Method for separating xylenes in a simulated moving bed by means of a zeolitic adsorbent solid having a particle size of between 150 and 500 microns

Abstract

Process for separating xylenes starting from a feed comprising cuts of isomers of aromatic hydrocarbons containing 8 carbon atoms, in a simulated moving bed, by selective adsorption of a xylene isomer in the presence of a desorbent, by means of particles of agglomerated zeolitic adsorbent based on zeolite crystals with a number-average diameter less than or equal to 1.2 m, wherein the number-average diameter of said particles of adsorbent is between 150 m and 500 m and the mechanical strength measured by the Shell method series SMS1471-74 adapted for agglomerates with a size below 500 m is greater than or equal to 2 MPa.

Claims

1. A process for separating xylenes starting from a feed comprising cuts of isomers of aromatic hydrocarbons containing 8 carbon atoms, the process comprising selectively adsorbing a xylene isomer from the feed in a simulated moving bed in the presence of a desorbent, using particles of agglomerated zeolitic adsorbent based on zeolite crystals with a number-average diameter less than or equal to 1.2 m, wherein the number-average diameter of said particles of adsorbent is between 150 m and 500 m, limits excluded, and the mechanical strength measured by the Shell method series SMS1471-74 adapted for agglomerates with a size below 500 m is greater than or equal to 2 MPa.

2. The process for separating xylenes according to claim 1, wherein the number-average diameter of said particles of agglomerated zeolitic adsorbent is from 200 m to 400 m, limits included.

3. The process for separating xylenes according to claim 1, wherein the granulometric distribution of said particles of adsorbent is such that there is no particle with size less than 100 m.

4. The process for separating xylenes according to claim 1, wherein the number-average diameter of the zeolite crystals is between 0.1 m and 1.2 m.

5. The process for separating xylenes according to claim 4, wherein the number-average diameter of the zeolite crystals is between 0.5 m and 0.8 m.

6. The process for separating xylenes according to claim 1, wherein the process is carried out in a simulated moving-bed unit having the following characteristics: number of beds between 4 and 24 number of zones: at least 4, wherein each zone has a zone flow rate.

7. The process for separating xylenes according to claim 6, wherein the cycle time, corresponding to the time between two injections of desorbent on a given bed, is between 4 and 18 min.

8. The process for separating xylenes according to claim 1, wherein the adsorption is carried out at a temperature from 100 C. to 250 C., and at a pressure between the bubble pressure of the xylenes at the process temperature and 3 MPa.

9. The process for separating xylenes according to claim 6, wherein the ratio of the flow rates of desorbent to feed is between 0.7 and 2.5 and a recycling rate, representing a ratio of the average flow rate of the zones weighted with the number of beds present in each zone to a feed flow rate, is between 2.0 and 12.

10. The process for separating xylenes according to claim 1 wherein the xylene which is selectively adsorbed is para-xylene and wherein the agglomerated zeolitic adsorbent is based on zeolite X or LSX having an Si/Al atomic ratio such that 1.0Si/Al<1.5.

11. The process for separating xylenes according to claim 10, wherein the agglomerated zeolitic adsorbent further comprises: i. a content of barium oxide BaO and a content of potassium oxide K.sub.2O such that the ratio of the number of moles of the total barium oxide+potassium oxide (BaO+K.sub.2O) to the number of moles of the total (BaO+K.sub.2O+Na.sub.2O) is greater than 90%; ii. a content of potassium oxide K.sub.2O such that the ratio of the number of moles of potassium oxide K.sub.2O to the number of moles of barium oxide BaO is less than 0.5; and iii. a total content of oxides of alkali-metal or alkaline-earth ions other than barium and potassium is below 5% relative to the total weight of the agglomerated zeolitic adsorbent.

