MXPA97004055A

MXPA97004055A - Paraxylene separation procedure comprising at least two stages of avert temperat crystallization

Info

Publication number: MXPA97004055A
Application number: MXPA/A/1997/004055A
Authority: MX
Inventors: Mikitenko Paul; R Macpherson Stuart
Original assignee: Institut Francais Du Petrole
Priority date: 1994-12-29
Filing date: 1997-06-02
Publication date: 1997-08-01

Abstract

The present invention relates to a process for the production of paraxylene of very high purity from a charge containing a mixture of aromatic hydrocarbons of 7 to 9 carbon atoms, in which at least a part of the charge is circulated in a zone adapted to enrich a first fraction of paraxylene and at least a part of the first fraction is purified by at least one crystallization at high temperature in at least one crystallization zone, the method being characterized in that the first enriched fraction of paraxylene is crystallized in a crystallization zone comprising at least two stages (50.70) of crystallization at high temperature, advantageously between +10 and -25 ° C. The paraxylene enrichment zone can be a very low temperature crystallization, a selective adsorption or a tolue dismutation zone

Description

PARAXYLENE SEPARATION PROCEDURE COMPRISING AT LEAST TWO STAGES OF HIGH CRYSTALLIZATION TEMPERATURE DESCRIPTION OF THE INVENTION The present invention relates to a separation process and to the preparation of paraxylene from a mixture of aromatic hydrocarbons comprising isomers of xylene. The isomers of xylene are orthoxyl ester. etaxylene. paraxylene and ethylbenzene. The main application of the present invention is to obtain paraxylene with a sufficient degree of purity for the synthesis, for example, of terephthalic acid. used in the preparation of synthetic fibers. mainly polyester. The applicant has described in the FR Patent No. 2,681,066 (US Pat. No. 5,284,992), a process for separating and recovering paraxylene contained in a hydrocarbon filler comprising essentially aromatic hydrocarbons of 8 carbon atoms. This method comprises the combination of an enrichment step. which is a selective adsorption on a simulated mobile bed adsorbent, of a charge containing essentially xylene isomers, which allows to substantially enrich paraxylene REF: 24728 a first adsorption effluent and purification, which is a crystallization when minus one crystallization unit operating at high temperature of the paraxylene enriched effluent. to obtain paraxylene with very high purity. where the mother liquor is recycled to the adsorption stage. This crystallization at high temperature corresponds to the second stage of crystallization of conventional crystallization processes (Chevron, Arco) which generally comprise a first crystallization at low temperature (from -40 to -70 * C) and a second crystallization to purify high temperature (from 0 to -20 * C, for example) of the crystals previously melted. In addition, a second fraction depleted of paraxylene and therefore enriched with orthoxylene and metaxylene. as well as ethylbenzene. which exits through the selective adsorption unit, is sent to an isomerization unit to increase its paraxylene concentration with a value close to equilibrium and very close to or greater than the composition of the initial hydrocarbon charge and then the isomerization product obtained is recycled to the adsorption. In the adsorption linkage. crystallization and isomerization described. Various types of impurities can appear in the different effluents and cause disturbances in the operation of the units that decrease the yield obtained and the purity of the recovered paraxylene. Firstly, in the isomerization of the paraxylene depleted fraction, olefinic hydrocarbons can be produced in a variable amount. according to the values of the partial pressures of hydrogen introduced. The subsequent formation of polymers and their passage through the adsorption unit can cause serious circulation problems through the adsorbent, including deactivation. Further. the accumulation of paraffinic and naphthalene hydrocarbons with 8 and 9 carbon atoms. whose volatility is between that of toluene. which is the deadsorsion solvent and that of the xi logs. which are intermediate products of the transformation of ethylbenzene in xylenes during isomerization. It can be harmful. In addition, the aromatic hydrocarbons of 9 carbon atoms present in a low proportion and do not separate well in the distillation columns. they can be harmful to the procedure, as are the aldehydes and ketones heavier than the initial charge, which are formed when oxygen is accidentally dissolved.

Finally, another problem is linked to the presence of methanol. This alcohol is added in a low proportion to the mixtures of wood to be crystallized, to avoid the co-crystallisation of water and paraxylene. Indeed, mixtures of aromatic compounds of 8 dry carbon atoms. they are relatively hygroscopic and during the centrifugation stage of the suspension of paraxylene crystals in mother liquors, the water contained in the environment can be absorbed in the mother liquors and later crystallized by the temperature of these mother liquors. Further. Certain exchangers can present leaks that allow the accidental passage of water to the mixture to be crystallized. An objective of the present invention is to improve the recovery of the produced paraxylene and minimize the cost of energy. mainly in the purification stage. Another object of the present invention is to solve these drawbacks and to limit mainly the content of these various impurities in the adsorption section to optimize, since the adsorbent is very sensitive to the impurities of the charge of the adsorption zone.

The present invention relates to a process for the production of paraxylene of very high purity, from a filler containing a mixture of aromatic hydrocarbons of 7 to 9 carbon atoms, in which at least a part of the load is circulated. in an enrichment zone adapted to enrich a first fraction of paraxylene and at least a part of said first fraction is purified in a purification zone, by at least one crystallization at high temperature in at least one crystallization zone, wherein the process is characterized in that the first paraxylene enriched fraction is crystallized in a purification zone comprising at least two stages of crystallization at high temperature and advantageously between +10 and -25"C. Each crystallization step may comprise one or more crystallizers. The term high temperature crystallization of paraxylene, refers to a crystallization of a Paraxylene solution or suspension already enriched with it. which corresponds to that which scientific literature calls a purification stage. For example, US Pat. No. 2,866,833. mention a stage of purification of paraxylene at -34 * C.

