JPH10128071A - Separation of isotopic gas using velocity separation type adsorbent - Google Patents

Abstract

PROBLEM TO BE SOLVED: To inexpensively separate an isotopic gas by using the adsorbent having a similar sized microhole as the molecular diameter of the isotopic gas and finishing an adsorption process before the adsorbing quantity of the isotopic gas reaches the saturated adsorbing quantity to increase the ratio of the isotopic gas more than in a vapor phase. SOLUTION: At the time of separating the isotope by adsorbing NH3 to an Na-A type zeolite, a gaseous starting material discharged from a gas holder 1 is introduced into an adsorption column 6 from a valve 5 through a mass rate controller 3. NH3 is adsorbed from the front side of the adsorption column 6 with valves 5 and 8 opened and valve 9 and 10 closed and gradually moves to the rear side. At the time of reaching a prescribed adsorption time, the adsorption column 6 is evacuated to vacuum by a vacuum pump 11 with the valves 5, 8 and 9 closed and the valve 10 opened and, when the absorption column 6 reaches a prescribed pressure, the valve 9 is opened and He is introduced at a flow rate corresponding to the displacement of the vacuum pump 11 into the adsorption column 6 by controlling a flow control valve 12 and the adsorbed NH3 is recovered from the rear side of the vacuum pump 11.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for separating an isotope gas using a rate separation type adsorbent.

[0002]

2. Description of the Related Art Isotope separation, concentration and recovery or removal techniques are important as nuclear reaction control techniques in the nuclear industry and labeling techniques for specific elements in chemistry or medicine. Conventionally, the most widely used method for separating isotope gases is called a diffusion method, in which a high-pressure gas is diffused to a low-pressure side through a membrane having micropores. Graham's law is widely known for diffusion in micropores. This law states that the rate at which an isothermal, isobaric gas flows out through a pore to the lower pressure under the same conditions is proportional to the reciprocal of the square root of its mass number.

[0003] That is, the subscript of the lighter isotope gas is 1,
When 2 the index of heavier isotopes gas, the concentration ratio u ₁ / u ₂ of the proportional gas flow rate in the pores of the mass number of the isotope gas Rights

[Outside 1] Concentrates, leaving heavy isotope gas on the high pressure side.

[0004] Since the concentration in one stage stops at 1% or less, a method is adopted in which a concentration unit is configured in multiple stages in a cascade type and the concentration is asymptotically performed.

Another method is a centrifugation method.
This is because when isotopic gas is centrifuged in a high-vacuum state, the proportion of heavier isotope gas increases as it goes radially outward, so gas is collected from the center and the outer periphery. To separate isotopes. Even in the separation in this case, the concentration in one step stops at 1% or less.
A method has been adopted in which a concentration unit is configured in multiple stages in a cascade type and concentration is asymptotically performed.

Light elements such as deuterium, tritium, deuterium water and tritium water have slight differences in their physical properties. Therefore, the precise distillation method, the isotope conversion method, the electrolysis method, etc. Etc. are employed.
These methods are characterized by having relatively high separation efficiency.

[0007]

However, the above-mentioned conventional diffusion method and centrifugal separation method require a vacuum operation under cascade conditions of multiple stages because the efficiency of separation in one stage is very small. There was a problem that the cost was extremely large. The methods such as the precision distillation method, the isotope conversion method, and the electrolysis method were applied only to hydrogen isotopes, and the other methods were not applied because the separation efficiency was lowered.

The present invention has been made in view of the above circumstances, and an object of the present invention is to provide a method for separating an isotope gas at low cost.

[0009]

SUMMARY OF THE INVENTION The present invention has been made to solve the above-mentioned object, and the gist of the present invention is to provide a method for separating an isotope gas, wherein the size of the micropores is reduced by the molecular diameter of the isotope gas. Using an adsorbent close to, the adsorption process is completed before the adsorption amount of the isotope gas reaches the saturated adsorption amount, and the adsorption rate of the isotope gas with a small mass number is reduced to the adsorption rate of the isotope gas with a large mass number. By utilizing the fact that it is larger than the velocity, the ratio of the isotope gas having a small mass number in the adsorbent is to be higher than that in the gas phase.

[0010]

BEST MODE FOR CARRYING OUT THE INVENTION Hereinafter, a method for separating an isotope gas using a rate separation type adsorbent according to the present invention will be described in detail. In a zeolite-based adsorbent, gas diffuses through the voids of the crystal and is adsorbed inside. Therefore, these voids are called windows, and the size of the window is 3 to 10 mm depending on the shape of the crystal and the type of exchanged ions. Having.

