Stable isotope concentration method

Abstract

The purpose of the present invention is to provide a stable isotope concentrating method which reduces equipment costs and power without prolonging activation time, and by which efficient concentration is possible. The present invention provides a stable isotope concentrating method that separates stable isotopes and uses a plurality of cascade-connected distillation towers (first through mth towers, m being an integer of 2 or higher), wherein one of either a gas or liquid is supplied from a position near the bottom of an (n-1)th tower to a position near the top of an nth (1<n≤m) tower, the other of either the gas or liquid is returned to the position near the bottom of the (n-1)th tower from the position near the top of the nth tower, and, in each distillation tower, when the flow rate of gas rising within the tower is defined as a first flow rate and the flow rate of gas or liquid supplied from a previous tower or next tower is defined as a second flow rate, the second flow rate is set to be 4 vol% or more of the first flow rate.

Description

Stable isotope enrichment method

[0001] The present invention relates to a method for enriching stable isotopes.

[0002] Known methods for separating stable isotopes that exist only in extremely small amounts in nature include thermal diffusion separation, centrifugal separation, laser separation, chemical exchange separation, and distillation separation. Among these separation methods, distillation separation is suitable for mass production of light elements, so for example, distillation separation of water and distillation separation of oxygen are used as industrial methods for separating stable isotopes of oxygen.

[0003] A feature of the method for separating stable isotopes by distillation is that the separation factor is extremely close to 1, so several thousand theoretical plates are required to obtain a high concentration of stable isotopes. However, since the height of the distillation column is limited, it is necessary to divide the distillation column into multiple columns and connect these multiple columns in series (hereinafter also referred to as "cascade connection"). Furthermore, since the natural abundance of stable isotopes is low, the amount of product is very small compared to the amount of raw material fed.

[0004] Therefore, among multiple distillation columns connected in series, the column into which the raw material is fed has the largest column diameter, and since the product concentration in the column increases toward the end of the stripping section and the rectifying section, the column diameter can be made smaller (also called a "stepped cascade"). This not only reduces equipment costs but also contributes to shortening start-up time by reducing the amount of liquid holdup. On the other hand, in order to reduce the column diameter, it is necessary to provide a condenser and a reboiler in each column with a smaller column diameter, and increasing the number of condensers and reboilers increases power consumption.

[0005] In general, when connecting multiple distillation columns, liquid from the bottom of the nth distillation column is supplied to the top of the next column (the (n+1)th distillation column), and gas from the top of the nth distillation column is supplied to the bottom of the previous column (the (n-1)th distillation column). On the other hand, in Patent Document 1, when connecting multiple distillation columns, gas is supplied from the bottom of the nth distillation column to the top of the next column (the (n+1)th distillation column), and liquid is supplied from the top of the nth distillation column to the bottom of the previous column (the (n-1)th distillation column).

[0006] Patent No. 5132822

[0007] As mentioned above, in isotope separation, a very long distillation column is divided into multiple columns and connected in series to perform separation. By providing a condenser and reboiler for each column, the concentration can be increased, and by reducing the column diameter, the required power consumption can be reduced and the startup time can be shortened. To reduce the required power consumption, it is necessary to reduce the operating power of the reboiler that generates the ascending gas and the condenser that generates the descending liquid in each column, as well as to ensure the flow rate of the raw material required to connect each column. If this connecting flow rate is excessive, the energy consumption of the condenser and reboiler increases, and the piping becomes larger, resulting in increased equipment costs. On the other hand, if the connecting flow rate is insufficient, differences in concentration will occur between the distillation columns, making efficient concentration impossible.

[0008] The present invention has been made in consideration of the above circumstances, and an object of the present invention is to provide a stable isotope enrichment method that reduces equipment costs and power consumption without prolonging start-up time and enables efficient enrichment.

