SIMULATED MOVING BED SEPARATION METHOD AND DEVICE WITH REDUCED NUMBER OF BEDS AND BYPASS FLUID FLOW

Abstract

Method for the simulated moving bed (SMB) separation of a feedstock (F), in which: at least one zone (1, 2, 3, 4) contains fewer than three beds, if the stream (D, E, F, R) delimiting said zone and situated upstream of said zone is injected or withdrawn at the plate P.sub.i via the bypass line L.sub.i/i+1, then the stream delimiting said zone and situated downstream of said zone is injected/withdrawn at the plate P.sub.j via the bypass line L.sub.j/j+1, and if the stream delimiting said zone and situated downstream of said zone is injected or withdrawn at the plate P.sub.i via the bypass line L.sub.i1/i, then the stream delimiting said zone and situated upstream of said zone is injected/withdrawn at the plate P.sub.j via the bypass line L.sub.j1/j.

Claims

1. Method for the simulated moving bed separation of a feedstock (F) in a simulated moving bed separation device, the device comprising: at least one column comprising a plurality of beds of adsorbent (A.sub.i) which are separated by plates (P.sub.i) each comprising a distribution/extraction system; and external bypass lines (L.sub.i/i+1) directly joining two successive plates (P.sub.i, P.sub.i+1), each external bypass line comprising fluid (F, D) feed points and effluent (E, R) withdrawal points, in which method: the at least one column is fed with the feedstock (F) and a desorbent (D) and at least one extract (E) and at least one raffinate (R) is withdrawn from the at least one column, the feed and withdrawal points being shifted during the course of time by an amount corresponding to one adsorbent bed with a switchover period (ST) and determining a plurality of operating zones of the device, and notably the following main zones: a zone 1 for the desorption of the compounds from the extract, this zone being comprised between the feed for the desorbent (D) and the withdrawal of the extract (E), a zone 2 for the desorption of the compounds from the raffinate, this zone being comprised between the withdrawal of the extract (E) and the feed for the feedstock (F), a zone 3 for the adsorption of the compounds from the extract, this zone being comprised between the feed for the feedstock (F) and the withdrawal of the raffinate (R), and a zone 4 situated between the withdrawal of the raffinate (R) and the feed for the desorbent (D); in which method: at least one zone contains fewer than three beds, if the stream (D, E, F, R) delimiting said zone containing fewer than three beds (1, 2, 3, 4) and situated upstream of said zone (1, 2, 3, 4) is injected or withdrawn at the plate P.sub.i via the bypass line L.sub.i/i+1, then the stream (E, F, R, D) delimiting the zone and situated downstream of said zone (1, 2, 3, 4) is injected/withdrawn at the plate P.sub.j via the bypass line L.sub.j/j+1, and if the stream (E, F, R, D) delimiting said zone containing fewer than three beds (1, 2, 3, 4) and situated downstream of said zone (1, 2, 3, 4) is injected or withdrawn at the plate P.sub.i via the bypass line L.sub.i1/i, then the stream (D, E, F, R) delimiting the zone (1, 2, 3, 4) and situated upstream of said zone (1, 2, 3, 4) is injected/withdrawn at the plate P.sub.j via the bypass line L.sub.j1/j, the plate P.sub.i corresponding to one plate of the column, the plate P.sub.j corresponding to a plate other than P.sub.i, the bypass line L.sub.i1/i being the line joining the two successive plates P.sub.i1 and P.sub.i, the bypass line L.sub.i/i+1 being the line joining the two successive plates P.sub.i and P.sub.i+1, the bypass line L.sub.j1/j being the line joining the two successive plates P.sub.j1 and P.sub.j, the bypass line L.sub.j/j+1 being the line joining the two successive plates P.sub.j and P.sub.j+1.

2. Method according to claim 1, in which: if zone 1 contains on average fewer than three beds, then when the desorbent (D) is injected on the plate P.sub.i via the bypass line L.sub.i/i+1, the extract (E) is withdrawn on the plate P.sub.j via the bypass line L.sub.j/j+1; if zone 2 contains on average fewer than three beds, then when the feedstock (F) is injected on the plate P.sub.i via the bypass line L.sub.i1/i, the extract (E) is withdrawn on the plate P.sub.j via the bypass line L.sub.j1/j; if zone 3 contains on average fewer than three beds, then when the feedstock (F) is injected on the plate P.sub.i via the bypass line L.sub.i/i+1, the raffinate (R) is withdrawn on the plate P.sub.j via the bypass line L.sub.j/j+1; and if zone 4 contains on average fewer than three beds, then when the desorbent (D) is injected on the plate P.sub.i via the bypass line L.sub.i1/i, the raffinate (R) is withdrawn on the plate P.sub.j via the bypass line L.sub.j1/j.

