Adsorptive separation of para-xylene.

Abstract: Para-xylene is separated from other C₈ isomers and/or C₉ aromatics by contacting the para-xylene-containing feed mixture with an X or Y zeolite adsorbent having Group IA or IIA cations, e.g., barium and/or potassium at exchangeable cationic sites. The para-xylene is selectively adsorbed onto the adsorbent. The feed is then removed from the adsorbent and the para-xylene recovered from the resulting rich adsorbent by desorption with diethyltoluene. The C₉'s and the other xylene isomers are typically recovered in the desorbent-containing raffinate and can be separated from this desorbent by fractionation of the raffinate and the desorbent recycled to the process.

Description: FIELD OF INVENTION The field of art to which the claimed invention pertains is adsorptive hydrocarbon separation. More specifically, the invention relates to a process for separating para-xylene from a feed mixture comprising at least two xylene isomers, including the para-isomer, and also containing one or more C₉ aromatic hydrocarbons, which process employs a zeolitic adsorbent and a particular desorbent. BACKGROUND OF THE INVENTION In numerous processes described in the patent literature, for example U.S. Patents 3,626,020 to Neuzil, 3,663,638 to Neuzil, 3,665,046 to deRosset, 3,668,266 to Chen et al., 3,686,342 to Neuzil et al., 3,700,744 to Berger et al., 3,734,974 to Neuzil, 3,894,109 to Rosback, 3,997,620 to Neuzil and B426,274 to Hedge, particular zeolitic adsorbents are used to separate the para isomer of dialkyl substituted monocyclic aromatics from the other isomers, particularly para-xylene from other xylene isomers. Many of the above patents teach the use of benzene, toluene, or p-diethylbenzene as the desorbent. P-diethyl-benzene (p-DEB) has become a commercial standard for this separation. However, p-DEB is a "heavy" desorbent (higher boiling than p-xylene) which causes problems in a process for adsorptive separation of p-xylene when the feed mixtures also contain C₉ aromatics because the boiling point of p-DEB is too close to the boiling point of C₉ aromatics in the feed and thus separation by simple fractionation is difficult. Because the C₉ aromatics are difficult to separate from p-DEB by simple fractionation, the C₉ aromatics, if allowed in the feed to the adsorptive separation step, would gradually build up in the desorbent, which must be recycled for economic reasons. In the prior art process for recovering p-xylene from feed mixtures containing isomers using p-DEB as the desorbent, therefore, it has been necessary to reduce C₉ aromatics in the feed to below about 0.1 vol.% prior to the adsorptive separation of p-xylenes. This is usually done by distillation in a so-called xylene splitter column. Or course, substantial costs associated with this practice, such as capital costs of the xylene splitter and utilities necessary to achieve substantially complete removal of the C₉ aromatics, could be reduced greatly or eliminated if it were not necessary to first remove C₉ aromatics. Thus, while U.S. Patent 3,686,342, supra, mentions other substituted benzenes as possible heavy desorbents for the para-xylene separation process, that reference clearly states that p-DEB is the best desorbent for the separation and, further, does not address the problem that the preferred desorbents may have in separating feeds containing C₉ aromatics. Therefore, a higher boiling point material, that meets the selectivity requirements for desorbents and can be separated from C₉ aromatics, has been long sought and is still desirable. US Patent 3,663,638 and Japanese Tokkyo Koho 79-38089 (Chemical Abstracts Vol 92 No 180802) refer to the use of toluene, p-diethylbenzene, o-, m- and p-ethyltoluene, 1,3,5-, 1,2,4- and 1,2,3-trimethylbenzene, cumene and propylbenzene as desorbents for p-xylene adsorbed on aluminosilicate material. It is also known that crystalline aluminosilicates or zeolites are used in adsorption separations of various mixtures in the form of agglomerates having high physical strength and attrition resistance. Methods for forming the crystalline powders into such agglomerates include the addition of an inorganic binder, generally a clay comprising a silicon dioxide and aluminum oxide, to the high purity zeolite powder in wet mixture. The blended clay zeolite mixture is extruded into cylindrical type pellets or formed into beads which are subsequently calcined in order to convert the clay to an amorphous binder of considerable mechanical strength. As binders, clays of the kaolin type, water permeable organic polymers or silica are generally used. The invention herein can be practiced in fixed or moving adsorbent bed systems, but the preferred system for this separation