RU2166982C2 - Method of separation of carbon isotopes - Google Patents

Abstract

FIELD: extraction of carbon isotopes from industrial waste, monitoring of environment, recovery of radioactive ¹⁴C isotope from radioactive waste. SUBSTANCE: mixture of carbon isotopes containing 1.104 at % of ¹³C isotope is purified by cryogenic rectification, is separated in cascade of rectification packing columns. The resulting semiproduct is hydrogenated with hydrogen to methane. Hydrogenation uses burnt gas resulting from production of deuterium by rectification of liquid hydrogen containing 0.0015 at % of deuterium. Hydrogenation product is subjected to drying and fed into continuously operating counterflow adsorption column filled with zeolite NaX, NaA, CaA at bottom when, heated to 470 K, at tip methane is absorbed with zeolite when cooled to 200 K. Pressure in column is maintained at level of 10⁵ Pa. Concentration of ¹³C isotope is at least 99.95 wt.%, separation coefficient is at least 1.02; enrichment coefficient exceeds identical characteristic for the well known method by 6 times. EFFECT: more efficient separation method. 4 cl, 4 ex, 3 tbl

Description

The invention relates to the field of isotope chemistry and can be used for the separation of isotopic mixtures of carbon: in the production of its isotopes from natural isotopic mixtures; during isotopic purification of carbon-14 preparations produced using the nuclear reaction ¹⁴ N (n, p) ¹⁴ C; upon extraction (regeneration) of carbon isotopes from technological wastes accompanying the production of chemical compounds labeled with these isotopes; during isotopic purification from radioactive carbon-14 circulating in the production, as well as to the technological flows of the enterprises of the nuclear technological cycle discharged into the environment and, in addition, during isotopic concentration (in order to increase the sensitivity and accuracy of analysis) of carbon samples taken for isotopic analysis in the course of various technological processes, environmental monitoring, as well as a variety of scientific research.

 A known method for the separation of carbon isotopes by chemical isotope exchange using two-phase carbon-containing working systems in continuously working countercurrent columns with the circulation of flows at their ends by chemical or thermal means (see Boreskov G.K., Katalnikov S.G. Technology of chemical isotope exchange processes. M. : Publishing House of Moscow Art Theater named after D.I. Mendeleev. 1974, S. 95-102).

The disadvantage of this method is: the low rate of interfacial isotope exchange of carbon in working systems, due to the occurrence of isotopic exchange reactions between them in phases along with diffusion processes of carbon-containing substances (the consequence is the large dimensions of the mass transfer columns, as well as their long duration for putting them into stationary operation ); high consumption of high-purity chemicals for the circulation of flows in the columns and the associated significant volume of chemically aggressive waste; high toxicity, as well as low thermochemical stability of working substances, requiring special measures to ensure the safety of maintenance personnel, as well as the organization, along with the main process of separation of carbon isotopes, auxiliary processes for the deep cleaning of substances circulating in the apparatus from their decomposition products, and, finally, low radiation stability of the substances that make up the working systems, not allowing the use of this method for concentration of the radioactive carbon isotope a - ¹⁴ C.

 A known method of separation of carbon isotopes, based on the selective excitation of atoms and molecules in electric discharges (pulsed, constant, variable) (see Oziashvili ED, Egiazarov AS Separation of stable carbon isotopes. // Successes of chemistry. 1989. T 58, N 4.P. 545-565).

 The disadvantage of this method are: high energy intensity, the complexity of the accumulation of separation products, as well as the multiple multiplication of the elementary separation effect.

 A known method of separation of carbon isotopes, based on the selective action of laser radiation on the atoms of carbon isotopes or on molecules containing them (see Oziashvili E.D., Egiazarov A.S. Separation of stable carbon isotopes. // Successes of chemistry. 1989. T. 58 N 4.P. 545-565). The specified method, characterized by high selectivity of the elementary act of separation, relatively low energy costs and the quick conclusion of the separation equipment to a stationary mode of operation, at the same time, has several disadvantages.

