IL32417A - Aromatic hydrocarbon separation process - Google Patents

Description

Aromatic hydrooarbon separation prooess UNIVERSAL OIL PRODUCTS COMPANY C. 30742 The present invention relates to a process for the separation of aromatic hydrocarbons using a solid adsorbent which selectively removes at least one aromatic component from the feed. The selectively adsorbed aromatic component is recovered from the solid adsorbent through a desorption step.

It is known in the separation art that certain crystalline aluminosilicates can be used to separate individual hydrocarbons from mixtures thereof. In the separation of aromatic hydrocarbons, the acidic nature of certain crystalline aluminosilicates may be used to separate a desired component of a mixture of aromatic components. This manner of separation is particularly useful where the components to be separated have similar physical properties such as freezing and boiling points. In aromatic hydrocarbon separation and in particular xylene separation, the differences in the acidic nature of the xylene isomers coupled with the acidic nature of the selected crystalline aluminosilicate permits a system which w ll selectively separate a predetermined xylene from a mixture of xylene isomers, In adsorptive-separation processes employing molecular sieve adsorbents which display known acidic characteristics, the separation of para-xylene from ortho- and meta-xylene may be accomplished by using an adsorbent which is more acidic than para-xylene or an adsorbent which approaches the basic characteristics of the meta- and ortho-xylenes. In the former case the more basic meta- and ortho-xylene isomers would be preferentially adsorbed within the mol sieve adsorbent leaving a para-xylene enriched external phase. In the latter case the para-xylene which is more acidic than the ortho- or meta-xylene would be preferentially adsorbed by the basic adsorbent leaving an external phase enriched in xylene meta- and ortho-xylene. The separation of the Cg •a-rem&'fcie isomers becomes much more difficult when ethylbenzene is present together with the ortho-, meta- and para-xylene isomers. Ethylbenzene is xylene considered to be. the most acidic Cg ea?9m¾%i€ isomer and its presence during adsorptive-separation operations of the prior art interferes with the adsorbents ability to efficiently separate para-xylene isomers. The sodium and calcium forms of both the type X and Y. zeolites selectively adsorb meta- and ortho-xylene from a mixture of ortho-, meta- and para-xylene. But when ethylbenzene is present with the three xylene isomers, the adsorbent selectively adsorbs para-xylene over the ethylbenzene. This gives an adsorbed phase rich in ortho- and meta-xylene but contaminated with para-xylene because of the para-xylene 1 s preferred selectivity when compared to ethylbenzene.

It has now been found that type X and Y zeolite adsorbents can provide selectivities which preferentially allow a single com- xylene ponent of the Cg a-remateie isomers to be adsorbed by the adsorbent without interference from the presence of ethylbenzene during adsorption if the inventive combination of cations are used. The essential feature of the present invention resides in employing a combination of at least one cation selected from cations which show preferential adsorption of para-xylene when compared to ethylbenzene.

It is therefore an object of the present invention to provide a process for the separation of a selected xylene isomer xylene from a mixture containing at least two Cg aromatic isomers.

Accordingly, the present invention provides a process xylene for separating at least one Cft xrrvrra-rf-n isomer from a feed stock xyrene- and ethylbenzene containing mixtures of CQ aromaLie isomers/ which comprises : a.) contacting said feed stock with a bed of crystalline aluminosilicate adsorbent comprising faujasite containing at least one cation selected from potassium, rubidium, cesium, barium and silver, in combination with at least one different cation selected from lithium, potassium, barium, magnesium, strontium, beryllium, cadmium, cobalt, nickel, copper, manganese, silver and zinc, xylene b. ) adsorbing the more selectively adsorbable Cg a¥ema%*e isomer of said feed stock with said adsorbent, c. ) withdrawing from said bed of adsorbent a raffinate stream comprising less selectively adsorbed CQ xylene ° asemafeie isomers, d. ) contacting the adsorbent bed with a desorbent material, an desorbing the selectively . adsorbed Cg isomer from the adsorbent,' and ' e. ) withdrawing from said adsorbent bed a "product stream comprising desorbent and said selectively xylene adsorbed Cg .acem-a^ie isomer.

