MXPA97006334A - Procedure for the separation of para-xylene quecomprende a pretratamiento by hydrogenacionselectiva and by land activ - Google Patents

Abstract

The present invention relates to an improved process for the separation and recovery of p-xylene from an initial feed to be treated, containing a mixture of xylenes containing olefin impurities comprising mono and diolefins, the process comprises circulating through at least one portion of a feed containing a mixture of xylenes in an enrichment zone to enrich a first fraction in p-xylene and to provide a second depleted fraction of p-xylene and containing the olefinic impurities; an isomerization zone, recovering an isomerization product and recirculating the isomerization product to the enrichment zone, the improvement is characterized in that it comprises circulating at least a portion of the initial feed, the isomerization product or a mixture thereof , in at least one zone of selective hydrogenation in the presence of hydrogen ba Such conditions are necessary to hydrogenate only a portion of the olefinic impurities, to circulate the resulting hydrogenated stream in at least one clay treatment zone to further remove the olefinic impurities from the hydrogenated stream, to remove excess hydrogen in at least a separation zone before or after the clay treatment, recovering an effluent resulting from the clay treatment and passing the effluent to the enrichment zone

Description

SEPARATION PROCEDURE FOR PARA-XYLENE COMPRISING A PRE-TREATMENT THROUGH HYDROGEN SELECTIVE AND BY ACTIVE LAND Description of the invention The present invention is concerned with a new concatenation of catalytic processes, purification procedures and separation procedures that allow to obtain aromatic compounds with a high degree of purity, such as, for example, benzene, ortho-xylene and -xylene. It is also concerned with a new process for the treatment of cuts or fractions of aromatic-rich hydrocarbons and, more particularly, with a process for the elimination and reduction of diolefins and, possibly, the elimination of olefins from mixtures of aromatic-rich hydrocarbons. by the hydrogenation reaction by means of particular catalysts. It is more particularly concerned with the production of very pure para-xylene, particularly for the synthesis of terephthalic acid used mainly in the textile industry. Blends of hydrocarbons rich in aromatic compounds are used in petrochemical complexes where after various separation and purification treatments lead to the production of benzene, toluene, ethylbenzene, meta-xylene, ortho-xylene, para-xylene, ethylbenzene and styrene. The cuts rich in aromatic compounds that have variable contents of diolefins and olefins come, in general: from unrefined hydrocarbon distillation processes such as unrefined oil, REF: 25323 condensed natural gas, coal; thermal processes such as catalytic steam pyrolysis of naphtha; Processes dedicated to the production of aromatics from light aliphatic cuts (from 3 to 5 carbon atoms more in particular from 3 atoms and from 3-4 carbon atoms), aliphatics from 6 carbon atoms and from 6-7 carbon atoms. carbon and heavy ñaña cuts (> 6 carbon atoms for the various catalytic reforming processes); and methods of transforming the aromatic products such as the trans-alkylation and isomerization processes of ortho- and meta-xylenes to para-xylene. The catalytic reforming process is the main procedure that allows to produce the mixtures rich in aromatic compounds. At the beginning, catalytic reforming is practiced in two types of facilities, according to their use in refining or in petrochemicals. Later, this distinction, together with the severity of the operating conditions, fades. Now, to respond to increased energy restrictions, industrialists are looking for more specific procedures again. It has thus resorted to refining in catalytic reforming units operating at a high severity, but with a greater stability of operation and improved performance in essence and in petrochemical in the optimization of the production of aromatics (benzene, toluene and xylene) through the use of a reactor that operates at low pressure. The use of reformers operating at high severity is accompanied by an increase in the content of olefins and diolefins in the reformed. In fact, the bromine number of the stabilized reformate can reach a maximum value that exceeds 7000 mg of bromine per 100 g of the product for the units operating at a very low pressure. The The presence of these olefins and diolefins is particularly detrimental to the separation processes of the aromatic compounds. For example, olefins and diolefins have a tendency to polymerize in the solvents used for