The invention relates to adsorbents based on agglomerated crystals of zeolite X comprising barium and strontium.

These adsorbents can be used more particularly for the production in liquid phase or gas phase of very pure para-xylene from an aromatic hydrocarbon feedstock containing isomers with 8 carbon atoms.

[0003] The high purity paraxylene market is considered to be a rapidly expanding market, its outlets being mainly the production of terephthalic acid (PTA) obtained by oxidation of paraxylene, the origin of the polyester fibers used for clothing and polyethylene terephthalate (PET) resins and films.

[0004] High purity para-xylene is today most often obtained produced by upgrading xylenes according to a process called "C8-aromatic loop", including separation steps (elimination of heavy compounds in the "column of xylenes ”, extraction of paraxylene) and isomerization of xylenes. The extraction of high purity paraxylene by selective adsorption is moreover well known from the prior art.

[0005] The technological background describing the production of very high purity paraxylene is illustrated in patent FR2681066 (Institut Français du Pétrole), and is based on the separation of paraxylene from a feed of essentially aromatic hydrocarbons. with 8 carbon atoms within an adsorber by contact with a bed of zeolitic adsorbent in the presence of a suitable desorption solvent (desorbent).

[0006] It is also known in the prior art that zeolitic adsorbents comprising at least Faujasite zeolite (FAU) of type X or Y and comprising, in addition to sodium cations, barium, potassium or strontium ions, alone or in mixtures, are effective in selectively adsorbing paraxylene in a mixture of aromatic hydrocarbons.

[0007] Patents US3558732 and US4255607 show that zeolitic adsorbents comprising aluminosilicates based on potassium or based on barium, or else based on potassium and barium, are effective for the separation of para-xylene present in aromatic cuts C8 (cuts comprising aromatic hydrocarbons with 8 carbon atoms).

[0008] Patent US3997620 presents agglomerates, in which the agglomeration binder is not zeolithized, said agglomerates being exchanged with barium and strontium, so that the Ba / Sr weight ratio is between 1: 1 and 15 : 1.

[0009] More particularly, this document teaches that adsorbents based on type X or type Y zeolites and comprising barium and strontium, at their exchangeable cationic sites, with a barium / strontium weight ratio of approximately 1: 1 to about 15: 1, can be advantageously used in a para-xylene recovery process. With this ratio, the advantage observed is an improved selectivity of para-xylene over the desorbent, para-diethylbenzene, but also an improved selectivity of para-xylene over other xylene isomers.

Apart from this document showing a certain advantage, having regard to selectivity, for agglomerates containing large proportions of strontium, the fact remains that in the scientific literature, as in the patent literature, the documents mainly teach that zeolites comprising barium cations or barium and potassium cations prove to be particularly effective in terms of selectivity for the separation of isomers of aromatic hydrocarbons with 8 carbon atoms. The industrial zeolites used in these isomer separation applications are conventionally prepared from zeolites comprising sodium as a compensating cation, then the sodium ions are exchanged with barium ions or barium and potassium ions.

These exchange operations are usually carried out from an aqueous solution of barium halide, and in particular of barium chloride (BaCh). However, the costs of barium chloride of very high purity, as required for the synthesis of said zeolites, being high, the exchange with this very pure salt proves to be less and less competitive for large-scale zeolite preparations.

In fact, barium chloride is most often obtained from ores, such as for example barite or benstonite, these ores generally comprise other alkali or alkaline earth metals including strontium, in particular. Consequently, the various sources of barium ions, for example in the form of barium chloride, available commercially, and when they are not of very high purity, can contain non-negligible amounts of impurities in the form of strontium salts. , for example of the order of 500 ppm by weight to a few percent by weight of strontium chloride.

The costs of sources of barium in the form of very high purity barium chloride as well as the environmental impact of barium chloride purification operations lead to finding alternatives which overcome these problems, while making it possible to maintain maximum efficiency, in particular in terms of selectivity and productivity, when the agglomerates exchanged with barium are used for the recovery of para-xylene in C8 aromatic cuts.

In addition, it is always advantageous to be able to have several suppliers of sources of barium, in particular of barium chloride, while being able to dispense with drastic conditions of purity, and to have available different sources, some of which may s 'prove to be less "pure" in terms of barium contents, than others which contain varying amounts of strontium chloride.

It has now been discovered that the aforementioned objectives can be achieved, in whole or at least in part, by virtue of the present invention which is now set out in the description which follows. Still other objectives may appear in the detailed description below.

Thus, and according to a first aspect, the present invention relates to an adsorbent based on agglomerated zeolite crystals (s), said agglomerate comprising:

- at least crystals of FAU zeolite (s), with an Si / Al molar ratio of between 1.00 and 1.50, limits included,

- a content by weight of barium ions (Ba 2+ ), expressed by weight of barium oxide (BaO), strictly greater than 30%, preferably strictly greater than 33%, more preferably a content of between 34% and 42 %, limits included, and quite preferably between 34% and 40%, limits included, relative to the total weight of the adsorbent,

- a weight content of strontium ions (Sr 2+ ), expressed in weight of strontium oxide (SrO), strictly greater than 0.1% and strictly less than 3%, preferably between 0.15% and 2, 9%, limits included, more preferably between 0.15% and 2.5% and quite preferably between 0.15% and 2.4%, limits included, relative to the total weight of the adsorbent.

In the present invention, it should be understood that the weight contents expressed by weight of oxides are expressed relative to the total weight of the anhydrous adsorbent (weight corrected for loss on ignition).

The adsorbents according to the invention can also comprise a non-zeolitic phase, that is to say a non-crystalline phase which is essentially inert with respect to adsorption, as explained below. In the case where the adsorbent according to the invention comprises such a non-zeolitic phase, the oxide contents defined above take account of the oxides included in said non-zeolitic phase.

