Solid desiccant resistant to alkaline hydroxides

Abstract

The present invention relates to the use of a solid desiccant comprising at least one kaolin compound for drying wet gases containing trace amounts of alkaline hydroxides. The present invention also relates to a process for drying a wet gas containing trace amounts of alkaline hydroxides, comprising at least one step of contacting the wet gas with a solid desiccant comprising at least one kaolin compound.

Description

[Technical field]

[0001] The present invention relates to the field of molecular hydrogen production, in particular to the production of dry molecular hydrogen, and more particularly to the production of molecular hydrogen dried over molecular sieves. [Background technology]

[0002] The production of molecular hydrogen by basic electrolysis is now a well-known and sufficiently developed technology that it can be the subject of major installations in production units. In this basic electrolysis process, water is converted into molecular hydrogen and molecular oxygen under the action of an electric current. As with any kind of electrolysis, an electrolyte is often necessary to facilitate the movement of ions in the solution to be electrolyzed.

[0003] Different electrolytes can therefore be used, very particularly basic electrolytes being selected, in particular electrolytes based on alkali metal hydroxides, very particularly sodium hydroxide and potassium hydroxide. One of the most effective electrolytes consists of an aqueous potassium hydroxide solution, i.e. KOH solution.

[0004] In the basic electrolysis process, molecular hydrogen is obtained on the cathode by reduction of two protons. The gaseous molecular hydrogen thus recovered is wet, i.e. it contains more or less traces of water. In addition, the wet molecular hydrogen recovered may, but generally contains traces of alkaline hydroxides, e.g., traces of potassium hydroxide.

[0005] In fact, it has been found that such traces of alkaline hydroxides can be detrimental to the dry solid, which appears in the preferred solution for drying the molecular hydrogen resulting from electrolysis. Many fields of application require the use of dry molecular hydrogen, in particular molecular hydrogen treated on dry solids. It can therefore be envisaged to remove any traces of alkaline hydroxide present in the molecular hydrogen to be dried, maintaining a good production of dry molecular hydrogen exhibiting the highest possible purity. Before use, the molecular hydrogen must therefore be purified, but the presence of potassium hydroxide, a strong base, greatly limits the range of sieves that can be used.

[0006] The various techniques conventionally used today for drying gases are well known to those skilled in the art. Thus, moist gases are usually dried by various techniques, for example by permeation through membranes, by scrubbing operations of the gas with formulations based on organic compounds, for example based on glycols.

[0007] Furthermore, the use of solid desiccants, such as activated alumina, silica gel or molecular sieves, for drying organic liquids or gases is known, as described for example in patent EP1597197B1, where zeolite 3A aggregates allow drying of esters and alcohols. Thus, moist molecular hydrogen is generally dried today using solid desiccants. Zeolite aggregates, also known as molecular sieves, are one of the most efficient desiccants, making it possible to achieve very low residual water contents, in the ppm range.

[0008] This extreme ability to adsorb water molecules is one of the properties of zeolites, which are aluminosilicates of controlled crystallinity. However, to be usable in industrial processes, the zeolite crystals (which exist in the form of a very fine powder) must be shaped so that they can be more easily handled, for example in the form of beads or threads.

[0009] To shape these zeolite crystals, and because the cohesion of zeolites is very low, it is necessary to use binders, called agglomerating binders, which are generally clay-type binders that are well known today and commonly used to control the final shape of the zeolite aggregates.

[0010] Nevertheless, these clay-based flocculating binders are in most cases very sensitive to the action of inorganic bases, such as sodium hydroxide or potassium hydroxide, etc. This vulnerability results in molecular sieves that are at risk of not being sufficiently stable under the dry conditions of the gases resulting from electrolysis using basic electrolytes, in particular electrolytes based on alkali metal hydroxides, very particularly electrolytes based on sodium hydroxide and potassium hydroxide. [Prior art documents] [Patent documents]

[0011] [Patent Document 1] European Patent No. 1597197 Summary of the Invention [Problem to be solved by the invention]

[0012] The technical problem that the present invention proposes to solve is therefore to provide a solid desiccant for drying wet gas produced by basic electrolysis, which is resistant to traces of alkaline hydroxide present in said wet gas. Another object of the present invention is to make available a solid desiccant which is resistant to traces of potassium hydroxide present in wet gas. Yet another object of the present invention is to provide a solid desiccant which is resistant to traces of potassium hydroxide present in wet hydrogen produced by basic hydrolysis, the basic electrolyte of which comprises potassium hydroxide. [Means for solving the problem]

[0013] It has been found that the above-mentioned objects are achieved in whole or at least in part by the subject matter of the invention described herein. Further objects will become apparent in the remainder of the description. In particular, it has been quite surprisingly discovered by the inventors that certain solid desiccants can be used to dry wet gases without undergoing significant decomposition even in the presence of trace amounts of basic compounds, such as alkali hydroxides.

