WO2015067904A1

WO2015067904A1 - Support for gas separation

Info

Publication number: WO2015067904A1
Application number: PCT/FR2014/052844
Authority: WO
Inventors: Christos SARANTOPOULOS; Stéphane RAFFY; Nathalie Petigny; Nabil Nahas
Original assignee: Saint-Gobain Centre De Recherches Et D'etudes Europeen
Priority date: 2013-11-08
Filing date: 2014-11-06
Publication date: 2015-05-14
Also published as: FR3012977A1

Abstract

A ceramic support for gas separation, consisting of a zirconia stabilised with yttrium and/or with calcium of which the porosity is greater than 20%, said support being obtained by sintering a feedstock comprising molten grains of yttrium-stabilized zirconia or calcium-stabilised zirconia or zirconia stabilised with yttrium and with calcium.

Description

SUPPORT FOR GAS SEPARATION

The present invention relates to the field of gas separation, and relates in particular to supports for the separation of gases present for example in gas separating ceramic membranes. Such membranes are used for energy applications, particularly in certain methods of oxycombustion of coal. More particularly, the present invention is related to the ceramic support present in such membranes.

In a well known manner, a conductive ceramic membrane comprises a set of materials having the ability to efficiently transfer charges (ions or electrons) between two compartments. If these two compartments have different oxygen partial pressures, the crossing of the oxygen becomes possible. More specifically, according to a well-known mechanism, the oxygen of the air is dissociated and reduced to 0 ^{2 ~} ions on a first surface of the membrane (called cathode) where it is present in greater concentration, then the ions 0 ^{2 ~} diffuse through the crystal lattice of different solid materials constituting the separating layer to the opposite surface. It is then brought into contact with a fuel such as a hydrocarbon, according to an oxidation or combustion reaction. These species react in a functional layer called anode itself consisting of ionic and electronic conductor allowing the catalytic reaction. The oxygen concentration is reduced on this opposite surface of the membrane, thus promoting the separation of oxygen from one compartment to another. Unlike batteries SOFC fuel, the system must operate without an external electronic circuit and at least the different layers constituting the membrane must have a conductivity that is both ionic (for the transport of ions 0 ^{2 ~} through the membrane) and electronic (to ensure the associated transport electrons through the separator layer). This double conduction is made possible by using a two-phase composite consisting of an ion conductor and an electrical conductor or alternatively using a mixed conductor.

In such membranes, the separating layer is relatively dense and must therefore be as thin as possible so as not to penalize the amount of oxygen transported therethrough. The layers constituting the anode and the cathode must also be thin and porous. It is therefore necessary for all of these layers to be deposited on another so-called support layer, of greater thickness and giving the entire membrane the necessary mechanical strength.

This support must also have a certain porosity to also allow the diffusion of the gas (oxidizer) or fuel in its thickness. The choice of the material constituting the support turns out to be particularly important because it is necessary to maintain good mechanical strength.

One of the essential criteria for measuring the performance of a support is its diffusivity, which can be evaluated by measuring its ability to let oxygen through its entire thickness.

This support must also be able to operate at high temperatures, often between 900 ° C and 1100 ° C, which correspond to the combustion temperature of the fuel in its porosity. Unlike SOFC-type applications, the material used as a support must also be able to withstand pressures of up to 20 bar.

According to another of its essential characteristics, the material constituting the support must have dimensional stability and in particular resist creep. Creep means, within the meaning of the present invention, the capacity of the material to deform under the effect of the stresses undergone, at the operating temperature of the ceramic membrane separating gas.

In order to solve the preceding technical problems, the patent application US 2013/0072375 describes a membrane consisting of a first dense layer made from a mixture of an electrically conductive material and an ionically conductive material, a intermediate layer for the catalytic reaction and a porous support layer, of tubular form, and consisting of an yttrium-stabilized zirconia YSZ. It is described in this application that it is possible to obtain a YSZ porous support layer of very good mechanical strength and having good oxygen diffusivity by using PMMA as pore-forming agent in the manufacture thereof. again using different size fractions of YSZ.