 The disadvantages of this method include: the complexity of multiple multiplication of the elementary effect of separation; the need to use, as a source, complex in construction, specially synthesized for the purpose of separation of carbon isotopes, expensive chemical compounds; the high cost and complexity of the separation equipment, as well as the chemical inertness of the resulting isotopically enriched compounds, complicating their further processing into widely used chemicals labeled with carbon isotopes.

 A known method of separating carbon isotopes by mass diffusion of a carbon-containing gas through a porous septum or through a steam stream of an auxiliary substance (see Oziashvili E.D., Egiazarov A.S. Separation of stable carbon isotopes. // Successes of chemistry. 1989. T. 58, N 4.P. 545-565). The specified method, when using methane as a carbon-containing gas, and as an excipient, mercury vapor allows, along with the concentration of stable carbon isotopes, radioactive carbon-14 to be concentrated from isotopic mixtures.

The disadvantage of this method are: the complexity and high cost of hardware; high energy intensity associated with the pumping of gas streams, as well as with the evaporation and pumping of auxiliary substances; the need, as a rule, to work with highly toxic substances (mercury vapor); low productivity. For example, upon separation of an isotopic mixture of ¹² CH ₄ - ¹³ CH _{4 by} mass diffusion of methane with an initial ¹³ C content of 2.7% at. through a stream of mercury vapor at a pressure of 1333 Pa in a cascade consisting of 70 mass diffusion separation pumps it is possible at a concentration of ¹³ C in the product of 88% at. ensure productivity (selection) of only 30 cm ^{3 of} gas per day.

A known method for the separation of carbon isotopes in gas thermal diffusion columns using methane (CH ₄ ) or carbon monoxide (II) (CO) as a working substance. The method allows along with stable carbon isotopes to concentrate from isotopic mixtures and radioactive carbon-14 (see Oziashvili E.D., Egiazarov A.S. Separation of stable carbon isotopes. // Advances in chemistry. 1989. T. 58, N 4. C 545-565).

The disadvantages of this method are: the need for preliminary extremely deep purification of the source gases from chemical impurities, as well as the continuous removal of the latter from the thermal diffusion columns during the separation of carbon isotopes; low productivity; high energy intensity and, as a result, the high cost of the target products. For example, for the enrichment of methane with an initial content of ¹³ C of 1.104% at. thermal diffusion method up to 20% at. it is necessary to expend a power of 6700 kWh / g ¹³ C. Using the thermal diffusion method for separating carbon isotopes to concentrate ¹³ C from natural isotopic mixtures is economically justified only with production volumes not higher than ~ (100-200) g ¹³ CH ₄ per year with a concentration of ¹³ C up to 60% at. Thermal diffusion of natural methane or CO without special measures does not make it possible to obtain highly concentrated carbon isotopes in columns, in particular for ¹³ C above 94.8% at. The latter is due to the concentration in the columns, along with the target product, for example ¹³ CH ₄ , diluting it and equal in weight, for example, ¹² CH ₃ D, of forms isotopically substituted for hydrogen.

A known method of separating carbon isotopes by distillation of carbon-containing compounds in continuously operating distillation columns (see Oziashvili V. D., Egiazarov A. S. Separation of stable carbon isotopes. // Successes in chemistry. 1989. T. 58, N 4. S. 545-565 ) The highest technical and economic indicators, as well as the possibility of concentration along with stable isotopes of carbon and radioactive ¹⁴ C, are achieved when carbon monoxide (II) (CO) is used as a carbon-containing compound in it. Using this compound, the rectification process is carried out in the cryogenic temperature range (near 80 K) at a pressure close to atmospheric (optimal pressure 86.6 kPa). Under these conditions, it is characterized by a relatively large value of a single separation coefficient (α ( ¹³ CO- ¹² CO) ≈ 1.007), a high rate of interfacial isotope exchange (the height of the equivalent theoretical separation stage in VETS packed columns is ≈ (2.2-2.6) cm ), as well as high density irrigation of mass transfer columns with carbon.

