EP1027135A1

EP1027135A1 - Method for separating oxygen from a gas mixture containing it and device for implementing said method

Info

Publication number: EP1027135A1
Application number: EP98952804A
Authority: EP
Inventors: Jean-Claude Boivin; Pascal Del Gallo; Jacques Fouletier; Michel Kleitz; Philippe Labrune; Gilles Lagrange; Ga[Tan Mairesse; Guy Nowogrocki; Marlu César STEIL
Original assignee: Air Liquide SA; LAir Liquide SA pour lEtude et lExploitation des Procedes Georges Claude
Current assignee: Air Liquide SA; LAir Liquide SA pour lEtude et lExploitation des Procedes Georges Claude
Priority date: 1997-10-29
Filing date: 1998-10-28
Publication date: 2000-08-16
Also published as: US6143162A; FR2770149B1; CN1278191A; ZA989843B; JP2001520929A; FR2770149A1; IL135815A0; WO1999021642A1

Abstract

The invention concerns a method for separating oxygen from a mixture of gas containing said oxygen, using an electrochemical cell with solid electrolyte, characterised in that said cell comprises a homogeneous structure consisting of one or several BIMEVOX derivatives, with reversible and self-adaptive dynamic electrodes generated 'in situ' and at least two current collectors. The invention also concerns the electrochemical cell for implementing the method and its applications.

Description

PROCESS FOR SEPARATING OXYGEN FROM A GAS MIXTURE CONTAINING SAME AND DEVICE FOR CARRYING OUT SAID METHOD.

The invention relates to a process for the separation of oxygen from a mixture of gases containing it and a device for carrying out this process.

In the field of separation of oxygen from air or from a mixture of gases containing it, by using an electrochemical cell with solid electrolyte, various electrolyte-electrode pairs can be used. The operating principle of an elementary cell is illustrated in FIG. 1. The oxygen molecules of said gas mixture are reduced at the cathode (C) to O ^{2 "} ions, which are then transported through the solid electrolyte ( E) impermeable to gases under the effect of an electric field; on the other side of the cell, these ions are oxidized at the anode (A) to generate the oxygen molecules. the ionic conductivity of the electrolyte and the ability of the electrode materials to reduce oxygen or oxidize O ^{2 "} ions at working temperature.

. The majority of solid electrolytes used for the separation of oxygen from the air are stabilized zirconia (for example with yttrium). These compounds make it possible to work between 700 and 800 ° C. (and are described in American patent US 4,879,016.

A new class of oxides derived from Bi ₄ V ₂ O _n , in which a variable part of vanadium is replaced by an element, for example an alkaline earth metal, a transition metal, a rare earth or an element of groups III to V of the periodic table of the elements, is described in the international patent publication WO 91/01274. These oxides are conductors of O ^{2 "} ions and their anionic conductivity at 500 ° C. is of the same order of magnitude as that at 800 ° C. of the stabilized zirconia.

The partial substitution of the constituent elements of Bi ₄ V ₂ O _π stabilizes the structural type of the gamma phase and maintains, in the network of O ^{2 "} ions, a rate of vacancies sufficient to allow anionic conductivity. of a essentially two-dimensional conduction, the oxygen atoms of the Bi ₂ layers. _x M _x O ₂ , strongly linked to bismuth atoms, unable to move.

The anionic conductivity of these phases is remarkable, since it reaches ÎO ^ Ω ^"1 cm ^'1 around 200 ° C. These derivatives are often designated by the generic name BIMENOX.

But if BIMEVOX-based electrolytes allow the transfer of O ^{2 "} ions through their thickness at low temperature (300 ° C), the metal electrodes which are commonly associated with them (Au, Pt) are low flux electrodes which do not allow correct catalytic dissociation of the oxygen molecules brought into contact with them, and the very structure of this type of electrode considerably reduces the developed surface of the electrode-electrolyte interface. these drawbacks can also be encountered when these electrodes are used with conventional electrolytes, other than those based on BIMENOX, for example with zirconia stabilized by a doping agent such as yttrium, or calcium, or with oxides of bismuth Bi ₂ O ₃ doped or not doped with elements such as magnesium, calcium, yttrium or erbium.

