Test results and discussion FIG. 1 shows the heat treatment temperature dependence on the external surface area of ACF-2. FIG. 2 shows the heat treatment temperature dependence on the total surface area of ACF-2. Processing time is 1 hour and 2 hours or 4 hours. The heating rate is 10 K / min. It can be seen that ACF-2 has a small ratio of the outer surface area to the total surface area, and most contributes to the surface area due to the pores. Until the heating temperature was 1573 K, there was no significant change in the total surface area. However, the total surface area suddenly decreased at 1673 K. 3 and 4 show the heating temperature dependence of the micropore capacity and average pore diameter of ACF-2. Similar to the total surface area, the micropore capacity decreased rapidly at 1673 K. The reduction of the total surface area and micropore volume needs to be suppressed as much as possible when performing isotope separation with high yield. Therefore, it can be said that 1123-1573 K is appropriate for controlling the pore structure of ACF-2 by heat treatment. FIG. 4 also shows that the average pore diameter can be controlled on the order of 0.01 nm by heat treatment. The separation of methane isotopes by the quantum molecular sieve effect is affected by a slight difference in the size of the pore entrance. Therefore, it was found that the surface modification method performed here is useful for searching for the optimum nanopore body for methane isotope separation.

«Equilibrium adsorption method by ¹² CH ₄ - ¹² CD ₄ based quantum molecular sieve separation effect study of»
(1) Purpose Next, ¹² CH ₄ - ¹² CD ₄ raises the equilibrium adsorption experiment for separation, it performed ¹² CH ₄ and ¹² adsorption experiments of single components of CD ₄ at room temperature side from the boiling point of about methane, methane The orientation of nanostructures that are promising for the quantum molecular sieve effect in the system was grasped.

(2) Test method Activated carbon fiber (ACF-1) was used for the nanoporous material. After vacuum heating pretreatment at 1 mPa and 423 K, a single-component adsorption isotherm of ¹² CH ₄ and ¹² CD ₄ at 112 K was equilibrated and adsorbed using a self-made high-resolution N ₂ adsorption measurement device. Obtained by law.

(3) Test results and discussion FIG. 5 shows adsorption isotherms of ¹² CH ₄ and ¹² CD ₄ at the boiling point of methane (112 K) of ACF-1 having a pore diameter of 0.7 nm. ¹² CD ₄ had a slightly larger amount of adsorption than ¹² CH _4, and a result suggesting a quantum sieve effect was obtained. In the H ₂ -D ₂ system, the quantum sieving effect is remarkable, and it is known that it appears as a difference in adsorption amount of about 10%. ¹² CH ₄ - In ¹² CD ₄ system, but quantum sieving effect is less pronounced for the mass ratio compared to H ₂ -D ₂ system is small, adsorption amount difference of about 1% was observed. Although this test is a measurement at a temperature around the boiling point of methane, it is expected that a more remarkable difference in adsorption amount can be obtained by measurement at a lower temperature.
As described above, it is possible to control the average pore diameter of activated carbon fibers on the order of 0.01 nm by heat treatment in an Ar gas atmosphere. It is effective to improve the separation ability of ¹² CH ₄ and ¹² CD ₄ by preparing nanopores with a pore diameter of 0.7 nm or less and conducting equilibrium adsorption experiments.

Next, the separation effect of the quantum molecular sieve of the carbon isotope methane mixed gas (mixed gas of ¹² CH ₄ and ¹³ CH ₄ ) was investigated.
FIG. 6 is an explanatory diagram showing an outline of the adsorption device.
Reference numeral 10 denotes an initial gas reservoir, which includes a pressure gauge 11. Reference numeral 12 denotes a separation cell, which contains a nanoporous material (adsorbent: ACF-3) and has been cooled to −196 ° C. (77 K) with liquid nitrogen in advance. The separation cell 12 is connected to the initial gas reservoir 10 through a valve 14 by a pipe 13. Reference numeral 16 denotes an analysis gas reservoir. The analysis gas reservoir 16 includes an analyzer (quadrupole mass spectrometer) 17. The analysis gas reservoir 16 communicates with the separation cell 12 through the pipe 13.

