METHOD FOR SEPARATING META-XYLENE USING A ZEOLITIC ADSORBENT WITH A LARGE EXTERNAL SURFACE AREA

Abstract

The invention relates to a method for separating meta-xylene from C8 aromatic fractions, using a zeolitic adsorbent based on sodium-exchanged or sodium-and-lithium-exchanged agglomerated crystals of zeolite Y, with a large external surface area.

Claims

1. A process for separating meta-xylene using a zeolite adsorbent comprising at least one faujasite-type zeolite Y comprising sodium or sodium and lithium, in which process the external surface area of said zeolite adsorbent, measured by nitrogen adsorption, is greater than 40 m.sup.2.Math.g.sup.−1, said external surface area being associated with a population of mesopores with a mean diameter of between 2 nm and 50 nm.

2. The process according to claim 1, wherein the pore size distribution of said zeolite adsorbent, as determined via the Barrett-Joyner-Halenda method from the nitrogen adsorption isotherm at 77 K, for pores between 2 nm and 50 nm, is unimodal and narrow.

3. The process according to claim 1, wherein the zeolite adsorbent is an adsorbent based on FAU-type zeolite(s) Y, the Si/Al atomic ratio of said adsorbent being greater than 1.50.

4. The process according to claim 1, in wherein said zeolite adsorbent has a mass fraction of FAU-type zeolite(s) Y of greater than or equal to 80% relative to the total weight of the adsorbent.

5. The process according to claims 1, wherein said zeolite adsorbent has a lithium content of less than 8%, expressed as weight of lithium oxide Li.sub.2O relative to the total mass of the adsorbent.

6. The process according to claim 1, wherein the zeolite adsorbent simultaneously comprises pores whose diameter is greater than 50 nm, pores whose diameter is between 2 nm and 50 nm, limits not included, and pores whose diameter is less than 2 nm.

7. The process according to claims 1, in wherein the zeolite adsorbent has 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 0.20 em.sup.3.Math.g.sup.−1 and 0.70 cm.sup.3.Math.g.sup.−1.

8. The process according to claim 1, wherein the zeolite adsorbent has a ratio (macropore volume)/(macropore volume+mesopore volume) of between 0.2 and 1.

9. The process according to claims 1, wherein the zeolite adsorbent used has a micropore volume, evaluated by the t-plot method from the nitrogen (N.sub.2) adsorption isotherm at a temperature of 77 K, of between 0.145 cm.sup.3.Math.g.sup.−1 and 0.350 cm.sup.3.Math.g.sup.−1.

10. The process according to claim 1, wherein the zeolite adsorbent comprises mesoporous crystals of at least one zeolite Y, said crystals having a number-mean diameter of between 0.1 μm and 20 μm.

11. The process according to claim 1, wherein the zeolite crystals of said zeolite adsorbent are prepared by direct synthesis using one or more structuring agents or sacrificial templates.

12. The process according to claims 1, wherein the desired product (meta-xylene) is separated by preparative adsorption liquid chromatography.

13. The process according to claim 1, wherein the external surface area of said zeolite adsorbent, measured by nitrogen adsorption, is greater than 50 m.sup.2.Math.g.sup.−1.

14. The process according to claim 1, wherein the external surface area of said zeolite adsorbent, measured by nitrogen adsorption, is between 60 m.sup.2.Math.g.sup.−1 and 200 m.sup.2.Math.g.sup.−1.

15. The process according to claim 1, wherein the zeolite adsorbent is an adsorbent based on FAU-type zeolite(s) Y, the Si/Al atomic ratio of said adsorbent being between 1.50 and 6.50.

16. The process according to claim 1, wherein said zeolite adsorbent has a lithium content of between 0 and 4%, expressed as weight of lithium oxide Li.sub.2O relative to the total mass of the adsorbent.

17. The process according to claim 1, wherein the zeolite adsorbent has 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 0.20 cm.sup.3.Math.g.sup.−1 and 0.50 cm.sup.3—g.sub.−1.

18. The process according to claim 1, wherein the zeolite adsorbent has a ratio (macropore volume)/(macropore volume +mesopore volume) of between 0.4 and 0.8.

19. The process according to claim 1, wherein the zeolite adsorbent used has a micropore volume, evaluated by the t-plot method from the nitrogen (N.sub.2) adsorption isotherm at a temperature of 77 K, of between 0.205 cm.sup.3.Math.g.sup.−1 and 0.320 cm.sup.3.Math.g.sup.−1.

20. The process according to claim 1, wherein the zeolite adsorbent comprises mesoporous crystals of at least one zeolite Y, said crystals having a number-mean diameter of between 0.7 μm and 10 μm.

Description

EXAMPLE 1

Synthesis of Zeolite Y with a Large External Surface Area

[0123] Zeolite Y with a large external surface area is synthesized directly.