12. The process for separating xylenes according to claim 10, wherein the agglomerated zeolitic adsorbent has a grain density between 1.1 and 1.4 g/mL, as measured by mercury intrusion (expressed relative to the dry mass of the zeolitic adsorbent) and a total pore volume measured by mercury intrusion (pore volume contained in the macropores and the mesopores with apparent diameter greater than 4 nm) between 0.20 and 0.35 mL/g (expressed relative to the dry mass of the zeolitic adsorbent).

13. The process for separating xylenes according to claim 10, wherein the process carried out at a temperature from 165 C. to 185 C. and wherein the water content of the hydrocarbon effluents is adjusted between 20 ppm and 150 ppm, by adding water to the feed comprising the cuts of isomers of aromatic hydrocarbons containing 8 carbon atoms and/or to the desorbent.

14. The process for separating xylenes according to claim 10, wherein the desorbent is selected from the group consisting of toluene and para-diethylbenzene.

15. The process for separating xylenes according to claim 1, wherein meta-xylene is selectively adsorbed and wherein the agglomerated zeolitic adsorbent is based on zeolite Y having an Si/Al atomic ratio such that 1.5<Si/Al<6.

16. The process for separating xylenes according to claim 15, wherein the agglomerated zeolitic adsorbent further comprises: i. a content of sodium oxide Na.sub.2O and a content of lithium oxide Li.sub.2O such that the ratio of the number of moles of sodium oxide to the number of moles of the total sodium oxide+lithium oxide (Na.sub.2O+Li.sub.2O) is greater than 65%; and ii. a total content of oxides of alkali-metal or alkaline-earth ions other than sodium and lithium below 5% relative to the total weight of the zeolitic adsorbent.

17. The process for separating xylenes according to claim 15, wherein the process is carried out at a temperature from 120 C. to 180 C. and wherein the water content in the hydrocarbon effluents is adjusted between 0 ppm and 80 ppm, by adding water to the feed comprising the cuts of isomers of aromatic hydrocarbons containing 8 carbon atoms and/or to the desorbent.

18. The process for separating xylenes according to claim 15, wherein the desorbent is selected from toluene and indane.

19. The process for separating xylenes according to claim 10, wherein para-xylene is separated in a purity greater than or equal to 90%.

20. The process for separating xylenes according to claim 15, wherein meta-xylene is separated in a purity greater than or equal to 90%.

Description

LIST OF FIGURES

(1) FIGS. 1 to 3 illustrate the invention and are presented non-exhaustively.

(2) FIG. 1: Schematic diagram of a multistage column with simulated moving-bed operation

(3) FIG. 2: Photograph of the results obtained in cold mock-up for the process for separating xylenes using the formulations of adsorbent according to the invention A to D: absence of formation of furrows or banks on the surface of the granular bed at the maximum linear surface velocity of 1.65 cm/s after 4 hours under flow, i.e. absence of deformation on the surface of the granular bed relative to the horizontal line passing by the surface of the bed at the beginning of the test (Example 2).

(4) FIG. 3: Photograph of the results obtained in cold mock-up for the process for separating xylenes using the formulations of adsorbent E and F (comparative tests): formation of furrows or banks on the surface of the granular bed for linear surface velocities below 1.65 cm/s, appearing after the first 20 minutes of the test. On the photograph, taken after 4 hours under flow, a deformation of the surface of the bed is observed in such a way that the heights of the bed at the lowest point and at the highest point of the height profile of the bed at the end of the test differ respectively by 70% and by 40% of the initial height of the granular bed (Example 3).

BRIEF DESCRIPTION OF FIG. 1

(5) The appended FIG. 1 shows a multistage column with distributor plates with simulated moving-bed operation. This figure is provided purely for purposes of illustration.

(6) The column in the enclosure (1) is divided into a certain number of granular beds (2). A distributor plate (5), supported on beams (7), is interposed between two successive granular beds, designated upstream bed and downstream bed. An upper grid (4) supports the granular medium (2) while allowing the fluid to go back into the plate.

