Zeolite adsorbent in the form of low-tortuosity agglomerates

Description

Scope of the invention

[0001] The present invention relates to the field of zeolitic adsorbents, and more particularly to zeolitic adsorbents in agglomerate form. The present invention also relates to the preparation of said zeolitic agglomerates, as well as their use for separating gaseous or liquid mixtures. Previous art

[0002] The synthesis of zeolites leads to crystals, generally in powder form, whose use on an industrial scale is particularly difficult. Indeed, it is now possible to synthesize zeolite crystals ranging in size from a few nanometers to a few micrometers, sizes required to give zeolites optimal adsorption capacities. However, the drawbacks associated with these small crystals are numerous, including the difficulty of handling the powdery substance and the significant pressure drops during their use.

[0003] To overcome these drawbacks, it has been proposed to use agglomerated forms of these crystals, for example as filaments, beads, and other agglomerated shapes. The manufacture of such agglomerates from powdered zeolite crystals is now well established, and the scientific and patent literature provides numerous examples of zeolite agglomerate preparation, notably by extrusion, pelletizing, and other agglomeration techniques known to those skilled in the art.

[0004] These agglomerates are usually in size on the order of a few tens of micrometers, or even a few hundred micrometers, or even a few millimeters, and do not present the disadvantages inherent in powdery materials such as the zeolite crystals previously defined.

[0005] These agglomerates, whether in the form of plates, beads, extrudates, and others, are generally made up of zeolite crystal(s), which constitute the active element (in the sense of adsorption) and an agglomeration binder.

[0006] This agglomerating binder is designed to ensure the cohesion of the crystals within the agglomerated structure, but it must also provide sufficient mechanical strength to the agglomerates in order to prevent, or at least minimize as much as possible, the risk of fractures, breaks, or breaks that could occur during industrial applications where the agglomerates are subjected to numerous stresses, such as vibrations, strong and / or frequent pressure variations, movement, and other factors. It is therefore crucial that zeolitic agglomerates subjected to these various stresses remain cohesive and do not generate fine, powdery particles that lead to the aforementioned problems.It is therefore very important to have access to zeolitic agglomerates with improved properties, such as improved stability over time, i.e., whose separation performance over time is not deteriorated.

[0007] However, the adsorption properties of these agglomerates are obviously reduced compared to crystal powder, due to the presence of an agglomeration binder that is inert with respect to adsorption.

[0008] Several methods have already been proposed to overcome the drawback of the agglomerating binder's inertness with regard to adsorption performance, including the transformation of all or at least part of the agglomerating binder into an adsorption-active zeolite. This operation is now well known to those skilled in the art, for example under the name "zeolithization." To easily carry out this operation, zeolithizable binders are used, most often clays belonging to the kaolinite family, and preferably pre-calcined at temperatures generally between 500°C and 700°C, that is to say, to bake the clay ("calcinate" or "fire" in English).

[0009] Key factors influencing the performance of an adsorption separation process include adsorption selectivity, adsorption capacity, and mass transfer kinetics within the adsorbent, which controls the adsorption and desorption rates of the different compounds. The adsorbent must therefore exhibit good mass transfer properties to ensure a sufficient number of theoretical plates for efficient separation of the mixed species, as Ruthven indicates in the work entitled «Principles of Adsorption and Adsorption Processes, John Wiley & Sons, (1984), pages 326 and 407. To estimate the improvement in transfer kinetics, the plate theory described in the aforementioned work, pages 248-250, can be used. This approach is based on representing a column with a finite number of ideally stirred, hypothetical reactors (theoretical stages). The equivalent height of these theoretical stages is a direct measure of the axial dispersion and mass transfer resistance of the system.

[0010] Ruthven specifies (ibid., page 124) that, in general, the transport of molecules within the porous network of the adsorbent occurs not by flow, but by diffusion. The diffusion mechanism within the porous network depends in particular on the pore size, and consequently, within an adsorbent composed of agglomerated zeolite crystals, two diffusion regimes coexist: intracrystalline diffusion in the micropores of the zeolite and intra-particle diffusion in the macropores and mesopores of the intercrystalline space. The overall transfer time within an agglomerated zeolite adsorbent will then be the sum of the transfer time by intra-particle diffusion through the macro- and mesoporous network and the transfer time by intracrystalline diffusion.The characteristic diffusion transfer time within a spherical particle is generally expressed as the square of the particle's radius divided by the diffusivity of the molecules transported within that particle. Therefore, to improve mass transfer within zeolite adsorbents in agglomerate form, the primary objective is to reduce the size of the zeolite crystals and / or the size of the agglomerates.

[0011] In practice, we will seek to reduce the size of the agglomerates, but only up to a certain point, because this parameter determines the pressure loss and the uniformity of the filling within the industrial unit when using the adsorbent in the industrial application.

[0012] Similarly, reducing crystal size improves microporous transfer but could potentially affect intra-particle diffusivity in macropores. Therefore, excessive crystal size reduction is not desirable, as it risks reducing intra-particle diffusivity to an unacceptable level. Other avenues for improving transfer thus remain to be investigated. Document WO 2016 / 075280 concerns an adsorbent comprising a zeolite phase including at least one FAU-structured zeolite of type X and a non-zeolite phase. This adsorbent has an external surface area less than or equal to 30 m² / g and a pore diameter distribution of 100 to 250 nm. Document FR 3004966 concerns a zeolite adsorbent based on agglomerated EMT zeolite crystals containing barium and / or potassium.

[0013] When molecules diffuse through a porous material, their effective diffusivity through the porous network Δp is reduced compared to their diffusivity Δ0 when they diffuse in a free volume. The reduction factor is equal to the ratio of the porosity εp to the tortuosity factor τ of the porous material, according to Equation 1 (Kärger, Ruthven and Theodorou, "Diffusion in Nanoporous Materials", Volume 1, Wiley Edition, 2012, p. 95): D p D 0 = ε p τ

[0014] A high porosity value εp, such as macroporosity and / or mesoporosity, promotes diffusion. However, significantly increasing macroporosity and / or mesoporosity is not desirable, as this porosity does not contribute to adsorption capacity. Therefore, those skilled in the art would not attempt to increase it to reduce the intra-particle diffusion time within the macropores and mesopores of the intercrystalline space, knowing that this would be detrimental to volumetric adsorption capacity.

[0015] The tortuosity factor τ is a property of the adsorbent and does not depend on the diffusing molecule. This factor τ accounts for the diffusion length when molecules diffuse through a three-dimensional porous space made up of pores interconnected by restrictions, compared to straight cylindrical pores of the same mean pore diameter. Kärger specifies (ibid., p. 96) that experimentally measured tortuosity factor values ​​τ are typically between 2 and 5.