In adsorptive-separation processes, an important factor that is used to determine the ability of a particular adsorbent to separate components of a feed is the selectivity of the adsorbent for one component as compared to another component. The selectivity (B) as used throughout this specification is defined as the ratio of the concentration of the two components in the adsorbed phase over the ratio of the same two components in the unadsorbed phase at equilibrium conditions, and is expressed in equation form in equation 1 below.

Selectivity=Bc/D= [vol% C/Vol% D[ A ( Χ } yol% C/Vol% Dj υ where C and D are two components of the feed represented in volumn % and the subscripts A and U represent the adsorbed and unadsorbed phases respectively. The equilibrium conditions are reached when the feed passing over a bed of adsorbent does hot change composition after contacting the bed of adsorbent, or in other words, when no net transfer of material occur^s between the unadsorbed and adsorbed phases.

As is apparent from the above equation, where the selectivity of two components approaches 1.0 there is no preferential adsorption of one component by the adsorbent. As the value of B becomes greater than unity there is a preferential selectivity by the adsorbent of one component. When comparing the selectivity of component C over component D, a B larger than 1.0 indicates preferential adsorption of component C within the adsorbent, while a B less than 1.0 would indicate that component D is preferentially ' adsorbed leaving an unadsorbed phase richer in component C and an adsorbed phase richer in component D.

In adsorptive-separation processes the separation of para- and meta-xylene may be effected through the use of a crystalline aluminosilicate faujasite adsorbent. Common faujasites which may separate the xylene isomers are the synthetically prepared type X and Y zeolites containing selected cations at the exchangeable cationic sites within the zeolite crystal structure.

Both the natural and synthetic aluminosilicates may be used as adsorbents in the present invention, A crystalline zeolitic aluminosilicate encompassed by the present invention for use as an adsorbent includes aluminosilicate cage structures in which the alumina and silica tetrahedra are intimately connected with each other in an open three-dimensional crystalline network. The tetrahedra are crossiinked by the sharing of oxygen atoms. The spaces between the tetrahedra are occupied by water molecules prior to dehydration. Subsequent partial or total dehydration results in crystals interlaced with channels of molecular dimensions. Thus, the crystalline aluminosilicates are often referred to as molecular sieves. In the hydrated form, the crystalline aluminosilicates may be represented by the formula of equation 2, M2 nO:Al203:wSi02 :yH20 (2) where is a cation which balances the electrovalence of the tetrahedra, n represents the valence of the cation, w represents the mols of SiC^, and Y, the mols of water. The cations may be any one of a number of cations such as for example the alkali metal cations or the alkaline earth cations or other selected cations.

Crystalline aluminosilicates which find use as adsorbents in the process of the present invention possess relatively well-defined pore structures. The exact type aluminosilicate is generally referred to by the particular silica-alumina ratio and the pore dimensions of the cage structures. The faujasites are commonly represented as type X and type Y aluminosilicates and are defined by their varying silica to alumina ratios.

The zeolite type X can be represented in terms of the mole ratio of oxides as represented in the following equation 3: 0.9+0.2M2 nO:Al2O3:2.5+0.5SiO2:yH2O (3) where M represents at least 1 cation having a valence of not more than 3, n represents the valence of M, and Y is a value up to about 8 depending upon the identity of M and the degree of hydration of the crystal. Zeolite type X is described in U. S. Patent No. 2,882,244.

The type Y zeolite may be represented in the terms of the mole ratio of oxides for the sodium form as represented in the following equation 4: 0.9+0.2 a20:Al203:wSi02yH20 (4) The exchangeable cationic sites for the type X and Y zeolites, in general, can be defined as represented in equation 2 above as "M". Cationic exchange or base exchange methods are generally known to those familiar With the field of crystalline aluminosilicate production and are generally performed by contacting a zeolite with an aqueous solution of soluble salts of the cations or cation desired to be exchanged on the sieve. The desired degree of cation exchange is allowed to take place before the sieves are removed from the aqueous solution and dried to a desired water content. It is contemplated that in cationic exchange or base exchange methods that the cation exchange may take place using individual solutions of desired cations to be placed on the molecular sieve or can use exchange solutions containing mixtures of the cations which are desired to be exchanged onto the crystalline aluminosili It is preferred that the type X and Y zeolite adsorbents contain at their exchangeable cationic sites at least one cation selected from potassium, rubidium, cesium, barium and silver, in combination •with at least one different cation selected from lithium, potassium, barium, magnesium, strontium, beryllium, cadmium, cobalt, nickel, copper, manganese, silver and zinc. The cations of the former group display a pronounced para-xylene selectivity as compared to meta- and ortho-xylene while the cations of the latter group display a pronounced para-xylene selectivity as compared to ethylbenzene. It is, therefore, preferred to employ type X or Y zeolite adsorbents containing both cations from the former group of cations and cations from the latter group of cations to effectively separate para-xylene from a mixture containing para-, meta-, ortho-xylene and ethylbenzene. The type X and Y zeolites which demonstrated the best selectivities for para-xylene separation, and which are most preferred, are those zeolites containing both barium and potassium cations, or potassium and beryllium cations, or potassium and magnesium cations, or rubidium and barium cations or potassium and cesium cations.