(7) The distributing devices also comprise a distribution network (6) immersed in the granular medium (2), for injecting or withdrawing an auxiliary fluid into or from the plate. In the case of injection, the auxiliary fluid injected is thus mixed with the main fluid coming from the upstream bed.

(8) The distributor plate (5) also comprises a lower grid (3) or perforated plate, or any other means for distributing the flow on the downstream granular bed. There is an empty space (8) between the lower grid (3) and the upper surface of the downstream granular bed.

DETAILED DESCRIPTION OF THE INVENTION

(9) The invention relates to a process for separating xylenes starting from C8 cuts of aromatic isomers in a simulated moving bed (SMB) consisting of using, as adsorbent solid selectively retaining a xylene isomer, an agglomerated zeolitic adsorbent having in addition particular characteristics of granulometry at the level of the size of the zeolitic adsorbents. The particles of agglomerated zeolitic adsorbent solid used in the process according to the invention have a (number-) average particle diameter between 150 m and 500 m, preferably between 200 m and 400 m. Preferably, the particles have a granulometric distribution such that there is no particle with size less than 100 m.

(10) More particularly, the following will be selected, for selectively adsorbing: (1) para-xylene, zeolitic adsorbents based on zeolite X or LSX, exchanged with barium (to at least 90% expressed in degree of exchange on the final agglomerate, estimated by evaluating the ratio of the number of moles of barium oxide, BaO, to the number of moles of the total (BaO+Na.sub.2O) of the final agglomerate), or exchanged very predominantly with barium, and to a minor extent with potassium (the degree of exchange with barium and potassium ions being at least 90%, estimated by evaluating the ratio of the number of moles of the total barium oxide+potassium oxide (BaO+K.sub.2O) to the number of moles of the total (BaO+K.sub.2O+Na.sub.2O)) of the final agglomerate, or (2) meta-xylene, zeolitic adsorbents based on zeolite Y, with sodium or exchanged with sodium and lithium such that the ratio of the number of moles of sodium oxide Na.sub.2O to the number of moles of the total sodium oxide+lithium oxide (Na.sub.2O+Li.sub.2O) of the final agglomerate is greater than 65%, this adsorbent solid moreover having particular characteristics of granulometry at the level of the size of the zeolitic adsorbents.

(11) The process according to the present invention can be carried out both in the liquid phase and in the gas phase.

(12) The invention relates more particularly to a process for separating para-xylene or meta-xylene at high purity (i.e. a purity greater than or equal to 90%) in a simulated moving bed starting from a feed of aromatic hydrocarbons containing isomers with 8 carbon atoms comprising the following steps:

(13) a) a step of bringing the feed in contact, in suitable conditions of adsorption, with a bed containing the selected adsorbent, so as to adsorb para-xylene or meta-xylene preferentially,

(14) b) a step of bringing the bed of adsorbent in contact, in conditions of desorption, with a desorbent, the desorbent preferably being either toluene, or para-diethylbenzene,

(15) c) a step of withdrawing, from the bed of adsorbent, a stream containing the desorbent and the feed products the least selectively adsorbed,

(16) d) a step of withdrawing, from the bed of adsorbent, a stream containing the desorbent and the required product, namely para-xylene or meta-xylene,

(17) e) separating the stream resulting from step c) into a first stream containing the desorbent and a second stream containing the feed products the least selectively adsorbed,

(18) f) separating the stream resulting from step d) into a first stream containing the desorbent and a second stream containing para-xylene or meta-xylene at a level of purity greater than or equal to 90%, preferably greater than or equal to 99%, and very preferably greater than or equal to 99.7%.

