WO2009130402A1

WO2009130402A1 - Reactive simulated mobile bed for producing paraxylene

Info

Publication number: WO2009130402A1
Application number: PCT/FR2009/000317
Authority: WO
Inventors: Ghislain Bergeot; Catherine Laroche; Philibert Leflaive; Damien Leinekugel Le Cocq; Luc Wolff
Original assignee: Ifp
Priority date: 2008-04-25
Filing date: 2009-03-24
Publication date: 2009-10-29
Also published as: FR2930549A1; FR2930549B1; SA109300238B1

Abstract

The invention relates to a method for producing high-purity paraxylene by coupling a step of selective paraxylene adsorption and a step of xylene isomerisation from an aromatic C8 mixture optionally containing ethylbenzene.

Description

SIMILAR REACTIVE MOBILE BED FOR PARAXYLENE PRODUCTION

Field of the invention

The invention relates to a process for the production of paraxylene with a high level of purity from a mixture of aromatic C8 compounds, which may in particular contain ethylbenzene, by coupling of an adsorption step and a step of reaction carried out in the same unit simulated moving bed.

Examination of the prior art:

The integration of different functionalities within a single unit is a means of intensifying the processes. This integration can for example be achieved by combining a reaction with a simultaneous separation of products and reagents. Such methods, called reactive separations, are now relatively common. Among the most important examples of reactive separation are the methods of reactive distillation, reactive absorption, reactive adsorption or reactive extraction.

In a reactive separation, the reaction and the separation can, for example, take place in the same area of a unit; the conversion of reagents into products and the separation of the products being done simultaneously. A reactive separation process often requires smaller and less expensive equipment than a conventional separation and conversion process sequence loop with unconverted reagent recycling to achieve complete conversion of reagents. Such a process is generally significantly more energy efficient than a non-integrated process.

The integration of a balanced reaction leading to one or more products (eg A o B or A (+ B) <=> C + D), catalyzed by a heterogeneous catalyst, within a separation unit by Simulated moving bed adsorption is well known in the prior art. Ganetsos et al. (1993, Preparative and Production Scale Chromatography, PE, Eds., Marcel Dekker Inc.) and Kulprathipanja (2002, Reactive Separation Processes, New York (NY), Taylor & Francis, pages 115-153) provide many details on the principle simulated moving bed reactive according to the type of reaction involved, and the type of experimental tools to study this implementation.

In the case of a reaction of type A (+ B) <-> C + D giving two products separated by adsorption, one can for example mention the article published by Lode et al. (2001, Eng Eng ScL, vol 56, pages 269-291) which shows the implementation of a laboratory-scale simulated moving bed applied to the esterification of acetic acid and methanol to produce methyl acetate. In this particular case, catalyst and adsorbent are mixed in the columns of the unit. Baur and Krishna (2005, Eng Chem Journal, Vol 109, pages 107-113) and Ray and Carr (1995, Eng Eng Sci, vol 50, pages 2195-2202 & 1995, Chem Eng ScL, Vol 50, pages 3033-3041) propose another implementation of the reactive simulated moving bed applied to reactions A o B, having two zones in which adsorbent and catalysts are mixed. This type of implementation is only applicable to reactions with a balance largely in favor of the formation of product B.

In the case of a reaction of type A <=> B, mention may be made of the articles published by Hashimoto et al. (1983, Biotechnology and Bioengineering, Vol 25, pp. 2371-2333) and Zhang et al. (2007, Biochemical Engineering Journal, Vol 35, pages 341-351) which applied a simulated moving bed responsive to glucose isomerization to fructose. In this particular case, the reaction must be located within a part of the volume of the unit, in order to avoid any reaction at the point where it is desired to recover with high purity the desired product. (fructose). This is why catalyst and adsorbent are placed in separate columns. While Hashimoto et al. propose placing the catalyst beds in series with the adsorbent beds in a particular area of the simulated moving bed, Zhang et al. propose placing the catalyst beds in parallel with certain adsorbent beds in a particular area of the simulated moving bed.

