PROCESS FOR THE SEPARATION OF PARA-XYLENE USING AN ETHER AS DESORBENT

Abstract: A process for the production of paraxylene from a C8 aromatic hydrocarbon stream using simulated moving bed countercurrent adsorption in combination with an ether desorbent and preferably a Ba X zeolite adsorbent. The adsorbent/ether desorbent system has been found to be particularly good when the water content of the system is carefully controlled. Preferred ether desorbents are diisopropyl ether and methyl tertiary-butyl ether.

Description: PROCESS FOR THE SEPARATION OF PARA-XYLENE USING AN ETHER AS DESORBENT Field of the Invention This invention relates to the separation of xylene isomers and ethylbenzene by adsorption to produce high purity paraxylene. Background of the Invention Paraxylene is a valuable petrochemical precursor for the production of terephthalate polymers such as polyethylene terephthalate. Polyethylene terephthalate is the polymer in several large scale end uses including polyester fibers and PET bottle resin. The large market for paraxylene for fiber and PET resin creates a need for large scale production and purification facilities for paraxylene. Paraxylene is typically isolated in refineries from the reformate produced by catalytic reformers although it can come other sources such as pygas. Catalytic reformers produce xylene molecules by the dehydrocyclization of straight and branched chain paraffins, the dehydrogenation of naphthenes, and to a small extent the dealkylation of polyalkylaromatics. The C 8 aromatic portion of the reformate contains more than just paraxylene. The other aromatic isomers present in C 8 boiling range reformate are orthoxylene (OX), metaxylene (MX), and ethylbenzene (EB). These isomers are difficult or impossible to separate from the paraxylene by distillation because they have close boiling points. Two methods that have been developed to isolate the paraxylene (PX) form the other C 8 isomers are crystallization and adsorption. Continuous simulated moving bed adsorption is currently used in most grass roots PX separation plants. Examples of such PX adsorption processes include Parex (UOP) and Eluxyl (IFP). These processes utilize an X or Y zeolite adsorbent in combination with either toluene or para-diethylbenzene as a desorbent. Selection of a desorbent is an important consideration and has significant impact on the design and economics of the process. In general the desorbent should be adsorbed about as well as the feed components on the selected adsorbent. If the desorbent is adsorbed too tightly to the adsorbent, an excessive fraction of the total capacity of the adsorbent is occupied by the desorbent. This effectively reduces the portion of the adsorbent that is available to adsorb and separate the C 8 aromatic isomers, and increases the amount of adsorbent that must be used. If the desorbent is not adsorbed tightly enough large amounts of desorbent must be used causing a high desorbent to feed ratio. A high ratio of desorbent to feed results in the need for larger extract and raffmate distillation columns. The extract column separates the PX product from the desorbent. In a PX process the raffmate column separates the the desorbent from the other C 8 aromatic isomers. These distillation columns are a major part of the expense of a PX separation process. They are both a large portion of the capital cost of the process and cause a large portion of the operating cost of the PX separation plant. Thus it is generally best to minimize the amount of desorbent used in the process. Another important parameter of the desorbent is its boiling point. Since the desorbent is separated from the raffmate and the extract by distillation it is desirable to select a desorbent with a boiling point that is substantially different from the extract and raffmate boiling ranges. The closer the desorbent boiling point to the PX extract or C aromatic raffmate boiling range, the larger the column required and the higher the utility costs to operate the column. Also it is important to select a desorbent that does not form an azeotrope with any of the feed components in order to have a clean separation. A patent that discusses the use of diisopropyl ether as a desorbent in a paraxylene adsorption process is U.S.P. 4,255,607 which is herein incorporated by reference. U.S.P. 4,255,607 is silent with regards to water content or control in their process. Summary of the Invention The present invention relates to an adsorptive separation process for the separation of paraxylene from a hydrocarbon feed comprising a mixture of C 8 aromatic hydrocarbons comprising: a) contacting the feed, at adsorption conditions with an adsorbent bed comprising crystalline aluminosilicate selected from the group consisting of type X and type Y zeolites thereby adsorbing paraxylene from the feed; b) contacting the adsorbent bed at desorption conditions with a desorbent material comprising diisopropyl ether; c) withdrawing a stream comprising desorbent and less selectively adsorbed components of the feed from the adsorbent bed; and d) withdrawing from the adsorbent bed a stream comprising desorbent and said paraxylene, wherein, the water content is maintained between 1 and 8 wt % based on the weight of the adsorbent The process may additionally include the steps of e) separating the stream comprising desorbent and