The adsorbent according to the present invention is an adsorbent based on crystals of X-type FAU zeolite (s). The term "X-type FAU zeolite" means zeolites whose Si / Al atomic ratio is between 1, 00 and 1, 50 included, preferably between 1, 00 and 1, 45, more preferably between 1, 05 and 1, 45, limits included and even more preferably between 1, 10 and 1, 45 included.

Among the X zeolites, it is now commonly accepted to recognize two subgroups called LSX zeolites and MSX zeolites. The LSX zeolites have an Si / Al atomic ratio equal to approximately 1 and the MSX zeolites have an Si / Al atomic ratio of between approximately 1.05 and approximately 1.15, limits included.

The definition of FAU zeolite also includes the FAU type X zeolites defined above with hierarchical porosity, that is to say the type X zeolites with hierarchical porosity (or XPH zeolite), the MSX type zeolites with hierarchical porosity (or MSXPH) and LSX type zeolites with hierarchical porosity (or LSXPH), and more particularly FAU zeolites with hierarchical porosity and Si / Al atomic ratio between 1.00 and 1.50, limits included, preferably between 1.05 and 1.50, more preferably between 1.05 and 1.40, limits included, and even more preferably, between 1.15 and 1.40, limits included.

By "zeolite with hierarchical porosity" is meant a zeolite having both micropores and mesopores, in other words a zeolite that is both microporous and mesoporous. By “mesoporous zeolite” is meant a zeolite whose microporous zeolite crystals exhibit, together with the microporosity, internal cavities of nanometric size (mesoporosity), easily identifiable by observation by means of a Transmission Electron Microscope (TEM or “TEM »In English), as described for example in US7785563: observation by transmission electron microscopy (TEM) makes it possible to verify whether the zeolite crystals are solid zeolite crystals (ie non-mesoporous) or aggregates of solid zeolite crystals or mesoporous crystals or aggregates of mesoporous crystals.

[0023] The adsorbent according to the invention also encompasses adsorbents comprising mixtures of two or more FAU zeolites as they have just been defined, but also comprising mixtures of one or more FAU zeolites with one or more other zeolites well known to those skilled in the art. However, adsorbents based on FAU type X zeolite crystals, as defined above, are preferred.

The structure of the zeolite crystals (s) in the adsorbent of the present invention is easily identifiable by any method well known to those skilled in the art and in particular by X-ray diffraction (also called "XRD analysis" ).

According to a preferred embodiment, the zeolitic adsorbent has an Si / Al atomic ratio of between 1.00 and 2.00, preferably between 1.00 and 1.80 limits inclusive, more preferably between 1.15 and 1.80, limits included and even more preferably between 1.15 and 1.60, limits included.

In this document, the term "number-average diameter" or "size" is used for the zeolite crystals (s) and for the adsorbent according to the invention. The method of measuring these quantities is explained later in the description. According to a preferred embodiment of the present invention, the number-average diameter of the zeolite crystals (s) is less than or equal to 1.5 μm, preferably between 0.1 μm and 1.2 μm, moreover preferred between 0.1 µm and 1.0 µm, limits included.

According to a preferred embodiment, the total weight content of alkali or alkaline-earth ions other than Ba 2+ and Sr 2+ , expressed by weight of alkali or alkaline-earth oxides respectively, is less at 5% and preferably is between 0% and 2% and advantageously between 0% and 1%, limits included, relative to the total weight of the adsorbent.

[0028] In the context of the present invention, adsorbents comprising a weight content of sodium ions (Na + ), expressed by weight of sodium oxide (Na 2 0), strictly less than 0.3%, are also preferred , preferably strictly less than 0.2%, relative to the total weight of the adsorbent.

According to yet another preferred embodiment of the invention, the adsorbent comprises a content of potassium ions (K + ), expressed by weight of potassium oxide (K2O), less than 9%, preferably less at 8%, better still less than 5%, and more preferably between 0% and 2%, advantageously between 0% and 1%, limits included, relative to the total weight of the adsorbent.

Preferably, the total weight content of alkali or alkaline earth ions other than Ba 2+ , K + and Sr 2+ , expressed by weight of alkali or alkaline earth oxides respectively, is less than 5 % and preferably is between 0% and 2% and advantageously between 0% and 1%, limits included, relative to the total weight of the adsorbent.

According to one embodiment of the present invention, the barium / strontium weight ratio in the adsorbent is greater than 15: 1, typically between 15: 1 and 400: 1, preferably between 16: 1 and 300 : 1, more preferably between 16: 1 and 200: 1.

The adsorbent of the present invention is preferably in the form of agglomerates, that is to say it consists of crystals of zeolite (s) and at least one non-zeolitic phase comprising at least less an agglomeration binder allowing the cohesion of the crystals between them.

The agglomeration binder can be zeolite. It then contains at least 80%, preferably at least 90%, more preferably at least 95%, more particularly at least 96%, by weight, of zeolitisable clay and can also contain other mineral binders such as bentonite, attapulgite, and others. By zeolitisable clay is meant a clay or a mixture of clays which are capable of being converted into zeolitic material (that is to say active material in the sense of adsorption), most often by the action of a basic alkaline solution. Zeolitisable clay generally belongs to the family of kaolins, kaolinites, nacrites, dickites, halloysite and / or metakaolins. Kaolin is preferred and most commonly used.

Other clays such as in particular sepiolite or attapulgite can also be used. In all cases, the clays can be used in their raw state or can be subjected beforehand to one or more treatments, for example chosen from calcination, treatment with acid, chemical modification, and others.

