ZEOLITE ADSORBENTS WITH LOW BINDER CONTENT AND LARGE EXTERNAL SURFACE AREA, METHOD FOR PREPARATION OF SAME AND USES THEREOF

Abstract

The present invention relates to a zeolite absorbent comprising at least one FAU zeolite with hierarchical porosity and comprising barium or barium and potassium, and the external surface area of which is greater than 20 m.sup.2.Math.g.sup.−1, and the non-zeolite phase content being between 6% and 12% by weight with respect to the total weight of the absorbent. The present invention also relates to the use of such a zeolite absorbent as an adsorption agent, as well as the method for separation of para-xylene from aromatic isomer fractions with 8 carbon atoms.

Claims

1. A zeolite adsorbent comprising at least one FAU zeolite with hierarchical porosity and comprising barium or barium and potassium, wherein the zeolite adsorbent has: an outer surface area, measured by nitrogen adsorption, of greater than 20 m.sup.2.Math.g.sup.−1, said outer surface area being combined with a population of mesopores with a mean diameter of between 2 nm and 50 nm, and a content of non-zeolite phase of between 6% and 12% by weight relative to the total weight of the adsorbent.

2. The zeolite adsorbent according to claim 1, wherein the FAU zeolite with hierarchical porosity of the zeolite adsorbent is a zeolite in the form of crystals having: a number-average diameter of between 1 μm and 20 μm, an outer surface area, measured by nitrogen adsorption, greater than 40 m.sup.2.Math.g.sup.−1.

3. The zeolite adsorbent according to claim 1, having a total volume contained in the macropores and the mesopores (sum of the macropore volume and of the mesopore volume) measured by mercury intrusion of between 0.15 cm.sup.3.Math.g.sup.−1 and 0.5 cm.sup.3.Math.g.sup.−1.

4. The zeolite adsorbent according to claim 1, having a (macropore volume)/(macropore volume+mesopore volume) ratio of between 0.2 and 1.

5. The zeolite adsorbent according to claim 1, having a mass fraction of FAU zeolite greater than or equal to 88% relative to the total weight of adsorbent of the present invention, the remainder to 100% consisting of non-zeolite phase.

6. A process for preparing a zeolite adsorbent according to claim 1, said process comprising at least the steps of: a) agglomerating crystals of at least one FAU-type zeolite with hierarchical porosity, having an outer surface area of greater than 40 m.sup.2.Math.g.sup.−1, the number-average diameter of the crystals being between 1 μm and 20 μm, with a binder and also with the amount of water that allows the forming of an agglomerated material, followed by drying and calcination of the agglomerated material to obtain agglomerates; b) cation exchanging of the agglomerates from step a) by placing in contact with a solution of barium ions and/or of barium ions and potassium ions; c) optional additional cation exchanging of the agglomerates from step b) by placing in contact with a solution of potassium ions; d) washing and drying of the agglomerates obtained in steps b) or c), at a temperature of between 50° C. and 150° C.; and e) producing the zeolite adsorbent according to claim 1 by activating the agglomerates obtained in step d) under a stream of oxidizing and/or inert gas, at a temperature of between 100° C. and 400° C.

7. The process according to claim 6, wherein, in step a), the at least one FAU zeolite with hierarchical porosity is prepared in the presence of a sacrificial template that is intended to be removed.

8. The process according to claim 6, wherein the agglomeration and the forming (step a) are performed according to one or more of the techniques selected from the group consisting of extrusion, compacting, agglomeration on a granulating plate, granulating drum, and atomization.

9. The process according to claim 6, wherein agglomeration binder and zeolite are used in a proportion of from 8 parts to 15 parts by weight of binder per 92 parts to 85 parts by weight of zeolite.

10. The process according to claim 6, wherein the binder is selected from the group consisting of clays and mixtures of clays, silicas, aluminas, colloidal silicas and alumina gels, and mixtures thereof.

