WO2013011210A1

WO2013011210A1 - Method for separating paraxylenes using an adsorbent from the family of zifs of sod structure.

Info

Publication number: WO2013011210A1
Application number: PCT/FR2012/000293
Authority: WO
Inventors: David Peralta; Karin Barthelet; Gerhard Pirngruber; Gérald CHAPLAIS; Angélique SIMON-MASSERON; Joël Patarin
Original assignee: IFP Energies Nouvelles; Centre National De La Recherche Scientifique (C.N.R.S.); Universite De Haute Alsace
Priority date: 2011-07-21
Filing date: 2012-07-13
Publication date: 2013-01-24
Also published as: FR2978063A1

Abstract

The invention describes a method for separating paraxylene from an aromatic hydrocarbon feedstock containing isomers with 8 carbon atoms by liquid-phase or gas-phase adsorption. The adsorbent used belongs to the family of ZIFs of SOD structure, containing an inorganic network of metal centres based on Zn²⁺ cations connected to one another by organic imidazolate ligands substituted in position 2.

Description

PROCESS FOR PARAXYLENE SEPARATION USING SIF STRUCTURAL TYPE ZIF FAMILY ADSORBENT

Technical field of the invention

The invention relates to a process for the production of paraxylene from an aromatic hydrocarbon feedstock containing isomers containing 8 carbon atoms.

It is particularly applicable to the production of paraxylene to produce a petrochemical intermediate, phthalic acid.

PRIOR ART The production of high purity paraxylene from a mixture of aromatic hydrocarbons by separation adsorption is well known in the prior art. Zeolites X and Y, exchanged by means of ions such as barium, potassium or. strontium, alone or in admixture, selectively adsorb paraxylene in a mixture containing at least one other C8 (eight carbon atom) aromatic isomer. For example, US Patents 3,558,730, US 3,558,732, US 3,626,020, US 3,663,638 and US 6,410,815 disclose adsorbents comprising barium and potassium exchanged aluminosilicates which effectively separate paraxylene from a mixture of C8 aromatic isomers.

Nevertheless, the adsorbents used for said separation remain perfectible. The performance of an adsorbent in a paraxylene separation process depends mainly on two criteria. The first is the ability to adsorb paraxylene in a mixture of C8 aromatics. This capacity is often expressed as the number of moles of paraxylene per mass or volume of adsorbent. An increase in the adsorption capacity makes it possible to reduce the quantity of solid necessary for the separation, which reduces the cost of the process. The second criterion is the adsorption selectivity. The adsorption selectivity between two compounds A and B is defined as follows:

SA B ⁼ QA.ads / BB.ads / CA CB Û _A _d = adsorbed amount of compound A (in moles per gram or mole per volume of adsorbent)

c _A = concentration of A in the fluid phase (in moles per volume)

The aim is to maximize the adsorption of paraxylene relative to the other C8 aromatic isomers, in particular metaxylene, orthoxylene and ethylbenzene. A Increased selectivity produces paraxylene with better purity or reduces the amount of adsorbent to produce paraxylene with the same purity as a less selective adsorbent. The SA / B adsorption selectivity is often governed either by the boiling point of the two compounds A and B, or by their dipole moment. As a general rule, the compound with the higher boiling point or the larger dipole moment is selectively adsorbed. Among the C8 aromatic isomers, neither the boiling points nor the dipole moment favor the adsorption of paraxylene. Indeed, the para-selectivity of zeolites exchanged with barium and / or potassium comes from a very complex mechanism that is difficult to predict. The only physico-chemical property that can promote the adsorption of paraxylene is its kinetic diameter, which is lower than that of the other C8 isomers. This property can be exploited by a so-called molecular sieve separation, i.e., a separation depending on the size of the molecule. A molecular sieve is an adsorbent that has a pore opening close to the kinetic diameter of the molecules to be separated. It selectively adsorbs the smallest molecule relative to the larger molecule. The disadvantage of molecular sieving for the separation of xylenes is that the kinetic diameters of paraxylene and ethylbenzene are close. It will therefore be a priori difficult to obtain a good selectivity S _P X _{/ E} B (PX = paraxylene, EB = ethylbenzene).

Table 1: Kinetic diameter, boiling point (T _e b) and dipolar moment of aromatic isomers C8.