Note also that in the articles published by Minceva et al. (2007, Chemical Engineering Journal, in press) and Wolff and Leinekugel-the-Cocq (2007, ^11th Congress of the French Society of Process Engineering), the principle of the simulated reactive moving bed is extended to the production of para-xylene coupling of the isomerization reaction of xylenes with the separation of paraxylene by adsorption. note that in these two cases, only fillers devoid of ethylbenzene are considered. If in the article by Minceva et al. the nature of the desorbent is not specified, in Wolff and Leinekugel-le-Cocq, it is explained that the use of toluene is recommended, preferably paradiethylbenzene which is likely to be isomerized in the same way as xylenes. In Minceva's article, there is talk of zeolite of type Y or X exchanged with barium or potassium.

The prior art as represented by Minceva et al. is limited to ethylbenzene free feeds. In the presence of ethylbenzene in the feed, the method described in the article by Minceva et al. does not produce paraxylene with high purity. In this paper, to treat a feedstock containing ethylbenzene, the authors recommend changing the catalyst and using a bifunctional acid / noble metal catalyst, and injecting hydrogen to isomerize ethylbenzene to xylene. . The isomerization then takes place in the gas phase, and the coupling of the steps of selective adsorption of paraxylene and isomerization of xylenes in a single unit is then no longer possible because the reaction is carried out in the gas phase and the separation into liquid phase.

The object of the present invention is to overcome the disadvantages of the prior art and to provide a reactive simulated moving bed method operating in the liquid phase for producing high purity paraxylene from a mixture of aromatic C8 compounds. , and especially for ethylbenzene rich feeds.

Furthermore, the prior art teaches that the adsorbents consisting of zeolites X or Y exchanged with ions such as barium, potassium or strontium, alone or as a mixture, are effective for selectively adsorbing paraxylene in a mixture of aromatic hydrocarbons.

US Patent 3,558,730 / US 3,558,732 / US 3,626,020 / and US 3,663,638 show that adsorbents comprising aluminosilicates exchanged with barium and potassium or with barium alone (as in US 3,960,774) effectively separate paraxylene from a mixture of isomers aromatic compounds with 8 carbon atoms (C8). These adsorbents are used as adsorption agents in liquid phase processes, preferably of the simulated countercurrent type, similar to those described in US Pat. No. 2,985,589, which apply inter alia to aromatic C 8 cuts. The zeolites found in the prior art for the separation of xylenes belong to the structural type of faujasite, first described in US 2,882,244 and US 3,130,007, which are crystallized silico-aluminates having cages of perfectly defined size. connected in three dimensions. No. 6,884,918 recommends a faujasite X having an Si / Al atomic ratio of between 1.15 and 1.5. No. 6,410,815 teaches that zeolitic adsorbents as described in the prior art but for which the faujasite has a low silica content and has a Si / Al atomic ratio close to 1 (which is called LSX, abbreviation of Low Silica X whose French translation is zeolite X with a low silica content), can also be used for the separation of paraxylene.

In the references listed above, the zeolitic adsorbents are in the form of powder or in the form of agglomerates consisting mainly of zeolite and up to 15 to 20% by weight of inert agglomeration binder.

The synthesis of zeolites X or LSX is most often carried out by nucleation and crystallization of silico-aluminate gels, one obtains powders whose use on an industrial scale is particularly difficult (significant losses of loads during handling). Agglomerated forms, for example in the form of granules or grains, which do not have the disadvantages inherent to the pulverulent materials, are preferred. These agglomerates, whether in the form of platelets, beads or extrudates, generally consist of a zeolite powder, which constitutes the active element (in the sense of the adsorption) and a binder intended to ensure the cohesion of the crystals in the form of agglomerates. A method for preparing such agglomerates is for example given in US Pat. No. 6,884,918.