[0016] Optimal diffusion properties (i.e., optimal mass transfer), optimal adsorption capacities, while maintaining maximum mechanical strength of zeolitic structure adsorbents in agglomerate form, were obtained by specifically selecting both porosity and tortuosity factor. Object of the invention

[0017] The present invention has as its primary objective the provision of zeolitic adsorbents in the form of agglomerates with properties optimized for the separation of gaseous or liquid mixtures of isomers and more particularly for the separation of xylenes, in gas or liquid phase, notably para-xylene from C8 aromatic fractions. The zeolitic adsorbents of the invention exhibit, in particular, maximum selectivity properties towards para-xylene and mass transfer properties, while also exhibiting improved strength and high adsorption capacity per volume of adsorbent, and are particularly suitable for use in a liquid-phase para-xylene separation process, preferably of the simulated countercurrent type. Summary of the invention

[0018] The invention relates to a zeolitic adsorbent in the form of agglomerates, according to claim 1, said adsorbent having: a tortuosity factor τ, calculated from the pore distribution determined by mercury intrusion porosimetry, strictly greater than 1 and strictly less than 3; a porosity ε p = Vma + Vme Vg determined by mercury intrusion porosimetry, where Vma denotes the macroporous volume, Vme the mesoporous volume, and Vg the grain volume, between 25% and 35%, the volumes being expressed in cm3.g-1

[0019] Preferably, the tortuosity factor τ is between 1.5 and 2.7.

[0020] The high mechanical resistance to bed crushing (REL), measured by the Shell series SMS1471-74 method adapted for agglomerates smaller than 1.6 mm, is generally greater than or equal to 1.0 MPa.

[0021] The said adsorbent advantageously has a size between 0.1 mm and 1 mm, including terminals.

[0022] The said adsorbent may comprise a zeolite selected from zeolites of FAU structure, preferably zeolite X, alone or in mixture with other zeolites.

[0023] The said adsorbent may comprise more than 90% by weight of zeolite(s).

[0024] The zeolite(s) said are advantageously in the form of crystals with a size ranging from 10 nm to 1500 nm. Preferably, the pore size distribution satisfies the following inequalities a) and / or b): has) Vme Vme + Vma ≤ 0 , 1 , b) 0 , 4 ≤ Vmi Vma + Vme + Vmi . Vmi designates the microporous volume expressed in cm³ 3 .g -1 , determined by nitrogen adsorption.

[0025] The zeolitic adsorbent may also include barium and / or potassium.

[0026] The invention also relates to a method for preparing said zeolitic adsorbent comprising at least the following steps: a) a step of mixing crystals of at least one zeolite, with an agglomerating binder containing at least 80%, preferably at least 90%, preferably still at least 95% by weight of zeolithizable clay, and possibly a source of silica, followed by shaping and firing of the agglomerates at a temperature between 500 and 700°C;b1) a first zeolithization step by contacting the zeolitic agglomerates obtained in step a) with an alkaline basic solution, with a concentration between 0.2 M and 0.9 M inclusive; b2) a second zeolithization step by contacting the zeolitic agglomerates obtained in step b1) with an alkaline basic solution, with a concentration between 1.2 M and 4.0 M inclusive, b1 and b2 being carried out in any order and b1 and / or b2 being repeatable; c) an optional cation exchange step of the cations contained in the reaction medium from the zeolithization steps by contacting them with a solution of barium ions, or of barium and potassium ions; d) a washing and drying step of the zeolitic agglomerates thus obtained; and e) an activation step by heating at a temperature between 100°C and 400°C, of ​​the zeolitic adsorbent in the form of agglomerates obtained in step d). ;

[0027] The process may include one or more additional shaping steps carried out after any of the steps a), b1) / b2), c), d), e).

[0028] The invention also relates to a method for separating para- xylene from cuts of aromatic hydrocarbon isomers containing 8 carbon atoms, in liquid phase, by adsorption of para- xylene by means of said adsorbent or of an adsorbent capable of being prepared according to the preparation process in the presence of a desorbent.

[0029] The invention also relates to a method for separating para-xylene from cuts of aromatic hydrocarbon isomers containing 8 carbon atoms, in the gas phase, by adsorption of para-xylene using said adsorbent or an adsorbent that can be prepared according to the preparation method in the presence of a desorbent.

[0030] The said separation process may be of the simulated moving bed type. Detailed description of the invention

[0031] The zeolite adsorbent in agglomerate form of the invention comprises macropores, mesopores, and micropores. "Macropores" are defined as pores with an opening greater than 50 nm. "Mesopores" are defined as pores with an opening between 2 nm and 50 nm, excluding these limits. "Micropores" are defined as pores with an opening less than 2 nm, typically greater than 0 and less than or equal to 2 nm.

[0032] In the description of the present invention, "porous distribution" means the distribution of the porous volume as a function of the diameter of the pores.

[0033] Furthermore, "Vma" designates the macroporous volume expressed in cm³ .g⁻¹ of adsorbent, "Vme" the mesoporous volume expressed in cm³ .g⁻¹ of adsorbent, and "Vmi" the microporous volume expressed in cm³ .g⁻¹ of adsorbent.

[0034] “Vg” represents the grain volume of the adsorbent expressed in cm³ .g⁻¹ of adsorbent.

[0035] More specifically, the present invention relates to a zeolitic adsorbent in the form of agglomerates, said adsorbent having: a tortuosity factor τ, calculated from the pore distribution determined by mercury intrusion porosimetry, strictly greater than 1 and strictly less than 3, preferably between 1.5 and 2.7; a porosity ε p = Vma + Vme Vg , determined by mercury intrusion porosimetry, where Vma denotes the macroporous volume, Vme the mesoporous volume and Vg the grain volume, between 25% and 35%, the volumes being expressed in cm 3< .g -1< .

[0036] In the description of the invention, "size" means the number-average diameter of an object, or its largest number-average dimension when it is not spherical. The zeolite adsorbent in the form of agglomerates of the invention advantageously has a size typically between 0.1 mm and 1 mm, preferably between 0.2 mm and 1 mm, and in particular between 0.3 mm and 0.8 mm, more generally between 0.4 mm and 0.7 mm, inclusive.

[0037] The zeolite adsorbent in agglomerate form according to the present invention advantageously exhibits high bed crush resistance (BCR), measured by the Shell series SMS1471-74 method adapted for agglomerates smaller than 1.6 mm, as described later. The BCR of the zeolite adsorbent in agglomerate form of the invention is generally greater than or equal to 1.0 MPa, more generally greater than or equal to 1.5 MPa, preferably greater than or equal to 2.0 MPa, typically greater than or equal to 2.1 MPa.