11. The process according to claim 10, wherein the binder is a clay selected from the group consisting of kaolins, kaolinites, nacrites, dickites, halloysites, attapulgites, sepiolites, montmorillonites, bentonites, illites and metakaolins, and also mixtures of two or more, in all proportions.

12. The process according to claim 11, wherein the clay(s) are formulated in the form of dry-ground and selected powders, or in the form of gel, and dispersed, and optionally ground.

13. A process, comprising using a zeolite adsorbent according to claim 1 as an adsorption agent in: separating C8 aromatic isomer fractions, separating substituted toluene isomers separating cresols, or separating polyhydric alcohols.

14. A process for separating xylene isomers in gas phase or in liquid phase using at least one zeolite adsorbent according to claim 1.

15. The process according to claim 14, wherein the process is a process for separating para-xylene from aromatic isomer fractions containing 8 carbon atoms, using, as para-xylene adsorption agent, a zeolite adsorbent according to claim 1.

16. The zeolite adsorbent according to claim 1, wherein the outer surface of the zeolite adsorbent, measured by nitrogen adsorption, is between 30 m.sup.2.Math.g.sup.−1 and 200 m.sup.2.Math.g.sup.−1.

17. The zeolite adsorbent according to claim 1, wherein the outer surface of the zeolite adsorbent, measured by nitrogen adsorption, is between 30 m.sup.2.Math.g.sup.−1 and 150 m.sup.2.Math.g.sup.−1.

18. The zeolite adsorbent according to claim 1, wherein the content of non-zeolite phase of between 6% and 11% by weight relative to the total weight of the adsorbent.

19. The zeolite adsorbent according to claim 1, wherein the content of non-zeolite phase of between 6% and 10% by weight relative to the total weight of the adsorbent.

20. The zeolite adsorbent according to claim 1, wherein the FAU zeolite with hierarchical porosity of the zeolite adsorbent is a zeolite in the form of crystals having: a number-average diameter between 1.8 μm and 10 μm, and an outer surface area, measured by nitrogen adsorption, between 40 m.sup.2.Math.g.sup.−1 and 200 m.sup.2.Math.g.sup.−1.

Description

CHARACTERIZATION TECHNIQUES

Particle Size of the Zeolite Crystals—Detection of the Mesopores:

[0152] The estimation of the number-average diameter of the zeolite FAU crystals contained in the zeolite adsorbents according to the invention is performed by observation with a scanning electron microscope (SEM).

[0153] In order to estimate the size of the zeolite crystals in the adsorbents, a set of images is taken at a magnification of at least 5000. The diameter of at least 200 crystals is then measured using dedicated software. The accuracy is of the order of 3%.

[0154] As indicated in U.S. Pat. No. 7,785,563, TEM also makes it possible to check whether the zeolite crystals contained in the adsorbent are filled zeolite crystals (i.e. non-mesoporous) or aggregates of filled zeolite crystals or mesoporous crystals (cf. the comparison of the TEM images in FIG. 1, in which the mesoporosity is clearly visible, and FIG. 2 which show filled crystals). TEM observation thus makes it possible to visualize the presence or absence of the mesopores. Preferably, the adsorbents of the process according to the invention very predominantly contain, i.e. typically more than 80% and preferably more than 90% by number, mesoporous zeolite crystals rather than filled crystals. This statistical analysis is advantageously performed by analysing at least 50 TEM or SEM images (SEM on sections of samples obtained by ionic polishing).

Chemical Analysis of the Zeolite Adsorbent—Si/Al Ratio and Degree of Exchange:

[0155] An elemental chemical analysis of the zeolite adsorbent may be performed according to various analytical techniques known to those skilled in the art. Among these techniques, mention may be made of the technique of X-ray fluorescence chemical analysis as described in standard NF EN ISO 12677: 2011 on a wavelength dispersive spectrometer (WDXRF), for example the Tiger S8 machine from the company Bruker.