An isotope methane mixed gas having an arbitrary pressure P ₀ is placed in the initial gas reservoir 10. The separation cell 12 and the analysis gas reservoir 16 are evacuated in advance.
In this state, the valve 14 was opened, and the isotope methane mixed gas was fed into the separation cell 12 and the analysis gas reservoir 16 using the differential pressure. The molar fraction of the gas component of the mobile phase component collected in the analysis gas reservoir 16 was calculated using the analyzer 17 and compared with the initial molar fraction of the introduced gas.

Table 1 shows the results.
Table 1
Gas 2 is ¹² CD ₄ and ¹³ CH ₄ respectively, and ACF-3 is an activated carbon fiber having a pore diameter of approximately 0.7 nm.

As shown in Table 1, the ratio of the gas mixture of the mobile phase component to the initial gas mixture ratio decreased by 4.1% for ¹² CD ₄ and ¹² CH ₄ for ¹³ CH ₄ It has decreased by 1.9%. That is, in the adsorbent, ¹² with respect to CH _4, is often adsorbed in a proportion of cases 4.1% ¹² CD _4, it shows that it is often adsorbed in a proportion of cases 1.9% ¹³ CH _4. In addition, the result of desorbing the gas component adsorbed by the adsorbent and measuring the ratio is also a separation result complementary to the above.

Table 2 is a table showing the selective absorptivity of various adsorbents with respect to various mixed gases, measured using the apparatus of FIG.
Table 2

Gas 2 is ¹² CD ₄ and ¹³ CH ₄ respectively, and ACF-2 is an activated carbon fiber having a pore diameter of approximately 1.1 nm.
MSC is a porous carbon having a pore diameter of approximately 0.3 nm.
Zeolite-4A has a pore diameter of approximately 0.4 nm.
As shown in Table 2, there is a difference in the adsorption rate depending on the type of mixed gas and the type of adsorbent.
Since ACF-3 has a pore size of about 0.7 nm and a methane molecule size of about 0.4 nm, the adsorption ratio of ¹² CD ₄ and ¹³ CH ₄ having a large molecular weight increases due to a good quantum molecular sieve effect. Yes.
The adsorbent ACF-2 with a large pore diameter of approximately 1.1 nm has an excessively large pore diameter with respect to methane molecules, and conversely, the adsorption ratio of ¹² CH ₄ having a small molecular weight increases.
The adsorbent MSC having a pore size of approximately 0.3 nm, which is smaller than the molecular size of methane, does not provide a very good quantum molecular sieving effect.
The adsorbent Zelite-4A, which has a pore size of approximately 0.4 nm, which is about the size of a methane molecule, also varies in its adsorptivity. Not all Zelite can be used, and there are some that show a good quantum molecular sieve effect of about 10% depending on the type.
As described above, it is preferable to adjust the size of the pore diameter by heat treatment and select an adsorbent according to the separation gas species.

Next, FIG. 7 is an explanatory view showing the principle of adsorption of a mixed gas of ¹² CH ₄ and ¹⁴ CH ₄ using a nanoporous material (adsorbent). ¹⁴ CH ₄ molecules have a molecular size of about 0.002 nm smaller than ¹² CH ₄ molecules at a temperature of 111 K, and are adsorbed in the pores more than ¹² CH ₄ molecules.
As shown in FIG. 8, after the selective adsorption of the mixed gas, if the temperature of the nanopore body is raised, the adsorbed gas is released and a mixed gas enriched with ¹⁴ CH ₄ is obtained. It is possible to completely separate ¹² CH ₄ and ¹⁴ CH ₄ by repeating adsorption and release of gas in the nanopore body.