A first approach consisted in producing a volume electrode defined as being a material allowing mixed ionic and electronic conduction. This invention is described in international publication WO 95/32050. In the volume electrode, the constituents are at least two in number, that is to say a BIMEVOX and a metal, or a metal oxide, as electronic conductor. These constituents are co-sintered in order to obtain the electrode. The respective particle sizes of these constituents and the sintering time are determined so that the structure of the composite layers is sufficiently porous to allow the diffusion of oxygen in their respective volumes. In the context of this invention, BIMEVOX ensures chemical compatibility and physical compatibility with the electrolyte, the catalytic dissociation of oxygen, while the role of metal or metal oxide is to distribute the electrons brought to the surface of the electrode by the current collector throughout the three-dimensional building of this composite electrode. The devices described in the state of the art do not however allow sufficient satisfactory results to be obtained for them to lead to suitable industrial and commercial exploitation. The Applicants have therefore sought to develop a new process for implementing the BIMEVOX derivatives which does not have the drawbacks developed above.

The subject of the present invention is a process for the separation of oxygen from a mixture of gases containing it, using an electrochemical cell with a solid electrolyte, characterized in that said cell comprises a homogeneous structure consisting of one or more BIMEVOX derivatives, with dynamic electrodes created "in situ", reversible and self-adapting and at least two current collectors.

By homogeneous structure, it is meant in the preceding definition that, unlike the devices of the prior art, which consist of a solid electrolyte and two electrodes connected to current collectors, said electrodes being able to be physically distinguished from said electrolyte, the cell according to the method, object of the invention, is a nucleus of one or more BIMEVOX behaving both as an electrolyte and as electrodes.

In the above definition, a person skilled in the art will readily appreciate the difference between a current collector, the function of which is only to allow the circulation of electric current by bringing the electrons to the cathode and by collecting them at the anode, and an electrode, the function of which is to catalyze electrochemical dissociation.

By dynamic electrodes created "in situ", it is meant in the preceding definition, that the electronic conductivity is due to the vanadium ^v * - "vanadium ^ transformation on the cathode side.

By created "in-situ", it is meant in the preceding definition, that the homogeneous structure becomes an electrode-electrolyte-electrode structure only by the application of a non-zero potential difference on the opposite faces of said device; in the following description, we will speak of “electrode zones” and “electrolyte zone” of said structure.

By reversible, it is meant in the preceding definition, that the device can operate in one direction or another by simply reversing the polarity of the current generator.

By self-adaptive, it means that the device adapts by itself to the two types of functions mentioned above, namely: the dynamic function and the reversibility function.

Those skilled in the art will also appreciate that one of the advantages of the homogeneous structure implemented by the method, object of the present invention, is that the thickness of the “electrode zones” and of the “electrolyte zone” of said structure varies in particular according to the temperature and the intensity of the electric current which are applied to it, and that this dynamic character thus makes it possible to regulate the rate of extraction of oxygen. Thus, the more the intensity of the applied current increases at constant temperature, the more the thickness of the “electrolyte” zone decreases, for an increasing oxygen extraction rate.

The term BIMEVOX derivative denotes the compound of formula (I) (Bi _2. _X M _x O ₂ ) (V _L , M ' _y O ₂ ) (I) in which:

M represents one or more substitution metals for bismuth, chosen from those having an oxidation number less than or equal to 3,

- M 'represents one or more vanadium substitution elements chosen from those having an oxidation number less than, equal to or greater than 5, the limit values of x, y and therefore z being a function of the nature of the substituent elements M and

Me.

Among the compounds of formula (I), there may be mentioned those in which only the vanadium atom is partially substituted by one or more elements. These compositions correspond to formula (II): (Bi ₂ O ₂ ) (V ,. _y M ' _y O ₂ ) (II) in which M ′ is as defined above, being there non-zero. - M 'is advantageously selected from alkali metals, alkaline earth metals, transition metals, or elements from groups III to V of the periodic table, or from rare earths. Compositions, of great stability and high conductivity at low temperature, include, as a metal substituting for vanadium, a transition metal such as Zn, Cu, Ni, Co, Fe, Mn and Cd.

When M 'is an alkaline earth metal, it is in particular Ca, Sr, or Ba. M 'is in another variant a metal having an oxidation state equal to 3, such as in particular Sb, In or Al.

In yet another variant, M ′ has an oxidation state of 4. Among these metals, mention will be made of Ti, Sn or Ru.

M 'can also represent a substitution element with an oxidation state equal to 5 such as Nb, Ta or P.