(19) The process for separating para-xylene or meta-xylene can also optionally include the following steps:

(20) g) a step of crystallization in a crystallizer consisting of crystallization of the para-xylene resulting from step f), giving on the one hand crystals of para-xylene impregnated with their mother liquor, and on the other hand a mother liquor that can be partly, or even completely, recycled mixed with the fresh feed at the inlet of the simulated moving-bed adsorption unit,
h) a step of washing the crystals resulting from step g), at the end of which para-xylene or meta-xylene is recovered at a purity of at least 99.7%, and preferably of at least 99.8%.

(21) More precisely, the aim of the invention is to optimize the adsorbent solid employed in the process for separating para-xylene or meta-xylene by simulated moving-bed adsorption, to maximize the performance of this process. In general, the performance required for separating a feed containing xylenes is maximum productivity for a purity of the required product in the stream of extract at least equal to 99.5% and even 99.9%, and an overall yield of the required product at least equal to 90%, or even greater than 95% and preferably greater than 97%.

(22) In the present invention, the adsorbent solid used is a zeolitic adsorbent based on zeolite crystals, preferably of zeolite X, LSX or Y, and optionally of non-zeolitic phase (i.e. residual binder, amorphous phase, crystalline phases such as quartz etc. after zeolitization etc.), and in said adsorbent the crystals have a number-average diameter less than or equal to 1.2 m, preferably between 0.1 m and 1.2 m, and preferably between 0.5 m and 0.8 m.

(23) When the agglomerate according to the present invention is prepared starting from zeolite X or LSX, the Si/Al atomic ratio is between 1 and 1.5, preferably between 1.2 and 1.3, and the agglomerate comprises: i. a content of barium oxide BaO and a content of potassium oxide K.sub.2O such that the ratio of the number of moles of the total barium oxide+potassium oxide (BaO+K.sub.2O) to the number of moles of the total (BaO+K.sub.2O+Na.sub.2O) is greater than 90%. ii. a content of potassium oxide K.sub.2O such that the ratio of the number of moles of potassium oxide K.sub.2O to the number of moles of barium oxide BaO is less than 50%. iii. and a total content of oxides of alkali-metal or alkaline-earth ions other than barium and potassium preferably less than 5% and preferably ranging from 0 to 2 wt % and advantageously ranging from 0 to 1 wt % relative to the total weight of the anhydrous zeolitic adsorbent.

(24) When the zeolitic adsorbent according to the present invention is prepared starting from zeolite Y, the Si/Al atomic ratio is between 1.5 and 6, preferably between 2.5 and 3, and the zeolitic adsorbent comprises i. A content of sodium oxide Na.sub.2O and a content of lithium oxide Li.sub.2O such that the ratio of the number of moles of sodium oxide Na.sub.2O to the number of moles of the total sodium oxide+lithium oxide (Na.sub.2O+Li.sub.2O) is greater than 65%. ii. and a total content of oxides of alkali-metal or alkaline-earth ions other than sodium and lithium preferably less than 5% and preferably ranging from 0 to 2 wt % and advantageously ranging from 0 to 1 wt % relative to the total weight of the anhydrous zeolitic adsorbent.

(25) The zeolitic adsorbents according to the invention have a (number-) average particle diameter between 150 m and 500 m, preferably between 200 m and 400 m and preferably with a granulometric distribution such that there is no particle with size less than 100 m.

(26) When the zeolitic adsorbents according to the present invention are prepared starting from zeolite X or LSX, they preferably have a grain density between 1.1 and 1.4 g/mL, and preferably between 1.1 and 1.3 g/mL as measured by mercury intrusion (expressed relative to the dry mass of the zeolitic adsorbent) and a pore volume measured by mercury intrusion (pore volume contained in the macropores and the mesopores with apparent diameter greater than 4 nm) between 0.20 and 0.35 mL/g (expressed relative to the dry mass of the zeolitic adsorbent).