Since the 1990s, there has been a particular interest in hybrids with mixed organic-inorganic matrix, also called MOFs (Metal-Organic Frame Orks) or coordination polymers. MOFs are porous crystalline solids in which the sub-networks of metal cations (dimers, trimers, tetramers, chain, plane) are interconnected by organic molecules serving as multidentate ligands to form a two- or three-dimensional crystalline structure.

A subfamily of the MOFs is constituted by the ZIF (Zeolitic Imidazolate Framework) family whose structure and preparation are described, for example, in US 2007/202038 A1.

ZIFs consist of assemblages of tetrahedral units that consist of a bivalent M ²⁺ cation (Zn ²⁺ or Co ²⁺ ) in the center of the tetrahedron and four imidazolates Im ^" at the vertices of the tetrahedron. between them by the vertices, that is to say that each imidazolate is shared between two tetrahedrons.The units M ²⁺ (lm ^" ) _4/2 are similar to the units Si0 _{/ 2} in the zeolites and the angle M- lm-M is close to the Si-O-Si angle in a zeolite. Therefore, ZIF structures are obtained with the same topology or structural type as the zeolites. Since the length of the Im-M-1m bond is greater than that of the O-Si-O bond, the pore size and the pore volume of a ZIF solid may be greater than those of the analogous zeolite structure, conditions that the pores of ZIF are not clogged by organic ligands.

ZIFs are known to have already been used in the separation of compounds present in a gaseous mixture. In particular, the patent application WO 2008/140788 teaches the use of ZIF-8 for the separation of C0 ₂ present in a CO ₂ / CH ₄ and CO ₂ / CO mixture.

A particular subgroup of the ZIF family is the solids that have the SOD topology or structural type. The metal center is the Zn ²⁺ cation or the Co ²⁺ cation and the organic ligand is an imidazolate substituted in the 2-position by a -methyl, chlorine or bromine group. The best known example is the solid ZIF-8. The pore opening size of ZIF-8 is 3.4A, i.e., significantly smaller than the critical diameter of all C8 aromatic isomers. In view of its pore opening size, the ZIF-8 solid does not seem to be a good candidate for the separation of xylenes. The small pore opening size of ZIF-8 suggests that this material is not expected to adsorb any of the C8 aromatic isomers. Summary and interest of the invention

The present invention relates to a paraxylene separation process included in a hydrocarbon feedstock comprising, in addition, aromatic isomers containing 8 carbon atoms, such as metaxylene, orthoxylene and ethylbenzene, comprising contacting said feedstock with at least one a zeolitic adsorbent of structure type SOD belonging to the family of ZIF containing an inorganic network of metal centers based on Zn ²⁺ cations connected to each other by organic ligands imidazolates substituted in position 2 so as to produce at least one enriched stream in paraxylene. This process is carried out by adsorption in the liquid phase or in the gas phase.

According to a variant of the process of the invention, the process is advantageously carried out by pressure swing adsorption (PSA). According to a second variant of the process according to the invention, the process is advantageously carried out by a simulated moving bed (L S) type process.

It has been very surprisingly discovered that solids belonging to the ZIF family, having the SOD structural type and a 2-substituted imidazolate ligand, have both a good paraxylene adsorption capacity and adsorb very selectively the paraxylene in a mixture containing aromatic isomers C8, and preferably in a mixture containing orthoxylene, metaxylene and ethylbenzene. The adsorption capacity and the paraxylene selectivity are higher than those of a barium-exchanged X zeolite which is currently used for paraxylene separation on an industrial scale.

Said adsorbent is preferably selected from ZIF-8, ZIF-90, ZIF-91 solids and ZIF solids for which the substituent at the 2-position is chlorine (2-chloroimidazolate ligand) or bromine (2-bromoimidazolate ligand).

Detailed description of the invention

The present invention relates to a paraxylene separation process included in a feed containing aromatic C8 isomers, namely, paraxylene, metaxylene, orthoxylene, ethylbenzene (PX, MX, OX, EB), comprising the contacting of said filler with at least one adsorbent of structural type SOD belonging to the ZIF family containing an inorganic network of metal centers based on Zn ²⁺ cations connected to each other by organic ligands imidazolates substituted in position 2 so as to produce a flow called the highly enriched paraxylene extract compared to other isomers. This process is carried out by adsorption in the liquid phase or in the gas phase.