The present inventors have found the following facts by making the size of the zeolite window close to the diameter of the adsorbed molecule. That is, when the window diameter of the zeolite is brought close to the molecular diameter of the isotope gas, the adsorption rate of the isotope having a larger mass number (hereinafter, referred to as a heavy isotope) is significantly reduced, while the adsorption rate of the gas having a larger mass number is significantly reduced. The adsorption rate of the isotope (hereinafter referred to as light isotope) gas does not decrease so much. In addition, in the case of adsorption for a relatively short time, light isotope gas is adsorbed more than heavy isotope gas, and the adsorbent exhibits rate separation type adsorption in which light isotope gas is concentrated. The degree of closeness between the window diameter and the molecular diameter may be within a range in which a significant difference in the target adsorption rate can be obtained, and is not limited to the values in the following examples.

[0012]

[Outside 2] The mass of the light isotope, m ₂ , greatly exceeds the mass of the heavy isotope), and can be used for isotopic gas separation.

As for the recovery of the isotope gas adsorbed by the adsorbent, a pressure swing method (PSA) for recovering by introducing a relatively reduced pressure, and a temperature swing method (TSA) for recovering by introducing a relatively high temperature. And a replacement swing method (DSA) in which an isotope gas is replaced with a gas which can be easily separated. But,
It is important to separate the isotope gas by the difference of the adsorption speed by the contact with the adsorbent for a relatively short time in the adsorption process,
Collection and regeneration may be performed by any method. If one-stage concentration is not sufficient, a multi-stage adsorption tower is constituted, and the above-mentioned steps are repeatedly performed on the recovered gas, whereby the isotope can be concentrated to a predetermined concentration.

First Example As an embodiment of the present invention, isotope separation was attempted by adsorbing NH ₃ having a molecular diameter of 3.8 ° on Na-A type zeolite (window diameter 4 °). This will be described with reference to FIG. In FIG. 1, 1 is NH ₃ 10 vol%, He
Since the gas holder is 90 vol% and the abundance ratio of NH ₃ isotope is ¹⁴ NH ₃ : ¹⁵ NH ₃ = 1: 0.0035, the concentration of ¹⁴ NH _{3 is} 10 vol% and ¹⁵ NH _{3 is} 350 ppm. The raw material gas leaving the gas holder 1
From the valve 5 via the mass flow controller 3 and the flow path 4 to the adsorption tower 6
Leads to. At this time, Na-
The A-type zeolite is packed, the adsorbent 7 is regenerated, and the gas phase is filled with He at the same pressure of 1.2 atm as in the adsorption step. When the valves 5 and 8 are opened and the valves 9 and 10 are closed, NH ₃ is adsorbed from the front of the adsorption tower 6 and gradually moves backward. A pressure regulating valve 18 is provided behind the valve 8 to maintain an adsorption pressure of 1.2 atm.

When a predetermined adsorption time is reached, the valves 5,
8 and 9 are closed, the valve 10 is opened, and the adsorption tower 6 is evacuated by the vacuum pump 11. When the adsorption tower 6 reaches a predetermined pressure, the valve 9 is opened, and the flow rate is adjusted to the displacement of the vacuum pump 11. When He is introduced into the adsorption tower 6 by adjusting the flow control valve 12, NH gradually adsorbed from the rear of the adsorption tower 6.
₃ is removed and collected from behind the vacuum pump 11. Here, regarding the isotope composition at the outlet of the adsorption step, the adsorption tower 6
Of the mass branch meter 1
Lead to 5 and measure. On the other hand, regarding the isotope composition on the desorption side, the outlet flow path 16 of the vacuum pump 11 is branched to form the flow path 1
The measurement was performed from 7 using a mass branch meter 19. When the decompression regeneration is completed, the valves 5, 8, and 10 are closed, the valve 9 is opened, and He is led to the adsorption tower 6 to increase the pressure to the same pressure as in the adsorption step, and then returns to the first adsorption step. Table 1 shows the operating conditions at this time.