In separating the para-xylene isomer in the process of the present invention, a bed of solid adsorbent is contacted with a feed mixture. The para-xylene is preferentially adsorbed on the adsorbent. The unadsorbed (or raffinate mixture) is then removed from the adsorbent bed, and the adsorbed para-xylene is removed from the solid adsorbent. The adsorbent may be contained in a · single chamber where, through programmed flow into and out of the chamber, a separation of a para-xylene stream is effected.

Swing bed operational technique where a series of adsorbent chambers are available, or simulated moving bed countercurrent operations similar to the general pattern of operations as disclosed in U. S. Patent 2,985,589 may be used. In the latter method of operation, the selection of a suitable desorbent requires that it be capable of readily displacing adsorbed para-xylene from the adsorbent and also that the para-xylene in the feed mixture be able to displace adsorbed desorbent from a previous desorption step. This requires that a desorbent used in a simultated moving bed process where there is a continuous transfer of para-xylene and desorbent into and out of the adsorbent have a selectivity, compared to para-xylene, close to unity and preferable slightly less than unity. Mass action effects are used to desorb adsorbed para-xylene with the desorbent when collecting para-xylene product, and mass action effects are also used to desorb adsorbed desorbent with para-xylene when the para-xylene is being adsorbed on the adsorbent.

The desorbent used in the process of this invention should be a material that is separable from the mixture that is fed to the solid adsorbent. In desorbing the preferentially adsorbed * component of the feed both desorbent and the desorbed feed component are removed from the adsorbent bed in admixture, and without a method of separation of these two materials the purity of the selectively adsorbed component of the feed would not be very high. Therefore, it is contemplated that a desorbent that is of a different boiling range than the feed mixture fed to the solid adsorbent be used in this separation process. The use of a desorbent of a differing boiling range would allow fractionation or other separation methods to be used to separate the selectively adsorbed feed component as a relatively pure product stream and allow recovery of the desorbent for possible reuse in the process.

Desorbents which are preferred for use in the process of the present invention include benzene, toluene, ethers, alcohols, cyclic dienes and the ketones, all of which have lower boiling points than para-xylene. However, desorbents which have a higher boiling point than the feed may also be used. Benzene and toluene are particularly preferred desorbents for use in the process of the present invention. Gaseous materials such as nitrogen, hydrogen, methane, ethane, etc., may also be used as desorbent materials.

Both liquid and vapor phase operations may be used in the process of this invention. The liquid phase operations are preferred because of the lower temperature requirements and slightly improved selectivities associated with the lower tem-peratures employed in liquid phase operations. Temperature ranges which may be used in adsorption of the preferred xylene isomer within the adsorbent include the range of from about 40°c. to about 200°C. Pressures preferred in the operation of this invention are included in the range of from about atmospheric to about 34 atmospheres, gauge (500 psig.). Desorption conditions include the same range of temperatures and pressures as used for adsorption. The desorption of the selectively adsorbed aromatic isomer may be effected at reduced pressures or elevated temperatures, in which case the desorbent would be used to strip the adsorbed component from the adsorbent.

Feed streams which may be used in the process of the present invention comprise at least two components selected from ortho-, meta-, para-xylene and ethylbenzene with possible inclusion of portions of straight and branched-chain paraffins, cyclo paraffins and aromatics including benzene, toluene, and naphthalenes. xylene It is preferred, however, to use feed streams having Cg axxxms-fci.© isomer concentrations of from about 80 to 100 volume % of the total feed contacting the adsorbent bed.