(27) The zeolitic adsorbents used in the present invention are obtained conventionally by a process comprising the following steps:

(28) 1/ mixing the crystals of zeolite X, LSX or Y as powder of the desired granulometry, in the presence of water with at least one binder based on a clay or a mixture of clays,

(29) 2/ forming the mixture obtained in 1/ to produce agglomerates, optionally followed by a step of sieving and/or of cycloning,

(30) 3/ calcination of the agglomerates obtained in 2/ at a temperature in the range from 500 C. to 600 C.,

(31) 4/ (optionally) zeolitization of the binder by contacting the product resulting from 3/ with a basic alkaline aqueous solution followed by washing;

(32) 5/ ion exchange of the zeolitic agglomerates based on zeolite X or LSX obtained in 3/ or in 4/ with barium ions alone or with barium ions and potassium ions and optionally partial ion exchange of the zeolitic agglomerates based on sodium zeolite Y obtained in 3/ or in 4/with lithium ions, followed by washing and drying of the product thus treated;
6/ activation of the product resulting from step 5 at a temperature in the range from 200 to 300 C.

(33) Step 2/ of forming can give zeolitic agglomerates having sufficient mechanical strength for use in a process for separating xylenes in a simulated moving bed. However, the presence of binder reduces the proportion of material that is active in the sense of adsorption (zeolite X, LSX or Y).

(34) The optional step 4/ of zeolitization of the binder thus allows some or all of the binder to be converted into material that is active in the sense of adsorption (zeolite X, LSX or Y) in order to obtain binderless agglomerates, i.e. no longer comprising non-zeolitic phase or in an amount typically less than 1% or binderlow agglomerates, i.e. comprising little non-zeolitic phase, i.e. generally from the non-zeolitized residual binder or any other amorphous phase after zeolitization, in an amount typically between around 2 and 5% in the final agglomerate, while maintaining the mechanical strength. The proportion of non-zeolitic phase (i.e. non-zeolitized residual binder, amorphous phase, after zeolitization) in the final agglomerate can be quantified by reference to an adsorbent composed solely of zeolite, in the form of powder, on the basis of measurements of adsorption or on the basis of intensities of XRD peaks. The reference zeolite is the zeolite used in step 1/ of the process for producing the adsorbent, and that has undergone the same ion exchange.

(35) The crystals of zeolites X, LSX or Y resulting from zeolitization of the binder (conversion of the binder to zeolite) are generally of smaller diameters than the initial crystals. Consequently, in the final agglomerate, the crystals whose average diameter is less than or equal to 1.2 m, preferably between 0.1 m and 1.2 m, and preferably between 0.5 m and 0.8 m, conventionally display a monomodal granulometric distribution, but we remain within the scope of the invention if the distribution of the diameters of the crystals is multimodal, and in particular bimodal, owing to the presence of the population of crystals resulting from zeolitization of the binder.

(36) The performance of the zeolitic adsorbents, as described above, for the process for separating para-xylene or meta-xylene, in terms of purity of para-xylene or of meta-xylene in the extract and yield of the process, is influenced by various parameters of the process, namely the operating conditions, the composition of the initial feed, the water content and the type of desorbent.

(37) The operating conditions of the industrial unit for simulated counter-current adsorption using the zeolitic adsorbents as described above are typically as follows: number of beds between 4 and 24 number of zones: at least 4 temperature 100 C. to 250 C., preferably 150 C. to 190 C., pressure between the bubble pressure of the xylenes at the process temperature and 3 MPa cycle time, corresponding to the time between two injections of desorbent on a given bed, between 4 and 18 min ratio of the flow rates of desorbent to feed 0.7 to 2.5 recycling rate (ratio of the average flow rate of the zones weighted with the number of beds present in each zone to the feed flow rate) from 2 to 12, preferably 2.5 to 4.

(38) The desorbent is preferably either toluene or para-diethylbenzene when the compound to be separated is para-xylene and preferably either toluene or indane when the compound to be separated is meta-xylene.

(39) The water content of the hydrocarbon effluents is preferably adjusted between 20 ppm and 150 ppm for the adsorbents based on zeolite X and LSX for a process temperature from 165 C. to 185 C., in order to obtain results that are optimum for productivity.