[0016]

[Table 1]

Under these conditions, the suction process outlet side^FifteenNH_Three/
¹⁴NH_ThreeThe concentration ratio was measured with a mass spectrometer and^Fifteen
NH_Three,¹⁴NH_ThreeThe concentration measurements were analyzed. Adsorption time and output
Mouth side ^FifteenNH_Three/¹⁴NH_ThreeThe relationship of the concentration ratio is shown in the graph of FIG.
Show. As can be seen, the shorter the adsorption time,^FifteenNH
_Three/¹⁴NH_ThreeConcentration ratio is large and with an adsorption time of 10 seconds
^FifteenNH_Three/¹⁴NH_ThreeIs 0.007 and the concentration ratio on the inlet side is 2
However, the concentration on the inlet side is increased for 300 sec.
It decreases to almost the same level as the power ratio. From this, N
a-A type zeolite isotope-isolated by the behavior of adsorption rate separation type
It was also confirmed that it showed. Show such behavior
For the adsorbent, the time per cycle is 0.1 to
Rapid PSA that repeats adsorption and desorption in 30 seconds
Suitable. In this embodiment, one step^FifteenNH_ThreeIs about twice as concentrated
Was done.

Second Example It is known that the window diameter of Na-A type zeolite used as an adsorbent is reduced when recalcining after adsorbing moisture. Considering the molecular size 3.8Å and Na-A type zeolite Mado径4Å of NH _3, further selectivity when scaled down to Mado径is expected to be improved. For this reason, after adsorbing moisture to the Na-A type zeolite, the temperature is 680-78.
The window diameter was reduced by re-firing at 0 ° C. for one hour in the heat treatment time, and the separation of ¹⁵ NH ₃ and ¹⁴ NH ₃ was attempted in the same manner as in the first embodiment. FIG. 3 shows the heat treatment temperature and the outlet side ^{15 of the} adsorption tower 6.
NH ^_3/14 NH ₃ shows the relationship between the concentration ratio in the conditions of the adsorption time of 10 seconds. 760 ° C., isotopic ratios of ¹⁵ NH ^_3/14 NH ₃ is 1 hour re-burned Na-A zeolite 0.0098
It is. This is 2.8 times the inlet-side concentration ratio, and is greatly improved as compared with the case where no heat treatment is performed.

Third Embodiment The crystal lattice Na which determines the window diameter of Na-A type zeolite
The binding force greatly changes depending on the temperature, and the lower the temperature, the more the adsorption is alienated. For this reason, the adsorption temperature is changed in the range of 75 ° C. to 60 ° C., and the outlet side ¹⁵ N
To verify the effect of the H _^3/14 NH ₃ concentration ratio. The result is shown in FIG. In the figure, ○ indicates the re-heat-treated Na-A type zeolite, and ● indicates the untreated Na-A type zeolite. In the re-heat-treated Na-A type zeolite, ¹⁵ NH ₃
/ ¹⁴ NH ₃ concentration ratio shows a maximum value of 0.011 and 3.2 times of the concentration ratio on the inlet side, and in the untreated Na-A type zeolite, −
¹⁵ NH around 30 ℃ _^3/14 NH ₃ concentration ratio 0.0087
5 and 2.5 times the concentration ratio on the inlet side. This shows that the adsorption of ¹⁵ NH ₃ is excluded even if the adsorption temperature is set to a low temperature side in combination with the shortening of the adsorption time and the heat treatment of the adsorbent.

With respect to the fourth embodiment C, separation of ¹² CH ₄ and ¹³ CH ₄ was attempted. Table 2 shows differences in operating conditions at this time from Table 1 of the first embodiment. CH _{4 has} a molecular diameter of 4 ° and Na-A
Since adsorption by the zeolite was possible, adsorption was performed for a relatively short time as in the first to third examples, and the enrichment ratio of the isotope was evaluated.

[0021]

[Table 2]

[0022] Figure 5 shows the adsorption time and the outlet side ¹³ CH _^4/12 CH ₄ concentration ratio. ¹³ CH ₄ / at an adsorption time of 10 seconds
^The concentration ratio of ¹² CH ₄ is 0.286, which is 2.6 times that of the inlet.
Here also, it was shown that the Na-A type zeolite has an isotope separation ability as a rate separation type adsorbent.

Fifth Embodiment Instead of the Na-A type zeolite of the first embodiment, the above-mentioned ¹⁵ NH ₃ and ¹⁴ NH by a carbon molecular sieve 3 A having a window of almost the same size (3.8 to 4 °). Tried to separate ₃ . And an outlet side ¹⁵ NH _^3/14 NH ₃ concentration ratio reached 2.5 times the 0.00875 and the inlet side of the concentration ratio in the adsorption time of 10 seconds. Molecular sieve carbon also has the potential to separate isotope gases as a rate separating adsorbent.

