Zeolitic adsorbent for the highly productive separation of xylenes

Abstract

The invention relates to agglomerated zeolitic adsorbents based on faujasite X zeolite crystals having a controlled Si / Al molar ratio, said zeolite being exchanged with barium, or based on faujasite X zeolite having a controlled Si / Al molar ratio, this zeolite being exchanged both with barium and potassium. The invention further relates to the use of the adsorbent in the separation of sugars, polyhydric alcohols, substituted toluene isomers, or cresols, or in the recovery of very high purity paraxylene, as well as to the xylene separation method using the adsorbent.

Description

[0001] Zeolite adsorbent for high productivity xylenes separation

[0002] Technical field

[0003] The invention relates to agglomerated zeolite adsorbents based on Faujasite (FAU) type zeolite crystals with a controlled silicon to aluminum molar ratio, Si / AI, exchanged with barium or barium and potassium and exhibiting selective behavior with respect to para-xylene present in the aromatic hydrocarbon feedstock containing isomers with 8 carbon atoms to be separated.

[0004] Prior art

[0005] The use of zeolite adsorbents consisting of faujasite zeolite (FAU) of type X or Y comprising, in addition to sodium cations, barium and / or potassium and / or strontium ions, alone or in mixtures, for selectively adsorbing para-xylene in a mixture of aromatic hydrocarbons, is well known in the prior art.

[0006] Patents US3558730, US3558732, US3626020 and US3663638 show that zeolite adsorbents based on aluminosilicates and comprising sodium and barium (US3960774) or comprising sodium, barium and potassium, are effective for the separation of para-xylene present in C8 aromatic cuts (cuts comprising aromatic hydrocarbons with 8 carbon atoms).

[0007] A method of preparing these adsorbents is for example described in US3878127 and consists of treating, in hot sodium hydroxide (soda), agglomerates comprising a zeolite X and binder with a Na2O / Al2O3 ratio strictly less than 0.7 in order to replace the exchangeable cations of the zeolite (such as protons or cations of the HA Group), with sodium prior to an exchange with barium or barium and potassium, the prior exchange with sodium allowing a greater quantity of barium ions or barium and potassium ions to be added to the zeolite structure.

[0008] These adsorbents are used as adsorption agents in liquid phase processes, preferably of the simulated countercurrent type, similar to those described in US2985589, which apply among others to C8 cuts (cut comprising aromatic hydrocarbons with 8 carbon atoms).

[0009] The prior art zeolites for xylenes separation belong to the Faujasite structural type, first described in US2882244 and US3130007, which are crystallized silico-aluminates having cages of perfectly determined size and connected in three dimensions.

[0010] US6884918 recommends a Faujasite X with an Si / Al atomic ratio between 1.15 and 1.5. US6410815 teaches that zeolite adsorbents as described in the prior art, but for which the Faujasite has a low silica content and has an Si / Al atomic ratio close to 1 (which will be called LSX, abbreviation of Low Silica X, the French translation of which is zeolite X with a low silica content) are advantageously used for the separation of paraxylene.

[0011] Both zeolite X and low silica zeolite X therefore have good performance in terms of paraxylene selectivity, but the synthesis of low silica zeolite X is rather difficult compared to the synthesis of zeolite X. Indeed, to lower the Si / Al atomic ratio of a Faujasite type zeolite, it is necessary to increase the consumption of sodium hydroxide used in the zeolite synthesis process. Furthermore, to crystallize according to the Faujasite structural type when the Si / Al atomic ratio is 1, high concentrations of potassium hydroxide (potash) must be added to inhibit the formation of zeolite A and obtain only low silica zeolite X. These high consumptions of sodium hydroxide and potash increase the manufacturing cost of this type of zeolite and pose problems of effluent discharge.

[0012] In the references listed above, zeolite adsorbents are in the form of crystals or in the form of agglomerates consisting mainly of zeolite and inert agglomeration binder, generally in a proportion of between 0.1% and 20% by weight.

[0013] Since the synthesis of X and X zeolites with low silica content is most often carried out by nucleation and crystallization of silico-aluminate gels, crystals are obtained which are particularly difficult to use on an industrial scale (significant pressure losses during handling) and agglomerated forms are preferred, for example in the form of granules or grains, which do not have the disadvantages inherent in powdered materials.

[0014] The preparation of these agglomerates is carried out for example by pasting zeolite crystals with a binder, most often a clay or a mixture of clays, possibly zeolithizable, in proportions of the order of 80% to 99.9% by weight of zeolite crystals for 0.1% to 20% by weight of binder, then shaping into granules, grains, balls, plates or extrudates, and high temperature heat treatment for cooking the clay and reactivating the zeolite, the exchange with barium and / or potassium being able to be carried out before and / or after the agglomeration of the powdered zeolite with the binder.

[0015] Zeolite agglomerates are obtained whose particle size is generally a few millimeters, and which, if the choice of binder and granulation are made according to the rules of the art, present a set of satisfactory properties, in particular porosity, mechanical resistance, and resistance to abrasion.

[0016] Patent FR2925366 describes a process for manufacturing agglomerates based on LSX zeolite crystals with a number-average diameter less than or equal to 4 pm, with an Si / AI atomic ratio such that (1.00 ± 0.05) < Si / AI < 1.15 and preferably with an Si / AI atomic ratio = 1.00 ± 0.05, of which at least 90% of the exchangeable cationic sites are occupied either by barium ions alone or by barium ions and potassium ions, in which the mechanical strength measured by the Shell series SMS1471-74 method adapted to agglomerates with a size less than 1.6 mm is greater than or equal to 2 MPa.

