SEPARATING HYDROGEN FROM GAS MIXTURES

Field of the Invention

The present invention relates to the field of hydrogen energetics, separation hydrogen from gas mixtures, and production of high-purity hydrogen.

Background of the Invention Ongoing sharp increase of hydrogen consumption is connected particularly to a specific role that is assigned to a direct conversion, by-passing the thermal cycle, of chemical energy of hydrogen to electricity using fuel cells (cars, submarines, notebooks, smart homes, etc.), which operation requires hydrogen having purity at least 99.999 %.

The most part of hydrogen is produced now and will be produced in the nearest future using the reforming of organic raw material, obtaining consequently a gas mixture comprising hydrogen. The critical stage of that process is a step of separating pure hydrogen from the obtained gas mixture.

It is admitted that the most effective method for separating pure hydrogen consists in diffusion purification thereof using metal membrane filters of various types, which vast majority is produced up to date on the basis of palladium and its alloys.

Known is, for example, a technical solution (see [1] RU 2416460 C2, Int. CI. B01D 63/00, 63/08, 72/02, publ. 20.04.2011) that discloses a hydrogen- permeable membrane, filter element, membrane apparatus. Therewith, the method for producing a hydrogen-permeable membrane consists in a step of forming, at the surface of a foil from the palladium alloy, a special relief having alternate protrusions and valleys surrounding each protrusion, wherein the palladium alloy includes one or more elements from I, III, IV, and VIII groups of the Period- ic table, and, in the step of forming said relief, the ratio of maximal length L of an arc at the surface of the protrusions in the cross-section thereof to the length D of the projection of that arc onto the basis area being made within a range of 1.05 to l+δ, where δ is ductility of the membrane alloy material. The known technical solution is destined for separating hydrogen from gas mixtures.

Despite a high degree of sophistication of the known technical solution and high purity of hydrogen separated by means thereof, that technical solution maintains disadvantages inherent to methods for separating hydrogen using membranes manufactured on the basis of palladium and/or its alloys:

· high cost due to the use of alloy from the precious metal palladium as the main membrane material,

• for a number of applications, insufficient capacity of hydrogen separation, which is explained by unsatisfactory thermodynamic characteristics of palladium alloys in respect of dissolving/transmitting hydrogen,

· origination of micro leakages in the membrane material during the thermo- and hydrogen-cycling.

On the other hand, it is known that transition metals of the 5th group of the Periodic table (vanadium, niobium and tantalum) possess higher permeability to hydrogen than palladium and palladium-argentum alloys due to a unique combination of large heat of dissolution and high rate of interstitial diffusion of hydrogen dissolved in metal, which ratio is much greater than in case of palladium. Metals of the 5th group, especially vanadium and niobium are also much lower in cost and much more accessible than palladium, they possess good mechanical properties and could be easily processed, in particularly, they possess good ductility allowing to obtain thin foils by the rolling method. However, the use of favorable characteristics of those metals is hindered to a certain extent because of high chemical activity of the surface thereof usually covered with dense oxide films formed rapidly in interaction of the metals with air, water vapor, etc. The oxide films reduce cardinally the rates of hydrogen dissolution and separation through a metal surface, thus making the membranes from those metals low-permeable for hydrogen.

The indicated problem is overcome by applying thin layers (about micron) of palladium to both surfaces of the membrane made from metal of the 5th group. Such composite membrane consisting from relatively thick (mm fractions) vanadium, niobium or tantalum or alloys thereof and thin palladium coatings (thickness of micron fractions) at both membrane surfaces allows to unite successfully favorable properties of both metals: high hydrogen permeability of the main membrane metal and high rates of hydrogen dissolution/separation through the precious metal palladium surface unexposed to oxidation, chemically resistant and stable.

Known is the technical solution "Palladium coated high-flux tubular membranes" (see [2] CA 2249126, Int. CI. B01D 53/22, publ. 02.04.2000), in which a membrane having an outer surface and an inner surface defining a cy- lindrical form is produced from niobium, tantalum, vanadium or other metals possessing characteristics needed for hydrogen penetration. In this case, the membrane having the cylindrical form is produced from non-palladium hydrogen-permeable materials, for example, from metals of the 5th group of the Periodic table, and coated with a thin palladium layer at both inner and outer surfac- es. The known technical solution is destined for separating hydrogen from gas mixtures.

It should be noted, however, that such producing method results in a small operational life of the membrane and continuous decrease of capacity of hydrogen separation by the membrane during its maintenance in the atmosphere of gas mixtures containing hydrogen. The indicated effects occur, in particular, as a result of that great hydrogen concentrations take place in the membrane material during penetration of great hydrogen flows through the membrane. This causes a non-uniform enlargement of different parts of the membrane material crystal lattice (hydrogen dilatation), which leads to an integrity breaking both the material of the membrane itself and the protective-catalytic coating on the surface thereof. Upon that, the cracks appear, and the coating material lift-off from the main membrane material, which results, in the end, in appearing, at the membrane surface, the main membrane material, i.e., the metals of the 5 ^th group and alloys thereof, and in poisoning the surface catalyst properties.

