PROCESS FOR THE SEPARATION OF C9AROMATIC ISOMERS

Abstract: A process for separating a C₉ aromatic isomer from a feed stream containing a mixture of C₉ aromatic isomers by contacting the feed stream with a bed of the adsorbent zeolite beta. The adsorbed C₉ aromatic isomer is removed from the adsorbent by desorption.

Description: The field of art to which the claimed invention pertains is hydrocarbon separation. More specifically, the claimed invention relates to the separation of a C₉ aromatic isomer from a hydrocarbon feedstream containing a mixture of C₉ aromatic isomers by use of the adsorbent zeolite beta which selectively removes a C₉ aromatic isomer from the feed stream. The selectively adsorbed C₉ aromatic isomer is removed from the adsorbent through a desorption step. DESCRIPTION OF THE PRIOR ART It is known in the separation art that certain adsorbents generally comprising crystalline aluminosilicates can be utilized to separate certain hydrocarbons from mixtures thereof. In the separation of aromatic hydrocarbon isomers with certain crystalline aluminosilicates containing selected cations at the zeolitic cationic sites selectivity of the zeolite for a given aromatic isomer is enhanced. This manner of separation is particularly useful when the components to be separated have similar physical properties, such as freezing and boiling points which renders the components difficult to separate by distillation or crystallization. A number of processes describing the separation of paraxylene from a mixture of at least one other xylene isomer utilizing a crystalline aluminosilicate adsorbent, are shown in U.S. Patent Nos. 3,558,730, 3,558,732, 3,626,020 and 3,663,638. Other processes which describe the adsorption separation of ethylbenzene from a mixture of xylene isomers utilizing a crystalline aluminosilicate adsorbent are shown in U.S. Patent Nos. 3,943,182, 3,997,619, 3,998,901, and 4,021,499. U.S. Patent 3,793,385 discloses the use of the crystalline aluminosilicate zeolite beta to separate C₈ aromatic isomers, specifically to separate p-xylene and ethylbenzene from a mixture containing at least one other C₈ aromatic isomer. While the use of various zeolites to separate C₈ aromatic isomers is known, less is known about adsorbents which effectively separate the C₉ aromatic isomers. The C₉ aromatic isomers are useful in the chemical arts as desorbents in separation processes and as precursors in preparing other chemicals. More specifically para-ethyltoluene is used in preparing plastics, pseudocumene which is used in the manufacture of dyes, plasticizers and pharmaceuticals and mesitylene as a precursor for high temperature resins. However, the availability of these C₉ aromatic isomers is restricted due to the difficulty of effectively separating a C₉ aromatic isomer from a mixture of the C₉ aromatic isomers. SUMMARY OF THE INVENTION It is accordingly a broad objective of this invention to provide a process of separating a C₉ aromatic isomer from a hydrocarbon feed stream containing a mixture of C₉ aromatic isomers. In brief, the invention comprises an adsorptive separation process for the separation of the C₉ aromatic isomers from a hydrocarbon feed stream containing a mixture of C₉ aromatic isomers by contacting the hydrocarbon feed stream with a bed of the adsorbent zeolite beta. A raffinate stream is then withdrawn from the bed, this stream containing less of the selectively adsorbed C₉ aromatic isomer. The adsorbed C₉ aromatic isomer on the bed is then desorbed to effect displacement of the isomer, followed by withdrawing from the adsorbent bed an extract stream containing the adsorbed C₉ aromatic isomer. The preferred zeolite beta adsorbent is cation exchanged to increase the C₉ aromatic selectivity of the adsorbent. DETAILED DESCRIPTION OF THE INVENTION Hydrocarbon feed streams which can be utilized in the process of this invention contain mixtures of C₉ aromatic isomers. Specifically, these include the difficult to separate trimethyl­ benzenes and ethyltoluenes. The ethyltoluene isomers include the ortho, para and meta isomers, while the trimethylbenzene isomers including 1,2,3-trimethylbenzene (hemimellitene), 1,2,4-trimethyl­ benzene (pseudocumene), and 1,3,5-trimethylbenzene (mesitylene). The close boiling points of these C₉ aromatic isomers renders the isomers difficult to separate by distillation. Mixtures containing substantial quantities of C₉ aromatic isomers and other aromatics generally are produced by reforming processes, processes which are well known to the refining and