[0017] Patent FR2925367 describes a method for manufacturing agglomerated zeolite adsorbents which comprises a mixture of crystals of zeolite X exchanged at least 90% by barium ions alone or by barium ions and potassium ions, the exchangeable sites occupied by potassium being able to represent up to a third of the exchangeable sites occupied by barium + potassium ions (the possible remainder being generally provided by alkali or alkaline-earth ions other than barium and potassium); crystals of zeolite LSX exchanged at least 90% by barium ions alone, or by barium ions and potassium ions, the exchangeable sites occupied by potassium being able to represent up to a third of the exchangeable sites occupied by barium + potassium ions (the possible remainder being generally provided by alkali or alkaline-earth ions other than barium and potassium);and a binder in a proportion less than or equal to 20% by weight of the total mass of the agglomerate.;

[0018] Patent US6410815 teaches that the performance of the industrial process for separating paraxylene depends to a large extent on the adsorbent, its adsorption capacity and the selectivity it shows for paraxylene in a medium consisting of Cs aromatic compounds, typically para-xylene, meta-xylene, ortho-xylene, ethylbenzene, as well as on the other hand on the ability of desorbents, such as toluene and para-diethylbenzene, to desorb the adsorbed para-xylene. The selectivity OA / B of the adsorbent for a component A compared to a compound B is defined as the ratio of the concentrations of the compounds in the adsorbed phase divided by the ratio of the concentrations of the compounds in the non-adsorbed phase at equilibrium:

[0019] OA / B —Aads / Bads XB|jq / A|jq where Aads and Bads are the concentrations of compound A and compound B in the adsorbed phase respectively and A| iq and B| iq are the concentrations of compound A and compound B in the fluid phase.

[0020] Summary of the invention

[0021] Surprisingly, it appears that agglomerated zeolite adsorbents comprising a Faujasite type zeolite having a controlled silicon to aluminum Si / AI molar ratio and in particular a Si / AI molar ratio such that 1.10 < Si / AI < 1.18, preferably 1.10 < Si / AI < 1.17, more preferably 1.10 < Si / AI < 1.16, more preferably 1.11 < Si / AI < 1.16 (which will be called MSX, abbreviation of Medium Silica X, the French translation of which is zeolite X with medium silica content) exchanged at least 90% by barium ions alone or by barium ions and potassium ions, can advantageously replace the zeolite adsorbents described in the literature based on zeolite X and / or based on zeolite LSX, alone, or in mixtures and exchanged with barium or exchanged with barium and potassium.The agglomerated zeolite absorbents according to the invention show in particular unexpected performances in terms of selectivity, in a process for separating para-xylene. For obvious reasons of ease of industrial implementation and exploitation, the agglomerated zeolite absorbents usable in the context of the process of the present invention are used alone, preferably without other zeolite adsorbent(s), whether in layers or in mixtures.

[0022] The invention relates to an agglomerated zeolite adsorbent based on MSX zeolite crystals, having an Si / AI atomic ratio such that 1.10 < Si / AI < 1.18, preferably 1.10 < Si / AI < 1.17, more preferably 1.10 < Si / AI < 1.16, more preferably 1.11 < Si / AI < 1.16, of which at least 90%, preferably at least 95% of the exchangeable cationic sites are occupied, either by barium ions alone or by barium ions and potassium ions. More preferably, the invention relates to an agglomerated zeolite adsorbent based on MSX zeolite crystals, having an Si / AI atomic ratio such that 1.11 < Si / AI < 1.15, and more preferably 1.12 < Si / AI < 1.15, and advantageously 1.12 < Si / AI < 1.14, of which at least 90%, preferably at least 95% of the exchangeable cationic sites are occupied, either by barium ions alone or by barium ions and potassium ions.

[0023] The micropore volume of the adsorbent, measured according to the Dubinin method by nitrogen adsorption at 77 K after pretreatment at 500°C for 12 hours under vacuum, may be greater than or equal to 0.200 cm 3 / g, preferably greater than or equal to 0.220 cm 3 / g, preferably still greater than or equal to 0.225 cm 3 / g, even more preferably greater than or equal to 0.250 cm 3 / g.

[0024] The exchangeable sites occupied by potassium can represent up to 1 / 3 of the exchangeable sites occupied by barium + potassium ions, and the possible complement of which is generally provided by alkali or alkaline-earth ions other than barium and potassium.

[0025] The adsorbent may comprise an inert binder in a proportion less than or equal to 20% by weight, preferably 15% by weight, of the total mass of the adsorbent.

[0026] The adsorbent may have a size distribution such that the number average diameter is between 0.4 mm and 2.0 mm, preferably between 0.4 mm and 0.8 mm. The mechanical strength of the adsorbent, measured by the Shell method SMS1471-74 series suitable for agglomerates of size less than 1.6 mm, is advantageously greater than or equal to 2 MPa, preferably greater than or equal to 2.5 MPa.

[0027] The loss on ignition measured at 900°C is advantageously less than or equal to 7.7%, preferably between 0 and 7.7%, preferably between 3.0% and 7.7%, more preferably between 3.5% and 6.5% and advantageously between 4.5% and 6.0%, limits included.

[0028] The MSX zeolite crystals may have a number average diameter of between 0.01 pm and 5 pm, preferably between 0.05 pm and 5 pm, very preferably between 0.1 pm and 4 pm, even more preferably between 0.1 pm and 3 pm and even more preferably between 0.1 pm and 2 pm.

[0029] The invention also relates to a process for separating sugars, polyhydric alcohols, substituted toluene isomers, cresols or for recovering para-xylene, using an agglomerated zeolitic adsorbent according to any one of the variants described, in the presence of a desorbent, in the liquid phase or in the gas phase.