[0038] The adsorbent according to the present invention is a zeolite material, indicating that it comprises crystals of at least one zeolite, selected from zeolites with the FAU structure, in particular from zeolites X and Y. Among these zeolites, zeolite X is preferred, without excluding mixtures of zeolite X with one or more of the other zeolites listed above. The Si / Al atomic ratio of the zeolite crystal(s) is advantageously between 1.00 and 1.50, preferably between 1.05 and 1.50, and even more preferably between 1.10 and 1.50, inclusive, measured by chemical analysis using X-ray fluorescence.

[0039] Preferably, the zeolite adsorbent of the present invention comprises only one zeolite form, which is zeolite X, identified by XRD, with a Si / Al ratio between 1.00 and 1.50, preferably between 1.05 and 1.50, and even more preferably between 1.10 and 1.50, inclusive, the ratio being measured by chemical analysis by X-ray fluorescence.

[0040] Advantageously, the zeolite adsorbent in the form of agglomerates of the invention comprises more than 90% by weight of zeolite, preferably more than 94% by weight of zeolite(s), most preferably more than 96% by weight of zeolite(s), and even more preferably between 96% and 98% by weight of zeolite(s), inclusive, relative to the total weight of the zeolite adsorbent. The remaining 100% by weight consists of the non-zeolite phase (NZP), comprising in particular the unconverted binder or amorphized zeolitic binder, identified by XRD.

[0041] According to a preferred embodiment, the zeolite(s) in the zeolite adsorbent of the invention are in the form of crystals with a size generally between 10 nm and 1500 nm, preferably between 100 nm and 1200 nm, more preferably between 200 nm and 1100 nm, and most particularly preferred between 300 nm and 900 nm, inclusive.

[0042] In one embodiment, the zeolitic adsorbent of the present invention has an atomic Si / Al ratio advantageously between 1.00 and 3.00, preferably between 1.00 and 2.00, more preferably between 1.00 and 1.50, inclusive, measured by chemical analysis by X-ray fluorescence.

[0043] Furthermore, the zeolite adsorbent according to the present invention may contain one or more alkali and / or alkaline earth metals, generally in cation form. Preferably, the alkali and / or alkaline earth metals present are sodium, potassium, barium, and mixtures of two or more of them in any proportion.

[0044] The zeolitic adsorbent in the form of agglomerates of the present invention further has a specific pore distribution, where the macroporous and mesoporous volumes are measured by mercury intrusion and the microporous volume is measured by nitrogen adsorption.

[0045] According to a particularly preferred embodiment of the present invention, the porous distribution of the zeolite adsorbent in the form of agglomerates satisfies the following inequalities a) and / or b): has) Vme Vme + Vma ≤ 0 , 1 preferably 0 , 01 ≤ Vme Vme + Vma ≤ 0 , 1 , more preferably 0 , 01 ≤ Vme Vme + Vma ≤ 0 , 06 b) 0 , 4 ≤ Vmi Vma + Vme + Vmi preferably 0 , 4 ≤ Vmi Vma + Vme + Vmi ≤ 0 , 6 preferably even 0 , 45 ≤ Vmi Vma + Vme + Vmi ≤ 0 , 55 .

[0046] The low value of the tortuosity factor of the zeolitic agglomerate according to the present invention allows for the most direct possible transport of molecules through said agglomerate.

[0047] According to another aspect, the invention also relates to a method for preparing zeolitic granular materials as defined above, a method which includes at least the following steps: a) mixture of crystals of at least one zeolite, with an agglomerating binder containing at least 80%, preferably at least 90%, preferably still at least 95% by weight of zeolithizable clay, and possibly a source of silica, shaping of the resulting mixture, and firing at a temperature between 500°C and 700°C, for a duration advantageously between a few minutes and a few hours, preferably between 2 and 6 hours; b1) a first zeolithization step by contact with an alkaline basic solution, of concentration between 0.2 M and 0.9 M, preferably between 0.2 M and 0.8 M, inclusive; b2) a second zeolithization step by contact with an alkaline basic solution, of concentration between 1.2 M and 4.0 M, preferably between 1.2 M and 3.5 M, preferably between 1.5 M and 3.5 M, particularly preferably between 1.5 M and 3.0 M.including the steps b1 and b2, which can be carried out in any order and b1 and / or b2 can be repeated, preferably each of steps b1 and b2 being carried out only once, c) possible cation exchange of the cations contained in the reaction medium from the zeolithization steps by contacting it with a solution of barium ions, or of barium and potassium ions, d) washing and drying of the zeolitic agglomerates thus obtained, and e) activation by heating to a temperature generally between 100°C and 400°C, preferably between 200°C and 300°C, of ​​the zeolitic adsorbent in the form of agglomerates obtained in step d).

[0048] A sodium hydroxide solution or a mixture of sodium hydroxide and potassium hydroxide can be used as an alkaline base solution for the zeolithization steps. Zeolithization is carried out in at least two steps with different concentrations, the duration of each step advantageously ranging from a few seconds to several hours, preferably between 30 seconds and 10 hours. Each zeolithization step may have the same or different durations, as explained later.

[0049] Without being bound by any particular theory, it would appear that the two zeolithization steps with alkaline solutions of different concentrations allow for the production of adsorbent aggregates with the required tortuosity factor and porosity. The resulting adsorbents combine both maximum adsorption capacity and improved properties.

[0050] The zeolite crystals used in the context of the present invention preferably have a size greater than or equal to 100 nm and less than or equal to 1500 nm, preferably greater than or equal to 150 nm and strictly less than 1200 nm, and even better greater than or equal to 150 nm and less than or equal to 1100 nm.

[0051] As previously stated, zeolite crystals of faujasite structure with an atomic ratio of Si / Al between 1.00 and 1.50 are preferred, preferably between 1.05 and 1.50, and even more preferably between 1.10 and 1.50, inclusive limits measured by chemical analysis by X-ray fluorescence.

[0052] During step a), in addition to the zeolite crystal(s) and the agglomerating binder, one or more additives may also be added, for example additives such as silica, particularly in the case where the zeolite used is zeolite X. The possible source of silica may be of any type known to a person skilled in the art, specializing in the synthesis of zeolites, for example colloidal silica, diatoms, perlite, fly ash, sand, or any other form of solid silica.

[0053] The zeolite crystals used in step a) can thus advantageously be produced from the synthesis of zeolite X crystals comprising mainly, or even exclusively, sodium cations, as is the case for example for NaX (or 13X) zeolites, but it would not depart from the scope of the invention by using crystals that have undergone one or more cation exchanges between the synthesis in NaX form and its implementation in step a). In this case, the cation exchange step d) consequently becomes unnecessary.

[0054] The shaping operation may involve the use of a bonding agent. According to a preferred embodiment of the invention, said bonding agent comprises at least 80% of a clay or a mixture of clays, optionally zeolitic, and up to 5% of additives known to those skilled in the art, such as, for example, but not limited to: carboxymethylcellulose, silica, alumina, and others, as well as the quantity of water necessary for shaping the agglomerated material. The zeolitic adsorbent in agglomerated form can thus be formed into beads, extrudates, or other shapes, with a size ranging from 0.1 mm to 1 mm, as previously described.