In testing various adsorbents the selectivity (BC//D) as defined previously was determined using apparatus and procedures as described below. The apparatus used to measure the selectivity of a particular adsorbent consisted of a chamber of approximately 40cc volume having inlet and outlet ports at opposite ends of the chamber. The chamber was contained within a temperature controlled heating element. Pressure control equipment was used to operate the chamber at a constant predetermined pressure. Attached to the outlet line from the chamber was chromatographic analysis equipment which was used to analyze the effluent stream leaving the adsorbent chamber.

The following general procedures were used to determine the B for various adsorbents texted in the chamber. A feed mixture having a known composition was passed through the adsorbent chamber at a regulated pressure and temperature until the effluent composition flowing from the adsorbent chamber remained at a constant composition indicating that there was not net transfer between the adsorbed phase within the adsorbent and the unadsorbed or external phase surrounding the sorbent particles. A second mixture containing a hydrocarbon which was able to desorb the previously adsorbed component of the feed from the adsorbent was then passed through the adsorbent chamber. The chromatographic analysis equipment was used to monitor the unadsorbed or external phase and the material desorbed from within the adsorbent. Knowing the compositions of θ these two streams, the B for various components present in the feed stream could be determined.

The feed streams which were used to illustrate the process of this invention in the aforementioned testing apparatus consisted of equal quantities of ethylbenzene, para-xylene and meta-xylene mixed with 2, 2, 4-trimethylpentane rendering a feed mixture containing 75 vol. % paraffinic material and 25 vol. % Cg xylene -a*e»at-ie- isomer material (8.33 vol. % of each Cg compound). The xylene Cg a^?©HwHc isomers were diluted in the paraffin material so that a more accurate analysis of the adsorbed and unadsorbed phases could be made. Ortho-xylene was excluded, since its presence complicates the analytical procedures, and previous experiments indicated that the ortho-xylene isomer behaved substantially the same as the meta-xylene isomer. The desorbent material consisted of 25 vol. % toluene, 74 vol. % 2, 2, -trimethylpentane and 1 vol. % neohexane which was used as a tracer to signal desorbent breakthrough in the effluent stream leaving the adsorbent chamber.

The adsorbents used herein to illustrate the process of the present invention were originally the sodium type X or type Y zeolites which contained cations as is indicated by their individual description. The adsorbents indicated as containing a single cation were essentially totally ion exchanged, and generally contained less than about 2 wt. % residual sodium based on volatile-free adsorbent. That is, less than 2 wt. % residual sodium remained on the adsorbent after being subjected to 900°C. calcination temperatures to drive off volatile material. The adsorbents which contained two different cations were also essentially totally ion exchanged and contained the two indicated cations.

EXAMPLE I In this example, type Y zeolites are used. The ' zeolite was essentially totally ion exchanged with the indicated cation and was tested for para-xylene/ethylbenzene selectivity (Bp_x Eg), and for para-xylene/meta-xylene selectivity (Βρ_χ Μ_χ) as previously described. The results are indicated in Table I below.

TABLE I EXAMPLE II In this example the zeolite was essentially totally ion exchanged with an aqueous mixture containing the two cations desired to be placed on the zeolite adsorbent. The adsorbents which contained both Group IA and Group IIA cations were ion exchanged in a manner which resulted in a mol ratio of the Group IIA metal over the Group IA metal of about 3:1 while the adsorbents containing the Group IA metals combination were exchanged in a manner which resulted in a mol ratio of the two Group IA metals of about 1:1.

The copper-potassium sieve tested contained a mol ratio of copper over potassium of about 3:1. The adsorbents were tested in accordance with the previously described procedures and the results of the test are reported in Table II below.

TABLE II SELECTIVITY DESCRIPTION B-v-xM-x ! Β-,-χ/ΕΒ Group IA+IIA Metals: Type Y, K+Ba exchanged . 3.76 2.10 Type Y, +Be exchanged 2.11 1.44 Type Y, K+Mg exchanged 2.25 1.41 Type Y, Rb+Ba exchanged 2.05 1.41 Type Y, Cs+Ba exchanged 1.57 1.30 Type X, K+Ba exchanged ' 2.49 2.03 Group IA Metals: Type Y, K+Rb exchanged 1.80 1.06 Type Y, K+Cs exchanged 1.79 1.03 As can be seen in Table II, the combination of the Group IA and Group IIA metals tested all displayed the ability to separate xylene para-xylene from a mixture containing all the Cg arOma-Hrc isomers. As is clear from a comparison of the data in Tables I and II, there is a synergistic effect produced by coexchanging certain Group IA and Group IIA metals that is particular to these two classes of elements. The barium and potassium coexchanged adsorbents shown in Table II display greater selectivities for para-xylene than either the single cation exchanged barium or potassium adsorbents shown in Table I of Example I.