[0009] In order to achieve the above object, the present invention employs the following configuration: [1] A method for separating stable isotopes using a plurality of cascaded distillation columns (first column to mth column; m is an integer of 2 or greater), wherein one of gas and liquid is supplied from a position near the bottom of the (n-1)th column to a position near the top of the nth column (1<n≦m), and the other of gas and liquid is returned from a position near the top of the nth column to a position near the bottom of the (n-1)th column, and wherein, in each distillation column, the flow rate of the gas ascending within the column is defined as a first flow rate, and the flow rate of the gas or liquid supplied from the preceding column or the next column is defined as a second flow rate, and the second flow rate is set to 4% by volume or more of the first flow rate. [2] A method for enriching stable isotopes, wherein the stable isotopes １８ 0, and the second flow rate is set to 4% by volume or more of the first flow rate.

[0010] According to the stable isotope enrichment method of the present invention, it is possible to reduce the equipment cost and power consumption without prolonging the start-up time, and to achieve efficient enrichment.

[0011] BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a system diagram showing the configuration of a stable isotope enrichment device applicable to a stable isotope enrichment method according to an embodiment of the present invention. Fig. 2 is a graph for verifying the effects of the present invention. Fig. 3 is a graph for verifying the effects of the present invention. Fig. 4 is a graph for verifying the effects of the present invention. Fig. 5 is a graph for verifying the effects of the present invention.

[0012] The present invention will be described in detail below by showing embodiments with reference to the accompanying drawings. Note that the drawings used in the following description may show characteristic parts enlarged for convenience in order to make the characteristics easier to understand, and the dimensional proportions of each component may not necessarily be the same as those in reality.

[0013] <Stable Isotope Enrichment Apparatus> Figure 1 is a system diagram showing the configuration of a stable isotope enrichment apparatus that can be applied to a stable isotope enrichment method according to an embodiment of the present invention. The stable isotope enrichment apparatus 300 of this embodiment comprises a distillation column group in which thirteen distillation columns 1 to 13 are cascade-connected, an isotope scrambler 25, thirteen condensers 21, thirteen reboilers 22, a raw material supply line 30, a product discharge line 31, an isotope-enriched gas extraction line 36, and an isotope-enriched gas return line 37. The stable isotope enrichment apparatus 300 of this embodiment is an apparatus that enriches, to a high concentration, stable isotope compound components (hereinafter, sometimes simply referred to as "stable isotopes") containing any stable isotope element contained in a raw material compound.

[0014] The stable isotope enrichment apparatus 300 also includes, between each distillation column, a supply line 34 for supplying raw material from the preceding column to the next column, and a return line 35 for returning raw material from the next column to the preceding column. Each supply line 34 is provided with an on-off valve 23. Each return line 35 is provided with an on-off valve 24.

[0015] Hereinafter, the n-th distillation column from the upstream end of the distillation column group will be referred to as the n-th distillation column. Due to space limitations, of the first to thirteenth distillation columns 1 to 13, only the first distillation column 1 to which a high-purity raw material compound (hereinafter sometimes simply referred to as the "raw material") is supplied, and the ninth to thirteenth distillation columns 9 to 13 are shown in FIG.

[0016] The first to thirteenth distillation columns 1 to 13 are cascade-connected in the following order: first distillation column 1 to which the raw material is supplied, second distillation column 2, third distillation column 3, fourth distillation column 4, fifth distillation column 5, sixth distillation column 6, seventh distillation column 7, eighth distillation column 8, ninth distillation column 9, tenth distillation column 10, eleventh distillation column 11, twelfth distillation column 12, and thirteenth distillation column 13 (the distillation column at the downstream end of the group of distillation columns).

[0017] The first to thirteenth distillation columns 1 to 13 distill stable isotope compounds containing stable isotopes (performing a distillation cascade process) to concentrate stable isotope compounds of light components with low boiling points at the top of the column and stable isotope compounds of heavy components with high boiling points at the bottom of the column. Of the first to thirteenth distillation columns 1 to 13, the first distillation column 1, to which the raw material is supplied, has the largest distillation load and therefore the largest column diameter. Furthermore, since the abundance ratio of stable isotopes of heavy components is usually low, the distillation load gradually decreases (the column diameter decreases) from the upstream end of the first distillation column 1 to the downstream end of the first distillation column 1. Each of the first to thirteenth distillation columns 1 to 13 is equipped with rectification (tray) trays, structured packing, random packing, or the like (none of which are shown).