The apparatus described in the [2] is chosen as the closest analog.

A result achieved in the proposed technical solution consists in ensuring a uniform hydrogen concentration distribution through the membrane thickness.

Summary on the Invention

The indicated result is achieved in the proposed method for producing a membrane for separating hydrogen from gas mixtures, which method comprising a step of applying a protective-catalytical coating from palladium or palladi- um alloys at an input surface and output surface of the membrane made on the basis of metals of the 5th group of the Periodic table, which metals being alloyed with each other or with another metals, wherein the membrane material being produced from an alloy containing impurities of doping ingredients, which concentration being changed in the direction from the input membrane surface to the output membrane surface by means of increasing the hydrogen solubility in the membrane material in the direction from the input membrane surface to the output membrane surface in accordance with a formula:

Pout J

where S(x) represents the hydrogen-in-metal/alloy solubility constant,

x represents a coordinate in the direction normal to the membrane surface,

Si„ represents a value of solubility constant in the membrane material near the input surface,

P _in represents an input hydrogen pressure, P _ou( represents an output hydrogen pressure,

L represents a membrane thickness.

Detailed Description of the Invention The indicated technical result is achieved by the above characteristic features as follows.

When hydrogen permeates through the hydrogen-permeable membranes, the following steps take place consequently: hydrogen is absorbed by the membrane input surface, the absorbed hydrogen is dissolved and diffused in the membrane material, and then hydrogen is desorbed from the output membrane surface. The concentration of hydrogen dissolved in the membrane material depends on a membrane temperature and hydrogen pressure above the membrane, and is subjected to the Sieverts' law that interrelates the hydrogen pressure above the membrane and the hydrogen concentration in the given metal: (1) where C represents the concentration of hydrogen dissolved in the material,

P represents the hydrogen pressure above the membrane,

K represents the Sieverts' constant depending on the temperature and gas-metal system.

Thus, in accordance with the Sieverts' law, the hydrogen solubility C in the metal, given equal temperature of gas and metal, is proportional to the square root from the partial hydrogen pressure P above the membrane. In order for ensuring an effective separation/exhaustion of hydrogen from the gas mixture (for ensuring a high permeating hydrogen stream), the hydrogen pressure at the input side of the membrane should be substantially higher than the hydrogen pressure at the output side of the membrane. The hydrogen pressure is usually equal to tens of atmospheres at the input side and units of atmospheres and even lower at the output side. Correspondingly, the concentration of hydrogen dissolved in the membrane in accordance with the Sieverts' law is different near the input membrane surface and output membrane surface. Specifically, the distribution of the dissolved hydrogen concentration through the membrane thickness could be found from the Fick's law (see [3] Fromm E., Gebhardt E. Gases and carbon in metals. M.: Metallurgy, 1980. pp. 126-130, 426-430): j = -D^- , (2) ax

where j represents the hydrogen flow penetrating through the membrane, dC/dx represents the gradient of the hydrogen concentration through the membrane thickness,

D represents the coefficient of hydrogen diffusion in a given material.

A typical example of the hydrogen concentration distribution through the thickness of, e.g., vanadium membrane is shown in Fig. 1 for the following conditions:

the pressure at the input membrane surface is 20 at,

the pressure at the output membrane surface is 0.5 at,

the temperature of the membrane is 400 °C,

the thickness of the membrane is 220 microns.

As is clear from Fig. 1, the dissolved hydrogen concentration in the membrane near the input surface thereof exceeds substantially the same at the output surface.

On the other hand, as has been said above, the dissolution of hydrogen in the metal crystal lattice is accompanied with an enlargement of that lattice (hydrogen dilatation), the amount of that dilatation being defined by the dissolved hydrogen concentration. In addition, the enlargement owing to the phenomenon of the dilatation is extremely great in its magnitude and exceeds substantially the thermal dilatation. As a result, various portions (layers) of the membrane are dilated in different degrees, since the concentration of hydrogen dissolved in the membrane differs in various portions (layers) of the membrane (Fig. 1), decreasing considerably in the direction from the input membrane surface to the output membrane surface. This results in emerging the significant internal stresses lead- ing to both appearing the mechanical defects of the protective-catalytic coating (breaking the integrity of that coat and exposing a part of a base), and breaking the membrane form up to disruption thereof. Defects in the coating result in that the main membrane material, i.e., an alloy of metals from the 5th group of the Mendeleev's Periodic table - niobium, tantalum, or vanadium - appears at the membrane surface instead of the protective-catalytic coating, which metals having a high chemical activity of the surface thereof undergoes a reaction with the gas mixture components, thus forming the oxide compounds practically non-permeable for hydrogen. Herewith, those processes are especially in- tensive during the repetitive thermo- and hydrogen-cycling, i.e., repetitive heating/cooling of the membrane in the course of interaction with high hydrogen pressure.