the extractions. Purification treatments using natural silicon aluminates, generally active, (for example: attapulgite, bentonites and montmorillonites activated by treatment in the presence of acids) are currently used. These purification materials are generally referred to as activated earths. The use of purification treatments on activated earth have numerous drawbacks, among which can be mentioned a short lifespan (4 to 6 months in general and sometimes also less than 1 month for loads that have significant concentrations of olefins, >; 0.6% by weight), a weak catalytic activity (hourly space velocities comprised between 0.5 to 3 load volumes / volume of catalyst / hour), the greatest difficulty in purifying loads containing more than 1.5% by weight of olefins and diolefins, which limits the severity of the reformer's functioning. The purification of the charge is accompanied by the production of high molecular weight compounds by alkylation reaction of the aromatic compounds. These products are necessarily separated later. On the other hand, the use of these activated lands presents problems with the protection of the environment. In effect, they are not economically regenerable and thus, they are eliminated in the discharges with their residual toxic aromatic products. Conventional production schemes for aromatic compounds comprising 6 to 8 carbon atoms per molecule are complex. They comprise a succession of separation, purification and catalytic procedures. An aromatic production complex of para-xylene and ortho-xylene can be described as consisting of at least the concatenation of steps described hereinabove. The source of aromatics, that is to say, the reformed used, is introduced in a separation column that allows obtaining a cut of 7 carbon atoms containing benzene and toluene in the head, this column is currently called "de-heptanizer". At the bottom of the de-heptanizer, an aromatic cut of 8 carbon atoms is recovered which is purified by passing over at least one bed of activated earth. This purification aims to eliminate most of the olefins and diolefins of this cut of more than 8 carbon atoms. The life time of these activated earth beds is relatively short, two beds or more than two beds are arranged in parallel so as to be able to pass from one to the other to operate the discharge of the used earth without stopping the production of the cut of more of 8 purified carbon atoms. At the exit of the treatment by earth, the aromatic cut of more than 8 carbon atoms is introduced into a separation column, in which an aromatic cut of more than 8 carbon atoms is extracted in the head (xylene + ethylbenzene) and a heavy fraction containing ortho-xylene and compounds of higher molecular weight than ortho-xylene in the bottom. It is customary to operate a second separation on this second cut to recover the ortho-xylene at the head of this second column and an aromatic cut of more than 9 carbon atoms at the bottom of which, the aromatic compounds of 9 atoms could eventually be separated of carbon to produce, by transalkylation reaction with benzene, a complementary amount of para-xylene and ortho-xylene.

The aromatic cut of 8 carbon atoms obtained at the top of the first column, which contains the three isomers of xylene (ortho-xylene, meta-xylene and para-xylene) and ethylbenzene is sent to a separation unit of the xylenes, for example, it uses molecular sieves or a crystallization process, from which it is obtained on the one hand para-xylene and on the other hand an aromatic section of 8 carbon atoms. The latter is introduced into a catalytic isomerization unit, possibly after the addition of hydrogen, in which the xylenes and optionally ethylbenzene are isomerized to produce an aromatic mixture of 8 carbon atoms close to the thermodynamic equilibrium and thus containing -xylene. An isomerization scheme is described in U.S. Patent 4,224,141. At the output of the isomerization reactor, a separation column of the light fractions of less than 5 carbon atoms, produced in the isomerization reactor, is used, then another separation column that allows to recover in the bottom a cut of more of 8 carbon atoms containing the aromatic compounds of 8 carbon atoms close to the thermodynamic equilibrium and the heavy aromatic compounds resulting from the transalkylation reactions of the isomerization process. This aromatic cut of more than 8 carbon atoms can contain olefins and diolefins. Thus, it is customary to treat this cut through at least one bed of activated soil before recycling it to the deheptanizer treated by the reformer. A scheme as such is described in patent application FR-94/15896 which also combines a method of enrichment of