[0030] The process may be a process for recovering para-xylene from aromatic Cs isomer cuts, by adsorption of para-xylene using said agglomerated zeolite adsorbent, in the presence of a desorbent, in the liquid phase or in the gas phase.

[0031] The para-xylene recovery process can be implemented in simulated moving bed, simulated co-current or simulated counter-current.

[0032] The process may be a process for the production of high-purity, high-productivity para-xylene from an aromatic hydrocarbon feedstock containing 8-carbon isomers comprising the following steps: a) a step of contacting the feedstock with a bed of agglomerated zeolite adsorbent, so as to preferentially adsorb the para-xylene, b) a step of contacting, under desorption conditions, the adsorbent bed with a desorbent, which is preferentially either toluene or para-diethylbenzene, c) a step of withdrawing from the adsorbent bed a stream containing the desorbent and the least selectively adsorbed feed products, d) a step of withdrawing from the adsorbent bed a stream containing the desorbent and the para-xylene, e) a step of separating the stream from step c) into a first stream containing the desorbent and a second stream containing the least selectively adsorbed charge products,and f) a step of separating the stream from step d) into a first stream containing the desorbent and a second stream containing para-xylene at a purity level greater than or equal to 75%.,

[0033] Said method may further comprise: g) a crystallization step in a crystallizer consisting of the crystallization of the para-xylene from step f), making it possible to obtain, on the one hand, para-xylene crystals soaked in their mother liquor, and on the other hand, a mother liquor which may be partly, or even entirely, recycled in a mixture with the fresh feed at the inlet of the simulated moving bed adsorption unit, and h) a step of washing the crystals from step g) at the end of which para-xylene is recovered at a purity of at least 99.7%, and preferably at least 99.8%.

[0034] More generally, the invention finally relates to the use of an agglomerated zeolite adsorbent based on MSX zeolite crystals having an Si / AI atomic ratio such that 1.10 < Si / AI < 1.18, preferably 1.10 < Si / AI < 1.17, more preferably 1.10 < Si / AI < 1.16, more preferably 1.11 < Si / AI < 1.16 of which at least 90% of the exchangeable cationic sites are occupied, either by barium ions alone or by barium ions and potassium ions for the separation of sugars, polyhydric alcohols, substituted toluene isomers, cresols or for the recovery of para-xylene, in the presence of a desorbent, which is preferably either toluene or para-diethylbenzene, in phase liquid or gaseous phase.

[0035] This use may concern the recovery of para-xylene from aromatic C8 isomer cuts, by adsorption of para-xylene in a simulated moving bed, simulated co-current or simulated counter-current type reactor.

[0036] Description of the embodiments

[0037] The present invention relates to zeolite adsorbents which can be used in particular for the separation of para-xylene from a mixture of Cs aromatic compounds having excellent performance, in particular in terms of selectivity for para-xylene, said adsorbents being particularly suitable for use in a process for the separation of para-xylene in the liquid phase and very particularly in a process for the separation of para-xylene in the liquid phase with high productivity, preferably of the simulated counter-current type.

[0038] The agglomerated zeolite adsorbents according to the present invention comprise an MSX zeolite having an Si / AI atomic ratio such that 1.10 < Si / AI < 1.18, preferably

[0039] 1.10 < Si / AI < 1.17, more preferably 1.10 < Si / AI < 1.16, more preferably

[0040] 1.11 < Si / AI < 1.16, the lower values ​​of which reflect the analytical uncertainties in the measurement of this ratio, and the higher values, either the same analytical uncertainty or a tolerable deviation in the purity of the product, exchanged at least 90% by barium ions alone or by barium ions and potassium ions, the exchangeable sites occupied by potassium being able to represent up to 1 / 3 of the exchangeable sites occupied by barium + potassium ions (the possible remainder being generally provided by alkali or alkaline-earth ions other than barium and potassium); the mechanical resistance of the agglomerated zeolite adsorbents according to the invention is measured by the Shell method SMS1471-74 series adapted to agglomerates of size less than 1.6 mm and is advantageously greater than or equal to 2 MPa.

[0041] Advantageously, the zeolitic adsorbents according to the invention may comprise a binder in a proportion less than or equal to 20% by weight, preferably 15% by weight, of the total mass of the agglomerate. This binder may contain one or more zeolithizable clays and preferably at least 80% by weight of zeolithizable clay(s) and optionally one or more additives.

[0042] The number average diameter of the zeolite crystals in the zeolite adsorbents according to the invention is advantageously between 0.01 pm and 5 pm, preferably between 0.05 pm and 5 pm, very preferably between 0.1 pm and 4 pm, even more preferably between 0.1 pm and 3 pm and, even more preferably between 0.1 pm and 2 pm.

[0043] Generally, the zeolite adsorbents according to the invention have a volume average diameter of 0.4 mm to 2 mm, and in particular between 0.4 mm and 0.8 mm.

[0044] In this document, the term "number average diameter" or "size" is used for zeolite crystals and zeolite agglomerates. The accuracy is of the order of 3%.

[0045] The invention also relates to a process for preparing the zeolite adsorbents according to the invention. The agglomerated zeolite adsorbents according to the invention can be prepared according to a preparation process which comprises the following steps:

[0046] - a / agglomeration of MSX zeolite crystals with a binder containing at least 80% by weight of zeolitizable clay and possibly additives, and shaping, then drying and calcination and,

[0047] - b / possible zeolitization of the binder by the action of a basic alkaline solution,

[0048] - replacement of at least 90% of the exchangeable sites of the MSX zeolite by barium, followed by washing and drying of the product thus treated,

[0049] - d / possibly replacement of at most 33% of the exchangeable sites of zeolite X by potassium, followed by washing and drying of the product thus treated,

[0050] - e / activation.