petrochemical arts. The hydrocarbon feed stream is contacted with a bed of the adsorbent, entitled zeolite beta. Zeolite beta and its method of manufacture is described in U.S. Patent 3,308,069 and its Reissue Patent 28,341 by Wadlinger et al., this disclosure being fully incorporated herein by reference. Also included within this definition of zeolite beta is where the aluminum is fully or partially substituted with the elements of gallium or boron to provide a gallo silicate or borosilicate or where the silicon is fully or partially substituted with the elements of germanium or phosphorus but retains the same structure and similar x-ray diffraction pattern as defined in the Wadlinger et al. patents. After synthesizing the zeolite beta, it is necessary to calcine the zeolite at a temperature and for a time effective to remove any tetraethylammonium ions remaining after its synthesis. While the zeolite beta composition is fully described in this Wadlinger et al. patent, it has been surprisingly found that zeolite beta can be used to separate C₉ aromatic isomers from a feedstream containing a mixture of C₉ aromatic isomers. Further, the C₉ selectivity can be substantially increased by cation exchanging the zeolite beta with a suitable cation. The process of this invention may be used to separate all the C₉ aromatic isomers from one another by the use of various stages or adsorption zones. The preferred separation is of para-ethyl­ toluene since it is the most strongly adsorbed C₉ aromatic isomer. In general the ethyltoluenes are preferentially adsorbed over the trimethylbenzenes with an order for the decreased preference of adsorption on zeolite beta as follows: p-ethyltoluene > O-­ ethyltoluene > m-ethyltoluene > pseudocumene + hemimellitene > mesiSHY tylene. In one embodiment of this invention the C₉ aromatic isomers are ethyltoluenes which are selectively adsorbed in the order of para-ethyltoluene > ortho-ethyltoluene > meta-ethyltoluene. In another embodiment the C₉ aromatic isomers are trimethylbenzenes which are selectively adsorbed in the order of hemimellitene and pseudoc umene>mesitylene, with either hemimellitene or pseudocumene being preferentially adsorbed depending upon the particular cation exchanged form of the zeolite beta and the conditions of adsorption. In order to substantially increase the selectivity of the adsorbent for C₉ aromatic isomers, the adsorbent which is available in its tetraethylammonium (TEA)-sodium form is preferably cation exchanged. After the TEA is removed the hydrogen-sodium form of zeolite beta can be exchanged with suitable cations which include base metal or transition metal cations, such as copper, rubidium, nickel, magnesium, strontium, cobalt, potassium, lead, barium, lithium, cadmium, cesium and calcium, or mixtures thereof or other cations, such as ammonium. The preferred cations for increased selectivity are potassium, lithium, cesium, with the most preferred cation being cesium. The zeolite beta adsorbent can be combined with a binder, such as natural or synthetic clays (e.g. Kaolin), and inorganic oxides and can be in any form acceptable to the separation process such as extrudates, spheres, granules or tablets. Certain characteristics of adsorbents are highly desirable, if not absolutely necessary, to the successful operation of a selective adsorption process. Among such characteristics are: the level of adsorptive capacity for the C₉ aromatic isomer and the selective adsorption of a C₉ aromatic isomer with respect to a raffinate component and the desorbent material. The adsorbent must be capable of adsorbing the C₉ aromatic isomer; without such capacity the adsorbent is useless for adsorptive separation. Furthermore, the higher the adsorbent's capacity for the C₉ aromatic isomer, the better is the adsorbent. Increased capacity of a particular adsorbent makes it possible to reduce the amount of adsorbent needed to separate the C₉ aromatic isomer contained in a particular charge rate of feed mixture. A reduction in the amount of adsorbent required for a specific adsorptive separation reduces the cost of the separation process. It is important that the good initial capacity of the adsorbent be maintained during actual use in the separation process over some economically desirable life. Generally, the adsorbent of this invention has a capacity of at least 3% of hydrocarbon by weight of adsorbent and preferably at least 5% of hydrocarbon by weight of adsorbent. The adsorbent is also capable of separating components of the feed; or, in other words, that the adsorbent