para-xylene by selective adsorption or crystallization at low temperature, purification by crystallization in at least one stage at high temperature and an isomerization process. It is proposed thus, to have at least two reactors and the rejects of used earth that become more problematic for the reasons of contamination of the environment. The North American patent 4, 118, 429 discloses a process for the i.i rogenation of olefins in the effluents of isomerization units of aromatic compounds which use as a catalyst a metal chosen from the metals ruthenium, radium, palladium, osmium, iridium, platinum, or a mixture of these metals. The stated objective of this invention is to improve the recovery of para-xylene by treatment by the adsorbent. It is indicated in the example that an olefin-free effluent is obtained after a treatment at 177 ° C under 9.7 bars of pressure, at a spacing rate of 3 h "1 and on a composite catalyst of 0.375 wt% of platinum on alumina. The loss of the aromatic compounds is never specified in the description of the invention; but this loss is significant. Indeed, the hydrogenation of the xylene-rich cuts by group VIII metals leads to a significant loss of xylenes in the form of dimethylcydohexanes, as hereinafter indicated. On the other hand, the effluent also contains monoolefins that alter the treatment downstream, particularly an adsorption on molecular sieve. The object of the invention is to remedy the drawbacks mentioned hereinabove. More precisely, the invention is concerned with a method of separating and recovering p-xylene from a charge to be treated containing a mixture of xylenes, in which at least a portion of a charge containing a mixture of xylenes in a so-called enrichment zone (14) adapted to enrich a first fraction of para-xylene and supply a second depleted fraction of p-xylene, the second fraction is circulated in an isomerization zone (19) and an isomer is recovered (20). ) which is recirculated towards the enrichment zone, the process is characterized in that at least a selected part is circulated in the group formed by the initial charge, the isomer and its mixture, in at least one zone of selective hydrogenation ( 3) in the presence of hydrogen, then in at least one zone (8) of treatment by earth, a separation stage (5) of excess hydrogen is carried out in at least one separation zone, before or after the treatment in land and an effluent is recovered and sent to the enrichment zone (14). Several variants can be proposed. According to a first variant, the isomer and the charge to be treated can be hydrogenated in the zone (3) of hydrogenation, then subjected to the treatment by earth (8) before or after the separation (5) of hydrogen, then the effluent The resultant is sent to the enrichment zone (14). In a second variant, the charge to be treated can be hydrogenated in the hydrogenation zone (3), then subjected to the treatment (8) by earth, before or after the separation (5) of the hydrogen, the isomer (20) is made recirculate between the hydrogenation and land treatment zones and the effluent is recovered, which is sent to the enrichment zone (14). According to a third embodiment, the charge to be treated can be hydrogenated in the zone (3) of hydrogenation, then be subjected to a treatment on land before or after the separation of the hydrogen and the isomer (20) can be recirculated to the treatment area (8) to the land of the Hydrogenated charge, the isomer has previously undergone a treatment on land in a different treatment area (24). According to a fourth embodiment, the isomer can be hydrogenated in the hydrogenation zone (3), then subjected to the treatment (8) by soil before or after the separation of the hydrogen and the charge to be treated, from where they are removed at less in part diolefins, can be introduced upstream of the enrichment zone (14). The process according to the present invention makes it possible to purify mixtures of hydrocarbons rich in aromatic compounds and which do not have the drawbacks inherent to conventional treatments that use only activated soil treatments as a purification system. The process according to the present invention can purify numerous hydrocarbon charges of which the content and nature of aromatic compounds can vary over a very large range. In addition, the process is not limited by the content of olefins and diolefins of the fillers to be purified. Among the advantages of the procedure can be mentioned: * the possibility of treating fillers characterized by high olefin and diolefin contents, for example from 0.1 to more than 3% by weight of olefins and advantageously less than 0.3% by weight of dienes. * the content of aromatic compounds