The present invention relates to a porous ceramic support for the gas separation (or a use of said support for said separation), having further improved performance, especially whose diffusivity and / or creep resistance are substantially improved compared with at the current state of the art in question, for values of substantially equal porosity, or even lower. By porosity is meant in the sense of the present invention the open porosity of the material, as it can be measured according to well-known mercury porosimetry techniques.

According to a first aspect of the present invention, it relates to a support for a gas separating ceramic membrane, constituted by an yttrium stabilized zirconia and / or calcium whose porosity is greater than 20%, said support being obtained by sintering a feedstock comprising a mixture of fused yttrium and / or calcium stabilized zirconia grains.

Preferably, the mineral portion of said filler, that is to say in particular all of the inorganic oxides present in said filler, consists essentially of said molten grains of yttrium and / or calcium stabilized zirconia.

For the purposes of the present invention, the following definitions are given:

- By gas separation means generally any separation of a specific gas contained in a gaseous fluid, including the purification of a gas.

- A "melted grain" is a grain obtained by a manufacturing process comprising at least one melting step, a solidification step and a grinding step.

- A "melting" of a granular mixture is called a heat treatment at a sufficiently high temperature for all the constituents of the granular mixture are in the molten state (liquid).

- Sintering, conventionally in the field of ceramics (that is to say in the sense indicated in the International Standard ISO 836: 2001, point 120), consolidation by heat treatment to more than 1100 ° C of a granular agglomerate. The sintering according to the invention is carried out in exclusively solid phase, and in particular all the constituents of the melted grains. remain in the solid phase during said sintering. The heat treatment at more than 1100 ° C., in particular between 1300 ° C. and 1500 ° C., of the melted grains used as feedstock for obtaining the supports according to the invention thus makes it possible to join and develop their feed interfaces. contact by movement of the atoms inside and between said melted grains.

According to the invention, the sintering temperature of the melted grains is normally between 1300 ° C. and 1500 ° C.

Said feedstock may also include unavoidable impurities.

By "impurities" is meant inevitable constituents necessarily introduced with the raw materials or resulting from reactions with these constituents. The impurities can in particular be introduced during the preliminary step of manufacturing the melted grains. Impurities are not necessary constituents, but only tolerated. In particular, the compounds forming part of the group of oxides, nitrides, oxynitrides, carbides, oxycarbides, carbonitrides and metallic species of sodium and other alkalis are impurities. By way of examples, mention may be made of alumina or silica. It is considered that a total content of impurities of less than 2% does not substantially modify the results obtained.

On the other hand, hafnium oxide is not considered according to this definition as an impurity. The zirconium sources currently marketed contain hafnium, since it is recognized that Hf0 ₂ is not chemically separable from Zr0 _2. For the purposes of the present invention, the term "zirconium oxide" or "Zr0 ₂ " therefore conventionally refers to total content of these two oxides. According to the present invention, Hf0 ₂ is thus not voluntarily added to the feedstock but is always naturally present in zirconia sources at levels generally below 2%.

For the sake of clarity, the present description thus denotes indifferently the zirconia and hafnium oxide content by "ZrO 2 + HfO 2" or "ZrO 2", again by "zirconia content".

Yttrium-stabilized zirconia or yttria-stabilized zirconia is a zirconia also incorporating element Y in an amount of about 2.0 to 6.0 mol%, expressed as the oxide. Y2O ₃ , to stabilize the quadratic and / or cubic structural forms of zirconia at room temperature.

"Zirconia stabilized with calcium" is a zirconia also incorporating the element Ca in an amount of about 5.0 to 15.0 mol%, preferably 6.0 to 10.0 mol%, expressed in the form of CaO oxide, to stabilize the quadratic and / or cubic structural forms of zirconia at room temperature.

- "Zirconia stabilized with yttrium and calcium", a zirconia incorporating the elements Y and Ca in a total amount of about 5.0 to 15.0 mol%, expressed as 2 ^χ Y 2 O 3 + CaO, to stabilize the quadratic and / or cubic structural forms of zirconia at room temperature. According to the invention, such yttrium and calcium stabilized zirconia very preferably comprises less than 6% yttrium in molar percentage and expressed as yttrium oxide Y2O3.