[0055] The agglomerating binder used in step a) contains at least 80%, preferably at least 90%, preferably still at least 95%, by weight, of clay or mixture of clays from among kaolins, kaolinites, nacrites, dickites, halloysite and / or metakaolins and may contain up to 20%, preferably up to 10%, preferably still up to 5%, by weight, of one or more additives and / or one or more other mineral binders such as bentonite, attapulgite, sepiolite and others.

[0056] The proportions of agglomeration binder and zeolite(s) used in step a), and possibly in one or more subsequent steps, are between 5 parts to 20 parts by weight of agglomeration binder for 95 parts to 80 parts by weight of zeolite(s).

[0057] In all cases, clays can be used in their raw state or can be pre-treated with one or more treatments, for example chosen from calcination, acid treatment, chemical modification, and others.

[0058] As previously mentioned, before the zeolite stages, the mixture obtained in step a) is heated (or "calcined") at a temperature generally between 500°C and 700°C. The principle is explained in "Zeolite Molecular Sieves" by DW Breck, John Wiley and Sons, New York, (1973), pp. 314-315.

[0059] The zeolithization steps b1 and b2 by contact with an alkaline basic solution are typically carried out by immersing the mixture obtained in step a) in an alkaline basic solution, generally aqueous, for example an aqueous solution of sodium hydroxide and / or potassium hydroxide, whose concentrations are different, as defined previously.

[0060] The zeolithization steps are carried out either cold or hot, preferably hot, at a temperature above ambient temperature, and typically between ambient temperature (approximately 20°C) and the boiling point of the alkaline solution, typically between ambient temperature and 100°C and preferably between 60°C and 100°C and even more preferably between 80 and 100°C.

[0061] The duration of each zeolithization stage is generally between a few seconds and a few hours. Generally, for the low-concentration zeolithization stage (stage b1 as previously described), a duration of between a few tens of minutes and a few hours is preferred, for example, between 15 minutes and 8 hours, typically between 30 minutes and 6 hours. For the high-concentration zeolithization stage (stage b2 as previously described), a duration of between a few seconds and a few tens of minutes is preferred, for example, between 30 seconds and 2 hours.

[0062] Each of the zeolithization stages is carried out under agitation or in the absence of agitation.

[0063] However, it is preferable to carry out one or the other, or even both, stages of zeolithization under agitation in order to maintain homogeneity of the reaction medium.

[0064] The intermediate product obtained between each of the zeolithization steps may or may not be isolated. Preferably, the intermediate product is not isolated: the two zeolithization steps are carried out one after the other without isolating the intermediate product.

[0065] Step c) of possible exchange of cations is carried out according to the classic methods known to the person skilled in the art, and most often by bringing the agglomerates from step b) into contact with a salt, such as a chloride, for example barium chloride (BaCl 2) for barium exchange and / or potassium chloride (KCl) for potassium exchange, in aqueous solution at a temperature between ambient temperature and 100°C, and preferably between 80°C and 100°C.

[0066] The activation following drying is carried out in a conventional manner, according to methods known to those skilled in the art, for example, at a temperature generally between 100°C and 400°C, preferably between 200°C and 300°C. This activation step (f) aims to fix the moisture content and the loss on ignition of the adsorbent optimally for the intended use. This is generally achieved through thermal activation, preferably carried out between 200°C and 300°C for a duration determined according to the desired moisture content and loss on ignition, typically from 1 to 6 hours.

[0067] The process of the invention comprises one or more shaping steps which may be carried out using any techniques known to those skilled in the art, such as, for example, extrusion, compaction, agglomeration, and others. Preferably, only one shaping step is carried out during step a) before baking. Optionally, one or more additional shaping steps may be carried out after any one of steps a), b), c), d), e).

[0068] The zeolitic adsorbent in agglomerate form according to the invention is particularly suitable for liquid-phase separation processes, and especially for processes in which said material is subjected to significant mechanical stresses, for example, co-current or counter-current liquid-phase separation processes, and more particularly simulated moving bed liquid-phase separation processes. The zeolitic adsorbent in agglomerate form according to the invention is particularly suitable for liquid-phase xylene separation processes. The zeolitic adsorbent according to the invention can also be used: for the separation of polyhydric alcohols, for the separation of substituted toluene isomers, for the separation of cresols.

[0069] Thus, and according to yet another aspect, the present invention relates to the use of at least one zeolitic adsorbent in the form of agglomerates as just defined, as an adsorbent material in co-current or counter-current liquid or gas separation processes, and more particularly in simulated moving bed liquid separation processes, typically in processes for separating aromatic fractions comprising mixtures of 8-carbon aromatic isomers, and more particularly in simulated moving bed liquid separation processes for xylenes, alone or coupled with a crystallization unit, and especially in processes for recovering para-xylene of high purity from cuts of aromatic isomers with 8 carbon atoms.

[0070] Finally, the invention also relates to the process for separating aromatic fractions comprising mixtures of 8-carbon isomers in liquid or gaseous phase. More particularly, the invention relates to the liquid-phase process for separating xylenes in a simulated moving bed, alone or coupled with a crystallization unit, and especially to the recovery process of para-xylene of high purity from 8-carbon aromatic isomer cuts, as described for example in application WO2009 / 081024, and wherein at least one zeolite adsorbent in agglomerate form as described above is used. The process is carried out in the presence of a desorbent, preferably selected from toluene and the para -diethylbenzene.

[0071] The invention also relates to the gas-phase process for separating xylenes in a simulated moving bed, by adsorption of the para-xylene by means of an adsorbent as described above in the presence of a desorbent, preferably chosen from toluene and the para -diethylbenzene. Preferably, the process is carried out in a simulated moving bed, most preferably in a simulated counter-current manner. Characterization techniques

[0072] The rate of non-zeolitic phase, for example non-zeolithized residual binder or any other amorphous phase, after zeolithization is calculated according to the following equation: PNZ = 100 - Σ (PZ),

[0073] PZ represents the sum of the quantities of the zeolitic fractions X as defined in the invention. The quantity of zeolitic fractions in the zeolitic adsorbent in agglomerate form is measured by X-ray diffraction analysis, known to those skilled in the art as XRD. This analysis is performed on a Bruker instrument, and then the quantity of the zeolitic fractions PZ and their sum Σ(PZ) are evaluated using Bruker's TOPAS software. The non-zeolitic phase, PNZ, is also evaluated by difference: PNZ = 100 - Σ(PZ).