[0035] Thus, the adsorbent according to the present invention comprises crystals of zeolite (s), and at least one non-zeolitic phase (PNZ), that is to say a non-crystalline phase which is essentially inert towards -vis of adsorption. The degree of crystallinity of the adsorbent according to the invention is measured by X-ray diffraction analysis, known to those skilled in the art by the acronym DRX, as indicated below. It is generally greater than 80%, that is to say the PNZ is less than 20%.

In a preferred embodiment of the present invention, the weight content of non-zeolitic phase (PNZ) is less than 15%, preferably less than 10%, more preferably less than 5%. According to an entirely preferred aspect, the PNZ of the adsorbent is between 0 and 15%, preferably between 0 and 10%, more preferably between 0 and 5%, limits not included, very preferably between between 0.1% and 5%, better still between 0.5% and 5% and very particularly preferably between 2% and 5%, limits not included, relative to the total weight of the anhydrous adsorbent (weight corrected by loss on ignition).

The number-average diameter of the adsorbent according to the invention is advantageously and most often between 0.2 mm and 2 mm, more particularly between 0.2 mm and 0.8 mm and preferably between 0, 2 mm and 0.65 mm, terminals included.

According to a preferred embodiment, the adsorbent according to the invention has a loss on ignition measured at 950 ° C according to standard NF EN 196-2 of between 4.0% and 7.7%, preferably between 4 , 5 and 6.5% and advantageously between 4.8 and 6%, limits included.

The adsorbent according to the present invention preferably has a mechanical strength generally greater than or equal to 1.8 MPa, typically greater than or equal to 2.1 MPa. This mechanical resistance is measured by the Shell method SMS1471-74 series suitable for agglomerates of size less than 1.6 mm.

The adsorption capacity of the adsorbent according to the invention is for its part measured by measuring the microporous volume of the adsorbent evaluated according to the Dubinin equation-

Raduskevich by adsorption of nitrogen (N 2 ) at a temperature of 77K, after degassing under vacuum at 300 ° C for 16 hours. The microporous volume of the adsorbent of the invention is thus measured as being generally strictly greater than 0.245 cm 3 . g 1 , preferably strictly greater than 0.250 cm 3 . g 1 , more preferably between 0.250 cm 3 . g 1 and 0.300 cm 3 . g 1 , terminals not included.

The adsorbent according to the present invention, which advantageously comprises a reduced content, or even very reduced, in non-zeolitic phase, that is to say essentially inert vis-à-vis the adsorption, is an all adsorbent. very efficient for the separation of xylenes and the manufacturing costs of which are reduced compared to adsorbents which are equivalent in terms of adsorption and known from the prior art.

The reduction in manufacturing costs of these adsorbents is mainly obtained by using a barium salt of lower purity than that usually used to carry out the cation exchange.

[0043] The adsorbent according to the invention can in fact be obtained according to any technique well known to those skilled in the art, and for example as described in application WO2014090771. The adsorbent synthesis processes, and in particular those intended for the separation of xylenes, generally comprises a step of agglomeration of zeolite crystals with a binder, most often a clay, a shaping step, then drying and calcination, and finally a cation exchange step by bringing the agglomerate into contact with a solution of alkali metal and / or alkaline earth ions.

In preferred embodiments, the agglomerate, before and / or after cation exchange, is subjected to one or more zeolitization steps well known to those skilled in the art, as described for example in WO2014090771, it is that is to say, converting all or at least part of the agglomeration binder into a zeolitic crystalline fraction, active in the sense of adsorption.

In the case of the separation of xylenes, the optimal cation exchange recognized in this technical field consists in exchanging all or at least the majority of the sodium ions present in the starting zeolite crystals (s), by barium ions or barium ions and potassium ions. This ion exchange can be carried out equally well on the zeolite crystals (s) or on the agglomerates of zeolite crystals (s) with the agglomeration binder, before and / or after optional zeolite, preferably after zeolite, of said binder.

The steps of exchanging the cations of the zeolite fractions are carried out according to conventional methods known to those skilled in the art, and most often by bringing the crystals of zeolite (s), agglomerated or not, into contact with a binder , before and / or after optional zeolitization, preferably after zeolitization, of said binder, with a salt of the cation to be exchanged, such as barium chloride (BaCh) for exchange with barium and / or potassium chloride (KCl) for exchange with potassium, in solution, most often in aqueous solution, at a temperature between room temperature and 100 ° C, and preferably between 80 ° C and 100 ° C.

To quickly obtain low sodium oxide contents, as described above, it is preferred to operate with a large excess of barium and / or potassium ions relative to the cations of the zeolite that one wishes to exchange, typically an excess of the order of 10 to 12, advantageously by proceeding by successive exchanges.

As indicated previously, this cation exchange is most often carried out with an aqueous, organic or hydro-organic solution, preferably an aqueous solution, comprising barium ions. The barium ion solutions used generally have a Ba 2+ ion concentration which can vary between 0.2 M and 2 M.

It has now been discovered quite surprisingly that solutions of barium ions which may contain a level of impurities, in the form of strontium salts, which may range up to 4% by weight, make it possible to obtain adsorbents exhibiting adsorption performance, in particular in the xylene separation application, completely comparable to those of the adsorbents known from the prior art and for which the cationic exchanges are carried out with solutions of barium ions of much more high purity and therefore more expensive.

More specifically, it has been observed that it is possible to use barium salts, in particular barium chloride, in particular aqueous solutions of barium chloride, which may contain up to 4% by weight of impurities in the form of strontium salt, preferably between 0.2% and 3% by weight, preferably between 0.2% and 2.5% by weight, limits included with respect to the total weight of the barium salt considered.

The impurity in the form of strontium salt is most often strontium chloride (SrCh).

