MXPA99005773A - Method for separating paraxylene comprising adsorption with water injection and crystallisation - Google Patents

Abstract

The invention concerns a method for separating paraxylene on a simulated fluid bed from a C8 feedstock comprising a step of adsorption on a zeolite and of desorption by a desorption agent delivering an extract and a raffinate and a step of crystallisation of the extract. A water supply is introduced in the feedstock, in the desorption agent and/or in a flux recycling current in the column, such that the weighted mean of water contents measured in the extract and in the raffinate ranges between 1 and 200 ppm. The S/F ratio of the flow rate of the desorption agent over that of the feedstock during the step of adsorption and desorption ranges between 0.6 and 2.5.

Description

METHOD FOR SEPARATING PARAXYLENE COMPRISING ADSORPTION WITH WATER INJECTION AND CRYSTALLIZATION DESCRIPTION OF THE INVENTION The invention relates to a process for separating para-xylene from a mixture of aromatic isomers of 8 carbon atoms containing para-xylene, with improved productivity and reduced operating costs. This is applicable for the preparation of para-xylene of very high purity for the synthesis of terephthalic acid-, an intermediate in the industry of the manufacture of nylon. The prior art is described in French patent application FR-A-2 089 639. United States Patent US-A-5 401 476, which is incorporated herein by reference, discloses a combination of simulated countercurrent adsorption separation and crystallization to produce high purity para-xylene in a more economical manner than in a single step. The basic principles of this combination are as follows: REF .: 30577 • The separation stage in simulated moving bed is controlled in a different way than that for the direct production of high purity paraxylene. The feed flow rate is such that the flow rates in zone 2 and zone 3 (feed flow velocity = flow velocity in zone 2 - flow velocity in zone 3) can not simultaneously produce an extract containing pure para-xylene and a refined liquid that is free of para-xylene. The feed flow velocity (and thus productivity) is increased by reducing the flow velocity in zone 2. It is thus impossible to obtain pure paraxylene (+ 98%). In addition, the solvent flow rate is also reduced with respect to a "high purity" system - in particular by increasing the flow velocity in zone 4 above a threshold. The increase in productivity and the reduction in operating costs combined with the use of less solvent is for the detriment of purity. • In the crystallization step, a mixture containing 75% to 98% para-xylene, more advantageously 85% to 95% para-xylene, is treated to reuse the pre-existing crystallizers in units based on crystallization schemes in two main stages. A crystallization temperature which is preferably from -15 ° C to + 15 ° C is used so as not to consume frigorias in a low level of heat. In the most favorable cases, there is an additional synergy in using the same solvent as the desorbent in the adsorption stage and as a rinsing solvent for the crystals in the crystallization stage, which is different to the case for the direct production of -xylene only of high purity. However, when the adsorption stage of the feed is carried out on the adsorbent with a high ratio of desorbent to feed, for example greater than 1.3, a very high purity of the desired product is obtained, generally greater than 95%. The higher the purity of the product, the more it can fluctuate. It is thus easy to imagine that when the crystallization purification step, which can produce more than 99% purity, is calculated for a speed of a fixed purity that does not fluctuate, the operation of the crystallizer is disturbed.