Brief description of the figure:

FIG. 1 represents the operating principle of a reactive simulated moving bed unit for the production of high purity paraxylene. For this, we take the example of a unit comprises 12 adsorbent beds (denoted C1 to C12) and 2 catalyst beds (denoted R1 and R2). This figure shows schematically the 6 first steps of a cycle which has 12, and distinguished zone Z3 which contains two catalytic beds R1 and R2.

Brief description of the invention:

The invention relates to a high purity para xylene production unit from a mixture of xylenes which can contain ethylbenzene up to a content of

20% in said mixture.

The unit operates in a reactive simulated moving bed and can generally be defined as having n adsorption beds operating in series, and p catalytic beds interconnected to certain adsorption beds, the unit being fed with the charge (C). and a desorbent (S) and producing an extract (E) and at least one raffinate (R), the adsorbent solid filling the n adsorption beds, and the isomerization catalyst of the xylenes filling the p catalytic beds, the unit being characterized in that the desorbent is toluene and the adsorbent solid is an agglomerated zeolite solid based on LSX zeolite crystals of atomic ratio

Si / Ai such that 0.95 <Si / Ai ≤ 1, 15 and preferably of atomic ratio

Si / Al = 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, the catalytic beds being distributed between the point of introduction of the feedstock (C) and the point of introduction of the desorbent (S), and preferably between the point of introduction of the feedstock (C) and the drawdown point of the raffinate closest to the point of introduction of the desorbent (S), and the p catalytic beds operating in series in the direction of the main flow.

The unit according to the invention is divided into 4 zones defined as follows:

Zone 1: between the injection of the desorbent and the withdrawal of the extract,

Zone 2: between withdrawal of the extract and injection of the charge,

Zone 3: between the injection of the charge and the withdrawal of the raffinate,

Zone 4: between the withdrawal of the raffinate and the injection of the desorbent,

The totality of the catalytic beds is then distributed in zone 3.

Usually zone 1 contains 1 to 6 beds, zone 2 contains 3 to 10 beds, zone 3 contains 2 to 9 beds, and zone 4 contains 1 to 5 beds. The invention also relates to a process for producing high purity paraxylene using the unit according to the present invention, the operating conditions of the process being defined by the following pages:

the temperature of the adsorption beds is between 120 ^° C. and 260 ^° C., preferably between 180 ^° C. and 240 ^° C., and more preferably between 200 ° C. and 220 ^° C.,

the pressure of the adsorption beds is between the bubble pressure of toluene at the operating temperature and 4 MPa.

According to one characteristic of the present invention, the high purity paraxylene production unit operating in reactive simulated moving bed comprises a number of catalytic beds between 1 and 13, and preferably between 2 and the number of adsorption beds of zone 3 decreased by one.

Detailed description of the invention:

The invention relates to a process for the production of high purity paraxylene by coupling a step of selective adsorption of paraxylene and a reaction step of isomerization of xylenes from a mixture of aromatic C8 compounds, and especially mixtures containing ethylbenzene. The invention is characterized by the use of toluene as a desorbent, and by the use of an agglomerated zeolite adsorbent solid based on LSX zeolite crystals of Si / Al atomic ratio such that 0.95 <Si / Al <1 , And preferably of Si / Al 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 order to understand the different stages of a cycle and the notion of zones, we first specify the nomenclature used.

The abbreviation LMS stands for simulated moving bed, and LMSR stands for simulated moving bed reagent. In general, an adsorption column (also called adsorber) operating in LMS is composed of several adsorption beds, each bed being delimited by an injection point and a draw-off point, knowing that these injection points and racking are only used at certain points in the cycle. The feedstock and the desorbent are injected at two distinct points of the adsorber. The raffinate and the extract are withdrawn at two distinct points of the adsorber, themselves distinct from the two injection points. A zone is defined as a set of consecutive beds delimited at one of its ends by an injection or withdrawal point, and at the other end by, respectively, a withdrawal or injection point.

At any time, the adsorber is therefore traversed by at least four streams, two distinct inlet streams, generally the feedstock (C) and the desorbent (S), and two separate output streams, the extract (E) and the raffinate (R).