(40) The water content of the hydrocarbon effluents is preferably adjusted between 0 and 80 ppm for the adsorbents based on zeolite Y for a process temperature from 120 C. to 180 C., in order to obtain results that are optimum for productivity.

(41) Techniques for Characterization of the Zeolitic Adsorbents

(42) For estimating the number-average diameter of the adsorbent solids obtained at the end of step 2/ of forming, it is necessary to carry out an analysis of the granulometric distribution of a sample of adsorbent by imaging according to standard ISO 13322-2: 2006 using a conveyor belt to enable the sample to pass in front of the camera lens.

(43) The number-average diameter is then calculated from the granulometric distribution, applying standard ISO 9276-2: 2001. In the present document, the designation number-average diameter or else size is used for the particles of zeolitic adsorbents. The precision is of the order of 10 m for the size range of zeolitic adsorbents of the invention.

(44) The number-average diameter of the crystals of zeolite X, LSX or Y contained in the zeolitic adsorbents is estimated by observation with the scanning electron microscope (SEM). In order to estimate the size of the zeolite crystals from the samples, a set of images is obtained at a magnification of at least 5000. The diameter of at least 200 crystals is then measured using dedicated software, for example the Smile View software (publisher: LoGraMi). The number-average diameter is then calculated from the granulometric distribution, applying standard ISO 9276-2: 2001.

(45) The designation number-average diameter or else size is employed for the zeolite crystals. The precision is of the order of 3%.

(46) SEM observation of the zeolitic adsorbents also makes it possible to confirm the presence of non-zeolitic phase comprising for example residual binder (not converted during the zeolitization step) or any other amorphous phase in the agglomerates.

(47) Elemental chemical analysis of the adsorbent can be carried out by various analytical techniques known by a person skilled in the art. Among these techniques, we may mention the technique of chemical analysis by X-ray fluorescence as described in standard NF EN ISO 12677: 2011 on a wavelength dispersive spectrometer (WDXRF), for example Tiger S8 from the company BRUKER.

(48) X-ray fluorescence is a spectral technique that offers precise determination, both quantitative and qualitative, except for the lightest elements such as lithium, sodium or potassium present at very low contents. In this case, inductively coupled plasma atomic emission spectrometry (ICP-AES) will be preferred, described in standard NF EN ISO 21079-3, for example on apparatus of the Perkin Elmer 4300DV type.

(49) These elemental chemical analyzes make it possible both to verify the Si/Al atomic ratio of the zeolitic adsorbent, and measure the contents of oxides of alkali-metal or alkaline-earth ions and notably BaO, K.sub.2O, Na.sub.2O, Li.sub.2O.

(50) It should be noted that the contents of different oxides are given, regardless of the technique, as percentage by weight relative to the total weight of the anhydrous adsorbent.

(51) The mercury intrusion technique is used for characterizing the intragranular pore volume contained in the pores of the adsorbent with diameters greater than 3.6 nm, and for measuring its grain density. A mercury porosimeter of the Autopore 9500 Micromritics type is used for analysing the distribution of the pore volume contained in the macropores with pore diameter >50 nm and in the mesopores between 3.6 and 50 nm. The micropore volume within the zeolite crystals as well as the pore volume contained in the small mesopores between 2 and 3.6 nm are not accessible with the existing porosimeters. The experimental method described in the operating manual of the apparatus consists of putting a previously weighed sample of adsorbent (of known loss on ignition) in a cell of the porosimeter, then, after first degassing (evacuation pressure of 30 m Hg for at least 10 min), filling the cell with mercury at a given pressure (0.0036 MPa), and then applying a pressure increasing in stages to 400 MPa for gradual penetration of the mercury into the porous network of the sample. The relation between the pressure applied and the diameter of the pores is established on the assumption of cylindrical pores, a contact angle between the mercury and the wall of the pores of 140 and a surface tension of the mercury of 485 dynes/cm.

