MXPA96004312A - Production of paraxylene from an effluent of paralelective dismutation of toluene, by a procedure of crystallization associated to an adsorption in simulated mobile bed - Google Patents

Abstract

The para-xylene production is carried out in a para-selective toluene dismutation reactor, the effluent (3) is distilled, to eliminate the toluene and the benzene, in at least two distillation columns (C1) (C2), the xylenes ( 8) are subjected to at least one crystallization (10) at a temperature between +10 and -30 ° C. The separated mother liquors (14) are subjected to an adsorption in the presence of toluene on a zeolitic screen in a simulated moving bed (15). In this way, a raffinate (16) containing depleted toluene of paraxylene is produced, which is distilled in a distillation column (C4), and an enriched extract of para-xylene containing toluene, which is recycled in the columns (C1 , C2). Para-xylene crystals are purified by washing with toluene and distillation, or by partial melting (30) followed by washing with high-purity molten para-xylene.

Description

PRODUCTION OF PARAXYLENE FROM AN EFFLUENT OF PARASELECTIVE DISMUTATION OF TOLUENO. BY A CRYSTALLIZATION PROCEDURE ASSOCIATED WITH A ADSORPTION IN SIMULATED MOBILE BED The present invention relates to an improved production and separation process of aromatic hydrocarbons of 8 carbon atoms (C8), from a charge rich in para-xylene, for example a mixture of C8 of the mobile process of dismutation of the toluene After a long time it has been recognized that certain zeolitic materials catalyze certain hydrocarbon transformations, as described for example in US Pat. No. 4,305,808. The transformations of hydrocarbons claimed are particularly alkylation, transalkylation and dismutation. Dismutation of toluene is claimed, for example, in U.S. Patent Applications Nos. 4,052,476; 4,007,231; 4,011,276; 4,016,219 and 4,029,716. In an article published in Oil and Gas Journal Vol. 69 No. 48 (1971). Grandio et al. describe a process of dismutation of toluene in liquid phase, using zeolitic catalysts in the absence of hydrogen.

REF: 23207 It should be noted that in the first known processes, the composition of xylenes obtained corresponds to the thermodynamic equilibrium at the operating temperature, either from 20 to 25% p-xylene over all the xylenes and ethylbenzene. The most recent patents, for example, U.S. Patent No. 4,380,685 disclose the alkylation, transalkylation and paramutation of substituted aromatics to form diakylene benzenes. In the case of dismutation of toluene, the p-xylene content of the effluent product exceeds the thermodynamic equilibrium. The catalyst used is based on zeolites characterized by a tension index of 1 to 12, a silica / aluminum ratio of at least 12/1 and comprising various metals and phosphorus; the zeolites used are. for example, ZSM5, ZSM11, ZSM12 or ZSM35. In addition, these catalysts must be treated to make more limited diffusion through the pores of the ortho-xylene and meta-xylene crystal and reduce the reisomerization of the p-xylene formed. This treatment consists essentially of a precooking carried out under well-defined conditions, possibly accompanied by the addition of a small amount of a hard-to-reduce oxide (antimony, phosphorus, boron or magnesium) or a surface treatment. These methods of preparing an effective catalyst are described, for example, in US Pat. No. 5,173,461. The procedure used by these catchers. called MSTDP, are described in two publications: Selective toluene disproportionation process proven at Italian refinery. Cap F .. Brec enridge L.L, Guy. M .. Sailor R. A., Oil and Gas Journal V90, N41, 60-67 (1992) et Mobil 's toluene to PX process proves itself Mobil Research and Deveiopment Corp. European Chemical News V54. N1418 (1990). These publications describe a production of xylenes containing from 80 to 95% p-xylene, by a 30% conversion of toluene per pass. Numerous patents claim this process and describe catalyst preparations, for example, European patent application EP 26,962; U.S. Patent Nos. 4,260,843; 4,274,982; 4,908,342; 5,173,461; WO 93/1787. In all these cases, the effluent of dismutation has a simplified composition close to: toluene 70%, benzene 15%, xylenes 15%. being the composition of the xylenes fraction. 85% p-xylene, or + m-xylene and 15% ethylbenzene. The present invention relates to an improved process for the separation of mixtures of aromatic compounds of 8 carbon atoms (C8). to obtain p-xylene with a purity sufficient for the production, for example, of terephthalic acid. A known method of separation of p-xylene from the mixture of aromatic compounds of 8 carbon atoms (C8) consists in effecting a fractional crystallization; among the existing procedures, we can cite those of Chevron, Krupp. Amoco, Maruzen and Arco (North American patents Nos. 3,177,255 and 3,467,724). The procedures are as follows: the charge containing at least 20% p-xylene is cooled to a temperature between -50 and -70 * C. which causes a crystallization, where the crystals contain from 85 to 90% p-xylene and the mother liquor from 7 to 8%; the crystals are re-crystallized and recrystallized at -10 * C. After filtering and washing the crystals. for example, with toluene, we obtain para-xi wood with a purity of 99.5%. Another method of separating p-xylene from an aromatic mixture of C8. is the simulated countercurrent liquid phase chromatographic method, claimed in US Patent Application No. 2,985,589, which uses zeolites to selectively absorb the p-xi wood. The Parex and Aromax procedures use this method or that of the simulated countercurrent (US Patent No. 4,402,832). The advantages and disadvantages of these procedures are described in detail in the different applications. The most recent technological approach is illustrated, for example, in the US patent US-A-5, 329, 060 and European patent EP-A-531191, which describe a combination of adsorption, crystallization and isomerization steps of aromatic mixtures of C8, to obtain para-xylene with a very high purity. The particularity of the mixture obtained at the output of a paraselective dismutation reactor in the MSTDP, after separation of the lighter hydrocarbons by distillation, is that it is particularly rich in p-xylene, since the content is 75 to 85% p-xylene. This mixture can be treated in different ways: a) by crystallization at high temperature The term "high temperature" refers to a temperature comprised, for example, between +10 and -25"C.