US-A-3 734 974 discloses the use of X or Y zeolites exchanged with cations from group IA and / or group IIA (or a combination of the 2) with a controlled amount of water in zeolite (1% to 5%) for the separation of para-xylene in a simulated moving bed (which is presumed to be of high purity in the absence of any mention to para-xylene of low purity). The desorbents used are, for example, toluene, or para-diethylbenzene or diethylbenzenes as a mixture or a mixture of these constituents with a paraffinic cut. Adding water, in particular to the K BaX and BaX zeolites, causes a very significant improvement in the selectivities of para-xylene-ethylene-benzene and para-xylene-meta-xylene. US-A-4 778 946 describes the use of KY zeolite containing up to 10% water and up to 4% of either methanol or ammonia for the separation of ethylbenzene and meta-xylene from feeds that are free of para-xylene, not only to maximize the selectivity of ethylbenzene-meta-xylene, but also to obtain a desorbent-ethylbenzene selectivity as close as possible to it. The used ratio of desorbent / feed is 2/1. This document specifies that a desorbent which is too strongly adsorbed does not allow a good separation and a desorbent which is too weakly adsorbed causes too much demand for the desorbent. This recommends that the selectivity of ethylbenzene-meta-xylene should be at least 3.0 and that the selectivity of ethylbenzene-desorbent be in the range of 1 to 2. US-A-5 401 476 does not suggest operating in the presence of water, since that the hydrocarbons are anhydrous. US-A-3 734 974 does not suggest the possible importance of injecting water into a system where high purity para-xylene is not desired and does not recognize the role that water could play in the selectivity of para-xylene-desorbent. US-A-4 778 946 discloses that water can modify the selectivity of the ethylbenzene desorbent only in the case of feeds that are free of para-xylene. Neither US-A-3 734 974 nor US-A-4 778 946 suggest a simple and practical means not to control the water content in the zeolite but rather to pretend to control the water content in the hydrocarbons that are in contact with it. the zeolite.

The invention is directed to overcoming the disadvantages of the prior art. More precisely, the first object of the invention is to carry out a process for separating high purity para-xylene by combining an adsorption step of the simulated moving bed and a crystallization step, wherein the simulated moving bed adsorption stage includes in a reduced ratio of solvent to feed due to the continuous injection of water into the streams that supply the column or columns of adsorption. The second object of the invention is to favorably change the selectivity of para-xylene-desorbent to obtain a maximum value that can reduce the demand of desorbent. The skilled person will be surprised that injecting water does not increase the purity in a constant yield and productivity neither increases the yield in a fixed purity and productivity, nor increases the productivity in a constant yield and purity, but reduces the demand of desorbent in a purity and constant productivity. A third object of the invention is to optimize the amount of water in the hydrocarbon effluents as a function of the nature of the adsorbent zeolite and the compensation cations and as a function of the temperature. Introducing an adequate amount of water into the adsorption columns by means of the incoming streams, combined with a limited amount of desorbent with respect to the amount of feed produces very good results. More precisely, the invention relates to a process for separating para-xylene from a feed comprising a mixture of aromatic isomers of 8 carbon atoms containing paraxylene, comprising a step for the adsorption and desorption of isomers in the mixture in minus a column containing a zeolite, the adsorption and desorption stage supplies, under suitable conditions, at least one desorbent, a fraction that is rich in para-xylene and a fraction that is depleted in para-xylene, the process further comprising At least one step to crystallize the fraction that is rich in para-xylene that supplies pure para-xylene, the process is characterized in that a stream of water is introduced into the feed and / or the desorbent and / or into a recirculation stream for a flow in the column, in such a way that the weighted average of the water contents measured in the fraction that is rich in paraxylene and in the fraction that is The paraxylene batch is in the range of 1 to 250 ppm, advantageously in the range of 3 to 120 ppm (parts per million), and because the S / F ratio of the flow rate of the desorbent to that of the feed during the stage of adsorption and desorption is in the range of 0.6 to 2.5, advantageously in the range of 0.8 to 1.5, and preferably in the range of 1 to 1.35. The optimum results resulting from the above combination have been determined as a function of the nature of the zeolite, its compensation cations and the operating temperature. Thus, in a first implementation of the process, the adsorption and desorption stage is carried out at a temperature of 140 ° C to 160 ° C with a Y zeolite exchanged with barium and potassium, the weighted average of the water contents. it is in the range of 3 to 6 ppm and the ratio of S / F is in the range of 1.15 to 1.35. In a second implementation of the process, the adsorption and desorption stage is carried out at a temperature of 165 ° C to 185 ° C with a Y zeolite exchanged with barium and potassium, the weighted average of the water contents is in the range from 6 to 12 ppm and the S / F ratio is in the range of 1.10 to 1.35. In a third implementation of the process, the adsorption and desorption stage is carried out at a temperature of 140 ° C to 160 ° C with a barium exchanged with zeolite X, the weighted average of the water contents is in the range of 45 ° C. at 70 ppm and the S / F ratio is in the range of 1 to 1.25. In a fourth implementation of the process, the adsorption and desorption stage is carried out at a temperature of 165 ° C to 185 ° C with a zeolite X exchanged with barium, the weighted average of the water contents is in the range of 60 to 130 ppm, preferably in the range of 90 to 110 ppm, and the S / F ratio is in the range of 0.95 to 1.2. In a fifth implementation of the process, the adsorption and desorption stage is carried out at a temperature of 140 ° C to 160 ° C with a Y or X zeolite exchanged with potassium, the weighted average of the water contents is in the range from 5 to 10 ppm and the S / F ratio is in the range of 1.2 to 1.4.