[0124] Step 1): Preparation of the Growth Gel in a Stirred Reactor

[0125] A growth gel is prepared in a 3-liter stainless-steel reactor equipped with a heating jacket, a temperature probe and a stirrer, by mixing an aluminate solution containing 1146 g of Ludox AM silica sol containing 30% silica in an aluminate solution containing 198 g of sodium hydroxide (NaOH), 138 g of alumina trihydrate (Al.sub.2O.sub.3.3H.sub.2O, containing 65.2% by weight of Al.sub.2O.sub.3) and 800 g of water at 25° C. for 25 minutes with a stirring speed of 300 rpm.

[0126] The stoichiometry of the growth gel is as follows: 2.75 Na.sub.2O/Al.sub.2O.sub.3/8.20 SiO.sub.2/120 H.sub.2O. Homogenization of the growth gel is performed with stirring at 300 rpm for 40 minutes at 25° C.

[0127] Step 2): Introduction of the Nucleation Gel into the Reaction Medium

[0128] 153 g of a nucleation gel (12 Na.sub.2O, Al.sub.2O.sub.3, 10 SiO.sub.2, 180 H.sub.2O matured for 1 hour at 40° C.) are introduced into the reaction medium; after 5 minutes of homogenization at 300 rpm, the stirring speed is lowered to 100 rpm for 30 minutes.

[0129] Step 3): Introduction of the Structuring Agent

[0130] 87.5 g of a solution of TPOAC at 60% in methanol are added at a stirring speed of 300 rpm (TPOAC/Al.sub.2O.sub.3 mole ratio=0.12). After 1 hour of homogenization, the stirring speed is lowered to 100 rpm.

[0131] Step 4): Maturation Phase

[0132] The reaction medium is kept stirring at 100 rpm at 25° C. for 17 hours, and crystallization is then started.

[0133] Step 5): Crystallization

[0134] The stirring speed is maintained at 100 rpm and the nominal temperature of the reactor jacket is set at 108° C. so that the reaction medium rises in temperature to 98° C. over 80 minutes. After 28 hours at the 98° C. stage, the reaction medium is cooled by circulating cold water in the jacket to stop the crystallization.

[0135] Step 6) Filtration/Washing

[0136] The solids are recovered on a sinter and then washed with deionized water to neutral pH.

[0137] Step 7): Drying/Calcination

[0138] In order to characterize the product, drying is performed in an oven at 90° C. for 8 hours; the loss on ignition of the dried product is 22% by weight.

[0139] 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., then 1 hour at a steady stage of 200° C., then 3 hours of increase to 550° C., and finally 1.5 hours at a steady stage at 550° C.

[0140] Pure zeolite Y crystals (identification via the x-ray diffraction spectrum) with a large external surface area of atomic ratio Si/Al 2.30 are obtained. The micropore volume and the external 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, 0.214 cm.sup.3.Math.g.sup.−1 and 115 m.sup.2.Math.g.sup.−1. The number-mean diameter of the crystals is 1.3 μm.

EXAMPLE 2

Preparation of Mesoporous Zeolite Y Adsorbent

[0141] In the text hereinbelow, a mass expressed as anhydrous equivalent means a mass of product minus its loss on ignition.

[0142] A homogeneous mixture formed from 16 g of zeolite Y crystals with a large external surface area obtained in Example 1, 4 g of kaolin and also the amount of water allowing extrusion of the mixture is prepared. The loss on ignition of the paste before extrusion is 40%.

[0143] Extrudates 1.6 mm in diameter are formed. The extrudates 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 dry decarbonated air.

[0144] The extrudates are then crushed so as to recover grains whose equivalent diameter is equal to 0.6 mm. The mechanical bulk crushing strength of the grains obtained above is 2.2 MPa.

[0145] The micropore volume and the external 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, 0.170 cm.sup.3.Math.g.sup.−1 and 137 m.sup.2.Math.g.sup.−1. Analysis by x-ray diffraction confirms the presence of only one FAU-type zeolite Y phase.

EXAMPLE 3

(Comparative) Preparation of Non-Mesoporous Zeolite Y Adsorbent (Conventional)

[0146] For comparative purposes for the preparation of the agglomerates, a pure commercial non-mesoporous zeolite Y is used, CBV100 sold by the company Zeolyst International, with an Si/Al atomic ratio equal to 2.6, a number-mean diameter of 0.6 μm and a micropore volume and external 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, which are, respectively, equal to 0.345 cm.sup.3.Math.g.sup.−1 and 23 m.sup.2.Math.g.sup.−1.

[0147] The operations of Example 2 are repeated in an identical manner, replacing the mesoporous zeolite Y with reference non-mesoporous zeolite Y (CBV 100 from Zeolyst).

[0148] The micropore volume and the external 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, equal to 0.278 cm.sup.3.Math.g.sup.−1 and 42 m.sup.2.Math.g.sup.−1.