M 'can also be a rare earth metal.

In another embodiment, M ′ is an alkali metal such as sodium or can represent Pb at the oxidation state 2. Mention may also be made, among the compounds of formula (I), in which only the bismuth atom is partially substituted by one or more metals. These derivatives correspond to formula (III): (Bi _2. _X M _x O ₂ ) (VO _z ) (III) in which:

- x is non-zero and

- M is as defined above and is chosen in particular paπni rare earths such as lanthanum.

Mention may also be made, among the compounds of formula (I), of those in which the oxygen atom is partially substituted by fluorine, or else those comprising mixed substitutions of bismuth and vanadium and correspond to formula (I) ci - above in which x and y are non-zero. As composition of this type, mention will be made of: (Bi _2. _X Pb _x O ₂ ) (V _μy Mo _y O _z )

The BIMEVOX derivative, or the mixture of BIMEVOX derivatives constituting said homogeneous structure, can be distributed uniformly or according to the porosity gradients adapted to the use which is made of it, in order in particular to promote the penetration and dissociation of the gaseous oxygen, as well as the recombination of oxygen ions into gaseous oxygen.

When the structure is composed of several BIMEVOX derivatives, the latter are chosen and distributed in said structure so as to retain its characteristics of homogeneity, dynamics, reversibility and self-adaptation as described above. In general, the particle size of the BIMEVOX derivative (s) is between 0.01 and 50 microns.

In a first variant of the device which is the subject of the present invention, the homogeneous structure of the electrochemical cell used comprises a single BIMEVOX. It goes without saying that the indication of the essential constituents of the structure used according to the invention is not exclusive of the presence of other constituents, since these would not significantly interfere with the properties required of this structure (ionic and electronic conductivities, porosity, etc.).

This homogeneous structure is advantageously applicable to the constitution of electrochemical cells for the separation or the extraction of oxygen out of a gas containing it and brought to the cathode surfaces, and to the recovery of this oxygen at the anodic sides of this structure, when 'They are themselves inserted in an electric current supply circuit allowing the creation of a potential difference between their opposite faces, by means of current collectors in contact with said structure. When the voltage, induced between the opposite “electrode zones” of said homogeneous structure according to the invention, is sufficient to allow the reduction of the oxygen admitted on the cathode side into O ^{2 ′} ions, and the oxidation of O ^{2 ″} ions , transferred through the electrolyte zone ", the extracted oxygen molecules can be recovered from the anode side of the structure. 1/2 O2 (gas) + Vo + 2e <= => O ^2" . The electrochemical chain according to the process which is the subject of the invention can therefore be represented in particular according to the diagram:

CC / BIMEVOX '/ ELECTROLYTE / BIMEVOX "/ CC in which: ELECTROLYTE represents the electrolyte zone", solid and impermeable to gases, BIMEVOX whose dopant Me can be in particular and in a nonlimiting manner at least one of the elements chosen among titanium, cobalt, nickel, copper, niobium, manganese or zinc.

- CC represents the current collectors which ensure both the supply of electrons in the "cathode zone" and their evacuation from "the anode zone". These collectors, for example gold, must naturally be compatible with BIMEVOX.

- BIMEVOX 'and BIMEVOX "represent the two characteristic constituents of the" electrode zones ".

The invention particularly relates to a process as defined above in which the current collectors connected to each of the “electrode zones” of the homogeneous structure of the electrochemical cell used, are independently of one another in a or several metals chosen from gold, silver, platinum, palladium or copper, in an alloy of metals such as stainless steel. The shape of the current collectors for implementing the method according to the invention can be determined so as to optimize the supply of electrons in the

"Cathode zone" and their evacuation from the "anode zone" of said structure. In a preferred variant of the process object of the present _invention. a non-zero part of each of the current collectors of the electrochemical cell used, is located inside the homogeneous structure, said part having a network shape each having more than 500 nodes / cm ^{2 in} particular a grid shape and, more particularly, a grid shape having more than 1000 meshes / cm ² .

In a second preferred variant of the process of the present invention, the homogeneous structure of the electrochemical cell used is a structure volume at thickness e, and in which the parts of the current collectors included in said structure are arranged parallel to each other.

In another variant of the process which is the subject of the present invention, the homogeneous structure of the electrochemical cell used is a hollow cylindrical structure with circular or oval section, comprising two coaxial cylindrical faces, in which the parts of each of the current collectors included in said structure are cylindrical grids coaxial with the faces of said structure.

