FI89904B - Adsorption separation of para-xylene using diethyltoluene desorbent - Google Patents

Description

1 89904

Adsorption Separation of Parate Cysene Using Diethylene with Deso rben 11 i a - Ad sorption s s par paration of para-xy1en genome användn i ng av dietary

This invention relates to the adsorption separation of hydrocarbons. More specifically, the present invention relates to a process for separating para-xylene from a feed mixture containing at least two xylene isomers, including the para-isomer, which process uses a zeolite adsorbent and a specific desorbent. This is particularly advantageous in a process in which the feed also contains Cg 10 aromatic hydrocarbons which are known to cause problems in previous adsorption separation processes of this type.

Numerous patents described in the patent literature, such as U.S. Patents 3,626,020 (Neuzil), 3,663,638 (Neuzil), 3,665,046 (deRosset), 3,668,266 (Chen et al.), 3,686,342 (Neuzil et al.), 3,700,744 (Berger et al.), 3,734, etc. , 3,894,109 (Rosback), 3,997,620 (Neuzil) and 8426,274 (Hedge), certain zeolite adsorbents are used to separate the para-isomer of dialkyl-substituted monocyclic aromatics from 20 other isomers, especially to separate para-xylene from other xylene isomers. Several of the above patents describe the use of benzene, toluene or p-diethylbenzene as a desorbent. P-Diethylbenzene (p-DEB) has become a commercial standard for this separation. However, the P-DEB is! ' ! 25 "heavy" desorbent (higher boiling point than with p-xylene), which causes problems in the p-xylene adsorption separation process when the feed mixture also contains Cg aromatics, as the boiling point of p-: DEB is too close to that of Cg aromatics. boiling points, which makes simple fraction separation difficult. 30 Since it is difficult to separate Cg aromatics from p-DEB by simple fractionation, Cg aromatics, if allowed to be present in the feed, will gradually accumulate in the desorbent which, for economic reasons, must be recycled. In previous p-xylene recovery methods for feed mixtures containing isomers in which p-DEB acts as a desorbent, the amount of C8 aromatics in the feed must therefore be reduced to less than about 0.1% by volume before adsorption separation of p-xylene. This 2 89904 is usually made by distillation on a so-called xylene separator column.

The costs associated with this process, such as the cost of capital-5 for the xylene separator column and the necessary equipment for the almost complete removal of Cg aromatics, could, of course, be significantly reduced or eliminated if Cg aromatics did not need to be removed in advance. The aforementioned U.S. Patent 3,686,342 mentions other substituted benzenes as potential heavy desorbents for the para-xylene adsorption separation process, which clearly suggests that p-DEB is the best desorbent for separation, and further that the problem possibly caused by preferred desorbents has not been addressed. feeds containing aromatics. A substance with a higher boiling point, which meets the selectivity requirements for a desorbent 15 and which can be distinguished from C8 aromatics, has therefore long been sought and is still desired.

It is also known to use crystalline aluminosilicates or zeolites in the adsorption separation of various mixtures as agglomerates which are physically very strong and wear-resistant. To form crystalline powders as such agglomerates, a method is used in which an inorganic binder, usually a clay of silica and alumina, is added to a high-purity zeolite powder as a wet mixture.

The mixed clay-zeolite mixture is compressed into cylindrical bars or formed into sheets which are subsequently calcined to convert the clay into an amorphous binder of considerable mechanical strength. Kaolin-type clays, water-permeable organic polymers or silica are used as binders.

The present invention can be applied to fixed or mobile adsorbent filling systems, but a preferred system for this separation is considered to be a countercurrent simulated mobile filling system, as described in U.S. Patent 2,985,598 (Broughton), which is incorporated herein by reference. The cyclic propagation of the input and output current can be realized by a residual system that i.

3,899,04 are also known, for example, from the rotary poppet valves disclosed in U.S. Patents 3,040,777 and 3,422,848. Equipment that follows these principles is familiar and ranges in size from a test scale (U.S. Patent 3,706,812 (deRossett)) to 5 commercial scales, from flow rates of a few cubic centimeters per hour to several thousand gallons per hour.

The invention can also be used in parallel current pulse batch processes, as described in U.S. Patent No. 4,159,284.

10

In some cases described herein, it is further necessary to remove three waste streams from the adsorption separation step to obtain the desired product between the adsorption power of the extract and refinery streams.