[0051] Agglomeration and shaping (step a / ) can be carried out using any technique known to those skilled in the art, such as extrusion, compaction, agglomeration. The agglomeration binder represents a proportion less than or equal to 20% by weight, preferably 15% by weight, of the total mass of the agglomerate.

[0052] According to a preferred embodiment, the MSX zeolite crystals have a diameter less than or equal to 5 pm, preferably between 0.01 pm and 5 pm, preferably between 0.05 pm and 5 pm, very preferably between 0.1 pm and 4 pm, even more preferably between 0.1 pm and 3 pm and, even more preferably between 0.1 pm and 2 pm.

[0053] At the end of step a / , the finest agglomerate particles can be removed by cycloning and / or screening and / or the larger particles by screening or crushing, in the case of extrudates, for example.

[0054] The agglomeration binder used in step a / contains at least 80% by weight of zeolithizable clay and may also contain other mineral binders such as bentonite, attapulgite. By zeolithizable clay is meant a clay or a mixture of clays which are capable of being transformed into zeolitic material by the action of an alkaline basic solution. Zeolithizable clay generally belongs to the family of kaolins, kaolinites, nacrites, dickites, halloysite and / or metakaolins. Kaolin is commonly used.

[0055] During step a / , in addition to the MSX zeolite crystals and the binder, one or more additives may also be used, for example additives intended to facilitate agglomeration or to improve the hardening of the agglomerates formed.

[0056] Among the additives possibly used in step a), there may be found a source of silica of any type known to those skilled in the art, specialists in the synthesis of zeolites, for example colloidal silica, diatoms, perlite, calcination ash (“fly ash” in English), sand, or any other form of solid silica.

[0057] The MSX zeolite crystals used in step a / may be derived from the synthesis of sodium-exchanged MSX zeolite crystals, but it would not be outside the scope of the invention to use crystals having undergone one or more cationic exchanges, between the synthesis in NaMSX form and its use in step a / .

[0058] The calcination which follows the drying is carried out at a temperature generally between 500°C and 600°C. According to a preferred embodiment, step b / of zeolitization is carried out.

[0059] When step b / of zeolitization is carried out, the transformation of at least 50% of the inert zeolitizable binder into zeolitic material is obtained; it is noted that the zeolitization is intended in particular to increase the mechanical resistance of the agglomerated zeolitic adsorbents. Zeolitization can be carried out by immersing the agglomerate in an alkaline basic solution, generally aqueous, for example an aqueous solution of sodium hydroxide and / or potassium hydroxide, the concentration of which is preferably greater than 0.5 M. It is preferably carried out hot (temperature above room temperature) typically at temperatures of the order of 80°C to 100°C in order to improve the kinetics of the process and reduce the immersion times to less than 8 hours; but it would not be outside the scope of the invention to operate at lower temperatures and longer immersion times. According to this procedure, zeolitization (i.e.the transformation of the inert binder in the sense of adsorption into active material in the sense of adsorption) of at least 50% by weight of the binder. This is then washed with water followed by drying.

[0060] Step cl of barium exchange of the zeolite cations is carried out by bringing the agglomerates resulting from step b / (or d / ) into contact with a barium salt, such as BaC in aqueous solution at a temperature between room temperature and 100°C, and preferably between 80°C and 100°C. To quickly obtain a high barium exchange rate, i.e. greater than 90%, it is preferable to operate with a large excess of barium relative to the cations of the zeolite that it is desired to exchange, typically such that the BaO / A^Os ratio is of the order of 10 to 12, proceeding by successive exchanges so as to reach the minimum targeted exchange rate of at least 90% and preferably at least 95%. Throughout the text, the exchange rates are calculated in equivalent and not in molarity.

[0061] The possible exchange with potassium (step d / ) can be carried out before and / or after the exchange with barium (step c / ) and / or simultaneously using a solution containing barium and potassium ions. As indicated previously, it is also possible to agglomerate in step a1 MSX zeolite crystals already containing potassium ions (pre-exchange of the NaMSX zeolite with potassium ions before step a / ) and to avoid (or not) step d / .

[0062] Activation (step e / ), the final step in the process for obtaining the adsorbents according to the invention, aims to set the water content and the loss on ignition of the adsorbent within optimal limits. This is generally carried out by thermal activation, preferably carried out between 200 and 300°C for a certain time depending on the desired water content and loss on ignition, typically from 1 to 6 hours.

[0063] In one embodiment of the present invention, the loss on ignition of the agglomerated zeolite adsorbent according to the invention, measured at 900°C according to standard NF EN 196-2, is less than or equal to 7.7%, preferably between 0 and 7.7%, preferably between 3.0% and 7.7%, more preferably between 3.5% and 6.5% and advantageously between 4.5% and 6.0%, limits included.

[0064] The agglomerates resulting from step e / , whether in the form of balls or extrudates, generally have a number average diameter ranging from 0.4 mm to 2.0 mm, and in particular between 0.4 mm and 0.8 mm.

[0065] In general, the number-average diameter of the zeolite crystals in these zeolite adsorbents resulting from steps a1 to e / is between 0.01 pm and 5 pm, preferably between 0.05 pm and 5 pm, very preferably between 0.1 pm and 4 pm, even more preferably between 0.1 pm and 3 pm and, even more preferably between 0.1 pm and 2 pm. The invention also relates to the uses of at least one zeolite adsorbent according to any one of the variants described as adsorption agents capable of advantageously replacing the adsorption agents described in the literature based on zeolite X or based on zeolite LSX, exchanged with barium or exchanged with barium and potassium, and in particular in the following uses:

[0066] - the separation of aromatic isomers in Cs and in particular xylenes,

[0067] - the separation of sugars,

[0068] - the separation of polyhydric alcohols,

[0069] - the separation of isomers of substituted toluenes such as nitrotoluene, diethyltoluene, toluenediamine,

[0070] - the separation of cresols,

[0071] - the separation of dichlorobenzenes.