In a sixth implementation of the process, the adsorption and desorption stage is carried out at a temperature of 165 ° C to 185 ° C with a zeolite X exchanged with potassium, the weighted average of the water contents is in the range of 10. at 20 ppm and the S / F ratio is in the range of 1.2 to 1.4. In a seventh implementation of the process,. the adsorption and desorption stage is carried out at a temperature of 110 ° C to 130 ° C with a zeolite X exchanged with barium, the weighted average of the water contents is in the range of 20 to 30 ppm and the ratio S / F is in the range of 1.2 to 1.4. Clearly, the fraction that is rich in paraxylene and the fraction that is depleted in paraxylene can be distilled to free them from the desorbent. The fraction that is depleted in solvent and free of desorbent, possibly containing a minimum amount of water, can be isomerized using known isomerization processes, and isomerized material that is rich in para-xylene, possibly free of light compounds, can be recycled at least in part to the adsorption and desorption zone.

The fraction that is rich in para-xylene can be crystallized, usually at high temperature, for example at more than -30 ° C according to US-A-5 401 476 and International application WO 96-20 908, incorporated in the present by reference, which describe the stages of crystallization at one or more temperatures. The solvent that is generally used is toluene or para-diethylbenzene, for example. This can be recycled at least in part to the adsorption and desorption stage, in a substantially anhydrous state. It may be advantageous to introduce methanol, in a proportion that is generally below 500 ppm, in the fraction that is para-xylene rich and free of desorbent, but that contains a minimum amount of water, which is proposed for crystallization. The crystallization stage thus supplies a mother liquor containing water and methanol which is removed, for example by adsorption, and the mother liquor which is thus free of water and methanol is recycled at least in part to the adsorption and desorption stage. The amounts of water in the hydrocarbon phases are, of course, connected to the amounts of water in the zeolite by an adsorption isotherm which substantially does not depend on the nature of the adsorbed hydrocarbon. As indicated by the prior art, a distinction must be made between the water adsorbed on the zeolite which is measured by a loss of ignition at 400 ° C as measured in the prior art by a loss of ignition at 500 ° C (called "free base"). relative volatile ") and between water that is much more strongly retained, which is measured by a loss of ignition at 900 ° C or 1000 ° C. During the measurements at 900 ° C or 1000 ° C, the structure of the zeolite is destroyed and the difference in water desorbed between 400 ° C and 1000 ° C can be considered to represent the constitution water. The difference between these two terms is of the order of 1.5% to 2% by weight in the faujasites. The water adsorption isotherm is measured as follows: a batch of solid to be tested is allowed to hydrate in the ambient atmosphere. A plurality of columns each 1 meter in length and one centimeter in diameter (78.5 cm3) is filled with this zeolite and the columns are placed in an oven at 250 ° C in a stream of very dry nitrogen (less than 10 ppm Water) . Each column is allowed to dehydrate for different times to obtain different water contents (which are measured by weight). Each column is then placed in a closed circuit comprising a dry hydrocarbon reserve (minimum volume), a positive displacement piston pump for liquid chromatography and the column. The hydrocarbon reserve is provided with a sample outlet to measure the water content in the hydrocarbon once equilibrium has been reached. The total amount of hydrocarbon is 100 cm3. In this way, the amount of water contained in the hydrocarbon is insignificant with respect to that which remains retained in the zeolite. The adsorption isotherms presented in Figures 1 and 2 are thus obtained, which are respectively related to the KBaY and BaX zeolites measured by the loss of ignition at 400 ° C which represents the amount of water (%) adsorbed by the zeolites as a function of the concentration (ppm) of the water in the hydrocarbon phase at different temperatures. The