WE CLAIM:
1. Process for producing para-xylene from a feedstock containing xylenes, ethylbenzene and C9+ hydrocarbons, comprising
- a single step A of separation in a simulated moving bed of said feedstock performed with a zeolite as adsorbent and a desorbent, at a temperature of between 20 and 250°C, at a pressure of between 1.0 and 2.2 MPa, and with a volume ratio of the desorbent to the feedstock in the simulated moving bed separation unit of between 0.4 and 2.5;
said step A allowing the production of
■ a first fraction (Al) containing a mixture of para-xylene and of desorbent, and
i ■ at least one second fraction (A2 or A21) containing ethylbenzene (EB), ortho-xylene (OX)
and meta-xylene (MX) and desorbent,
- a step B of fractionation by distillation in at least one first distillation column (BC3) of said second fraction derived from step A, allowing the production of three fractions:
■ a first fraction (B2) containing EB, OX and MX,
■ a second fraction (B3) containing OX and MX, and
■ a third fraction (B42) containing desorbent.
2. Process according to Claim 1, in which the first fraction (B2) derived from step B has an EB content greater than the EB content of the second fraction (B3) derived from the first column (BC3) used in step B.
i 3. Process according to Claim 2, in which the EB content of the first fraction is at least 1.0% greater than that of the second fraction.
4. Process according to any one of Claims 1 to 3, in which the distillation column used in step B comprises between 30 and 80 theoretical plates.
5. Process according to any one of Claims 1 to 4, in which the separation step A also allows the production of a third fraction A22 depleted in EB, containing a mixture of MX, OX and desorbent, said fractions A21 and A22 being sent into said fractionation step B.
6. Process according to any one of Claims 1 to 4, in which the second fraction (B3) is engaged in the fractionation step B in a second distillation column (B-C4), allowing the production of a
fraction (B31) free of desorbent, containing MX and OX, and a fraction B43 consisting of desorbent.
7. Process according to Claim 5, in which the third fraction (A22) derived from step A is engaged in a second distillation column (B-C4) allowing the production of a fraction (B31) free of desorbent, containing MX and OX, and a fraction B43 consisting of desorbent.
8. Process according to Claim 7, in which the third fraction (A22) derived from step A is introduced into the second distillation column (B-C4) below the lateral point of injection of the second fraction (B3) derived from the first distillation column (B-C3).
9. Process according to any one of the preceding claims, in which the distillation step B uses a column comprising an internal wall.
10. Process according to any one of the preceding claims, comprising a step C of vapour-phase isomerization of the first fraction (B2) derived from a fractionation step containing EB, OX and MX.
11. Process according to any one of Claims 1 to 5, comprising a step D of liquid-phase isomerization of the second fraction (B3) containing OX and MX derived from the first distillation column (BC3) used in the fractionation step B.
12. Process according to any one of Claims 6 to 10, comprising a step D of liquid-phase isomerization of the fraction (B31) containing OX and MX derived from the second distillation column (BC4) used in the fractionation step.
13. Process according to Claim 10, in which the vapour-phase isomerization step C is performed at a temperature above 300°C, a pressure of less than 4.0 MPa, a space velocity of less than 10.0 h"1, a mole ratio of hydrogen to hydrocarbon of less than 10.0, and in the presence of a catalyst including at least one zeolite having channels whose aperture is defined by a ring of 10 or 12 oxygen atoms (10 MR or 12 MR), and at least one metal from group VIII in a content of between 0.1% and 0.3% by weight.
14. Process according to Claim 13, in which the catalyst used in step C comprises from 1% to 70% by weight of a zeolite of EUO structural type, said zeolite comprising silicon and at least one element T preferably chosen from aluminium and boron, the Si/T ratio of which is between 5 and 100.
15. Process according to either of Claims 11 and 12, in which the liquid-phase isomerization step D is performed at a temperature of less than 300°C, a pressure of less than 4.0 MPa, a space velocity of less than 5.0 h"1 and preferably between 2.0 and 4.0 h"1, and in the presence of a catalyst including at least one zeolite having channels whose aperture is defined by a ring of 10 or 12 oxygen atoms (10 MR or 12 MR); preferably, said zeolite is of ZSM-5 type.