in the hydrogenated effluents remains almost unchanged, * the absence of production of high molecular weight compounds through the hydrogenation stage, the eventually transformed olefins and the diolefins are converted to paraffins, * the noise in front of the medium environment is significantly reduced because the catalyst is regenerable and the non-hydrogenated olefins are the least Annoying for procedures downstream of the treatment. The associated life time of the land is clearly prolonged, the quantities of land placed in the discharge are therefore much lower in relation to conventional procedures. For example, it has been found that when the charge is treated according to the process of the invention and when the isomer is recycled upstream from the treatment zone to the earth, you can not use more than a single reactor of treatment to the earth, even to avoid the operations of change of land. According to a characteristic of the invention, it is possible simultaneously to the separation of the excess hydrogen resulting from the selective hydrogenation, to carry out a separation of the compounds of less than 7 carbon atoms. It is possible according to another variant to refine the separation by effecting the separation of the hydrogen in a first separation zone, then the separation of the compounds of less than 7 carbon atoms in at least one other separation zone. It is still possible to improve the separation of these compounds according to the considered use of intermediate cuts. Thus, the separation of hydrogen in a first separation zone can be effected, then the separation of the compounds of less than 5 carbon atoms in a second separation zone and the separation of the compounds of less than 7 carbon atoms remaining in the second separation zone. a third separation zone. It is also possible to carry out the separation of hydrogen in a first separation zone, then the separation of the compounds of less than 4 carbon atoms in a second separation zone and the separation of the compounds of less than 7 remaining carbon atoms in a third separation zone. The separated hydrogen can, at least in part, be recycled in the selective hydrogenation zone. These last three alternatives advantageously allow recovering intermediate fractions, cutting heptane, cutting pentane, cutting substantially butane and particularly devoid of diolefins or heavier products that can plug the molecular sieves of the reactors. The light compounds, in particular the hydrocarbons of less than 7 carbon atoms that are formed after the isomerization step, can be prepared, by distillation for example, after the isomerization step. In general, the filler to be treated has variable contents of diolefins and olefins in an unrefined reforming, an unrefined reforming from which at least in part the compounds of less than 3 carbon atoms, an unrefined reforming of the which compounds of less than 4 carbon atoms, an unrefined reforming from which compounds of less than 5 carbon atoms, a cut of 8 carbon atoms, have been removed at least in part have been removed at least in part. rich in aromatic compounds that contains a major part of xylenes and their mixtures. According to another feature of the invention, it is possible to circulate the effluent to be enriched in at least one distillation column called "rerun" located at the entrance of the enrichment zone of paraxylene, to recover a mixture of xylenes-ethylbenzene and a mixture ortho-xylene-heavy products. According to a first embodiment, this enrichment zone can be at least one crystallization zone at a very low temperature which, after a separation step, supplies a liquid (the so-called second fraction) which is isomerized in the region of isomerization and crystals that are basically only a part of at least the first fraction. According to a second particularly advantageous embodiment, the para-xylene enrichment zone can be a selective adsorption zone containing a zeoiitic adsorbent, a selective adsorption of the charge is carried out, in the presence of a desorption solvent, the first enriched fraction of para-xylene and the second depleted fraction of para-xylene, the second fraction is isomerized in an isomerization zone containing an isomerization catalyst under suitable conditions to produce an isomer containing para-xylene and is recycled at less in part the isomer towards the adsorption zone. According to another characteristic of the invention, the selective hydrogenation of diolefins can be carried out in the presence of a catalyst containing at least one element of group VIII in which sulfur has been introduced before the passage of the charge to be hydrogenated. This sulfur can be introduced by treatment