The adsorbent of the separation process of the invention is a structural type material SOD and belonging to the ZIF family containing an inorganic network of metal centers based on Zn ²⁺ connected to each other by organic ligands imidazolates substituted in the 2-position. . More specifically, each of said organic ligand is a heterocyclic aromatic compound, negatively charged, containing two nitrogen atoms separated by a carbon atom bearing a substituent preferably selected from -CH _3l -Cl, -Br, -CH ₂ OH and - CHO. The developed formulas of each of said 2-substituted ligands with a substituent selected from -CH ₃ , -Cl, -Br, -CH ₂ OH and -CHO are given below.

Said adsorbent used in the separation process of the invention has a chemical composition having the base unit Zn [N- (CH-CH) -N-CR] ₂ , simplified in the form Zn [lm- R] ₂ where R is the 2-substituent preferably selected from -CH _3, -Cl, -Br, -CH ₂ OH and -CHO and Im represents the imidazolate ligand. The adsorbents with a zeolitic framework, of structural type SOD and belonging to the ZIF family, for which the substituent in position 2 is respectively the methyl group (-CH ₃ ), the group -CHO and the group -CH ₂ OH, put in the separation process of the present invention are known respectively as ZIF-8, ZIF-90 and ZIF-91.

The structure and synthesis of ZIF-8 are described in detail in US Patent Application 2007/202038 A1, as well as in Park et al., PNAS 2006, 103, p. 10186-10191 and Huang et al., Angew. Chemie Int. Ed. 2006, 45, 1557. Said ZIF-8 solid advantageously implemented in the separation process of the invention has a three-dimensional structure in which the inorganic network formed by metal centers based on Zn ²⁺ cations acting as Connectors are bonded together by 2-methylimidazolate ligands (1m-CH ₃ ) to obtain Zn (1m-CH ₃ ) ₂ stoichiometry. Said solid ZIF-8 belongs to the cubic crystallographic system, the space group is l-43m and the mesh parameter (a = b = c) is 17.0 +/- 0.2 A. Said solid ZIF-8 presents porosity very predominantly microporous. It comprises cages with a diameter of approximately 1, 16 nm (1 nm = 10 ^"9 m), which are interconnected in three dimensions of space via an aperture of approximately 0.34 nm in diameter .

The ZIF-90 solid for which the substituent at the 2-position is the -CHO aldehyde function, having the base motif [Zn (lm-CHO) ₂ ], and the solid ZIF-91 for which the substituent at the 2-position is the group -CH ₂ OH, having the basic motif [Zn (1m-CH ₂ OH) ₂ ], are described in Morris et al., J. Am. Chem. Soc. 2008, 130, 12626. The solid ZIF-91 is obtained by reducing the aldehyde function, present in the solid ZIF-90, in alcohol. The zeolitic framework adsorbents, of structural type SOD and belonging to the ZIF family, for which the substituent at the 2-position is chlorine (2-chloroimidazolate ligand) or bromine (2-bromoimidazolate ligand) are also used in the separation process of the invention. The SOD structural ZIF solid having the base motif [Zn (lm-Cl) ₂ ] and the SOD structural ZIF solid having the base motif [Zn (lm-Br) ₂ ] are described in Li and al., J. Am. Chem. Soc. 131, 2009, 10368.

According to the mode of synthesis of the adsorbents used in the separation process of the present invention, the specific surface of the adsorbent advantageously varies in a range between 400 and 1600 m ² / g, preferably between 1000 and 1600 m ² / g more preferably between 1200 and 1600 m ² / g. The filler treated in the process according to the invention contains mainly, preferably more than 80%, preferably more than 90% of aromatic compounds having 8 carbon atoms per molecule. In particular, said feed contains paraxylene, orthoxylene, metaxylene and ethylbenzene (respectively denoted PX, OX, MX, EB). Advantageously, in addition to the C8 isomers, the filler may also contain traces of benzene and toluene.

According to the separation process according to the invention, the fractionation of the charge to be treated is carried out in a separation unit containing one or more adsorbents, at least one of the adsorbents being a zeolite structural adsorbent of SOD structural type belonging to the family. ZIF as described above in the present description.