In some cases, there may be several injections of desorbent or

/ or more withdrawals of extract or raffinate. In the remainder of the description, for the sake of clarity, only one unit having two injection points, one for the load, the other for the desorbent, and two draw points, one for the extracted, the other for the raffinate, Such a unit therefore has four zones.

The attached FIG. 1 is a schematic representation of an LMSR unit comprising 12 adsorption beds denoted C1 to C12, and two reagent beds denoted R1 and R2.

A cycle of LMSR is defined from an initial position of the injection and withdrawal points, and corresponds to the coordinated movement of these points along the adsorber, from bed to bed until finding the initial position.

One stage of the cycle is defined by an elementary displacement of a bed of injection and withdrawal points along the adsorber.

A cycle therefore comprises a finite number of steps, corresponding to the total number of beds of the adsorber.

The adsorber is generally subdivided into zones. These areas are defined as follows:

Zone 1: zone of the adsorber situated between the injection of the desorbent and the withdrawal of the extract;

Zone 2: zone of the adsorber situated between the withdrawal of the extract and the injection of the charge;

Zone 3: zone of the adsorber situated between the injection of the charge and the withdrawal of the raffinate;

Zone 4: zone of the adsorber situated between the extraction of the raffinate and the injection of the desorbent. An LMSR unit according to the invention further comprises adsorption beds one or more catalyst beds which are positioned, using a set of adequate valves, in series or in parallel adsorbent beds located in the zone. 3, or in zone 4, or distributed in zones 3 and 4.

Preferably, all the catalytic beds are positioned in zone 3.

In FIG. 1 attached, the two catalytic beds R1 and R2 have been placed in an illustrative way in zone 3 situated between the point of injection of the feedstock (C) and the raffinate withdrawal point (R). This zone 3 moves during the cycle, and its displacement can be followed by an increment equal to an adsorption bed in FIG. 1 which reproduces the different positions of the zone 3 and the other zones which are deduced therefrom, during a complete cycle of the process. At any moment of the cycle, the relative position of the different zones is kept the same.

This is the description of a LMSR operating with 4 zones, but it is possible to envisage units operating with a greater number of zones, in connection with the multiplication of injection or withdrawal points.

In this case, the catalyst beds are all positioned between the point of charge injection and the desorbent injection point, in the direction of flow, and preferably between the point of charge injection and the point of injection. withdrawing the raffinate closest to the desorbent injection point. Indeed, when the unit has more than 4 zones, this situation generally corresponds to the existence of a second raffinate, the extraction of which is situated between the injection of the desorbent and the extraction of the extract. It is therefore possible to distinguish the first and the second raffinate by their respective distance from the injection point of the desorbent.

When the LMSR unit has only one raffinate, the term "raffinate closest to the desorbent injection point" refers to the raffinate in question.

In general, a catalytic bed is interposed between two successive adsorbent beds such as n and n + 1, ie the flow leaving the adsorption bed n feeds at least part of the catalytic bed p, and the effluent exiting the catalytic bed p feeds at least partly the n + 1 adsorption bed.

In a variant of the process according to the invention, it may be that for certain catalytic beds, the reinjection of the effluent of the catalytic bed in question takes place at least in part, either towards the adsorption bed n, or towards the bed adsorption n + 2. The expression "at least in part" means that the entire stream may be involved, or only a part, the other party then following another reinjection among the different possible reinjections.

By way of non-limiting example, we can have an operation of the unit in

LMSR according to the invention wherein a part of the flow leaving the adsorption bed n is introduced into the catalyst bed p, the other part of the flow entering the adsorption bed n + 1 and the effluent of the catalyst bed p is for a first part directed to the n + 1 adsorption bed, for a second part directed to the n + 2 adsorption bed, and for a third part directed to the adsorption bed n.

Preferably, the catalytic bed p operates in series with respect to the adsorption beds n and n + 1, that is to say that the flow leaving the adsorption bed n feeds the catalyst bed p, and that the effluent the catalytic bed p supplies the n + 1 adsorption bed.