less selectively adsorbed components into a second stream comprising desorbent and a stream comprising less selectively adsorbed components; and f) separating the stream comprising desorbent and said paraxylene into a third stream comprising desorbent and a stream comprising paraxylene Among other factors the present invention is based on the finding that control of the water content of the process has a surprising impact on the paraxylene adsorption process when using an X or Y zeolite molecular sieve adsorbent in combination with diisopropyl ether desorbent or MTBE desorbent. Also, it has been surprisingly found that diisopropyl ether is a particularly good desorbent for PX adsorptive separation when the water content of the system is maintained between 1% and 8 wt% based on the weight of the X or Y zeolite adsorbent. It has been found that both the selectivity and the capacity of the adsoφtion system changes substantially depending on the water content of the system. To maintain both 1) the proper selectivities for PX, the other C 8 aromatic hydrocarbons, and the desorbent, and 2) the proper capacity of the system, one must be careful to control the water content of the process. The invention is based on the suφrising finding that ethers can be particularly good desorbents in a paraxylene (PX) adsoφtion separation processes where PX is separated from mixed xylenes. Oxygenates have been generally thought to be poor candidates for desorbents because they can have too high an affinity for the adsorbents. A good desorbent must not be adsorbed too strongly yet must be able to displace the C 8 isomers from the adsorbent. Paraxylene adsoφtion processes include simulated moving-bed, countercurrent adsoφtion processes such as Parex and Eluxyl processes. The particularly preferred desorbent of the present invention is diisopropyl ether (IPE). Another preferred desorbent of the present invention is methyl tertiary- butyl ether (MTBE). Detailed Description of the Invention In particular Diisopropyl ether (IPE) has been found to be an effective desorbent for an adsoφtive separation system using an adsorbent comprising barium exchanged X or Y zeolite. It has also been suφrisingly found that the selectivities for an adsorbent/IPE system can be better than an adsorbent/toluene system when the water content of the adsorbent/IPE system is controlled. In particular the selectivity of PX vs EB is better in the IPE system. The PX vs EB selectivity is 2.8 in the IPE system compared to 1.7 in the toluene system. The PX vs EB selectivity is important because , except for the PX, the EB is the most strongly adsorbed of the C 8 aromatic isomers. Thus the EB/PX separation is the most difficult. Also the selectivity of the PX vs. the desorbent is an extremely important parameter. A selectivity of greater than 1 means that the PX is more strongly adsorbed than the desorbent. A selectivity of less than 1 means that the PX is less strongly adsorbed than the desorbent. It is preferable that the desorbent be about as strongly adsorbed as the C 8 aromatic isomers. Most preferably the desorbent is adsorbed less strongly than the desired PX isomer but more strongly than the next most strongly adsorbed C 8 aromatic isomer. If the desorbent is adsorbed too strongly, an excessive fraction of the total capacity of the adsorbent is occupied by the desorbent. This effectively reduces the portion of the adsorbent that is available to adsorb and separate the C 8 aromatic isomers, and increases the amount of adsorbent that must be used. If the desorbent is not adsorbed strongly enough, large amounts of desorbent must be used, causing a high desorbent to feed ratio and increasing the capital and operating costs of the process. It has been suφrisingly found, as the examples will show, that when using diisopropyl ether desorbent, carefull control of the water content allows a state to be achieved where the PX is the most strongly adsorbed component, followed by the diisopropyl ether desorbent, followed by the other C 8 aromatics while maintaining good capacity. Another aspect of the present invention is that the water content of the adsoφtion/desoφtion system must be maintained at between 1 wt % and 8 wt % based on the dry weight of the adsorbent when the desorbent is diisopropyl ether. If the water level is too low the ether is weakly adsorbed, and is not an effective desorbent. If the water level is too high the capacity of the adsorbent is greatly decreased requiring more adsorbent and making the process uneconomical. The water content on the adsorbent can be measured by loss on ignition (LOI) where the adsorbent is weighted then heated to a predetermined temperature for a set time period, then weighted again to determine how much water was evolved from the adsorbent. As mentioned above diisopropyl ether is a preferred desorbent of the present invention. Diisopropyl ether has been found to have suφrising advantages over conventional-prior art desorbents such as toluene and para-diethyl benzene. One advantage is that separation of the diisopropyl ether desorbent from the PX or other C isomers is easier due to a wider difference in boiling point. An advantage of an ether desorbent and in particular the preferred diisopropyl ether (IPE) desorbent is that it is easily separated by distillation from the extract containing the PX (Bp = 138.4 