[0156] X-ray fluorescence is a non-destructive spectral technique exploiting the photoluminescence of atoms in the X-ray range, to establish the elemental composition of a sample. Excitation of the atoms, generally with a beam of X-rays or by bombardment with electrons, generates specific radiations after returning to the ground state of the atom. The X-ray fluorescence spectrum has the advantage of depending very little on the chemical combination of the element, which offers a precise determination, both quantitatively and qualitatively. A measuring uncertainty of less than 0.4% by weight is conventionally obtained after calibration for each oxide.

[0157] These elemental chemical analyses make it possible both to check the Si/Al atomic ratio of the zeolite used during the preparation of the adsorbent, and also the Si/Al atomic ratio of the adsorbent and to check the quality of the ion exchange described in step b) and the optional step c). In the description of the present invention, the measuring uncertainty of the Si/Al atomic ratio is ±5%.

[0158] The quality of the ion exchange is linked to the number of moles of sodium oxide, Na.sub.2O, remaining in the zeolite agglomerate after exchange. More specifically, the degree of exchange with barium ions is estimated by evaluating the ratio between the number of moles of barium oxide, BaO, and the number of moles of the combination (BaO+Na.sub.2O). Similarly, the degree of exchange with barium and/or potassium ions is estimated by evaluating the ratio between the number of moles of the combination barium oxide+potassium oxide (BaO+K.sub.2O) and the number of moles of the combination (BaO+K.sub.2O+Na.sub.2O). It should be noted that the contents of various oxides are given as weight percentages relative to the total weight of the anhydrous zeolite adsorbent.

Particle Size of the Zeolite Adsorbents:

[0159] The determination of the volume-average diameter of the zeolite adsorbents obtained after the agglomeration and forming step a) is performed by analysis of the particle size distribution of a sample of adsorbent by imaging according to standard ISO 13322-2:2006, using a conveyor belt which allows the sample to pass before the objective lens of the camera.

[0160] The volume-average diameter is then calculated from the particle size distribution by applying standard ISO 9276-2:2001. In the present document, the name “volume-average diameter” or “size” is used for the zeolite adsorbents. The precision is of the order of 0.01 mm for the range of sizes of the adsorbents that are useful in the context of the present invention.

Mechanical Strength of the Zeolite Adsorbents:

[0161] The crush strength of a bed of zeolite adsorbents as described in the present invention is characterized according to the Shell method series SMS1471-74 (Shell Method Series SMS1471-74 Determination of Bulk Crushing Strength of Catalysts. Compression-Sieve Method), associated with the BCS Tester machine sold by the company Vinci Technologies. This method, initially intended for the characterization of catalysts from 3 mm to 6 mm, is based on the use of a 425 μm screen, which makes it possible especially to separate the fines created during the crushing. The use of a 425 μm screen remains suited to zeolite adsorbents with a diameter of greater than 1.6 mm, but should be adapted according to the particle size of the adsorbents that it is desired to characterize.

[0162] The adsorbents of the present invention, generally in the form of beads or extrudates, generally have a volume-average diameter or a length, i.e. longest dimension in the case of non-spherical adsorbents, of between 0.2 mm and 2 mm, in particular between 0.2 mm and 0.8 mm and preferably between 0.40 mm and 0.65 mm. Consequently, a 100 μm screen is used instead of the 425 μm screen mentioned in the Shell method standard SMS1471-74.

[0163] The measuring protocol is as follows: a sample of 20 cm.sup.3 of agglomerated adsorbents, screened beforehand with the appropriate screen (100 μm) and dried beforehand in an oven for at least 2 hours at 250° C. (instead of 300° C. mentioned in Shell method standard SMS1471-74), is placed in a metal cylinder of known internal cross section. An increasing force is imposed in stages on this sample by means of a piston, through a bed of 5 cm.sup.3 of steel balls so as better to distribute the force exerted by the piston on the agglomerated absorbents (use of balls 2 mm in diameter for particles of spherical shape with a diameter strictly less than 1.6 mm). The fines obtained at the various pressure stages are separated out by screening (appropriate 100 μm screen) and weighed.