According to another aspect, the use of a barium salt comprising strontium in the proportions indicated above, leads to an adsorbent according to the invention for which the strontium exchange rate is at most equal to 1 1%, preferably at most equal to 10%, more preferably at most equal to 9% and very particularly preferably between 0.5% and 8%. This exchange rate corresponds to the molar ratio of strontium oxide to the sum of the oxides of barium, strontium, potassium and sodium.

Thus, the present invention provides the use of barium salts, the specifications of which in terms of purity are lowered, while allowing the properties of the barium exchanged adsorbent to be maintained, in the application considered.

It is therefore possible, thanks to the present invention, to have high-performance adsorbents which may contain strontium, the content of which, expressed as oxide (SrO), is strictly less than 3% by weight relative to the total weight of the adsorbent, which corresponds to a strontium exchange rate of approximately 1 1% at the most and to an impurity content (strontium salt) of approximately 3% by weight in the barium solution used for the ion exchange.

These adsorbents can be used more particularly for the production in liquid phase or gas phase of very pure para-xylene from a feed of aromatic hydrocarbons containing isomers with 8 carbon atoms.

Thus, and according to another aspect, the present invention provides a process for separating xylenes using an adsorbent as described above, allowing the production of high purity para-xylene with optimized productivity from a charge of aromatic hydrocarbons containing isomers with 8 carbon atoms.

More particularly, the invention relates to a process for recovering high purity para-xylene from fractions of aromatic isomers with 8 carbon atoms, consisting in using an adsorbent according to the invention, implemented in processes in the liquid phase but also in the gas phase as adsorption agent for para-xylene in the presence of a desorbent, preferably chosen from toluene and para-diethylbenzene.

By high purity para-xylene is meant a product suitable for use in the production of terephthalic acid or dimethyl terephthalate, that is to say a purity of at least 99.5% by weight, preferably at least 99.7% by weight, more preferably at least 99.8% by weight, and more preferably at least 99.9% by weight. The purity of para-xylene can be determined by chromatographic methods. A gas chromatography method usable both for the determination of the purity of para-xylene and the specific amounts of impurities is the ASTM D-3798 method.

It is thus possible to separate the desired product (para-xylene) by preparative adsorption liquid chromatography (in batch), and advantageously continuously in a simulated moving bed, that is to say against the simulated current or at simulated co-current, and more particularly simulated counter-current.

The para-xylene recovery process according to the invention using the adsorbent described according to the invention has the advantage of maximizing productivity, that is to say of maximizing the feed rate to be treated. This is particularly true under the following simulated counter-current type industrial adsorption unit operating conditions:

• number of beds: 6 to 30,

• number of zones: at least 4 operating zones, each located between a supply point and a draw-off point,

• temperature between 100 ° C and 250 ° C, preferably between 150 ° C and 190 ° C,

• industrial unit pressure between the xylenes bubble pressure at process temperature and 3 MPa,

• ratio of desorbent / feed flow rates of between 0.7 and 2.5, for example between 0.9 and 1.8 for an adsorption unit alone (“stand alone”) and between 0.7 and 1.4 for an adsorption unit combined with a crystallization unit,

• recycling rate (ie ratio of the average recycling flow (average of the flow rates of zones weighted by the number of beds per zone) to the load flow) between 2.5 and 12, preferably between 3.5 and 6.

On this subject, reference may be made to the teaching of patents US2985589, US5284992 and US5629467. The operating conditions of an industrial simulated co-current adsorption unit are generally the same as those operating with simulated counter-current, except for the recycling rate which is generally between 0.8 and 7. In this aspect, reference may be made to patents US4402832 and US4498991.

The desorption solvent can be any desorbent known to a person skilled in the art and whose boiling point is lower than that of the feed, such as toluene but also a desorbent whose boiling point is greater than that of the filler, such as para-diethylbenzene (PDEB). The selectivity of the adsorbents according to the invention for the adsorption of para-xylene contained in C8 aromatic cuts is optimal when their loss on ignition measured at 950 ° C is generally between 4.0% and 7.7%, and preferably between 4.5% and 6.5%, and very preferably between 4.8% and 6.0%, limits included.

In addition to the aforementioned use of separating xylene isomers, the present invention also relates to the uses of the adsorbents described above as adsorption agents capable of advantageously replacing the adsorption agents of the prior art for:

• the separation of substituted toluene isomers such as nitrotoluene, diethyltoluene, toluenediamine, and others,

• separation of cresols, and

»Separation of polyhydric alcohols, such as sugars.

The invention finally relates to a process for recovering high purity para-xylene from fractions of aromatic isomers with 8 carbon atoms comprising the following successive steps:

a) a step of bringing the feed into contact with an adsorbent bed comprising at least one zeolitic adsorbent as defined according to any one of claims 1 to 8;

b) a step of bringing the adsorbent bed into contact with a desorbent, preferably chosen from toluene and para-diethylbenzene in the liquid phase or in the gas phase.

CHARACTERIZATION TECHNIQUES

Granulometry of crystals:

The estimation of the number-average diameter of the X zeolite crystals used in step a) and of the X zeolite crystals contained in the agglomerates is carried out by observation under a scanning electron microscope (SEM) or by observation under a microscope electronic transmission (MET).

In order to estimate the size of the zeolite crystals on the samples, a set of photographs is taken at a magnification of at least 5000. The diameter of at least 200 crystals is then measured using a dedicated software, for example Smile View software from the LoGraMi editor. The precision is of the order of 3%.