degrees C) and from the raffmate containing the PX depleted xylenes and EB (boiling range 136-145 degrees C). The boiling point of IPE is 69 degrees C versus 110.6 degrees for toluene which is the current light desorbent of choice. This large difference in boiling points would allow smaller extract and raffmate columns and provide both capital and operating cost savings. The savings can be particularly important to the economics of a hybrid process, where the cost for separating the desorbent from the extract and the raffmate streams can be as great or greater than the cost for separating the xylenes in the adsoφtion column. A hybrid PX process is one which includes both adsoφtion and crystallization sections. Typically the adsoφtion section of a hybrid process is used as the first stage of the process and produces a PX enriched stream. Crystallization is then used to purify the PX enriched stream to produce high purity PX. When using Diisopropyl ether desorbent, I have found that the water content of the adsoφtion system should be maintained at between 1 % and 8 wt % based on the weight of the adsorbent. Preferably the water content of the system should be between 1 % and 8 wt %, more preferably 2 and 6 wt %, and still more preferably between 2 and 5 wt %. When using Methyl tertiary-butyl ether desorbent, I have found that the water content of the adsoφtion system should be below 4 wt % based on the weight of the adsorbent. Preferably the water content of the system should be less than 3 wt %,more preferably less than 2 %, and still more preferably less than 1 %. The adsorbent used in the present invention is preferably a type X or type Y molecular sieve zeolite. The zeolite adsorbent is preferably modified to contain one or more alkaline or alkaline earth metal ions at exchangeable cationic sites. More preferably the X or Y zeolite is modified to contain one or more alkaline or alkaline earth metal ion selected from the group consisting of barium, potassium, calcium, strontium and cesium at exchangeable cationic sites. Most preferably the X or Y zeolite contains barium at exchangeable cationic sites. As mentioned above adsorbents to be used in the process of this invention comprise specific crystalline aluminosilicates or molecular sieves, namely X and Y zeolites. The zeolites have known cage structures in which the alumina and silica 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 dehydration 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: (0.9 +- 0.2)M 2 . n O:Al 2 O 3 :(2.5 +- 0.5)SiO 2 :yH 2 O Formula 1 where "M" is a cation having a valence of not more than 3 which balances the electrovalence of the 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 SiO 2 /Al 2 O 3 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: (0.9 +- 0.2)M 2 n O:Al 2 O 3 :wSiO 2 :yH 2 O Formula 2 where "M", "n" and "y" are the same as above and "w" is a value greater than about 3 up to about 6. The SiO 2 /Al 2 O 3 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 I A or II A, e.g., barium or potassium or mixtures thereof, is necessary to achieve the separation. The preferred adsorbent useful in the present invention is a barium exchanged X zeolite. Typically, adsorbents used in separative processes contain the crystalline material dispersed in an amoφhous, 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) (1.9 mm to 250 micron). Feed mixtures which can be utilized in the process of this invention will comprise para-xylene, at least one other C 8 aromatic isomer, and may also contain one or more C 9 aromatics as impurities. Mixtures containing substantial quantities of para-xylene and other C 8 aromatic isomers and C 9 aromatics generally are produced by reforming and isomerization processes, processes which are well known to the refining and petrochemical arts. Many of the C 9 aromatics have boiling points in the range of 160- 170. degrees C. and cannot be easily removed by distillation from the standard desorbent, p-diethylbenzene. In the current process, therefore, the C 9 's are usually removed from the feed by distillation before the adsoφtive separation and consequent contact with the normal desorbent. I have discovered a desorbent which can be easily separated from the C 9 aromatics by fractionation and does not require the large column and quantity of utilities to pretreat the feed, resulting in substantial cost savings. Patents that teach about faujasitic-type, X and Y zeolites are United States Patents 2,882,244, 3,130,007, 3,789,107, 3,949,059, 4,007,253, 4,340,573, 4,440,366, 4,436,708, and 4,678,651 all herein incoφorated by reference. 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 C 8 aromatic isomers. Generally, the reformate is then fractionated to concentrate the C 8 aromatic isomers in a C 8 fraction which will contain the C 8 aromatic isomers as well as C 8 nonaromatics and C 9 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 C 8 aromatic isomers, e.g., enriched in p-xylene, as well as C 8 nonaromatics and C 9 aromatics. The C 9 aromatic content of isomerized xylene isomers can be as much as 1-2% depending on isomerization conditions. Likewise, transalkylation of mixtures of C and C 9 aromatics produces xylene isomers which contain C 9 aromatics. In all of these catalytic processes, the xylene splitter column must be employed to remove C 9 aromatics from C 8 aromatics before conventional adsoφtive xylene separation methods can be employed. Thus, the feed mixtures to the process of this invention can contain quantities of C 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 C 8 aromatic and C 9 aromatics, the mixture is contacted with the adsorbent at adsoφtion 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 a desorbent material at desoφtion 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 adsoφtion 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 raffmate 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 raffmate 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 raffmate product would not be very high nor would the desorbent material be available for reuse in the process. 