possess adsorptive selectivity, (α), for one component as compared to another component. Relative selectivity can be expressed not only for one feed component as compared to another but can also be expressed between any feed mixture component and the desorbent material. The separation factor, (α), as used throughout this specification is defined as the ratio of the two components of the adsorbed phase over the ratio of the same two components in the unadsorbed phase at equilibrium conditions. Relative selectivity is shown as Equation 1 below: where C and D are two components of the feed represented by weight and the subscripts A and U represent the adsorbed and unadsorbed phases respectively. The equilibrium conditions were determined when the feed passing over a bed of adsorbent did not change composition after contacting the bed of adsorbent. In other words, there was no net transfer of material occuring between the unadsorbed and adsorbed phases. Where selectivity of two components approaches 1.0 there is no preferential adsorption of one component by the adsorbent with respect to ther other; they are both adsorbed (or nonadsorbed) to about the same degree with respect to each other. As the selectivity (α) becomes less than or greater than 1.0 there is preferential adsorption by the adsorbent for one component with respect to the other. When comparing the selectivity by the adsorbent of one component C over component D, at (α) larger than 1.0 indicates preferential adsorption of component C within the adsorbent. An (α) 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. For optimum performance desorbent materials should have a selectivity equal to about 1 or less than 1 with respect to all extract components so that all of the extract components can be extracted as a class and all raffinate components clearly rejected into the raffinate stream. When the adsorbent of this invention is cation exchanged it is preferably exchanged with a cation which will impart an (α) separation factor of at least 2.0 of the C₉ aromatic isomer (component C) over at least one of the other components (component D) of the hydrocarbon feed stream. In order to test the various cation exchanged zeolite beta adsorbent materials with a particular feed mixture to measure the characteristics of adsorptive capacity and selectivity, a static testing procedure was employed. The procedure consisted of contacting a known weight of adsorbent with a known weight of mixed hydrocarbon feed stream. After allowing this mixture to reach equilibrium, a sample was removed and analyzed by gas chromatography. The amount of isomers in the raffinate were measured and the amount of isomers adsorbed were obtained by difference from the standard feed stream. In a separation process, after the hydrocarbon feed stream is contacted with the bed of adsorbent, a raffinate stream is withdrawn from the adsorbent bed, this stream containing less of the selectively adsorbed C₉ aromatic isomer of the feed stream. Then the adsorbed aromatic isomer on the bed is desorbed to effect displacement thereof. The desorbing step which can be used in the various processing schemes employing this adsorbent will vary depending on the type of operation employed. Preferably a desorbent material is passed through the bed. The term "desorbent material" as used herein shall mean any fluid substance capable of removing a selectively adsorbed C₉ aromatic isomer from the adsorbent. In the swing­ bed system in which the selectively adsorbed C₉ aromatic isomer is removed from the adsorbent by a purge stream, desorbent materials comprising gaseous hydrocarbons such as methane, ethane, etc., or other types of gases such as nitrogen or hydrogen may be used at elevated temperatures or reduced pressures or both to effectively purge the adsorbed C₉ aromatic isomer from the adsorbent. However, in an adsorptive separation process which employs the adsorbent and which is generally operated at substantially constant pressures and temperatures to insure a liquid phase, the desorbent material relied upon must be judiciously selected to satisfy several criteria. First, the desorbent material must displace the C₉ aromatic isomer from the adsorbent with reasonable mass flow rates without itself being so strongly adsorbed as to unduly prevent the C₉ aromatic isomer from displacing the desorbent material in a following adsorption cycle. Expressed in terms of the selectivity, it is preferred that the adsorbent be more selective for the C₉ aromatic isomers with respect to a raffinate (e.g. other