This intermediate stream may be referred to as a second refining stream, as in U.S. Patent 4,313,015, or a second extraction stream, as in U.S. Patent 3,723,302, both of which are incorporated herein by reference. Such a case arises when a contaminating component of the feed, such as p-ethyltoluene, is adsorbed more efficiently than the desired product, p-xylene. It is not always necessary to remove p-ethylene toluene from p-xylene, for example when terephthalic acid is the end product of p-xylene oxidation, since oxidation of p-ethyltoluene gives the same product. However, if the concentration of the contaminating component in the product is to be kept as high as possible 'small, remove the first extract rich in the desired component and low contaminating product, then remove the second extract rich in contaminating and low contaminated product between the desorbent inlet and the first extraction site. However, the second desorbent need not be used if the desorbent is capable of first desorbing the lightly adhering product and then desorbing the remaining, more strongly adhering contaminants, as described in the aforementioned U.S. Patent 3,723,302. If the contaminating product is desired to be the largest ·; ·; concentrations and pure, it can be achieved by removing 35 second extracts from the above parallel batch pulse batch process.

4,89904

The modes of operation and properties of adsorbents and desorbents in the chromatographic separation of liquid components are well known, but reference should be made to U.S. Patent 4,642,397 (Zinnen et al.).

5

A process has now been invented using a zeolite adsorbent to separate p-xylene from its other isomers using a desorbent which provides a significant improvement in the process of separating xylene isomers whose feed also contains C8 aromatic impurities.

In summary, the invention is a chromatographic process for separating p-xylene from a feed mixture containing p-xylene and one or more other xylene isomers (including ethylbenzene) and in addition C8 aromatic hydrocarbons, and wherein said feed mixture is contacted with X- or Y- with a type IA zeolite having exchanged Group IA or IIA metal ions exchanged at the cationic sites to enhance the selective adsorption of said p-xylene and to produce a refining containing other xylene isomers, including ethylbenzene and C8 aromatics. P-xylene is recovered by contacting the adsorbent with a desorbent containing diethyltoluene. As used herein, diethyltoluene refers to all isomers and mixtures thereof. More specifically, the following diethyltoluenes can be used in this invention: 2,3-DET; 2,4-DET; 2,5-DET; 2,6-DET; 3,4-DET; Mixtures of 3,5-DET and two or more monomers. The desorbent has a higher boiling point (e.g. 3,5-diethyltoluene has a boiling point of 198-200 ° C) than Cg-aro-30 mates, so that Cg aromatics can be separated from the desorbent by simple fractionation so that the desorbent can be reused in the process without Cg accumulation of aromatics in the recycled desorbent. In another aspect, the invention is also a process for separating C8 aromatics from a feed mixture comprising C8 aromatics and p-xylene and at least one other xylene isomer.

li 5 89904

Figure 1 shows a chromatographic description of the separation of p-xylene from a mixture of xylene isomers and C8 aromatics with a K-exchanged Y zeolite and a 30/70 ratio of a desorbent containing diethyltoluene and n-heptane.

Figure 2 is similar to Figure 1, except that the adsorbent is a BaX-exchanged zeolite and the desorbent is 100% diethyltoluene isomers.

The adsorbents used in the process of this invention are specific crystalline aluminosilicates or molecular sieves, namely X and Y zeolites. Zeolites have known frame structures in which aluminum- and silica-exchangeable tetra-edges are closely interconnected into an open three-dimensional network, forming frame-like structures with pore-like windows. The tetrahedra are crosslinked by dividing the oxygen atoms between the openings in the tetrahedra that had water molecules before partial or complete dehydration of the zeolite. Dehydration of the zeolite yields crystals and the molecular size cells between them, which is why crystalline aluminosilicates are often referred to as "molecular sieves" when the separation they affect depends mainly on differences in the size of the feed molecules, e.g. In the process of this invention: the term "molecular sieves", although in a broad sense, is not entirely appropriate, as the separation of certain aromatic isomers is clearly dependent on the electrochemical attraction and adsorbent of the various isomers rather than on the physical size of the purely isomeric molecules.

30; The crystalline aluminosilicates correspond in hydrate form to the X-type zeolites described in formula 1 on the basis of oxide moles: 35 Formula 1 · ': (0.9 + 0.2) M2 / n0: AlgOg: (2.5 + 0.5) SiO2: yH2O 6 89904 wherein "M" is a cation having a valence of not more than 3 that balances the electric valence of the aluminum tetrahedron and is generally referred to as an exchangeable cationic site, "n" is the valence of the cation, and "y" is equal to about 9 ". Depending on the nature of M "and the degree of hydration of the crystal. As shown in Formula 1, the molar ratio of SiO 2 / AlgO 2 is 2.5 + 0.5. The cation "M" may be a monovalent, divalent or trivalent cation or a mixture of such cations.