[0074] The size of the zeolite crystal(s) in step a) and of the zeolite crystal(s) in the adsorbents is measured by scanning electron microscopy (SEM), by taking a series of images at a magnification of at least 5000x. The diameter of at least 200 crystals is then measured using dedicated image analysis software. The accuracy is on the order of 3%.

[0075] Zeolite crystals, such as those used in step a) of the process according to the invention, as well as the zeolite adsorbent in agglomerated form, are evaluated with respect to the Si / Al atomic ratio and the cation exchange rate by elemental chemical analysis of the zeolite adsorbent, and more specifically by chemical analysis by X-ray fluorescence as described in the standard NF EN ISO 12677: 2011 on a wavelength dispersive X-ray fluorescence (WDXRF) spectrometer, for example Tiger S8 from Bruker.

[0076] X-ray fluorescence spectroscopy has the advantage of being largely independent of the element's chemical composition, thus providing precise quantitative and qualitative determination. Following calibration, a measurement uncertainty of less than 0.4% by weight is typically obtained for each oxide (SiO₂ and Al₂O₃), as well as for sodium, potassium, and barium oxides. The measurement uncertainty for the Si / Al atomic ratio is ±5%.

[0077] The adsorbent size is determined by analyzing the particle size distribution of an agglomerate sample using imaging according to ISO 13322-2:2006, with a conveyor belt allowing the sample to pass in front of the camera lens. The object size (number-average diameter) is then calculated from the particle size distribution by applying ISO 9276-2:2001. The accuracy is on the order of 0.01 mm for the size range of the zeolitic adsorbent in agglomerate form of the invention.

[0078] The microporous volume V miis determined according to all methods known to those skilled in the art, for example by measuring the adsorption isotherm of a gas at its liquefaction temperature, for example nitrogen, argon, oxygen, and others. Preferably, nitrogen is used. Prior to this adsorption measurement, the zeolite adsorbent in the form of agglomerates of the invention is degassed between 300°C and 450°C for a period of 9 to 16 hours, under vacuum (P < 6.7 x 10⁻⁴ Pa). For example, for a zeolite with FAU structure, the measurement of the nitrogen adsorption isotherm at 77K is then carried out on a Micromeritics ASAP 2020 type device, taking at least 35 measurement points at relative pressures with a P / P0 ratio between 0.002 and 1. The microporous volume is determined according to the Dubinin and Raduskevitch equation from the isotherm obtained, applying the ISO 15901-3:2007 standard.The microporous volume thus evaluated is expressed in cm³ of liquid adsorbent per gram of anhydrous adsorbent. The measurement uncertainty is ± 0.003 g / cm³.

[0079] Macroporous volumes V ma and mesoporous V me grain density dg and porosity ε p Macroporosity and mesoporosity are measured by mercury intrusion porosimetry. A Micromeritics Autopore®< 9500 mercury porosimeter is used to analyze the distribution of pore volume within macropores and mesopores.

[0080] The experimental method, described in the instrument's operating manual referencing ASTM D4284-83, consists of placing a pre-weighed sample of adsorbent (zeolitic adsorbent in agglomerate form to be measured) (with a known loss on ignition) into a porosimeter cell. After initial degassing (a venting pressure of 30 µm Hg for at least 10 min), the cell is filled with mercury at a given pressure (0.0036 MPa). A gradually increasing pressure is then applied up to 400 MPa to progressively penetrate the sample's porous network, using at least 15 pressure steps down to 0.2 MPa, followed by increments of 0.1 MPa up to 1 MPa, then 0.5 MPa up to 10 MPa, then 2 MPa up to 30 MPa, and finally 5 MPa. up to 180 MPa, and finally 10 MPa up to 400 MPa.

[0081] The relationship between the applied pressure and the characteristic dimension of the pore entrance threshold (corresponding to an apparent pore diameter) is established using the Laplace-Young equation and assuming a cylindrical pore opening, a contact angle between the mercury and the pore wall of 140°, and a mercury surface tension of 485 dynes / cm. Volume increments ΔVi of mercury introduced at each pressure stage Pi are recorded, which then allows us to plot the cumulative volume of mercury introduced as a function of the applied pressure V(Pi), or depending on the apparent diameter of the pores V(li).The value at which mercury fills all intergranular voids is set at 0.2 MPa, and it is considered that beyond this pressure, mercury penetrates the pores of the adsorbent. The grain volume (Vg) is then calculated by subtracting the cumulative volume of mercury at this pressure (0.2 MPa) from the volume of the porosimeter cell, and dividing this difference by the mass of the equivalent anhydrous adsorbent, that is, the mass of said material corrected for loss on ignition. The grain density dg is the inverse of the grain volume Vg defined previously.

[0082] The macroporous volume Vma The volume of the adsorbent is defined as the cumulative volume of mercury introduced at a pressure between 0.2 MPa and 30 MPa, corresponding to the volume contained in pores with an apparent diameter greater than 50 nm. The mesoporous volume VmeThe adsorbent volume is defined as the cumulative volume of mercury introduced at a pressure between 30 MPa and 400 MPa. Since the mercury intrusion method for measuring pore volume does not allow access to the microporous volume, the total pore volume Vtot, as measured by mercury intrusion, corresponds to the sum of the macroporous volume Vma and the mesoporous volume Vme.

[0083] In this document, macroporous and mesoporous volumes V ma And V me , as well as their sum (total porous volume V tot The density of zeolitic adsorbents, expressed in cm³ .g⁻¹, is thus measured by mercury intrusion porosimetry and related to the anhydrous equivalent mass of the sample, i.e., the mass of said adsorbent corrected for loss on ignition. The grain density dg is expressed in g.cm -3< and refers to the mass of the sample in anhydrous equivalent.

[0084] Porosity ε pof the macroporosity and mesoporosity type, is the product of grain density dg by the sum of the macroporous and mesoporous volumes Vma And Vme: ε p = dg x ( V ma + V me ).

