RU2605561C1 - Method of extracting hydrogen isotopes from gas mixtures - Google Patents

Abstract

FIELD: physics; chemistry.

SUBSTANCE: invention relates to physical chemistry, vacuum engineering, nuclear power engineering and can be used for separation of hydrogen isotopes from gas mixtures, and also for evacuation of vacuum systems, in which hydrogen isotopes serve as working gas. Method of extracting hydrogen isotopes from gas mixes involves heating of a membrane to temperature of 350±35 °C and supplying oxygen to output surface of membrane for oxidation of hydrogen isotopes penetrating through membrane until change of partial pressure at output of membrane stops, wherein membrane is a composite membrane based on metals of 5-th group of periodic table of niobium, vanadium, tantalum or their alloys with each other and other metals, both surfaces of which are coated with a thin layer of palladium or its alloys.

EFFECT: invention provides high rate of pumping hydrogen isotopes from gas mixture.

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Description

The invention relates to physical chemistry, vacuum technology, thermonuclear energy and can be used to separate hydrogen isotopes (protium, deuterium, tritium) from gas mixtures, as well as for pumping vacuum systems in which hydrogen isotopes serve as the working gas.

One of the most common methods for the separation of hydrogen isotopes (in particular, protium) from gas mixtures in industry at present is the use of a membrane separation method carried out using membranes that selectively pass hydrogen isotopes, do not pass any other gases and thereby provide a high degree of selectivity discharge. Including metal membranes of various shapes and sizes, made on the basis of palladium and its alloys, are widely used.

A technical solution is known (see [1] patent for the invention of the Russian Federation No. 2416460, M. C. B01D 63/00, 63/08, 72/02 publ. 04/20/2011), in which a hydrogen-permeable membrane, a filter element and a membrane apparatus. Moreover, the hydrogen-permeable flat membrane is made on the basis of a palladium alloy with a relief external surface with alternating protrusions and depressions surrounding each protrusion, characterized in that the palladium alloy contains one or more elements from groups Ib, III, IV, and VIII of the Periodic Table of Elements, and the ratio is maximum the length L of the arc on the surface of the protrusions in their cross section to the length D of its projection onto the base area is in the range from 1.05 to 1 + δ, where δ is the ductility of the material of the membrane alloy. Known technical solution is intended for the allocation of molecular hydrogen from gas mixtures.

Despite the high degree of perfection of the known technical solution, it has several disadvantages inherent to membranes made on the basis of palladium and its alloys, namely:

- the high cost of the device associated with the use as the main material of the membranes of an alloy of precious metal palladium,

- the specific productivity of hydrogen isotope extraction insufficient for a number of applications, which is explained by the unsatisfactory thermodynamic characteristics of palladium alloys with respect to the dissolution / transmission of hydrogen isotopes,

- the impossibility of pumping hydrogen isotopes from the low-pressure region of the gas mixture to the region of higher pressure.

Known technical solution that eliminates a number of these disadvantages, "High-performance cylindrical membranes coated with palladium" (Palladium coated high-flux tubular membranes) (see [2] Canadian patent CA No. 2249126, Ml. B01D 53/22, publ. 02.04. 2000), which is a composite membrane made of niobium, tantalum, vanadium or other non-palladium metals and coated with a thin layer of palladium, both on the inner and the outer surfaces. The known technical solution, adopted as a prototype, is intended for the separation of hydrogen isotope molecules from gas mixtures, which is due to the presence on the membrane surface of a protective-catalytic palladium coating with a high absorption coefficient of hydrogen isotope molecules.

A disadvantage of the known solution is the inability to separate / pump out hydrogen isotopes from the low-pressure region of the gas mixture to the region of higher pressure and, as a result, the rate of pumping / separation of hydrogen isotopes from gas mixtures is insufficient for a number of applications.

The technical result of the proposed method is to increase the rate of pumping of hydrogen isotopes from a gas mixture.

The achievement of the specified technical result is provided in the method of separation of hydrogen isotopes from gas mixtures using a composite membrane based on metals of the 5th group of the Periodic system of elements of niobium, vanadium, tantalum or their alloys with each other and other metals, both sides of which are coated with a thin layer of palladium or of its alloys, characterized in that oxygen is supplied to the output surface of the composite membrane to oxidize hydrogen isotopes penetrating through the membrane until the change in partial stops of the outlet pressure of the membrane, the composite membrane and the temperature was maintained at 350 ± 35 ° C. Achieving the specified technical result by the above differences is as follows.

