Zeolite adsorbent in the form of low-tortuosity agglomerates
Provided is a zeolite-based adsorbent in the form of agglomerates, where the adsorbent having a tortuosity factor, calculated from the pore distribution determined by mercury intrusion porosimetry, of greater than 1 and less than 3. The adsorbent also has a porosity as determined by mercury intrusion porosimetry of between 25% and 35%. The adsorbent is useful in the field of separations in particular in a process for separating para-xylene from aromatic hydrocarbon isomer fractions containing 8 carbon atoms.
1. A zeolite-based adsorbent in the form of agglomerates, said adsorbent having:
a tortuosity factor
t
==
(
2.23
-
1
,
13
·
v
tot
·
d
g
)
(
0
,
92
4
S
S
∑
Pi
=
0
,
2
MPa
Pdc
Δ
Vi
li
)
1
+
δ
calculated from the pore distribution determined by mercury intrusion porosimetry, strictly greater than 1 and strictly less than 3, in which Vtot is the total porous volume: sum of macroporous and mesoporous volumes expressed in cm 3 ·g −1 ; δ=0 for liquid phase or 1 for gas-phase; Ss is the surface area of the pores of macropore and mesopore type, expressed in m 2 per gram of sample as anhydrous equivalent; ΔVi is the mercury incremential volume; dg is the grain density expressed in g·cm −3 ; Pdc=87 MPa; and
a porosity
e
p
=
Vma
+
Vme
Vg
,
determined by mercury intrusion porosimetry, in which V ma denotes the macropore volume, V me denotes the mesopore volume and Vg denotes the grain volume, of between 25% and 35%, the volumes being expressed in cm 3 ·g −1 .
2. The zeolite-based adsorbent as claimed in claim 1 , in which the tortuosity factor t is between 1.5 and 2.7.
3. The zeolite-based adsorbent as claimed in claim 1 , wherein the high mechanical bulk crushing strength (BCS), measured via the Shell method series SMS1471-74 adapted for agglomerates less than 1.6 mm in size, is greater than or equal to 1.0 MPa.
4. The zeolite-based adsorbent as claimed claim 1 , wherein said adsorbent has a size of between 0.1 mm and 1 mm, limits inclusive.
5. The zeolite-based adsorbent as claimed in claim 1 , wherein said adsorbent comprises a zeolite chosen from the zeolites of FAU structure.
6. The zeolite-based adsorbent as claimed in claim 5 , said zeolite being zeolite X, alone or as a mixture with zeolites chosen from among zeolites Y, EMT, LTA and MFI.
7. The zeolite-based adsorbent as claimed in claim 1 , wherein said adsorbent comprises more than 90% by weight of zeolite.
8. The zeolite-based adsorbent as claimed in claim 1 , wherein said zeolite is in the form of crystals between 10 nm and 1500 nm in size.
9. The zeolite-based adsorbent as claimed in claim 1 , wherein the pore distribution satisfies the inequalities a) and/or b) below:
Vme
Vme
+
Vma
≤
0.1
,
a
)
0.4
≤
Vmi
Vma
+
Vme
+
Vmi
b
)
Vmi denoting the micropore volume expressed in cm 3 ·g −1 , determined by nitrogen adsorption.
10. The zeolite-based adsorbent as claimed in claim 1 , further comprising barium and/or potassium.
11. A process for preparing a zeolite-based adsorbent as claimed in claim 1 , comprising at least following steps:
a) a step of mixing crystals of at least one zeolite with an agglomeration binder containing at least 80 of zeolitizable clay, and optionally a source of silica, followed by forming and a firing step at a temperature of between 500 and 700° C.,
b1) a first step of zeolitization by placing the material obtained in step a) in contact with an alkaline basic solution, with a concentration of between 0.2 M and 0.9 M, limits inclusive,
b2) a second step of zeolitization by placing the material obtained in step b1) in contact with an alkaline basic solution, with a concentration of between 1.2 M and 4.0 M, limits inclusive,
b1 and b2 possibly being performed in any order and b1 and/or b2 possibly being repeated,
c) an optional step of cationic exchange of the cations contained in the reaction medium obtained from the zeolitization steps by placing in contact with a solution of barium ions or of barium ions and potassium ions,
d) a step of washing and drying the material thus obtained, and
e) a step of activation of the material obtained in step d), by heating to a temperature of between 100° C. and 400° C., and recovery of the zeolite-based adsorbent in the form of agglomerates.
12. The process as claimed in claim 11 , further comprising a step of extruding the zeolite-based adsorbent to an agglomerated form and/or compacting the material formed after any one of steps a), b1)/b2), c), d) or e).
13. A process for separating para-xylene from aromatic hydrocarbon isomer fractions containing 8 carbon atoms, in the liquid phase, comprising selectively adsorbing the para-xylene onto the adsorbent as claimed in claim 1 by contacting a mixture of the para-xylene and the aromatic hydrocarbon isomer fractions containing 8 carbon atoms with the adsorbent, in the presence of a desorbent.
14. A process for separating para-xylene from aromatic hydrocarbon isomer fractions containing 8 carbon atoms, in the gaseous phase, comprising selectively adsorbing the para-xylene onto the adsorbent as claimed in claim 1 by contacting a mixture of the para-xylene and the aromatic hydrocarbon isomer fractions containing 8 carbon atoms with the adsorbent, in the presence of a desorbent.
15. The process as claimed in claim 13 , wherein the contacting is conducted in a simulated moving bed column.
16. The process as claimed in claim 14 , wherein the contacting is conducted in a simulated moving bed column.