According to a first modality. the enrichment zone of the first fraction with at least 30% by weight of paraxylene. it can be at least a crystallization zone with very low temperature. for example less than -40 * C, called the recovery section (section recovery) into which a charge containing a mixture of aromatic hydrocarbons of 8 carbon atoms is introduced. as described in US Pat. No. 2,866,833 or as those described in US Pat. No. 5,329,061. This enrichment zone produces a crystal suspension that separates in the separation zone and the recovered crystals melt and constitute at least a part of the first fraction to be purified. In addition, the mother liquor resulting from the separation can be isomerized in an isomerization zone and the isomerized portion is at least partly recycled to the enrichment zone (recovery section). According to a second variant, the paraxylene enrichment zone can be a selective charge adsorption zone containing a mixture of aromatic hydrocarbons of 8 carbon atoms and producing the paraxylene enriched fraction. According to a third variant, the paraxylene enrichment zone can be a dismutation zone of a charge consisting essentially of toluene and using a selective coke or silicon coal catalyst in accordance with US Patents 4,117,026; 4,097,543; 4,851,604; 5,173,461; 5,243,117 and 5,321,183. The effluent of the dismutation comprising xi logs, benzene and toluene that did not react, is advantageously removed by the distillation of benzene and toluene. It is advantageous to dispose at the outlet of the enrichment zone, ie, for example, of the selective adsorption zone as described in US Pat. No. 5,284,992 of the Applicant, of an effluent containing more than 50% by weight of paraxylene. and preferably from 75 to 98%. Another alternative of paraxylene production can be referred to the combination of an adsorption step. of isomerization and crystallization with a single stage at high temperature. According to one embodiment of the invention, the method comprises, in combination, an adsorption operation and a crystallization operation. According to this variant, at least a part of the charge and / or the products resulting from the crystallization, for example the mother liquor, are subjected to a treatment operation with soil. At least a part of the effluent resulting from the land treatment operation is recycled to be subjected to an adsorption operation. The present invention also applies to a method comprising in combination, in addition to the adsorption and crystallization operations, an isomerization stage. In this case, at least a part of the isomerization product may be treated with soil. The present invention also relates to a process for separating and recovering paraxylene from a filler that may contain metaxylene. wherein this process comprises in combination, an adsorption operation and a crystallization operation. According to this method, at least a part of the mother liquor resulting from the crystallization operation is purified by distillation before being recycled to the adsorption operation. The present invention also relates to a process for separating and recovering the paraxylene contained in a hydrocarbon charge comprising aromatic hydrocarbons of 8 carbon atoms, in which: the charge is brought into contact in at least one adsorption zone. contains at least ethaxylene and paraxylene with at least one adsorbent in the presence of at least one suitable desorption solvent, under adsorption conditions such that a first fraction containing solvent, metaxylene and optionally ethylbenzene and / or orthoxylene and a second fraction which is obtained is obtained. contains solvent and substantially enriched paraxylene; the first fraction is distilled in order to recover a mixture of metaxylene and eventually ethylbenzene and / or orthoxylene; - said mixture is at least partially isomerized under appropriate conditions in at least one isomerization zone and an isomerization product is recovered, which is recycled at least in part to the adsorption zone; the second fraction is distilled in order to obtain substantially enriched paraxylene and a crystallization of the substantially enriched paraxylene is carried out, obtaining, on the one hand, mother liquors which are recycled at least partly towards the adsorption stage and on the other hand, crystals of paraxylene. According to another embodiment of the method according to the invention, at least one part that is selected from at least a part of the load, at least a part of the isomerization product and at least a part of the mother liquor, can be circulated, in at least one treatment zone with soil or an equivalent material and a first effluent is recovered, which is introduced at least in part to the adsorption zone, to produce said first and second fractions stripped from the adsorption zone. More precisely. The present invention relates to a process for separating and recovering paraxylene contained in a hydrocarbon charge comprising aromatic hydrocarbons of 8 carbon atoms, wherein the process comprises the following steps: a) it is contacted in at least one zone of adsorption, the load containing metaxylene. paraxylene and optionally ethylbenzene and / or orthoxylene. with an adsorbent in the presence of an appropriate desorption solvent, under conditions of adsorption such that a first solvent-containing fraction is obtained. metaxylene and optionally ethylbenzene and / or orthoxylene. and a second fraction containing substantially enriched solvent and paraxylene; b) the first fraction is distilled to separate the solvent on the one hand and the mixture of metaxylene and optionally ethylbenzene, and / or orthoxylene on the other hand; c) said mixture is εomerized under appropriate conditions, in an isomerization zone and an isomerization product is recovered. which is recycled at least in part to the stage of subsection (a); d) the second fraction is distilled and solvent and substantially enriched paraxylene are recovered; e) a crystallization of the paraxylene of the stage of part (d) is carried out in at least one crystallization zone at a high temperature. advantageously comprised between +10 and -25 * C and by means of this separation, on one hand, mother liquor is obtained. which are recycled at least partly to the stage of subsection (a) and on the other hand paraxylene crystals embedded in the mother liquor; f) the paraxylene crystals are washed with an appropriate solvent in at least one wash zone and the recovered paraxylene crystals have a high degree of purity; According to another variant of the invention, at least one part selected from the load can be circulated. the product of isomerization and the mother liquor, in at least one treatment zone with soil and a first effluent is recovered, which is introduced to the adsorption zone to produce the first and second fractions of the stage of part (a). At least a part of the hydrocarbon charge can be pretreated in a land treatment reactor. It may be advantageous if the isomerization product is sent to a soil treatment reactor before being subjected to adsorption. These reactors can be - independent or it can be one, possibly common to the one that treats the mother liquors. These treatments with soil allow to eliminate at least a part of the olefins created mainly in the isomerization stage and at least a part of the heavy impurities circulating in the adsorption, crystallization and isomerization circuit. Different variants can be devised: The mother liquor, advantageously, can be introduced at least partly into a distillation column downstream of the isomerization zone.

This column also treats the effluent from the isomerization zone and produces a head fraction that contains light compounds (air, water, methanol, hydrocarbons less than 7 carbon atoms) and another fraction that contains a distilled mixture of mother liquor and product. isomerization, which is then introduced to the treatment zone with soil. A final or tail fraction of the distillation containing heavy compounds can also be transferred to this distillation column, which makes it possible to reduce the size of downstream installations. A part of the mother liquors can also be mixed with the effluent that leaves the treatment zone with earth, which can be an effluent resulting from the circulation of the isomerization product, the mother liquor and the cargo, in the area of treatment with soil or the effluent resulting from the circulation in the treatment zone with earth of the latter and the fraction of distillation containing the distilled mixture of mother liquors and isomerization product. before being introduced into the selective adsorption zone. The effluent resulting from these latter modalities can be distilled in at least one distillation column (called retreatment column), which produces a glue fraction containing heavy compounds and a head fraction that is introduced to the adsorption zone, possibly with a part of the mother waters. The conditions of adsorption or elimination of undesirable compounds on the earth, as a rule, are the following: Temperature: from 100 to 300 * C. preferably from 160 to 230'C Space velocity per hour: 1 to 8. Preferably from 1 to 4 (volume per hour of loading per volume of earth) Type of earth: activated natural aluminosilicates, for example the land called F54 of ENGELHARD - Pressure: from 3 to 100 bars, preferably from 4 to 20 bars. The distillation column following the isomerization, in general has the following characteristics: - Pressure: from 1 to 20 bar, preferably from 3 to 8 bars Bottom temperature: from 150 to 280 * C. preferably from 200 to 240 * C Number of dishes: from 30 to 80, preferably from 50 to 70.