of the catalyst by means of a sulfur compound in the hydrogenation zone or it can be introduced into the catalyst by means of a sulfurized agent, prior to loading the catalyst in the hydrogenation zone. More precisely, in this latter case, at least one sulfur-containing agent based on an organic compound containing sulfur can be incorporated in the catalyst prior to its use and prior to its activation by hydrogen. The sulfurized agent may be chosen from at least one thioether R1-S-R1, at least one thiol R SH, at least one organic disulfide R1-SS-R2 or HO-R1-SS-R2-OH and at least one organic polysulfide R (S) n-R, the organic radicals Ri and R2 are identical or different, taken separately or in combination and n is an integer from 1 to 20. Advantageously, this agent can be chosen in the group of sulfurized hydrogen, dimethylsulfide, dimethyldisulfide or the ditertiononilpolisulfide (TPS-37), Elf Atochem). These sulfur-containing agents and their incorporation into the catalyst are described in patent FR-2 664 610 and in patent application FR-95/00977 incorporated as references. Preferred catalysts contain at least one metal from the group of nickel, platinum and palladium and more in particular from 0.1 to 1% by weight of platinum or palladium in relation to the support or from 5 to 70% by weight of nickel in relation to the support. In general, 0.05 to 10% of sulfur expressed by weight is added in relation to the mass of the catalyst and more particularly 0.05 to 1% of sulfur when palladium is used, particularly when the content of diolefins to be extracted from the load is reduced . The selective hydrogenation conditions are, as a rule, the following: * Temperature: 20 to 250 ° C, preferably 80 to 180 ° C, * Pressure: 4-50 bars, preferably 10 to 40 bars, * WH: 0.2 to 25 h "1, preferably 2 to 15 h'1, * Molar ratio: hydrogen / monoolefins + polyolefins and / or acetylenics: 0.3-100, preferably 1-50. Under these conditions, the conversion of the aromatic compounds of the filler is less than 0.15% and preferably less than 0.1% Charges which are suitable for the present invention have a bromine number of 10,000 to 100 mg per 100 g of the product, for example. it usually comprises at least one catalytic hydrogenation reactor in which the charge to be treated is mostly in the liquid state at the reactor inlet, preferably at least 80% by weight of the charge is in the liquid state At the reactor inlet, the hydrogen and the charge to be treated are injected upstream or downstream into the reactor, which is preferably a fixed bed of catalyst.After hydrogenation, the conversion of the diolefins is generally greater than 75%. % ym s often exceeding 90%. In contrast, the conversion of the monolefins is significantly less important (15 to 40% for example). A treatment by activated earth is thus generally carried out according to the conditions described in the patent applications FR-94/15896 and FR-94/15897 of the applicant, incorporated by reference.

For example, the conditions of adsorption or elimination of the undesirable compounds are the following: * Temperature: 100 to 300 ° C, preferably 160 to 230 ° C, * Hourly space velocity: 1 to 8, preferably 1 to 4 ( volume per hour of load per volume of earth), * Type of earth: activated natural aluminosilicates, for example the land called F54 - of Engelhard, * Pressure 3 to 100 bars, preferably 4 to 20 bars. The xylenes fraction devoid of the most impurities can be enriched by selective adsorption on molecular sieve, as described in the applicant's patents incorporated by reference US Pat. No. 5,284,992 or by low temperature crystallization as described in US Patents 5,329,061 and 2,866,833, incorporated by reference. The enriched fraction can then be purified in a so-called purification zone by at least one crystallization, at high temperature and preferably in a purification zone comprising at least two stages of high temperature crystallization, as described in the applications of patents incorporated by reference: FR 94/15896, FR 94/15897 and FR 95/00746. At least one part of mother liquor resulting from the stage, the coldest in the purification zone, can then be recycled at the entrance of the selective hydrogenation zone where it can be mixed with the initial charge and / or the isomer. According to another variant, it can be recycled at any point upstream of the enrichment zone, for example before or after the land treatment reactor. The invention will be better understood in view of the figures given below 1 to 6 illustrating schematically and not limiting several variants of the method. The organs of the device that have well-defined functions have the same references in all the figures.