For the purposes of the present description, stabilized zirconia is more than 80%, even more than 90% or even more than 95%, or even substantially 100%, by volume, of quadratic and / or cubic phase, the complement being 100%. % consisting of monoclinic phase. A "set of particles", denoted D ₅ o, is the "median size", the size dividing the particles of this set into a first and a second population equal in mass, these first and second populations comprising only particles having a size higher or lower, respectively, to said median size.

- The term "percentile" 99.5 (denoted by D _99, _5), the particle size corresponding to a percentage equal to 99.5%, by weight, of the cumulative particle size distribution curve of the sizes of the powder particles, said particle sizes being ranked in ascending order. According to this definition 99.5% by mass of the powder particles and have a size of less than D _99, 5 and 0.05% of the particles by mass have a size greater than D _99, _5.

The average sizes and percentiles are determined using a particle size distribution made using a laser granulometer, for example the Partica laser granulometer LA-950 from HORIBA, using the "Version 4" algorithm. XX Compatible ".

The "maximum size" of a powder is called the 99.5 percentile.

- The "aspect ratio" is the ratio of the largest apparent dimension to the smallest apparent dimension.

- We call "median size" grains of a sintered part, the dimension measured according to the method of "Mean Linear Intercept" described in the IS013322-1 method.

- "containing one", "comprising one" or "comprising one" means "containing at least one", unless otherwise indicated.

In a conventional manner, a "precursor" of an oxide is a compound or a set of compounds which, by a heat treatment, especially under air, lead to the formation of said oxide. By way of example, yttrium nitrate is a precursor for yttrium oxide. A mixture of yttrium oxide and zirconia is a yttria precursor. An amount of a precursor of an oxide is said to be "equivalent" to a quantity of said oxide when, during the heat treatment, it leads to said quantity of said oxide.

According to certain preferred aspects of implementation of the support according to the invention, which can of course possibly be combined with one another:

- The mineral part of said filler, in particular all of the inorganic oxides present in said filler, consists essentially of said melted zirconia grains stabilized with yttrium and / or calcium.

- The median size of the melted grains after grinding is 0.1 to 10 micrometers.

The stabilized zirconia is yttrium stabilized zirconia, the yttrium content of which is expressed in the form of Yttrium oxide Y ₂ O ₃ , is between 2% and 6% by moles.

The stabilized zirconia is calcium-stabilized zirconia whose calcium content, expressed in the form of CaO oxide, is between 5.0 and 15.0 mol%, preferably between 6.0 and 10.0. molar%.

- The stabilized zirconia is yttrium and calcium stabilized zirconia, the total content of yttrium and calcium, expressed as 2 ^χ Y2O3 + CaO, is between 5.0 and 15.0 mol%.

The thickness of said support is between 0.5 and

4 mm.

- The porosity of zirconia stabilized with yttrium and / or calcium is between 20 and 40%. Sintering is carried out starting from at least two granulometric fractions of melted grains.

Sintering is carried out starting from a first granulometric fraction of melted grains whose median size is between 0.3 and 1.5 micrometers and from a second granulometric fraction of melted grains whose median size is between 2 and 6 micrometers.

The support according to one of the preceding claims being in the form of a layer or in the form of tubes. More specifically and in other aspects, the mixture of molten grains of the feedstock preferably has a median size greater than 0.1 micrometers, preferably greater than 0.3 micrometers, preferably greater than 0.5 micrometers.

The melted grain mixture of the feedstock preferably has a median size of less than 10 micrometers, preferably less than 8 micrometers, preferably less than 6 micrometers, preferably less than 5 micrometers.

The melted grain mixture of the feedstock preferably has a maximum size of less than 12 microns, preferably less than 10 microns, preferably less than 8 microns, preferably less than 6 microns.

The grains of the feedstock have a form factor of less than 2, preferably less than 1.5.

According to one embodiment, the mixture of grains of the feedstock is obtained by mixing two or more grain size fractions which differ in their respective median size to form a two-modal or multi-modal mixture, their chemical nature being identical or different.