The column with retreat distiller located between the treatment zone with soil and the zone of selective adsorption, usually has the following characteristics: - Pressure: from 1 to 20 bar, preferably from 3 to 8 bars - Bottom temperature: from 160 to 290 * C, preferably from 210 to 250'C Number of dishes: from 40 to 200, preferably from 50 to 90. According to another characteristic of the invention, the content can be maintained of intermediate volatility components at a tolerable level. In this case, at least a part of the mother liquor can be purged before being introduced to the treatment zone with soil. In the same way, it may be advantageous to purge at least a part of the desorption solvent resulting from steps (b) and (d) of the distillation of the depleted fraction or the paraxylene enriched fraction. before it is recycled and compensate the purge of the solvent with a new solvent supply, either in the charge, or upstream of the adsorption zone. As already indicated, it is possible to recycle the crystallization mother liquors in different sites of the facilities, according to the importance of the content of undesirable compounds, but it can be advantageous to combine these different recyclables, for example, when it comes to reusing the existing equipment for the distillation of the isomerization product. treatment with soil or retreatment distillation and when one of these equipment already operates at maximum. You can also combine these different recycles and purges when you want to decrease the content of an impurity in the cycle without completely eliminating it. The crystallization step of the paraxylene enriched fraction from the enrichment zone is generally carried out, as already mentioned above, at a high temperature, for example between +10 and -30 * C and preferably between +10 and -25. 'C. Normally, the temperature is selected according to the concentration of paraxylene desired in the mother liquor and the monetary cost of the crystal operation. Under these conditions, a crystallization of the paraxylene can be carried out in a single step, when the paraxylene is collected. for example in the form of an extract in the adsorption unit, it has a higher purity, for example at 85%.

But it can also be advantageous, mainly for economic reasons, to carry out a crystallization in several stages, for example a first crystallization at a temperature TI between +10 and -10 * C and a second crystallization at a temperature T2 lower than TI, comprised between 0 and -25 'C. According to a first embodiment of the invention, a single stage crystallization of the paraxylene of the stage of part (b) is carried out, a crystal suspension is recovered in the mother liquor, the crystals are separated from the mother liquor in a separation area. they are washed with a solvent, washing waters are recovered, at least a part of the mother liquor and a part of the washing waters are recycled and the paraxylene crystals are completely melted, so that a liquid stream of water is obtained. molten paraxylene. According to a second embodiment of the invention, which uses a first variant of a two-step crystallization of the paraxylene enriched fraction, a first crystallization of the paraxylene from the stage of part (d) is carried out in a first crystallization zone at a temperature IT. a suspension of first paraxylene crystals in first mother liquors is recovered, the first crystals are separated from the first mother liquors in a first separation zone, at least a part of the first mother liquors is recycled towards the adsorption zone or crystallization at very low temperature. the first separated crystals melt and crystallize in a second crystallization zone at a temperature T2 higher than TI. a suspension of second paraxylene crystals is recovered in a second mother liquor, the second crystals of the second mother liquors are separated in a second separation zone, the second crystals are washed with the appropriate washing solvent in the second separation zone, washing waters are recovered, at least a part of the recycle is recycled. the second mother liquors and at least a part of the washing water to the first crystallization zone and possibly a part to the second crystallization zone and the second paraxylene crystals are completely melted »so that a liquid stream of liquid is obtained. molten paraxylene. For example, the temperature TI of the first crystallization can be from -5 to -25 * C and then the temperature T2 of the second crystallization can be from +10 to -5'C. According to another embodiment of the invention, which uses a second variant of two-stage crystallization, a first crystallization of the paraxylene from the stage of part (d) is carried out in a first crystallization zone at a temperature TI. a suspension of first paraxylene crystals in first mother liquors is recovered, the first crystals of the first mother liquors are separated in a first separation zone, optionally the first crystals are washed with the washing solvent in the first separation zone, at least a part of the first mother liquors crystallizes in a second crystallization zone at a temperature T2 lower than the temperature TI, a suspension of second paraxilens crystals is recovered in a second mother liquor, the second crystals are separated from the second ones mother liquors in a second separation zone, the second crystals are washed with the washing solvent in the second separation zone, second mother liquors are recovered which are recycled at least partly towards the adsorption or crystallization zone at temperature very low. the first and second paraxylene crystals are mixed. they melt completely and a stream of molten paraxylene is recovered. For example, the first crystallization can be performed at a temperature TI between +10 and -10 * C and then the second temperature T2 which is lower than TI. It can be between 0 and -25'C.

In accordance with another embodiment of the present invention, which uses a crystallization with several steps. a first crystallization of the paraxylene of the stage of part (d) and of the partially melted or optionally molten crystals recycled coming from a second crystallization mentioned above, in a first crystallization zone at a temperature TI takes place. a suspension of paraxylene crystals in a first mother liquor is recovered, the crystals are separated from the first mother liquors in a first separation zone, said crystals are washed, melted and a stream of molten paraxylene is recovered. at least a part of the first mother liquors crystallizes in a second crystallization zone at a temperature T2 lower than the temperature TI. a suspension of second paraxylene crystals in second mother liquors is recovered, the second crystals are separated from the second mother liquors in a separation zone, second mother liquors are recovered which are recycled at least partly towards the adsorption zone or crystallization at a very low temperature, the second crystals are eventually melted and the crystals which are optionally melted are recycled to the first crystallization zone in order to recrystallize them with the paraxylene of the stage of part (d) at the temperature TI.