Chemical analysis of zeolitic adsorbents - Si / Al ratio, weight content of oxides and exchange rate:

An elementary chemical analysis of the final product obtained at the end of the steps of the process of the invention described above can be carried out according to various analytical techniques known to those skilled in the art. Among these techniques, mention may be made of the technique of chemical analysis by X-ray fluorescence as described in standard NF EN ISO 12677: 2011 on a wavelength dispersive spectrometer (WDXRF), for example Tiger S8 from the company Bruker.

X fluorescence is a non-destructive spectral technique using the photoluminescence of atoms in the X-ray field, to establish the elemental composition of a sample. The excitation of atoms generally by a beam of X-rays or by bombardment with electrons, generates specific radiations after return to the ground state of the atom. The X-ray fluorescence spectrum has the advantage of being very little dependent on the chemical combination of the element, which offers an accurate determination, both quantitative and qualitative. In a conventional manner, after calibration, a measurement uncertainty of less than 0.4% by weight is obtained for each oxide. In the present invention, the contents of barium, strontium,

On the other hand, for the lighter elements (relative to their atomic weight, such as sodium or potassium present in the adsorbent), atomic emission spectrometry with high frequency induced plasma will be preferred for greater precision ( ICP-OES for “Inductively Coupled Plasma-Optical Emission Spectroscopy” according to the English terminology) according to the UOP 961-12 standard.

ICP is a method of analysis by atomic emission spectrometry, the source of which is a plasma generated by inductive coupling. This method is also commonly used to determine the contents of various elements such as silicon, aluminum, potassium, sodium, barium and strontium. In the present invention, the sodium contents (and possibly potassium for low contents of less than 0.5% by weight of oxide relative to the total mass of oxides) are preferably measured by the ICP method according to the UOP standard. 961-12. In this case, for sodium, an uncertainty on the measurement of less than 0.01% is obtained for the content by weight of sodium oxide in the adsorbent and for potassium an uncertainty on the measurement of less than 0,

These elementary chemical analyzes make it possible both to verify the Si / Al atomic ratio of the zeolite within the agglomerate, and to verify the quality of the ion exchange carried out in the process described above. In the description of the present invention, the measurement uncertainty of the Si / Al atomic ratio is 0.05.

The quality of the ion exchange is linked to the number of moles of sodium oxide

(Na 2 0), remaining in the agglomerated zeolitic adsorbent after exchange. More precisely, the rate of exchange by the barium ions is estimated by evaluating the ratio between the number of moles of barium oxide, BaO, and the number of moles of the whole (BaO + SrO + Na 2 0 + K 2 0). Likewise, the rate of exchange by strontium ions is estimated by evaluating the ratio between the number of moles of strontium oxide (SrO) and the number of moles of the whole (BaO + SrO + Na 2 0 + K 2 0) .ll be noted that the contents of the various oxides are given in percent by weight relative to the total weight of the dry adsorbent (corrected weight loss on ignition).

Granulometry of zeolitic adsorbents:

The determination of the number-average diameter of the zeolitic adsorbents obtained at the end of the agglomeration and shaping step is carried out by analyzing the particle size distribution of an agglomerated sample by imaging according to the standard ISO 13322-2: 2006, using a conveyor belt allowing the sample to pass in front of the camera lens.

The number-average diameter is then calculated from the particle size distribution by applying the ISO 9276-2: 2001 standard. In the present document, the term “number-average diameter” or “size” is used for the adsorbents according to the invention, with a precision of the order of 0.01 mm.

Mechanical resistance of zeolitic adsorbents:

The technique for characterizing the mechanical strength representative of the crushing of the adsorbent within a bed or a reactor is the technique for characterizing the mechanical strength in bed, as described in the Shell method SMS1471-74 series (Shell Method SMS1471-74 Serials Determination of Bulk Crushing Strength of Catalysts. Compression-Sieve Method "), associated with the" BCS Tester "device marketed by the company Vinci Technologies. This method, initially intended for characterization of catalysts of 3 to 6 mm is based on the use of a sieve of 425 μm which will make it possible in particular to separate the fines created during the 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 of the adsorbents which it is sought to characterize. The ASTM D7084-04 standard which also describes a method of measuring the crush strength in catalyst bed ("Determination of Bulk Crush Strength of Catalysts and Catalyst Carriers") defines the passage of the sieve to be used as being equal to the half the diameter of the catalyst particles to be characterized.

The method provides for a preliminary step of sieving the sample of catalysts or adsorbents to be characterized. If an amount equal to 10% by weight of the sample passes through the screen, a smaller pass screen will be used.

The adsorbents of the present invention, generally in the form of beads or extrudates, generally have a number-average diameter or a length, ie largest dimension in the case of non-spherical agglomerates, between 0.4 mm and 2 mm, and in particular between 0.4 mm and 0.8 mm and preferably between 0.4 mm and 0.65 mm. Therefore, a suitable sieve such that less than 10% by weight of the sample passes through the screen in a preliminary sieving step is used instead of the 425 µm sieve mentioned in the standard Shell method SMS1471-74 .

The measurement protocol is as follows: a sample of 20 cm 3 of agglomerates, previously sieved with the suitable sieve (200 μm) and previously dried in an oven for at least 2 hours at 250 ° C (instead of temperature of 300 ° C mentioned in the standard Shell method SMS1471-74), is placed in a metal cylinder of known internal section. An increasing force is imposed in stages on this sample by means of a piston, through a bed of 5 cm 3steel balls in order to better distribute the force exerted by the piston on the adsorbents (use of 2 mm diameter balls for spherical-shaped particles with a diameter strictly less than 1.6 mm). The fines obtained at the various pressure levels are separated by sieving (suitable sieve of 200 μm) and weighed.

