HYBRID PROCESS FOR PRODUCING HIGH-PURITY PARA-XYLENE WITH TOLUENE SOLVENT

Abstract

Provided is a hybrid process for producing high-purity para-xylene from a feedstock of aromatic hydrocarbon isomer fractions having 8 carbon atoms, in a liquid phase. The process includes a liquid chromatography separation step and a crystallization step of the para-xylene from the purified stream of para-xylene obtained at the separation step.

Claims

1. Hybrid process for producing high-purity para-xylene from a feedstock of aromatic hydrocarbon isomer fractions having 8 carbon atoms, in a liquid phase, comprising: a) a liquid chromatography separation step via simulated counter-current adsorption of para-xylene using a zeolitic adsorbent and a desorption solvent, said adsorbent comprising: at least one majority zeolite of faujasite type having a lattice parameter a higher than 25.100 , barium such that the content of barium oxide BaO is comprised between 30% and 41% by weight, limits included, relative to the total weight of the adsorbent, to obtain a stream of purified para-xylene; b) a crystallization step of the para-xylene from the purified stream of para-xylene obtained at the separation step, at a temperature between 0 and -25 C., followed by washing of the crystals with a washing solvent, to obtain high-purity para-xylene, wherein the desorption solvent at the separation step and the washing solvent at the crystallization step comprises toluene.

2. The process for producing para-xylene according to claim 1, wherein the adsorbent comprises an LSX zeolite.

3. The process for producing para-xylene according to claim 2, wherein the outer surface area of said zeolitic adsorbent, measured by nitrogen adsorption, is less than 100 m.sup.2.Math.g.sup.1.

4. The process for producing para-xylene according to claim 1, wherein the adsorbent comprises a mixture of zeolites of faujasite type.

5. The process according to claim 4, wherein the adsorbent comprises a mixture of X-type zeolite and LSX zeolite, the LSX zeolite being in majority.

6. The process for producing para-xylene according to claim 1, wherein the content of barium oxide is between 33 and 41%, limits included.

7. The process for producing para-xylene according to claim 6, wherein the content of barium oxide is between 33 and 38%, limits included.

8. The process according to claim 1, wherein the temperature of the crystallization step is between 5 C. and 15 C.

9. The process according to claim 1, wherein the wash ratio of the crystals is between 0.8 and 2 volumes of toluene per volume of para-xylene crystals.

10. The process according to claim 1, wherein the number of beds at the adsorption separation step is between 4 and 24.

11. The process according to claim 10 wherein the number of beds at the adsorption separation step is between 8 and 12.

12. The process according to claim 1, wherein the adsorption separation step is conducted with a number of beds comprised between 4 and 24, a number of zones of at least 4, a temperature between 100 C. and 250 C., pressure between toluene bubble point pressure at processing temperature and 3 MPa, a cycle time corresponding to the time interval between two injections of desorbent into a given bed of between 4 and 18 min, a ratio of desorbent flow rate to feed flow rate of 0.7 to 2.5, and a recycle ratio comprised between 2 and 12.

13. The process according to claim 12, wherein the adsorption separation step is conducted at a temperature compriscd between 150 C. and 180 C.

14. The process according to claim 12, wherein the adsorption separation step is conducted with a recycle ratio of between 2.5 and 4.

15. The process according to claim 12, wherein the adsorbent is prepared according to the following steps: a) mixing the faujasite zeolite crystals in powder form of desired particle size, in the presence of water, with at least one binder containing a clay or mixture of clays comprising at least 80% by weight of zeolitizable clay and optionally a silica source; b) forming the mixture obtained at a) to produce aggregates, followed by drying and optionally a screening and/or cyclonic separation step; c) calcining the aggregates obtained at b) at a temperature preferably in the range of 500 C. to 600 C.; d) optional zeolitization of the binder by contacting the calcined aggregates obtained at step c) with an alkaline basic aqueous solution followed by washing; e) ion exchange of the zeolitic aggregates containing LSX zeolite or X and LSX zeolite obtained at c) or d) with barium ions, followed by washing and drying of the product thus treated; f) heat activation of the aggregates exchanged at step e).

Description

LIST OF FIGURES

[0108] FIG. 1A: Diffractogram obtained with X-ray diffraction on adsorbents C (comparative) and F (conforming to the invention) with zoom on the split peak at 35.