[0072] The invention relates in particular to a process for recovering para-xylene from aromatic Cs isomer cuts consisting of using as para-xylene adsorption agent a zeolitic adsorbent according to the invention used in liquid phase processes, but also in gas phase.

[0073] The invention particularly relates to a process for producing high-purity, high-productivity para-xylene from an aromatic hydrocarbon feedstock containing 8-carbon isomers comprising the following steps: a) a step of bringing the feedstock into contact, under suitable adsorption conditions, with an adsorbent bed according to the invention, so as to preferentially adsorb the paraxylene, b) a step of bringing the adsorbent bed into contact, under desorption conditions, with a desorbent, which is preferentially either toluene or para-diethylbenzene, c) a step of withdrawing from the adsorbent bed a stream containing the desorbent and the least selectively adsorbed feedstock products, d) a step of withdrawing from the adsorbent bed a stream containing the desorbent and the paraxylene,e) a step of separating the stream from step c) into a first stream containing the desorbent and a second stream containing the least selectively adsorbed feed products, and f) a step of separating the stream from step d) into a first stream containing the desorbent and a second stream containing paraxylene at a purity level greater than or equal to 75% and preferably greater than or equal to 99.7%.,

[0074] The process may also optionally include the following steps: g) a crystallization step in a crystallizer consisting of the crystallization of the paraxylene from step f), making it possible to obtain, on the one hand, paraxylene crystals soaked in their mother liquor, and on the other hand, a mother liquor which may be partly, or even entirely, recycled in a mixture with the fresh feed at the inlet of the simulated moving bed adsorption unit, and h) a step of washing the crystals from step g) at the end of which para-xylene is recovered at a purity of at least 99.7%, and preferably at least 99.8%.

[0075] The desired product can thus be separated by preparative adsorption liquid chromatography (in "batch"), advantageously in a simulated moving bed, i.e. with simulated countercurrent or simulated cocurrent, and more particularly with simulated countercurrent.

[0076] Simulated countercurrent moving bed chromatographic separation is well known in the state of the art. Generally, a simulated moving bed separation unit comprises at least one adsorption column containing a plurality of beds of an adsorbent, interconnected in a closed loop. The simulated moving bed separation unit comprises at least three chromatographic zones, and possibly four or five, each of these zones consisting of at least one bed or column portion and located between two successive feed or draw-off points.

[0077] Typically, at least one fractionation feed and one desorbent (sometimes called eluent) are fed and at least one raffinate and one extract are withdrawn. The feed and withdrawal points are changed over time, typically shifted towards the bottom of a bed and synchronously.

[0078] By definition, each of the operating zones is designated by a number:

[0079] • Zone 1 = desorption zone of the desired product (contained in the extract) between the injection of the desorbent and the sampling of the extract;

[0080] • Zone 2 = desorption zone of the raffinate compounds, between the extraction of the extract and the injection of the charge to be fractionated;

[0081] • Zone 3 = adsorption zone of the desired product, between the injection of the feedstock and the withdrawal of the raffinate, and;

[0082] • Zone 4 located between the raffinate withdrawal and the desorbent injection.

[0083] The operating conditions of an industrial simulated counter-current adsorption unit are generally as follows: number of beds 6 to 30 number of zones at least 4 temperature 100 to 250°C, preferably 150 to 190°C • pressure between the bubble pressure of xylenes at the process temperature and 3 MPa

[0084] • ratio of desorbent flow rates to load 0.7 to 2.5 (for example 0.9 to 1.8 for a stand-alone adsorption unit and 0.7 to 1.4 for an adsorption unit combined with a crystallization unit)

[0085] • recycling rate of 2.5 to 12, preferably 3.5 to 6. The recycling rate is defined as the ratio between the average flow rate flowing in the different beds of the adsorber and the charge injection flow rate in this adsorber

[0086] Reference may be made to the teaching of patents US2985589, US5284992 and US5629467.

[0087] The operating conditions of an industrial adsorption unit with simulated co-current are generally the same as those operating with simulated counter-current, with the exception of the recycling rate, which is generally between 0.8 and 7. Reference may be made to patents US4402832 and US4498991.

[0088] The desorption solvent may be a desorbent whose boiling point is lower than that of the feedstock, such as toluene, but also a desorbent whose boiling point is higher than that of the feedstock, such as para-diethylbenzene (PDEB). The selectivity of the adsorbents according to the invention for the adsorption of paraxylene contained in Cs aromatic cuts is optimal when their loss on ignition measured at 900°C is generally less than or equal to 7.7%, preferably between 0 and 7.7%, preferably between 3.0% and 7.7%, more preferably between 3.5% and 6.5% and advantageously between 4.5% and 6.0%, limits included.

[0089] One of the techniques of choice for characterizing the adsorption of molecules in the liquid phase on a porous solid is to perform a breakthrough curve. In his book "Principles of Adsorption and Adsorption processes", Ruthven defines the breakthrough curve technique as the study of the injection of a step of adsorbable constituents.

[0090] The present invention is now described with the aid of the following examples, which are intended to illustrate certain embodiments of the invention, without however limiting the scope of said invention, as claimed in the appended claims.

[0091] Analytical techniques

[0092] Identification of zeolite phases

[0093] The MSX zeolite in the zeolite adsorbents of the invention is identified by X-ray diffraction analysis, known to those skilled in the art by the acronym DRX. This analysis is carried out on a DRX D8 Advance device from Bruker. Phase identification is carried out using Bruker EVA software and databases known to those skilled in the art containing a large number of diffractograms such as the ICCD PDF-2 release 2011 database. Si / Al molar ratio and exchange rate

[0094] The measurement of the Si / Al molar ratio and the exchange rate is carried out by any analytical chemical analysis techniques known to those skilled in the art.