3. Method according to claim 1, in which: if zone 1 contains on average fewer than three beds, then when the extract (E) is withdrawn on the plate P.sub.i via the bypass line L.sub.i1/i, the desorbent (D) is injected on the plate P.sub.j via the bypass line L.sub.j1/j; if zone 2 contains on average fewer than three beds, then when the extract (E) is withdrawn on the plate P.sub.i via the bypass line L.sub.i/i+1, the feedstock (F) is injected on the plate P.sub.j via the bypass line L.sub.j/j+1; if zone 3 contains on average fewer than three beds, then when the raffinate (R) is withdrawn on the plate P.sub.i via the bypass line L.sub.i1/i, the feedstock (F) is injected on the plate P.sub.j via the bypass line L.sub.j1/j; and if zone 4 contains on average fewer than three beds, then when the raffinate (R) is withdrawn on the plate P.sub.i via the bypass line L.sub.i/i+1, the desorbent (D) is injected on the plate P.sub.j via the bypass line L.sub.j/j+1.

4. Method according to claim 1, in which the plate P.sub.i is connected to the bypass line L.sub.i1/i and to the bypass line L.sub.i/i+1.

5. Method according to claim 1, in which each plate (P.sub.i) comprises a plurality of distributionmixingextraction panels of the parallel sectors type with asymmetric feed.

6. Method according to claim 1, in which the feedstock (F) contains paraxylene or metaxylene within a mixture of C8 aromatic hydrocarbons.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0058] FIG. 1 depicts an SMB device used in the method according to embodiments of the present description, the device comprising a column having a succession of plates (P.sub.i1, P.sub.i, P.sub.i+1, P.sub.i+2) and of beds (A.sub.i1, A.sub.i, A.sub.i+1, A.sub.i+2), and external bypass lines (L.sub.i1/i, L.sub.i/i+1, L.sub.i+1/i+2).

[0059] FIG. 2 depicts an SMB device used in the method according to embodiments of the present description, the device being in a mode of operation in which zone 1 for the desorption of the compounds from the extract is comprised between the desorbent feed at plate P.sub.i and the extract withdrawal at plate P.sub.j.

[0060] FIG. 3 depicts an SMB device used in the method according to embodiments of the present description, the device being in a mode of operation in which zone 2 for the desorption of the compounds from the raffinate is comprised between the extract withdrawal at plate P.sub.j and the feedstock F feed at plate P.

[0061] FIG. 4 depicts an SMB device used in the method according to embodiments of the present description, the device being in a mode of operation in which zone 3 for the absorption of the compounds from the extract is comprised between the feedstock feed at plate P.sub.i and the raffinate withdrawal at plate P.sub.j.

[0062] FIG. 5 depicts an SMB device used in the method according to embodiments of the present description, the device being in a mode of operation in which zone 4 is comprised between the raffinate withdrawal at plate P.sub.j and the desorbent feed at plate P.sub.i.

DETAILED DESCRIPTION

[0063] The purpose of the invention is to improve the performance of a simulated bed separation method in comparison with the teachings of U.S. Pat. Nos. 5,972,224, 6,110,364 and FR 2,935,100, when use is made of an SMB device comprising at least one zone comprising on average fewer than three beds.

[0064] With reference to FIG. 1, in order to achieve high separation performance using SMB technology with a limited number of beds, the invention proposes a method for the SMB separation of a feedstock F in an SMB device possessing at least one column, said column being made up of a plurality of beds of adsorbent A.sub.i, separated by plates P.sub.i each comprising a distribution/extraction system. The SMB device further comprises external bypass lines L.sub.i/i+1 directly joining two successive plates P.sub.i, P.sub.i+1, notably allowing said plates to be flushed. It being possible for each of these bypass lines L.sub.i/i+1 to comprise automated means for regulating the flushing flow rate.

[0065] According to one or more embodiments, the column comprises n adsorbent beds A.sub.i. According to one or more embodiments, n is a natural whole number comprised between 6 and 15, preferably between 8 and 12, i being a natural whole number comprised between 1 and n.