An adsorbent analysis of the type X, K+Ba exchanged adsorbent tested in Example II is shown in Table III below.

TABLE III Type X, K+Ba Coexchanged Adsorbent Chemical analysis based on volatile free adsorbent: Na, wt. % 0.7 K, w . % 3.7 Ba, wt. % 19.1 Si02/Al203 mol ratio 2.4 Volatile material driven off at 500°C,wt.¾ 14.7 Physical analysis: Surface Area, m /gm 415 Pore Volume, cc/gm 0.24 Apparent Bulk Density (ABD), gm/cc 0.83! Particle Size (mm. ) 0.84+. , wt. % 0.0 0.59 to 0.84, wt. % 32.9 0.42 to 0.59, wt. % 44.5 0.30 to 0.42, wt. % 21.1 0.25 to 0.30, wt. % 0.7 0.25- , wt. % 9.8

Claims (17)

xylene

1. A process for separating at least one Cg arnm i r xylene isomer from a feed stock containing mixtures of Cg a«w¾e½ie- isomers and ethylbenzene ,which comprises: a. ) contacting said feed stock with a bed of crystalline aluminosilicate adsorbent comprising faujasite containing at least one cation selected from potassium, rubidium, cesium, barium and silver, in combination with at least one different cation selected from lithium, potassium, barium, magnesium, strontium, beryllium, cadmium, cobalt, nickel, copper, manganese, silver and zinc, xylene b. ) adsorbing the more selectively adsorbable Cg aromatic isomer of said feed stock with said adsorbent, c. ) withdrawing from said bed of adsorbent a raffinate stream comprising less selectively adsorbed Cg xylene iaromatic isomers, d. ) contacting the adsorbent bed with a desorbent material, and desorbina the selectively adsorbed Cg isomer from the adsorbent, and e. ) withdrawing from said adsorbent bed a product stream comprising desorbent and said selectively xylene adsorbed Cg aromat.-€ isomers.

2. The process of claim 1, further characterized in that the feed stock comprises para-xylene and at least one other xulene Cg afom i<& isomer, and para-xylene is preferentially adsorbed.

3. The process of either of claims 1 or 2, further characterized in that the feed stock comprises para-xylene, ortho-xylene, meta-xylene, and ethylbenzene, and para-xylene is preferentially adsorbed.

4. The process of any of claims 1 to 3, further cations characterized in that the faujasite contains potassium and barium.

5. The process of any of claims 1 to 3, further cations characterized in that the faujasite contains potassium and beryllium.

6. The process of any of claims 1 to 3, further cations characterized in that the faujasite contains potassium and rubidium.

7. The process of any of claims 1 to 3, further cations characterized in that the faujasite contains potassium and cesium.

8. The process of any of claims 1 to 3, further cations characterized in that the faujasite contains potassium and magnesium.

9. The process of any of claims 1 to 3, further cations characterized in that the faujasite contains rubidium and barium.

10. The process of any of claims 1 to 3, further cations characterized in that the faujasite contains cesium and barium.

11. The process of any of claims 1 to 10, further characterized in that the adsorption is effected at a temperature within a range of from about 40°C. to about 200°C, and at a pressure within a range of from about atomspheric to about 34 atmospheres, gauge.

12. The process of any of claims 1 to 11, further characterized in that the adsorption is effected in substantially

13. The process of any of claims 1 to 12, further characterized in that the desorbent has a boiling range different from that of the feed stock.

14. The process of claim 13, further characterized in that the desorbent is separated from the selectively adsorbed Cg aromatic isomer by fractionation.

15. The process of any of claims 1 to 14, further characterized in that the desorbent comprises toluene.

16. The process of any of claims 1 to 15, further characterized in that the desorption is effected in substantially the liquid phase. xylene

17. A process for separating at least one C arQmatic xylene isomer from a feed stock containing a mixture of Cg aremerie isomers substantially as hereinbefore described.