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 mixture or any of its components, i.e., more than about 20 degrees centrigrade difference, to allow separation of at least a portion of the desorbent material from feed components in the extract and raffmate 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. As discussed above there are several criteria that an adsorbent/desorbent pair must meet. A suitable desorbent or desorbents for a particular separation with specific adsorbent are generally not predictable. I have further found that the water content of the adsorbent/desorbent system can have a significant impact on the selection criteria and on the effectiveness of the adsorbent/desorbent pair particularly using an ether desorbent. Examples I performed the following tests in order to determine the effectiveness of Ethers as desorbents and to determine the effect of varying water concentrations on the X type zeolite adsorbent/ ether desorbent systems. Example 1 Preparation of Ba-X Zeolite via Ion Exchange. The starting material used was Na-X zeolite (molar Si/Al ratio = 1.0, i.e., molar SiO 2 /Al 2 O 3 ratio = 2.0). Suitable Na X zeolites can be synthesized using the procedures outlined in U.S. Patents 5,370.859 and 3,789,107 which are herein incoφorated by reference. The procedure which follows was used to make the Barium exchanged X-Zeolite: 7.84 g Ba(NO 3 ) 2 was added water to get a total volume of 100 ml. 1.0 g Na-X-Zeolite was then added to 10.0 g of the Ba(NO 3 ) 2 solution. The aqueous Na-X/ Ba(NO 3 ) 2 suspension was then placed into a 25 ml Teflon-lined Parr autoclave and rotated at 43 φm at 80 °C for 16.5 hours. The resulting Ba-X zeolite was filtered, then washed with -200 ml water. The Ba-X zeolite was then Air-dryed at 65 °C overnight. Prior to use in the adsoφtion tests that follow it was dried at 122 C for 16 hours and then 300 C for 3 hours. Example 2 Adsorbent/IPE Desorbent System Without Water Added In this experiment a Barium exchanged X zeolite was used as the adsorbent. All of the reagents and materials used were carefully kept dry using appropriate means unless otherwise noted. About 4 g of the adsorbent was carefully weighed into a vial along with about 2 g of the ether desorbent and about 2 g of a mixed xylene with known amounts of ethyl benzene, meta-xylene, paraxylene, and ortho xylene. 0.1 grams of a GC tracer (triisopropylbenzene) was also added to the adsorbent/desorbent/xylenes system. It is known that the tracer, in this case triisopropylbenzene, is not adsorbed by the adsorbent thus can be used as a standard or reference to determine the change in composition of the free liquid in the system and thus by difference the amount each component adsorbed by the adsorbent. Said system was allowed to equilibrate for 3 hours at 140 degrees F. A sample of the free liquid in the system was removed by syringe and injected into a Gas Chromatograph (GC). Using the GC results and the known amounts of components added to the system, the capacity and the selectivity of the system was calculated by difference. The calculated results for this example are given in Table 1. Example 3 Adsorbent/IPE Desorbent System With 4 Wt % Water Added The procedure used in this experiment was the same as that used in example 2 except that a controlled amount of water was added to the system along with the triisopropylbenzene tracer. In this example 0.16 g of water was added in order to make the system 4 wt % water based on the dry weight of the adsorbent. The calculated results for this example are given in Table 1. Example 4 Adsorbent/IPE Desorbent System With 5.9 Wt % Water Added The procedure used in this experiment was the same as that used in example 2 except that a controlled amount of water was added to the system along with the triisopropylbenzene tracer. In this example 0.236 g of water was added in order to make the system 5.9 wt % water based on the dry weight of the adsorbent. The calculated results for this example are given in Table 1. Example 5 Adsorbent/IPE Desorbent System With 9.6 Wt % Water The procedure used in this experiment was the same as that used in example 2 except that a controlled amount of water was added to the system along with the triisopropylbenzene tracer. In this example 0.384 g of water was added in order to make the system 9.6 wt % water based on the dry weight of the adsorbent. The calculated results for this example are given in Table 1. TABLE 1 Selectivity and Capacity Results for Examples 2-5 Using Ba X Zeolite Adsorbent and Diisopropyl Ether Desorbent Example 6 Ba X Zeolite Adsorbent/MTBE Desorbent System Without Water Added In this experiment a Barium exchanged X zeolite was used as the adsorbent. All of the reagents and materials used were carefully kept dry using appropriate means unless otherwise noted. About 4 g of the adsorbent was weighed into a vial along with about 2 g of the ether desorbent and about 2 g of a mixed xylene with known amounts of ethyl benzene, meta-xylene, paraxylene, and ortho xylene. 