The adsorbent used in the adsorber is an agglomerated zeolite adsorbent based on LSX zeolite crystals of Si / Al atomic ratio such that 0.95 <Si / Al <1.15 and preferably of Si / Al 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 the context of the present invention, the desorbent is toluene.

The volume ratio of the desorbent on the filler is between 1.0 and 10 and preferably between 4.5 and 6.8.

The unit in simulated reactive moving bed is operated at a temperature between

120 ^° C. and 260 ^° C., preferably between 180 ° C. and 240 ^° C., and more preferably between 200 ^° C. and 220 ^° C., and under a pressure ranging from the bubble pressure of toluene to the operating temperature, and 4 MPa. (MPa symbolizes the mega pascal, 10 ⁶ Pascal)

The catalytic isomerization zone preferably works in the liquid phase and is operated under the following conditions:

- Temperature below 350 ⁰ C, preferably between 180 ⁰ C and 300 ⁰ C,

- Pressure less than 4 MPa, preferably between 2 and 3 MPa,

Hourly mass flow rate (PPH) less than 20 hours ^-1 , preferably between 2 and 10 hours ^-1 ,

Catalyst containing a zeolite of the ZSM-5 type. Any of the catalysts capable of isomerizing hydrocarbons with 8 carbon atoms are suitable for the catalytic isomerization zone of the present invention. Preferably, a catalyst containing an acidic zeolite, for example of structural type MFI, MOR, MAZ, FAU and / or EUO, is used. Even more preferably, a catalyst containing a ZSM-5 zeolite is used.

Examples:

The invention will be better understood on reading the following examples which illustrate the invention without limiting its scope.

- Example 1 is representative of the prior art in which it is considered the separation of a mixture of ethylbenzene-free aromatic C8 using an LMSR device with toluene as a desorbent and a BaX-type adsorbent solid (zeolite X exchanged with Barium).

- Example 2 describes the performance of a LMSR device identical to that of Example 1, representative of the prior art, but in which this time it is considered the separation of a mixture of C8 aromatic compounds containing ethylbenzene.

- Example 3 is according to the invention in which it is considered the separation of an ethylbenzene-free aromatic C8 mixture using an LMSR device with toluene as desorbent and employing a BaLSX type zeolite as adsorbent (LSX zeolite exchanged with Barium).

Example 4 describes the performance of an LMSR device identical to that of Example 3, according to the invention, but in which this time it is considered the separation of a mixture of aromatic C8 containing ethylbenzene .

Example 1 (according to the prior art)

The production of paraxylene from an aromatic feed containing 8 carbon atoms devoid of ethylbenzene is considered on an LMSR device equipped with 24 adsorbent beds containing a BaX-type zeolite (zeolite X exchanged with

Barium), 6 catalyst beds containing a zeolite HZSM-5 and positioned in series between 2 adsorbent beds located in zone 3, and using toluene as desorbent. This LMSR device comprises 24 adsorbent beds length 1, 13m and internal section 3.5x10 ⁴ m ² , with a charge injection, desorbent injection, extract extraction and raffinate withdrawal. The zone configuration is:

• 5 adsorbent beds in zone 1;

• 9 adsorbent beds in zone 2;

• 7 adsorbent beds in zone 3;

• 3 adsorbent beds in zone 4.

The catalyst beds are of length 2m and inner section 3.5x10 ^"4 m ^2.

The temperature is 210 ^° C., and the pressure is 2.0 MPa.

Charge F is composed of 21, 05% PX, 28% OX and 50.95% MX. The permutation time used is 70.3 seconds. The liquid flow rates in the different zones are as follows:

• 181.0 cm ³ / min in zone 1;

• 127.0 cm ³ / min in zone 2;

• 143.0 cm ³ / min in zone 3;

• 94.0 cm ³ / min in zone 4.