influence of the water content on the adsorption separation is fully studied independently of the isotherm measurements. In this way, the need for cumulative balances in the water inlets and outlets in the Separation unit is prevented, where accumulated errors are often very large. The separation unit is filled with zeolite, which has been activated in a rotary kiln at the time of the final phase of its manufacture. Depending on the manufacturing lot, the ignition loss at 400 ° C in the zeolite before loading is in the range of 2% to 5%. During loading, the adsorbent is partially rehydrated to an undetermined level (always less than 7%). When anhydrous conditions are required, dry desorbent is passed over the zeolite until only water contents of less than 1 ppm are measured in effluents from the unit. This operation requires a long time (2 to 3 weeks). In contrast, when a known water content is required, two controlled streams of desorbent are injected, one of anhydrous product, the other of product saturated with water, and a controlled flow of anhydrous feed is injected (the opposite can also be done). As an example, if the average water content is going to be 50 ppm in the streams entering the unit with respective flow rates of 5 cm3 / minute of feed and 7 c-mVminute of desorbent, 5 cm3 / minute of anhydrous feed, 5.6 cm3 / minute of anhydrous solvent and 1.4 cm3 / minute of solvent that is saturated with water at room temperature (430 ppm in the case of toluene). These conditions are maintained until the weighted average of the water contents in the outlets is substantially 50 ppm; As an example, if the water contents of 43 ppm in an extract flow of 5.3 cm3 / minute and 54 ppm in a refined liquid flow of 6.65 cm3 / minute are obtained respectively, the weighted average in the outputs is 49 ppm , which is considered to be acceptable with the accuracy of the water content measurements in mind. The water contents in the inlet and outlet streams are measured using the KARL FISCHER method for contents above 15 ppm. When these contents are below 15 ppm, online measurements provided by online analytical probes (PA-NAMETRIC apparatus, series 1) are relied upon. The calibration is performed between 15 ppm and 200 ppm, and the extrapolation of this calibration curve to determine the contents in the range of 1 to 15 ppm is considered to be valid. The following examples compare the use of a KBaY-toluene zeolite system with and without water, a KBaY-PDEB system with and without water, a BaX-toluene system with and without water and a BaX-PDEB system with and without Water. In the attached figures: • Figures 1 and 2 represent the amount of water adsorbed by various zeolites as a function of the concentration (ppm) of water in the hydrocarbon phase, at different temperatures; • Figures 3, 4 and 5 show the solvent ratio for constant performances as a function of the weighted average of the water cations (ppm) contained in the outlets (extract and refined liquid) at different temperatures; • Figures 6, 7 and 8 show a performance index as a function of the weighted average water content at the outputs, expressed in ppm; and • Figure 9 represents a performance index for various desorbents as a function of the ratio of desorbent to feed (S / F).

EXAMPLE 1 A continuous liquid chromatography pilot unit was produced from 24 columns, 1 m long and 1 cm in diameter, in series, the circulation between the 24th and the first column was effected by means of a recirculation pump . Either a feed to be separated or a solvent could be injected into each inter-column connection. Either a refined liquid or an extract could also be removed. This unit has been described in "Preparative and production scale chromatography processes with applications" edited by G. Barker, G. Ganestos, in the chapter "From batch elution to simulated counter current chromatography" by B. Balannec and G. Hotier (published by Marcel Dekker Inc., New York, 1992).

The adsorbent is constituted by zeolite Y exchanged for potassium and barium, the degree of exchange, expressed in normalities, was approximately 50% for each of the two cations. The zeolite was in the form of spherules of 0.315 to 0.5 mm in diameter. The assembly of the columns and the distribution valves was placed in an oven at 150 ° C.