The disadvantage of this method is the difficulty in achieving high concentrations of the heavy carbon isotope in it due to the presence of a heavy oxygen isotope of ¹⁸ O in the composition of carbon monoxide (II). For example, natural carbon monoxide (II) is basically a mixture of ¹² C ¹⁶ O + ¹³ C ¹⁶ O + ¹² C ¹⁸ O. The partial vapor pressures of ¹² C ¹⁸ O and ¹³ C ¹⁶ O differ so little (α = 1.002 at 74.4 K) that during rectification only ¹² C ¹⁶ O from the binary mixture of ¹² C is practically separated in the column. ¹⁸ O + ¹³ C ¹⁶ O. This limits the possible concentration level at ¹³ C to the distillation Onne limit of <95% at. In really working columns, this level does not exceed 93% at.

The closest solution to the proposed invention is a method for the separation of carbon isotopes, including: pre-concentration of the target isotope by cryogenic distillation of carbon monoxide (II) in continuously operating distillation columns; subsequent hydrogenation of the pre-enriched product to methane with hydrogen obtained by electrolysis of water of natural isotopic composition, in which the deuterium content is 5 • 10 ⁵ (0.005% at.); final concentration of the target isotope in countercurrent columns by thermal diffusion of the hydrogenation product (methane) (see G.D. Rabinovich, Separation of isotopes and other mixtures by thermal diffusion. M: Atomizdat. 1981, 98 pp.) (prototype). The specified method provides the concentration of isotopic mixtures of carbon as stable ( ¹² C; ¹³ C), and radioactive ( ¹⁴ C) isotopes. Due to the use of hydrogen with a reduced deuterium content in the hydrogenation stage of CO, the final isotope, for example ¹³ C, is subjected to the final thermal diffusion concentration in this method. Methane with a reduced content of deuterium substituted forms (in particular, CH ₃ D) is fed in this method. This provides the ability to achieve high concentrations of the target isotope in the product. For example, when ¹³ C is concentrated by the specified method from a natural isotopic mixture, it is possible to achieve the content of this isotope in the product at the level of 99-99.8% at.

The disadvantages of this method include the use in it at the stage of hydrogenation of CO of hydrogen obtained by electrolysis of water with a natural isotopic composition, as well as the final concentration of the target isotope by thermal diffusion of methane in the columns. Hydrogen obtained by electrolysis of water contains up to 0.5% vol. impurities, including up to 25 g / cm ³ moisture. In addition to moisture, air components (O ₂ , up to 0.02% N ₂ , Ar), as well as CO _2, are present as impurities in it. These impurities, coming together with hydrogen to the stage of hydrogenation of CO, can not only contaminate the methane obtained in this case and, as a result, subsequently have a harmful effect on the operation of thermal diffusion columns (reducing their productivity and also causing corrosion of structural materials at high temperatures) but also lead to the loss of the enriched product due to the chemical interaction of some of them (O ₂ , H ₂ O) with CO and CH ₄ , as well as due to its isotopic dilution by isotope exchange and mixing with carbon odomeric CO ₂ natural isotopic composition. The presence of these impurities in the hydrogen used necessitates either its deep chemical purification before the hydrogenation stage or the subsequent deep chemical purification of methane obtained with its use, which is then used to power the thermal diffusion columns. Both processes are complex in hardware design, their implementation leads to an increase in the cost of the target isotope product. Hydrogen obtained during the electrolysis of water with a natural isotopic composition contains a reduced amount of deuterium (~ 0.005% at.) Relative to the natural level (0.015% at.). This, by reducing the content of isotopic substituted forms (for example, CH ₃ D) at the stage of CO hydrogenation in methane subjected to thermal diffusion, helps to increase the attainable limit of the concentration of target isotopes (in particular, ¹³ C) in the product. For example, for ¹³ C from 94.7 to 99 - 99.8% at. However, the amount of deuterium in electrolytic hydrogen remains at a high level and, as a result, the content of deutero-substituted forms of methane in the gas supplied to thermal diffusion remains at a high level. This limits the ability of the final thermal diffusion concentration of carbon isotopes in the considered method to a level of 99 - 99.8% at. The use of methane in the method under consideration at the stage of the final concentration of thermal diffusion leads to a sharp complication of the hardware design of the final concentration process, to the need to use precious metals for its hardware design, as well as to the low efficiency of the final concentration process, which is determined by the isotope separation coefficient (for example, for a mixture ¹² CH ₄ - ¹³ CH ₄ α = 1,009) and the height of the equivalent theoretical stage (VETS ≈ 4 cm) of separation in the thermal diffusion ring he is not. In this regard, it can be noted that because of the high temperatures in the thermal diffusion columns (~ 695 K), the hot wires inside them are made of platinum-iridium alloy (80% Pt and 20% Ir). In the prototype, to obtain several liters of methane with a concentration of ¹³ C 99 - 99.8% at. from preliminarily enriched by distillation of CO to ~ 70 at. ¹³ C methane required a cascade of eight series-connected columns with an inner diameter of 12.1 mm, an effective height of 2.85 m with a hot wire with a diameter of 0.4 mm. The low α value for thermal diffusion and the high VETS = h lead to a large volume (V ~ h / (α - 1) ² ) and a length (H ~ h / (α - 1)) of mass transfer columns, to a large amount of reflux flows circulating inside them (L ~ 1 / (α -1)), as well as to a high level of energy consumption (E ~ 1 / (α -1)) for the implementation of the separation process.