[0014] Thus, according to a first aspect, the present invention relates to the use of a solid desiccant comprising at least one kaolin compound for drying a wet gas containing traces of alkaline hydroxides. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0015] The moist gas that can be dried using the above mentioned solid desiccants can be of any type known to those skilled in the art, for example, but not limited to, the gas is selected from industrial gases, such as nitrogen, oxygen, hydrogen, noble gases, carbon dioxide and mixtures thereof, in particular hydrogen, optionally in mixture with one or more of the other gases listed above, very particularly hydrogen obtained by electrolysis in a basic medium.

[0016] The moisture content of the gas to be dried in the context of the use of the present invention can vary in wide ranges, depending in particular on the nature of the gas to be dried (wet gas) and on the nature of the solid desiccant used. As a rule, the moisture content is between 5 ppm and 2% by volume. The term "moisture content" is understood to mean the volumetric amount of water contained in the gas to be dried.

[0017] The use according to the invention is particularly well suited for wet gases containing trace amounts of alkaline hydroxides, such as lithium hydroxide, sodium hydroxide, potassium hydroxide and mixtures thereof, more particularly sodium hydroxide, potassium hydroxide and mixtures thereof, more particularly potassium hydroxide. The term "trace amount" is more particularly understood to mean a content of 1 ppm to 1000 ppm by volume, preferably 1 ppm to 500 ppm by volume.

[0018] The solid desiccants intended for use according to the invention may be of any type known to those skilled in the art and may be selected, by way of non-limiting example, from activated alumina, silica gel, molecular sieves, etc., and mixtures thereof in any proportion. Very particular preference is given to molecular sieves and, above all, to zeolite aggregates, more particularly those comprising zeolite crystals and at least one kaolin compound.

[0019] According to a particularly preferred embodiment of the use of the present invention, the kaolin compound is a kaolin binder that binds with the zeolite crystals to give said zeolite aggregates cohesive strength. Such zeolite aggregates with kaolin binders are very well known to the person skilled in the art and are commercially available or can be prepared according to known procedures available in the scientific and patent literature and on the Internet.

[0020] According to one embodiment of the present invention, the solid desiccant is a zeolite aggregate comprising 70% to 99.99% by weight, preferably 70% to 99.9% by weight, more preferably 80% to 99.9% by weight (limits included) of crystals of at least one zeolite selected from LTA type zeolite, FAU type zeolite, SOD type zeolite, P type zeolite, and mixtures thereof, preferably from zeolite 3A, 4A, 5A, 13X, and mixtures thereof, more preferably from zeolite 3A and 4A, and mixtures thereof.

[0021] The zeolite aggregates which can be used in the context of the present invention may further comprise crystals of one or more zeolites other than those already listed above, but it is preferred to use zeolite aggregates whose crystals are perfectly well suited for drying operations on gases, such as zeolites 3A, 4A, 5A, 13X and mixtures thereof, more preferably zeolites 3A and 4A and mixtures thereof.

[0022] As mentioned above, the kaolin compound is advantageously a kaolin binder. The term "kaolin binder" is understood to mean a kaolin clay or a kaolin clay precursor, more particularly a clay selected from kaolin, kaolinite, nacrite, dickite, halloysite and metakaolin, and mixtures thereof.

[0023] The zeolite aggregates that can be used in the context of the present invention can also include one or more other binders, as well as one or more inert fillers, for the purpose of further strengthening the aggregation of the solid desiccant, modifying its density and generating porosity. Among other possible aggregate binders, mention may be made, for example, but not limited to, bentonite. Among possible inert fillers, mention may be made, for example, but not limited to, any type of silica source known to those skilled in the art who are experts in the synthesis of zeolites, such as colloidal silica, diatomaceous earth, perlite, fly ash, sand, or any other form of solid silica, as well as glass fibers, carbon fibers, carbon nanotubes, etc., and mixtures thereof.

[0024] According to a preferred embodiment, the other binders and / or inert fillers do not exceed 33% by weight, relative to the total weight of the kaolin compound, the other binders and fillers.

[0025] The kaolin compound may further comprise one or more additives intended to facilitate the preparation of the solid desiccant, in particular the handling of the zeolite / kaolin compound paste by modifying the rheology and / or adhesion, or to impart satisfactory properties to the solid desiccant, in particular macroporosity, preferably organic additives, such as lignin, starch, methylcellulose and its derivatives, surface-active (cationic, anionic, nonionic or amphoteric) molecules, which are introduced during the preparation of the solid desiccant in a proportion of 0% to 5% by weight, preferably 0.1% to 2% by weight, relative to the total weight of the adsorbent.