Following the principle of simulated counter-current chromatography, solvent injection, extract removal, feed injection and removal of the refined liquid were advanced by three columns every six minutes in a direction that was co-current with the circulation of the liquid. According to the invention, the number of beds to be considered was thus eight. There were six columns (ie two beds) between the solvent injection and the withdrawal of the extract, nine columns (three beds) between the removal of the extract and the injection of the feed, three columns (one bed) between the injection of the feed and the removal of the refined liquid and the last six columns (two beds) were between the removal of the refined liquid and the injection of solvent. The performances of the simulated mobile bed unit were observed as a function of the weighted average of the water contents in the extract and the refined liquid, the water having been continuously introduced into the columns by means of anhydrous desorbent and saturated desorbent as described in the above. For the point on the curve in Figure 3, the conditions of Example 1 of US-A-5 401 476 were repeated. 7.2 cm 3 / minute of toluene and 5 cm 3 / minute of feed were injected continuously (ambient conditions). 5.40 cm3 / minute of extract and 6.74 cm3 / inute of refined liquid were removed, also continuously; The loss was approximately 5%. During the first cycle period, eight in total, solvent was injected into column 1, extract was withdrawn from the outlet of column 6, feed was injected into column 15, and refined liquid was withdrawn from the outlet of column 18 During the first two periods of the cycle, the flow velocity of the recirculation pump was 38.7 cm3 / minute at room temperature; during the third period, the flow rate was 45.5 cm3 / minute; during the next three periods this was 40.5 cm3 / minute and during the last two periods the flow rate was 45.9 cm3 / minute. The average recirculation flow rate was thus 42 cm3 / minute. Para-xylene was obtained with a purity of 92.2% and the recovery ratio was 98.1%. The pressure decreased approximately linearly from 30.57 kg / cm2 to 5.1 kg / cm2 (30 bar to 5 bar). The following table shows the balance of the unit in the steady state: A first mode of operating the unit consisted of reducing the flow rate of desorbent (toluene) while maintaining the flow velocity of the constant feed. Purity and performance had to be kept approximately constant; this was achieved by adjusting the refined extract-liquid balance, the flow rates in zones 2 and 3 and the permutation periods remained constant, while the flow rates in zones 1 and 4 were adjusted by increasing the flow velocity in the zone 4 and reducing the flow velocity in zone 1. This mode of operation approached the type of operation in an industrial unit, however it was difficult to obtain the points for iso-purity and iso-performance. Variations on the order of 1% in purity and yield were tolerated, provided that the geometric mean of purity and yield did not vary by more than 0. 3 % . Figure 3 shows that the flow velocity of the solvent was minimized at 150 ° C for an average water content of 5 ppm; the ratio of solvent to feed was 1.27 / 1 while that for an anhydrous solvent (as in US-A-5 401 476) was 1.44 / 1.

EXAMPLE 2 (FIGURE 4) The experiment was repeated, increasing the operating temperature from 150 ° C to 175 ° C. The efficiencies of purity and yield remained identical to the experiments at 150 ° C for solvent ratios that were practically identical for the anhydrous zeolite and for the optimum water content. However, Figure 4 shows that the optimum water content was not 5 ppm but 10 ppm. This corresponded to an optimum content of water in the zeolite from 0.4% to 0.5% (loss by ignition at 400 ° C), which was impossible to measure directly with a sufficiently high precision.

EXAMPLE 3 (FIGURE 5) The experiment was repeated at 175 ° C with para-diethylbenzene with 98% purity as the desorbent (the impurities consisted mainly of meta- and ortho-diethylbenzene). Figure 5 shows that the solvent ratio could be reduced to approximately 0.1 / 1 and the optimum efficiency was again in 10 ppm of water in the hydrocarbon. This gain in the solvent ratio could be related to the selectivity of PX-desorbent, which was 0.55 for toluene and 0.72 for para-diethylbenzene.

