RU2812219C1 - Method for obtaining highly concentrated o-18 oxygen isotope - Google Patents

Abstract

FIELD: isotope separation.

SUBSTANCE: method for separating isotopes by rectification and chemical isotope exchange. A method has been proposed for obtaining a highly concentrated ¹⁸O oxygen isotope by distillation of water under vacuum with concentration of ¹⁸O isotope from its natural content to a high concentration of at least 95 at.%. in several stages. At one of the stages, water with intermediate ¹⁸O enrichment is removed in the form of selection of the first or the second kind. Water flow is subjected to isotope exchange with hydrogen in a counter-current mass transfer column with a platinum hydrophobic catalyst at the water distillation temperature. After this, the flow is sent to the next stage of rectification into a cascade of columns with ¹⁸O concentration and exhausting parts if the first type is selected or to a separate stage of the concentrating cascade if the second type is selected at the previous stage of rectification.

EFFECT: reduced consumption of the working gas used for chemical isotope exchange, reduced volume of separation equipment, and increased specific production of ¹⁸O in the form of water.

3 cl, 3 dwg, 2 tbl, 2 ex

Description

The invention relates to methods for separating isotopes by physical and chemical methods, namely, rectification and chemical isotope exchange, and can be used to obtain highly concentrated oxygen isotopes O-17 and 0-18.

There is a known method for separating oxygen isotopes by the gas centrifugal method, when the initial oxygen-containing inorganic compound, enriched with the target oxygen isotope, is chemically converted into a gaseous monoatomic inorganic oxygen compound with monoisotopic elements, such as P ¹⁸ OF ₃ (phosphorus oxyfluoride) or F ₂ ¹⁷ O (fluorine oxide) , which is further used as a working substance in gas centrifuges (Petrov Yu.V. et al. Method for separating oxygen isotopes. Patent RU No. 2092234 dated 10.10.1997).

This method is suitable only at the stage of final concentration ^{of 18} O and, as the authors point out, starting from 10% at. ¹⁸ O, that is, after increasing the natural concentration ^{of 18} O by 50 times or more.

There is a known method for separating oxygen isotopes with a concentration of ¹⁸ O during chemical isotope exchange between gaseous nitrogen oxides (NO, NO ₂ ) and an aqueous solution of nitric acid (Oziashvili E.D., Nikolaev Yu.V., Myasoedov N.F. On the possibility of using isotopic exchange between NO and H ₂ O in solutions of nitric acid for the concentration of O ¹⁸ // Communications of the Academy of Sciences of the Georgian SSR, 1962, vol. XXIX, No. 3, pp. 289-292) with the concentration of the heavier oxygen isotope ¹⁸ O in the gas phase:

N ¹⁶ O _(gas) + HN ¹⁸ O ¹⁶ O _{2 (solution, aq} .) = N ¹⁸ O _(gas) + HN ¹⁶ O _{3 ( solution, aq.)} ; (1)

N ¹⁶ O _(gas) + H ₂ ¹⁸ O _(liquid) = N ¹⁸ O _(gas) + H ₂ ¹⁶ O _(liquid) ; (2)

N ¹⁶ O _{2 (gas)} + HN ¹⁸ O ¹⁶ O _{2 (p-relative)} = N ¹⁸ O ¹⁶ O _(gas) + HN ¹⁶ O _{3 (pp, aq} ); (3)

N ¹⁶ O _{2 (gas)} + H ₂ ¹⁸ O _(liquid) = N ¹⁸ O ¹⁶ O _(gas) + H ₂ ¹⁶ O _(liquid) , (4)

starting from the natural content of ¹⁸ O.

The process of separating oxygen isotopes is carried out at room temperature and a nitric acid concentration of 8 M, and the value of the oxygen isotope separation coefficient a under the specified conditions is 1.018.