Another variant may consist of partially melting the second crystals and recycling the obtained suspension to the first crystallization zone, to recrystallize it with the paraxylene of the stage of part (d) at the temperature TI. In accordance with another particularly advantageous embodiment. a first crystallization of the paraxylene of the stage of part (d) is carried out in a first crystallization zone at a temperature TI, a suspension of first paraxylene crystals is recovered in a first mother liquor, the first crystals of the first waters are separated mothers in a first separation zone, at least a part of the first mother liquors crystallizes in a second crystallization zone at a temperature T2. lower than TI, the second crystals are separated from the second mother waters in a second separation zone, the second mother liquors are recovered, which are recycled at least partly towards the adsorption or crystallization zone at a very low temperature, recovering the first crystals and the second crystals and washed at least in a separation and washing area, countercurrent with the appropriate washing solvent, a part of the washing waters is recovered which is recycled at least partly towards the First crystallization zone, the other part of the paraxylene crystals is recovered, completely melted in a melting zone and a stream of molten paraxylene is collected. The present invention also relates to a process for separating and recovering paraxylene contained in a hydrocarbon charge comprising aromatic hydrocarbons of 8 carbon atoms, wherein the process comprises the following steps: a) it is contacted in at least one adsorption zone (8), the load (1) containing metaxylene. paraxylene and optionally ethylbenzene and / or orthoxylene. with an adsorbent in the presence of an appropriate desorption solvent, under conditions of adsorption such that a first solvent-containing fraction is obtained. metaxylene and optionally ethylbenzene and / or orthoxylene. and a second fraction containing solvent and essentially substantially enriched paraxylene; b) the first fraction is distilled (12) to separate the solvent on the one hand and, on the other hand, the mixture of metaxylene and optionally ethylbenzene. and / or orthoxylene; c) the mixture is isomerized under appropriate conditions in an isolating zone (21) and an isomerization product (2) is recovered which is recycled at least in part towards the stage of part (a); d) the second fraction is distilled and on the one hand the solvent is recovered and on the other hand the substantially enriched paraxylene; e) the crystallization of the paraxylene from the stage of part (d) is carried out in at least one crystallization zone (5a, 5b) at a high temperature, advantageously comprised between +10 and -25'C and you get. thh this separation. on the one hand, mother liquors which are recycled (3) at least partly towards the stage of part (a) and on the other hand, paraxylene crystals embedded in the mother liquor; f) the paraxylene crystals are washed with an appropriate washing solvent in at least one washing zone and paraxylene crystals are recovered with a very high degree of purity; the process is characterized in that at least a part of the mother liquor (3) (33) is introduced into a distillation column, obtaining a fraction containing purified mother liquors. which is subsequently sent to the adsorption zone (8).

It should be understood that within the scope of the present invention it is included to use another desorption solvent other than toluene, such as paradiethylbenzene (PDEB). where the solvent is heavier than the xi logs, which will be recovered at the bottom of certain columns leaving the toluene in the head. The present invention will be better understood with reference to the following Figures, which illustrate in a non-limiting manner various embodiments of the present invention, wherein: Figure 1 schematically depicts the process and the various recycling possibilities of the isomerization product and the mother liquor of crystallization towards adsorption thh a treatment with earth; Figures 2 and 3 illustrate a purification of paraxylene by crystallization in a single step; Figures 4 and 5 show a purification of paraxylene by double stage crystallization, the first being colder than the second, - Figures 6 to 10 represent a purification of paraxylene by double stage crystallization, the first being warmer than the second .