[0018] One end of the supply line 34 is connected to a position near the bottom of the (n-1)th distillation column (1 < n ≦ m; m is an integer of 2 or greater). The other end of the supply line 34 is connected to a position near the top of the nth distillation column. Gas from the (n-1)th distillation column is discharged to the supply line 34. The flow rate of the raw material gas discharged to the supply line 34 is controlled by the on-off valve 23, and then the raw material gas is introduced into the nth distillation column.

[0019] One end of the return line 35 is connected to a position near the top of the nth distillation column. The other end of the return line 35 is connected to a position near the bottom of the (n-1)th distillation column (1 < n ≦ m; m is an integer of 2 or greater). The liquid in the nth distillation column is discharged to the return line 35. The raw material liquid discharged to the return line 35 is introduced into the (n-1)th distillation column after its flow rate is controlled by the on-off valve 24.

[0020] One condenser 21 is provided for each distillation column (first to thirteenth distillation columns 1 to 13). The condenser 21 is provided on a circulation line 32, both ends of which are connected to different positions at the top of each distillation column. The condenser 21 liquefies gas that rises within the distillation column through heat exchange and then allows the gas to descend again within the distillation column. The condenser 21 can reduce the pressure at the top of each column, preventing a decrease in the separation factor due to pressure increases when columns are connected in series. In this example, a heat transfer medium fluid circulation line 38 is provided to circulate a heat transfer medium fluid through multiple condensers 21, and heat exchange is performed by the heat transfer medium fluid circulating through this heat transfer medium fluid circulation line 38.

[0021] One reboiler 22 is provided for each distillation column. The reboilers 22 are provided on circulation lines 33, both ends of which are connected to different positions in the bottom of each distillation column. The reboilers 22 have the function of vaporizing the liquid that has descended within the distillation column through heat exchange and causing it to rise again within the distillation column. In this example, a heat transfer medium fluid circulation line 39 is provided to circulate a heat transfer medium fluid through multiple reboilers 22, and heat exchange is performed by the heat transfer medium fluid circulating through this heat transfer medium fluid circulation line 39.

[0022] One end of the raw material supply line 30 is connected to the intermediate section of the first distillation column 1. The raw material supply line 30 is a line for supplying a high-purity raw material to the intermediate section of the first distillation column 1. A valve is provided on the raw material supply line 30. The intermediate section of the distillation column refers to a position other than the top and bottom of the distillation column. High purity means that the total impurity concentration of the raw material is 0.001% or less.

[0023] The stable isotope compounds that can be enriched by the stable isotope enrichment method of this embodiment are not particularly limited, and examples of the stable isotope compounds that can be enriched include oxygen, water, carbon monoxide, and nitric oxide.

[0024] One end of the isotope-enriched gas extraction line 36 is connected to an intermediate portion of the tenth distillation column 10, and the other end is connected to the isotope scrambler 25. The isotope-enriched gas extraction line 36 is provided with a valve 26. When the valve 26 is open, the isotope-enriched gas extraction line 36 extracts some or all of the raw material compound from the tenth distillation column 10 and supplies it to the isotope scrambler 25. The raw material compound extracted from the tenth distillation column 10 contains stable isotope compounds enriched by the first to tenth distillation columns 1 to 10.

[0025] The isotope scrambler 25 converts a stable isotope compound containing one stable isotope element into a stable isotope compound containing another stable isotope element through an isotope exchange reaction (isotope scrambling). This allows for the production of a concentrate with an even higher concentration of the stable isotope compound containing the desired stable isotope element. The concentrate obtained by the isotope scrambler 25 is returned to the tenth distillation column 10 via the isotope-enriched gas return line 37.