The disadvantages of this method lie in the complex system of phase flow circulation, and chemically, when in order to obtain concentrated ¹⁸ O it is necessary to obtain both HN ¹⁸ O ₃ and H ₂ ¹⁸ O in a single circuit of the flow circulation system, and without the use of oxygen-containing substances having natural isotopic composition. In addition, due to the peculiarities of flow circulation by this method ^{, 18} O is produced with a relatively low concentration of ≈70-80% at.

There is a known method for separating oxygen and nitrogen isotopes by low-temperature rectification of nitric oxide NO, implemented at a temperature of 120 K and characterized by the separation coefficient of oxygen isotopes α = 1.037 for molecules ¹⁴ N ¹⁶ O ¹⁴ N ¹⁸ O (Borman V.D. et al. Method of simultaneous enrichment of oxide nitrogen (II) isotopes ¹⁸ O, ¹⁷ O, ¹⁵ N. Patent RU No. 2309788).

Significant disadvantages of this method include:

- the need to obtain nitrogen oxide NO of high purity, which is complicated by the disproportionation of the latter in the reaction

4 NO=N ₂ O+N ₂ O ₃ (5)

with the formation of nitrous oxide and sesquinitric oxide;

- the proximity of the melting and boiling points of NO, the values of which are also higher than the boiling point of liquid nitrogen, which makes its direct use as a refrigerant impossible, complicating the design of the condenser and determining the use of an intermediate refrigerant;

- relatively high costs of the main refrigerant - liquid nitrogen;

- high degree of environmental hazard due to NO toxicity.

Another known method for separating oxygen isotopes is low-temperature rectification. This is the rectification of molecular oxygen O ₂ (temperature 93 K, α = 1.006). The process does not require the use of an intermediate refrigerant, but is implemented in combination with air separation (in combination with an air separation unit), which ensures in a single technological cycle both the production of raw material O ₂ and the refrigerant - liquid nitrogen (Igarashi T., Kambe T., Kihara N. Industrial separation of oxygen isotopes by oxygen distillation // J. labeled Compounds and Radiopharmaceuticals, 2019, Vol. 1-5, p. 1-5).

The disadvantages of this method include high refrigerant consumption, as well as the need to carry out a homomolecular oxygen isotope exchange reaction to achieve a high concentration of ¹⁸ O ₂

¹⁶ O ¹⁸ O= ¹⁶ O ₂ + ¹⁸ O ₂ (6)

between two stages of cryogenic distillation (initial and final concentration).

There is a known method for producing water enriched in oxygen-18 by rectifying water under vacuum (Kostylev A.M. et al. Method for producing water enriched in oxygen-18 and an installation for its production. Patent No. RU 2632697), according to which the method for producing water enriched in for oxygen-18, from natural water by the method of rectification of water under vacuum includes preliminary enrichment of water for oxygen-18 in parallel columns operating in an open circuit with the selection of the first type of intermediate concentrate of oxygen-18 and the final enrichment of the intermediate concentrate in a cascade of columns consisting of concentrating oxygen-18 and oxygen-exhaustive 16 columns, that is, they use the so-called Dostrovsky scheme (Andreev B.M. et al. Separation of isotopes of biogenic elements in two-phase systems. - M.: Publishing house AT, 2003, p. 350). In this case, to power the installation, water is used circulating in the pre-enrichment columns, in which the natural content of oxygen-18 is maintained through chemical isotope exchange with carbon dioxide, which in turn maintains the natural content of oxygen-18 through chemical isotope exchange with natural water, performing the role of transport agent between the raw material column and the plant feed flow.

The authors of the patent indicate that the invention provides the production of water enriched in oxygen-18 with a concentration of 95% at. and normalized isotope composition for deuterium, with high efficiency and low level of technological losses.

The main disadvantage of the prototype is the need to use maximum flows of working substances at the initial stage of technological operations, which represent an isotope exchange of oxygen between the source water and carbon dioxide and, further, carbon dioxide with water circulating in the separation equipment, that is, the exchange is carried out in two stages: H ₂ O-CO ₂ -H ₂ O.