is a countercurrent simulated moving bed system, such as described in Broughton U.S. Patent 2,985,589, incorporated herein by reference. Cyclic advancement of the input and output streams can be accomplished by a manifolding system, which are also known, e.g., by rotary disc valves shown in U.S. Patents 3,040,777 and 3,422,848. Equipment utilizing these principles are familiar, in sizes ranging from pilot plant scale (deRossett U.S. Patent 3,706,812) to commercial scale in flow rates from a few cc per hour to many thousands of gallons per hour. The invention may also be practiced in a cocurrent, pulsed batch process, like that described in U.S. Patent 4,159,284. Also, in some cases illustrated herein, It is necessary to remove three effluent streams from the adsorptive separation step in order to obtain a desired product intermediate in adsorption strength from an extract and a raffinate stream. This intermediate stream can be termed a second raffinate stream, as in U.S. Patent 4,313,015 or a second extract stream, as in U.S. Patent 3,723,302, both incorporated herein by reference. This case pertains when a contaminating component in the feed, such as p-ethyltoluene, is more strongly adsorbed than the desired product, p-xylene. It is not always necessary to remove p-ethyltoluene from p-xylene, e.g., where terephthalic acid is the final product of the oxidation of p-xylene, since oxidation of p-ethyltoluene results in the same product. However, if it is desired to keep the concentration of the contaminating component in the product as low as possible, a first extract is taken off, high in concentration of the desired component and lower in the contaminating product followed by a second extract withdrawn at a point between the desorbent inlet and the first extract point, containing a high concentration of the contaminant and a lower concentration of the desired product. It is not necessary, however, to use a second desorbent, if the desorbent is able to first desorb the lightly held product and then desorb the remaining more strongly held contaminants, as disclosed in the aforementioned '302 patent. If the contaminating component in high concentrations and purity is desired, this can be achieved by withdrawing a second extract in the cocurrent pulsed batch process mentioned above. The functions and properties of adsorbents and desorbents in the chromatographic separation of liquid components are well-known, but for reference thereto, Zinnen et al. U.S. Patent 4,642,397 is incorporated herein. A process employing a zeolite adsorbent for the separation of p-xylene from its isomers using a desorbent which is a substantial improvement in a process for separating xylene isomers where the feed mixture also contains C₉ aromatic impurities has now been discovered. SUMMARY OF THE INVENTION In brief summary, the invention is a chromatographic process for separating p-xylene from a feed mixture comprising p-xylene, one or more additional xylene isomers (a term which includes ethylbenzene) and one or more C₉ aromatic hydrocarbons comprising contacting said feed mixture with a crystalline aluminosilicate adsorbent, preferably an X- or Y-type zeolite, exchanged with Group IA or IIA metal ions at exchangeable cationic sites, to effect the selective adsorption of said p-xylene and produce a raffinate comprising the less strongly adsorbed other xylene isomer(s), (including ethylbenzene) and the C₉ aromatics. P-xylene is recovered by contacting the adsorbent with a desorbent comprising diethyltoluene. As used herein, the term diethyltoluene is intended to signify each of the isomers and any mixture thereof. More specifically, the instant invention covers the use of 2,3-DET; 2,4-DET; 2,5-DET; 2,6-DET; 3,4-DET; 3,5-DET and mixtures of 2 or more of these isomers. The desorbent is higher boiling (e.g., 3,5-diethyltoluene: b.p. = 198-200°C) than the C₉ aromatics, making it possible to separate the C₉ aromatics from the desorbent by simple fractionation so that the desorbent can be reused in the process without building up C₉ aromatics in the recycled desorbent. The invention, in another aspect, is a process for separating C₉ aromatics from a feed mixture of C₉ aromatics and p-xylene and at least one other xylene isomer. BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 is a chromatographic representation of the separation of p-xylene from a mixture of xylene isomers and C₉ aromatics with a K-exchanged Y zeolite and a desorbent comprising a 30/70 mixture of diethyltoluene and n-heptane. Figure 2 is similar to Figure 1 except that the adsorbent is BaX-exchanged zeolite and the desorbent is 100% diethyltoluene isomers. DETAILED DESCRIPTION OF THE INVENTION Adsorbents to be used in the process of this invention