The conditions of operation and adsorption in simulated moving bed (for example, countercurrent) are selected in such a way that the first fraction containing metaxylene. orthoxylene and ethylbenzene. be a refined and the second fraction containing essentially paraxylene. be an extract. These conditions are described in US Pat. No. 5,284,992. A charge comprising approximately 20% ethylbenzene, 18% paraxylene, 45% metaxylene and 17% orthoxylene is conducted on line 1. Thh a line 2 a recycled effluent is collected whose ethylbenzene content is appreciably lower, typically from 8 to 13% and which contains impurities. Thh lines 3 and 300. another recycled effluent is introduced whose paraxylene content is higher, typically from 25 to 45%. A line 4 recovers the load and these two effluents. obtaining a mixture of approximately 20 to 22.5% paraxylene composition. from 9 to 14% ethylbenzene. from 20 to 22.5% of orthoxylene and from 45 to 50% of metaxylene. which is introduced into a countercurrent adsorption zone 8 comprising one or more columns 6 and / or 7 filled with a zeolitic adsorbent, each of which has a limited number of beds. the number of beds of each column is between 4 and 20. The productivity expressed with respect to the paraxylene product is approximately 0.07 m 3 per m 3 of sieve. per hour, in environmental conditions. It is desadsorbed by toluene. at a ratio of approximately 1.45 m 3 of toluene per m3 load, with the operating temperature close to 160 * C. From this unit a refined depleted paraxylene is passed thh a line 10. containing essentially toluene. metaxylene, ethylbenzene and orthoxylene, and by line 9 a paraxylene enriched composition extract. containing essentially toluene and paraxylene, with the major impurity being ethylbenzene. The raffinate is introduced to a distillation column 12 (head temperature 125 * C, bottom temperature 160 * C, for example). In the head, toluene (approximately 30% of the amount introduced in the adsorption, for example) containing toluene is passed thh line 14., for example, less than 2% of aromatic compounds of 8 carbon atoms, and through the bottom of this column a liquid (solvent-free refining) rich in ethylbenzene is transferred by line 15. metaxylene and orthoxylene, and paraxylene depleted (eg, less than 3%). which is sent to an isomerization unit 21. This raffinate is contacted with the hydrogen introduced by the line 20 and with a catalyst based on mordenite and platinum on aluminum, at approximately 380'C. A line 22 conducts the isomerization product from the reactor outlet to a gaseous component separation balloon (not shown in the Figure), then to a distillation column 23 (e.g., head temperature 90 * C. bottom 160"C) The hydrocarbons of 1 to 5 carbon atoms, hexane, cyclohexane, benzene and toluene are passed through the head through a line 24 through a line through the bottom of this column. 2. an effluent containing 8 to 13% ethylbenzene, 21 to 24% paraxylene, 21 to 24% orthoxylene, 45 to 50% metaxylene and impurities, which is recycled to the adsorption zone. Line 9 introduces the extract to a distillation column 16. from which toluene is decanted with less than 2% aromatic compounds of 8 carbon atoms (approximately 70% of the amount introduced in the adsorption. ) which is recycled by lines 17 and 11 to the feeding of desorption solvents of the adsorption unit. At the bottom of column 16 at about 160 * C, a paraxylene enriched stream (with approximately 90% paraxylene) is transferred via line 19. which leads to a one-stage crystallization unit 5a, for example. running at approx. -ÍO'C. In this unit 5a. 5b, paraxylene crystals suspended in mother liquors are produced. The crystals are separated in at least one centrifuge 5b, for example, and washed in the centrifuge. Part of the paraxylene depleted mother liquor is recovered (approximately 54%), which is recycled through line 3 to the adsorption zone 8 through the land treatment and distillation zones already described; and on the other hand, paraxylene crystals. which merge The washing solvent, for example toluene. it is led by a line 18 and can come, as shown in the Figure, from the refining distillation unit 12 and / or even from the extract distillation unit 16. From the unit 5b. after a distillation of the molten crystals (not shown), liquid paraxylene is recovered with a purity, for example, equal to 99.75% through line 25 and toluene, which is recycled (line not shown). Figure 2 more accurately illustrates the steps of crystallization and downstream treatment of the paraxylene crystals with a toluene wash. According to this Figure, the crystallization unit 50, of a single stage, receives the crystallization charge (the distillate extract) by line 19. By line 51, crystals suspended in mother liquors are recovered, which are separated when less in part in at least one centrifuge 52. A mother liquor containing, for example, 54% paraxylene, is transferred and recycled at least partly to the adsorption zone 8 via lines 53 and 3, and another part can be recycled to the crystallization zone 50 by a line 53a. A part of the crystals suspended in the mother liquor can also be recycled to the crystallization zone by line 51a. Subsequently, the re-dissolved toluene from a distillation column 60 driven by a line 56 and toluene supplied by a solvent line 57, coming from the line 18 is introduced as a washing solvent in the centrifuge 52. Separately, some washing water by a line 54 connected to the centrifuge 52 and these are recycled at least partly to the refining distillation unit 12. The washed crystals of paraxylene are decanted by a line 55. they are completely melted in a melting zone 58 and are introduced to the distillation column 60 by a line 59. At the bottom of the column liquid paraxylene is recovered with very high purity and at the top of the column toluene, which is recycled at least partly to the centrifuge 52 At least one centrifuge is used to separate the crystals and the mother liquor. You can also use at least one rotary filter instead of the centrifuge. According to a variant not illustrated, the centrifuge can be replaced at least by a column of separation and backwash, such as that described in US Pat. Nos. 4,475,355 and US 4,481,169, for example the column NIRO. In this case, the mother liquor and the washing water are the same. which is eventually distilled before being recycled, at least in part, to the adsorption zone through the treatment zones with earth and distillation, and in some cases eventually towards the crystallization zone. According to another embodiment illustrated by Figure 3, another wash solvent such as molten paraxylene from the melting zone 58 can be used. In this case the Figure contains the same references for the same components as those in Figure 1. at least a portion of the molten paraxylene is introduced upstream of the suspension 51, by a line 59b in a counter current column 80. for example a NIRO type column and used to wash the paraxylene crystals in the column. At least a part of the molten paraxylene is introduced to the column and crystallized. The crystals recovered from the column by line 55 are subsequently melted in the melting zone 58 and the liquid paraxylene of very high purity is recovered through line 59. The washing waters and mother liquors are recovered at the same time by line 53 and are recycled to the adsorption zone 8. where a part can be recycled to the crystallization zone 50. If a washing solvent is used. for example pentane. different to the desorption solvent (toluene) and molten paraxylene. the procedure described in Figure 2 will be reproduced, except that the washing waters from the centrifuge 52 must be depleted of solvent by a subsequent distillation (not shown in the Figure) before being recited to the adsorption or to the crystallization. After. The distilled solvent is recycled in the centrifuge. In the same manner, the procedure of Figure 3 can be reproduced with a countercurrent wash column and a wash solvent different from the molten paraxylene and the desorption solvent. In this case, after the complete melting of the crystals, a stream of molten paraxylene containing washing solvent is distilled, the solvent is recovered in the head, which is recycled at least partly towards the washing column and at the bottom paraxylene of very high purity is recovered. The mother liquors comprising the washing waters are stripped from the washing column, distilled and recycled at least partly towards the selective adsorption zone and eventually another part towards the crystallization zone. Figures 4 and 5 illustrate a two-stage crystallization, wherein the temperature of the second stage of crystallization is higher than that of the first. According to Figure 4, the first crystallization