Abstract:
This present disclosure relates to processes and apparatuses for toluene methylation in an aromatics complex for producing paraxylene. More specifically, the present disclosure relates to processes and apparatuses wherein a toluene methylation zone is integrated within an aromatics complex for producing paraxylene thus allowing no benzene byproduct to be produced. This may be accomplished by incorporating a toluene methylation process into the aromatics complex and recycling the benzene to the transalkylation unit the aromatics complex.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
       [0001]    This application claims priority from Provisional Application No. 62/216,425 filed Sep. 10, 2015, the contents of which are hereby incorporated by reference. 
     
    
     FIELD 
       [0002]    This present disclosure relates to processes and apparatuses for toluene methylation in an aromatics complex for producing paraxylene. More specifically, the present disclosure relates to processes and apparatuses for toluene methylation within an aromatics complex for producing paraxylene where no benzene byproduct is produced. 
       BACKGROUND 
       [0003]    The xylene isomers are produced in large volumes from petroleum as feedstocks for a variety of important industrial chemicals. The most important of the xylene isomers is para-xylene, the principal feedstock for polyester, which continues to enjoy a high growth rate from large base demand. Ortho-xylene is used to produce phthalic anhydride, which supplies high-volume but relatively mature markets. Meta-xylene is used in lesser but growing volumes for such products as plasticizers, azo dyes and wood preservers. Ethylbenzene generally is present in xylene mixtures and is occasionally recovered for styrene production, but is usually considered a less-desirable component of C 8  aromatics. 
         [0004]    Among the aromatic hydrocarbons, the overall importance of xylenes rivals that of benzene as a feedstock for industrial chemicals. Xylenes and benzene are produced from petroleum by reforming naphtha but not in sufficient volume to meet demand, thus conversion of other hydrocarbons is necessary to increase the yield of xylenes and benzene. Often toluene is de-alkylated to produce benzene or selectively disproportionated to yield benzene and C 8  aromatics from which the individual xylene isomers are recovered. 
         [0005]    An aromatics complex flow scheme has been disclosed by Meyers in the Handbook of Petroleum Refining Processes, 2d. Edition in 1997 by McGraw-Hill, and is incorporated herein by reference. 
         [0006]    Traditional aromatics complexes send toluene to a transalkylation zone to generate desirable xylene isomers via transalkylation of the toluene with A 9+  components. A 9+  components are present in both the reformate bottoms and the transalkylation effluent. 
         [0007]    Paraxylene is most often produced from a feedstock which has a methyl to phenyl ration of less than 2. As a result, the paraxylene production is limited by the available methyl groups in the feed. In addition, paraxylene production also typically produces benzene as a byproduct. Since paraxylene is more valuable than benzene and the other byproducts produced in an aromatics complex, there is a desire to maximize the paraxylene production from a given amount of feed. There are also cases where a paraxylene producer would prefer to avoid the production of benzene as a byproduct or paraxylene production. However, there are also cases where a paraxylene producer would prefer to limit the production of benzene as a byproduct or paraxylene production by making adjustments. 
       SUMMARY 
       [0008]    The present subject matter relates to processes and apparatuses for toluene methylation in an aromatics complex for producing paraxylene. More specifically, the present disclosure relates to processes and apparatuses for toluene methylation within an aromatics complex for producing paraxylene where no benzene byproduct is produced. Integrating a toluene methylation process within an aromatics complex has several benefits. First, the integrated process may increase the amount of paraxylene that can be produced form a given amount of reformate. The integrated process may also reduce the amount of reformate required to produce a fixed amount of paraxylene. Second, the integrated process may avoid the production of benzene as a byproduct from the aromatics complex. These two benefits may be accomplished by incorporating a toluene methylation process into the aromatics complex and recycling the benzene to the transalkylation unit the aromatics complex. 
         [0009]    Additional objects, advantages and novel features of the examples will be set forth in part in the description which follows, and in part will become apparent to those skilled in the art upon examination of the following description and the accompanying drawings or may be learned by production or operation of the examples. The objects and advantages of the concepts may be realized and attained by means of the methodologies, instrumentalities and combinations particularly pointed out in the appended claims. 