A yield of xylene is then obtained by simulation, defined as the amount of PX withdrawn from the extract on all the xylenes (PX + MX + OX) injected into the feed, of 55.05%. The purity of PX in the extract of 99.73% weight.

Example 2 (according to the prior art)

Considering the production of paraxylene from an aromatic charge containing 8 carbon atoms containing ethylbenzene on an LMSR device equipped with 24 adsorbent beds containing a BaX type zeolite, with 6 catalyst beds containing a zeolite HZSM 5 and positioned in series between 2 adsorbent beds located in zone 3, and using toluene as desorbent.

This LMSR device comprises 24 adsorbent beds length 1, 13m and internal section 3.5x10 ⁴ m ² , with a charge injection, desorbent injection, extract extraction and raffinate withdrawal. The zone configuration is:

• 5 adsorbent beds in zone 1; • 9 adsorbent beds in zone 2;

• 7 adsorbent beds in zone 3;

• 3 adsorbent beds in zone 4.

The catalyst beds are of length 2m and internal section 3.5x10 ⁴ m ² .

The temperature is 210 ^° C., and the pressure is 2.0 MPa.

Load F is composed of 20% PX, 27% OX, 48% MX and 5% EB. The permutation time used is 70.3 seconds. The liquid flow rates in the different zones are as follows:

• 181, 0 cm / min in zone 1;

• 127.0 cm ³ / min in zone 2;

• 143.0 crrvVmin in zone 3;

• 94.0 cm ³ / min in zone 4.

Then, by simulation, a yield of xylene, defined as the amount of PX withdrawn from the extract on all the xylenes (PX + MX + OX) injected into the feedstock, of 56.8% is obtained. The level of purity of PX in the extract is insufficient since it is 92.6% by weight and a purity level of at least 99.7% is required for the commercialization of paraxylene.

Example 3 (according to the invention)

The production of paraxylene from an aromatic feed containing 8 carbon atoms devoid of ethylbenzene is considered on an LMSR device according to the invention, equipped with 24 adsorbent beds containing a BaLSX type zeolite, with 6 catalyst beds. containing a zeolite HZSM-5 and positioned in series between 2 adsorbent beds located in zone 3, and using toluene as desorbent.

This LMSR device comprises 24 adsorbent beds of length 1, 13m and internal section 3.5x10 ^"4 m ² , with charge injection, desorbent injection, extraction of extract and raffinate withdrawal. of areas is:

• 5 adsorbent beds in zone 1;

• 9 adsorbent beds in zone 2;

• 7 adsorbent beds in zone 3; • 3 adsorbent beds in zone 4.

The catalyst beds are of length 2m and internal section 3.5x10 ^'4 m ² .

The temperature is 210 ^c C, and the pressure of 2.0 MPa.

• 181.0 cm ³ / min in zone 1;

• 127.0 cm ³ / min in zone 2;

• 143.0 cm ³ / min in zone 3;

• 94.0 cm ³ / min in zone 4.

A yield of xylene is then obtained by simulation, defined as the amount of PX withdrawn from the extract on all the xylenes (PX + MX + OX) injected into the feedstock, of 56.81%. The purity of PX in the extract is 99.73% weight.

Example 4 (according to the invention)

Considering the production of paraxylene from an aromatic charge containing 8 carbon atoms containing ethylbenzene on an LMSR device according to the invention, equipped with 24 beds of adsorbent containing a BaLSX type zeolite, 6 beds of catalysts containing a zeolite HZSM-5 and positioned in series between 2 adsorbent beds located in zone 3, and using toluene as desorbent.

This LMSR device comprises 24 adsorbent beds length 1, 13m and internal section 3.5x10 ^'4 m ² , with a charge injection, desorbent injection, extraction of extract and raffinate withdrawal. The zone configuration is:

• 5 adsorbent beds in zone 1;

• 9 adsorbent beds in zone 2;

• 7 adsorbent beds in zone 3;

• 3 adsorbent beds in zone 4.