comprise crystalline aluminosilicates (molecular sieves), especially X and Y zeolites. The zeolites have known cage structures in which the alumina and silica -exchanged tetrahedra are intimately connected in an open three-dimensional network to form cage-like structures with window like pores. The tetrahedra are cross-linked by the sharing of oxygen atoms with spaces between the tetrahedra occupied by water molecules prior to partial or total dehdyration of this zeolite. The dehydration of the zeolite results in crystals interlaced with cells having molecular dimensions and thus, the crystalline aluminosilicates are often referred to as "molecular sieves" when the separation which they effect is dependent essentially upon differences between the sizes of the feed molecules as, for instance, when smaller normal paraffin molecules are separated from larger isoparaffin molecules by using a particular molecular sieve. In the process of this invention, however, the term "molecular sieves", although widely used, is not strictly suitable since the separation of specific aromatic isomers is apparently dependent on differences in electrochemical attraction of the different isomers and the adsorbent rather than on pure physical size differences in the isomer molecules. In hydrated form, the crystalline aluminosilicates encompass type X zeolites which are represented by Formula 1 below in terms of moles of oxides: Formula 1 (0.9±0.2)M 2/n O:Al₂O₃:(2.5±0.5)SiO₂:yH₂O where "M" is a cation having a valence of not more than 3 which balances the electrovalence of the alumina tetrahedra and is generally referred to as an exchangeable cationic site, "n" represents the valence of the cation, and "y", which represents the moles of water, is a value up to about 9 depending upon the identity of "M" and the degree of hydration of the crystal. As noted from Formula 1, the SiO₂/Al₂O₃ mole ratio is 2.5±0.5. The cation "M" may be monovalent, divalent or trivalent cations or mixtures thereof. The type Y structured zeolite, in the hydrated or partially hydrated form, can be similarly represented in terms of moles of oxides as in Formula 2 below: Formula 2 (0.9±0.2)M 2/n O:Al₂O₃:wSiO₂:yH₂O where "M", "n" and "y" are the same as above and "w" is a value greater than about 3 up to about 6. The SiO₂/Al₂O₃ mole ratio for type Y structured zeolites can thus be from about 3 to about 6. For both zeolites, the cation "M" may be one or more of a variety of cations but, as the zeolites are initially prepared, the cation "M" is also usually predominately sodium. The type Y zeolite containing predominately sodium cations at the exchangeable cationic sites is, therefore, referred to as a sodium-exchanged type-Y, or NaY, zeolite. Depending upon the purity of the reactants used to make the zeolite, other cations mentioned above may be present, however, as impurities. The zeolites useful in the invention are typical as described above. However, the exchange of the cation of the as-manufactured zeolite by ions from Group IA or IIA, e.g., barium or potassium or mixtures thereof, is necessary to achieve the separation. Typically, adsorbents used in separative processes contain the crystalline material dispersed in an amorphous, inorganic matrix or binder, having channels and cavities therein which enable liquid access to the crystalline material. Silica, alumina, clay or mixtures thereof are typical of such inorganic matrix materials. The binder aids in forming or agglomerating the crystalline particles which otherwise would comprise a fine powder. The adsorbent may thus be in the form of particles such as extrudates, aggregates, tablets, macrospheres or granules having a desired particle range, preferably from about 16 to about 60 mesh (Standard U.S. Mesh) (250 to 1190 microns). Feed mixtures which can be utilized in the process of this invention will comprise para-xylene, at least one other C₈ aromatic isomer, and also one or more C₉ aromatics as impurities. Mixtures containing substantial quantities of paraxylene and other C₈ aromatic isomers and C₉ aromatics generally are produced by reforming and isomerization processes, processes which are well known to the refining and petrochemical arts. Many of the C₉ aromatics have boiling points in the range of 160-170°C and cannot be easily removed by distillation from the standard desorbent, p-diethylbenzene. In the current process, therefore, the C₉'s are usually removed from the feed by distillation before the adsorptive separation step and consequent contact with the normal desorbent. A desorbent has now been discovered which can be easily separated from the C₉ aromatics by fractionation after the adsorptive separation step and does not thus require the large column and quantity of