[0085] The tortuosity factor τ is determined by a standard measurement detailed in the Autopore® user manual and modified for the case of microporous materials. The measurement is based on the following equation, from Carniglia in "Construction of the tortuosity factor from porosimetry" J. of Catalysis 102, 401-418 (1986): τ = 2.23 − 1 , 13 ⋅ V tot ⋅ d g 0 , 92 4 S S ∑ Pi = 0 , 2 MPa Pdc Δ Vi li 1 + δ

[0086] The parameters δ and S are two user parameters: δis an exponent representing the diffusion mechanism: it is equal to 0 or 1 depending on whether the transport properties of zeolite adsorbents are being evaluated for use in liquid-phase or gas-phase processes, respectively. Ss is the surface area of ​​macropores and mesopores, expressed in m² per gram of anhydrous sample equivalent. The user manual suggests assigning to the parameter Ss the value of the BET specific surface area measured by nitrogen adsorption porosimetry. However, in the case of the materials according to the invention, obtained by agglomerating individual zeolite crystals, the BET specific surface area includes not only the surface area of ​​macropores and mesopores, but also an equivalent surface area representing adsorption in micropores.In the present invention, the tortuosity factor is therefore determined using a user parameter Ss adapted to the case of agglomerates with a zeolitic structure as follows: the parameter Ss can be approximated by the external surface area of ​​the constituent crystals of the adsorbent relative to their mass. For faujasite zeolite crystals, for which the volume-to-external surface area ratio is assumed to be equal to the diameter of the crystals divided by 6 (the same ratio as for a sphere), the surface parameter Ss is calculated according to the following equation 3: . S S = 6 ⋅ V mi f ⋅ d cristaux where f is the fraction of empty space in the crystals, and crystalsis the number-average diameter of the zeolite crystals in the zeolite adsorbent measured by scanning electron microscopy (SEM). The value of f is taken as 0.5 in the case of a NaX zeolite with a Si / Al ratio of 1.25 (Table 5.9, p. 429 of the book "Zeolite Molecular Sieves" by DW Breck, John Wiley and Sons, New York, (1973)) and this value will be retained in the case of zeolite adsorbent after exchange of sodium cations with barium and / or potassium cations, because this volume ratio is not modified during the exchange with this type of cation.

[0087] In equation 2, the summation is performed for mercury intrusion pressures between 0.2 MPa and a pressure Pdc corresponding to an apparent pore entry threshold diameter Idc equal to: Idc= 25 times the molecular radius of the molecules that will be transported in the zeolite adsorbent for use in liquid mixture separation, i.e., 17 nm in the context of the invention applied to xylene molecules, that is to say, a pressure Pdc equal to 87 MPa. Idc =5 times the mean free path of the gas, the zeolitic adsorbent for use in gas mixture separation.

[0088] The measured mechanical resistance is the bed crushing strength (REL) characterized according to the Shell Method Series SMS1471-74 (Shell Method Series SMS1471-74 “Determination of Bulk Crushing Strength of Catalysts. Compression-Sieve Method”), associated with the “BCS Tester” device marketed by Vinci Technologies.

[0089] This method for measuring REL, initially designed for characterizing catalysts from 3 mm to 6 mm in size, is based on the use of a 425 µm sieve, which allows, in particular, the separation of fines created during crushing. The use of a 425 µm sieve remains suitable for particles with a diameter greater than 1.6 mm, but must be adapted according to the particle size distribution of the material being characterized. For the zeolite adsorbent in agglomerate form of the present invention, a 200 µm sieve is used instead of the 425 µm sieve mentioned in the standard Shell method SMS1471-74.

[0090] The measurement protocol is as follows: a 20 cm³ sample of the material to be analyzed, previously sieved with a suitable 200 µm sieve and oven-dried for at least 2 hours at 250°C (instead of the 300°C mentioned in the standard Shell method SMS1471-74), is placed in a metal cylinder of known internal cross-section. Increasing forces are applied to this sample in stages via a piston, through a 5 cm³ bed of steel balls to better distribute the force exerted by the piston on the material (using 2 mm diameter balls for spherical particles with a diameter strictly less than 1.6 mm). The fines obtained at the different pressure stages are separated by sieving (using a suitable 200 µm sieve) and weighed.

[0091] The REL (Resistance to Efficiency of the Sieve) is determined by the pressure in megapascals (MPa) at which the quantity of accumulated fines passing through the sieve reaches 0.5% by weight of the sample. This value is obtained by plotting the mass of fines obtained as a function of the force applied to the zeolite bed and interpolating to 0.5% by mass of accumulated fines. The mechanical crush resistance in the bed is typically between a few hundred kPa and a few tens of MPa, and generally between 0.3 MPa and 3.2 MPa. The accuracy is typically less than 0.1 MPa.

[0092] The following examples illustrate the object of the invention. They are provided for illustrative purposes only and are not intended in any way to limit the various embodiments of the present invention. Example A: Preparation of zeolite adsorbents based on zeolite X Synthesis of zeolite X crystals, with an atomic ratio of Si / Al = 1.25, an average number diameter of 1.0 µm and an atomic ratio of Na / Al = 1.

[0093] A gel with a molar composition of 3.5 Na₂O - 2.8 SiO₂ - Al₂O₃ - 130 H₂O is prepared by mixing the following reagents: sodium silicate, sodium aluminate, and water. The gel is matured at 35°C for 20 hours, and crystallization is carried out for 4 hours at 100°C.

[0094] The crystals obtained after filtration and washing were identified by X-ray diffraction (XRD analysis) as faujasite crystals. Chemical analysis of the solid yielded a Si / Al atomic ratio of 1.25. The microporous volume, evaluated according to the Dubinin-Raduskevich equation as described in the technical characterization section and expressed in cm³ per gram of dry adsorbent, was 0.345 ± 0.003 cm³ / g. Analysis of the zeolite crystal size by scanning electron microscopy showed that their number-average diameter (d crystals) was 1.0 µm. Preparation of zeolitic adsorbents

[0095] A homogeneous mixture is prepared, and 800 g of NaX zeolite crystals, prepared according to the procedure described above, are agglomerated with 105 g of kaolin (expressed as calcined equivalent) and 45 g of colloidal silica sold under the trade name Klebosol™ < 30N50 (containing 30% by weight of SiO₂ and 0.5% by weight of Na₂O) with enough water to allow the mixture to be extruded. The extrudates are dried, then calcined at 550°C (clay firing) under a stream of nitrogen for 2 hours, and finally crushed to obtain agglomerates with an average number diameter of 0.5 mm. Example 1 (comparative):

[0096] 20 g of agglomerates obtained as described above from the zeolite crystals X synthesized in example A are placed in a glass reactor with a double jacket regulated at a temperature of 100°C ± 1°C then 150 mL of an aqueous sodium hydroxide solution of concentration of 2.5 M is added, and the reaction medium is left under stirring for a period of 4 to 5 hours.

[0097] The agglomerates are then washed in three successive water washing operations followed by draining the reactor. The effectiveness of the washing is verified by measuring the final pH of the wash water, which is between 10.0 and 10.5.

[0098] These agglomerates are exchanged by contacting them with a 0.5 M barium chloride solution at 95°C in four stages. At each stage, the solution volume-to-solid mass ratio is 20 mL / g, and the exchange is carried out for four hours each time. Between each exchange, the solid is washed several times to remove excess salt. The agglomerates are then dried at 80°C for two hours and finally activated at 250°C for two hours under a stream of nitrogen.

[0099] The porosity distribution and mechanical strength (REL) of the agglomerates are characterized by the characterization techniques described above. The results are given in Table 1 below. The values ​​of the porosity εp, calculated according to Equation 2 taking δ=0, and of the tortuosity factor τ are given in Table 2 below.