       Definitions 
       [0010]    As used herein, the term “stream”, “feed”, “product”, “part” or “portion” can include various hydrocarbon molecules, such as straight-chain, branched, or cyclic alkanes, alkenes, alkadienes, and alkynes, and optionally other substances, such as gases, e.g., hydrogen, or impurities, such as heavy metals, and sulfur and nitrogen compounds. Each of the above may also include aromatic and non-aromatic hydrocarbons. 
         [0011]    Hydrocarbon molecules may be abbreviated C 1 , C 2 , C 3 , Cn where “n” represents the number of carbon atoms in the one or more hydrocarbon molecules or the abbreviation may be used as an adjective for, e.g., non-aromatics or compounds. Similarly, aromatic compounds may be abbreviated A 6 , A 7 , A 8 , An where “n” represents the number of carbon atoms in the one or more aromatic molecules. Furthermore, a superscript “+” or “−” may be used with an abbreviated one or more hydrocarbons notation, e.g., C 3+  or C 3− , which is inclusive of the abbreviated one or more hydrocarbons. As an example, the abbreviation “C 3+ ” means one or more hydrocarbon molecules of three or more carbon atoms. 
         [0012]    As used herein, the term “zone” can refer to an area including one or more equipment items and/or one or more sub-zones. Equipment items can include, but are not limited to, one or more reactors or reactor vessels, separation vessels, distillation towers, heaters, exchangers, pipes, pumps, compressors, and controllers. Additionally, an equipment item, such as a reactor, dryer, or vessel, can further include one or more zones or sub-zones. 
         [0013]    As used herein, the term “rich” can mean an amount of at least generally 50%, and preferably 70%, by mole, of a compound or class of compounds in a stream. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0014]      FIG. 1  schematically illustrates an aromatics complex. 
           [0015]      FIG. 2  illustrates an aromatics complex having an integrated toluene methylation zone. 
       
    
    
       [0016]    Corresponding reference characters indicate corresponding components throughout the several views of the drawings. Skilled artisans will appreciate that elements in the figures are illustrated for simplicity and clarity and have not necessarily been drawn to scale. For example, the dimensions of some of the elements in the figures may be exaggerated relative to other elements to help to improve understanding of various embodiments of the present disclosure. Also, common but well-understood elements that are useful or necessary in a commercially feasible embodiment are often not depicted in order to facilitate a less obstructed view of these various embodiments of the present disclosure. 
       DETAILED DESCRIPTION 
       [0017]    The following description is not to be taken in a limiting sense, but is made merely for the purpose of describing the general principles of exemplary aspects. The scope of the present disclosure should be determined with reference to the claims. 
         [0018]    The feedstream to the present process generally comprises alkylaromatic hydrocarbons of the general formula C 6 H (6-n) R n , where n is an integer from 0 to 5 and each R may be CH 3 , C 2 H 5 , C 3 H 7 , or C 4 H 9 , in any combination. The aromatics-rich feed stream to the process of the present disclosure may be derived from a variety of sources, including without limitation catalytic reforming, steam pyrolysis of naphtha, distillates or other hydrocarbons to yield light olefins and heavier aromatics-rich byproducts (including gasoline-range material often referred to as “pygas”), and catalytic or thermal cracking of distillates and heavy oils to yield products in the gasoline range. Products from pyrolysis or other cracking operations generally will be hydrotreated according to processes well known in the industry before being charged to the complex in order to remove sulfur, olefins and other compounds which would affect product quality and/or damage catalysts or adsorbents employed therein. Light cycle oil from catalytic cracking also may be beneficially hydrotreated and/or hydrocracked according to known technology to yield products in the gasoline range; the hydrotreating preferably also includes catalytic reforming to yield the aromatics-rich feed stream.  FIG. 1  is a simplified flow diagram of an exemplary aromatics-processing complex of the known art directed to the production of at least one xylene isomer. The complex may process an aromatics-rich feed which has been derived, for example, from catalytic reforming in a reforming zone  6 . The reforming zone generally includes a reforming unit  4  that receives a feed via conduit  2 . The reforming unit typically comprises a reforming catalyst. Usually such a stream will also be treated to remove olefinic compounds and light ends, e.g., butanes and lighter hydrocarbons and preferably pentanes; such removal, however, is not essential to the practice of the broad aspects of this disclosure and is not shown. The aromatics-containing feed stream contains benzene, toluene and C 8  aromatics and typically contains higher aromatics and aliphatic hydrocarbons including naphthenes. 