The yield of pure p-xylene is close to. 83% when the para-xylene content of the C8 mixture of the effluent is 80% by weight. This performance is limited by the fact that the mother liquor still contains, due to the thermodynamic equilibrium liquid-solid, para-xylene in an amount greater than 40%. 2) By crystallization at two temperatures In this case a first crystallization at a temperature between +10 and -25 ° C leads to the production of pure para-xylene, the mother liquors are recrystallized at a lower temperature (-50 at -7Q "C), the crystals being rich in p-xylene recycled in the charge of the first crystallization, and the mother liquors of the second crystallization do not contain more than about 10% by weight of p-xylene. para-xylene in the two-stage crystallization, for an initial charge of 80% by weight of para-xylene, is approximately 97% by weight, The drawback of this process is the high energy consumption The objective of the present invention is to solve the aforementioned drawbacks More precisely, the present invention relates to a process for preparing para-xylene from a charge of toluene, comprising: a) A para-selective dismutation step of l toluene in the presence of hydrogen and a catalyst, in a zone of dismutation, producing an effluent containing benzene, toluene, xi logs enriched with para-xylene. etiibenzene and aromatic hydrocarbons having at least 9 carbon atoms; b) A stage of distillation of the effluent in at least two distillation columns, producing separately benzene, toluene and xylenes: c) A stage of crystallization of xylenes at high temperature, preferably comprised between +10 and -30 ° C, when less a crystallization zone, producing para-xylene with a very high purity and impoverished para-xylene mother liquors, the process being characterized in that: d) it is contacted, in at least one adsorption zone in simulated moving bed, the mother liquor with an adsorbent zeolite bed in the presence of a desorbent which is toluene, under conditions such that a first fraction depleted of para-xylene and containing toluene is obtained, and a second fraction enriched with para-xylene which contains toluene; e) At least a part of said second fraction is recycled in the crystallization zone after substantially all the toluene has been distilled.

The term "high temperature crystallization" means a crystallization in at least one catalyst for each crystallization step, of a solution or suspension of para-xylene, already enriched in para-xylene. that corresponds to that which the scientific literature calls a purification stage. For example, US Pat. No. 2,866,833, incorporated herein by reference, mentions a step of purifying para-xylene at a high temperature, and can go up to a temperature of -34'C. In accordance with a feature of the present invention, the distillation step (b) may comprise the distillation of the effluent from step (a) in a first distillation column (Cl). which produces a distillate containing benzene and a first residue, the distillation of the first residue in a second distillation column (C2) that produces a distillate containing toluene and a second residue, and the distillation of the second residue in a third distillation column (C3) ) which produces a distillate containing the xi logs and a third residue containing the aromatic hydrocarbons of at least 9 carbon atoms and wherein at least a part of the second fraction is distilled in the second distillation column (C2).