FIG. 9 is a schematic explanatory diagram of a process of separating isotope carbon from graphite in the present embodiment.
That is, first, graphite (used graphite reflector) is placed in a combustion furnace, supplied with oxygen and burned (temperature of about 850 ° C) to produce ¹² CO ₂ , ¹⁴ CO ₂ (and ¹² CO, ¹⁴ CO) (Step A).
This ¹² CO ₂ , ¹⁴ CO ₂ (and ¹² CO, ¹⁴ CO) is introduced into the methanation reactor, supplied with hydrogen, and methanated with nickel as a catalyst (temperature of about 350 ° C.), ¹² CH ₄ , ¹⁴ CH ₄ is produced (step B).
Next, this mixed gas of ¹² CH ₄ and ¹⁴ CH ₄ is introduced into an adsorbent filling chamber filled with nanopores (adsorbent) and adjusted to a temperature of 77 K, and adsorbs the mixed gas to the adsorbent. (Step C). As described above, the proportion of ¹⁴ CH _{4 in} the adsorbed mixed gas is adsorbed more than the proportion of ¹⁴ CH ₄ in the original mixed gas (about several percent more) due to the quantum molecular sieve effect.

The adsorbed mixed gas is released by raising the temperature of the adsorbent filling chamber. Concentration operation in which adsorption and release of the mixed gas with the adsorbent is repeated a plurality of times to recover only ¹⁴ CH ₄ as a gas (step D). Only ¹² CH ₄ is recovered as a gas. ¹² CH ₄ can be used as fuel. Step D will be described in detail later.
The recovered ¹⁴ CH ₄ is introduced into a carbonization production furnace and subjected to a decomposition reaction using palladium as a catalyst, and separated into solid ¹⁴ C and gaseous H ₂ (step E). Solid ¹⁴ C can be processed safely by hardening with concrete. Gaseous H ₂ is reused in step B. Or you may discharge | release to air | atmosphere.
The separation of ¹² C and ¹³ C can be performed in the same process.

Next, FIG. 10 shows a schematic diagram of a processing apparatus 20 for separating ¹³ C or ¹⁴ C from graphite containing ¹² C and ¹³ C or ¹⁴ C carbon isotopes.
In FIG. 10, F1 is a graphite combustion furnace and has a graphite combustion chamber C1.
F2 is a methanation reactor and has a nickel catalyst-filled chamber C2. F3 is a carbonization production furnace and has a palladium catalyst-filled chamber C3. Further, the part indicated by reference numeral 22 is a methane isotope gas adsorption / concentration part.
G1 is an oxygen filling cylinder, G2 is a hydrogen filling cylinder, and G3 is a methane isotope gas storage cylinder. V1 to V24 are valves. CT is a carbon dioxide trap, GC1 and GC2 are gas reservoirs, and AC1 and AC2 are adsorbent-filled chambers. P1, P2, P3, and P4 are suction pumps, and MS is a mass spectrometer.
Each part is connected by piping so that gas flows in the order shown below.

A process of separating ¹³ C or ¹⁴ C from graphite containing ¹² C and ¹³ C or ¹⁴ C carbon isotopes using the processing apparatus 20 will be described (hereinafter, it will be described as separating ¹³ C).
1. A required amount of graphite to be processed is stored in advance in the graphite combustion chamber C1. The adsorbent filling chambers AC1 and AC2 are also preliminarily filled with a necessary amount of adsorbent (nanopore body: ACF-3 or the like).
2. The valves V1 to V8 and V13 and V14 are opened, and suction is performed by the suction pump P1 to evacuate the system.
3. When the evacuation is finished, the valve V13 is closed.
The inside of the graphite combustion furnace F1 is kept at 850 ° C., and the inside of the methanation reaction furnace F2 is kept at 350 ° C. Further, the carbon dioxide trap CT is cooled to −79 ° C. or lower, and the adsorbent-filled chambers AC1 and AC2 are cooled to −196 ° C. with liquid nitrogen.

4). Through the valve V1, oxygen is supplied from the oxygen filling cylinder G1 into the graphite combustion chamber C1, and the graphite is burned to generate a carbon dioxide isotope mixed gas.
Reaction formula: C (graphite) + O ₂ → CO ₂
The produced carbon dioxide isotope mixed gas is sent to the methanation reactor F2, and reacts with hydrogen fed from the hydrogen filling cylinder G2 via the valve V2 under a nickel catalyst to produce a methane isotope mixed gas. . Reaction formula: CO ₂ + 4H ₂ → CH ₄ + 2H ₂ O
The unreacted carbon dioxide isotope mixed gas is collected in a carbon dioxide collection trap CT as dry ice. The methane isotope mixed gas passes through the carbon dioxide collecting trap CT as a gas.