[0149] Observation with a transmission electron microscope TEM (FIGS. 2a and 2b, TEM of the agglomerates according to Example 2) and FIGS. 3a and 3b of the conventional agglomerates prepared in Example 3 reveals the presence of cavities of nanometric size (mesopores) in the agglomerates according to the invention of Example 2 (FIGS. 2a and 2b).

[0150] Comparison of the distributions of the pore sizes determined by the BJH method, from the nitrogen adsorption isotherm at 77 K and illustrated by FIG. 4 which clearly shows that the agglomerates according to the invention have a narrow unimodal pore size distribution in the mesopore range (i.e. pores with a mean diameter of between 2 nm and 50 nm) centered on 3.4 nm and a full width at half maximum of 2 nm (cf. curve materialized by dark diamonds). The conventional agglomerates of non-mesoporous zeolite Y (cf. curve materialized by light triangles) does not have a unimodal distribution in the mesopore range (i.e. pores with a mean diameter of between 2 nm and 50 nm); in fact, no peak having a maximum is observed on the pore size distribution.

[0151] Table 1 below presents the characteristics of the adsorbents prepared in Examples 2 and 3.

TABLE-US-00001 TABLE 1 Adsorbent of Adsorbent of Example 2 Example 3 (according to the invention) (conventional) Micropore volume by t-plot 0.170 0.278 (cm.sup.3 .Math. g.sup.−1) External surface area 137 42 (m.sup.2 .Math. g.sup.−1) Mesopore volume 0.268 0.111 (cm.sup.3 .Math. g.sup.−1) Mesopore diameter - Mode 3.4 not defined (nm) dV/dD mode 0.031 not defined (cm.sup.3 .Math. g.sup.−1 .Math. nm.sup.−1) Full width at half maximum 2.0 not defined (nm)

EXAMPLE 4

Breakthrough Test on Adsorbents of Examples 2 and 3

[0152] A breakthrough test (frontal chromatography) is then performed on these adsorbents to evaluate their efficiency. The amount of adsorbent used for this test is about 26 g.

[0153] The procedure for obtaining the breakthrough curves is as follows: [0154] filling the column with the sieve and inserting it in a test bench. [0155] filling with the solvent at room temperature. [0156] gradual raising to the adsorption temperature under a stream of solvent (5 cm.sup.3−min.sup.−1). [0157] injecting solvent at 30 cm.sup.3.Math.min.sup.−1 when the adsorption temperature is reached. [0158] solvent/feedstock exchange to inject the feedstock (30 cm.sup.3.Math.min.sup.−1). [0159] injection of the feedstock is then maintained for a time sufficient to reach thermodynamic equilibrium (i.e. until the concentration of solvent in the effluent is zero). [0160] collecting and analyzing the breakthrough effluent.

[0161] The solvent used is para-diethylbenzene. The composition of the feedstock is as follows: [0162] meta-xylene: 45% by weight [0163] ortho-xylene: 45% by weight [0164] isooctane: 10% by weight (this is used as tracer for estimating the non-selective volumes and does not participate in the separation)

[0165] The test is performed with an adsorption temperature of 140° C. The pressure is sufficient for the feedstock to remain in the liquid phase, i.e. 1 MPa. The circulation surface speed (flow rate/cross section of the column) of the liquid at the test temperature is about 1.2 cm s.sup.−1 for all the tests.

[0166] The selectivity for meta-xylene (MX) relative to ortho-xylene (OX) (α.sub.MX/OX) is calculated from the adsorbed volumes q.sub.MX and q.sub.OX of the compounds MX and OX (the latter being determined by the material balance from analysis of the breakthrough effluent) and from the composition of the feedstock (feedstock in which the volume fraction f the compounds is y.sub.MX and Y.sub.OX):

[00001]${\alpha }_{\mathrm{MX}/\mathrm{OX}}=\frac{q_{\mathrm{MX}}}{q_{\mathrm{OX}}}.Math.\frac{y_{\mathrm{OX}}}{y_{\mathrm{MX}}}.$

[0167] The breakthrough results are given in Table 2 below:

TABLE-US-00002 TABLE 2 MX/OX Adsorption capacity EHTP MX Adsorbent selectivity (%) (%) of Example 2 1.70 11.3 6.7 (invention) of Example 3 1.83 13.4 10.0 (comparative)

[0168] Key [0169] Adsorption capacity expressed in % (cm.sup.3 of C.sub.8-aromatics adsorbed per cm.sup.3 of column) [0170] EHTP=Equivalent Height of Theoretical Plates measured on meta-xylene expressed in % of column length [0171] MX=meta-Xylene; OX=ortho-Xylene

[0172] It is observed that the use of the adsorbent according to the invention makes it possible to considerably reduce the equivalent height of theoretical plates measured for meta-xylene, indicating an improvement in the matter transfer.