[0026] Preference may be given to, but non-exhaustively mentioned, methylcellulose and its derivatives, such as carboxymethylcellulose, lignosulfonates, polycarboxylic acids and carboxylic acid copolymers, their amino derivatives and their salts, especially the alkali and ammonium salts.

[0027] In yet another preferred embodiment, the kaolin compounds can be preferentially zeolitized, either in whole or in part, i.e. all or part of the kaolin compounds or the kaolin binder are converted to zeolite material before or during use or before and during use. Zeolitization can be carried out by any means known to the skilled artisan, for example as described in EP1697042. Zeolitization can also be carried out during actual use of drying gases under certain conditions. Without wishing to be bound by theory, it is believed that the presence of trace amounts of alkaline hydroxides, optionally in combination with at least a local temperature increase in the solid desiccant, can result in at least partial zeolitization of the kaolin compounds or the binder.

[0028] It has thus been found that solid desiccants comprising zeolite compounds are particularly resistant to the presence of traces of alkaline hydroxides, in particular traces of potassium hydroxide, present in the moist molecular hydrogen to be dried, in particular in molecular hydrogen obtained by electrolysis, and that solid desiccants comprising kaolin compounds are particularly resistant, in particular better than the solid desiccants currently known and usually used for the drying of gases.

[0029] The use of the present invention is therefore particularly suitable for zeolite agglomerates with a kaolin binder, very particularly for zeolite agglomerates based on zeolites 3A, 4A, 5A and / or 13X which comprise kaolin as the agglomerate binder which may be fully or at least partially zeolitized, preferably for agglomerates based on zeolites 3A and / or 4A with a kaolin binder which is not zeolitized or which is partially or fully zeolitized.

[0030] According to a second aspect, the present invention relates to a process for drying a wet gas containing traces of alkaline hydroxides, comprising at least one step of contacting said wet gas with a solid desiccant comprising at least one kaolin compound as just defined.

[0031] The contacting of the wet gas with the solid desiccant can be carried out according to any method known to those skilled in the art, in particular, for example, in an adsorber, which is generally a column containing a solid desiccant.

[0032] The process of the invention can be carried out according to various techniques and methods, for example according to a process selected from the following: Pressure Swing Processes, for example of the PSA (Pressure Swing Adsorption) type or the VSA (Vacuum Swing Adsorption) type or the VPSA (a hybrid process of the two preceding ones) type or the RPSA (Rapid Pressure Swing Adsorption) type, preferably of the PSA type, Temperature Swing Processes of the TSA (Temperature Swing Adsorption) type, and · Pressure and Temperature Swing Processes of the PTSA (Pressure and Temperature Swing Adsorption) type.

[0033] The wet gas intended to be dried in the process of the invention may, if desired or desired, be previously subjected to a first drying, in particular if the water content of the wet gas is too high. This first drying stage can be carried out according to any method known to the person skilled in the art, for example by cooling the gas and gradually releasing the condensed water or by passing it through a membrane.

[0034] The process of the invention is generally carried out at pressures between atmospheric and 10 MPa, preferably between atmospheric and 5 MPa, at ambient or moderate temperatures, preferably below the boiling point of water at the pressure under consideration.

[0035] The process of the invention is very particularly highly suitable for drying wet molecular hydrogen obtained by electrolysis with a basic electrolyte, in particular an electrolyte based on potassium hydroxide. The process for drying molecular hydrogen obtained by basic electrolysis in the presence of potassium hydroxide is particularly effective when the solid desiccant is a zeolite aggregate as defined above, for example a zeolite aggregate based on zeolite 3A, 4A, 5A and / or 13X with kaolin as an aggregate binder, preferably a zeolite aggregate based on zeolite 3A and / or 4A with a kaolin binder.

[0036] The solid desiccants described in the present invention are much more stable and resistant to attack by bases than the solid desiccants used today, which reduces the contamination of the gas to be dried by dust, etc., and in particular limits the increase in pressure drop of the desiccant system, especially the adsorber, during its operation. The use of such solid desiccants for drying wet gases is therefore more effective and represents a major and certain economic advantage. EXAMPLES

[0037] The following examples are used to illustrate the invention, but are not intended to limit it, the scope of which is defined by the claims appended hereto.

[0038] <Example of resistance to potassium hydroxide> The stability of the solid desiccant was evaluated according to the following test: 5 g of sieves, previously activated at 550° C. for 2 hours, are suspended in 100 ml of a 110 g / l potassium hydroxide solution in an Erlenmeyer flask.