The third residue can be distilled in order to obtain a distillate containing ortho-xylene and a residue containing the hydrocarbons of at least 9 carbon atoms. According to a variant, the distillation stage (b) can comprise the distillation of the effluent from stage (a) in a first distillation column (Cl). which produces on the one hand a distillate (BT) containing benzene and toluene, and on the other hand a residue containing the xylenes and hydrocarbons of at least 9 carbon atoms, said residue being subjected to the stage (c) of crystallization, wherein at least a part of the second fraction enriched in para-xylene in the distillation column (Cl) is distilled. The distillate (BT) containing benzene and toluene is distilled in a distillation column (C2) so that benzene is recovered as a distillate and toluene as a residue and where at least a part of the distillate (BT) of the column (Cl) is recovered. used to provide the necessary heat to boil the column again (C2). The residue of the column (Cl) containing the xylenes can be sent. ethylbenzene and hydrocarbons of at least 9 carbon atoms, directly to at least one crystallization zone, but this residue can also be distilled in a column (C3) and recover a distillate containing the xylenes and ethylbenzene, which is subjected to the stage (c) of crystallization. Hydrocarbons of at least 9 carbon atoms are removed as waste. According to another secondary characteristic, at least a part of the toluene from the distillation column C2 can be recycled to the adsorption zone, the remaining part being recycled in the paraselective dismutation zone. According to a first characteristic relating to adsorption, the first dextrose fraction of para-xylene containing toluene and the other aromatic hydrocarbons of 8 carbon atoms or more. it can be distilled in a distillation column (C4), which produces as a distillate toluene, which is recycled, at least in part, in the adsorption zone and a residue substantially devoid of toluene containing the first fraction, that is, ortho -xylene, eta-xylene and ethylbenzene, and optionally hydrocarbons of at least 9 carbon atoms. When the hydrocarbons of at least 9 carbon atoms are not removed above the crystallization zone, they can be transferred to a distillation column (C3) below the column (C4), as a residue.