utilities to pretreat the feed, resulting in substantial cost savings. Reforming processes can provide feed mixtures for the process of this invention. In reforming processes, a naphtha feed is contacted with a platinum-halogen-containing catalyst at severities selected to produce an effluent containing C₈ aromatic isomers. Generally, the reformate is then fractionated to concentrate the C₈ aromatic isomers in a C₈ fraction which will contain the C₈ aromatic isomers as well as C₈ nonaromatics and C₉ aromatics. Feed mixtures for the process of this invention may also be obtained from isomerization and transalkylation processes. Xylene mixtures which are deficient in one or more isomers can be isomerized, at isomerization conditions, to produce an effluent containing the C₈ aromatic isomers, e.g., enriched in p-xylene, as well as C₈ nonaromatics and C₉ aromatics. The C₉ aromatic content of isomerized xylene isomers can be as much as 1-2 vol% depending on isomerization conditions. Likewise, transalkylation of mixtures of C₇ and C₉ aromatics produces xylene isomers which contain C₉ aromatics. In all of these catalytic processes, the xylene splitter column must be employed to remove C₉ aromatics from C₈ aromatics before conventional adsorptive xylene separation methods can be employed. Thus, the feed mixtures to the process of this invention can contain quantities of C₉ aromatics and may also contain quantities of straight or branched chain paraffins, cycloparaffins, or olefinic material. It is preferable to have these quantities at a minimum amount in order to prevent contamination of products from this process by materials which are not selectively adsorbed or separated by the adsorbent. Preferably, the above-mentioned contaminants should be less than about 20% of the volume of the feed mixture passed into the process. To separate the para-xylene from a feed mixture containing para-xylene, at least one other C₈ aromatic and C₉ aromatics, the mixture is contacted with the adsorbent at adsorption conditions and the para-xylene (and para-ethyltoluene, if present) is more selectively adsorbed and retained by the adsorbent while the other components are relatively unadsorbed and are removed from the interstitial void spaces between the particles of adsorbent and from the surface of the adsorbent. The adsorbent containing the more selectively adsorbed para-xylene is referred to as a "rich" adsorbent--rich in the more selectively adsorbed para-xylene. The para-xylene is then recovered from the rich adsorbent by contacting the rich adsorbent with the instant DET desorbent material at desorption conditions. In this process, which employs zeolitic adsorbents and which is generally operated continuously at substantially constant pressures and temperatures to ensure liquid phase, the desorbent material relied upon must be judiciously selected to satisfy several criteria. First, the desorbent material should displace an extract component from the adsorbent with reasonable mass flow rates without itself being so strongly adsorbed as to unduly prevent the extract component from displacing the desorbent material in a following adsorption cycle. Secondly, the desorbent material must be compatible with the particular adsorbent and the particular feed mixture. More specifically, they must not reduce or destroy the critical selectivity of the adsorbent for an extract component with respect to the raffinate component or react chemically with the feed components. Desorbent materials should additionally be easily separable from the feed mixture that is passed into the process. Both the raffinate components and the extract components are typically removed from the adsorbent in admixture with desorbent material and without a method of separating at least a portion of desorbent material, the purity of the extract product and the raffinate product would not be very high nor would the desorbent material be available for reuse in the process. It is, therefore, contemplated that the DET desorbent material used in this process will have a substantially different average boiling point than that of the feed mixture or any of its components, i.e., more than about 5°C difference, to allow separation of at least a portion of the desorbent material from feed components in the extract and raffinate streams by simple fractional distillation, thereby permitting reuse of desorbent material in the process. Finally, desorbent materials should be readily available and reasonable in cost. However, a suitable desorbent or desorbents for a particular separation with specific adsorbent are not always predictable. In the preferred isothermal, isobaric, liquid-phase operation of the process of this invention, when the feed material to the separation process contains more than about 0.1 vol.