isomers), than it is for the desorbent material with respect to a raffinate. Secondly, desorbent materials must be compatible with the particular adsorbent and the particular feed stream. More specifically they must not reduce or destroy the critical selectivity of the adsorbent for the C₉ aromatic isomers with respect to the raffinate. Desorbent materials to be used in the process of this invention should additionally be substances which are easily separable from the feed stream that is passed into the process. After desorbing the C₉ aromatic isomer of the feed, both desorbent material and the C₉ aromatic isomers are removed in a mixture from the adsorbent. Without a method of separating the desorbent material, such as distillation, the purity of either the C₉ aromatic isomer or the raffinate component would not be very high. It is therefore contemplated that any desorbent material used in this process will have a substantially different average boiling point than that of the feed stream. The use of a desorbent material having a substantially different average boiling point than that of the feed allows separation of desorbent material from feed components in the extract and raffinate streams by simple fractionation thereby permitting reuse of desorbent material in the process. The term "substantially different" as used herein shall mean that the difference between the average boiling points between the desorbent material and the feed mixture shall be at least about 8°C (15°F). The boiling range of the desorbent material may be higher or lower than that of the feed mixture. In a liquid-phase operation of the process of our invention, desorbent materials comprising mono-aromatic hydrocarbons are effective. Mixtures of toluene with paraffins are also effective as desorbent materials. Such paraffins must be compatible with the adsorbent and feed stream as described above and must be easily separated from the feed stream. The paraffins can include straight or branched chain paraffins or cycloparaffins which meet these criteria. Typical concentrations of toluene in such mixtures can be from a few volume percent up to 100 volume % of the total desorbent material mixture but such concentrations preferably will be within the range of from about 50 volume % to about 100 volume % of the mixture. Other desorbents include benzene, diethylbenzene, other polyalkylbenzenes, or more generally polycyclic hydrocarbons etc., and mixtures thereof. Following desorption, the extract stream containing the C₉ aromatic isomer is withdrawn from the adsorbent bed. Depending on the separation factor (α) this withdrawn extract can contain relatively pure fractions of C₉ aromatic isomer. However, it will be appreciated that the selectively adsorbed component is generally not completely adsorbed by the adsorbent, nor is the raffinate component generally completely non-adsorbed by the adsorbent. In general, this adsorptive separation process can be carried in the vapor or liquid phase, while the liquid phase is preferable. Adsorption conditions for the process of this invention may include temperatures within the range of from about ambient to 450°F. (232°C) and will include pressures in the range from about atmospheric to 500 psig (3450 kPa). Desorption conditions for the process of the invention shall generally include the same range of temperatures and pressures as described for the adsorption operation. The desorption of the selectively adsorbed C₉ aromatic isomer could also be effected at subatmospheric pressures or elevated temperatures or both, or by vacuum purging of the adsorbent to remove the adsorbed isomer, but this process is not primarily directed to these desorption methods The following Examples are now given, though by way of illustration only, to show certain aspects of the invention in more detail. Example I A crystalline aluminosilicate adsorbent zeolite beta having an Al/Si atom ratio of 0.07 in its hydrogen form was cation exchanged with the cations to the extent listed in Table I. A C₉ aromatic feedstream containing 0.3% of para-ethyltoluene (PE), 1.15% of meta-ethyltoluene (ME), 0.38% of ortho-ethyltoluene (OE), 1.99% of pseudocumene (PS), 0.55% of mesitylene (MS) and 0.52% of hemimellitene (HE), with the remainder n-hexane, all by weight, was added at ambient temperature to the various cation exchanged zeolite beta adsorbents, with the amount of feedstream being in excess of that which the zeolite can adsorb. After allowing this mixture to reach equilibrium, the mixture was allowed to settle and a sample was