[0095] Among these techniques, we can cite the chemical analysis technique by X-ray fluorescence as described in the standard NF EN ISO 12677: 2011 on a wavelength dispersive spectrometer (WDXRF), for example Tiger S8 from the Bruker company.

[0096] X-ray fluorescence is a non-destructive spectral technique that exploits the photoluminescence of atoms in the X-ray range to establish the elemental composition of a sample. The excitation of atoms, generally by an X-ray beam or by bombardment with electrons, generates specific radiation after returning to the ground state of the atom. The X-ray fluorescence spectrum has the advantage of depending very little on the chemical combination of the element, which offers a precise determination, both quantitative and qualitative. A measurement uncertainty of less than 0.4% by weight is typically obtained after calibration for each oxide.

[0097] These elementary chemical analyses make it possible to both verify the Si / Al molar ratio of the starting zeolite and to verify the quality of the ionic exchange described in step c) and in the optional step d).

[0098] The quality of ion exchange is related to the number of moles of sodium oxide, Na2O, remaining in the agglomerated zeolite adsorbent after exchange. More precisely, the exchange rate by 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 + Na2O). Similarly, the exchange rate by barium and potassium ions is estimated by evaluating the ratio between the number of moles of the whole barium oxide + potassium oxide (BaO + K2O) and the number of moles of the whole (BaO + K2O + Na2O). It should be noted that the contents of different oxides are given as a percentage by weight relative to the total weight of the anhydrous zeolite adsorbent.

[0099] The Si / Al molar ratio of the zeolite present in the agglomerated zeolite adsorbent is measured by solid-state Nuclear Magnetic Resonance (NMR) spectroscopy of silicon.

[0100] Grain size of zeolite crystals:

[0101] The estimation of the number-average diameter of the zeolite crystals used in step a) and of the zeolite crystals contained in the agglomerates is carried out by observation under a scanning electron microscope (SEM).

[0102] In order to estimate the size of the zeolite particles (i.e. crystals) on the samples, a set of images is taken at a magnification of at least 5000. The diameter of at least 200 particles is then measured using dedicated software, for example the Smile View software from the publisher LoGraMi. The accuracy is of the order of 3%. The measurement of the histogram formed from the said diameter measurements simultaneously allows the determination of the standard deviation o of its distribution.

[0103] This scanning electron microscope (SEM) observation of zeolite crystals also makes it possible to distinguish the crystalline structures of zeolites (LSX, MSX, X).

[0104] Particle size of zeolite adsorbents:

[0105] The determination of the number-average diameter of the zeolite adsorbents obtained from step a) of agglomeration and shaping is carried out by analyzing the particle size distribution of an agglomerate sample by imaging according to ISO 13322-2:2006, using a conveyor belt allowing the sample to pass in front of the camera lens.

[0106] The number-average diameter is then calculated from the particle size distribution by applying ISO 9276-2:2001. In this document, the term "number-average diameter" or "size" is used for zeolite agglomerates. The accuracy is of the order of 0.01 mm for the agglomerate size range of the invention.

[0107] Microporous volume:

[0108] The crystallinity of the agglomerates is also assessed by measuring their micropore volume by comparing it to that of an appropriate reference (100% crystalline zeolite under identical cationic treatment conditions or theoretical zeolite). This micropore volume is determined from the measurement of the adsorption isotherm of gas, such as nitrogen, at its liquefaction temperature.

[0109] Prior to adsorption, the zeolite adsorbent is degassed between 300°C and 450°C for a period of between 9 hours and 16 hours, under vacuum (P < 6.7.10 -4 Pa). The measurement of the nitrogen adsorption isotherm at 77 K is then carried out on a Micromeritics ASAP 2020 M type device, taking at least 35 measurement points at relative pressures with a P / Po ratio between 0.002 and 1.

[0110] Loss on ignition of zeolite adsorbents:

[0111] The loss on ignition is determined in an oxidizing atmosphere, by calcining the sample in air at a temperature of 900°C ± 25°C, following the procedure described in standard NF EN 196-2 (April 2006). The standard deviation of measurement is less than 0.1%.

[0112] Examples if): Preparation of a BaLSX type adsorbent with zeolitized binder

[0113] In this example, an adsorbent according to the prior art is produced and tested.

[0114] 840 g (expressed as calcined equivalent) of LSX zeolite crystals with a Si / Al ratio of 1.01 and 160 g of kaolin (expressed as calcined equivalent) are intimately mixed and agglomerated with the appropriate quantity of water to operate by extrusion. The extrudates are dried, crushed to recover grains with an equivalent diameter of 0.7 mm, then calcined at 550°C under a stream of nitrogen for 2 hours.

[0115] 200 g of granules thus obtained are placed in a glass reactor equipped with a double jacket regulated at a temperature of 95 ± 1 °C then 700 mL of an aqueous sodium hydroxide solution with a concentration of 220 g / L are added and the reaction medium is left stirring for 3 hours.

[0116] The granules are then washed in 4 successive water washing operations followed by emptying the reactor. The effectiveness of the washing is ensured by measuring the final pH of the wash water, which must be between 10 and 10.5.

[0117] A barium exchange is then carried out under operating conditions identical to those of example 1, followed by washing and then drying at 80°C for 2 hours and finally activation at 200°C for 2 hours under a stream of nitrogen.

[0118] The barium exchange rate of this adsorbent is 97%. The micropore volume measured according to the Dubinin method by nitrogen adsorption at 77 K after pretreatment at 500°C for 12 hours under vacuum is 0.235 cm 3 / g.