[0026] One end of the isotope-enriched gas return line 37 is connected to the isotope scrambler 25, and the other end is connected to the tenth distillation column 10. The isotope-enriched gas return line 37 is provided with a valve 27. When the valve 27 is open, the isotope-enriched gas return line 37 returns the concentrate, which has a further increased concentration of stable isotope compounds containing desired stable isotopes, from the isotope scrambler 25 to the tenth distillation column 10. The raw material compounds returned to the tenth distillation column 10 are distilled in the eleventh to thirteenth distillation columns 11 to 13, and the stable isotope compounds containing the desired stable isotopes are further enriched.

[0027] The product discharge line (discharge line) 31 is a path for discharging a stable isotope compound containing an enriched stable isotope element as a product. One end of the product discharge line 31 is connected to the lower part (position near the bottom) of the thirteenth distillation column 13.

[0028] <Stable Isotope Enrichment Method> Next, a stable isotope enrichment method according to an embodiment of the present invention will be described with reference to Fig. 1. In this embodiment, stable isotopes are enriched using a stable isotope enrichment apparatus 300 shown in Fig. 1.

[0029] The stable isotope enrichment method of this embodiment is a method for separating stable isotopes using a plurality of cascade-connected distillation columns (first to mth columns; m is an integer of 2 or greater), in which gas is supplied from a position near the bottom of the (n-1)th column to a position near the top of the nth column (1<n≦m), and liquid is returned from a position near the top of the nth column to a position near the bottom of the (n-1)th column. In each distillation column (in the stable isotope enrichment apparatus 300 shown in FIG. 1 , the second distillation column 2 to the thirteenth distillation column 13), the flow rate of the gas ascending through the column is defined as a first flow rate, and the flow rate of the gas supplied from the preceding column is defined as a second flow rate, and the second flow rate is set to be 4% by volume or more of the first flow rate.

[0030] According to the stable isotope enrichment method of this embodiment, by setting the second flow rate to 4% by volume or more of the first flow rate, there is no gap in the concentrations of the respective columns, thereby enabling efficient enrichment. It is more preferable that the second flow rate relative to the first flow rate be 5% by volume or more.

[0031] In addition, as will be explained in the examples below, stable isotopes １８ Even when the stable isotope enrichment apparatus 300 is in a stable state, it is preferable to set the second flow rate to 4% by volume or more of the first flow rate. By operating the stable isotope enrichment apparatus 300 in this manner, the proportion of stable isotope molecules in the oxygen gas discharged from the bottom of the thirteenth distillation column 13 when the apparatus is stable can be set to １８ The O content can be 97 atomic % or more.

[0032] Moreover, the upper limit of the second flow rate relative to the first flow rate is preferably set to 30% by volume or less from the viewpoint of reducing energy consumption.

[0033] On the other hand, the cascade process also requires a return line 35 to the preceding column. The flow rate of this line must be approximately the same as the flow rate of the supply line 34. The size of the piping and on-off valves related to this flow path, as well as the input energy of the reboiler and condenser, can be minimized.

[0034] According to the stable isotope enrichment method of this embodiment, the flow rate of the gas or liquid flowing between the distillation columns can be controlled to 4 to 30% by volume of the flow rate of the gas ascending through the columns (first flow rate), thereby maintaining a smooth transition in concentration between the columns. This reduces the energy consumed by the reboilers and condensers installed in each column, and enables the extraction of a high-purity product.

[0035] As described above, according to the stable isotope enrichment method of this embodiment, it is possible to reduce equipment costs and power consumption without prolonging the start-up time of the stable isotope enrichment apparatus 300, and to achieve efficient enrichment.

[0036] The technical scope of the present invention is not limited to the above-described embodiment, and various modifications can be made without departing from the spirit of the present invention. For example, the number of distillation columns (cascade-connected distillation columns) constituting the distillation column group of the stable isotope enrichment apparatus 300 is not limited to 13 columns, and can be any number. The distillation columns are not limited to packed columns packed with structured packing, and may be packed columns packed with random packing or plate columns.

[0037] The isotope scrambler is not limited to being connected to the tenth distillation column 10, and may be connected to a distillation column other than the tenth distillation column 10. However, at least one of the distillation columns to which the isotope scrambler 25 is connected is a distillation column other than the distillation column located at the downstream end of the group of distillation columns. Note that, although the stable isotope enrichment apparatus 300 of the above embodiment has been described as including the isotope scrambler 25 as an example, the stable isotope enrichment apparatus 300 may be configured such that the isotope scrambler 25 is not required depending on the fluid used.