A Y-type structured zeolite, in hydrogenated or partially hydrogenated form, can be similarly represented on the basis of oxide moles in formula 2 below:

Formula 2 15 (0.9 + 0.2) M 2 / n O: Al 2: w SiO 2: yH 2 O wherein "M", "n" and "y" are the same as above and "w" is a number between about 3 and 6. Thus, the molar ratio of Y-type zeolites to SiO 2 / Al 2 O 2 may be between about 3 and 6. The cation "M" in each zeolite may be one or more of a number of cations, but since the zeolites are initially prepared, the cation "M" is generally sodium. Type Y zeolites with predominantly sodium-cation exchange sites are therefore referred to as sodium-exchange type Y or NaY zeolite. Depending on the purity of the starting materials used to prepare the zeolite, the other cations mentioned above may also be present, albeit as impurities.

The zeolites useful in this invention are as described above. However, the exchange of the cation of the prepared zeolite for ions of group IA or IIA, e.g. barium or potassium or mixtures thereof, is necessary to achieve the separation.

35 Adsorbents used in separation processes generally contain a crystalline material distributed in an amorphous, inorganic matrix or binder with channels and li 7 89904 from the cavity which allow liquid to pass through the crystalline material. Silica, aluminum, clay or mixtures thereof are typical of such inorganic matrix materials. The binder otherwise helps the crystalline particles in the form of a fine powder 5 to form or agglomerate.

Thus, the adsorbent may be in particulate form, such as compressions, aggregates, tablets, macroparticles, or granules, and the desired particle sizes, preferably between about 2b0 and 1190 microns (16 and 60 mesh, standard US mesh).

The feed compositions useful in the process of this invention may contain para-xylene, at least one other C8 aromatic isomer, and also one or more C8 aromatics as impurities. Substantial amounts of para-xylene and other mixtures containing C8 aromatic isomers and C8 aromatics are generally obtained from reforming and isomerization processes well known in the art of petroleum refining and petrochemistry. Many C8 aromatics have boiling points in the range of 160-170 ° C and are not easily removed by distillation from a standard desorbent, p-diethyl 20 benzene. In the current process, therefore, the C8 aromatics are generally removed by distilling the feed before the adsorption separation step and the subsequent desorbent contacting step. A desorbent has now been found which is easy to separate from C8 aromatics by fractionation after the adsorption separation step and which does not: requires a large column and a large amount of equipment for feed pretreatment, which results in considerable cost savings.

Feed mixtures from reforming processes can be used in this invention. In the reforming process, the petroleum feed is contacted with a platinum- and halogen-containing catalyst with such stringency that a removal containing C8 aromatic isomers is obtained. The reformate is generally fractionated to concentrate the C8 aromatic isomers to a C8 fraction which, in addition to the C8 aromatic isomers, contains C835 non-aromatics and C8 aromatics. Feed mixtures from isomerization and transalkylation processes can also be used in this invention. Mixtures of xylene containing too little of one 8,899,04 or more isomers can be isomerized under such conditions to give a remover containing C8 aromatic isomers, e.g. enriched in p-xylene, as well as C8 non-aromatics and C8 aromatics. The Cg aromatic content of the isomerized xylene isomers can be up to 1-2% by volume depending on the isomerization conditions. Transalkylation of C and C8 aromatic mixtures gives xylene isomers containing C8 aromatics, respectively. All of these catalytic processes must use a xylene separator column to separate Cg aromatics from Cg aromatics before conventional xylene separation methods can be applied. Thus, feed mixtures containing C8 aromatics may be used in the process of this invention and may also contain straight or branched chain paraffins, cycloparaffins or olefinic substances. The amounts of these substances must be small so that the products of this process are not contaminated with substances which are not selectively adsorbed or separated by the adsorbent. The amount of the above contaminants should preferably be less than about 20% by volume of the mixture fed through the process.

20

To separate para-xylene from a feed mixture containing para-xylene, at least one other Cg aromatic and Cg aromatics, the mixture is contacted with an adsorbent under adsorption conditions and the para-xylene (and para-ethyltoluene, if present) is selectively adsorbed and remains in the adsorbent. being relatively non-adsorbent and leaving the openings between the adsorbent particles and the surface of the adsorbent. An adsorbent containing more selectively adsorbed para-xylene is called an "enriched" ad-30 sorbent. Para-xylene is recovered from the enriched adsorbent by contacting the rich adsorbent with the DET desorbent under desorption conditions.