removed and analyzed by gas chromatography. The amount of C₉ isomers in the raffinate were measured and the amount of isomers adsorbed were obtained by difference from the standard feed stream. The capacity and the (α) separation factor were calculated for para-ethyltoluene as compared to the other C₉ isomers and between the trimethylbenzene isomers as listed in Table I. As can be seen from Table I, the potassium and especially the cesium exchanged forms of zeolite beta were highly effective in the separation of para-ethyltoluene from the other C₉ aromatic isomers. Example II A sample of a C₉ ethyltoluene aromatic feedstream containing equimolar amounts of para-ethyltoluene (PE), meta-ethyltoluene (ME) and ortho-ethyltoluene (OE) was added to various cation exchanged aluminosilicate zeolite beta adsorbents in an amount equal to the capacity of the adsorbent. After agitation at room temperature to reach equilibrium a gas phase sample was removed and analyzed by gas chromatography. From the peaks of the chromatograms the (α) separation factors between the various component of the feedstream were measured. The capacity and (α) separation factor are listed on the following Table II. Table II Table I Example III A sample of a C₉ trimethylbenzene aromatic feedstream containing equimolar amounts of pseudocumene, mesitylene and hemi­ mellitene were added to a Cs aluminosilicate zeolite beta adsorbent as in Example II. The capacity and the α separation factor were calculated for the various components of the feedstream, as listed in Table III. TABLE III "α" Separation Factor pseudocumene/mesitylene = 6.9 pseudocumene/hemimellitene = 2.3 hemimellitene/mesitylene = 3 Adsorption Capacity Wt.% = 20.9 Example IV A gallosilicate zeolite beta adsorbent was cation exchanged with cations as listed in Table IV. A C₉ aromatic feedstream containing 0.3% p-ethyltoluene (PE), 1.0% m-ethyltoluene (ME), 0.3% o-ethyltoluene (OE), 2.0% pseudocumene (PS), 0.5% mesitylene (MS) and 0.5% hemimillitene (HE) with the remainder n-hexane was added to the adsorbent as in Example I. The capacity and the (α) separation factor are calculated as listed in Table IV. TABLE IV The above examples demonstrate the effectiveness of zeolite beta as an adsorbent for separating C₉ aromatic isomers.

Claims: 1. An adsorptive separation process for separating at least one C₉ aromatic isomer from a hydrocarbon feed stream containing a mixture of C₉ aromatic isomers comprising: (a) contacting said hydrocarbon feed stream with a bed of an adsorbent of zeolite beta; (b) withdrawing from said bed of adsorbent a raffinate stream containing less of the selectively adsorbed C₉ aromatic isomer of the feed stream; (c) desorbing the adsorbed C₉ aromatic isomer to effect displacement thereof; and (d) withdrawing from the adsorbent bed an extract stream containing the adsorbed C₉ aromatic isomer. 2. Process of Claim 1 wherein the adsorbent selectively adsorbs paraethyltoluene. 3. Process of Claim 1 wherein the mixture of C₉ aromatic isomers are trimethylbenzenes which are selectively adsorbed in the order of pseudocumene and hemimellitene>mesitylene. 4. Process of Claim 1 wherein the mixture of C₉ aromatic isomers are ethyltoluenes which are selectively adsorbed in the order of para-ethyltoluene > ortho-ethyltoluene > meta-ethyltoluene. 5. Process of any of Claims 1 to 4, wherein the adsorbent has a cation exchanged with cesium. 6. Process of any of Claims 1 to 4, in which the adsorbent contains at least one cation which imparts an (α) separation factor of at least 2.0 of the adsorbed aromatic isomer over at least one of the other components of the hydrocarbon feed stream. 7. Process of any of Claims 1 to 6, wherein the zeolite beta adsorbent is an aluminosilicate. 8. Process of any of Claims 1 to 7, wherein the adsorbed aromatic components are desorbed by passing a desorbent material through said bed. 9. Process of Claim 8, wherein the desorbent is selected from the group consisting of toluene, benzene, paraffin, diethylbenzene, polyalkyl benzenes, polycyclic hydrocarbons and mixtures thereof. 10. Process of any Claims 1 to 9, wherein the separation is carried out at a temperature within the range of ambient to 232°C and a pressure within the range of atmospheric to 3450 kPa. 11. Process of any of Claims 1 to 10 wherein the process is carried out in the liquid phase.

Inventors: BARTHOMEUF, DENISE MARY, ROSENFELD, DANIEL DAVID
Applicants: EXXON RESEARCH AND ENGINEERING COMPANY
Application year: 1985
Main classification: C07C7
IPC codes: C07C15, C07C1, C07C7, C07C67