[0119] The mechanical resistance is also measured according to the method presented in the description of the invention. The pressure required to obtain 0.5% fines is 2.70 MPa. Preparation of a BaX-type adsorbent with zeolitic binder

[0120] In this example, an adsorbent according to the prior art is produced and tested.

[0121] 900 g (expressed as calcined equivalent) of zeolite X crystals, with a Si / Al ratio of 1.25 and an average crystal size of 1.6 pm, are agglomerated by mixing them intimately with 170 g of kaolin (expressed as calcined equivalent), 70 g of colloidal silica sold under the trade name Klebosol® 30 (containing 30% by weight of SiC>2 and 0.5% of Na2<3) and the appropriate quantity of water for shaping the agglomerates by extrusion. The extrudates are dried, crushed to obtain agglomerates with an equivalent diameter of 0.7 mm, and then activated at a temperature of 550°C for 2 hours under a stream of nitrogen.

[0122] 200 g of granules thus obtained are placed in a glass reactor equipped with a double jacket regulated at a temperature of 100 ± 1 °C then 1.5 L of an aqueous sodium hydroxide solution with a concentration of 100 g / L are added and the reaction medium is left stirring for 3 hours. The granules are then washed in 3 successive operations of washing with water followed by emptying the reactor. The effectiveness of the washing is ensured by measuring the final pH of the washing water, which must be between 10 and 10.5.

[0123] A barium exchange is then carried out under operating conditions identical to those of example 1, followed by washing and then drying at 80°C for 2 hours and finally activation at 200°C for 2 hours under a stream of nitrogen.

[0124] The barium exchange rate is 95%.

[0125] The micropore volume measured by the Dubinin method by nitrogen adsorption at 77 K after pretreatment at 500°C for 12 hours under vacuum is 0.256 cm 3 / g.

[0126] The mechanical resistance is also measured according to the method presented in the description of the invention. The pressure required to obtain 0.5% fines is 2.50 MPa. 50:50 mixture weight of samples from examples 1 and 2

[0127] (BaLSX type zeolite adsorbent and BaX type zeolite adsorbent)

[0128] A mechanical mixture of the samples of examples 1 and 2 is carried out 50:50 by weight.

[0129] The apparent Si / Al molar ratio of this mixture is 1.13. Preparation of a BaMSX type adsorbent with zeolitized binder

[0130] BaMSX crystals (Si / AI = 1.14) are prepared according to Table 4 Example 27 of patent US6596256.

[0131] The analysis of the size of zeolite crystals is carried out by scanning electron microscopy. The average size of the crystals is 2.8 pm.

[0132] 840 g (expressed as calcined equivalent) of these MSX zeolite crystals are intimately mixed and agglomerated with 170 g of kaolin (expressed as calcined equivalent), 40 g of colloidal silica sold under the trade name Klebosol® 30 (containing 30% by weight of SiO2 and 0.5% of Na2O) and with the appropriate quantity of water to operate by extrusion. The extrudates are dried, crushed so as to recover grains with an equivalent diameter equal to 0.7 mm, then calcined at 600°C under a stream of nitrogen for 2 hours.

[0133] 200 g of granules thus obtained are placed in a glass reactor equipped with a double jacket regulated at a temperature of 95 ± 1 °C then 700 mL of an aqueous solution of sodium hydroxide with a concentration of 170 g / L are added and the reaction medium is left stirring for 3 hours. The granules are then washed in 3 successive operations of washing with water followed by emptying the reactor.

[0134] The effectiveness of the washing is ensured by measuring the final pH of the washing water, which must be between 10 and 10.5. A barium exchange is then carried out under operating conditions identical to those of Example 1, followed by washing and then drying at 80°C for 2 hours and finally activation at 200°C for 2 hours under a stream of nitrogen.

[0135] The barium exchange rate of this adsorbent is 97%. The micropore volume measured according to the Dubinin method by nitrogen adsorption at 77 K after pretreatment at 500°C for 12 hours under vacuum is 0.255 cm 3 / g.

[0136] The mechanical strength is also measured according to the method presented in the description of the invention. The pressure required to obtain 0.5% fines is 2.60 MPa.

[0137] An agglomerated zeolite adsorbent of BaMSX is obtained according to the invention.

[0138] Example 5: Piercing test (frontal chromatography)

[0139] The Loss on Ignition (LOI) for each sample is adjusted to a value of 6.0%.

[0140] A breakthrough test (frontal chromatography) is then carried out on these adsorbents to evaluate their effectiveness. The quantity of adsorbent used for this test is approximately 82 g.

[0141] The operating mode and the composition of the charge are identical to those of example 1. The operating mode for obtaining the drilling curves is as follows:

[0142] -Filling the column through the sieve and placing it in the test bench.

[0143] -Filling with solvent at room temperature.

[0144] -Progressive rise to the adsorption temperature under solvent flow (5 cm 3 / min).

[0145] -Solvent injection at 10 cm 3 / min when the adsorption temperature is reached - Solvent / charge permutation to inject the charge (10 cm 3 / min).

[0146] -The injection of the charge is then maintained for a sufficient time to reach thermodynamic equilibrium.

[0147] -Collection and analysis of drilling effluent.

[0148] The pressure is sufficient for the charge to remain in the liquid phase, i.e. 1 MPa. The adsorption temperature is 175°C.

[0149] The composition of the charge is as follows:

[0150] Paraxylene: 45% by weight

[0151] Metaxylene: 45% by weight

[0152] Iso-octane: 10% by weight (this is used as a tracer for non-selective volume estimation and is not involved in the separation)

[0153] The drilling results are shown in Table 1.