In this process, which uses zeolite adsorbents and which generally operates continuously at near constant pressures and temperatures to ensure a liquid phase, the material to be used as the desorbent must be carefully selected to meet a number of criteria. First, the desorbent should replace the extract component in the adsorbent at reasonable flow rates without being itself adsorbed very strongly so as not to unduly prevent the extract component from replacing the desorbent 5 during the next adsorption cycle. Second, the desorbent must be compatible with the adsorbent in question and the feed mixture in question. More specifically, they must not impair or destroy the critical selectivity of the adsorbent over the extract component and the refining component, or chemically react with the feed components. In addition, the desorbents must be easily separable from the feed mixture passing through the process. Both the refining components and the extraction components are generally removed from the adsorbent by the addition of a desorbent, and without a method for separating at least a portion of the desorbent, the extract product and the refined product are not very pure and the desorbent cannot be reused in the process. It must therefore be investigated that the boiling point of the DET desorbent used in this process differs significantly from that of the feed mixture or any of its components. . 20 boiling points, i.e. a difference of at least about 5 ° C, so that at least part of the desorbent can be separated from the feed components of the extract refining streams by simple fractional distillation, allowing the desorbent to be reused in the process.

25

Finally, desorbents should be readily available and reasonably priced. A suitable desorbent or suitable desorbents for a particular separation with a particular adsorbent are / are always difficult to predict. In a preferred isothermal, isobaric liquid phase process of the present invention in which the material fed to the separation process contains at least about 0.1% by volume of Cg aromatics, it has been found that a desorbent consisting of diethyltoluene (individual isomers and mixtures thereof) desorbs from the adsorbent of the extract and can be separated from the Cg-containing process by distillation.

The adsorption conditions comprise a temperature range of about 20 to 250 ° C, preferably about 60 to 200 ° C, and sufficient pressure to maintain the liquid phase, which can range from atmospheric pressure to 4140 kPa. The desorption conditions correspond to the adsorption temperature and pressure conditions.

5

A dynamic tester is used to test different adsorbents and desorbents for a particular feed mixture to measure the adsorption capacity properties and exchange rate of the adsorbent. The apparatus consists of a helical adsorbent chamber with a volume of about 70 cm and an inlet and outlet on each side. The chamber temperature is monitored and pressure monitoring devices are used to maintain the chamber pressure at a predetermined constant pressure. Quantitative and qualitative equipment such as refractometers, polarimeters. 15 chromatographs, etc., can be connected to the outlet line of the chamber and used to analyze the current leaving the adsorbent chamber "during use".

The pulse test performed with this device and according to the general procedure 20 is used to obtain the desired values, e.g.

selectivities for different adsorbent systems. The adsorbent is filled to equilibrium with a particular desorbent by passing the desorbent material through an adsorbent chamber. After a suitable time, a pulse feed containing known concentrations of tracer and a specific extract component or processing component, or both, all dissolved in a desorbent, is injected for several minutes. The desorbent stream is continued and the tracer and extract and refining components elute as in the liquid-solid chromatographic procedure. The effluent can be analyzed using chromatographic equipment connected to the effluent, whereby the various components can be detected as peaks in the chromatographic curve. Alternatively, the effluent samples may be collected periodically and subsequently analyzed separately by gas chromatography.

35 According to the data obtained from the test, the performance of the adsorbent can be determined by the porosity, extract or extraction components. i n with respect to the retention volume, and the desorption rate of the extract component, 11,89904 as adsorbent as well as selectivity. Pore volume refers to the non-selective volume of adsorbent, expressed as the amount of desorbent pumped over a period from the output stream to the center of the tracer envelope peak. The net retention volume of the extract or refining component can be described as the distance between the center of the envelope peak (gross retention volume) of the extract or refining component and the center of the envelope of the tracer (pore volume) or some other reference point. This is expressed in terms of the distance between the peaks of the envelope-10 as the cubic centimeter volume of the desorbent being pumped. The rate of exchange or desorption of the extract component from the desorbent can generally be described by the width of the envelope peaks at half intensity. The smaller the peak width, the higher the desorption rate. The desorption rate can also be described as the distance between the center of the tracer envelope peak and the disappearance of the freshly desorbed extract component. This distance again corresponds to the volume of desorbent pumped during this time interval. The selectivity, β, is determined as the net retention-volume ratios between the more strongly adsorbed component and each of the other components.

The following examples illustrate the process of the present invention.

However, it is not intended to limit the appended claims to Patent-25.