According to a second particularly advantageous characteristic in relation to the adsorption, at least a part of the second fraction enriched with para-xylene and containing toluene can be recycled in the distillation column iCl) when benzene is recovered only as a distillate from the column ( C2). Another variant may consist of recycling said second fraction in column (C2) when benzene is recovered only as a distillate in the column (Cl). The crystallization step of the xylenes recovered after distillation with or without hydrocarbons of at least 9 carbon atoms can be carried out according to several variants: According to a first variant. the xylenes can be crystallized according to the step (s). in a crystallization zone which produces a suspension of para-xylene crystals in mother liquors, said crystals are separated from the mother liquor in a separation zone, the crystals are washed with a suitable washing solvent, the recovered mother liquor they send to the adsorption zone according to step (d) in addition to the para-xylene crystals of very high purity, and wash waters are recovered which are recycled in the distillation zone if the washing solvent contains toluene, or in the crystallization zone or in the adsorption zone if the washing solvent is molten para-xylene. -In accordance with a second variant that may be preferred with certain types of crystallizers, for example the scraping crystallizers, the xylenes can be crystallized according to step (c) in at least two crystallization zones, of which one is at a colder temperature than the other, the mother liquors coming from the zone are recovered of colder crystallization and are sent according to step (d) to the adsorption zone. Whatever the variant. the para-xylene is crystallized as indicated above, at high temperature, for example between +10 and -34"C and in particular between +10 and -25 [deg.] C. More precisely, the low temperature zone is comprised, example, between 0 and -25"C while the zone of higher temperature is between +10 and -5 * C, for example. The crystallization with two temperature levels can be carried out according to at least two variants.- According to the first variant, the xylenes can be crystallized from the distillation stage in a first distillation zone at low temperature, the first crystals are separated of paraxylene from the first mother liquors, which are sent to the adsorption zone, the first crystals are melted. they are recrystallized in a second crystallization zone at a higher temperature, the second para-xylene crystals are separated from the second mother liquors, washed with a suitable washing solvent, melted, the para-xylene is recovered from very high purity and at least a part of the second mother liquor and possibly at least a part of the washing water in the first crystallization zone and / or in the second crystallization zone and / or in the adsorption zone is recycled. According to a second variant, the xylenes from the distillation stage can be crystallized in a first crystallization zone at high temperature, the first paraxylene crystals are separated from a first mother liquor, the first mother liquors are recrystallized in a second crystallization zone at lower temperature, the second para-xylene crystals are separated from the second mother liquors, which are sent to the adsorption zone, the first and the second crystals are washed with a suitable washing solvent, the para-xylene of very high purity is melted and recovered and at least part of the washing water is recycled in the first or in the second crystallization zone and / or in the adsorption zone. According to another embodiment of the process, the log after being crystallized and separated from its mother liquor can be resuspended in a partial melting zone as described in the French patent application FR 95/00746, which is incorporated herein as a reference. In a more detailed manner, in a first variant, the xylenes from the distillation stage can be crystallized according to the crystallization stage (c), in the crystallization zone, which produces a suspension of para-xylene crystals in the waters mothers, said crystals are separated from the mother liquors, partially melted in a partial melting zone, a second crystal suspension is recessed. said second suspension is separated and washed in a separation zone and washed with a washing solvent, which is toluene or para-xylene molten of very high purity, recovering para-xylene of very high purity which eventually melts, and washing waters which are recycled at least partly in the crystallization zone and / or in the adsorption zone after having been optionally distilled, if the washing solvent is toluene. In a second variant, the xylenes from the distillation stage can be crystallized in a first crystallization zone at high temperature, the first para-xylene crystals are separated from the first mother liquors, the first mother liquors are recrystallized a second zone of crystallization at a lower temperature, the second crystals of para-log are separated from the second mother liquors, which are sent to the adsorption zone. the first and second crystals are washed with a washing solvent, which is toluene or molten para-xylene of very high purity, the first and second crystals are partially remelted in at least a partial melting zone, a suspension of crystals, said suspension is separated and washed in a separation zone and washed with a washing solvent which is toluene or molten para-xylene of very high purity, the para-xylene of very high purity is recovered which eventually melts, and washing waters and at least partly recycle said washing waters in the first and / or second crystallization zone, after having been optionally distilled »if the washing solvent is toluene, and / or in the adsorption zone . The remaining part of the washings not recycled to the recrystallization or adsorption zone, can be introduced to the partial melting zone, where the obtained crystals, of smaller size, will be put in suspension.

The adsorption of the mother liquor on a selective adsorbent can be carried out in a movable bed simulated in at least three zones. This can be a simulated countercurrent moving bed (US patent US 2, 985,589) or simulated countercurrent (US Pat. Nos. 4,498,991 and 4,402,832), which are incorporated herein by reference. In accordance with a first variant. The adsorbent of the adsorption zone can be a selective adsorbent for para-xylene. This may be preferably, for example, a Y zeolite exchanged at least partially with barium and potassium, as described in US Pat. No. 3,894,109, which is incorporated herein by reference, and in US Pat. No. 3,558,732. , which cite toluene as desorbent. According to this variant, the extract recovered containing the fraction enriched for para-xylene and the desorbent, are recycled in the distillation step (Cl or C2) above the crystallization stage, while the enriched raffinate of ortho-xylene. meta-xylene and ethylbenzene. and the desorbent, are treated in a distillation unit (C4), where the desorbent is immediately recycled in the adsorption unit.