The introduction of two additional stages at the very beginning of the technological chain, where the maximum flows of working substances take place, is not only economically inexpedient, but also hardly feasible from a practical point of view. The fact is that the rate of oxygen isotope exchange between water and CO ₂ is very low and exchange practically does not occur without special additives: in a column with a diameter of 25 mm with a packing of wire spirals at a pressure of 14.6 atm, the HETS value was ≈7.7 m ( Andreev B.M. et al. Separation of isotopes of biogenic elements in two-phase systems. - M.: Publishing house AT, 2003, p. 370). To intensify the isotope exchange of oxygen, activating additives are dissolved in water. In particular, it can be monoethanolamine in combination with sodium selenide: 2 M solution of MEA + 9% Na ₂ SeO ₂ . When using such a solution in a similar column with a diameter of 20 mm at a temperature of 50°C and a pressure of ≥10 atm, the HETS value decreased to 6-10 cm (ibid.), reaching acceptable values.

Unfortunately, satisfactory results obtained in solving research problems cannot always be used in production practice. The introduction of the above-mentioned activating additives into the water circulating in the separation part of the installation will lead to the cessation of the separation of oxygen isotopes during water rectification due to the water-MEA mixture and concentration of sodium selenide in the evaporators of the columns. In addition, increasing the pressure in the exchange columns, for example, to 10 atm, will require appropriate equipment, but, in addition, will cause the dissolution of CO ₂ in the water supplied to the reflux of distillation columns operating under vacuum. This means that the struggle of technologists for the purity of rectified water and the stability of the rectification process can go to waste due to degassing of water and accompanying fluctuations in flows in the columns. Most likely, both the productivity of the installation and the claimed increase in the concentration of ¹⁸ O will not be achieved, since the negative impact will exceed the claimed (supposed) positive effect from reducing the concentration of deuterium in water.

The objective of the invention is to reduce the consumption of working gas used for chemical isotope exchange during the rectification of water under vacuum when separating oxygen isotopes with concentration of the ¹⁸ O isotope, reducing the volume of separation equipment and increasing the specific productivity of ¹⁸ O in the form of water.

The problem is solved by the method of separating oxygen isotopes by rectifying water under vacuum with concentrating ^{the 18} O isotope from its natural content to a high concentration of at least 95% at. in several stages, in one of which water with intermediate enrichment of ¹⁸ O is removed in the form of selection of the first or second kind, characterized in that the water flow is subjected to isotope exchange with hydrogen in a countercurrent mass transfer column with a platinum hydrophobic catalyst at the temperature of water rectification, after which sent to the next stage of rectification in a cascade of columns with concentrating and ¹⁸ O-exhausting parts during selection of the first type or to a separate stage of the concentrating cascade during selection of the second type at the previous stage of rectification.

Isotopic exchange of water enriched with the ¹⁸ O isotope with hydrogen of natural isotopic composition is used to normalize the hydrogen isotope composition of water.

Hydrogen isotope exchange is used at several stages of the oxygen isotope separation process.

Example 1

A diagram of the proposed process for separating oxygen isotopes by distillation of water under vacuum using chemical isotope exchange of hydrogen with water is shown in Fig. 1. The scheme includes two stages of water rectification, namely, the stage of obtaining an intermediate product (item I) and the final concentration stage (item II). At the first stage of rectification, pos. I in Fig. 1, when the process is carried out at a pressure of ≈13-15 kPa, water with a natural content of isotopes (food) is enriched to a concentration of 1.6% at. ¹⁸ O. In this case, a simultaneous concentration of deuterium occurs, the content of which increases from the natural level of 0.015% at. up to 0.7% at. The water flow in the form of a first-class selection from the first stage of rectification is directed to irrigate a mass transfer column with a mixed layer of a hydrophobic platinum catalyst and a hydrophilic packing in a ratio of 1:4 (isotope exchange column - IEC), operating at the same distillation temperature, and into which water is countercurrent hydrogen gas is supplied. In the column as a result of a chemical isotope exchange reaction

HDO _(l) + H _{2 (g)} = H ₂ O _(l) ) + HD _(g) (7)

the deuterium content in water is reduced to a concentration of ≈0.03-0.4% at. without changing the isotopic composition of oxygen. The water flow leaving the CIO is fed to the stage of final concentration of ¹⁸ O (item II), where the water is enriched with the heavy oxygen isotope up to 95.7% at.