The separation process according to the invention is carried out according to adsorption separation techniques well known to those skilled in the art. In particular, in a variant of the process of the invention, the process is advantageously carried out by pressure swing adsorption (PSA or pressure swing adsorption according to the English terminology). According to a second variant of the process according to the invention, the process is advantageously implemented by a simulated moving bed (LMS) method. The principle of the LMS process is well known to those skilled in the art and is described in detail in FIG. G. Ash, K. Barth, G. Hotier, L. Mank and P. Renard, Review of the French Petroleum Industry, 49, 541 (1994) and in US Patent 2,985,589. The LMS process, carried out in the liquid phase, uses at least one adsorption column in which the adsorption and desorption phases are carried out simultaneously within distinct zones. According to the invention, said adsorption column is provided with at least one bed of said SOD structural type adsorbent belonging to the ZIF family as described above in the present description. The LMS process comprises at least the following steps: a) a step of contacting, under adsorption conditions, the feedstock with an adsorbent bed containing a structural type SOD solid belonging to the family of ZIF whose framework consists of an inorganic network of metal centers based on Zn ²⁺ cations connected to each other by organic ligands imidazolates substituted in position 2.

b) a step of contacting, under desorption conditions, the adsorbent bed with an eluent. c) a step of withdrawing the adsorbent bed from a flow containing the eluent and the less adsorbed feed products (the raffinate)

d) a step of withdrawing the adsorbent bed from a stream containing the eluent and paraxylene (the extract) The process may optionally include the following two steps:

e) a separation of the stream of step c) into a first stream containing the eluent and a second stream containing the least adsorbed feed products

f) a separation of the stream of step d) into a first stream containing the eluent and a second stream containing paraxylene.

The eluent E (or desorbent) is chosen so that S _{PX / B} > SB is MX, OX or EB. Very preferably, the eluent used is paradiethylbenzene.

Advantageously, at the end of step f), the second flux containing paraxylene comprises 80% by weight of paraxylene, preferably 90% by weight of paraxylene, even more preferably 99% by weight of paraxylene relative to the total flow.

The LMS process is carried out in the liquid phase, but the latter can advantageously be carried out in the gas phase. The LMS process in the present invention is operated at a temperature between 50 and 200 ° C, preferably between 100 and 200 ° C, more preferably between 100 and 150 ° C. The pressure is between 0.1 and 4 MPa.

In a preferred variant, the separation process according to the invention is carried out by pressure swing adsorption (PSA), at least one bed of said SOD structural type adsorbent belonging to the ZIF family as described hereinabove. The invention is placed in at least one adsorption column. Several columns provided with said adsorbent are generally installed in parallel. Each column undergoes a cycle comprising at least one adsorption step and at least one desorption step, which are optionally interspersed with depressurization, pressure equalization and repressurization steps. The continuous treatment of the load is ensured by peremutation of the periods of the cycles practiced in the different columns placed in parallel. The very principle of the PSA process lies in the cyclic sequence of high adsorption phases. pressure and desorption at low pressure, possibly with additional steps of pressure equalization and purge. The operating mode of the PSA process is recalled in the patent application FR-A-2 910 457. For the implementation of the adsorption step: a) the hydrocarbon feedstock to be treated is introduced into at least one adsorption column by the feed end, b) the implementation of said adsorption step is preferably carried out at a total pressure of between 0.1 and 4 MPa. Said adsorption step leads to the production of a first depleted paraxylene stream. c) the desorption of paraxylene, carried out by lowering the pressure in the adsorption column (s), the desorption pressure being generally between 0.01 and 1 MPa, and / or by a purge gas which is introduced into the column (es) by the production end.

The purge gas is preferably formed of compounds that are only slightly adsorbed by the adsorbent. Preferably, the purge gas is either benzene or toluene. The separation process according to the invention, implemented by pressure swing adsorption (PSA), is advantageously carried out at a temperature of between 100 and 200 ° C., preferably between 125 and 175 ° C.

The adsorption capacity and adsorption selectivity of SOD structure type adsorbents, belonging to the ZIF family, were evaluated by the drilling curve method. In her book Principles of Adsorption and Adsorption Processes, Ruthven defines the technique of breakthrough curves as the study of injection molding. a step of adsorbable constituents. The technique of drilling curves is therefore to inject a load. containing one or more constituents, in the form of a step, in a column filled with the adsorbent material. This technique is applicable in the gas phase as well as in the liquid phase. In the liquid phase, the column is initially filled with eluent. The drilling curve is triggered by sending a load flow into the column. In the case of the C8 aromatics separation, the feedstock is composed of aromatic C8 components in the presence or absence of eluent, pure or in mixture, (PX, MX, OX, EB and eluent). In some cases, a non-adsorbed tracer is added to the feed. By fractionation of the column effluent and analysis of each sample, so-called drilling curves are plotted whose position and shape give information respectively on the thermodynamics and kinetics of the system under consideration. A material balance calculation on the column makes it possible to calculate the adsorbed quantities and the adsorption selectivity between the various compounds of the charge. The analysis of the shape of the curves makes it possible to extract information on the kinetics of the phenomena observed.