[0066] The SMB separation method comprises the following steps: the feedstock F and a desorbent D are fed, and at least one extract E and at least one raffinate R are withdrawn, the feed and withdrawal points being shifted over the course of time by an amount corresponding to one adsorbent bed, with a changeover period (period denoted ST between two successive feed/withdrawal changeovers) and determining a plurality of operating zones of the SMB device, and notably the following main zones:

a zone 1 for the desorption of the compounds from the extract, this zone being comprised between the feed for the desorbent D and the withdrawal of the extract E;
a zone 2 for the desorption of the compounds from the raffinate, this zone being comprised between the withdrawal of the extract E and the feed for the feedstock F;
a zone 3 for the adsorption of the compounds from the extract, this zone being comprised between the feed for the feedstock and the withdrawal of the raffinate R; and
a zone 4 situated between the withdrawal of the raffinate R and the feed for the desorbent D.

[0067] It should be noted that an external bypass line L.sub.i/i+1directly joining two successive plates P.sub.i, P.sub.i+1, is said to belong to a zone when the bed A.sub.i situated between the plates P.sub.i and P.sub.i+1 belongs to said zone. In addition, the n adsorbent beds A.sub.i are distributed between zones 1 to 4 in configurations referred to as being of type a/b/c/d, which means to say that the distribution of the beds is as follows:

a is the mean number of beds in zone 1;
b is the mean number of beds in zone 2;
c is the mean number of beds in zone 3; and
d is the mean number of beds in zone 4.

[0068] In the present description, a zone comprising on average fewer than three beds corresponds to a zone which may, at isolated points, comprise more than two beds for part of the changeover period ST (e.g. when the changeovers of the injection points and of the withdrawal points are not in phase), but in which the average number of beds per changeover period ST is strictly lower than three.

[0069] According to one or more embodiments:

a=(n*0.208)*(10.2);
b=(n*0.375)*(10.2);
c=(n*0.292)*(10.2);
d=(n*0.125)*(10.2).

[0070] When a fluid (feedstock F or desorbent D) is injected or when a fluid (extract E or raffinate R) is withdrawn at a plate P.sub.i, use is made of the corresponding injection line L.sub.F or L.sub.D or withdrawal line L.sub.E or L.sub.R which is connected to one of the two bypass lines (L.sub.i1/i or L.sub.i/i+1) connected to the plate P.sub.i.

[0071] The method according to the invention is characterized in that it respects the following rules: [0072] A/ if a zone contains fewer than 3 beds, [0073] then, if the stream delimiting the zone concerned and situated upstream of said zone is injected or withdrawn at the plate P.sub.i via the bypass line L.sub.i/i+1, then the stream delimiting the zone and situated downstream of said zone is injected/withdrawn at the plate P.sub.j via the bypass line L.sub.j/j+1, and [0074] B/ if the stream delimiting the zone concerned and situated downstream of said zone is injected or withdrawn at the plate P.sub.i via the bypass line L.sub.i1/i, then the stream delimiting the zone and situated upstream of said zone is injected/withdrawn at the plate P.sub.j via the bypass line L.sub.j1/j, [0075] the plate P.sub.i corresponding to one plate of the column, [0076] the plate P.sub.j corresponding to a plate other than P.sub.i, [0077] the bypass line L.sub.i1/i being the line joining the two successive plates P.sub.i1 and P.sub.i, [0078] the bypass line L.sub.i/i+1 being the line joining the two successive plates P.sub.i and P.sub.i+1, [0079] the bypass line L.sub.j1/j being the line joining the two successive plates P.sub.j1 and P.sub.j, [0080] the bypass line L.sub.j/j+1 being the line joining the two successive plates P.sub.j and P.sub.j+1.

[0081] j is a natural whole number comprised between 1 and n and different from i.

[0082] if a zone contains more than 3 beds, then it is possible to inject or withdraw via the bypass line L.sub.i/i+1 or the bypass line L.sub.i1/i, provided that rules A/ and B/ are respected.

[0083] j is a natural whole number comprised between 1 and n and different from i.

[0084] According to one or more embodiments, if zone 1 contains on average fewer than three beds, then when the desorbent is injected on the plate P.sub.i via the bypass line L.sub.i/i+1, the extract is withdrawn on the plate P.sub.j via the bypass line L.sub.j/j+1. For example, with reference to FIG. 2, if zone 1 contains on average fewer than three beds, then if the desorbent is injected on the plate P.sub.i via the bypass line L.sub.i/i+1, then the extract must be withdrawn on the plate P.sub.j via the bypass line L.sub.j/j+1.