0.1 grams of a GC tracer (triisopropylbenzene) was also added to the adsorbent/desorbent/xylenes system. It is known that the tracer, in this case triisopropylbenzene, is not adsorbed by the adsorbent thus can be used as a standard or reference to determine the change in composition of the free liquid in the system and thus by difference the amount each component adsorbed by the adsorbent. Said system was allowed to equilibrate for 3 hours at 140 degrees F. A sample of the free liquid in the system was removed by syringe and injected into a Gas Chromatograph (GC). Using the GC results and the known amounts of components added to the system, the capacity and the selectivity of the system was calculated by difference. The calculated results for this example are given in Table 2. Example 7 Ba X Zeolite Adsorbent/MTBE Desorbent System With 9.6 Wt % Water The procedure used in this experiment was the same as that used in example 6 except that a controlled amount of water was added to the system along with the triisopropylbenzene tracer. In this example 0.384 g of water was added in order to make the system 9.6 wt % water based on the dry weight of the adsorbent. The calculated results for this example are given in Table 2. TABLE 2 Selectivity and Capacity Results for Examples 6-7 Using Ba X Zeolite Adsorbent and Methyl-tert-butyl ether Desorbent Comparative Example 8 Ba X Zeolite Adsorbent/Toluene Desorbent System In this experiment a Barium exchanged X zeolite was used as the adsorbent. All of the reagents and materials used were carefully kept dry using appropriate means unless otherwise noted. About 4 g of the adsorbent was weighed into a vial along with about 2 g of toluene desorbent and about 2 g of a mixed xylene with known amounts of ethyl benzene, meta-xylene, paraxylene, and ortho xylene. About 0.1 grams of a GC tracer (triisopropylbenzene) was also added to the adsorbent/desorbent/xylenes system. It is known that the tracer, in this case triisopropylbenzene, is not adsorbed by the adsorbent thus can be used as a standard or reference to determine the change in composition of the free liquid in the system and thus by difference the amount each component adsorbed by the adsorbent. Said system was allowed to equilibrate for 3 hours at 140 degrees F. A sample of the free liquid in the system was removed by syringe and injected into a Gas Chromatograph (GC). Using the GC results and the known amounts of components added to the system, the capacity and the selectivity of the system was calculated by difference. The calculated results for this example are given in Table 3. Comparative Example 9 Ba X Zeolite Adsorbent/ para-Diethylbenzene Desorbent System In this experiment a Barium exchanged X zeolite was used as the adsorbent. All of the reagents and materials used were carefully kept dry using appropriate means unless otherwise noted. About 4 g of the adsorbent was weighed into a vial along with about 2 g of para-diethylbenzene desorbent and about 2 g of a mixed xylene with known amounts of ethyl benzene, meta-xylene, paraxylene, and ortho xylene. About 0.1 grams of a GC tracer (triisopropylbenzene) was also added to the adsorbent/desorbent/xylenes system. It is known that the tracer, in this case triisopropylbenzene, is not adsorbed by the adsorbent thus can be used as a standard or reference to determine the change in composition of the free liquid in the system and thus by difference the amount each component adsorbed by the adsorbent. Said system was allowed to equilibrate for 3 hours at 140 degrees F. A sample of the free liquid in the system was removed by syringe and injected into a Gas Chromatograph (GC). Using the GC results and the known amounts of components added to the system, the capacity and the selectivity of the system was calculated by difference. The calculated results for this example are given in Table 3. TABLE 3 Selectivity and Capacity Results for Examples 8-9 Using Ba X Zeolite Adsorbent and Methyl-tert-butyl ether Desorbent Results for comparitive Examples 8 and 9 shown in table 3 show selectivities and capacities for two commercial desorbents commonly used in PX adsoφtion processes on the same basis and under the same conditions as the earlier examples using ethers. These results show that the ethers can be as good or better from a selectivity and capacity perspective than the commercial desorbents. The ether desorbents are particularly advantaged over the commercial desorbents in that they have significantly lower boiling points thus are much easier and cheaper to separate. Diisopropylether and MTBE also have a significant advantage over p-DEB in that they are less expensive. The cost of the desorbent can be an important capital and operating cost for a PX complex. Make-up desorbent is often needed due to losses in the processes or contamination of the desorbent. Thus an inexpensive desorbent can provide an important benefit to the economics of a PX adsoφtion process.