[0038] Furthermore, in the stable isotope enrichment apparatus 300 of the above embodiment, a configuration in which the raw material supply tower is the first tower has been described as an example, but this is not limited to this, and the tower may be provided in a location other than the first tower depending on the composition of the raw material and the required yield.

[0039] In the above embodiment, the stable isotope enrichment apparatus 300 applied to the stable isotope enrichment method has been described as having a configuration in which the condenser 21 and the reboiler 22 are provided in each distillation column, but the present invention is not limited to this. For example, the condenser 21 and the reboiler 22 may be provided every few columns.

[0040] Furthermore, in the above-described embodiment, the stable isotope enrichment apparatus 300 applied to the stable isotope enrichment method is described as being configured to supply gas from a position near the bottom of the (n-1)th column to a position near the top of the nth column (1 < n ≦ m) and return liquid from a position near the top of the nth column to a position near the bottom of the (n-1)th column. However, this is not limiting. The stable isotope enrichment apparatus may also be configured to supply liquid from a position near the bottom of the (n-1)th column to a position near the top of the nth column (1 < n ≦ m) and return gas from a position near the top of the nth column to a position near the bottom of the (n-1)th column. Adopting such a configuration allows the capacity of the condenser 21 and reboiler 22 provided in each distillation column to be reduced, but requires a liquid pump to supply liquid to the next column, necessitating adjustment of energy consumption.

[0041] The present invention will be described in detail below with reference to verification tests, but the present invention is not limited thereto.

[0042] <Verification Test 1> (Test Example 1) Using the stable isotope enrichment device 300 shown in FIG. １８ The distillation column group consisted of 13 distillation columns, each of which was a packed column. Liquefied nitrogen was used as the refrigeration source for the condensers 21, and liquefied nitrogen was supplied to each condenser 21 via a heat transfer fluid circulation line 38. Gaseous nitrogen was used as the heat source for the reboilers 22, and gaseous nitrogen was supplied to each reboiler 22 via a heat transfer fluid circulation line 39.

[0043] The stable isotope enrichment apparatus 300 is also provided with an isotope scrambler 25, and one end of an isotope-enriched gas extraction line 36 is connected to the tenth distillation column 10 and the other end is connected to the isotope scrambler 25, and some or all of the oxygen is extracted and supplied to the isotope scrambler 25. The oxygen that has undergone the isotope exchange reaction in the isotope scrambler 25 is returned to the tenth distillation column 10 via an isotope-enriched gas return line 37.

[0044] The composition of the raw material high-purity oxygen was at natural abundance ratio. The pressure at the top of each distillation column was 20 kPaG. When the apparatus was stable, the abundance ratio of stable isotope molecules in the oxygen gas discharged from the bottom of the thirteenth distillation column 13 was: １８ The flow rate of the gas and liquid connecting each tower was set to 5% of the flow rate of the ascending gas of each tower.

[0045] 2 shows the concentration distribution in each distillation column in Test Example 1. As shown in FIG. 2, in Test Example 1, the target １８ The O concentration was 97.5 atomic %.

[0046] (Test Example 2) In Test Example 2, the equipment configuration and operating conditions were the same as in Test Example 1, except that the flow rates of the gas and liquid connecting each column were set to 4% of the ascending gas flow rate of each column. Figure 3 shows the concentration distribution in each distillation column in Test Example 2. As shown in Figure 3, in Test Example 2, the final column (thirteenth distillation column 13) was used to obtain the target １８ The O concentration was 97.1 atomic %.

[0047] (Test Example 3) In Test Example 3, the equipment configuration and operating conditions were the same as in Test Example 1, except that the flow rates of the gas and liquid connecting each column were set to 3% of the ascending gas flow rate of each column. Figure 4 shows the concentration distribution in each distillation column in Test Example 3. As shown in Figure 4, in Test Example 3, the final column (the thirteenth distillation column 13) was used to obtain the target １８ The O concentration was 96.3 atomic %, which was slightly below the set value.