According to a second variant, the adsorbent can be selective for the para-xylene depleted fraction and contains meta-xylene, ortho-xylene and ethylbenzene in accordance with US Pat. Nos. 4,940,830 and US 5,382,742. which are incorporated herein by reference. According to this variant, the raffinate containing para-xylene and the desorbent are recycled in the distillation unit above the crystallization, while the extract containing the other xylenes and ethylbenzene and the desorbent are treated in the distillation unit (C4). . wherein the desorbent is immediately recycled in the adsorption unit. The scheme of conformity with the prior art comprises a paraselective dismutation of toluene and a crystallization with one or two stages involving a crystallization at very low temperature if it is desired to achieve a very high yield of para-xylene, which is extremely costly in terms of of energy. In spite of everything, the loss of para-xylene can reach up to 3% by weight of para-xylene following the crystallization temperature adopted. In accordance with the following invention. By interposing an adsorption to treat the mother liquor of the crystallization, this crystallization is prevented at a very low temperature and the para-'xylene losses are limited by at least 0.1% by weight. On the other hand, using toluene as the desorbent, this can be regenerated in one of the treatment columns of the dismutation effluent without it being necessary to add a specific distillation column to separate the para-xylene from the desorbent. The present invention will be better understood in view of the following figures, which illustrate schematically some examples of the practice of the process leading to the preparation of para-xylene of very high purity, in which: Figure 1 shows the chain of the stages of the procedure: dismutation, fractionation. crystallization and purification of para-xylene and adsorption of para-xylene from the mother liquor. Figure 2 illustrates the chaining with a para-xylene purification by means of a para-xylene wash, preceded by a resuspension of the crystals. Figure 3 represents a particular embodiment of the fractionation by distillation. Figure 4 takes up a fractionation by distillation identical to that of Figure 3. but with a previous separation of the hydrocarbons of 9 carbon atoms or more (C9 +) above the crystallization. The toluene transported by a line the resultant of a line 1 and a recycling line 7a. -s introduced in a rector 2 of paraselective desamutation of toluene. to produce benzene and xylenes and more particularly para-xylene. An effluent is recovered, which is cooled and treated with earth after separation of the hydrogen, which is recycled, and the removal of the light hydrocarbons (not shown in the figure). The reactor effluent containing, after that treatment, approximately 15.5% by weight of xylenes and heavier compounds of which 80% is para-xylene, 16.5% benzene and 68% unreacted toluene, is transported by a line 3 and introduced to a distillation column (Cl), to produce a distillate consisting of benzene by a line 5 and a residue containing toluene, xylenes. ethylbenzene and hydrocarbons of at least 9 carbon atoms (C9 +) by a line 6. This residue feeds a distillation column (C2) from which part of the toluene is decanted as distilled by a line 7, of which a part is recycled in reactor 2 on line 7a. while the other part of the toluene is recovered by a line 7b for further washes of the para-xylene crystals, or as desorbent. As a residue of column (C2), xylenes and hydrocarbons (C9 +) are recovered. by a line 8. This line 8 feeds a distillation column (C3) adapted to produce a C9 + waste by a line 21 and a distillate by a line 9, containing essentially ortho-xylene and meta-xylene, para-xylene and eti-benzene. This distillate is introduced via line 9 to a crystallization zone comprising at least one crystallizer operating at a temperature between +10 and -25 ° C, for example. Para-xylene crystals suspended in impoverished paraxylene mother liquors and containing meta-xylene and ethylbenzene. they are transported by a line 11 and separated in a centrifuge 12. The crystals are washed in the centrifuge in the presence of a washing solvent, which is toluene transported by a line 24 connected to the outlet line 7b of the distillation column ( C2). The washing waters containing impurities and toluene are recycled by a line 12a to the distillation unit (C2), allowing to separate the toluene. The pure para-xi crystals, which still contain toluene, collected in the centrifuge, are melted in a foundry (not shown in the figure) and then introduced by a line 13 to a distillation column (C5). A toluene distillate is recycled by a line 22 and line 24 in the centrifuge washing device, while the para-xlleno of very high purity is decanted as residue of the column (C5), by a line 25. mother liquors collected from centrifuge 12 depleted of para-xylene and which also contain ethylbenzene. meta-xylene and ortho-xylene, are introduced by a line 14 to an adsorption zone 15, consisting of twelve beds of adsorbent and operating according to the simulated countercurrent principle, as described in US Pat. No. 5,284, 992. The adsorbent is a Y zeolite exchanged with barium and potassium. Toluene from the distillation column (C2) through lines 7 and 7b. from the column of purification of toluene (C5) by a line 22a and / or from the distillation column (C4) defined above by a line 18, is introduced as eluent or solvent of desorption, by a line 7c. to the adsorption zone. A raffinate containing toluene, depleted of paraxylene and enriched with meta-xylene and / or ortho-xylene and ethylbenzene. is collected by a line 16. An extract containing toluene and enriched para-xylene is desorbed and recycled by a line 17. to the distillation column (C2) of toluene.