According to figure 1, the aromatic charge, an unrefined reformed for example, devoid of hydrocarbons of less than 3 and less than 4 carbon atoms (not shown in the figure) is introduced by a line 1, in the presence of hydrogen optionally recycled via a line 2 in a selective hydrogenation reactor 3 comprising a fixed catalyst bed (transition alumina) containing from 0.1 to 1% by weight in relation to the palladium support previously sulphided, ex-situ and contains 0.05 at 1% sulfur (weight in relation to the mass of the catalyst). The hydrogenated effluent does not contain substantially more diolefins, it is sent by a line 4 in a separator 5 (main deheptanizer) from where it is extracted by a line 6., the hydrogen in excess of hydrocarbons of 7 carbon atoms, such as benzene and toluene of which the purity is improved. From the bottom of the separator 5, a line 7 containing a liquid effluent of more than 8 carbon atoms (xylenes, ethylbenzene and of more than 8 carbon atoms) is sent to at least one reactor 8 for treatment by activated earth (attapulgite for example). At the outlet thereof, a liquid effluent is introduced by a line 9 into a separator 11 from which, from the bottom, orthoxylene and heavy hydrocarbons of more than 8 carbon atoms are extracted by a line 12. The orthoxylene can be optionally separated in the head in another separator that is not represented in the figure. At the head of the separator, a line 13 recovers a fraction containing mainly aromatic compounds of 8 carbon atoms per molecule, xylenes and ethylbenzene and is introduced into an enrichment zone 14 of para-xylene, for example a zone of selective adsorption on Molecular sieve that operates in a simulated moving bed is desorbed by toluene or para-diethylbenzene. A line 15 retrieves an extract that contains para-xylene which can be further purified by crystallization in at least one stage, a high temperature, preferably in two steps between plus 10 and -25 ° C. An impoverished fraction of para-xylene and devoid of solvent is sent via a line 18 in an isomerization zone 19 which optionally operates in the presence of hydrogen, from which an isomer is recovered by line 20. It contains hydrocarbons of 5 carbon atoms, hydrocarbons of 7 carbon atoms and a fraction of 8 carbon atoms and is sent via line 20 to at least one separation zone 21 (two, for example, which are not shown in the figure). , one that drags the hydrocarbons of 5 carbon atoms and one that drags the hydrocarbons from 7 carbon atoms (secondary deheptanizer) from which benzene and toluene via line 22 at the head of zone 21. At the bottom, an effluent containing xylenes, ethylbenzene and compounds of more than 8 carbon atoms by a line 23 that introduces it into at least one reactor 24 for treatment by earth. The isomerization effluent treated by soil, devoid of the majority of the olefins, is recycled by a line 10 attached to line 9 to the separator 11 upstream of the adsorption zone 14. The enriched fraction of para-xylene from the zone of selective adsorption 14 via line 15 can be introduced after separation of the solvent to a purification zone which is a crystallization zone 16 of one or two stages at high temperature (+10 to -25 ° C for example). The crystal suspension supplied is separated by at least one centrifuge (not shown in the figure) and 99.9% purified crystals are recovered by a line 17. These purification steps of para-xylene are described in the patent applications of the cited applicant previously in the present. A mother liquor resulting from the centrifugation stage is recovered, that of the colder stage even when there are two and can be recirculated either at the entrance of the adsorption zone 14 via the lines 25, 25a, either to the entrance of the separation zone 11, via the lines 25, 25b, either to the entrance of the ground reactor activated via the lines 25, 25c, or to the entrance of the separator 21 via the lines 25, 25d. Figure 2 reproduces the same organs with the same references as those in figure 1. According to this figure the charge 1, selectively hydrogenated and devoid of excess hydrogen, ie the effluent of outlet of the separator 5 is introduced by the line 7 to the land treatment reactor 24. In this reactor, the isomer from which the light compounds have been removed by line 22 is also introduced via line 23. The line thus treated by earth is sent via line 10 to the separator. 11. According to figure 3 which reproduces the same organs with the same references as those of figure 1, the charge after having been selectively hydrogenated is stripped of excess hydrogen in the separator which also treats the unrefined isomer, in other terms, the hydrogenation effluent is introduced via line 4 to line 20 which leads the isomer to separator 21. Line 23 which receives the e heavy flow of the separator 21 feeds the treatment reactor 24 by earth. The effluent from the earth reactor is introduced as above by line 10 into the separator 11, which racks undesirable heavy compounds. Figure 4 is substantially the same as Figure 1, except that the hydrogenation effluent via line 4 is introduced into the reactor 8 of treatment by earth before being introduced by a line 8a to the separator 5 to decant light compounds and hydrogen. The hydrogenated effluent treated by the soil and devoid of the light compounds is then introduced by line 9, which also receives via line 10 the isomer treated by earth in the separator. 