(52) The cumulative amount of mercury introduced is recorded as a function of the pressure applied. The apparatus cannot differentiate between intergranular volume and intragranular volume: it is assumed that at about 0.2 MPa (corresponding to apparent pore diameters of 7 m), the mercury fills all the intergranular voids, and in addition the mercury penetrates into the pores of the adsorbent. The grain density of the adsorbent is thus calculated by dividing the sample mass by the sample volume evaluated from the volume of mercury introduced at a pressure of 0.2 MPa.

(53) The total pore volume of the adsorbent is then evaluated from the total volume of mercury introduced, corrected with the volume of mercury introduced at a pressure of 0.2 MPa.

(54) In the present document, the grain density and the pore volume of the zeolitic adsorbents measured by mercury intrusion are referred to the mass of the anhydrous sample (by correcting the loss on ignition of the sample analyzed).

(55) The technique for characterization of the mechanical strength representative of crushing of the adsorbent within a bed or a reactor is the technique for characterization of the mechanical strength in a bed, as described in the Shell method series SMS1471-74 (Shell Method Series SMS1471-74 Determination of Bulk Crushing Strength of Catalysts. Compression-Sieve Method), combined with the BCS Tester apparatus marketed by the company Vinci Technologies. This method, originally intended for characterization of catalysts from 3 to 6 mm, is based on the use of a 425 m sieve, which will notably make it possible to separate the fines created during crushing. The use of a 425 m sieve is still suitable for particles with a diameter greater than 1.6 mm, but must be adapted according to the granulometry of the zeolitic adsorbents that are to be characterized. Standard ASTM D7084-04, which also describes a method for measuring the crushing strength in a catalyst bed (Determination of Bulk Crush Strength of Catalysts and Catalyst Carriers) defines the passage of the sieve to be used as being equal to half the diameter of the catalyst particles to be characterized. The method specifies a preliminary step of sieving the sample of catalysts or adsorbents to be characterized. If an amount equal to 10 wt % of the sample passes through the grid, a sieve with smaller passage will be used.

(56) The zeolitic adsorbents of the present invention are in the form of beads or extrudates, and have a number-average diameter ranging from 150 m to 500 m, and preferably ranging from 200 m to 400 m. Preferably, no particle has a size smaller than 100 m. Consequently, a 100 m sieve will be used in place of the 425 m sieve mentioned in the Shell standard SMS1471-74 method.

(57) The method takes place as follows: a 20 cm.sup.3 sample of agglomerated adsorbents, sieved beforehand with the appropriate sieve (100 m) and previously dried in a stove for at least 2 hours at 250 C. (instead of 300 C. mentioned in the Shell standard SMS1471-74 method), is placed in a metal cylinder of known internal section. An increasing force is imposed in stages on this sample by means of a piston, through a 5 cm.sup.3 bed of steel beads for better distribution of the force exerted by the piston on the agglomerates of adsorbents (use of beads of 4 mm diameter for analysis of all the types of extrudates and the particles of spherical shape having a diameter >1.6 mm, and beads of 2 mm diameter for particles of spherical shape with diameter <1.6 mm). The fines obtained at the different pressure stages are separated by sieving (suitable 100 m sieve) and weighed. The bed crushing strength is determined by the pressure in megapascal (MPa) for which the amount of cumulative fines passing through the sieve rises to 0.5 wt % of the sample. This value is found by plotting a graph of the mass of fines obtained as a function of the force applied on the bed of adsorbent and by interpolating to 0.5 wt % of cumulative fines. The bed crushing strength is typically between some hundreds of kPa and some tens of MPa and is generally between 0.3 and 3.2 MPa.

(58) The precision is conventionally below 0.1 MPa.