[0085] According to one or more embodiments, if zone 2 contains on average fewer than three beds, then when the feedstock is injected on the plate P.sub.i via the bypass line L.sub.i1/i, the extract is withdrawn on the plate P.sub.j via the bypass line L.sub.j1/j. For example, with reference to FIG. 3, if zone 2 contains on average fewer than three beds, then if the feedstock is injected on the plate P.sub.i via the bypass line L.sub.i1/i, then the extract must be withdrawn on the plate P.sub.j via the bypass line L.sub.j1/j.

[0086] According to one or more embodiments, if zone 3 contains on average fewer than three beds, then when the feedstock is injected on the plate P.sub.i via the bypass line L.sub.i/i+1, the raffinate is withdrawn on the plate P.sub.j via the bypass line L.sub.j/j+1. For example, with reference to FIG. 4, if zone 3 contains on average fewer than three beds, then if the feedstock is injected on the plate P.sub.i via the bypass line L.sub.i/i+1 then the raffinate must be withdrawn on the plate P.sub.j via the bypass line L.sub.j/j+1.

[0087] According to one or more embodiments, if zone 4 contains on average fewer than three beds, then when the desorbent is injected on the plate P.sub.i via the bypass line L.sub.i1/i, the raffinate is withdrawn on the plate P.sub.j via the bypass line L.sub.j1/j. For example, with reference to FIG. 5, if zone 4 contains on average fewer than three beds, then if the desorbent is injected on the plate P.sub.i via the bypass line L.sub.i1/i, then the raffinate must be withdrawn on the plate P.sub.j via the bypass line L.sub.j1/j.

[0088] According to one or more embodiments, if zone 1 contains on average fewer than three beds, then when the extract is withdrawn on the plate P.sub.i via the bypass line L.sub.i1/i, the desorbent is injected on the plate P.sub.j via the bypass line L.sub.j1/j.

[0089] According to one or more embodiments, if zone 2 contains on average fewer than three beds, then when the extract is withdrawn on the plate P.sub.i via the bypass line L.sub.i/i+1, the feedstock is injected on the plate P.sub.j via the bypass line L.sub.j/j+1.

[0090] According to one or more embodiments, if zone 3 contains on average fewer than three beds, then when the raffinate is withdrawn on the plate P.sub.i via the bypass line L.sub.i1/i, the feedstock is injected on the plate P.sub.i via the bypass line L.sub.j1/j.

[0091] According to one or more embodiments, if zone 4 contains on average fewer than three beds, then when the raffinate is withdrawn on the plate P.sub.i via the bypass line L.sub.i/i+1, the desorbent is injected on the plate P.sub.j via the bypass line L.sub.j/j+1.

[0092] Each plate P.sub.i comprises two chambers for accomplishing the sequential operations of feeding the feedstock F or injecting the desorbent D and extracting the raffinate R or the extract E. The present invention relates to columns having two chambers per plate P.sub.i. There are a number of possible solutions for using the two chambers, each one of them being able to be used for the injection or the withdrawal of one or more streams. According to one or more embodiments, a first chamber performs the operations of injecting feedstock F or desorbent D, and the other chamber performs the operations of withdrawing raffinate R or extract E. According to one or more embodiments, one chamber is used for injecting the feedstock F and withdrawing the raffinate R, the other handling the injection of desorbent D and the withdrawal of the extract E. The above examples are nonlimiting, other uses of the two chambers being possible. Each bed i is equipped with a bypass line which connects one chamber of the upstream plate to one chamber of the downstream plate.

[0093] According to one or more embodiments, the feedstock is selected from the group consisting of a mixture of essentially C8 aromatic compounds (e.g. xylenes and ethylbenzene). According to one or more embodiments, the mixture comprises at least 95%, preferably at least 97% (e.g. at least 99%) of essentially C8 aromatic compounds.

[0094] The method according to the present invention more particularly applies to the separation of a feedstock containing paraxylene and/or metaxylene within a mixture of C8 aromatic hydrocarbons. According to one or more embodiments, the feedstock comprises at least 15 wt % of paraxylene and/or 30 wt % of metaxylene with respect to the total weight of the feedstock.