EXAMPLE 4 (FIGURE 6) The same unit was loaded with BaX zeolite in the form of spherules from 0.3 mm to 0.8 mm containing 22% clay-based binder. The residual sodium content after the exchange was less than 3% of the cations (expressed as normal). Only 12 columns were used instead of 24 and the permutations were carried out on all columns instead of every three columns as in the previous. There were three beds in each area. The permutation period was one minute, the ratio between the average recirculation rate and the feed was 4.5 / 1, the solvent ratio remained constant at 1.25 / 1, the ratio of extract to refined liquid was 0.40 / 1 and the flow velocity of the feed was 8 cm3 / minute. Productivity was much higher compared to the previous case due to a reduction in ignition loss. The unit was operated at 175 ° C and the desorbent was para-diethylbenzene. A performance index can be defined: IP = (% yield) x (% purity). The flow velocities in the zones were adjusted slightly to retain a reasonable balance between purity and performance (no more than 6% apart). Figure 6 shows that the performance index for the BaX zeolite was a maximum for a water content that was approximately ten times higher than with the KBaY zeolite; on the other hand, the performances of the BaX zeolite were much lower than those for the KBaY zeolite when these products were anhydrous, while they were substantially higher in the optimal water content. This had never been described in the prior art.

EXAMPLE 5 (FIGURE 7) The same experiment was repeated at 150 ° C.

Apart from a considerably greater loss by ignition (increase of approximately 70%), minor performances were observed together with a different optimal water content in the hydrocarbon (from 50 to 60 ppm (see Figure 7)).

EXAMPLE 6 (FIGURE 8) Examples 4 and 5 were repeated except that the solvent was toluene and the temperature was 120 ° C: this time a maximum performance was obtained for a water content in the hydrocarbon (weighted average of the outputs) of about 25 ppm, the ratio of extract to refined liquid was about 0.7 / 1 (see Figure 8).

EXAMPLE 7 (FIGURE 9) Example 4 was repeated, this time fixing the water content at the optimum, that is, 90 ppm, and the solvent / feed ratio was varied. The feed contained only 4% ethylbenzene for 22.5% para-xylene. It also contained 23.5% ortho-xylene, 1.5% toluene and 48.5% meta-xylene. Since this feeding was easier to treat, the injected flow rate could be higher, i.e. 10 cm3 / minute. The recirculation to feed ratio was reduced to 4.4 / 1 and the permutation period was reduced to 50 seconds. It was observed that above a ratio of solvent to feed of 1.45 / 1, the performances remained identical. By reducing the desorbent flow rate to 8 cm3 / minute, it was realized that it was possible to obtain again purities of more than 75% for solvent ratios of less than 1, sufficient for purification by crystallization, which constitutes the second stage of the process (see Figure 9). When the PDEB desorbent was replaced with toluene, the performances were slightly poorer and the flow rates were different (the flow rates in zone 1 and zone 4 were larger and the ratio of extract to refined liquid had to be increased). The importance of using toluene was to be able to treat feeds containing large amounts of aromatic compounds of 9 carbon atoms. Depending on whether toluene or para-diethylbenzene was used, the water contained in the extract had to be removed before sending the low purity para-xylene to the crystallization stage. When the desorbent was toluene, impure para-xylene (substantially free of water) was removed from the bottom of the extract column. Liquid water was removed from the top of the columns to distill extract and refined liquid, by means of the decanting plate, and constituents that were lighter than toluene were continuously or periodically purged. From about 3 to 10 plates below, substantially anhydrous toluene was removed from a removal plate. When the desorbent was para-diethylbenzene and the feed that was to be treated contained little toluene, it was possible to operate in this manner (water and toluene were removed from the top of the distillation column in this case) and para-xylene was removed impure substantially anhydrous approximately 10 plates below. It was also possible not to use this pasteurization zone, to cool the para-xylene, to decant the water at approximately 10 ° C, then to send the paraxylene saturated with water to the crystallization stage. In this case, if the crystallization was carried out at a temperature below 0 ° C, methanol was generally injected to prevent an accumulation of ice in the crystallization equipment. Before re-injecting the mother liquor from the crystallization stage to the adsorption step, the water and methanol had to be removed by adsorption or by distillation. With respect to the refined liquid that had been released from solvent, it could contain up to 200 ppm of water without any harm to the isomerization catalyst. 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.