[0109] FIG. 1B: Diffractogram obtained with X-ray diffraction on adsorbents C (comparative) and F (conforming to the invention) and zoom on the split peak at 35 (offset).

[0110] FIG. 2: Diffractogram obtained with X-ray diffraction on adsorbent G (conforming to the invention).

[0111] Appended hereto, Table 4 lists the peaks and the intensity thereof for each of the diffractograms in FIGS. 1A, 1B and 2.

[0112] FIGS. 3A and 3B: Breakthrough curves of each compound present in the effluent sampled at the outlet of the chromatographic column, showing changes in the concentrations of each compound as a function of eluted volume, at the breakthrough test (feed injection step) and at the reverse breakthrough test (injection of desorbent), for adsorbent A (BaX).

[0113] FIG. 4: Graphical representation of the R.sub.Des/Feed ratio of the different adsorbents tested in Examples 1 and 2, as a function of the lattice parameter of the majority zeolite.

EXAMPLES

Example 1 (Comparative)

Chromatography Column Separation Test with Prior Art Adsorbents

[0114] Separation tests were conducted using a chromatography column in which the zeolitic solid was packed.

[0115] The tested solids were: [0116] A: Adsorbent prepared according to Example 4 in patent FR2903987, containing X-type zeolite exchanged with barium, with measured lattice parameter of 25.025 ; [0117] B: Y-type zeolite adsorbent exchanged both with barium (45 to 65% of sites) and with potassium (35 to 55% of sites) such as described in patent FR2681066 and such as defined in U.S. Pat. No. 3,558,730, with measured lattice parameter of 24.712 . [0118] C: Adsorbent containing a mixture of X-LSX, X being in majority, prepared with a crystal molar ratio LSX/X of 0.54, exchanged with barium according to Example 5 in FR2925367, the measured lattice parameters being 25.034 for the X-type zeolite and 25.226 for the LSX zeolite. The majority zeolite is therefore the X-type zeolite having a lattice parameter smaller than 25.100 . [0119] The temperature for the tests is 163 C. corresponding to the temperature routinely recommended for paraxylene separation via hybrid process combining adsorption and crystallization.

[0120] The conducted tests are tests of breakthrough/reverse breakthrough type with feed containing 10% volume of iso-octane (iC8) used as tracer, 45% volume of paraxylene (PX) and 45% volume of metaxylene (MX), with a desorption solvent (desorbent) that is toluene. [0121] the breakthrough test corresponds to the feed injection step into the column initially saturated with desorbent; [0122] the reverse breakthrough test corresponds to the desorbent injection step and is performed at the end of the preceding feed injection step.

[0123] The curves shown in FIGS. 3A and 3B correspond to the breakthrough curves of each compound present in the effluent sampled at the outlet of the chromatography column; they represent the changes in concentrations of each compound as a function of eluted volume at the breakthrough test (feed injection step) and at the reverse breakthrough test (injection of desorbent), for adsorbent A (BaX).

[0124] The feed injection step is completed when the concentration of desorbent in the effluent sampled at the outlet of the chromatography column becomes zero i.e. when the composition of the sampled effluent is identical to the composition of the injected feed: the feed volume injected at this point is denoted Vfeed equilibrium and translates the amount of feed that needs to be injected for full saturation of the pore volumes in the column, namely all the non-selective volumes (inter-grain volume and macro and mesopore intra-grain volume) and selective volume (micropore volume of adsorbent).

[0125] The iso-octane (iC8) contained in 10% volume in the feed was used as tracer of non-selective volumes (inter-grain volume and macro and mesopore intra-grain volume): measurement of the first instant of the iC8 breakthrough curve therefore gives the estimation of the non-selective volume denoted Vnon selective.

[0126] Therefore, by subtracting this non-selective volume from the feed injection volume at equilibrium Vfeed_equilibrium, the necessary feed volume to be injected is obtained in order to saturate the micropore volume V of the adsorbent, namely (Vfeed_equilibriumVnon_selective).

[0127] Therefore, by multiplying the volume (Vfeed_equilibriumVnon-selective) by the fraction xpx of PX contained in the feed, the volume of paraxylene PX to be injected is obtained for the breakthrough test in order to reach adsorption equilibrium in the micropore volume.