EXAMPLES

Example 1: Reference Formulation Ref. (Comparative)

(59) In this example, a zeolitic adsorbent based on BaX forming 1.5 m crystals, formed as beads with diameter of 0.6 mm, is used for simulated moving-bed separation of para-xylene. The adsorbent is prepared by the method of manufacture described in patent application WO08/009845.

(60) A unit with simulated moving-bed operation is used, consisting of 24 beds, with a length of 1.1 m, with injection of feed, injection of desorbent, withdrawal of extract and withdrawal of raffinate. The beds are distributed in 4 chromatographic zones according to the configuration: 5/9/7/3.

(61) The feed is composed of 21.6% of para-xylene, 20.8% of ortho-xylene, 47.9% of meta-xylene and 9.7% of ethylbenzene. The desorbent is para-diethylbenzene. The temperature is 175 C., and the pressure is 15 bar. The water content is 95 ppm (by weight).

(62) The reference productivity is 70 kg of para-xylene/m.sup.3/h.

(63) The linear surface velocity in zone 3 is 1.63 cm/s.

Example 2: Formulations A to D (According to the Invention)

(64) The performance of the different formulations according to the invention in a simulated moving bed is evaluated and expressed as gain in productivity relative to the preceding reference example. The objective of maximum linear surface velocity in zone 3 is maintained at 1.65 cm/s.

(65) Formulation A: 300 m beads consisting of 1.2 m crystals

(66) Formulation B: 300 m beads consisting of 0.3 m crystals

(67) Formulation C: 200 m beads consisting of 0.8 m crystals

(68) Formulation D: 200 m beads consisting of 0.5 m crystals

(69) Moreover, mock-up tests reproducing a section of an adsorber (upper distributing element, bed of adsorbent and lower distributing element) were conducted with formulations A to D in order to observe possible formation of furrows or banks on the surface of the granular bed, which is detrimental for the productivity in the long term.

(70) For all the formulations A to D, no formation of furrows or banks on the surface of the granular bed is observed at the maximum linear surface velocity of 1.65 cm/s (FIG. 2), i.e on the photographs, no deformation of the surface of the granular bed is observed relative to the horizontal line passing by the initial bed surface at the beginning of the test.

Example 3: Formulations E and F (Comparative)

(71) Formulations with smaller granulometry than that of examples A to D are tested and the gain in productivity relative to the comparative example is evaluated. The objective of maximum linear surface velocity in zone 3 is maintained at 1.65 cm/s.

(72) Formulation E: 100 m beads consisting of crystals of 1.2 m

(73) Formulation F: 100 m beads consisting of crystals of 0.5 m

(74) Mock-up tests were also carried out with formulations E and F, and show formation of furrows or banks on the surface of the granular bed for linear surface velocities below 1.65 cm/s (FIG. 3). After 4 hours under flow, a deformation of the bed surface is observed in such a way that the lowest and/or the highest point of the height profile of the bed is at least 10% different from the initial height of the granular bed.

Example 4: Results

(75) Table 1 presents the results of the mock-up test on the different formulations A, B, C, D, E, F relative to the reference Ref.

(76) It appears that choosing a granulometry according to the invention for the particles of adsorbent solid, combined with small size of zeolite crystals, can give good initial productivity, while avoiding the formation of banks, which allows this productivity to be maintained in the long term.

(77) TABLE-US-00001 Diameter Diameter Mechanical Gain in of the of the crushing Velocity initial beads crystals strength in zone 3 productivity/ Formation (m) (m) (MPa) (cm/s) Ref. of banks Ref. 600 1.5 2.4 1.63 No (comparative) A 300 1.2 2.5 1.28 2.4 No B 300 0.3 2.0 1.60 6.7 No C 200 0.8 2.3 1.63 5.9 No D 200 0.5 2.1 1.66 9.0 No E 100 1.2 1.50 3.7 Yes (comparative) F 100 0.5 1.65 9.2 Yes (comparative)