Claims (12)

CLAIMS Having described the invention as above, the content of the following claims is claimed as property:

1. A process for separating para-xylene from a feed comprising a mixture of aromatic isomers of 8 carbon atoms containing para-xylene, comprising a step for the adsorption and desorption of isomers in the mixture in at least one column containing a zeolite, the adsorption and desorption stage supplies, under suitable conditions, at least one desorbent, a fraction that is rich in paraxylene and a fraction that is depleted in para-xylene, the process also comprising at least one step to crystallize the fraction which is rich in para-xylene which supplies pure para-xylene, the process is characterized in that a stream of water is introduced into the feed, in the desorbent and / or in a recirculation stream for a flow in the column, in such a way that the weighted average of the water contents measured in the fraction that is rich in para-xylene and in the fraction that is impoverished in para-xylene is in the range from 1 to 250 ppm, advantageously in the range from 3 to 120 ppm (parts per million), and because the S / F ratio of the flow rate of the desorbent to that of the feed during the adsorption and desorption stage is in the range of 0.6 to 2.5, advantageously in the range of 0.8 to 1.5, and preferably in the range of 1 to 1.35.

A process according to claim 1, characterized in that the adsorption and desorption stage is carried out at a temperature of 140 ° C to 160 ° C with a Y zeolite exchanged with barium and potassium, the weighted average of the water contents it is in the range of 3 to 6 ppm and the S / F ratio is in the range of 1.15 to 1.35.

A process according to claim 1, characterized in that the adsorption and desorption stage is carried out at a temperature of 165 ° C to 185 ° C with a Y zeolite exchanged with barium and potassium, the weighted average of the water contents it is in the range of 6 to 12 ppm and the S / F ratio is in the range of 1.10 to 1.35.

A process according to claim 1, characterized in that the adsorption and desorption stage is carried out at a temperature of 140 ° C to 160 ° C with a zeolite X exchanged with barium, the weighted average of the water contents is in the range of 45 to 70 ppm and the S / F ratio is in the range of 1 to 1.2.

A process according to claim 1, characterized in that the adsorption and desorption stage is carried out at a temperature of 165 ° C to 185 ° C with a zeolite X exchanged with barium, the weighted average of the water contents is in the range of 60 to 130 ppm, preferably in the range of 90 to 110 ppm, and the S / F ratio is in the range of 0.95 to 1.2.

A process according to claim 1, characterized in that the adsorption and desorption stage is carried out at a temperature of 140 ° C to 160 ° C with a Y or X zeolite exchanged with potassium, the weighted average of the water contents it is in the range of 5 to 10 ppm and the S / F ratio is in the range of 1.2 to 1.4.

A process according to claim 1, characterized in that the step of adsorption and desorption is carried out at a temperature of 165 ° C to 185 ° C with a zeolite X exchanged with potassium, the weighted average of the water contents is in the range of 10 to 20 ppm and the S / F ratio is in the range of 1.2 to 1.4.

A process according to claim 1, characterized in that the adsorption and desorption stage is carried out at a temperature of 110 ° C to 130 ° C with a zeolite X exchanged with barium, the weighted average of the water contents is in the range of 20 to 30 ppm and the S / F ratio is in the range of 1.2 to 1.4.

9. A process according to any of claims 1 to 8, characterized in that the fraction that is rich in para-xylene and the fraction that is depleted in para-xylene of the adsorption and desorption stage are distilled to free them from the desorbent, the fraction which is rich in para-xylene and substantially free of desorbent undergoes the crystallization step.

10. A process according to claim 9, characterized in that the desorbent is toluene or para-diethylbenzene, and is recycled at least in part to the adsorption and desorption step in a substantially anhydrous state.

11. A process according to claim 9 or claim 10, characterized in that methanol is introduced into the fraction that is rich in para-xylene free of desorbent but that contains a minimum amount of water, before the crystallization step.

12. A process according to claim 11, characterized in that the crystallization step supplies a mother liquor containing water and methanol, and in which the water and the methanol are removed from the mother liquor before being recycled at least in part to the step of adsorption and desorption. A process according to any of claims 9 to 12, characterized in that the fraction that is impoverished in para-xylene and free of desorbent is isomerized.