 The objective of the invention is to increase the concentration limit of target carbon isotopes, simplify the hardware design of the process of their separation, reduce capital and operating costs for its implementation, as well as increase the coefficient of separation of carbon isotopes at the stage of their final concentration.

 The problem is solved in that in a method for separating carbon isotopes, including preconcentrating the target isotope with cryogenic distillation of carbon monoxide (II), hydrogenating the concentrate with hydrogen with a deuterium content reduced to a natural level to methane and final concentration of the target isotope in countercurrent columns in the form of methane, in the quality of hydrogen is used in the waste gas produced by deuterium by cryogenic distillation of hydrogen of a natural isotopic composition, and the final concentration Cryogenic adsorption of methane on zeolite is carried out. In this case, zeolites NaX (13X), NaA (4A) or CaA (5A) are mainly used as zeolite.

 The invention is illustrated by the following examples.

Example 1. Natural isotopic mixture of carbon with a content of ¹³ C of 1.104% at. 81.6 K (α ≈ 1.007) is subjected to separation in a cascade of distillation packed columns in the form of preliminary purified cryogenic distillation in a packed column from chemical impurities of carbon monoxide (II) to a content of ¹³ C in the intermediate product 92% at. The intermediate thus obtained is hydrogenated with hydrogen on a commercially available methanation catalyst (TO-2) to methane. In this case, waste gas from the production of deuterium by distillation of liquid hydrogen is used as hydrogen. The specified gas contains 0.0015% at. deuterium, which is 10 times less than the deuterium content in natural hydrogen and more than 3 times less than the similar content in electrolytic hydrogen used in the prototype. The total content of chemical impurities in this gas does not exceed 108% at., Which is 5 • 10 ⁹ times lower than in electrolytic hydrogen used in the prototype. After deep drying, the hydrogenation product (methane) is fed into a continuously working countercurrent adsorption column filled with NaX zeolite with flow reversal at its ends: below, by desorption of methane from zeolite when heated to ~ 470 K, and above, by adsorption of methane by zeolite when cooled to ~ 200 K. in the adsorption tower supported pressure ~ 10 ⁵ Pa and a temperature of about 200 K (the coefficient of separation of a mixture CH ₄ ¹³ - ¹² CH ₄ under these conditions is equal to α = 1,02, corresponding enrichment coefficient ε = α - 1 higher than the enrichment factor at about otipe in ~ 2.22 times). The column is filled with granular zeolite NaX (13X) with a granule size of ≈ 1 mm. The heat of adsorption (desorption) of methane on zeolite NaX (13X) under these conditions is ~ 4.3 kJ / mol. When the specific value of the reflux stream of methane inside the column was ~ 7.64 mol / (m ² • s), the length of the concentrating part of the column was 2.7 m, a concentration of ¹³ C of 99.95% at. Was reached at the rich end, which exceeds the same concentration, achieved in the prototype (99-99.8%). The height of the equivalent theoretical separation stage under the indicated conditions of operation of the adsorption column was h ≈ 1.33 cm, which is ~ 3 times lower than the similar value in the prototype.