[0128] Selectivity .sub.PX/MX is the ratio of the adsorbed quantities of PX (paraxylene) and MX (metaxylene) divided by the ratio of PX and MX concentrations in the feed x.sub.PX and x.sub.MX. The adsorbed quantities of PX and MX are obtained starting from the first instants of the PX and MX curves denoted V.sub.R_PX and V.sub.R_NX, and from the first instant of the tracer curve giving the non-selective volume denoted Vnon_selective. Therefore, selectivity .sub.PX/MX is calculated with the following equation:

[00001]${}_{\mathrm{PX}/\mathrm{MX}}=\frac{\left(V_{\mathrm{R\_PX}}-V_{\mathrm{non\_selective}}\right).Math.x_{\mathrm{MX}}}{\left(V_{\mathrm{R\_PX}}-V_{\mathrm{non\_selective}}\right).Math.x_{\mathrm{PX}}}$

[0129] Similarly, the desorbent injection step is completed when the concentration of desorbent in the effluent sampled at the outlet of the chromatography column becomes 100%; the volume of desorbent injected at this stage is denoted Vdesorbent_equilibrium and translates the quantity of desorbent that needs to be injected for full regeneration of the pore volumes in the column, namely all the non-selective volumes (inter-grain volume and macro and mesopore intra-grain volume) and the selective volume (micropore volume of the adsorbent).

[0130] Measurement of the first instant of the iC8 reverse breakthrough curve gives the estimation of the non-selective volume denoted Vnon_selective.

[0131] By subtracting this non-selective volume from the desorbent injection volume at equilibrium Vdesorbent_equilibrium, the volume of desorbent to be injected is obtained that is needed for full regeneration of the micropore volume of adsorbent, namely (Vdesorbent_equilibriumVnon_selective).

[0132] FIGS. 3A and 3B show that for comparative adsorbent A, the volume of desorbent at equilibrium is greater that the volume of feed at equilibrium.

[0133] From the breakthrough/reverse breakthrough tests, the ratio R.sub.Des/Feed calculated by:

R.sub.Des/Feed=(Vdesorbent_equilibriumVnon_selective)/(Vfeed equilibriumVnon_selective)

[0134] must represent a process similar to the ratio D/F used for separation of paraxylene in a simulated moving bed i.e. the ratio between the desorbent flow rate and the feed flow rate. This parameter translates the quantity of desorbent needed for complete desorbing of the compounds in the feed previously adsorbed in the micropore volume of the adsorbent.

[0135] The volume ratios R.sub.Des/Feed calculated from the concentration curves of the different compounds for the different tested adsorbents are given in following Table 1:

TABLE-US-00001 TABLE 1 Results of breakthrough tests on adsorbents A, B, C in terms of PX/MX selectivity and desorbent/feed volume ratio. Comparison of zeolite peaks Si/Al of BaO Measured with lattice final content lattice parameter >25.100 Comparative product (wt %) parameter and <25.100 .sub.PX/MX R.sub.Des/Feed A 1.25 35.2 25.025 3.33 1.13 B 2.3 10.8 24.712 2.55 1.29 C 1.16 36.7 25.034 (X) X Peak surface 3.65 1.03 and area > LSX 25.226 (LSX) Peak surface area

Example 2 (of the Invention)

[0136] Tests such as described in Example 1 are conducted on zeolite solids of the invention. [0137] D: Adsorbent prepared according to Example 6 in patent FR2925366 containing LSX zeolite exchanged with barium, with measured lattice parameter of 25.200 . [0138] E: Adsorbent comprising at least one FAU-structure zeolite of LSX type comprising barium, with outer surface area measured by nitrogen adsorption of 64 m.sup.2.Math.g.sup.1, prepared according to Example 3 in patent application FR3028430 with measured lattice parameter of 25.204 . [0139] F: Adsorbent containing a mixture of X and LSX zeolites (35%-65%), LSX in majority, prepared with a crystal molar ratio LSX/X of 1.86 and exchanged with barium according to Example 5 in patent FR2925367, with measured lattice parameter of 25.031 (X) and 25.223 (LSX). [0140] G: Adsorbent containing a mixture of X-LSX zeolites, LSX the majority component, prepared with a crystal molar ratio LSX/X of 1, zeolitized and exchanged with barium according to Example 7 of patent FR2925367, with measured lattice parameter of 25.003 (X) and 25.181 (LSX).