unit 50 is for example at -20 * C and receives the crystallization charge (the extract distilled from the adsorption) by line 19. Its purity is about 80%. By means of line 51, crystals suspended in mother liquors are recovered, which are separated in a first centrifuge 52. First mother liquors containing, for example, 40% paraxylene are transferred and recycled at least in part towards the adsorption zone 8 by lines 53 and 3 through the land treatment and distillation zones, and another part can be recycled towards the first crystallization. The crystals recovered by a line 55 melt in a melting zone 58 and are introduced by line 59 to a second crystallization unit 70 operating, for example at 0 * C. A suspension of second crystals is recovered by line 71. which are introduced to at least one centrifuge 72 or a rotary filter. Second mother liquors are recovered by line 73, which are recycled at least in part towards the first crystallization unit 50 and possibly another part towards the crystallization zone. The separated crystals are washed with the desorption solvent. (toluene for example) used as washing solvent, which is introduced to the centrifuge by a line 56 and an additional line 57 coming from the line 17 and wash waters 74, which are recycled at least partly towards the first crystallization unit 50 and / or to the second crystallization unit 70 after being optionally distilled. They may also be recited to the refining distillation unit (stage b). In addition, they are recovered from a line 75 connected to the centrifuge 72. The second crystals which completely melt in a melting zone 76 are collected by the molten paraxylene line 77, which is distilled in the distillation column. 60. Toluene recovered in the head is re-terminated via line 56 while para-xylene of very high purity is decanted through the bottom of the column via line 61. Centrifuge 72 may be replaced by a back-wash column. In this case, the second mother liquors containing washing toluene can be distilled, as in the case of a single stage, before being recycled and the washing toluene is sent back to the washing column. Figure 5 illustrates the use of a counter current column of the NIRO column type as the second separation and washing zone of the second crystals instead of the centrifuge 72 of Figure 4 or of a rotary filter, which it no longer uses as a solvent of toluene washing. but a part of the molten paraxylene stream. In this Figure 5. which comprises the same components as Figure 4. the second crystals suspended in the second mother liquors are recovered by line 71 coming from the second crystallization unit and introduced to the NIRO 80 column, which is fed of wash solvent on one part of the molten paraxylene collected by line 77a. Paraxylene crystals of very high purity are collected by line 75. which melt in the melting zone 76 and a stream of molten paraxylene is recovered via line 77. The second mother liquors have 70%. for example, of paraxylene and the washing waters are recovered thereto and are recycled at least partly to the first crystallization unit 50 by line 73 and eventually another part to the second crystallization unit. In accordance with another variant not illustrated by the Figures. the washing solvent may be a solvent different from the stream of molten paraxylene and that of desorption, for example pentane. In this case, the molten paraxylene stream is distilled to recover washing solvent in the head. which is recycled at least partly to the separation zone, and in the background, very high purity paraxylene; the mother liquors comprising the washing waters are distilled before being recycled to the first crystallization zone and eventually partly to the second crystallization zone. if the second separation zone is a backwash column (for example, a NIRO column). On the other hand, if the second separation zone is a centrifuge or a rotating filter, the mother liquors are recycled to the first crystallisation zone and possibly partly to the second crystallization zone and the washing waters are distilled before being recycled to the first crystallization zone and possibly partly towards the second crystallization zone. The first suspended crystals 51a and the second suspended crystals 71a can be recycled. to the first and second crystallization zones, respectively. Figures 6 and 7 illustrate a two-stage paraxylene crystallization, wherein the temperature of the second crystallization stage of the mother liquor is lower than that of the first stage. According to FIG. 6, the crystallization charge (the extract distilled from the adsorption) is introduced via line 19 to the first crystallization unit 70 which operates at approximately 0 * C. Through the line 81, the first crystals suspended in first mother liquors are recovered, which are separated in at least one first centrifuge 82. They are washed with toluene driven by line 97 and recovered by line 84. The first mother liquors they contain, for example, 70% paraxylene. they are introduced at least partly through the line 83 to the second crystallization unit 50. which operates at -10'C. Another part can be recycled to the first crystallization unit by a line 83a. Through the line 85 the second crystals suspended in second mother liquors are recovered, and they are separated in at least one second centrifuge 86. They are collected by a line 88 after having been washed with toluene, which is introduced through the line 98 to the second centrifuge. The second mother liquors collected by line 87 contain a fraction of the washing toluene; it is recycled at least partly to the adsorption zone 8 through the earth treatment and distillation zones, and eventually another part via line 87a to the second crystallization unit. The first and second paraxylene crystals are mixed and introduced into a melting zone 89. A line 90 collects the molten paraxylene stream, which is introduced to a distillation column 91 which produces, in the bottom »very high paraxylene. purity and in the head toluene, which is reciled by line 92 and mixed with additional toluene supplied by line 95 or line 18. The obtained toluene mixture is introduced at least in part to some of the centrifuges 82 and 86 as a washing solvent. Figure 7 takes up the same components and references of Figure 6, except for the step of washing the crystals. it is used as a very pure molten paraxylene wash solvent recovered from the line 90. Indeed, at least a part of the very pure molten paraxylene is transported by line 91 and introduced to the first centrifuge 82 and to the second centrifuge 86 for washing , respectively, the first and the second crystals. The first mother liquors and the first washing waters are sent via line 83 to the second crystallization unit 50, while the second mother liquors and the second washing waters are collected via line 87 to be recycled at least in part towards the adsorption zone 8. In this Figure the use of the centrifuges 82 and 86 to separate the crystals from the mother liquors and wash them, but can also be replaced by counter current columns, of the NIRO column type, is described. In this case, the respective solutions collected will collect the mother liquor and the washing water from each column. Figure 8 illustrates another variant of the multi-stage crystallization process, in which the molten paraxylene of very high purity is collected at the exit from the crystallization zone at a high temperature. The crystallization charge (the extract distilled from the adsorption) is introduced through the line 19 to the first crystallization unit 70. which operates at approximately 0 * C. By line 81, crystals suspended in first mother liquors are recovered, which are separated, for example, in a NIRO 80 column. The crystals recovered by line 84 are melted in a melting zone 100. A para-xylene stream is collected. cast very high purity by line 101. which takes it out partly thanks to line 102. to wash the crystals on the NIRO column. The first mother liquors which are transferred from the NIRO column by the line 83 are introduced at least in part to a second crystallization unit 50 operating at a temperature of -15'C, for example. Another part of these first mother liquors can be recycled via line 83a and mixed with the charge of the first crystallization unit. From the second crystallization unit 50, a suspension of second crystals in second mother liquors is collected via line 85. which are separated in at least one centrifuge 86 or a rotary filter. By means of line 87, second mother liquors are recovered, which are recycled at least partly to the adsorption unit 8 through the land treatment and distillation zones, and another part can also be recycled to the second water treatment unit. crystallization 50 by line 105 connected to line 87.