11. The light effluent of the separator 11 is treated in the same manner as that of figure 1. Figures 5 and 7 illustrate the case where both the filler and the isomer are selectively hydrogenated, devoid of excess hydrogen before or after the treatment on earth. Thus, in figure 5, the charge introduced by line 1 is devoid of the compounds of 3 to 4 and 5 carbon atoms in at least one separator 30. The effluent received by line 1a is mixed with the unrefined isomer provided by line 20 and the total is introduced into the selective hydrogenation reactor 3 which receives hydrogen via line 2. The hydrogenation effluent received via line 4 is devoid of light compounds of 7 carbon atoms and excess hydrogen in the separator 5 while the heavier compounds are sent via line 7 to the treatment reactors 8 by activated earth. The effluent from the reactors 8 devoid of diolefins and most of the monoolefins is sent at least in part via line 9 to the separator 11 to remove the heavier compounds while the aromatic compounds of 8 carbon atoms recovered in the head, are introduced to the adsorption zone 14 from which the fraction enriched para-xylene that can be purified by crystallization 16 is recovered. The impoverished fraction of para-xilepo is recovered by line 18 that sends it to the zone of isomerization 19. The isomer does not refining is recovered by line 20 which causes it to recirculate to selective hydrogenation reactor 3, as indicated hereinafter. Preferably, the compounds of 3 carbon atoms can be separated in the separator 30 and the compounds of 4 and 5 carbon atoms, as well as the excess hydrogen in the separator 5. In this case, the effluent from the ground reactor it is sent by a line 9a to another separator (in dashed lines) 5a which is supplied to the head by a line 9c of benzene and toluene and to the bottom by a line 9b, the compounds of 8 carbon atoms that are fed to the separator 11. The hydrogen is separated upstream of the reactor in the earth 8, thus a better recovery of the compounds of more than 8 carbon atoms can be obtained downstream of this reactor 8. According to figure 6, the same organs that perform the same functions are represented with the same references as those of figures 1 to 5. The device of figure 5 is also available, except that the separator 5 of hydrogen and the compounds of less than 7 atoms carbon is located between the land treatment reactor 8 and the separator 11. Thus, the hydrogenated and land treated effluent is recovered from the reactor 8 by a line 8a and leads it to the separator 5 from which the hydrogen is particularly removed and the heavier effluent of more than 8 carbon atoms is introduced by line 9 into the separator 11 from which heavy compounds of more than 9 carbon atoms and optionally ortho-xylene are transported. It is noted that in relation to this date, the best method known by 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 (19)

1. Claims 1 A process for separating and recovering p-xylene from a charge to be treated containing a mixture of xylene, in which a part of at least one charge containing a mixture of xylenes is circulated in an area called enrichment, adapted to enrich a first fraction of p-xylene and supply a second depleted fraction of p-xylene, the second fraction is circulated in an isomerization zone and an isomer is recovered and recirculated to the enrichment zone , the process is characterized in that at least a selected part of the group formed by the initial charge, the isomer and its mixture, is circulated to at least one zone of selective hydrogenation, in the presence of hydrogen, then in at least one ground treatment zone, a stage of separation of excess hydrogen is carried out in at least one separation zone, before or after the treatment ediante and an effluent is recovered that is sent to the enrichment zone.
2. 2. The process according to claim 1, characterized in that the isomer and the charge to be treated are hydrogenated in the hydrogenation zone, then subjected to the treatment by earth, before or after the separation of hydrogen, then the resulting effluent is sent to the enrichment zone.
3. 3. The process according to claim 1, characterized in that the charge to be treated is hydrogenated in the hydrogenation zone, then it is subjected to a treatment by earth, before or after the separation of the hydrogen and in which the isomer is recirculated between the hydrogenation zone and the land treatment zone and recovered to the effluent that is sent to the enrichment zone.