[0100] The barium exchange rates of the agglomerates, calculated from the elemental analyses of barium and sodium oxides by X-ray fluorescence, as described in the characterization techniques, are 99.6 ± 0.2%. The loss on ignition, measured as described previously, is 5.2% ± 0.1%. Example 2 (according to the invention) : Step 1 :

[0101] 20 g of agglomerates obtained from the powder synthesized in example A are placed in a glass reactor with a double jacket regulated at a temperature of 100°C ± 1°C, then 100 mL of an aqueous sodium hydroxide solution of concentration 0.5 M is added and the reaction medium is left under stirring for 3 hours. 2nd step :

[0102] After 3 hours, 100 mL of a previously prepared 5.5 M sodium hydroxide solution, maintained at a temperature of 100°C ± 1°C, is added to this solution to obtain a sodium hydroxide concentration of 3 M in the reaction mixture. The reaction mixture is then stirred for 1 hour.

[0103] The agglomerates are then washed in three successive water washing operations followed by draining the reactor. The effectiveness of the washing is verified by measuring the final pH of the wash water, which generally remains between 10.0 and 10.5.

[0104] These agglomerates undergo a cation exchange reaction by contact with a 0.5 M aqueous barium chloride solution at 95°C in four steps. At each step, the solution volume-to-solid mass ratio is 20 mL / g, and the exchange is carried out for four hours each time. Between each exchange, the solid is washed several times to remove excess salt, for example, four 50 mL washes with water. The agglomerates are then dried at 80°C for two hours and finally activated at 250°C for two hours under a stream of nitrogen.

[0105] The products are characterized in order to determine by all the analytical techniques described above: the Dubinin-Raduskevitch volume is 0.260 cm3 / g and the REL is 3.1 MPa.

[0106] The measured loss on ignition, as described previously, is 5.2% ± 0.1% for each sample. The barium exchange rates of the agglomerates, calculated from the elemental analyses of barium and sodium oxides by X-ray fluorescence, as described in the characterization techniques, is 99.4 ± 0.2%. Table 1 REL (MPa) Vg (cm³ / g) Vmi (cm³ / g) Vme (cm³ / g) Vma (cm³ / g) Vme / (Vme+Vma) Vmi / (Vmi + Vme + Vma) Example 1 (comparative) 2,62 0,840 0,258 0,014 0,261 5,1% 48% Example 2 (according to the invention) 3,10 0,800 0,260 0,008 0,212 3,5% 55% Table 2 Surface parameter Ss of equation 2 (calculated by equation 3 with f=0.5) Tortuosity factor τ (equation 2 with δ=0 and Pdc=87MPa) Porosity εp Example 1 (comparative) 3,3 3,2 32% Example 2 (according to the invention) 3,5 2,1 28% Example 3 : Xylene separation test (drilling test)

[0107] A penetration test (frontal chromatography) is then performed on the adsorbents described in examples 1 and 2 to evaluate their effectiveness. The quantity of adsorbent used for these tests is 72 g.

[0108] The procedure for obtaining drilling curves is as follows: The column is filled with the sieve and placed in the test bench. It is then filled with the solvent (paradiethylbenzene) at room temperature. The flow rate is gradually increased to the adsorption temperature under a solvent flow rate of 5 cm³ / min. Solvent is injected at 20 cm³ / min when the adsorption temperature is reached. The solvent / feed mixture is then switched to inject the feed (20 cm³ / min). Feed injection is maintained until the solvent concentration in the effluent reaches zero, thus achieving thermodynamic equilibrium.

[0109] The pressure is sufficient to keep the charge in the liquid phase, i.e., 1 MPa. The adsorption temperature is 175°C.

[0110] The composition of the charge is as follows: Paraxylene: 45% by weight; Metaxylene: 45% by weight; Isooctane: 10% by weight (this is used as a tracer for estimating non-selective volumes and does not participate in the separation) Table 3 Nature of the solid PAF (1) < 950°C Capacity (2)< Selectivity (3)< α PX / MX HEPT iC8 (4)< HEPT PX (5)< Ex. 1 (comparative) 5,2% 0,211 3,57 3,3 7,1 Ex. 2 5,2% 0,218 3,63 2,8 6,1 (1) PAF: Loss on Ignition (2) Capacity is expressed in cm³ of C8-aromatics adsorbed per gram of adsorbent (3) PX: paraxylene, MX: metaxylene (4) HEPT iC8: Theoretical plateau height expressed in cm from the isooctane drilling front (5) HEPT PX: Theoretical plateau height expressed in cm from the paraxylene drilling front

[0111] The results in Table 3 show that agglomerates according to the invention lead to a lower theoretical platform height and consequently to better productivity and better purity of the expected product. Example 4 (comparative): 1st step :

[0112] 20 g of agglomerates obtained from the powder synthesized in example A are placed in a glass reactor with a double jacket regulated at a temperature of 100°C ± 1°C, then 200 mL of an aqueous sodium hydroxide solution of concentration 0.5 M is added and the reaction medium is left under stirring for 4 hours.

[0113] The agglomerates are then washed in three successive water washing operations followed by draining the reactor. The effectiveness of the washing is verified by measuring the final pH of the wash water, which generally remains between 10.0 and 10.5.

[0114] These agglomerates undergo a cation exchange reaction by contact with a 0.5 M aqueous barium chloride solution at 95°C in four steps. At each step, the solution volume-to-solid mass ratio is 20 mL / g, and the exchange is carried out for four hours each time. Between each exchange, the solid is washed several times to remove excess salt, for example, four 50 mL washes with water. The agglomerates are then dried at 80°C for two hours and finally activated at 250°C for two hours under a stream of nitrogen.

[0115] The products are characterized. The Dubinin-Raduskevitch volume is 0.250 cm³ / g. Example 5 (comparative) : 1st step :

[0116] 20 g of agglomerates obtained from the powder synthesized in example A are placed in a glass reactor with a double jacket regulated at a temperature of 100°C ± 1°C, then 200 mL of an aqueous sodium hydroxide solution of concentration 3 M is added and the reaction mixture is left under stirring for 4 hours.

[0117] The agglomerates are then washed in three successive water washing operations followed by draining the reactor. The effectiveness of the washing is verified by measuring the final pH of the wash water, which generally remains between 10.0 and 10.5.

[0118] These agglomerates undergo a cation exchange reaction by contact with a 0.5 M aqueous barium chloride solution at 95°C in four steps. At each step, the solution volume-to-solid mass ratio is 20 mL / g, and the exchange is carried out for four hours each time. Between each exchange, the solid is washed several times to remove excess salt, for example, four 50 mL washes with water. The agglomerates are then dried at 80°C for two hours and finally activated at 250°C for two hours under a stream of nitrogen.