The second crystals, once separated, are collected by line 88 and eventually melted in a melting zone 103. The molten paraxylene is recycled by a line 104 and mixed with the charge of the first crystallization unit to be recrystallized at the temperature of the first crystallization. Figure 9 represents a preferred variant of the two-step crystallization process, which advantageously operates at a temperature between +5 and -7 'C for the first stage and between -7 and -25 * C for the second. The crystallization charge (the distilled extract of the adsorption) is introduced via line 19 to a first crystallization unit 70. By line 81, the first crystals suspended in first mother liquors are recovered, which are separated by at least one centrifuge 82 or at least one rotary filter. The first mother liquors collected by line 83 are introduced at least partly to a second crystallization unit 50. and the other part can be recycled to the first crystallization unit 70. A second crystal suspension is collected by line 85, which are separated in at least one centrifuge or a rotary filter 86. Second mother liquors are transferred through line 87 and are recycled at least partly to the adsorption zone 8 through the land treatment and distillation zones. , and the other part can be taken out and recycled to the second crystallization unit by a line 87a connected to the line 87. The first and second crystals collected by the lines 84 and 88. respectively, are put together and introduced at least one washing column 110 of column type NIRO. where they are washed with a washing solvent. The paraxylene crystals are collected by a line 111, completely melted in a melting zone 112 and a high purity paraxylene stream is decanted. A part of the paraxylene stream is collected via line 114 and is introduced as a wash solvent to column 110. The wash waters collected in the column are recycled at least in part towards the first crystallization unit. In accordance with Figure 10. When the wash solvent in the wash column is the desorption solvent (toluene) or other appropriate solvent such as pentane. the molten paraxylene stream containing a small part of solvent can be distilled in a distillation unit 117. The very high purity paraxylene is collected via line 118. while the light fraction comprising the wash solvent is recycled into the NIRO column. By last, the wash waters passed through line 15 and containing solvent, are distilled in a distillation unit 120. The solvent is recycled at least in part in the column and the washing waters devoid of most of the solvent are recycled when less in part towards the first crystallization unit on line 121. For a given figure, it was already mentioned the recycling of the mother liquors coming from a stage A of crystallization in said stage A or of a stage B in said stage B. It should be understood that these recyclables can be applied to all the Figures. In the same way, it is known that a suspension of crystals from a crystallization stage can be recycled towards this stage and this recycling can also be applied to all the Figures (81a and 85a). In the description, the term separation zone is often used. It should be understood that it is at least one centrifuge or at least one rotary filter or at least one backwash column for a solvent. The separation zone where the crystals are washed can comprise at least one centrifuge or at least one rotary filter. However, it should be noted that when used as a separation zone where the washing of the crystals takes place, at least one backwash column, for example of the NIRO type and in particular with a part of the recovered very pure molten paraxylene, As a washing solvent, excellent results are obtained and the cost of the utilities is reduced. As is schematized in Figure 1. the mother liquor from the crystallization unit 5b is recycled to the adsorption unit 8. In the case of a crystallization of two stages or more. the mother liquor comes from the coldest stage of crystallization after the separation of the paraxylene crystals (line 53, Figure 5. line 87. Figures 7 to 9). The impurities circulating in the circuit of the adsorption device. crystallization. isomerization. they can be olefinic hydrocarbons. as well as paraffinic and naphthalene hydrocarbons or other oxygenated compounds. These can also come from both the load to be treated coming from a catalytic modification, and the isomerization. These circulating impurities can be removed from all fractions. mainly from the extract and the mother liquor resulting from the crystallization stage. These mother liquors can be introduced by line 32 connected to line 3 and by line 53 or 87 to at least one land treatment reactor 26, advantageously two, arranged upstream and connected by line 27 to the adsorption unit. 8. Line 32 containing the charge to be treated and line 2 containing the isomerization product can be connected to this line 32. where the three flows are treated mixed in a single reactor 26. Accordingly, with another variant. Load 1 can be pretreated in another land treatment reactor (not indicated in the Figure). The same applies to the isomerization product 2. which can also be initially pretreated after its passage through the distillation unit 23. According to a preferred variant, the mother liquor 3 can be introduced directly to the line 22 leading to the distillation unit 23 of the isomerization product. before being treated mixed with the distilled isomerization product, in the land treatment reactor 26. This variant allows to eliminate all the most volatile compounds sensibly. not only of the isomerization product. but also from mother waters. When the distillation unit is arranged to decant through the bottom a supplementary fraction containing most of the heavy compounds (hydrocarbons of 9 or more carbon atoms, aldehydes. ketones). the soil treatment of the distilled mixture comprising the isomerization products and the mother liquor is substantially improved. Likewise, a part of the mother liquor can be recycled via line 31 to the effluent 27 of reactor 26. The effluent of the treatment reactor with earth and possibly the mother liquor of crystallization 31 could still contain heavy hydrocarbons, such as hydrocarbons of 9 carbon atoms introduced by a line 27 into a distillation column 28 which produces, on the bottom (line 29) the undesirable impurities and in the head a distillate corresponding to the 8-carbon-purified portion, which is introduced by line 4 to an adsorption unit 8. A part of the mother liquor can also be introduced to line 4 through line 30. These recycling products can be combined with each other, when it comes, for example, to reuse the existing equipment for distillation 23. treatment with soil 26 or distillation 28 and when one of these equipment is already operating at its maximum, or even when searching for reduce the content of an impurity in the circuit without seeking to eliminate it completely. In other words, the mother liquor of the crystallization unit (colder stage), conducted by line 3 can be recycled in part to the adsorption unit 8, either directly by line 30, or indirectly by means of of lines 31. 32 or 33. In order to maintain the content of intermediate volatility components of toluene (desorption solvent) at a tolerable level, for example less than 5%, at least one purge of toluene contaminated by said components by means of the line 35 connected either to the line 17, or to the line 14 or to the line 11. bringing the recycled solvent mixture to the adsorption unit 8. In addition, a purge of the waters can be effected. mothers coming from crystallization if the content of intermediate volatility components is very high. This purging is carried out by means of line 34 connected to line 3. The toluene purge can be compensated by a toluene supplement. It is established that the most important sources of the aromatic portion of 8 carbon atoms (line 1) come from the catalytic modification of the dismutation (disproportionation) of toluene in benzene and xylene. and the transalkylation of toluene-aromatic compounds of 9 carbon atoms, and that the effluents of these units are generally purified in part in a chain of distillation units of which column 28 can be part, can be used at least in part as source of additional toluene, either that produced at the head (line 42) of a toluene distillation column 40 downstream of the reactor 26 containing the earth, or that produced by the passage through the line Ib of at least another fraction of the load (line 1) that is mixed with the effluent from the bottom of column 40. or that which is introduced into the purified load (line 1) for the deregulation of column 40. which allows to pass through the 8 carbon atom portion, the desired proportion of toluene. 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 an antecedent, what is contained in the following is claimed as property.