The catalyst beds are of length 2m and internal section 3.5x10 ⁴ m ² .

The temperature is 210 ^° C., and the pressure is 2.0 MPa. Load F is composed of 20% PX, 27% OX, 48% MX and 5% EB. The permutation time used is 70.3 seconds. The liquid flow rates in the different zones are as follows:

• 181.0 cm ³ / min in zone 1;

• 127.0 cnrvVmin in zone 2;

• 143.0 cm ³ / min in zone 3;

• 94.0 cm ³ / min in zone 4.

A yield of xylene is then obtained by simulation, defined as the amount of PX withdrawn from the extract on all the xylenes (PX + MX + OX) injected into the feedstock, of 56.79%. The purity of PX in the extract of 99.74% weight.

Claims

1. A high purity paraxylene production unit operating in a reactive simulated moving bed from a feedstock consisting of a mixture of aromatic C8 compounds, having n approximately identical adsorption beds in series, and p interconnected catalytic beds. certain adsorption beds and operating in series in the direction of the main flow, the unit being fed with the feed (C) and a desorbent (S) and producing an extract (E) and at least one raffinate (R) , the solid adsorbent filling the n adsorption beds, and the isomerization catalyst xylenes filling the p catalytic beds, the unit being characterized in that the desorbent is toluene and the adsorbent solid is a zeolitic solid agglomerated base of LSX zeolite crystals of Si / Al atomic ratio such that 0.95 ≤ Si / Al <1.15 and preferably of Si / Al 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, the unit comprising 4 zones defined as follows:

Zone 2: between withdrawal of the extract and injection of the charge,

Zone 4: between the raffinate withdrawal and the injection of the desorbent, all the p catalytic beds being placed in zone 3, the number p of catalytic beds being between 2 and the number of adsorption beds of the zone 3, the unit being characterized in that a catalytic bed p is fed by the effluent of an adsorption bed n, and the effluent of the catalytic bed p is reintroduced into the adsorption bed n + 1.

A high purity paraxylene production unit operating in a simulated reactive moving bed according to claim 1, wherein the zone 1 contains from 1 to 6 adsorbent beds, the zone 2 contains from 3 to 10 adsorbent beds, the Zone 3 contains 2 to 9 adsorbent beds, and Zone 4 contains 1 to 5 adsorbent beds.

A high purity paraxylene production unit operating in a simulated reactive moving bed according to any one of claims 1 to 2, wherein the catalyst used for the catalyst beds contains a structural type zeolite selected from the group consisting of: MFI, MOR, MAZ, FAU and / or EUO.

A high purity paraxylene production unit operating in a simulated reactive moving bed according to any one of claims 1 to 3, wherein the catalyst contains a ZSM5 zeolite.

A process for producing high purity paraxylene from a C8 aromatic feed using the unit of any one of claims 1 to 4, wherein the volume ratio of the desorbent to the feed is from 1 , 0 and 10, and preferably between 4.5 and 6.8.

A process for producing high purity paraxylene from a C8 aromatic feedstock utilizing the unit of any one of claims 1 to 5, wherein the temperature of the adsorption beds is in the range of about 120 ° ^C. C and 260 ^° C., preferably between 180 ^° C. and 240 ^° C., and more preferably between 200 ^° C. and 220 ^° C., and the pressure is between the bubble pressure of toluene at the operating temperature and 4 MPa .

A process for producing high purity paraxylene from a C8 aromatic feed using the unit of any one of claims 1 to 6, wherein the catalytic zones work under the following conditions:

- Temperature below 350 ⁰ C, preferably between 180 ⁰ C and 300 ⁰ C,

- Pressure less than 4 MPa, preferably between 2 and 3 MPa,

Hourly mass flow rate (PPH) less than 20 hours ^-1 , preferably between 2 and 10 hours ^-1 .

A process for producing high purity paraxylene from a C8 aromatic feedstock utilizing the unit of any one of claims 1 to 4, wherein the feedstock to be treated contains up to 20% of ethylbenzene.