It is also possible to perform a drilling, that is to say, to send an eluent stream in the column initially filled with the load. The breakthrough curve is plotted by an analysis of the effluent from the column. A material balance calculation makes it possible to calculate the quantity desorbed from the column.

In the gas phase, the column is initially filled with inert gas. The procedure is the same as for the liquid phase. The effluent from the column is, for example, analyzed by injection into a chromatograph in fixed time intervals. The examples which follow illustrate the invention without, however, limiting its scope.

EXAMPLE 1 Separating Performance of ZIF-8 for the Separation of a Paraxylene / Metaxylene Mixture in the Gas Phase

This example illustrates the PX / MX and PX OX separation by the ZIF-8 solid. The tests were carried out with equimolar mixtures of PX MX and PX OX, respectively, under the following conditions: temperature equal to 125 ° C., partial pressure of the hydrocarbons equal to 675 Pa, and flow rate of 1 NL / h.

0.7 gram of the solid ZIF-8 (Sigma-Aldrich) are placed in a column about 10 cm long. The solid ZIF-8 is pretreated under a stream of helium at a temperature equal to 150 ° C. Then the temperature of the column is stabilized at the adsorption temperature equal to 125X and the total pressure in the column is equal to 0.1 MPa. The drilling curve is triggered by switching the feed of the column consisting of a flow pure helium on a mixture of PX and MX or OX, respectively, diluted in helium. The concentration of PX, MX or OX at the outlet of the column is monitored over time by gas phase chromatography until all the concentrations stabilize at their input values. The piercing curve of each constituent of the load can thus be constructed.

The first moment of the drilling curve of a given compound makes it possible to calculate the adsorbed quantity of each compound by the well known method known as "moments" (Ruthven, DM principles of adsorption and adsorption processes, John Wiley & Sons ed, 1984 ).

The first moment of the curve is obtained by the integration

where C / o is the initial concentration of compound i in the feed and is the concentration at the output of compound i as a function of time.

The adsorbed amount of compound i is proportional to the first moment of the drilling curve (after correction for the dead time). It is given by the formula:

where P is the partial pressure of the compound i in the feed, P _tot the total pressure, m the mass of adsorbent, p the grain density of the adsorbent, F the total molar flow, _υ the first moment of the curve drilling of component i, and Q _at _js, i the adsorbed amount of the compound i.

The adsorption selectivity Si _{/ j} between compounds i and j is calculated according to the formula _{Si / i} = ^Qads - ' ^{X Pj}

where P 1 and P _j are the pressures of the compound i and the compound j and Q _adSj and Q _adsJ are the adsorbed amounts of the compound i respectively of the compound j. Table 2 summarizes the separation performance of ZIF-8 in terms of adsorption capacity and adsorption selectivity. Table 2: adsorption capacity and adsorption selectivity of ZIF-8 for the adsorption of the PX / MX or PX / OX mixture in the gas phase at 125 ° C.

The results in Table 2 demonstrate that ZIF-8 exhibits adsorption selectivity to paraxylene over metaxylene or orthoxylene.

Example 2: Separating performance of ZIF-8 for the separation of a PX / MX / OX / EB mixture in the liquid phase 1.3 grams of ZIF-8 are placed in a column of a length of about 10 cm. The column is filled with paradiethylbenzene and is heated to 70 ° C. The pressure is set at 12 bar (1 bar = 0.1 MPa). After stabilization of the temperature, a flow of 0.42 cm ³ / min of a mixture of PX / OX MX EB / TMB in mass proportions of 22.5 / 22.5 / 22.5 / 22.5 / 10 is sent in the column. TMB (1,3,5-trimethylbenzene) is used as a tracer. The effluent from the column is collected in several vials and each vial is analyzed by gas chromatography, which allows to draw the drilling curve of PX, OX, MX, EB, TMB.

The quantities PX, OX, MX, EB, TMB retained in the column are calculated by a material balance. The retained quantity Q _ret of the tracer TMB makes it possible to distinguish for the other compounds the quantity retained in the interstitial volume Q _inter of the column and the actually adsorbed quantity Q _a d _S -

QretJMB ⁼ Qinter.TMB

Qinter.i ⁼ Q CTMB * Qinter.TMB ⁼ PX, MX, OX, EB

Qads.i ⁼ Qretj "Qinter.i

The adsorption selectivities are then calculated according to The test described above is reproduced to evaluate the separating performance of the BaX zeolite (temperature 175 ° C., dodecane tracer, pressure 12 bar), conventionally used for the separation of paraxylene. The results are shown in Table 2.