[0039] The solid particles are left in contact with the solution for 1 hour, with occasional manual stirring. The solution is then recovered by filtration for quantitative determination of silicon and aluminum. The sieves are then dried at about 50°C for 8 hours (without washing with water) and then activated in a ventilated oven at 230°C for 3 hours (direct set point).

[0040] The following three samples are tested: Sample 1 (comparison): Zeolite 4A aggregate containing 20% ​​by weight of attapulgite binder (sample from Arkema sold under the name NK 10B 1.6 / 2.5 mm). Sample 2 (invention): Zeolite 4A aggregate with 20% by weight of kaolin binder (sample from Arkema sold under the name SRA B 1.6 / 2.5 mm). Sample 3 (invention): Zeolite 4A aggregates containing 20% ​​by weight of kaolin binder (2% residual binder) zeolitized with 110 g / l aqueous sodium hydroxide (NaOH) solution at 95° C. for 2 hours, with a weight ratio of NaOH solution to zeolite aggregates of 2.1.

[0041] An additional test (blank test) is carried out on three samples that have not been treated with potassium hydroxide. Tests and blank tests are carried out on each of the samples by water adsorption (H50) and mechanical strength (MS) measurements.

[0042] The water adsorption capacity (H50) (expressed in %) is determined by the ratio (multiplied by 100) of the increase in weight of 1 g of active solid desiccant after saturation with water at the end of 24 hours of residence in a closed chamber at 23±2°C (relative humidity equal to 50%) to the weight of a reference active solid desiccant (in this case 1 g).

[0043] The measured mechanical strength (MS), expressed in daN, corresponds to the grain crushing strength. The grain crushing mechanical strength is measured according to standards ASTM D4179 and D6175 using a grain crushing strength instrument sold by Vinci Technologies.

[0044] The H50 and MS (test and blank test) measurements of the three samples are recorded in Table 1 below.

[0045] [Table 1]

[0046] These results indicate that zeolite aggregates containing kaolin binder are more resistant to treatment with potassium hydroxide, even when the kaolin binder is partially zeolitized while retaining acceptable adsorption capacity.

Claims

1. Use of a solid desiccant comprising at least one kaolin compound for drying a wet gas containing trace amounts of alkaline hydroxide.

2. 2. The use according to claim 1, wherein the moist gas is selected from nitrogen, oxygen, hydrogen, the noble gases, carbon dioxide and mixtures thereof, preferably the moist gas is hydrogen.

3. 3. The use according to claim 1 or claim 2, wherein the moisture content of the wet gas is between 5 ppm and 2% by volume.

4. 4. Use according to any one of claims 1 to 3, wherein said solid desiccant is chosen from activated alumina, silica gel, molecular sieves etc. and mixtures thereof in all proportions, preferably from molecular sieves and, among the latter, from zeolite aggregates, more particularly from zeolite aggregates comprising zeolite crystals and at least one kaolin compound.

5. 5. Use according to any one of claims 1 to 4, wherein the solid desiccant is a zeolite aggregate comprising from 70% to 99.99% by weight, preferably from 70% to 99.9% by weight, more preferably from 80% to 99.9% by weight, inclusive of the limits, of crystals of at least one zeolite selected from LTA type zeolites, FAU type zeolites, SOD type zeolites, P type zeolites and mixtures thereof, preferably from zeolites 3A, 4A, 5A, 13X and mixtures thereof, more preferably from zeolites 3A and 4A and mixtures thereof.

6. 6. The use according to any one of claims 1 to 5, wherein the kaolin compound is a kaolin clay or a kaolin clay precursor, more particularly a clay selected from kaolin, kaolinite, nacrite, dickite, halloysite and metakaolin, and mixtures thereof.

7. Use according to any one of claims 1 to 6, wherein the kaolin compound is fully or partially zeolitized, preferentially partially zeolitized.

8. 8. A process for drying a wet gas containing trace amounts of alkali hydroxide, comprising at least one step of contacting said wet gas with a solid desiccant comprising at least one kaolin compound as defined in any one of claims 1 to 7.

9. 9. A process according to claim 8, characterized in that it is carried out at a pressure between atmospheric and 10 MPa, preferably between atmospheric and 5 MPa, at ambient or moderate temperatures, preferably below the boiling point of water at the pressure under consideration.

10. 10. A process according to claim 8 or claim 9 for drying wet molecular hydrogen obtained by electrolysis with a basic electrolyte, preferably for drying wet molecular hydrogen obtained by electrolysis with an electrolyte based on potassium hydroxide.

11. 11. The process according to any one of claims 8 to 10, wherein the solid desiccant is a zeolite aggregate based on zeolite 3A, 4A, 5A and 13X with kaolin as an aggregate binder, preferably a zeolite aggregate based on zeolite 3A and / or 4A with a kaolin binder.

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