The resistance to bed crushing is determined by the pressure in megaPascal (MPa) for which the amount of accumulated fines passing through the sieve amounts to 0.5% by weight of the sample. This value is obtained by plotting on a graph the mass of fines obtained as a function of the force applied to the adsorbent bed and by interpolating at 0.5% by mass of cumulative fines. The mechanical resistance to bed crushing is typically between a few hundred kPa and a few tens of MPa and generally between 0.3 MPa and 4 MPa. The precision is conventionally less than 0.1 MPa.

Non-zeolite phase of zeolite adsorbents:

The level of non-zeolitic phase, for example non-zeolitized residual binder or any other amorphous phase, after zeolitization is calculated according to the following equation:

PNZ = 100 - å (PZ),

where PZ represents the sum of the quantities of the zeolite fractions X within the meaning of the invention.

The amount of zeolitic X fractions is measured by X-ray diffraction analysis, known to those skilled in the art by the acronym DRX. This analysis is carried out on an apparatus of the Bruker brand, then the quantity of the zeolitic fractions X is evaluated by means of the TOPAS software from the Bruker company. This method also makes it possible to determine the nature of the various zeolitic fractions present in the adsorbent of the present invention.

Microporous volume:

The crystallinity of the agglomerates is also evaluated by measuring their microporous volume by comparing it with that of an appropriate reference (100% crystalline zeolite under identical cationic treatment conditions or zeolite

theoretical). This microporous volume is determined from the measurement of the adsorption isotherm of gas, such as nitrogen, at its liquefaction temperature. Prior to adsorption, the zeolitic adsorbent is degassed between 300 ° C and 450 ° C for a period of 9 hours to 16 hours, under vacuum (P <6.7.10 4 Pa). The measurement of the nitrogen adsorption isotherm at 77K is then carried out on a device of the ASAP 2010 M type from Micromeritics, by taking at least 35 measurement points at relative pressures with a P / Po ratio between 0.002 and 1. The microporous volume is determined according to Dubinin and Raduskevitch from the isotherm obtained, by applying the ISO 15901-3: 2007 standard. The microporous volume evaluated according to Dubinin and Raduskevitch is expressed in cm 3of liquid adsorbate per gram of anhydrous adsorbent. The measurement uncertainty is ± 0.003 cm 3 . g 1 .

Loss on ignition of zeolitic adsorbents:

The loss on ignition is determined in an oxidizing atmosphere, by calcining the sample in air at a temperature of 950 ° C ± 25 ° C, as described in standard NF EN 196-2 (April 2006). The standard deviation of the measurement is less than 0.1%.

Characterization of adsorption in liquid phase by drilling:

The technique used to characterize the adsorption of molecules in the liquid phase on a porous solid is the so-called piercing technique, described by Ruthven in "Principles of Adsorption and Adsorption Processes", Chapters 8 and 9, John Wiley & Sons, (1984), who defines the technique of breakthrough curves as the study of the response to the injection of a step of adsorbable constituents. Analysis of the mean exit time (first moment) of the piercing curves provides information on the quantities adsorbed and also makes it possible to evaluate the selectivities, that is to say the separation factor, between two adsorbable constituents. The injection of a non-adsorbable component used as a tracer is recommended for the estimation of non-selective volumes. The

Examples

The examples which follow illustrate the invention without, however, limiting it in any way and the scope of protection of which is specified by the appended claims.

General method of preparing an adsorbent according to the invention based on zeolite X with a molar ratio Si / Al = 1.25

A homogeneous mixture is prepared and 800 g of NaX zeolite crystals are agglomerated according to the procedure described in patent application WO2014090771 (synthesis of Example B) with 105 g of kaolin (expressed in calcined equivalent) and 45 g colloidal sold under the tradename silica Klebosol ® 30 (containing 30 wt% S1O 2 and 0.5% Na 2 0) with the amount of water which allows the extrusion of the mixture.

The extrudates are dried, crushed so as to recover grains whose number-average diameter is equal to 0.5 mm, then calcined at 550 ° C. under a stream of nitrogen for 2 hours.

The agglomerate obtained (200 g) is placed in a glass reactor provided with a double jacket regulated at a temperature of 100 ° C ± 1 ° C. 1.5 L of an aqueous solution of sodium hydroxide of concentration 2.5 M are then added and the reaction medium is left under stirring for a period of 4 hours.

The agglomerates are then washed in 3 successive washing operations with water followed by emptying the reactor. The efficiency of the washing is ensured by measuring the final pH of the washing water of between 10.0 and 10.5.

The sodium cations of the agglomerates obtained are exchanged at 95 ° C. with barium and strontium ions. For this, one adds to the barium salt, of formula BaCh, 2H 2 O, (pure, containing at most 0.2% by weight of strontium chloride) different amounts of strontium salt, of formula SrCh, 6H 2 O, such that the mass percentage of SrCh salt is equal to the percentage indicated in Table 1 below.

For example, an exchange solution is produced by dissolving 150 g of BaCh salt, 2 H 2 O with 1.4 g of SrCh salt, 6 H 2 O in 1 L of water. The amount of strontium salt corresponds to 0.9% by weight relative to the total mass of salt. Then, 10 g of agglomerates prepared above are contacted with this solution in order to carry out the cation exchange.

The exchange takes place in 4 stages. At each step, the volume of solution to mass of solid ratio is 25 mL.g 1 and the exchange is continued for 4 hours each time. Between each exchange, the solid is washed several times so as to get rid of excess salt. The agglomerates are then dried at 80 ° C. for 2 hours and finally activated at 250 ° C. for 2 hours under a stream of nitrogen.

The loss on ignition measured, as described above, is 5.5% ± 0.1% for each sample. The barium + strontium exchange rate of the adsorbents is calculated from elementary X-ray fluorescence analyzes of the oxides of barium, strontium and sodium as described in the characterization techniques.