% C₉ aromatics, it has now been found that a desorbent material comprising diethyltoluene (individual isomers or mixtures thereof) will desorb the extract from the adsorbent and can be separated from the C₉-containing raffinate by distillation. Adsorption conditions suitably include a temperature range of from about 20 to about 250°C, with about 60 to about 200°C being more preferred, and a pressure just sufficient to maintain liquid phase, which may be from about atmospheric to 600 psig. Desorption conditions suitably include the same range of temperatures and pressure as used for adsorption conditions. A dynamic testing apparatus is employed to test various adsorbents and desorbent material with a particular feed mixture to measure the adsorbent characteristics of adsorptive capacity and exchange rate. The apparatus consists of a helical adsorbent chamber of approximately 70 cc volume having inlet and outlet portions at opposite ends of the chamber. The chamber is contained within a temperature control means and, in addition, pressure control equipment is used to operate the chamber at a constant predetermined pressure. Quantitative and qualitative equipment, such as refractometers, polarimeters, chromatographs, etc., can be attached to the outlet line of the chamber and used to analyze, "on-stream", the effluent stream leaving the adsorbent chamber. A pulse test, performed using this apparatus and the following general procedure, is used to determine data, e.g., selectivities, for various adsorbent systems. The adsorbent is filled to equilibrium with a particular desorbent by passing the desorbent material through the adsorbent chamber. At a convenient time, a pulse of feed containing known concentrations of a tracer and of a particular extract component or of a raffinate component, or both, all diluted in desorbent material is injected for a duration of several minutes. Desorbent flow is resumed, and the tracer and the extract and raffinate components are eluted as in a liquid-solid chromatographic operation. The effluent can be analyzed by on-stream chromatographic equipment and traces of the envelopes of corresponding component peaks developed. Alternatively, effluent samples can be collected periodically and later analyzed separately by gas chromatography. From information derived from the test, adsorbent performance can be rated in terms of void volume, retention volume for an extract or a raffinate component, and the rate of desorption of an extract component from the adsorbent and selectivity. Void volume is the non-selective volume of the adsorbent, which is expressed by the amount of desorbent pumped during the interval from initial flow to the center of the peak envelope of the tracer. The net retention volume of an extract or a raffinate component may be characterized by the distance between the center of the peak envelope (gross retention volume) of the extract of raffinate component and the center of the peak envelope (void volume) of the tracer component or some other known reference point. It is expressed in terms of the volume in cubic centimeters of desorbent material pumped during this time interval represented by the distance between the peak envelopes. The rate of exchange or desorption rate of an extract component with the desorbent material can generally be characterized by the width of the peak envelopes at half intensity. The narrower the peak width, the faster the desorption rate. The desorption rate can also be characterized by the distance between the center of the tracer peak envelope and the disappearance of an extract component which has just been desorbed. This distance is again the volume of desorbent material pumped during this time interval. Selectivity, β, is determined by the ratio of the net retention volumes of the more strongly adsorbed component to each of the other components. The following examples are presented to illustrate the process of the present invention. EXAMPLE I In this experiment, a pulse test, using the apparatus as described above, was performed to evaluate the ability of the present invention to separate para-xylene (b.p. 138°C) from the other xylene isomers and ethylbenzene (b.p's. from 136-145°C) and from C₉ aromatics. The adsorbent used was a Y faujasite exchanged with potassium, dried at 400-450°C combined with 15 wt.