In the electrochemical chain as defined above, the BIMEVOX, BIMEVOX 'and BIMEVOX "can be identical or different. The physical conditions, which the homogeneous structure of the electrochemical cell used in the process according to the invention must meet, will be in each case determined by those skilled in the art.

In general, it will always be advantageous to use electrolytes as thin as possible, if only to reduce the internal tensions.

According to the process which is the subject of the present invention, the separation of oxygen from a gas mixture is carried out at a temperature between 250 and 700 ° C., at a potential difference between the "cathode zone" and the "anode zone »Sufficient to allow reduction of the oxygen admitted on the cathode side into O ^2" ions and the oxidation of said O ^" ions transferred through the solid electrolyte into oxygen, on the anode side.

This process is particularly suitable for removing oxygen from argon obtained by cryogenic distillation of air, or for extracting oxygen from air or even for separating oxygen from mixtures. nitrogen / oxygen, water vapor / oxygen, carbon monoxide or dioxide / oxygen, or NO _x or SO _x / oxygen. In general, the device which is the subject of the present invention is therefore suitable for gas purification operations or gas mixtures as well as for operations for analyzing the oxygen present in a given gas atmosphere.

It is thus possible, either to produce ultra-pure oxygen or, for applications requiring atmospheres without oxygen such as the component industry electronics or the food industry, removing oxygen from said atmosphere, said gaseous atmosphere being above a liquid or a solid.

By way of nonlimiting example, the method is implemented for removing oxygen from the gaseous atmosphere above food products, in particular fresh food products to improve their conservation.

According to a last aspect of the present invention, it relates to an electrochemical cell as defined above.

The following examples illustrate the invention without however limiting it:

Examples of implementation: Three examples of implementation are listed below. They concern the following systems:

- Au grid (CC) -BICOVOX7BICOVOX / BICOVOX "-Au (CC) grid (example 1)

- Au grid (CC) -BICUVOX '/ BICUVOX / BICUVOX "-Au (CC) grid (example 2)

- Au grid (CC) -BIZNVOX7BIZNVOX / BIZNVOX "- Au (CC) grid (example 3. Realization of electrochemical cells.

The electrochemical cells are prepared in the following manner:

- Solid electrolytes in the form of a disc are prepared from powders derived from Bi ₄ V ₂ O _π of formula Bi ₂ Coo.No.gO ₅ . ₃₅ , Bi ₂ Cu _0, ιV _0.9 O ₅ . ₃₅ and Bi ₂ Zno.jVo. ₉ O ₅ . ₃₅ (BICOVOX. IO, BICUVOX. IO and BIZNVOX.IO). To do this, the powders are ground so that the average grain diameter is at most of the order of a few microns, the particle size varying from 0.01 to 10 microns. The discs are obtained by pressing by applying a force of the order of one ton to the ground powders. They have an area of 2cm ² and a thickness of about 1.5mm. - the discs are then sintered for 1 to 12 hours between 600 and 900 ° C. The sinterings are carried out under an air atmosphere so as to obtain materials impermeable to gases and mechanically resistant.

- On each surface of the discs, a gold grid with controlled mesh is deposited by pressing (1024 meshes per cm ² ). - We insert the gold grid in a layer of BIMEVOX '(ME: Co, Cu, Zn) and in a layer of BIMEVOX "on both sides of the disc. In the following examples the BIMEVOX are identical The compounds BIMEVOX 'and BIMEVOX "also serve as a binder. The porosity and the size of the particles are checked. The system thus produced is then sintered at a sintering temperature at most equal to that of the BIMEVOX used to prepare the first disc.

- The structure thus formed is deposited on the edge of a stainless steel conductive tube. In order to keep the disc stably on the edge of the stainless steel tube and to ensure electrical contact, a gold seal is applied. The other electrical contact is provided by an external metal rod, one end of which is in contact with the gold-controlled mesh grid.