In the example given above, the barium exchange rate is 95.1%, the strontium exchange rate is 4.0%.

The other examples are carried out starting from 150 g of barium salt to which is added the amount of SrCh salt making it possible to obtain the% by weight indicated for the examples according to the invention and the comparative examples.

Reference example A

This example corresponds to Example 1 of application WO2014090771 and corresponds to an adsorbent exchanged for barium alone, with a very low sodium content (cf. Table 1).

The barium exchange rate calculated from the X-ray fluorescence analysis of this agglomerate is 99.1% and the loss on ignition is 5.4%.

Reference example B

For this second reference example, Example 1 of application WO2014090771 is repeated identically, but we stop at the second exchange. The agglomerates are engaged in a cation exchange reaction by the action of an aqueous solution of 0.5 M barium chloride at 95 ° C. in only 2 stages. At each step, the volume of solution to mass of solid ratio is 20 mL.g 1 and the exchange is continued for 4 hours each time. Between each exchange, the solid is washed several times so as to get rid of excess salt. The agglomerates are then dried at 80 ° C. for 2 hours and finally activated at 250 ° C. for 2 hours under a stream of nitrogen.

The barium exchange rate of this agglomerate is 92.6% and the loss on ignition 5.5%.

[0100] Examples 1 to 5 and Comparative Example are carried out according to Reference Example A using barium chloride solutions containing increasing strontium chloride contents, in order to simulate barium chloride solutions containing strontium as an impurity.

[0101] The details of each of Examples 1 to 5 and of the Comparative Example are reproduced in Table 1 below:

[0102]

- Table 1 -

[0103] In Table 1 above:

•% SrCh denotes the weight percentage of strontium chloride hexahydrate in the barium chloride solution (comprising barium and impurities) used to carry out the cation exchange;

•% BaO denotes the percentage by weight of barium oxide relative to the total weight of the anhydrous adsorbent;

•% SrO denotes the weight percentage of strontium oxide relative to the total weight of the anhydrous adsorbent;

•% Na 2 0 denotes the weight percentage of sodium oxide relative to the total weight of the anhydrous adsorbent;

• Ba / Sr denotes the barium / strontium weight ratio in the anhydrous adsorbent;

Drill test

A piercing test (frontal chromatography) is then carried out on the agglomerates obtained in Example 1 in order to evaluate their effectiveness. The amount of adsorbent used for this test is approximately 30 g.

The procedure for obtaining the drilling curves is as follows:

• Filling of the column through the sieve and installation in the test bench.

• Filling with a solvent (toluene) at room temperature.

• Gradual rise to the adsorption temperature under a flow of solvent (2 cm 3. Min 1 ). · Solvent injection at 2 cm 3 . min 1 when the adsorption temperature is reached.

• Solvent / load changeover to inject the load (2 cm 3. Min 1 ).

• The injection of the charge is then maintained for a sufficient time to reach thermodynamic equilibrium.

• Collection of the drilling recipe in a single bottle then analysis of the composition of the recipe by CPG.

The pressure is sufficient for the feed to remain in the liquid phase, ie 1 MPa. The adsorption temperature is 175 ° C. The composition of the load used for the tests is as follows:

• Para-xylene: 18% weight

• Meta- ylene: 18% by weight

• Orf / 70-xylene: 18% weight

• Ethylbenzene: 18% by weight

• Para-diethylbenzene: 18% by weight

• Iso-octane: 10% by weight (this is used as a tracer for the estimation of non-selective volumes and does not intervene in the separation)

The binary selectivities of two by two compounds, denoted binary selectivities cii / k are calculated from the adsorbed quantities q, and q k of compounds i and k, the latter being determined by material balance from the analysis of the composition of the drilling recipe and the composition of the charge (charge in which the fraction

qiVk

mass of compounds i and k is y, and y k ): (X j / fc =

The evaluation of the potential of these adsorbents during the implementation in simulated countercurrent, is made based on the theory of equilibrium applied to multi-component systems with constant selectivities as described by Mazotti, Storti and Morbidelli in “Robust Design of Countercurrent Adsorption Separation Processes: 2. Multicomponent Systems”, AlChE Journal, (November 1994), Vol. 40, no.11.

In particular, reference is made here to equation 8, which describes the conditions to be satisfied on the reduced flow rates p of the 4 sections (j = 1 to j = 4) of a countercurrent separation unit such as as shown schematically in Figure 1 of the cited article to obtain complete separation.

This equation 8 refers to the adsorptivities K, of the various constituents, as well as to the parameter d, of each section j defined by equation 7:

It should be noted here that by definition the binary selectivity a, / k between the compounds i and k is equal to the ratio of the adsorptivities K, / Kk.

The reduced flow rate "m" of each section of the unit is defined as being the ratio of the flow rate of the liquid phase to the flow rate of the adsorbed phase. Equation 8 indicates what are the limit reduced flow rates for each section. In a 4-section countercurrent separation unit, the feed flow corresponds to the difference between the reduced flow in zone 3 and the reduced flow in zone 2.

Therefore, when we want to evaluate the maximum productivity that can be achieved with a given adsorbent, we seek to evaluate the maximum amount of filler that can be treated, that is to say to evaluate the difference between the maximum reduced flow in zone 3 and the minimum reduced flow in zone 2.

The performance in terms of maximum productivity of two adsorbents can be compared by comparing their maximum reduced feed rate determined from the reduced flow rates of zones 2 and 3, respectively m 2 and rri 3 , according to the relationship:

max (mload) = max (m 3 ) - min (m 2 ).