In order for eliminating that negative effect, in accordance with the proposed method for producing a membrane, the membrane material is manufac- tured from an alloy containing impurities of doping ingredients, which concentration is changed in the direction from the input membrane surface to the output membrane surface, thus increasing the hydrogen solubility in the membrane material in the direction from the input membrane surface to the output membrane surface.

A possibility for reducing the hydrogen solubility upon introducing the doping ingredients is confirmed by experiment, which is demonstrated in Fig. 2 representing the relationship of the hydrogen solubility in the vanadium- palladium alloy depending on the concentration of the impurity (palladium) in the main membrane material (vanadium). As is clear from Fig. 2, even an insig- nificant doping of the main membrane material, vanadium, with palladium results in substantial reduction of the hydrogen solubility in the vanadium- palladium alloy.

Such introduction of the doping ingredients into the main membrane material results in compensating the effect of reducing the dissolved hydrogen con- centration (see Fig. 1) and in equalizing the hydrogen concentration through the membrane thickness. In this case, the difference in dilatation enlargements of various sections of the membrane material decreases, and internal stresses are lowered significantly.

In particular, a change of the hydrogen solubility in the membrane material can be ensured in accordance with such a law that, under given input and output hydrogen pressures, the hydrogen concentration in the membrane material will be constant. In this situation, the above indicated stresses do not appear at all, and the physical and mechanical properties of the membrane material and protective-catalytic coating are not deteriorated during the repetitive thermo- and hydrogen-cycling. The authors discovered that the above mentioned law of variation of the hydrogen solubility in the membrane material ensuring the constancy of the hydrogen concentration through the membrane takes the following form:

where S(x) represents the hydrogen-in-metal/alloy solubility constant,

x represents a coordinate in the direction normal to the membrane surface,

Si„ represents a value of solubility constant in the membrane material near the input surface,

P _in represents an input hydrogen pressure,

P _mt represents an output hydrogen pressure,

L represents a membrane thickness.

If the relationship S from the concentration of the doping impurity, then the equation (3) allows to find the required distribution of the impurity through the membrane body.

For example, the authors have discovered that in the case of doping vanadium with palladium

S = S _v e p{-\6A-S), (4) where «S> represents the hydrogen solubility in pure vanadium,

δ represents the atomic fraction of palladium in the alloy.

Correspondingly, in order for ensuring the constancy of the hydrogen concentration in the membrane made from the vanadium-palladium alloy under given input and output hydrogen pressures, the palladium concentration in the membrane material should be distributed in accordance with a linear function:

Fig. 3 shows, as an example, the distributions S(x) and δχ) for the membrane made from the vanadium-palladium alloy with the thickness L = 0.1 mm at the ratio of input and output pressures P P _out ⁼ 9 (P _in = 9 at, P _out = 1 at), which ratio ensures the constancy of the hydrogen concentration in the membrane material.

Practical realization of the method for producing the membrane having the required varying composition (having a varying concentration of impurities through the thickness) is performed by one of known technological methods, for example, gas-phase deposition, magnetron deposition, high-temperature fused electrolysis, thermal-diffusion method, or ion-beam implantation.

Figs. 4 to 7 show examples of the practical realization of the proposed method. Fig. 4 represents an electronic image of the cross section of the mem- brane made from the vanadium-palladium alloy with the palladium protective- catalytic coating on the input and output surfaces. Points (white squares) are indicated in Fig. 4, where the palladium content in the membrane material is determined using the local X-ray spectrum analysis. The corresponding data are brought in Table 1 and Fig. 5.

Another example of the practical realization is shown in Fig. 6 representing an electronic image of the cross section of the membrane made from the vanadium-nickel alloy. Data on the elemental content of the membrane material from the vanadium-nickel alloy are brought in Table 2 and Fig. 7. As is clear from Figs. 5 and 7, concentrations of doping ingredients (palladium in Fig. 5 and nickel in Fig. 7) decrease in the direction from the membrane input side to the membrane output side, thus ensuring the increase of the hydrogen solubility in the membrane material in the direction from the input membrane surface to the output membrane surface.

Table 1. Distribution of palladium in the vanadium-palladium membrane

Table 2. Distribution of nickel in the vanadium-nickel membrane

Spectrum Nos. Elemental content

V Ni

1 84.31 15.69

2 86.35 13.65

3 88.34 10.8

4 91.64 8.36

5 94.5 5.5

6 96.03 3.97

7 97.2 2.80

8 98.29 1.71

9 98.8 1.2

10 99.6 0.40

11 99.69 0.31