Claims

CLAIMS 1. A process for the production of paraxylene of very high purity from a charge containing a mixture of aromatic hydrocarbons of 7 to 9 carbon atoms, wherein at least a part of the charge is circulated in a so-called enrichment adapted to enrich a first fraction with more than 50% by weight of paraxylene and at least a part of the first fraction in a so-called purification zone is purified by at least one crystallization at high temperature in at least one crystallization zone. the process being characterized in that the first paraxylene enriched fraction is crystallized in a purification zone comprising at least two steps of crystallization at a high temperature comprised between +10 and -30 * C and advantageously between +10 and -25 * C; being the temperature TI of one of the stages different from the T2 of the other stage.
2. A process according to claim 1, characterized in that the paraxylene enrichment zone is at least one crystallization zone at a very low temperature in which the charge containing a mixture of aromatic hydrocarbons of 8 carbon atoms is introduced. it produces, after a separation step, a liquid which is isomerized in an isomerization zone and crystals, which melt and which are at least a part of said first fraction.
3. A process according to claim 1, characterized in that the paraxylene enrichment zone is a selective adsorption zone containing a zeolitic adsorbent. where the charge containing a mixture of aromatic hydrocarbons of 8 carbon atoms is introduced. a selective adsorption of the filler is carried out in the presence of a desorption solvent, the first paraxylene enriched fraction and a second paraxylene depleted fraction are recovered. the second fraction is isomerized in an isomerization zone containing an isomerization catalyst. under conditions suitable for producing an isomerization product containing paraxylene and at least a portion of the isomerization product is recycled to an adsorption zone.
4. A method according to any of claims 1 to 3, characterized in that at least a part of the mother liquor resulting from the coldest stage of the purification zone is recycled to the enrichment zone.
5. A process according to claim 1, characterized in that the paraxylene enrichment zone is at least one charge zone of the charge consisting essentially of toluene which produces an effluent containing benzene and xylene and which is distilled to remove benzene and toluene that have not reacted.
A process according to any of claims 1 to 5, characterized in that the purification zone comprises a first crystallization carried out at a temperature TI and a second crystallization carried out at a temperature T2. superior to IT.
7. A process according to any of claims 1 to 6, characterized in that a first crystallization of the paraxylene enriched fraction is effected in a first crystallization zone at a temperature TI. a suspension of first paraxylene crystals in first mother liquors is recovered, the first crystals of the first mother liquors are separated in a first separation zone, first mother liquors are recovered, the first separated crystals are melted and crystallized in a second crystallization zone at a temperature T2 higher than the TI, a suspension of second crystals of paraxi logs is recovered in second mother liquors, the second crystals are separated from the second mother liquors in a separation zone, the second crystals are washed with an appropriate washing solvent in the second separation zone, washing water is recovered, at least part of the second mother liquor is recycled and at least a part of the washing water is recycled to the first crystallization zone and eventually another part to the second crystallization zone. second paraxylene crystals. so that a liquid stream of molten paraxi wood is obtained.
8. A process according to any of claims 1 to 5, characterized in that the purification zone comprises a first crystallization carried out at a temperature TI and a second crystallization carried out at a temperature T2. inferior to IT.
9. A process according to any of claims 5 and 8. characterized in that a first crystallization of the enriched fraction of paraxyl wood is effected in a first crystallization zone at a temperature TI, a suspension of first paraxylene crystals is recovered. in first mother liquors, the first crystals of the first mother liquors are separated in a first separation zone, if the first crystals are washed with a washing solvent in the first separation zone, at least a part of the first crystals is crystallized mother liquors in a second crystallization zone at a temperature T2 lower than the temperature TI, a suspension of second paraxi-wood crystals is recovered in second mother liquors, the second crystals are separated from the second mother liquors in a second separation zone , the second crystals are washed with the appropriate washing solvent in the second zone of paration second mother liquors are recovered, the first and second paraxylene crystals are mixed. they melt completely and a stream of molten paraxylene is recovered.
10. A process according to any of claims 1 to 5 and 8. characterized in that a first crystallization of the enriched fraction of paraxyl wood and recycled crystals optionally molten coming from a second previous crystallization, in a first crystallization zone a a temperature IT. a suspension of paraxylene crystals in a first mother liquor is recovered, the crystals are separated from the first mother liquors in a first separation zone, the crystals are washed in the first zone, melted and a stream of molten paraxylene is recovered, at least a part of the first mother liquors crystallizes in a second crystallization zone at a temperature T2 lower than the temperature TI, a suspension of second paraxylene crystals in a second mother liquor is recovered, the second crystals are separated from the second ones mother liquors in a second separation zone, second mother liquors are recovered, optionally the second crystals are melted and the molten crystals are eventually recycled to the first crystallization zone to recrystallize them with the enriched fraction of paraxylene at the temperature TI.
11. A process according to any of claims 1 to 5 and 8. characterized in that a first crystallization of the fraction enriched for paraxylene and the crystals optionally molten recycled from a second previous crystallization, in a first crystallization zone a a temperature IT. a suspension of paraxylene crystals in a first mother liquor is recovered, the crystals are separated from the first mother liquors in a first separation, the crystals are washed in the first zone, melted and a stream of molten paraxylene is recovered, at least a part of the first mother liquors crystallizes in a second crystallization zone at a temperature T2 lower than the temperature TI, a suspension of second paraxylene crystals in a second mother liquor is recovered, the second crystals of the second waters are separated mothers in a second separation zone, second mother liquors are recovered. the second crystals are partially melted and the crystals in suspension are recycled to the first crystallization zone to recrystallize them with the fraction enriched for paraxylene at the temperature TI.
12. A process according to any of claims 1 to 5 and 8. characterized in that a first crystallization of the paraxylene enriched fraction is effected in a first crystallization zone at a temperature TI. a suspension of first paraxi-wood crystals is recovered in a first mother liquor, the first crystals are separated from the mother liquor in a first separation zone, at least a part of the first mother liquors is crystallized in a crystallisation zone at a time. T2 temperature. inferior to IT. the second crystals are separated from the second mother liquors in a second separation zone. the mother liquors are recovered, the first crystals and the second crystals are recovered and washed in at least one separation and countercurrent washing zone with an appropriate washing solvent. On the one hand, washing waters are recovered which are recycled at least partly towards the first crystallization zone. on the other hand, paraxylene crystals are recovered, which are completely melted in a melting zone and a stream of molten paraxylene is obtained.