In the general case, gas diffusion is described by Fick differential equations of the first and second order (see [3] E. Fromm, E. Gebhardt “Gases and carbon in metals” M., Metallurgy, 1980, pp. 126-129). In the case of gas diffusion (penetration) of interest to us through a solid-state metal partition / membrane, the gas flow is determined by the ratio:

where J is the gas flow penetrating through the membrane,

D _(T) is the diffusion coefficient of gas in the membrane. The diffusion coefficient depends on temperature and increases with its growth,

c ₁ - gas concentration in the membrane at its inlet surface,

C ₂ - the concentration of gas in the membrane on its output surface,

l is the thickness of the membrane.

If equilibrium with the gas phase is established on the membrane surface and there is an interaction with a diatomic gas, which ideally dissolves upon dissociation in the metal, which takes place in the case of hydrogen isotopes penetration through hydrogen-permeable membranes, the Sievers law is valid:

where S is the Siverts constant,

p is the gas pressure in the gas phase.

Then for p ₁ > p ₂ from (1) and (2):

where p ₁ is the gas pressure from the input surface of the membrane,

p ₂ - gas pressure from the outlet surface of the membrane.

As follows from equation (3), the flow of hydrogen isotopes penetrating through the membrane is directly proportional to the difference in their pressures at the inlet of the membrane and at its exit, decreasing when this difference decreases and, in particular, is equal to zero (pumping out of hydrogen isotopes ceases) when these pressures are equal. In this case, penetration of molecular hydrogen isotopes through the membrane from the low-pressure region to the higher-pressure region is impossible and, accordingly, it is impossible to realize both a high rate of hydrogen isotope pumping from the gas mixture and the accumulation of hydrogen isotopes at the membrane outlet.

At the same time, the task of deep separation / pumping of hydrogen isotopes from a gas mixture is very urgent for various applied projects, in particular, for pumping systems of existing and designed reactors of controlled thermonuclear fusion.

To achieve the specified technical result in the proposed method, oxygen is supplied to the output surface of the composite membrane. Oxygen on the outlet surface of the membrane coated with palladium or its alloys, which are catalysts for the hydrogen oxidation reaction, interacts with hydrogen isotopes penetrating through the membrane with the formation of hydrogen isotope oxides (water, heavy water), which accumulate in the outlet volume. As a result, hydrogen isotopes do not enter the output volume in molecular form, their pressure in it is practically zero, and their evacuation / separation from the gas mixture occurs until hydrogen isotopes are completely separated from the input volume, since there is no reverse flow of hydrogen isotopes from the membrane exit to its entrance .

According to (3), in the absence of hydrogen at the membrane outlet

In this case, the flow of hydrogen isotopes J penetrating from the input volume through the membrane, as well as the pumping rate Q (Q ~ J) proportional to the penetrating flow, reach their limiting value - (4).

Feasibility and practical implementation of the proposed technical solution are shown in FIG. 1 and FIG. 2.

In FIG. Figure 1 shows the effect of oxidation of hydrogen isotopes on the output surface of the membrane on the relative value of the hydrogen flux penetrating through the membrane J / J _max .

In FIG. 2 presents a device for implementing the inventive method.

In FIG. 2:

1 - a cylindrical membrane hermetically separating the inlet and outlet vacuum volumes,

2 - input volume,

3 - output volume,

4 - external current source for heating the membrane,

5 - mass spectrometer.

The proposed method for the separation of hydrogen isotopes from gas mixtures is implemented as follows - see Fig. 1 and FIG. 2.

Membrane 1 in the device of FIG. 2, using an external current source 4 is heated to a temperature of 350 ± 35 ° C by direct transmission of current. Then, a mixture of gases (time t ₁ in FIG. 1) is fed into the inlet volume, from which hydrogen isotopes must be separated. As seen in FIG. 1, hydrogen isotopes located in the inlet volume 2 penetrate through the membrane 1, which is manifested in the appearance of a penetrating stream - see Fig. 1, the time t ₁ , which is recorded using a mass spectrometer 5. However, as the pressure of hydrogen isotopes in the output volume 2 increases, they begin to be pumped from the output volume back to the input volume, respectively, the reverse flow of hydrogen isotopes from the outlet appears and begins to increase at the entrance, which reduces the penetrating flow. As a result, when the pressure of the hydrogen isotopes at the inlet and outlet of the membrane is reached, when the forward and reverse flows are compared, the penetrating flow and the rate of evacuation of the hydrogen isotopes become equal to zero - at time t _{2 the} evolution of hydrogen isotopes ceases.