Example 2. Using methane obtained as a result of catalytic hydrogenation (with hydrogen produced as waste gas in the production of deuterium by cryogenic distillation of liquid hydrogen of natural isotopic composition) previously enriched with cryogenic distillation of ¹³ C carbon monoxide (II), by methods of single and double isotopic carbon balancing in the gas and adsorbed on zeolite NaX (13X) phases, by mass spectrometric measurement of ¹³ C concentration in the equilibrated phases determined coefficient p Thousands separator isotopic mixture of ¹² CH ₄ - ¹³ CH ₄ at the stage of final concentration sorption on zeolite NaX (13X). The determination results are shown in table 1.

From the data of table 1 it is seen that the enrichment coefficient of the isotope mixture ¹³ CH ₄ - ¹² CH ₄ at the stage of final concentration of ¹³ C in the proposed method at (145 - 273) K is ~ 1.2-5.9 times greater than in the prototype.

Example 3. Under conditions similar to example 2, determine the separation coefficient of the mixture ¹³ CH ₄ - ¹² CH ₄ at the stage of its final concentration by sorption on zeolite NaA (4A). The determination results are shown in table 2.

From the data of table 2 it is seen that the enrichment coefficient of the isotope mixture ¹³ CH ₄ - ¹² CH ₄ at the stage of final concentration of ¹³ C in the proposed method at (143-188) K is ~ 2.9-5.8 times greater than in the prototype.

Example 4. Under conditions similar to example 2, determine the separation coefficient of the mixture ¹³ CH ₄ - ¹² CH ₄ at the stage of its final concentration by sorption on zeolite CaA (5A). The results of the determination are shown in table 3.

From the data of table 3 it is seen that the conversion coefficient of the isotopic mixture ¹³ CH ₄ - ¹² CH ₄ at the stage of final concentration of ¹³ C in the proposed method at (142-210) K is ~ 1.9-5.7 times greater than in the prototype.

 The above examples show that, in comparison with the prototype, the proposed method achieves higher concentrations of the target isotope in the final product, is characterized at the final concentration stage by higher (up to 6 times) enrichment coefficient values, as well as lower (~ 3 times) height values equivalent theoretical stage of separation, which indicates its greater efficiency. In addition, the proposed method allows for the final concentration of the target isotopes in simpler equipment (single column) of significantly smaller sizes made of glass or cheap metal alloys. Its use excludes the use of precious metals (platinum, iridium) for instrumentation of the process of final concentration of carbon isotopes. The above advantages together provide a reduction in capital and operational costs for the implementation of the process of separation of carbon isotopes and, as a result, the cost of the target isotopes. The proposed method, as well as the prototype, due to the high radiation resistance of the sorbents and gases used in it, provides the possibility of producing not only highly concentrated stable carbon isotopes, but also its radioactive carbon-14 isotope.

Claims (4)

 1. The method of separation of carbon isotopes, including pre-concentration of the target isotope with cryogenic distillation of carbon monoxide (II), hydrogenation of the concentrate with hydrogen with a low deuterium content to methane relative to the natural level and final concentration of the target isotope in countercurrent columns in the form of methane, characterized in that as hydrogen use the waste gas produced by deuterium by cryogenic distillation of liquid hydrogen of natural isotopic composition, and the final concentration of exist cryogenic adsorption of methane on zeolite.  2. The method according to claim 1, characterized in that as the zeolite use zeolite NaX (13X).  3. The method according to claim 1, characterized in that as the zeolite use zeolite NaA (4A).  4. The method according to claim 1, characterized in that the zeolite is CaA zeolite (5A).

Images