To avoid losses of ¹⁸ O (Fig. 2), the hydrogen leaving the CIO column I with an increased deuterium content is cooled in the TO heat exchanger, where condensation of H ₂ ¹⁸ O vapor occurs and liquid water is sent to stage II in addition to the main flow from KIO I.

To obtain water with a normalized hydrogen isotopic composition at the end of the installation, KIO II is installed on the product selection line of stage II, which operates similarly to KIO I.

The results of the separation of oxygen isotopes in comparison with the prototype method are shown in Table 1.

As follows from Table 1, the flow of working gas used for chemical isotope exchange is reduced compared to the prototype method by at least 7.6 times, the volume of separation equipment is reduced by 65% rel., and the specific productivity increases by 1. 65 times.

Example 2

The separation of oxygen isotopes by rectification of water under vacuum is carried out in N stages as part of a cascade with a reduction in flows, that is, using selection of the second kind to connect the stages or stages (Fig. 3). The rectification process is carried out at a pressure in the column condensers of 13-15 kPa and the concentration ^{of 18} O is changed from the natural level of 0.204% at. up to 98% at. In this case, the flow from stage J, before it is supplied to stage J+1, is directed to irrigate the isotope exchange column KIO J, into which hydrogen is supplied in countercurrent and where reaction (7) occurs, reducing the deuterium content in the water. After the isotope exchange stage, water is sent to irrigate stage J+1, and the return flow of water from this stage is supplied to the lower part of the previous stage (stage J). In a similar way, the concentration of deuterium in the water leaving stage N-2 is reduced (the stage is not shown in Fig. 3), which is supplied to irrigate the KIO column N-2, and after its passage it is sent to irrigate stage N-1 with the return of the flow of this steps to stage N-2. Thus, in the circuit in FIG. 3 isotope exchange to reduce the concentration of deuterium is carried out at each stage or stage of concentration, ¹⁸ O. A comparison of the results of the separation of oxygen isotopes in this case is given in Table 2.

The comparative results obtained in the example (Table 2) show that in the case of a cascade with a reduction in flows and with selection of the second kind to connect stages or steps along water flows, and with chemical isotope exchange at each stage or step, the advantages of the proposed method are preserved and when receiving ¹⁸ O with a higher concentration.

Thus, the flow of working gas for chemical isotope exchange decreases by 3.6 times, and the volume of separation equipment decreases by 1.49 times with an increase in specific productivity by almost 50% rel.

The considered examples of the process of separating oxygen isotopes by distillation of water under vacuum demonstrate, in general, significant advantages of using hydrogen isotope exchange in the proposed method compared to oxygen isotope exchange, as is the case in the prototype method.

Claims (3)

1. A method for producing a highly concentrated oxygen isotope ¹⁸ O by distillation of water under vacuum with concentration of the isotope ¹⁸ O from its natural content to a high concentration of at least 95 at. in several stages, in one of which water with intermediate enrichment of ¹⁸ O is removed in the form of selection of the first or second type, characterized in that the water flow is subjected to isotope exchange with hydrogen in a countercurrent mass transfer column with a platinum hydrophobic catalyst at the temperature of water rectification, after which sent to the next stage of rectification in a cascade of columns with concentrating and exhaustive ¹⁸ O parts during selection of the first type or to a separate stage of the concentrating cascade during selection of the second type at the previous stage of rectification. 2. The method according to claim 1, characterized in that the isotopic exchange of water enriched with the ¹⁸ O isotope with hydrogen of natural isotopic composition is used to normalize the hydrogen isotope composition of water. 3. The method according to claim 1, characterized in that isotope exchange with hydrogen is used at several stages of the process of separating oxygen isotopes.

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