According to Figure 2, which takes the same references of Figure 1 for the same devices, the suspension of para-xylene crystals in the mother liquor coming from the crystallization unit 10, is introduced by line 11 to the centrifuge 12. The collected mother liquors are treated in accordance with Figure 1, while the para-xylene crystals are suspended in a partial melting zone 30. to which they are introduced by line 13. The new suspension collected by line 31 is washed in a backwash column 32, d? NIRO type, for example, as described in US Patents 4, 475,355; US 4,481. 169; CH 515,730 and FR 95/00746. on the one hand of the very high purity molten para-xylene recovered from the column by a line 35. At least a part of the washing waters containing para-xylene is recovered via line 33 and is recycled in the crystallization zone 10 by the line 33b and / or in the adsorption zone 15 by a line 33c, while the remaining part can be introduced by a line 33a to the line 13, thus facilitating the suspension of the crystals and their transfer to the partial melting zone 30. Preferably, the wash waters are recycled in the crystallization zone 10 and the partial melting zone 30 due to their high paraxylene content. Finally, para-xylene of very high purity is recovered by a line 34. Figure 3 shows a variant of the distillation step, of the crystallization steps of para-xylene and of purification of the crystals, being the same as those of Figure 1 or those of Figure 2. The references of the various devices common to the devices of Figures 1 and 2 are the same. A mixture 317 of the effluent 3 from the paraleclective dismutation reactor of toluene and the content of a previously defined line 17 was distilled in a distillation column (Cl) adapted to produce a residue containing xylenes and hydrocarbons of at least 9 carbon atoms , which is sent to the crystallization zone by line 8. The column (Cl) also distributes a steam distillate containing benzene and toluene, which is condensed yielding its condensation heat to boil again the distillation column C2. After having been condensed, this distillate is partially recycled by a line or in the form of a reflux in the head of the column (Cl), while the remaining part feeds by a line 6 to the column (C2). Benzene is recovered as a distillate on a line 5 and toluene as a residue on lines 7 and 7a, which are connected to the dismutation reactor 2: and on lines 7 and 7b to the adsorption columns 15 and / or to the washing of crystals. once separated (line 24. figure 1). The separated para-xylene crystals are sent in the same way as figure 2 to the partial melting zone and purified as already indicated according to said figure 2. The mother liquors resulting from the centrifugation stage are sent to the adsorption columns 15. The resulting extract is recycled in the distillation column Cl by line 17, while the refining depleted para-xylene but containing the C9 + hydrocarbons. it is distilled in the distillation column (C4) to remove appreciably all the toluene (line 18). The waste also containing the hydrocarbons of at least 9 carbon atoms is sent by a line 19 to a distillation column (C3) of reduced dimensions, from which the hydrocarbons are traded as waste by a line 21 and as an orthogonal distillate. xylene and meta-xylene. in addition to ethebenzene. According to FIG. 4, instead of removing the hydrocarbons of at least 9 carbon atoms after the distillation step of the raffinate, as in FIG. 3, they are removed before the crystallization step. For this purpose, the distillation column (C3) of larger dimensions than those of FIG. 3 is fed., with the residue of the column (Cl) containing the xi logs and said hydrocarbons of at least 9 carbon atoms. By line 21, a residue that does not contain the hydrocarbons is recovered and by a line 9 the xylenes are introduced into the crystallization zone 10. The para-xylene crystals. Once separated from the mother waters and washed. they are purified as indicated in figure 1 in column (C5). the toluene being recycled via line 22 to the centrifuge-wash. The resulting wash waters are recycled by lines 12a and 17 to the distillation Cl column. The following example illustrates the present invention. Table 1 presents a material balance of the procedure described according to Figure 3. where the different lines represent the reference of this figure. The operating conditions of the various stages are summarized below: Paraselective dismutation reactor (MSTDP of Móvi 1) Catalyst ZSM5 activated H_ / hydrocarbons (molar): 1.5 Temperature: 450'C Pressure: 25 bars PPH: 3.5 Distillation columns: (Cl): 70 plates; reflow 1.5 (C2): 70 plates; reflux 2.5 (C3): 70 dishes: reflow 1.5 (C4): 70 dishes; reflux 1.5 Lifting glass Temperature: -20"C at atmospheric pressure Partial fusion: 7" C at atmospheric pressure Adsorption: simulated countercurrent Sieve: Zeolite Y exchanged with Ba and K Toluene / charge: 1.2 Temperature: 150 * C Pressure: 10-15 bar Glass washing: molten para-xylene In these conditions, almost all of the para-xylene product was recovered in the dismutation reactor, with a purity of 99.9 %.