Preferably, the sintering is carried out starting from a first granulometric fraction of melted grains. whose median size is between 0.3 and 1.5 micrometers and a second size fraction of molten grains whose median size is between 2 and 6 micrometers.

According to one embodiment, the molten grains of the feedstock consist mainly of yttria - containing zirconia, the content of yttrium oxide, expressed in the form of yttrium oxide Y2O ₃ relative to the total (ZrO 2 + HfO 2 ) + Y ₂ O ₃ is between 2% and 6% by mole, or even between 4% and 6% by mole.

After sintering, the sintered support has a porosity greater than 20%, even greater than 25% and / or less than 50%, even less than 45%, even less than 42 ~ 6, or even less than 40%.

After sintering, the sintered support has a structure in which the yttrium and / or calcium stabilized zirconia fused grains are found, said grains being sintered together. The median size of the zirconia grains of the sintered part is greater than 0.2 micrometers, even greater than 0.3 micrometers, or even greater than 0.5 micrometers.

According to one embodiment, the sintered support consists mainly of yttria-containing zirconia, the content of yttrium oxide, expressed in the form of Yttrium oxide Y ₂ 0 ₃ relative to the total (Zr0 ₂ + Hf0 ₂ ). + Y2O3 is between 2% and 6% by mole, or even between 4% and 6% by mole; its chemical composition has more than 90% by weight, or even more than 95% by weight, or even more than 98% by weight of Zr0 ₂ + HfO 2 + Y 2 O 3.

The invention also relates to a gas separating ceramic membrane comprising a support as previously described.

According to the invention, within said membrane, the support may be present in the form of a layer porous or tubes, as described in the publication US2013 / 0072375.

An example of a membrane according to the invention comprises a layer consisting of a composite based on an ionically conductive material and an electrically conductive material or consisting of a mixed ionic and electrical conductor, said layer being supported by a support such as as previously described.

The invention finally relates to a method of manufacturing a support as described above, comprising the following steps:

the melting of a stabilized zirconia powder with yttrium, calcium, or yttrium and calcium or a mixture of its precursors,

solidification and cooling of the molten liquid,

grinding the molten product to obtain melted grains whose median size is less than 10 micrometers,

the preparation of a feedstock comprising said melted grains,

- shaping the starting load,

sintering said grains, optionally in the presence of a pore-forming agent, at a temperature between 1300 ° C and 1500 ° C.

The feedstock may also comprise one or more porogens and / or deflocculant (s) and / or binder (s) and / or lubricants, preferably temporary, conventionally used in forming processes for the production of preforms to be sintered. for example a polyethylene glycol (PEG), a carboxylic acid, or a latex. The pore-generating agent (s) are for example chosen from graphite, carbon black, polymethyl methacrylate (PMMA), starch, etc. The particle size of the pore-forming agent (s) is between 0.5 and 5 microns. . According to one embodiment, two or more pore-forming agents differing in their respective median size are used to form a two-modal or multi-modal mixture; their chemical nature may be the same or different.

The shaping of the feedstock is carried out for example by extrusion to form, for example, tubes of the desired size.

Other techniques such as cold isostatic pressing, slip casting, uniaxial pressing, gel casting, vibro-casting, strip casting, injection molding or a combination of these techniques could be used. to form pieces of desired size.

The sintering is carried out, preferably under air, at atmospheric pressure and at a temperature of between 1300 ° C. and 1500 ° C., preferably between 1350 ° C. and 1450 ° C. The holding time at this temperature is preferably between 2 and 8 hours. The rise speed is typically between 10 and 100 ° C / h. The descent speed can be free. If porogens and / or deflocculants and / or binder (s) and / or lubricants are used, the sintering cycle preferably comprises a step of 1 to 4 hours at a temperature of between 200 ° C. and 800 ° C. to promote the elimination of said products.

According to one embodiment, the sintering is performed after deposition of the functional layers (cathode, ...) on the support. The advantages of the present invention are illustrated by the examples according to the invention and comparative which follow. It should be understood that these examples should not be construed as limiting the scope of the present invention in any of the aspects described therein.