5. Since the methane isotope mixed gas is sucked by the suction pump P1, it is sent into the adsorbent filling chamber AC1 through the valves V3 and V4. The adsorbent filling chamber AC1 is cooled to a low temperature of −196 ° C., and as described above, ¹³ CH ₄ in the methane isotope mixed gas has a size smaller than ¹² CH ₄ having a small molecular weight due to the quantum effect. It behaves as a molecule and is selectively adsorbed by adsorbent several wt% more than ¹² CH ₄ (quantum molecular sieving effect). Unadsorbed methane isotope mixed gas is collected in the gas reservoir GC1 through valves V5, V6, V8 and pumps P1, V14. The components of the methane isotope mixed gas are constantly monitored by the mass spectrometer MS.

6). Next, the valves V3 to V5 and V8 are closed, and the valves V9 to V12 are opened. At this stage, the temperature of the adsorbent filling chamber AC1 is raised to about −150 ° C. This increase in temperature can be achieved simply by stopping the cooling with liquid nitrogen. Of course, you may heat with a heater. On the other hand, the temperature of the adsorbent filling chamber AC1 is maintained at a low temperature of -196 ° C. Next, the valve V5 is opened. When the temperature in the adsorbent filling chamber AC1 rises to about −150 ° C., the methane isotope mixed gas adsorbed on the adsorbent is desorbed (released). Due to the operation of the pump P1, the released methane isotope mixed gas is introduced into the adsorbent filling chamber AC2 through the valves V5, V6, and V10, and is adsorbed by the adsorbent in the adsorbent filling chamber AC2. Also in this case, due to the quantum molecular sieve effect, ¹³ CH ₄ is adsorbed at a rate of several wt% more than ¹² CH ₄ . The methane isotope mixed gas adsorbed in the adsorbent filling chamber AC1 ( ¹³ CH ₄ is adsorbed in a large proportion) is desorbed, and ¹³ CH ₄ is adsorbed again in the adsorbent filled chamber AC2 in a large proportion. As a result, ¹³ CH ₄ is concentrated in the adsorbent. Unadsorbed methane isotope mixed gas is collected in the gas reservoir GC1 through the valves V11 and V9, the pump P1, and the valve V14.

7). Next, the valves V9 to V11 are closed, and V4, V5, and V8 are opened. At this stage, the temperature of the adsorbent filling chamber AC2 is raised to about −150 ° C. This increase in temperature can be achieved simply by stopping the cooling with liquid nitrogen. On the other hand, the adsorbent filling chamber AC1 is cooled to a temperature of −196 ° C. Next, the valve V11 is opened. When the temperature in the adsorbent filling chamber AC2 rises to about −150 ° C., the methane isotope mixed gas adsorbed on the adsorbent is desorbed (released). Due to the operation of the pump P1, the released methane isotope mixed gas is introduced into the adsorbent filling chamber AC1 through the valves V11, V12, and V4, and is adsorbed by the adsorbent in the adsorbent filling chamber AC1. Also in this case, due to the quantum molecular sieve effect, ¹³ CH ₄ is adsorbed at a rate of several wt% more than ¹² CH ₄ . The methane isotope mixed gas adsorbed in the adsorbent filling chamber AC2 ( ¹³ CH ₄ is adsorbed in a larger proportion) is desorbed, and the proportion of ¹³ CH ₄ is increased again in the adsorbent filled chamber AC1. ¹³ CH ₄ is further concentrated in the adsorbent. Unadsorbed methane isotope mixed gas is collected in gas reservoir GC1 through valves V5, V6, V8, pump P1, and valve V14.