[0164] The bulk crushing strength is determined by the pressure in megapascals (MPa) for which the cumulative amount of fines passing through the screen is 0.5% by weight of the sample. This value is obtained by plotting on a graph the mass of fines obtained as a function of the force applied to the adsorbent bed and by interpolating to 0.5% by mass of cumulative fines. The mechanical bulk crushing strength is typically between a few hundred kPa and a few tens of MPa and generally between 0.3 MPa and 3.2 MPa. The precision is conventionally less than 0.1 MPa.

Non-Zeolite Phase of the Zeolite Adsorbents:

[0165] The content of non-zeolite phase NZP, for example the content of agglomeration binder and of any other amorphous phase, is calculated according to the following equation:

NZP=100−Σ(ZP),

in which ZP represents the sum of the amounts of zeolite X fractions within the meaning of the invention.

Mass Amount of the Zeolite Fractions of the Zeolite Adsorbents:

[0166] The mass amount of the zeolite fractions is measured by X-ray diffraction analysis, known to those skilled in the art by the abbreviation XRD. This analysis is performed on a Brüker brand machine, and the amount of zeolite fractions is then evaluated from the peak intensities of the diffractograms by taking as reference the peak intensities of a suitable reference (zeolite of the same chemical nature assumed to be 100% crystalline under cationic treatment conditions identical to those of the adsorbent under consideration). The peaks for tracing back to the crystallinity are the most intense peaks of the angular 26 zone between 9° and 37°, namely peaks observed in the angular 28 ranges between, respectively, 11° and 13°, between 22° and 26° and between 31° and 33°.

Micropore Volume, Outer Surface Area and Diameter of the Mesopores:

[0167] The crystallinity of the zeolite adsorbents of the invention is also evaluated by measuring their micropore volume and comparing it with that of a suitable reference (100% crystalline zeolite under identical cationic treatment conditions or theoretical zeolite). This micropore volume is determined form the measurement of the adsorption isotherm of the gas, such as nitrogen, at its liquefaction temperature.

[0168] Prior to the adsorption, the zeolite adsorbent is degassed at between 300° C. and 450° C. for a time of between 9 hours and 16 hours under vacuum (P<6.7×10.sup.−4 Pa). Measurement of the nitrogen adsorption isotherm at 77 K. is then performed on an ASAP 2020 M machine from Micromeritics, taking at least 35 measurement points at relative pressures with a ratio P/P.sub.0 of between 0.002 and 1.

[0169] The micropore volume and the outer surface area are determined from the isotherm obtained, via the t-plot method by applying standard ISO 15901-3:2007 and calculating the statistical thickness t via the Harkins-Jura equation. The micropore volume and the outer surface area are obtained by linear regression on the points of the t-plot between 0.45 nm and 0.57 nm, respectively from the y-axis to the origin and from the slope of the linear progression. The evaluated micropore value is expressed in cm.sup.3 of liquid adsorbate per gram of anhydrous adsorbent. The outer surface area is expressed in m.sup.2 per gram of anhydrous adsorbent.

[0170] Interpretation of the nitrogen adsorption isotherm at 77 K via the Barrett-Joyner-Halenda method (BJH method, proposed in 1951) also makes it possible to obtain the pore size distribution, and especially the mesopore distribution. The mesopore size distribution by volume is represented by the curve dV/dDm as a function of the mean pore diameter Dm.

[0171] The full width at half maximum of the volume distribution dV/dDm is given by the difference between the two mean diameters for which the value dV/dDm would be equal to half of its maximum value f.sub.max, at the top of the peak. These two mean diameters are obtained by interpolation between the desired points on either side of the mode, for which dV/dDm surrounds the value f.sub.max/2. This is the full width at half maximum or FWHM of a distribution f(x) whose maximum value is f.sub.max.

Macropore and Mesopore Volume and Grain Density:

[0172] The macropore and mesopore volumes and the grain density are measured by mercury intrusion porosimetry. An Autopore® 9500 mercury porosimeter from Micromeritics is used to analyse the distribution of the pore volume contained in the macropores and in the mesopores.