Example 1 (comparative)

According to this first example, the procedure is as described in the application US2013 / 0072375.

67 parts (by weight) of a powder of 3YSZ (that is to say comprising about 3 mol% of Y ₂ O ₃ ) marketed by the company Tosoh, whose median grain size is of the order of 0 , 6 micrometers, are mixed with 23 parts of a pore-forming agent of the PMMA (polymethyl methacrylate) type and 10 parts by weight of carbon black until a homogeneous mixture of the three powders is obtained. The assembly is shaped by extrusion to obtain a tube of thickness 1 mm and by pressing in the form of a strip of dimensions 25 mm x 15 mm x 180 mm.

The resulting pieces are sintered at 1400 ° C for two hours.

The characteristics and properties of the porous supports thus obtained are measured according to the following techniques and protocols:

Porosity: The open porosity is measured according to the standard techniques of mercury porosimetry, using an apparatus of the company Micromeritics.

Creep: Creep is measured by placing a sample (25 mm x 15 mm x 180 mm) in a four-point configuration (distance between external supports L = 110 mm, distance between internal supports 1 = 40 mm). The sample is then heated to a temperature of 1100 ° C. The temperature is kept constant and the sample is stressed at 12.5 MPa in the middle. The variation of the sample deflection is recorded throughout the test. Df90 and DflOO denote the relative strain given respectively after 90 hours or 100 hours. A creep rate V100 = (Df100-Df90) / 36000, expressed in s ^{-1, is} then calculated.

Diffusivity:

The sample is positioned within a sealed system and equipped with means (oxygen probes marketed by Illinois Instrument) for the quantification of gaseous species.

A flow of air (2 liters / min) is sent to the oxygen-rich side (inner side of the tube), and at the same time, an exclusively nitrogen flow (2 liters / min) flows counter-clockwise to the side opposite of the sample (outer face of the tube).

A partial pressure gradient of oxygen is thus created, which induces an oxygen transfer from the richest oxygen side to the opposite side until the gradient becomes zero. The equilibrium oxygen content at the outlet of the sample is then measured in both the nitrogen stream and the air. By applying the law of Fick, we deduce the effective diffusion coefficient De, expressed in m ² . s ^-1 , oxygen through the thickness of the material constituting the tubular support.

The results obtained, as well as a summary of the operating conditions previously described, are reported in Table 1 below. Example 2 (according to the invention)

In this example, fused grains are prepared by melting at a temperature above 2200 ° C of a mixture of a zirconia powder and an yttrium oxide powder. The molten liquid is solidified and cooled. The molten product is then ground until a powder of melted grains is obtained, the median particle size of which is 0.8 micrometers and whose form factor is less than 2.

These molten grains have a yttrium oxide content of about 3 molar percent relative to the zirconia content. The main impurities are alumina (3370 ppm) and titanium dioxide T1O ₂ (1865 ppm), calcium oxide CaO (1135 ppm) and silica (350 ppm).

79 parts (by weight) of the powder of melted grains are mixed with 12 parts of the PMMA (polymethylmethacrylate) type pore-forming agent and 9 parts by weight of carbon black until a homogeneous mixture is obtained of the three powders. The assembly is shaped by extrusion to obtain a 1 mm thick tube and by pressing in the form of a bar of dimensions:

25 mm x 15 mm x 180 mm.

The parts thus obtained are then sintered under conditions of temperature (1380 ° C. for 8 hours) substantially identical to those described in Example 1. The same measurements of porosity and diffusivity are carried out on the sample according to Example 2 .

The results obtained, as well as a summary of the operating conditions previously described, are reported in Table 1 below. Example 3 (according to the invention)

In this example, the procedure is as in Example 2 with the only difference that the sintering temperature is raised to 1430 ° C for 6 hours.

The same measurements of porosity, diffusivity and creep resistance are carried out on the sample according to Example 3.

The results obtained, as well as a summary of the operating conditions previously described, are reported in Table 1 below.

Example 4 (according to the invention)

In this example, the procedure is as in Example 2 with the only difference that the sintering temperature is raised to 1450 ° C for 4 hours.