8). The above steps 6 and 7 are repeated until the methane isotope gas component ( ¹² CH ₄ ) having a small mass number is not detected by the mass spectrometer MS. As a result, the gas adsorbed by the adsorbent is only ¹³ CH ₄ . Although depending on the degree of enrichment, it is of course possible to stop the enrichment when a small amount of methane isotope gas component ( ¹² CH ₄ ) is present. As described above, adsorption / separation (concentration) of the methane isotope mixed gas can be efficiently performed only by adjusting the temperature.
In step 6, the flow of the mixed gas is AC1 (desorption) → AC2 (adsorption) → P1 → GC1, and in step 7, AC2 (desorption) → AC1 (adsorption) → P1 → GC1. That is, AC1 and AC2 are connected in series and repeat desorption and adsorption alternately. By using the two chambers alternately in this way, the size can be reduced. Three or more chambers may be connected in series and used, but the apparatus becomes large.
Although the methane isotope mixed gas can be concentrated by the pressure swing method, the concentration by the temperature change as described above has higher desorption efficiency (speed).

9. Next, the valves V8 and V9 are closed, the valve 18 or the valve V19 and the valve V23 are opened, the valves V24, V22 and V20 are closed, and the temperature of the adsorbent filling chamber AC1 or AC2 where the last adsorption operation is performed is increased. At the same time, by operating the pump P3, ¹³ CH ₄ desorbed from the adsorbent is collected in the isotope methane storage cylinder G3 via the valve V23.

10. Next, the valves V18 and V19 are closed, and the temperature of the carbonization furnace F3 is maintained at 550 ° C. By opening the valves V20, V22, V24 and operating the suction pump p2, ¹³ CH ₄ collected in the isotope methane storage cylinder G3 is introduced into the palladium catalyst-filled chamber C3. Thereby, ¹³ CH ₄ is decomposed into ¹³ C and H ₂ . Reaction formula: ¹³ CH ₄ → ¹³ C (solid) + 2H ₂

11. Unreacted ¹³ CH ₄ is collected in the gas reservoir GC 2 and again introduced into the palladium catalyst-filled chamber C 3 for decomposition.
12. Repeat steps 10 and 11 until all residual methane gas is converted to carbon (solid). Finally, only H ₂ is collected in the gas reservoir GC2. This H ₂ can be opened to the atmosphere by opening the valve V21. Of course, H ₂ may be effectively used elsewhere. For example, it can be reused as hydrogen gas for methanation in the methanation reactor F2.
13. Further, the unreacted carbon dioxide isotope mixed gas collected as dry ice in the carbon dioxide collection trap CT in the step 4 opens the valves V16 and V17, operates the suction pump p4 again, and recharges the nickel catalyst filled chamber C2. To make it completely methanated.

14 The methane isotope mixed gas collected by the gas reservoir GC1 closes the valves V18 and V19, cools the adsorbent filling chambers AC1 and AC2 to a temperature of −196 ° C., closes the valve V14, and V15. And is introduced into the adsorbent filling chamber AC1, and the methane isotope mixed gas is adsorbed to the adsorbent in the adsorbent filling chamber AC1 in the same manner as described above. Steps 6 and 7 are then repeated as above to concentrate ¹³ CH ₄ into the adsorbent. In this way, all methane ¹³ CH ₄ having a large mass number is recovered and recovered as ¹³ C (solid).
The ¹² CH _{4 that} is finally left after separation remains in the gas reservoir GC1 or the chambers AC1 and AC2, but may be recovered by opening the valve V13. This ¹² CH ₄ can be effectively used as a fuel as appropriate.

15. ¹³ C generated in the palladium catalyst-filled chamber C3 can be recovered and effectively used as a medical marker or the like.
When graphite is a used graphite reflector, radioactive ¹⁴ C (solid) is reduced and separated and recovered. This ¹⁴ C (solid) can be appropriately treated by being covered with concrete and buried in the ground.

Radiocarbon ¹⁴ C emits beta rays (electron beam) (half-life 5730 years).
In order to determine the shielding of β rays, the equation of R = 0.5E is simply used. R and E are the β ray range (g cm ⁻² ) and energy (MeV), respectively.
¹⁴ C Since the energy of β-rays emitted is 0.156 MeV, the range is 0.078 g cm ^-2 . Here, when aluminum is used as the β-ray shielding material, the aluminum density near room temperature is 2.70 g cm ^-3 , so the aluminum thickness necessary for shielding is 0.078 (g cm ^-2 ) ÷ 2.70 (g cm ^-3 ) = 0.028 cm. Therefore, it is possible to take measures against safety by covering the entire apparatus with an aluminum plate having a thickness of at least 0.3 mm.