[0095] One example of an SMB separation method of great industrial importance is the separation of C8 aromatic fractions in order to produce paraxylene of commercial purity, typically at a purity of at least 99.7 wt %, and a raffinate rich in ethylbenzene, orthoxylene and metaxylene.

[0096] According to one or more embodiments, the adsorbent is selected from the group made up of zeolites of the faujasite type, of type NaY, BaX, BaKX, BaLSX. For preference, the adsorbent is selected from the group made up of BaX, BaKX, NaY.

[0097] According to one or more embodiments, the desorbent is selected from the group made up of one or more isomers of diethylbenzene and toluene. For preference, the desorbent is selected from the group made up of paradiethylbenzene and toluene.

[0098] According to one or more embodiments, the temperature of the column is comprised between 120 C. and 190 C. For preference, the temperature of the column is comprised between 150 C. and 180 C.

[0099] According to one or more embodiments, the pressure in the column is comprised between 0.3 MPa and 3 MPa. According to one or more embodiments, the pressure in the column is comprised between 0.5 MPa and 3 MPa. According to one or more embodiments, the pressure in the column is comprised between 0.8 MPa and 3 MPa. For preference, the pressure in the column is comprised between 1 MPa and 2 MPa.

[0100] According to one or more embodiments, the changeover period ST used is comprised between 20 seconds and 120 seconds. For preference, the changeover period ST used is comprised between 40 seconds and 100 seconds.

[0101] Of course, these application examples are entirely nonlimiting, and other applications are possible, notably in the field of the separation of normal and iso paraffins or normal and iso olefins.

[0102] Without further elaboration, it is believed that one skilled in the art can, using the preceding description, utilize the present invention to its fullest extent. The preceding preferred specific embodiments are, therefore, to be construed as merely illustrative, and not limitative of the remainder of the disclosure in any way whatsoever.

[0103] In the foregoing and in the examples, all temperatures are set forth uncorrected in degrees Celsius and, all parts and percentages are by weight, unless otherwise indicated.

[0104] The entire disclosures of all applications, patents and publications, cited herein and of corresponding application No. FR 1855448, filed Jun. 20, 2018 are incorporated by reference herein.

EXAMPLES

[0105] The invention will be better understood from reading the following examples.

Example 1

Reference Method

[0106] Consider an SMB unit made up of 12 beds, of length 1.3 m and of internal radius 3.5 m, with an injection of feedstock L.sub.F, an injection of desorbent L.sub.D (which may also be referred to as eluent or solvent), a withdrawal of extract L.sub.E and a withdrawal of raffinate L.sub.R.

[0107] The adsorbent used is a zeolite of type BaX, and the eluent is paradiethylbenzene. The temperature is 175 C., and the pressure is 15 bar.

[0108] The feedstock is made up of 23 wt % of paraxylene, of 22 wt % of orthoxylene, of 50 wt % of metaxylene, and of 5 wt % of ethylbenzene with respect to the total weight of the feedstock. The changeover period ST employed is 45 seconds.

[0109] The feedstock and desorbent injection liquid flow rates are as follows:

565 m.sup.3.h.sup.1 for the feedstock;
710 m.sup.3.h.sup.1 for the desorbent;
namely a solvent ratio S/F=1.3.

[0110] The beds are distributed in the 2/5/3/2 configuration, which means to say that the distribution of the beds is as follows:

2 beds in zone 1;
5 beds in zone 2;
3 beds in zone 3;
2 beds in zone 4.

[0111] The plates have two mixing chambers. The total volume (V.sub.i+V.sub.i+1+V.sub.i/i+1), where VL.sub.i/i+1 is the volume of the bypass line from plate P.sub.i to plate P.sub.i+1 and where V.sub.i is the volume of the distribution/extraction system for plate P.sub.i, represents 3% of the volume of the bed comprised between plate P.sub.i and plate P.sub.i+1.

[0112] The synchronism is set at 100% for all the open bypass lines.

Reference Method

[0113] The effluent (raffinate or extract) withdrawal lines are situated downstream of the bypass line isolation valve (referred to more simply as downstream of the bypass line valve).

[0114] The feed lines (feeding feedstock or desorbent) are situated upstream of the isolation valve.

[0115] When a fluid (feedstock or desorbent) is injected at plate P.sub.i, use is made of an injection line connected to the bypass line L.sub.i/i+1. The isolation valve isolating the bypass line L.sub.i/i+1 is then closed to ensure that the injected fluid does indeed flow towards the plate P.sub.i.