TABLE 1 Partial and total mass expenditure in g. h It is noted that in relation to this date, the best method known to the applicant to carry out the aforementioned invention, is that which is clear from the present description of the invention. Having described the invention as above, property is claimed as contained in the following:

Claims (18)

1. CLAIMS \ \ 1. A procedure for preparation of para-xi log from a charge of toluene, comprising; a) a para-selective dismutation step of toluene in the presence of hydrogen and a catalyst, in a dismutation zone, producing an effluent containing benzene. toluene, xylenes enriched with para-xylene, ethylbenzene and aromatic hydrocarbons having at least 9 carbon atoms; b) a stage of distillation of the effluent in at least two distillation columns, producing separately benzene, toluene and xylenes; c) a stage of crystallization of xylenes at high temperature, preferably comprised between +10 and -30 ° C, in at least one crystallization zone, producing para-xylene with a very high purity and impoverished para-xylene mother liquors, the method being characterized in that; d) the mother liquor is contacted in at least one adsorption zone in a simulated mobile bed with an adsorbent zeolitic bed in the presence of a desorbent which is toluene, under conditions such that a first depleted fraction of paraffin is obtained. xylene and containing toluene, and a second enriched fraction of para-xylenes which contains toluene, -e) at least a part of said second fraction is recycled in the crystallization zone after having substantially distilled all the toluene.
2. 2. A process according to claim 1, characterized in that the distillation step (b) comprises the distillation of the effluent from step (a) in a first distillation column, which produces a distillate containing benzene and a first residue, the distillation of the first residue in a second distillation column that produces a distillate containing toluene and a second residue, and the distillation of the second residue in a third distillation column that produces a distillate containing the xylene and a third residue containing the aromatic hydrocarbons of at least 9 carbon atoms and where at least a part of the second fraction is distilled in the second distillation column.
3. 3. A process according to claim 2, characterized in that the third residue is distilled in order to obtain a distillate containing ortho-xylene and a residue containing the hydrocarbons of at least 9 carbon atoms.
4. 4. A process according to claim 1, characterized in that the distillation step (b) comprises the distillation of the effluent from stage (a) in a first distillation column, which on the one hand produces a distillate containing benzene and toluene . and on the other hand a residue containing the xylenes and the hydrocarbons of at least 9 carbon atoms, said residue being subjected to the stage (c) of crystallization, wherein at least a part of the second fraction enriched in para-xylene is distilled in In the first distillation column, the distillate containing benzene and toluene is distilled in a second distillation column, so that benzene is recovered as a distillate and as a toluene residue and where at least a part of the distillate of the first column is used to provide the heat needed to boil the second column again.
5. 5. A process according to any of claims 1 to 4. characterized in that at least a part of the toluene from the second distillation column is recycled in the adsorption zone, the remaining part being recycled in the dismutation zone.
6. 6. A process according to claim 4, characterized in that the residue of the first column is distilled in a third distillation column and a distillate containing the xylenes is recovered. which are subjected to the stage (c) of crystallization, and a residue containing the hydrocarbons of at least 9 carbon atoms.
7. 7. A process according to any of claims 1 to 6, characterized in that the first dextrose fraction of para-xylene containing toluene, it is distilled in a fourth distillation column, which produces toluene as a distillate, which is recycled. at least in part, in the adsorption zone, and a residue containing the first fraction substantially devoid of toluene.
8. 8. A process according to any of claims 1, 4 to 5, characterized in that the residue of the fourth distillation column is distilled in the third distillation column, in such a way that a distillate containing ortho-xylene is obtained, meta-xylene and ethylbenzene, and a residue containing the hydrocarbons of at least 9 carbon atoms.
9. 9. A process according to any of claims 1, 4 to 8, characterized in that at least a part of the second fraction enriched in para-xylene and containing toluene, is recycled in the first distillation column.
10. A process according to any of claims 2 or 3, characterized in that at least a part of the second fraction enriched in para-xylene and containing toluene, is recycled in the second distillation column.