[0154] Table 1

[0155] (1) PX: para-xylene, MX: meta-xylene

[0156] Surprisingly, the zeolite absorbents according to the invention exhibit a selectivity for para-xylene compared to meta-xylene that is much higher than those measured with the zeolite absorbents of the prior art.

Claims

CLAIMS 1. Agglomerated zeolite adsorbent based on MSX zeolite crystals, having an atomic ratio Si / AI such that 1.10 < Si / AI < 1.18, preferably 1.10 < Si / AI < 1.17, more preferably 1.10 < Si / AI < 1.16, more preferably 1.11 < Si / AI < 1.16, of which at least 90%, preferably at least 95% of the exchangeable cationic sites are occupied, either by barium ions alone or by barium ions and potassium ions.

2. Agglomerated zeolite adsorbent according to claim 1, the microporous volume of which measured according to the Dubinin method by nitrogen adsorption at 77 K after pretreatment at 500°C for 12 hours under vacuum is greater than or equal to 0.200 cm 3 / g, preferably greater than or equal to 0.220 cm 3 / g, preferably still greater than or equal to 0.225 cm 3 / g, even more preferably greater than or equal to 0.250 cm 3 / g.

3. Agglomerated zeolite adsorbent according to claim 1, the exchangeable sites occupied by potassium of which represent up to 1 / 3 of the exchangeable sites occupied by barium + potassium ions, and the possible complement of which is generally provided by alkali or alkaline-earth ions other than barium and potassium.

4. Agglomerated zeolite adsorbent according to claim 1, comprising an inert binder in a proportion less than or equal to 20% by weight, preferably 15% by weight, of the total mass of the agglomerate.

5. Agglomerated zeolite adsorbent according to one of the preceding claims, having a size distribution such that the number average diameter is between 0.4 mm and 2.0 mm, preferably between 0.4 and 0.8 mm.

6. Agglomerated zeolite adsorbent according to one of the preceding claims, the mechanical resistance of which, measured by the Shell series SMS1471-74 method adapted to agglomerates of a size less than 1.6 mm, is greater than or equal to 2 MPa, preferably greater than or equal to 2.5 MPa.

7. Agglomerated zeolite adsorbent according to one of the preceding claims, the loss on ignition of which measured at 900°C is less than or equal to 7.7%, preferably between 0 and 7.7%, preferably between 3.0% and 7.7%, more preferably between 3.5% and 6.5% and advantageously between 4.5% and 6.0%, limits included.

8. Agglomerated zeolite adsorbent according to one of the preceding claims, in which the MSX zeolite crystals have a number average diameter of between 0.01 pm and 5 pm, preferably between 0.05 pm and 5 pm, very preferably between 0.1 pm and 4 pm, even more preferably between 0.1 pm and 3 pm and even more preferably between 0.1 pm and 2 pm.

9. Process for the separation of sugars, polyhydric alcohols, substituted toluene isomers, cresols or for the recovery of paraxylene, using an agglomerated zeolite adsorbent according to any one of claims 1 to 8, in the presence of a desorbent, in the liquid phase or in the gas phase.

10. Process according to claim 9, for recovering paraxylene from aromatic Cs isomer cuts, by adsorption of paraxylene using said agglomerated zeolite adsorbent, in the presence of a desorbent, in the liquid phase or in the gas phase.

11. Method according to any one of claims 9 or 10, implemented in a simulated moving bed, in simulated co-current or in simulated counter-current.

12. Process according to one of claims 9 to 11 for the production of high-purity and high-productivity paraxylene from a feedstock of aromatic hydrocarbons containing isomers with 8 carbon atoms, comprising the following steps: a) a step of bringing the feedstock into contact with a bed of agglomerated zeolite adsorbent, so as to preferentially adsorb the paraxylene, b) a step of bringing the adsorbent bed into contact, under desorption conditions, with a desorbent, which is preferentially either toluene or para-diethylbenzene, c) a step of withdrawing from the adsorbent bed a stream containing the desorbent and the least selectively adsorbed feedstock products, d) a step of withdrawing from the adsorbent bed a stream containing the desorbent and the paraxylene, e) a step of separating the stream from step c) into a first stream containing the desorbent and a second stream containing the least selectively adsorbed load products,and f) a step of separating the stream from step d) into a first stream containing the desorbent and a second stream containing paraxylene at a purity level greater than or equal to 75%., 13. Method according to claim 12 further comprising: g) a crystallization step in a crystallizer consisting of the crystallization of the paraxylene from step f), making it possible to obtain on the one hand paraxylene crystals soaked in their mother liquor, and on the other hand a mother liquor which can be partly, or even entirely, recycled in a mixture with the fresh feed at the inlet of the simulated moving bed adsorption unit, and h) a step of washing the crystals from step g) at the end of which paraxylene is recovered at a purity of at least 99.7%, and preferably at least 99.8%.

14. Use of an agglomerated zeolite adsorbent based on MSX zeolite crystals having an Si / AI atomic ratio such that 1.10 < Si / AI < 1.18, preferably 1.10 < Si / AI < 1.17, more preferably 1.10 < Si / AI < 1.16, more preferably 1.11 < Si / AI < 1.16 of which at least 90% of the exchangeable cationic sites are occupied, either by barium ions alone or by barium ions and potassium ions for the separation of sugars, polyhydric alcohols, substituted toluene isomers, cresols or for the recovery of paraxylene, in the presence of a desorbent, which is preferably either toluene or para-diethylbenzene, in the liquid phase or in the gas phase.

15. Use of an agglomerated zeolite adsorbent according to claim 14, for the recovery of paraxylene from aromatic C8 isomer cuts, by adsorption of paraxylene in a simulated moving bed, simulated co-current or simulated counter-current reactor.

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