[0116] When an effluent (extract or raffinate) is withdrawn at plate P.sub.i, use is made of a withdrawal line connected to the bypass line L.sub.i1/i. The isolation valve isolating the bypass line L.sub.i1/i is then closed.

[0117] In order to gain a clear understanding of the configuration of the reference method, a description is given of the configuration of the method when the desorbent is injected at plate P.sub.1. At that moment, the desorbent is injected via the bypass line L.sub.1/2. The feedstock is then injected at plate P.sub.8 via the bypass line L.sub.8/9. The extract is withdrawn at plate P.sub.3 via the bypass line L.sub.2/3. The raffinate is withdrawn at plate P.sub.11 via the bypass line L.sub.10/11.

[0118] In the reference method, zone 1 contains fewer than 3 beds. When the desorbent is injected at plate P.sub.i via the bypass line L.sub.i/i+1, the extract is withdrawn at plate P.sub.j (i.e., P.sub.i+2) via the bypass line L.sub.j1/j (i.e., L.sub.i+1/i+2).

[0119] By simulation, a paraxylene purity of 99.88% and a paraxylene yield of 96.18% are obtained.

Method According to the Invention

[0120] The raffinate withdrawal line is situated downstream of the bypass line isolation valve (referred to more simply as downstream of the bypass line valve).

[0121] The feed lines (feeding feedstock or desorbent), and the extract line are situated upstream of the isolation valve.

[0122] When a fluid (feedstock or desorbent) is injected at plate P.sub.i, use is made of an injection line connected to the bypass line L.sub.i/i+1. The isolation valve isolating the bypass line L.sub.i/i+1 is then closed to ensure that the injected fluid does indeed flow towards the plate P.sub.i.

[0123] When the extract is withdrawn at plate P.sub.i, use is made of an injection line connected to the bypass line L.sub.i/i+1. The isolation valve isolating the bypass line L.sub.i/i+1 is then closed.

[0124] When the raffinate is withdrawn at plate P.sub.i, use is made of a withdrawal line connected to the bypass line L.sub.i1/i. The isolation valve isolating the bypass line L.sub.i1/i is then closed.

[0125] In order to gain a clear understanding of the configuration of the reference method, a description is given of the configuration of the method when the desorbent is injected at plate P.sub.1. At that moment, the desorbent is injected via the bypass line L.sub.1/2. The feedstock is then injected at plate P.sub.8 via the bypass line L.sub.8/9. The extract is withdrawn at plate P.sub.3 via the bypass line L.sub.3/4. The raffinate is withdrawn at plate P.sub.11 via the bypass line L.sub.10/11.

[0126] In the method according to the invention, zone 1 contains fewer than 3 beds. When the desorbent is injected at plate P.sub.i via the bypass line L.sub.i/i+1, the extract is withdrawn at plate P.sub.j (i.e., P.sub.i+2) via the bypass line L.sub.j/j+1 (i.e., L.sub.i+2/i+3).

[0127] By simulation, a paraxylene purity of 99.91% and a paraxylene yield of 96.53% are obtained.

Example 2

[0128] Consider an SMB unit made up of 12 beds, of length 1.3 m and of internal radius 3.5 m, with an injection of feedstock, an injection of desorbent (which may also be referred to as eluent or solvent), a withdrawal of extract and a withdrawal of raffinate.

[0129] The adsorbent used is a zeolite of type BaX, and the eluent is paradiethylbenzene. The temperature is 175 C., and the pressure is 15 bar.

[0130] The feedstock is made up of 23 wt % of paraxylene, of 22 wt % of orthoxylene, of 50 wt % of metaxylene, and of 5 wt % of ethylbenzene with respect to the total weight of the feedstock. The changeover period ST employed is 45 seconds.

[0131] The feedstock and desorbent injection liquid flow rates are as follows:

565 m.sup.3.h.sup.1 for the feedstock;
710 m.sup.3.h.sup.1 for the desorbent;
namely a solvent ratio S/F=1.3.

[0132] The beds are distributed in the 2/5/3/2 configuration, which means to say that the distribution of the beds is as follows:

2 beds in zone 1;
5 beds in zone 2;
3 beds in zone 3;
2 beds in zone 4.