[0173] The experimental method, described in the operating manual for the machine which refers to standard ASTM D4284-83, consists in placing a sample of adsorbent (zeolitic granular material to be measured) (known loss on ignition) weighed beforehand, in a porosimeter cell, and then, after first degassing (vacuum pressure of 30 μmHg for at least 10 minutes), in filling the cell with mercury at a given pressure (0.0036 MPa) and then in applying a pressure increasing in stages up to 400 MPa so as to make the mercury gradually penetrate into the pore network of the sample.

[0174] The relationship between the applied pressure and the apparent pore diameter is established by assuming cylindrical pores, a contact angle between the mercury and the pore wall of 140° and a mercury surface tension of 485 dynes/cm. The cumulative amount of mercury introduced as a function of the applied pressure is recorded. The value at and above which the mercury fills all the inter-granular voids is set at 0.2 MPa, and it is considered that beyond this value, the mercury penetrates into the pores of the granular material. The grain volume (Vg) is then calculated by subtracting the cumulative volume of mercury at this pressure (0.2 MPa) from the volume of the porosimeter cell, and by dividing this difference by the mass of the anhydrous equivalent granular material, i.e. the mass of said material corrected for the loss on ignition.

[0175] The grain density is the inverse of the grain volume (Vg), and is expressed in grams of anhydrous adsorbent per cm.sup.3.

[0176] The macropore volume of the granular material is defined as being the cumulative volume of mercury introduced at a pressure of between 0.2 MPa and 30 MPa, corresponding to the volume contained in the pores with an apparent diameter of greater than 50 nm. The mesopore volume of the granular material is defined as being the cumulative volume of mercury introduced at a pressure of between 30 MPa and 400 MPa.

[0177] In the present document, the macropore and mesopore volumes of the zeolite adsorbents, expressed in cm.sup.3.Math.g.sup.−1, are thus measured by mercury intrusion and related to the mass of the sample as anhydrous equivalent, i.e. the mass of said material corrected for the loss on ignition.

Loss on Ignition of the Zeolite Adsorbents:

[0178] The loss on ignition is determined under an oxidizing atmosphere, by calcination of the sample in air at a temperature of 950° C.±25° C., as described in standard NF EN 196-2 (April 2006). The measurement standard deviation is less than 0.1%.

Example A: Synthesis of FAU Zeolite with Hierarchical Porosity

[0179] The FAU zeolite with a high outer surface area is synthesized directly according to the article by Inayat et al. (Angew. Chem. Int. Ed., (2012), 51, 1962-1965).

Step 1): Preparation of the Growth Gel in a Reactor Stirred with an Archimedean Screw at 300 rpm.

[0180] A growth gel is prepared in a stainless-steel reactor equipped with a heating jacket, a temperature probe and a stirrer, by mixing a solution of aluminate containing 119 g of sodium hydroxide (NaOH) with 128 g of alumina trihydrate (Al.sub.2O.sub.3, 3H.sub.2O, containing 65.2% by weight of Al.sub.2O.sub.3) and 195.5 g of water at 25° C. over 25 minutes with a stirring speed of 300 rpm in a silicate solution containing 565.3 g of sodium silicate, 55.3 g of NaOH and 1997.5 g of water at 25° C.

[0181] The stoichiometry of the growth gel is as follows: 3.48 Na.sub.2O/Al.sub.2O.sub.3/3.07 SiO.sub.2/180 H.sub.2O. Homogenization of the growth gel is performed with stirring at 300 rpm for 25 minutes at 25° C.

Step 2): Introduction into the Reaction Medium of the Structuring Agent

[0182] 27.3 g of TPOAC at 60% in MeOH are introduced into the reaction medium with a stirring speed of 300 rpm (TPOAC/Al.sub.2O.sub.3 mole ratio=0.04). After homogenization for 5 minutes, the stirring speed is lowered to 50 rpm.

Step 3): Maturation Phase

[0183] The reaction medium is stirred at 50 rpm at 25° C. for 22 hours, and crystallization is then started.