11. 11. A process according to any of claims 1 to 10, characterized in that the xylenes are crystallized according to step (c), in a crystallization zone which produces a suspension of para-xylene crystals in mother liquors, are separated said crystals of the mother liquor in a separation zone, the crystals are washed with a suitable washing solvent, the mother liquor is recovered, which are sent to the adsorption zone according to step (d) and the para crystal. xylene of very high purity, and wash water is recovered which is recycled in the distillation zone if the washing solvent contains toluene, or in the crystallization zone or in the adsorption zone if the washing solvent is para- molten xylene
12. 12. A process according to any of claims 1 to 10, characterized in that the xylenes are crystallized according to step (c) in at least two crystallization zones, of which one is at a colder temperature than the other, They recover the mother liquor from the coldest crystallization zone and send it, according to stage (d), to the adsorption zone.
13. 13. A process according to claim 12, characterized in that the xylenes are crystallized in a first crystallization zone at low temperature, the first para-xi log crystals are separated from the first mother liquors, which are sent to the adsorption zone , the first crystals are melted, recrystallized in a second crystallization zone at a higher temperature, the second para-xylene crystals are separated from the second mother liquors, washed with a suitable washing solvent, melted, recover the para-xylene of very high purity and recycle at least a part of the second mother liquors and eventually at least a part of the washing waters in the first crystallization zone and / or in the second crystallization zone and / or in the adsorption zone.
14. 14. A process according to claim 12. characterized in that the xylenes are crystallized in a first crystallization zone at high temperature, the first para-xylene crystals are separated from a first mother liquor, the first mother liquors are recrystallized in a second crystallization zone at a lower temperature separates the second para-xylene crystals from second mother liquors, which are sent to the adsorption zone, the first and second crystals are washed with a suitable washing solvent, they are melted and the para-xylene of very high purity is recovered and at least a part of the wash water is recycled in the first or second crystallization zone and / or in the adsorption zone.
15. 15. A process according to any of claims 1 to 10, characterized in that the xylenes are crystallized according to step (c) in the crystallization zone, which produces a suspension of para-xylene crystals in the mother liquor, These crystals are separated from the mother liquor, partially melted in a partial melting zone, a second crystal suspension is recessed, said second suspension is separated and washed in a separation zone and washed with a washing solvent, which is toluene or para-xylene molten of very high purity, recovering para-xylene of very high purity, which is eventually melted, and washing waters that are recycled. at least in part, in the crystallization zone and / or in the adsorption zone after having been optionally distilled, if the washing solvent is toluene.
16. 16. A process according to any of claims 1 to 9 and 12. characterized in that the xylenes are crystallized in a first crystallization zone at high temperature, the first para-xylene crystals are separated from a first mother liquor, recrystallized The first mother liquors are placed in a second crystallization zone at a lower temperature, the second para-xylene crystals are separated from the second mother liquors, which are sent to the adsorption zone, the first and second crystals are washed with water. a washing solvent, which is toluene or fused para-xylene of very high purity, the first and second crystals are partially remelted in at least a partial melting zone. a crystal suspension is recovered, said suspension is separated and washed in a separation zone and washed with a washing solvent which is toluene or molten para-xylene of very high purity, the para-xylene of very high purity is recovered, the which optionally melts, and some washing waters, and at least part of the washing waters is recycled in the first and / or second crystallization zone, after having been optionally distilled if the washing solvent is toluene, and / or in the adsorption zone.
17. 17. A process according to any of claims 15 or 16, characterized in that the remaining part of the mother liquor is recycled in the partial melting zone.
18. 18. A process according to any of claims 11 to 17, characterized in that the washing solvent is toluene, the molten para-xylene of very high purity containing toluene is distilled in a fifth distillation column and the toluene is recycled as solvent of washing.