The same porosity and diffusivity measurements are made on the sample according to Example 4.

The results obtained, as well as a summary of the operating conditions previously described, are reported in Table 1 below:

Table 1

The results reported in Table 1 above show that very low creep values, including for high porosity levels, are obtained when the support is obtained by sintering molten grains of zirconia yttriee according to the present invention.

Also it appears that regardless of the porosity of the support, the effective diffusion coefficients are always significantly higher for the samples obtained according to the principles of the present invention. At a given porosity, the improvement is very significant, which makes it possible to envisage a reduction in the porosity (for example by reducing the amount of porogens) while maintaining a very good effective diffusion coefficient, the decrease in porosity being accompanied by an improvement of the mechanical strength. In the foregoing examples, the advantages of the support according to the invention have been illustrated in the context of their use in an oxygen-separating ceramic membrane. Of course the field of application of such media is not limited to such an example.

In particular, the porous ceramic supports according to the invention are useful for any application having high operating temperatures (from 500 to 1100 ° C.), in particular at pressures greater than atmospheric pressure, or even greater than 5 bars. This is for example any support application for dense layer (s) (membrane for the transport of oxygen) or support for porous layer (s): filters called "membrane" (monolith) by example for the separation of gases (N¾, N ₂ , H ₂ , CO ₂ ), so-called "membrane" contactors (monolith) for example for the purification of gases (elimination of VOCs, SO ₂ , Ox, H ₂ O) or support for particles filter (bed of particles divided) for example for the transformation of hydrocarbons (reforming, cracking).

Claims

Ceramic support for gas separation, constituted by an yttrium and / or calcium stabilized zirconia having a porosity greater than 20%, said support being obtained by sintering a feedstock comprising melted grains of yttrium stabilized zirconia or calcium stabilized zirconia or yttrium and calcium stabilized zirconia.

2. Support according to claim 1, wherein the mineral part of said filler, in particular all of the inorganic oxides present in said filler, consists essentially of said melted grains.

3. Support according to one of the preceding claims, wherein the median size of the molten grains of the feedstock is 0.1 and 10 micrometers.

4. Support according to one of the preceding claims, wherein the stabilized zirconia is yttrium stabilized zirconia whose yttrium content expressed in the form of yttrium oxide Y2O ₃ , is between 2% and 6% in moles.

5. Support according to one of claims 1 to 3, wherein the stabilized zirconia is calcium stabilized zirconia whose calcium content, expressed in the form of CaO oxide, is between 5.0 and 15, 0 mol%.

6. Support according to one of claims 1 to 3, wherein the stabilized zirconia is zirconia stabilized with yttrium and calcium, the total yttrium content and calcium, expressed as 2 ^χ Y2O3 + CaO, is between 5.0 and 15.0 mol%.

7. Support according to one of the preceding claims, of thickness between 0.5 and 4 mm.

8. Support according to one of the preceding claims, wherein the porosity of zirconia stabilized with yttrium and / or calcium is between 20 and 40%.

9. Support according to one of the preceding claims, wherein the sintering is carried out from a first size fraction of melted grains whose median size is between 0.3 and 1.5 micrometers and a second granulometric fraction of melted grains whose median size is between 2 and 6 micrometers.

10. Support according to one of the preceding claims being in the form of a layer or in the form of tubes.

11. Ceramic gas separating membrane comprising a support according to one of the preceding claims.

A gas separating ceramic membrane according to claim 11 comprising a layer consisting of a composite based on an ionically conductive material and an electrically conductive material or consisting of a mixed ionic and electrical conductor, said layer being supported by a support according to one of claims 1 to 10.

13. A method of manufacturing a support according to one of the preceding claims comprising the following steps: melting an yttrium stabilized zirconia or calcium stabilized zirconia or yttrium and calcium stabilized zirconia powder or a mixture of its precursors,

solidification and cooling of the molten liquid,

grinding the melted product until melted grains having a median size of less than 10 microns,

preparing a starting charge comprising said melted grains,

- shaping the starting load,

sintering said grains, optionally in the presence of a pore-forming agent, at a temperature of between 1300 ° C. and 1500 ° C.