4. 4. The process according to claim 1, characterized in that the charge to be treated is hydrogenated in the hydrogenation zone, then it is subjected to a treatment by earth, before or after the separation of the hydrogen and in which the isomer is recirculated afterwards. of the treatment zone by earth of the hydrogenated filler, the isomer has preferably previously undergone treatment by earth in a different treatment zone.
5. 5. The process according to claim 1, characterized in that the isomer is hydrogenated in the hydrogenation zone, then subjected to the treatment by earth, before or after the separation of the hydrogen and from which the charge to be treated, from where it is eliminated by at least in part diolefins, is introduced before the enrichment zone.
6. 6. The process according to any of claims 1 to 5, characterized in that in the so-called separation zone, the hydrogen separation is carried out simultaneously with a separation of the compounds of 7 carbon atoms.
7. 7. The process according to any of claims 1 to 5, characterized in that the separation of hydrogen takes place in a first separation zone, then the separation of the compounds of 7 carbon atoms in at least one other separation zone.
8. 8. The process according to any of claims 1 to 5, characterized in that the separation of the hydrogen in a first separation zone is carried out, then the separation of compounds of 5 carbon atoms in a second separation zone and separation of the compounds of 7 carbon atoms remaining in a third separation zone.
9. 9. The process according to any of claims 1 to 5, characterized in that the separation of the hydrogen is carried out in a first separation zone, then the separation of the compounds of 4 carbon atoms in a second separation zone and the separation of the Compounds of 7 remaining carbon atoms in a third separation zone.
10. 10. The process according to any of the preceding claims, characterized in that a separation of the compounds of 7 carbon atoms takes place after the isomerization step.
11. 11. The process according to any of the preceding claims, characterized in that the charge to be treated is chosen in the group formed by an unrefined reforming, an unrefined reforming from which the compounds of 3 carbon atoms have been removed at least in part, an unrefined refurbishment from which at least some of the compounds of 4 carbon atoms, an unrefined reforming from which the compounds of 5 carbon atoms have been removed, a cut of 8 aromatic carbon atoms containing a major part of xylene and their mixtures.
12. 12. The process according to any of the preceding claims, characterized in that the effluent to be enriched in at least one separator is recirculated, located at the entrance of the enrichment zone.
13. 13. The process according to any of the preceding claims, characterized in that the enrichment zone of para-xylene is at least one crystallization zone at a very low temperature which supplies, after a separation step, a liquid (the second fraction ) which is isomerized in the isomerization zone and crystals which in the background form part of at least the first fraction.
14. 14. The process according to any of claims 1 to 12, characterized in that the para-xylene enrichment zone is a selective adsorption zone containing a zeolitic adsorbent, selective adsorption is carried out in the charge, in the presence of a solvent of desorption, the first para-xylene enriched fraction and the second para-xylene depleted fraction are recovered, the second fraction is isomerized in the isomerization zone containing an isomerization catalyst under suitable conditions to produce an isomer containing para-xylene and the isomer is recirculated at least in the adsorption zone.
15. 15. The process according to any of the preceding claims, characterized in that the selective hydrogenation is carried out in the presence of a catalyst containing at least one element of group VIII in which sulfur has been introduced prior to the passage of the charge to be hydrogenated.
16. 16. The process according to claim 15, characterized in that the sulfur has been introduced by treatment of the catalyst by means of a sulfurized agent in the hydrogenation zone.
17. 17. The process according to claim 15, characterized in that the sulfur has been introduced into the catalyst by means of a sulfurized agent, prior to loading the catalyst in the hydrogenation zone.
18. 18. The process according to claim 17, characterized in that at least one sulfur-based agent based on an organic compound comprising sulfur is incorporated into the catalyst prior to its use and prior to its activation by hydrogen.
19. 19. The process according to any of claims 15, 17 or 18, characterized in that the catalyst contains from 0.1 to 1% (by weight in relation to the support) of palladium, the catalyst has been treated prior to its activation by the sulfurized agent. way to introduce 0.05 to 1% (by weight in relation to the mass) of sulfur.