% of an amorphous clay binder. For each pulse test, the column was maintained at a temperature of 150°C and at a pressure of 165 psig (1240 kPa) so as to maintain liquid-phase operations. Gas chromatographic analysis equipment was attached to the column effluent stream in order to determine the composition of the effluent material at given time intervals. The feed mixture employed for each test was 5 cc of a mixture containing 0.45 cc each of the xylene isomers and ethylbenzene and each of the following C₉ aromatics: cumene, n-propylbenzene, p-ethyltoluene, mesitylene, 1,2,4-trimethylbenzene and 1,2,3-trimethylbenzene. Normal nonane (0.45 cc) was used as a tracer and 4.95 cc desorbent material was added to the feed. The desorbent material comprised 30 vol.% of a diethyltoluene (DET) isomer mixture with the remainder being n-C₇ paraffin. The DET isomer distribution of the desorbent was that of mixture A, Table 1. A B 3,5-DET 47.7 18.6 3,4-DET trace trace 2,4-DET 2.5 1.6 2,3-DET 7.4 7.3 2,5-DET 18.6 41.9 2,6-DET 23.8 30.6 The operations taking place for the test were as follows: The desorbent material was run continuously at a rate of about 1.2 cc per minute. At some convenient time interval, the desorbent was stopped and the feed mixture was run for a 4.2 minute interval. The desorbent stream was then resumed and continued to pass into the adsorbent column until all of the feed aromatics had been eluted from the column as determined by chromatographic analysis of the effluent material leaving the adsorption column. The results of the tests shown in Table 2 and the chromatographic tracing of Figure 1 illustrate the invention. The table lists the gross retention volume (GRV) and net retention volume (NRV) for each component of the feed and, β, for each component with respect to the reference, p-xylene. Component Figure 1 Reference No. Gross Retention Volume (ml) Net Retention Volume (ml) Selectivity (Beta) Boiling Point (°C) n-Nonane 1 46.9 0.0 0.00 (Tracer) Ethylbenzene 8 73.8 26.9 1.86 136 p-Xylene 10 97.0 50.1 1.00 (Ref.) 138 Cumene 9 72.5 25.6 1.96 153 o-Xylene 4 59.0 12.1 4.14 144 n-Propylbenzene 7 66.9 20.0 2.50 159 p-Ethyltoluene 11 128.8 81.9 0.61 162 Mesitylene 2 53.4 6.5 7.69 163 1,2,4-Trimethylbenzene 6 66.8 19.8 2.52 168 1,2,3-Trimethylbenzene 5 61.4 14.5 3.45 175 m-Xylene 3 57.3 10.3 4.84 139 EXAMPLE II Another pulse test was run under the same conditions and with the same feed mixture as Example I, except that the desorbent was 100% diethyltoluene with isomer distribution B (Table 1) and the adsorbent was BaX. The results of the test are shown in Table 3 and Figure 2. Component Figure 2 Reference No. Gross Retention Volume (ml) Net Retention Volume (ml) Selectivity (Beta) n-Nonane 1 41.3 0.0 0.00 Ethylbenzene 8 56.7 15.4 1.52 p-Xylene 10 64.6 23.3 1.00 Cumene 9 58.3 17.0 1.37 o-Xylene 4 48.5 7.2 3.25 n-Propylbenzene 7 50.4 9.1 2.56 p-Ethyltoluene 11 69.0 27.7 0.84 Mesitylene 2 44.0 2.6 8.80 1,2,4-Trimethylbenzene 6 47.9 6.6 3.53 1,2,3-Trimethylbenzene 5 46.8 5.4 4.28 m-Xylene 3 47.9 6.6 3.53

Claims: A process for separating p-xylene from a mixture comprising p-xylene and at least one other isomer of xylene and one or more C₉ aromatic hydrocarbons, comprising contacting the mixture with an adsorbent comprising crystalline aluminosilicate containing Group IA or IIA metal ions at exchangeable cationic sites at adsorption conditions to effect the selective adsorption of said p-xylene by said adsorbent and to produce a raffinate stream comprising the less strongly adsorbed other xylene isomer(s) and thereafter contacting the resulting p-xylene-containing adsorbent with a desorbent at desorption conditions to effect the removal of p-xylene from said adsorbent as an extract stream, characterised in that the desorbent comprises a diethyltoluene isomer or a mixture of these isomers. A process as claimed in claim 1, characterised in that the desorbent is selected from 2,3-diethyltoluene; 2,5-diethyltoluene; 2,6-diethyltoluene; 3,4-diethyltoluene; 3,5-diethyltoluene and mixturees of two or more of these, optionally together with 2,4-diethyltoluene. A process as claimed in claim 1 or 2, characterised in that the adsorbent is selected from type X and type Y zeolites. A process as claimed in claim 3, characterised in that the zeolite is exchanged with potassium or barium or a mixture thereof. A process as claimed in any of claims 1 to 4, characterised in that it is practiced in a countercurrent, simulated moving bed system. A process as claimed in any of claims 1 to 4, characterised in that it is practiced in a cocurrent, pulsed batch system. A process as claimed in any of claims 1 to 6, characterised in that the desorbent is recycled to the process after removal of C₉ aromatic hydrocarbon(s) from the desorbent by fractionation. A process as claimed in any of claims 1 to 7, characterised in that the content of C₉ aromatic hydrocarbon in the mixture is between 0.1 and 20 vol %.

Inventors: ZINNEN HERMANN Z, ZINNEN, HERMANN Z.
Applicants: UOP INC, UOP
Application year: 1989
Main classification: C07C15
IPC codes: C07C15, B01D15, B01J20, C07C69, C07C67, C07C7