[0048] In Test Examples 1 and 2, the flow rates of the gas and liquid connecting each tower were 4% or more of the flow rate of the ascending gas in each tower, making the equipment an appropriate size and optimizing energy consumption. On the other hand, in Test Example 3, the flow rates of the gas and liquid connecting each tower were less than 4% of the flow rate of the ascending gas in each tower, creating gaps in the concentration distribution in each tower and making it impossible to extract high-concentration products.

[0049] <Verification Test 2> (Test Example 4) Using the stable isotope enrichment device 300 shown in FIG. １８ O was concentrated. １８ The conditions were the same as in Test Example 1, except that the O concentration was set to 98 atomic % or more. In other words, the flow rates of the gas and liquid connecting each column were set to 5% of the ascending gas flow rate of each column. Figure 5 shows the concentration distribution in each distillation column in Test Example 4. As shown in Figure 5, in Test Example 4, the final column (the thirteenth distillation column 13) was １８ The O concentration was 98.2 atomic %.

[0050] (Test Example 5) In Test Example 5, the apparatus configuration and operating conditions were the same as those of Test Example 4, except that the flow rates of the gas and liquid connecting each column were set to 4% of the ascending gas flow rate of each column. Figure 6 shows the concentration distribution of each distillation column in Test Example 5. As shown in Figure 6, in Test Example 5, the final column (the thirteenth distillation column 13) was used to obtain the target １８ The O concentration was 98.0 atomic %.

[0051] (Test Example 6) In Test Example 6, the apparatus configuration and operating conditions were the same as those of Test Example 4, except that the flow rates of the gas and liquid connecting each column were set to 3% of the flow rate of the ascending gas in each column. Figure 7 shows the concentration distribution in each distillation column in Test Example 6. As shown in Figure 7, in Test Example 6, the final column (the thirteenth distillation column 13) was used to obtain the target １８ The O concentration was 97.6 atomic %, which was below the set value.

[0052] In Test Examples 4 and 5, the flow rates of the gas and liquid connecting each tower were 4% or more of the flow rate of the ascending gas in each tower, making the equipment an appropriate size and optimizing energy consumption. On the other hand, in Test Example 6, the flow rates of the gas and liquid connecting each tower were less than 4% of the flow rate of the ascending gas in each tower, creating gaps in the concentration distribution in each tower and making it impossible to extract high-concentration products.

[0053] According to the results of verification tests 1 and 2, it is possible to １８ Under multiple O specification conditions (98 atomic % or more, 97 atomic % or more), by setting the flow rates of the gas and liquid connecting each tower to 4% or more of the ascending gas flow rate of each tower, it became possible to extract a highly concentrated product.

[0054] The present invention has applicability in a stable isotope enrichment method for enriching stable isotopes including stable isotope atoms using a distillation apparatus equipped with a distillation column group in which a plurality of distillation columns are connected in series (cascade connection).

[0055] 1, 9, 10, 11, 12, 13... Distillation column 21... Condenser 22... Reboiler 23, 24... On-off valve 25... Isotope scrambler 30... Raw material supply line 31... Product discharge line (discharge line) 34... Supply line 35... Return line 300... Stable isotope enrichment device

Claims

1. A method for separating stable isotopes using multiple cascaded distillation columns (1st column to mth column; m is an integer of 2 or greater), in which one of gas and liquid is supplied from a position near the bottom of the (n-1)th column to a position near the top of the nth column (1<n≦m), and the other of gas and liquid is returned from a position near the top of the nth column to a position near the bottom of the (n-1)th column, and in each distillation column, the flow rate of the gas ascending within the column is defined as a first flow rate, and the flow rate of the gas or liquid supplied from the preceding or succeeding column is defined as a second flow rate, where the second flow rate is 4% by volume or more of the first flow rate.

2. The stable isotope １８ 2. The stable isotope enrichment method according to claim 1, wherein the second flow rate is 4% by volume or more of the first flow rate.

Images