Step 4): Crystallization

[0184] The stirring speed is maintained at 50 rpm, and the nominal temperature of the reactor jacket is set at 80° C. so that the temperature of the reaction medium rises to 75° C. over 80 minutes. After 72 hours at a stage of 75° C., the reaction medium is cooled by circulating cold water in the jacket to stop the crystallization.

Step 5): Filtration/Washing

[0185] The solids are recovered on a sinter and then washed with deionized water to neutral pH.

Step 6): Drying/Calcination

[0186] In order to characterize the products, drying is performed in an oven at 90° C. for 8 hours; the loss on ignition of the dried product is 22% by weight.

[0187] Calcination of the dried product, which is necessary to release both the microporosity (water) and the mesoporosity by removing the structuring agent, is performed with the following temperature profile: 30 minutes of increase to 200° C., followed by 1 hour at a stage of 200° C., then 3 hours of increase to 550° C., and finally 1.5 hours at a stage of 550° C.

[0188] The micropore volume and the outer surface area measured according to the t-plot method from the nitrogen adsorption isotherm at 77 K after degassing under vacuum at 400° C. for 10 hours are, respectively, 0.260 cm.sup.3.Math.g.sup.−1 and 90 m.sup.2.Math.g.sup.−1. The number-average diameter of the crystals of the mesoporous zeolite (or zeolite with hierarchical porosity) thus obtained is 4.5 μm and the Si/Al ratio is equal to 1.24.

[0189] In the text hereinbelow, a mass expressed as anhydrous equivalent means a mass of product minus its loss on ignition.

Example 1: (Comparative)

[0190] Preparation of a Zeolite Adsorbent in the Form of Beads with a Zeolite of XHP Type, Zeolite Crystals 4.5 μm in Size and a Binder of Kaolin Type Such that the Content of Non-Zeolite Phase (NZP) of the Final Adsorbent is Equal to 16% by Weight Relative to the Total Weight of the Adsorbent.

[0191] A homogenous mixture is prepared consisting of 1600 g anhydrous equivalent of zeolite X crystals synthesized according to the procedure of Example A (crystal size 4.5 μm), 350 g anhydrous equivalent of kaolin, 130 g of colloidal silica sold under the trade name Klebosol® 30 (containing 30% by weight of SiO.sub.2 and 0.5% of Na.sub.2O) and also the amount of water that allows agglomeration of the mixture according to bead formation techniques, for instance granulating plate.

[0192] Distribution beads between 0.3 mm and 0.8 mm and with a volume-average diameter of 0.55 mm are formed. The beads are dried overnight in a ventilated oven at 80° C. They are then calcined for 2 hours at 550° C. under a stream of nitrogen, and then for 2 hours at 550° C. under a stream of decarbonated dry air.

[0193] Barium exchange is then performed with a 0.7M concentration of barium chloride solution, BaCl.sub.2, at 95° C. in 4 steps. At each step, the volume ratio of solution to mass of solid is 20 ml/g and the exchange is continued for 4 hours each time. Between each exchange, the solid is washed several times so as to free it of the excess salt. It is then dried at 80° C. for 2 hours and then activated at 250° C. for 2 hours under a stream of nitrogen.

[0194] The degree of barium exchange measured by WDXRF, as described above in the analytical techniques, is 97% and the loss on ignition (measured at 900° C.) is 5.5%. The micropore volume and the outer surface area measured according to the t-plot method from the nitrogen adsorption isotherm at 77 K after degassing under vacuum at 400° C. for 10 hours are, respectively, 0.192 cm.sup.3.Math.g.sup.−1 and 70 m.sup.2.Math.g.sup.−1.

[0195] The total volume contained in the macropores and the mesopores (sum of the macropore volume and of the mesopore volume) measured by mercury intrusion is 0.31 cm.sup.3.Math.g.sup.−1. The (macropore volume)/(macropore volume+mesopore volume) ratio is equal to 0.65.