Example 1 In this experiment, a pulse test was performed with equipment as described above to evaluate the ability of this invention to separate para-xylene (b.p. 138 ° C) from other xylene isomers and ethylbenzene (b.p. 136-145 ° C) and Cg aromatics. As the adsorbent, Y fajacite was used, which had been exchanged with potassium and dried at 400-450 ° C, in addition to which 15% by weight of amorphous clay binder was used.

35 - · In each pulse test, the column temperature was maintained: 150 ° C and the pressure 1240 kPa for the process to operate in the liquid phase. A gas chromatographic analysis apparatus was connected to the effluent of the column to determine the composition of the effluent at certain intervals. In each test 3 3 a 5 cm feed mixture containing 0.45 cm of each of the 5 xylene isomers and ethylbenzene and each of the following C8 aromatics was used: cumene, n-propylbenzene, p-ethyltoluene, mesitylene, 1,2,4-trimethylbenzene and 1, 2,3-trimethyl-3-benzene. Ordinary nonane (0.45 cm) was used as a tracer and 4.95 cm of desorbent was added to the feed.

The desorbent contained 30% by volume of a mixture of diethyltoluene (DET) isomers with a residue of n-C-paraffin. The DET isomer distribution of desorbent was as in Mixture A, Table 1.

Table 1 15

A B

3,5-DET: 47.7 18.6 3.4- DET: slightly low 2.4- DET: 2.5 1.6 20 2.3-DET: 7.4 7.3 2.5- DET: 18.6 41, 9 2.6- DET: 23.8 30.6

The experimental conditions were as follows: The desorbent was fed 3 continuously at a rate of about 1.2 cm per minute. The desorbent stream was turned off after an appropriate period and the feed mixture was fed for 4.2 minutes. The feed of desorbent stream to the adsorbent column was continued again until all feed aromatics had been eluted from the column, as determined by gas chromatographic analysis of the effluent from the adsorption column.

The test results shown in Table 2 and the chromatographic curve shown in Figure 1 illustrate the invention. The table shows the gross retention volume (GRV) and 35 net retention volume (NRV) and β for each input component for each component relative to the reference substance, p-xylene.

13 Ö 9 9 O 4

Table 2

Figure 1 Gross-Net

Reference ret.- ret.- Select. Boiling 5 No. volume volume point

Component_ (ml) _ (ml) _ (beta) -oC

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 (ver.) 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 '25 - - Example 2 ... "The second pulse test was carried out under the same conditions and with the same feed mixture as in Example 1 but with with the difference that 100% diethyltoluene with isomer-30 distribution B was used as the desorbent (Table 1) and BaX was the adsorbent.

The test results are shown in Table 3 and Figure 2.

14 39904

Table 3

Figure 2 Gross-Net

Reference- ret.- Selectivity 5 No. volume volume

Component (ml) - (ml) - (beta) - n-nonane 1 41.3 0.0 0.00 ethyl-benzene 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 2,444.0 2.6 8.80 1,2,4-trimethylbenzene 6 47.9 6.6 3.53 1,2,3-trimethylbenzene 5 46 Δ 5.4 4.28 m-xylene 3 47.9 6.6 3.53 L;

Claims (7)

A process for separating p-xylene from a mixture containing p-xylene and at least another isomer of xylene, characterized in that said mixture is contacted with an adsorbent comprising crystalline aluminum silicate, whose replaceable cation sites contain a metal ion from the group. IA or IIA, under adsorption conditions to selectively adsorb said p-xylene at said adsorbent and produce a refined stream containing the less strongly adsorbed second xylene isomer, after which the obtained p-xylene-containing adsorbent is contacted with a desorbent. which contains diethyltoluene at desorption conditions to deposit p-xylene from said adsorbent as an extract stream. 20 Process according to claim 1, characterized in that the adsorbent is selected from the group consisting of X and Y-type zeolites. 25 Process according to claim 1, characterized in that said mixture contains C 9 aromatic hydrocarbons. Process according to claim 2, characterized in that said zeolite is exchanged for potassium or barium or a mixture thereof. Process according to claim 1, characterized in that said desorbent is selected from the group consisting of 2,3-diethyltoluene; 2,5-diethyltoluene; 2,6-diethyltoluene, 3,4-diethyltoluene; 3,5-diethyltoluene and mixtures of two or more of these. ti 17 39904 6. A process according to any of claims 1-5, characterized in that the process is carried out in a simulated variable bed system and using countercurrent. Process according to one of Claims 1 to 5, characterized in that the process is performed in a pulsating batch system using co-current.