Claims: What is claimed is: 1. An adsoφtive separation process for the separation of paraxylene from a hydrocarbon feed comprising a mixture of C 8 aromatic hydrocarbons comprising: e) contacting the feed, at adsoφtion conditions with an adsorbent bed comprising crystalline aluminosilicate selected from the group consisting of type X and type Y zeolites thereby adsorbing paraxylene from the feed; f) contacting the adsorbent bed at desoφtion conditions with a desorbent material comprising an ether selected from the group consisting of diisopropyl ether and methyl tertiary-butyl ether; g) withdrawing a stream comprising desorbent and less selectively adsorbed components of the feed from the adsorbent bed; and h) withdrawing from the adsorbent bed a stream comprising desorbent and said paraxylene, wherein, if the desorbent material comprises diisopropylether water content is maintained between 1 and 8 wt % based on the weight of the adsorbent, if the desorbent material comprises methyl tertiary-butyl ether water content is maintained below 4 wt % based on the weight of the adsorbent 2. The process of claim 1 wherein said adsorbent bed further comprises one or more alkaline or alkaline earth metal ions at exchangeable cationic sites of the crystaline aluminosilicate. 3. The process of claim 1 wherein, if the desorbent material comprises diisopropylether water content is maintained between 2 and 6 wt % based on the weight of the adsorbent, if the desorbent material comprises methyl tertiary butyl ether water content is maintained below 3 wt % based on the weight of the adsorbent. 4. The process of claim 2 wherein said one or more alkaline or alkaline earth metal is selected from the group consisting of Barium, Potassium, Calcium, Strontium, and Cesium. 5. The process of claim 1 wherein said adsorbent bed further comprises Barium at exhangeable cationic sites of the crystaline aluminosilicate. 6. The process of claim 1 wherein, if the desorbent material comprises diisopropyl ether water in the process is maintained at between 2 and 5 wt % based on the weight of the adsorbent. 7. The process of claim 1 wherein said adsoφtive separation process is a simulated moving bed, countercurrent, adsoφtive separation process. 8. The process of claim 1 further comprising the following steps: e) separating the stream comprising desorbent and less selectively adsorbed components into a second stream comprising desorbent and a stream comprising less selectively adsorbed components; and f) separating the stream comprising desorbent and said paraxylene into a third stream comprising desorbent and a stream comprising paraxylene. 9. The process of claim 8 further comprising the following steps: g) combining said second stream comprising desorbent with said third stream comprising desorbent; and h) recycling at least a portion of the combined desorbent streams from step g) to supply at least a portion of the desorbent material of step b). 10. The process of claim 1 wherein the desorbent material comprises methyl tertiary butyl ether and water is maintained below 2 wt % based on the weight of the adsorbent. 11. The process of claim 1 wherein the desorbent material comprises methyl tertiary butyl ether and water is maintained below 1 wt % based on the weight of the adsorbent.

Inventors: CAIN JOHN J, CAIN, JOHN, J.
Applicants: CHEVRON CHEM CO, CHEVRON CHEMICAL COMPANY LLC
Application year: 2000
Main classification: C07C15
IPC codes: B01D15, C07C7, C07C15