TABLE-US-00002 TABLE 2 Results of breakthrough tests on adsorbents D, E, F, G in terms of PX/MX selectivity and desorbent/feed volume ratio. Comparison of zeolite peaks Si/Al of BaO Measured with lattice final content lattice parameter >25.100 Invention product (wt %) parameter and <25.100 .sub.PX/MX R.sub.Des/Feed D 1.03 39.3 25.200 3.98 0.92 E 1.01 33.4 25.204 3.03 0.89 F 1.10 38.3 25.031 (X) LSX Peak surface 3.75 0.93 and area > X Peak 25.223 (LSX) surface area G 1.12 37.4 25.003 (X) LSX Peak surface 3.65 0.98 and 25.181 area > X Peak (LSX) surface area

[0141] The volume ratio R.sub.Des/Feed of the different adsorbents A to G tested in Examples 1 and 2 are graphically illustrated in FIG. 4 as a function of the lattice parameter of the majority zeolite (LSX).

Example 3 (Comparative)

[0142] Breakthrough/reverse breakthrough tests with feed containing 10% by volume of iso-octane used as tracer, 45% by volume of paraxylene PX and 45% by volume of metaxylene MX were conducted on solid D having a lattice parameter conforming to the invention, using a desorbent not conforming to the invention namely para-diethylbenzene (PDEB) and at a temperature of 175 C. routinely recommended when PDEB is used as desorbent.

[0143] Table 3 below gives PX/MX selectivity and the volume of injected desorbent corresponding to Vdesorbent_equilibrium i.e. the volume needed so that the concentration of desorbent in the effluent sampled at the outlet of the chromatography column becomes 100%.

TABLE-US-00003 TABLE 3 PX/MX selectivity and volume of desorbent at equilibrium as a function of the desorbent used. Measured lattice Vdesorbent_equilibrium Adsorbent D parameter .sub.PX/MX (cm3) PDEB at 175 C. 25.200 3.89 130.5 (comparative) TOL at 163 C. 25.200 4.15 105.6 (invention)

[0144] For one same adsorbent comprising an LSX zeolite having a measured lattice parameter of 25.200 and exchanged with barium (adsorbent D), the Vdesorbent_equilibrium volume for the PDEB desorbent at 175 C. is much greater than the Vdesorbent_equilibrium volume for the Toluene desorbent at 163 C. This indicates that the quantity of desorbent needed to desorb the compounds of the feed adsorbed in the micropore volume is much smaller when the toluene solvent is used with adsorbent D.

TABLE-US-00004 TABLE 4 Table of the positions and intensities of the main rays corresponding to the diffractograms in FIGS. 1A, 1B and 2 of adsorbents C, F and G (values of the split peak at 35 shown in bold type). Adsorbent C, Adsorbent F, Adsorbent G, comparative invention invention Angle Intensity Angle Intensity Angle Intensity 2-Theta.sup.0 Count 2-Theta.sup.0 Count 2-Theta.sup.0 Count 6.10 447 6.08 527 6.12 431 11.67 39 11.65 47 11.93 72 12.22 370 12.16 377 12.23 418 14.04 146 14.04 220 14.08 191 14.12 155 15.43 57 15.32 43 15.34 53 18.29 84 18.27 90 18.30 93 18.41 140 18.40 92 18.44 136 23.14 169 23.12 234 23.18 236 23.35 371 23.38 503 23.44 462 23.56 429 23.56 269 23.60 420 24.46 58 24.45 84 24.50 81 26.45 94 26.43 135 26.49 109 26.63 146 26.64 101 26.67 126 27.16 61 27.14 85 27.21 58 27.34 70 27.36 55 27.41 63 29.20 79 29.18 75 29.24 77 30.05 75 30.05 107 30.11 89 30.29 100 30.28 68 30.34 86 30.70 84 30.69 123 30.75 104 30.91 165 30.90 170 30.96 173 31.13 114 31.13 75 31.17 107 31.72 240 31.71 383 31.78 322 31.96 362 31.95 218 32.01 325 32.56 82 32.57 56 32.64 70 33.56 75 33.56 59 33.60 65 33.86 58 33.90 72 33.95 72 34.83 124 34.82 199 34.89 175 35.10 153 35.11 105 35.15 151 36.85 76 37.02 129 37.01 175 37.11 133 37.10 162 37.29 143 37.29 97 37.34 138 40.43 128 40.43 216 40.50 157 40.75 146 40.75 101 40.80 131 42.36 92 42.37 146 42.43 139 42.70 151 42.70 97 42.75 152