Sixth Embodiment It is known that when trimethylsilane gas is brought into contact with the surface of Na-A type zeolite, it reacts with hydroxyl groups on the crystal surface to reduce the window diameter. For the case where the window diameter is reduced by coating 10% of the surface with silicic acid using trimethylsilane gas, ¹⁵ NH ₃ and ¹⁴ NH ₃ were obtained by the method of the first embodiment.
Attempted to separate. Outlet ¹⁵ at adsorption time 10 sec NH _^3/14
The NH ₃ concentration ratio was 0.00945, which is 2.
7-fold.

[0025]

As described above, according to the isotope gas separation method of the present invention, equipment costs and variable costs are inexpensive and can be applied to the separation of various isotope gases. is there.

[Brief description of the drawings]

FIG. 1 is a process chart showing an isotope gas separation process according to the present invention.

FIG. 2 is a graph showing a relationship between an isotope gas concentration ratio on the outlet side of an adsorption tower and an adsorption time in the first embodiment.

FIG. 3 is a graph showing a relationship between an isotope gas concentration ratio on the outlet side of an adsorption tower and a heat treatment temperature in a second embodiment.

FIG. 4 is a graph showing the relationship between the isotope gas concentration ratio on the outlet side of the adsorption tower and the adsorption temperature in the third embodiment.

FIG. 5 is a graph showing a relationship between an isotope gas concentration ratio on the outlet side of an adsorption tower and an adsorption time in a fourth embodiment.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Gas holder 2 Flow path 3 Mass flow controller 4 Flow path 5 Valve 6 Adsorption tower 7 Adsorbent 8 Valve 9 Valve 10 Valve 11 Vacuum pump 12 Flow control valve 13 Flow path 14 Flow path 15 Mass spectrometer 16 Flow path 17 Flow path 18 Pressure control valve 19 Mass spectrometer

 ────────────────────────────────────────────────── ─── Continuing on the front page (72) Inventor Shigeyuki Asana Nagase Prefecture Nagasaki-shi 5-717-1 Fukahori-cho Sanishi Heavy Industries, Ltd. Nagasaki Laboratory

Claims (9)

[Claims] In an isotope gas separation method, an adsorbent whose micropore size is close to the molecular diameter of the isotope gas is used, and the adsorbing step is performed before the amount of adsorbed isotope gas reaches a saturated adsorption amount. And the ratio of the isotope gas having the lower mass number in the adsorbent is determined by utilizing the fact that the adsorption rate of the isotope gas having the lower mass number is higher than that of the isotope gas having the higher mass number. A method for separating isotope gas, characterized in that it is higher than in phase. 2. After the isotope gas adsorption step is completed,
The method for separating isotope gas according to claim 1, wherein the isotope gas in the adsorbent is recovered by reducing the pressure in the adsorption tower. 3. The method for separating isotope gas according to claim 1, wherein ¹⁵ NH ₃ and ¹⁴ NH ₃ are separated by using Na-A type zeolite as an adsorbent. 4. Use of a Na-A type zeolite which has been subjected to a heat treatment after adsorbing water as an adsorbent, and using ¹⁵ Na ₃ and ¹⁴ NH
_{3. The method} for separating an isotope gas according to claim 1, wherein ₃ is separated. 5. The method for separating isotope gas according to claim 1, wherein ¹⁵ NH ₃ and ¹⁴ NH ₃ are separated using a Na-A type zeolite coated with silicic acid as an adsorbent. 6. The method for separating isotope gas according to claim 1, wherein ¹³ CH ₄ and ¹² CH ₄ are separated by using Na-A type zeolite as an adsorbent. 7. Use of a Na-A type zeolite which has been adsorbed as an adsorbent and re-heat treated, to obtain ¹³ CH ₄ and ¹² CH.
_{4. The method} for separating an isotope gas according to claim 1, wherein ₄ is separated. 8. The method for separating isotope gas according to claim 1, wherein ¹³ CH ₄ and ¹² CH ₄ are separated by using Na-A type zeolite coated with silicic acid as an adsorbent. 9. The method according to claim 1, wherein Na-A type zeolite and heat-treated Na-A type zeolite are used as the adsorbent, and the isotope gas is recovered at a low temperature below room temperature.
4. The method for separating an isotope gas described in 1. above.