DESCRIPTION OF SYMBOLS 10 Initial gas reservoir 11 Pressure gauge 12 Separation cell 13 Piping 14 Valve 16 Analysis gas reservoir 17 Analyzer 20 Processing apparatus G1: Oxygen filling cylinder,
G2: Hydrogen filled cylinder,
G3: Isotope methane storage cylinder V1 to V24: Valve F1: Graphite combustion furnace F2: Methanation reactor F3: Carbonization reactor C1: Graphite combustion chamber C2: Nickel catalyst filling chamber C3: Palladium catalyst filling chamber CT: Carbon dioxide collection trap GC1-GC2: Gas reservoir AC1, AC2: Adsorbent-filled chamber P1-P4: Suction pump MS: Mass spectrometer

Claims (8)

^In a carbon isotope separation method for separating a carbon isotope from graphite containing ¹² C and ¹³ C or ¹⁴ C,
A process of burning graphite to produce a carbon dioxide isotope mixed gas;
Reacting hydrogen with the generated carbon dioxide isotope mixed gas to produce a methane isotope mixed gas;
The resulting methane isotopic gas mixture, flows through the chamber which the adsorbent is filled, by quantum molecular sieve effect, ¹² CH molecular weight than ₄ large ¹³ CH ₄ or ¹⁴ CH ₄ into more adsorbent than ¹² CH ₄ A process of concentrating ¹³ CH ₄ or ¹⁴ CH ₄ by repeatedly desorbing the adsorbed methane isotope mixed gas from the adsorbent and adsorbing to the adsorbent;
Separating the concentrated ¹³ CH ₄ or ¹⁴ CH ₄ into ¹³ C or ¹⁴ C and hydrogen ,
A method for separating carbon isotopes from graphite, wherein activated carbon fibers having pores having a pore entrance diameter adjusted to 0.5 to 1 nm by heat treatment are used as the adsorbent .   In the concentration step, the methane isotope mixed gas is adsorbed on the adsorbent by setting the temperature in the chamber to a low temperature state where the quantum molecular sieving effect occurs, and the temperature inside the chamber is higher than the temperature at which the quantum molecular sieving effect occurs 2. The method for separating carbon isotopes from graphite according to claim 1, wherein the methane isotope mixed gas adsorbed on the adsorbent is desorbed from the adsorbent by adjusting the temperature.   The method for separating carbon isotopes from graphite according to claim 1 or 2, wherein the adsorption of the methane isotope mixed gas onto the adsorbent in the concentration step is performed at -196 ° C which is the boiling point of liquid nitrogen.   In the concentration step, the methane isotope mixed gas is allowed to flow through a plurality of chambers, and the methane isotope mixed gas adsorbed on the adsorbent is repeatedly desorbed and adsorbed and collected, and the methane isotope not absorbed is 4. The methane isotope mixed gas is re-separated by collecting the mixed gas in a gas reservoir and passing the collected methane isotope mixed gas through the chamber. A method for separating a carbon isotope from graphite according to claim 1. The method for separating carbon isotopes from graphite according to claim _{4, wherein} only ¹² CH ₄ is finally collected in the gas reservoir.   2. The concentration step is characterized in that the methane isotope mixed gas adsorbed and desorbed in another chamber is alternately introduced into the two chambers, and the adsorption and desorption are repeated to perform concentration. A method for separating a carbon isotope from graphite according to any one of -5.   The method for separating carbon isotopes from graphite according to any one of claims 1 to 6, wherein an adsorbent having pores smaller than 1 nm is used as the adsorbent.   In the step of generating the methane isotope mixed gas, the unreacted carbon dioxide isotope mixed gas is collected as dry ice and gasified to generate a methane isotope mixed gas again. A method for separating a carbon isotope from graphite according to any one of claims 1 to 7.