[0133] The plates have two mixing chambers. The total volume (V.sub.i+V.sub.i+1+VL.sub.i/i+1), where VL.sub.i/i+1 is the volume of the bypass line from plate P.sub.i to plate P.sub.i+1 and where V.sub.i is the volume of the distribution/extraction system for plate P.sub.i, represents 3% of the volume of the bed comprised between plate P.sub.i and plate P.sub.i+1.

[0134] The synchronism is set at 100% for all the open bypass lines.

Reference Method

[0135] The effluent (raffinate or extract) withdrawal lines are situated upstream of the bypass line isolation valve (referred to more simply as upstream of the bypass line valve).

[0136] The feed lines (feeding feedstock or desorbent) are situated downstream of the isolation valve.

[0137] When a fluid (feedstock or desorbent) is injected at plate P.sub.i, use is made of an injection line connected to the bypass line L.sub.i1/i. The isolation valve isolating the bypass line L.sub.i1/i is then closed to ensure that the injected fluid does indeed flow towards the plate P.sub.i.

[0138] When an effluent (extract or raffinate) is withdrawn at plate P.sub.i, use is made of a withdrawal line connected to the bypass line L.sub.i/i+1. The isolation valve isolating the bypass line L.sub.i/i+1 is then closed.

[0139] In order to gain a clear understanding of the configuration of the reference method, a description is given of the configuration of the method when the desorbent is injected at plate P.sub.1. At that moment, the desorbent is injected via the bypass line L.sub.12/1. The feedstock is then injected at plate P.sub.8 via the bypass line L.sub.7/8. The extract is withdrawn at plate P.sub.3 via the bypass line L.sub.3/4. The raffinate is withdrawn at plate P.sub.11 via the bypass line L.sub.11/12.

[0140] In the reference method, zone 4 contains fewer than 3 beds. When the raffinate is withdrawn at plate P.sub.i via the bypass line L.sub.i/i+1, the desorbent is injected at plate P.sub.j (i.e. P.sub.i+2) via the bypass line L.sub.j1/j (i.e. L.sub.i+1/i+2).

[0141] By simulation, a paraxylene purity of 99.76% and a paraxylene yield of 96.95% are obtained.

Method According to the Invention

[0142] The effluent (raffinate or extract) withdrawal lines and the desorbent injection line are situated upstream of the bypass line isolation valve (referred to more simply as upstream of the bypass line valve).

[0143] The feedstock feed line is situated downstream of the isolation valve.

[0144] When the desorbent is injected at plate P.sub.i, use is made of an injection line connected to the bypass line L.sub.i/i+1. The isolation valve isolating the bypass line L.sub.i/i+1 is then closed to ensure that the injected fluid does indeed flow towards the plate P.sub.i.

[0145] When the feedstock is injected at plate P.sub.i, use is made of an injection line connected to the bypass line L.sub.i1/i. The isolation valve isolating the bypass line L.sub.i1/i is then closed to ensure that the injected fluid does indeed flow towards the plate P.sub.i.

[0146] When an effluent (extract or raffinate) is withdrawn at plate P.sub.i, use is made of a withdrawal line connected to the bypass line L.sub.i/i+1. The isolation valve isolating the bypass line L.sub.i/i+1 is then closed.

[0147] In order to gain a clear understanding of the configuration of the reference method, a description is given of the configuration of the method when the desorbent is injected at plate P.sub.1. At that moment, the desorbent is injected via the bypass line L.sub.1/2. The feedstock is then injected at plate P.sub.8via the bypass line L.sub.7/8. The extract is withdrawn at plate P.sub.3 via the bypass line L.sub.3/4. The raffinate is withdrawn at plate P.sub.11 via the bypass line L.sub.11/12.

[0148] In the method according to the invention, zone 4 contains fewer than 3 beds. When the raffinate is withdrawn at plate P.sub.i via the bypass line L.sub.i/i+1, the desorbent is injected at plate P.sub.j (i.e. P.sub.i+2) via the bypass line L.sub.j/j+1 (i.e. L.sub.i+2/i+3).

[0149] By simulation, a paraxylene purity of 99.81% and a paraxylene yield of 96.99% are obtained.

[0150] The preceding examples can be repeated with similar success by substituting the generically or specifically described reactants and/or operating conditions of this invention for those used in the preceding examples.

[0151] From the foregoing description, one skilled in the art can easily ascertain the essential characteristics of this invention and, without departing from the spirit and scope thereof, can make various changes and modifications of the invention to adapt it to various usages and conditions.