If we consider a system with constant selectivities, the composition of the liquid phase which gives the highest stress in zone 2 and in zone 3 is the composition of the liquid phase at the point of injection of the feed into the tank. unit. Indeed, from this point the concentration of para-xylene, which is the most adsorbed compound, increases in the direction of circulation of the solid in zone 2, and decreases in the direction of circulation of the liquid in zone 3. It is possible to approximate the composition of this point to the composition of the feed to be treated, and it is this composition which will be used to evaluate the term d 2 and d 3 of equation 8. The terms d 2 and d 3 being defined by l equation 7 mentioned above.

For each adsorbent, this maximum reduced flow rate (mload) is calculated from the binary selectivity values ​​measured experimentally. Table 2 makes it possible to compare the maximum reduced feed rate "max (mcharge)", for each of the adsorbents tested. The maximum reduced load flow rate "max (mc harge )" is representative of productivity, the higher its value, the better the productivity.

[0117]

- Table 2 -

It can be seen that the maximum reduced flow rate (mload) remains substantially the same whether it is an adsorbent for which the ion exchange has been carried out with a "pure" barium chloride solution (ie without strontium impurity) (Example A) or an adsorbent prepared from a solution of barium chloride also containing strontium chloride (Example 2).

On the other hand when the barium chloride contains sodium at levels comparable to strontium (due to the partial exchange with barium), it is noted that the maximum productivity (mload) is reduced. This observation effectively confirms, in addition to the effect on selectivity, the interest of barium exchange for the separation of xylenes, which is perfectly known to those skilled in the art.

Likewise, it was observed that when the content of strontium impurities in the barium chloride solution is too high, in particular greater than 4% (cf. Comparative example, strontium chloride content> 4.8% ), the maximum productivity (mload) drops drastically.

These examples confirm in all points the object of the present invention and make it possible to demonstrate that it is quite possible to envisage the presence of strontium ions in adsorbents exchanged with barium which can be used for the separation of xylenes, without affecting the productivity of para-xylene. Furthermore, an improvement in selectivity for para-xylene is obtained.

CLAIMS

1. Adsorbent based on agglomerated zeolite (s) crystals, said agglomerate comprising:

- at least crystals of FAU zeolite (s), with an Si / Al molar ratio of between 1.00 and 1.50, limits included,

- a content by weight of barium ions (Ba 2+ ), expressed by weight of barium oxide (BaO), strictly greater than 30%, preferably strictly greater than 33%, more preferably a content of between 34% and 42 %, limits included, and quite preferably between 34% and 40%, limits included, relative to the total weight of the adsorbent,

- a weight content of strontium ions (Sr 2+ ), expressed in weight of strontium oxide (SrO), strictly greater than 0.1% and strictly less than 3%, preferably between 0.15% and 2, 9%, limits included, more preferably between 0.15% and 2.5% and quite preferably between 0.15% and 2.4%, limits included, relative to the total weight of the adsorbent.

2. Adsorbent according to claim 1, wherein "X-type FAU zeolite (s)" denotes zeolites whose Si / Al atomic ratio is between 1.00 and 1.50 limits inclusive, preferably between 1.00 and 1, 45, more preferably between 1, 05 and 1, 45, limits included and even more preferably between 1, 10 and 1, 45 limits included.

3. Adsorbent according to one of claims 1 or 2, having an Si / Al atomic ratio of between 1.00 and 2.00, preferably between 1.00 and 1.80 limits inclusive, more preferably between 1.15 and 1, 80, limits included and even more preferably between 1, 15 and 1, 60, limits included.

4. Adsorbent according to any one of the preceding claims, in which the total weight content of alkali or alkaline earth ions other than Ba 2+ and Sr 2+ , expressed by weight of alkali or alkaline earth oxides respectively, is less than 5% and preferably is between 0% and 2% and advantageously between 0% and 1%, limits included, relative to the total weight of the adsorbent.

5. Adsorbent according to any one of the preceding claims, wherein the weight content of sodium ions (Na + ), expressed by weight of sodium oxide (Na 2 0), is strictly less than 0.3%, preferably strictly less than 0.2%, relative to the total weight of the adsorbent.

6. Adsorbent according to any one of the preceding claims, in which the content of potassium ions (K + ), expressed by weight of potassium oxide (K 2 0), is less than 9%, preferably less than 8%. , better still less than 5%, and more preferably still between 0% and 2%, advantageously between 0% and 1%, limits included, relative to the total weight of the adsorbent.

7. Adsorbent according to any one of the preceding claims, in which the barium / strontium weight ratio in the adsorbent is greater than 15: 1, typically between 15: 1 and 400: 1, preferably between 16: 1 and 300: 1, more preferably between 16: 1 and 200: 1, limits included.

8. Adsorbent according to any one of the preceding claims, in which the weight content of non-zeolitic phase (PNZ) is less than 15%, preferably less than 10%, more preferably less than 5%, and typically between 0 and 15% preferably between 0 and 10%, more preferably between 0 and 5% limits not included, quite preferably between 0.1% and 5%, better still between 0.5% and 5 % and very particularly preferably between 2% and 5%, limits not included, relative to the total weight of the anhydrous adsorbent.

9. Use of an adsorbent according to any one of claims 1 to 8 for the production in liquid phase or gas phase of very pure para-xylene from an aromatic hydrocarbon feedstock containing isomers with 8 atoms of carbon.

10. Process for recovering high purity para-xylene from fractions of aromatic isomers with 8 carbon atoms comprising the following successive steps:

a) a step of bringing the feed into contact with an adsorbent bed comprising at least one zeolitic adsorbent as defined according to any one of claims 1 to 8;

b) a step of bringing the adsorbent bed into contact with a desorbent, preferably chosen from toluene and para-diethylbenzene in the liquid phase or in the gas phase.