Table 3: Adsorption capacity and adsorption selectivity of ZIF-8 for separation of a PX / MX / OX / EB mixture in liquid phase

Table 3 shows that ZIF-8 has an advantage both in terms of adsorption capacity and in terms of selectivity with respect to BaX zeolite. It is surprisingly found that the ZIF-8 PX / EB selectivity is very high, despite the fact that ZIF-8 separates the isomers of xylenes in principle according to their size.

Claims

A process for separating paraxylene from a hydrocarbon feedstock comprising, in addition, aromatic isomers containing 8 carbon atoms, such as metaxylene, orthoxylene and ethylbenzene, comprising contacting said feed with at least one adsorbent with zeolite framework of SOD structural type belonging to the ZIF family containing an inorganic network of metal centers based on Zn ²⁺ cations connected to each other by organic ligands imidazolates substituted in the 2-position so as to produce at least one flux enriched in paraxylene, said process being carried out by adsorption in the liquid phase or in the gas phase.

2. Paraxylene separation process according to claim 1 wherein the process is carried out by pressure swing adsorption (PSA).

3. Process for separating paraxylene according to claim 1 wherein the process is carried out by implementation by a simulated moving bed (LMS) type process.

4. Process for separating paraxylene according to one of claims 1 to 3 wherein said adsorbent has a chemical composition having the base unit Zn [N- (CH-CH) - N-CR] ₂ , where R is the substituent in position 2 selected from the group consisting of -CH ₃ , -Cl, -Br, -CH ₂ OH and -CHO.

5. Process for separating paraxylene according to one of claims 1 to 4 wherein said adsorbent has a specific surface area of between 400 and 1600 m ² / g.

6. Process for separating paraxylene according to one of claims 1 to 5 wherein the feedstock to be treated contains more than 80% of aromatic compounds having 8 carbon atoms.

7. Process for separating paraxylene according to one of claims 1 to 5 wherein the feedstock to be treated contains more than 90% of aromatic compounds having 8 carbon atoms.

8. Process for separating paraxylene according to claims 1 and 3 to 7 wherein, when implementing simulated moving bed type method, the method comprises:

a) a step of bringing the charge into contact with an adsorbent bed under adsorption conditions, b) a step of contacting, under desorption conditions, the adsorbent bed with an eluent.

c) a step of withdrawing the adsorbent bed from a stream containing the eluent and the products of the least adsorbed filler,

d) a step of withdrawing the adsorbent bed from a stream containing the eluent and paraxylene

The paraxylene separation process of claim 8 wherein the process further comprises the steps of:

e) a separation of the stream of step c) into a first stream containing the eluent and a second stream containing the least adsorbed feed products,

f) a separation of the stream of step d) into a first stream containing the eluent and a second stream containing paraxylene. . 10. Paraxylene separation process according to claims 8 and 9 wherein the eluent E is chosen so that the selectivity of the paraxylene relative to the element B is greater than the selectivity of the eluent with respect to the element. B, the element B being selected from the group consisting of metaxylene, orthoxylene and ethylbenzene.

The paraxylene separation process of claim 10 wherein the eluent is paradiethylbenzene.

10. Paraxylene separation process according to claims 8 to 11 wherein one operates at a temperature between 50 and 200 ° C and a pressure between 0.1 and 4

MPa.

12. Process for separating paraxylene according to one of claims 1 to 2 and 4 to 7 wherein, during implementation by adsorption pressure modulated, the method comprises: a) the introduction of the load in at least an adsorption column by the feed end,

b) carrying out said adsorption step at a total pressure of between 0.1 and 4 MPa producing a first depleted paraxylene stream, c) the desorption of paraxylene by lowering the pressure in the adsorption column (s), and / or by introducing a purge gas through the production end of the column, the pressure of desorption being between 0.01 and 1 MPa.

The paraxylene separation process according to claim 12, wherein the purge gas of the paraxylene desorption step c) is selected from the group consisting of benzene and toluene.

14. Paraxylene separation process according to claims 12 and 13 as implemented at a temperature between 100 and 200 ° C.