The general experimental setup is shown in FIG. 2. This setup includes a first compartment 21 provided with an air intake 22. Inside this compartment is housed a stainless steel tube 23 provided with an outlet 24. It is in its upper part provided with a disc 25 formed of a homogeneous structure according to the invention. In order to maintain the disc in a stable manner on the edge of the stainless steel tube and to ensure electrical contact on the anode side, a gold seal (or cement known under the brand "CERASTIL C3") is applied. The electrical contact on the cathode side is provided by an external metal rod, one of the ends of which is in contact with the gold,

The homogeneous structure 25 is itself interposed in an electrical circuit 26 making it possible to apply, via appropriate collectors, a potential difference between the two opposite “electrode zones” of the disc 25. The operation can be reversible. In operation for the electrochemical separation of oxygen from the air, the

"Cathodic zone" of the disc is brought into contact with air. The pure oxygen is recovered in the stainless steel tube, through the "anode zone" of the cell. The stainless steel tube and the metal rod are connected to an electric generator. Cell operating conditions.

All these cells defined by examples 1, 2 and 3 operated over a wide temperature range (between 350 and 600 ° C) under variable potential. The results are reported in the tables below:

Characteristics of an elementary electrochemical cell:

Au, BIZNVOXJ0 / BIZNVOXJ0 / BIZNVOXJ0, Au

(t = 485 ° C; thickness 3.6 mm; diameter 16mm)

Comparison of the faradic yields measured (%) for different elementary electrochemical cells:

Au, BIMEVOXJ0 / BIMEVOXJ0 / BIMEVOXJ0, Au (t = 585 ° C; thickness 3.6 mm; diameter 16 mm)

In all cases, the faradaic yields are greater than 95% and this for temperature zones between 350 and 600 ° C. The cell lifetimes exceed several tens of days. In reality, no drop in electrochemical performance and no aging of the cells is observed during the experiments.

Claims

1) Process for the separation of oxygen from a mixture of gases containing it, using an electrochemical cell with a solid electrolyte, characterized in that said cell comprises a homogeneous structure consisting of one or more BIMEVOX derivatives, with electrodes dynamic created "in situ" reversible and self-adaptive and at least two current collectors.

2) Method as defined in claim 1 wherein the homogeneous structure of the electrochemical cell used comprises a single BIMEVOX.

3) Method as defined in one of claims 1 or 2, in which the current collectors connected to each of the “electrode zones”, of the homogeneous structure of the electrochemical cell used, are independently one of the 'other, in one or more metals chosen from gold, silver, platinum, palladium or copper or in a metal alloy such as stainless steel.

4) Method as defined in one of claims 1 to 3, in which a non-zero part of each of the current collectors of the electrochemical cell used is located inside the homogeneous structure, said part having a form in networks each presenting more than 500 knots / cm ² . 5) Method as defined in claim 4 in which the part of each of the current collectors located inside the homogeneous structure has a form of grids and, more particularly, a form of grid having more than 1000 meshes / cm ² .

6) Method as defined in one of claims 4 or 5, in which the homogeneous structure of the electrochemical cell used is a volume structure with thickness e, and in which the parts of the current collectors included in said structure, are arranged parallel to each other.

7) Method as defined in one of claims 4 or 5, wherein the homogeneous structure of the electrochemical cell used is a hollow cylindrical structure with circular or oval section, comprising two cylindrical faces coaxial, in which the parts of each of the current collectors included in said structure are cylindrical grids coaxial with the faces of said structure.

8) Use of the method as defined in one of claims 1 to 7 for the elimination of the oxygen from the argon obtained by cryogenic distillation of the air, the extraction of the oxygen from the air, the separation oxygen from nitrogen / oxygen mixtures, water vapor / oxygen, carbon monoxide or dioxide / oxygen, or NO _x or SO _x / oxygen.

9) Use of the method as defined in one of claims 1 to 8 either to produce ultra-pure oxygen, or to remove oxygen from a gaseous atmosphere above a given liquid, or d 'a solid.

10) Use of the method as defined in claim 9, to remove oxygen from the gaseous atmosphere above food products.

11) electrochemical cell used in the process as defined in one of claims 1 to 5. 12) electrochemical cell used in the process as defined in

One of claims 6 or 7.

13) Electrochemical cell as defined in one of claims 11 or 12, represented by one of the following electrochemical chains:

- Au grid (CC) -BICOVOX7BICOVOX / BICOVOX "-Au (CC) grid - Au grid (CC) -BICUVOX '/ BICUVOX / BICUVOX' '-Au (CC) grid

- Au grid (CC) -BIZNVOX7BIZNVOX / BIZNVOX "- Au (CC) grid.