Note that this is the effect that is observed when using the prototype.

At time t ₃ (Fig. 2) oxygen is injected into the outlet volume, which oxidizes hydrogen isotopes penetrating through the membranes on the outlet surface of the membrane. The resulting water molecules (heavy water) in gaseous form accumulate in the output volume. At the same time, hydrogen isotopes in molecular form penetrating through the membrane no longer enter the outlet volume and do not increase the pressure in it. Accordingly, the flow of hydrogen isotopes and the rate of their pumping from the input volume begin to increase and reach their maximum (time t ₄ ), determined by the rate of hydrogen isotope evolution by the particular membrane system used.

The penetration process is controlled using a mass spectrometer 5, which is located in the output volume and registers the partial pressures of the gases in the output volume. The oxygen flow is carried out in such a way as to ensure complete oxidation of the hydrogen isotope molecules penetrating the membrane, namely, the oxygen flow is increased until the change in the partial pressure of the hydrogen isotopes caused by their penetration through the membrane ceases to be observed.

The choice of temperature at which hydrogen is produced is determined from the following considerations. The flow of hydrogen penetrating through the membrane is greater, the higher the temperature of the membrane. On the other hand, the temperature of the membrane is limited by its performance. The fact is that at high temperature there is a phenomenon of interdiffusion of the material of the protective-catalytic coating deposited on the inlet and outlet surfaces of the membrane and the main membrane material - metal of the 5th group or its alloys. As a result, metals of the 5th group appear on the membrane surface, which have a significantly lower absorption coefficient of molecular hydrogen than palladium, which leads to a radical decrease in the flow of hydrogen isotopes penetrating through the membrane.

The authors performed special experiments to determine the range of membrane operating temperatures, in which stable, long-term evolution / extraction of hydrogen isotopes from gas mixtures is observed. The results of these experiments are shown in FIG. 3 and FIG. four.

Example 1

In FIG. Figure 3 shows the dependences of the relative magnitude of the pumping rate of hydrogen isotopes (protium, deuterium) on the operating time for various membrane temperatures. As follows from the presented results, an increase in the temperature of the membrane leads, over time, to a decrease in the rate of pumping of hydrogen isotopes by the membrane. Moreover, the higher the temperature of the membrane, the faster the pumping speed decreases. The reason is a violation of the protective-catalytic coating and the appearance on the surface as a result of the interdiffusion of metals of the 5th group. The maximum temperature at which no decrease in the pumping rate was observed was 350 ° C.

Example 2

In FIG. Figure 4 shows the detailed temperature dependence of the relative value of the rate of evacuation of hydrogen isotopes 300 hours after the start of the experiment in a temperature range of about 350 ° C. As can be seen from the data obtained, an increase in the temperature of the membrane leads to a decrease in the rate of pumping out due to the phenomenon of interdiffusion (see also Fig. 3), and a decrease in the temperature of the membrane leads to a decrease in the rate of pumping out due to a decrease in the diffusion coefficient - cm (1). The operating temperature range was chosen, as is usually accepted, so that at its borders the rate of evolution / extraction of hydrogen isotopes decreased by no more than 5% - FIG. 4, which is 350 ± 35 ° C.

Thus, due to the conversion of the flow of hydrogen isotope molecules into water / heavy water at the exit surface of the composite membrane, it is possible to drastically increase the rate of evacuation of hydrogen isotopes from gas mixtures tenfold. Moreover, due to the absence of a reverse (from the exit to the entrance) stream of hydrogen isotopes, it becomes possible to deeply purify the gas mixture from hydrogen isotopes. This significantly reduces the requirements for gas mixture pumping systems, since it will not contain radioactive isotopes of hydrogen. In addition, due to the absence of restrictions on the accumulation of hydrogen isotope oxides at the membrane outlet, it becomes possible to accumulate and compress water molecules containing hydrogen isotopes for the subsequent separation of hydrogen isotopes at a sufficiently high pressure for their further use.

Claims (1)

The method of separation of hydrogen isotopes from gas mixtures using a composite membrane based on metals of the 5th group of the Periodic system of elements of niobium, vanadium, tantalum or their alloys with each other and other metals, both surfaces of which are coated with a thin layer of palladium or its alloys, characterized in that oxygen is supplied to the output surface of the composite membrane to oxidize hydrogen isotopes penetrating through the membrane, until the change in the partial pressure at the membrane outlet stops, and the temperature of the composite membrane anes maintained at 350 ± 35 ° C.

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