[0119] The products are characterized. The Dubinin-Raduskevitch volume is 0.240 cm³ / g. Table 4 Surface parameter Ss of equation 2 (calculated by equation 3 with f=0.5) Tortuosity factor τ (equation 2 with δ=0 and Pdc=87MPa) Porosity εp Example 4 (comparative) 3,2 3,3 36% Example 5 (comparative) 3,1 3,4 36%

Claims

1. Zeolite-based adsorbent in the form of agglomerates, said adsorbent having: - a tortuosity factor τ, of formula : τ = 2.23 − 1 , 13 − V tot − d g 0 , 92 4 S S ∑ Pi − 0.2 MPa Pck Δ Vi li 1 + 4 wherein Vtot represents the total pore volume, corresponding to the sum of the macropore and mesopore volumes Vma and Vme expressed in cm3.g-1, Vtot, Vma and Vme being measured by mercury intrusion porosimetry and related to the mass of the sample as anhydrous equivalent, δ = 0 for liquid phase or 1 for gaseous phase, Ss is the surface area of the pores of macropore and mesopore type, expressed in m2 per gram of sample as anhydrous equivalent, calculated according to the following equation 3 : S S = 6 ⋅ V mi f ⋅ d cristaux in which f is the void fraction in the crystals, dcrystals is the number-mean diameter of the zeolite crystals of the zeolite-based adsorbent measured by observation with a scanning electron microscope (SEM), and Vmi denoting the micropore volume expressed in cm3.g-1, determined by nitrogen adsorption, the micropore volume being evaluated by means of the Dubinin-Raduskevitch equation, from the isotherm obtained, by applying the standard ISO 15901-3:2007, the micropore volume corresponding to pores whose aperture is less than 2 nm, ΔVi is the volume increment of mercury and li is the apparent pore diameter, dg is the grain density expressed in g.cm-3 and refers to the mass of the sample as anhydrous equivalent, Pdc = 87 MPa, the tortuosity factor being calculated from the pore distribution determined by mercury intrusion porosimetry, strictly greater than 1 and strictly less than 3; the tortuosity factor being determined by a standard measure detailed in the user manual of Autopore® 9500 mercury porosimeter from Micromeritics ; - a porosity ε p = Vma + Vme Vg determined by mercury intrusion porosimetry, in which Vma denotes the macropore volume, Vmedenotes the mesopore volume and Vg denotes the grain volume, of between 25% and 35%, the volumes being expressed in cm3.g-1 ; the macropores volume corresponding to pores volume whose aperture is greater than 50 nm, the mesopores volume corresponds to pores volume whose aperture is between 2 nm and 50 nm, limits not inclusive, said adsorbent comprising a zeolite chosen from the zeolites of FAU structure, the porosimetry measurements by mercury intrusion being performed according to the standard ASTM D4284-83.

2. Zeolite-based adsorbent according to claim 1, in which the tortuosity factor τ is between 1.5 and 2.7.

3. Zeolite-based adsorbent according to claim 1 or 2, wherein the high mechanical bulk crushing strength (BCS), measured via the Shell method series SMS1471-74 adapted for agglomerates less than 1.6 mm in size, is greater than or equal to 1.0 MPa, the size means the number-mean diameter of the adsorbent or its number-mean largest dimension when it is not spherical.

4. Zeolite-based adsorbent according to any one of claims 1 to 3, wherein said adsorbent has a size of between 0.1 mm and 1 mm, limits inclusive, the size means the number-mean diameter of the adsorbent or its number-mean largest dimension when it is not spherical, the determination of the size being performed by particle size distribution analysis on a sample of agglomerate by imaging according to the standard ISO 13322-2:2006, using a conveyor belt for passing the sample before the objective lens of the camera, the size of the object being calculated from the particle size distribution by applying the standard ISO 9276-2:2001.

5. Zeolite-based adsorbent according to any one of claims 1 to 4, said zeolite being zeolite X, alone or as a mixture with other zeolites.

6. Zeolite-based adsorbent according to any one of claims 1 to 5, wherein, said adsorbent comprising more than 90% by weight of zeolite(s).

7. Zeolite-based adsorbent according to any one of claims 1 to 6, wherein said zeolite(s) are in the form of crystals between 10 nm and 1500 nm in size, the size corresponding to the number-mean diameter of the adsorbent or its number-mean largest dimension when it is not spherical, the size being measured by observation with a scanning electron microscope (SEM).

8. Zeolite-based adsorbent according to any one of claims 1 to 7, wherein the pore distribution satisfies the inequalities a) and / or b) below: a) Vme Vme + Vma ≤ 0 , 1 , b) 0 , 4 ≤ Vmi Vma + Vme + Vmi . Vmi denoting the micropore volume expressed in cm3.g-1, determined by nitrogen adsorption, the micropore volume corresponding to pores whose aperture is less than 2 nm.

9. Zeolite-based adsorbent according to any one of claims 1 to 8, further comprising barium and / or potassium.

10. Process for preparing a zeolite-based adsorbent according to any one of claims 1 to 9, comprising at least following steps: a) a step of mixing crystals of at least one zeolite with an agglomeration binder containing at least 80%, preferably at least 90%, more preferably at least 95% by weight of zeolitizable clay, followed by forming and a firing step at a temperature of between 500 and 700° C, b1) a first step of zeolitization by placing the material obtained in step a) in contact with an alkaline basic solution, with a concentration of between 0.2 M and 0.9 M, limits inclusive, b2) a second step of zeolitization by placing the material obtained in step b1) in contact with an alkaline basic solution, with a concentration of between 1.2 M and4.0 M, limits inclusive, b1 and b2 possibly being performed in any order and b1 and / or b2 possibly being repeated, d) a step of washing and drying the material thus obtained, and e) a step of activation of the material obtained in step d), by heating to a temperature of between 100° C. and 400° C, and recovery of the zeolite-based adsorbent in the form of agglomerates.

11. Preparation process according to claim 10, comprising a step c) of cationic exchange of the cations contained in the reaction medium obtained from the zeolitization steps by placing in contact with a solution of barium ions or of barium ions and potassium ions, this step being performed between step b2) et d) or between steps b1) and d).

12. Preparation process according to claims 10 or 11, comprising one or more additional forming steps performed after any one of steps a), b1) / b2), optionally c), d) or e).

13. Preparation process according to any one of claims 10 to 12, wherein a source of silica is present in the mixture of crystals of at least one zeolite, with the agglomeration binder in step a).

14. Process for separating para-xylene from aromatic hydrocarbon isomer fractions containing 8 carbon atoms, in the liquid phase or in the gaseous phase, by absorption of para-xylene using an adsorbent according to anyone of the claims 1 to 9, in the presence of a desorbent.

15. Process according to claim 14, of simulated moving bed type