[0196] The content of non-zeolite phase of the 97% barium-exchanged adsorbent is 16% by weight relative to the total weight of the adsorbent.

Example 2

[0197] Adsorbent with XHP Crystals 4.5 μm in Size and an Agglomeration Binder of Kaolin Type Such that the Content of Non-Zeolite Phase of the Final Adsorbent is Between 4% and 20% by Weight Relative to the Total Weight of the Adsorbent.

[0198] Example 1 is reproduced, varying the content of agglomeration binder so as to obtain adsorbents whose content of non-zeolite phase after exchange ranges between 4% and 20%. The adsorbents are subjected to the same treatments as in Example 1. The results are collated in Table 1 below:

TABLE-US-00001 TABLE 1 NZP content V.sub.micro by t-plot Ex. (%) REL (Mpa) (cm.sup.3 .Math. g.sup.−1) comparative 4 0.5 0.220 comparative 5 1.0 0.211 according to the 6 1.6 0.215 invention according to the 8 1.8 0.211 invention according to the 10 2.0 0.206 invention according to the 12 2.2 0.202 invention comparative 16 2.4 0.192 comparative 20 3.0 0.183

[0199] A piercing test (frontal chromatography) is then performed on a selection of 6 adsorbents to evaluate their efficacy. The adsorbents containing 4% and 5% NZP are not tested, since such adsorbents could not be used in the para-xylene separation application on account of their low mechanical strength. The amount of adsorbent used for this test is about 34 g. The loss on ignition (LOI) is set at between 5.4% and 5.6%.

[0200] The procedure for obtaining the piercing curves is as follows: [0201] packing of the column with the sieves and installation in the test bed; [0202] packing with the solvent at room temperature; [0203] gradual rise to 175° C. under a stream of solvent (5 cm.sup.3.Math.min.sup.−1); [0204] injection of solvent at 30 cm.sup.3.Math.min.sup.−1 when the adsorption temperature (175° C.) is reached; [0205] solvent/feedstock exchange to inject the feedstock (30 cm.sup.3.Math.min.sup.−1); [0206] collection and analysis of the piercing effluent; the injection of the feedstock will be maintained until the concentration of solvent in the effluent is zero.

[0207] The solvent used is para-diethylbenzene. The composition of the feedstock is as follows: [0208] para-xylene: 45% by weight, [0209] meta-xylene: 45% by weight, [0210] isooctane: 10% by weight (this is used as tracer for estimating the non-selective volumes and does not participate in the separation).

[0211] The pressure is sufficient for the feedstock to remain in liquid phase at the adsorption temperature, i.e. 1 MPa. The surface speed is 1.3 cm/s.

[0212] The selectivity of para-xylene relative to meta-xylene is calculated from the adsorbed amounts of each compound, the latter being determined by material balance from the first moments of the piercing curves for all of the constituents present in the effluent. The evaluation of the quality of the matter transfer is performed by estimating the EHTPs from the para-xylene piercing curves. The results are given in Table 2 below:

TABLE-US-00002 TABLE 2 NZP content Xylene adsorption PX transfer Example (%) capacity (cm.sup.3 .Math. g.sup.−1) (=EHTP) according to the 6 0.205 4.4 invention according to the 8 0.200 4.6 invention according to the 10 0.194 4.7 invention according to the 12 0.189 5.1 invention comparative 16 0.181 5.4 comparative 20 0.172 6.8

[0213] In the above table: [0214] the xylene adsorption capacity is expressed in cm.sup.3 of aromatic C8 adsorbed per gram of adsorbent; [0215] “PX” means para-xylene; and finally [0216] “EHTP” represents the equivalent height of theoretical plates and is expressed in cm.

[0217] The adsorbents comprising 16% and 20% by weight of NZP have a loss of adsorption capacity of greater than 10% relative to the adsorbent with the highest xylene adsorption capacity (0.205 cm.sup.3.Math.g.sup.−1). Moreover, the increase in the diffusional resistance to PX transfer (EHTP) is increasingly pronounced beyond 12% NZP.