         [0019]    The feed stream is passed via conduit  10  via a heat exchanger  12  to reformate splitter  14  and distilled to separate a stream comprising C 8  and heavier aromatics, withdrawn as a bottoms stream via a bottoms outlet  15  in conduit  16 , from toluene and lighter hydrocarbons recovered overhead via conduit  18 . The toluene and lighter hydrocarbons are sent to extractive distillation process unit  20  which separates a largely aliphatic raffinate in conduit  21  from a benzene-toluene aromatics stream in conduit  22 . The aromatics stream in conduit  22  is separated, along with stripped transalkylation product in conduit  45  and overhead from para-xylene finishing column in conduit  57 , in benzene column  23  into a benzene stream in conduit  24  and a toluene-and-heavier aromatics stream in conduit  25  which is sent to a toluene column  26 . Toluene is recovered overhead from this column in conduit  27  and may be sent partially or totally to a transalkylation unit  40  as shown and discussed hereinafter. 
         [0020]    A bottoms stream from the toluene column  26  is passed via conduit  28 , along with bottoms from the reformate splitter in conduit  16 , after treating via clay treater  17 , and recycle C 8  aromatics in conduit  65 , to fractionator  30 . The fractionator  30  separates concentrated C 8  aromatics as overhead in conduit  31  from a high-boiling stream comprising C 9 , C 10  and heavier aromatics as a bottoms stream in conduit  32 . This bottoms stream is passed in conduit  32  to heavies column  70 . The heavy-aromatics column provides an overhead stream in conduit  71  containing C 9  and at least some of the C 10  and C 11  aromatics, with higher boiling compounds, primarily higher alkylaromatics, being withdrawn as a bottoms stream via conduit  72 . 
         [0021]    The C 9+  aromatics from heavies column in conduit  71  is combined with the toluene-containing overhead contained in conduit  27  as feed to transalkylation reactor  40 , which contains a transalkylation catalyst as known in the art to produce a transalkylation product comprising benzene through C 11+  aromatics with xylenes as the focus. The transalkylation product in conduit  41  is stripped in stripper  42  to remove gases in conduit  43  and C 6  and lighter hydrocarbons which are returned via conduit  44  to extractive distillation  20  for recovery of light aromatics and purification of benzene. Bottoms from the stripper are sent in conduit  45  to benzene column  23  to recover benzene product and unconverted toluene. 
         [0022]    The C 8 -aromatics overhead provided by fractionator  30  contains para-xylene, meta-xylene, ortho-xylene and ethylbenzene and passes via conduit  31  to para-xylene separation process  50 . The separation process operates, preferably via adsorption employing a desorbent, to provide a mixture of para-xylene and desorbent via conduit  51  to extract column  52 , which separates para-xylene via conduit  53  from returned desorbent in conduit  54 ; the para-xylene is purified in finishing column  55 , yielding a para-xylene product via conduit  56  and light material which is returned to benzene column  23  via conduit  57 . A non-equilibrium mixture of C 8 -aromatics raffinate and desorbent from separation process  50  is sent via conduit  58  to raffinate column  59 , which separates a raffinate for isomerization in conduit  60  from returned desorbent in conduit  61 . 
         [0023]    The raffinate, comprising a non-equilibrium mixture of xylene isomers and ethylbenzene, is sent via conduit  60  to isomerization reactor  62 . The raffinate is isomerized in reactor  62 , which contains an isomerization catalyst to provide a product approaching equilibrium concentrations of C 8 -aromatic isomers. The product is passed via conduit  63  to deheptanizer  64 , which removes C 7  and lighter hydrocarbons with bottoms passing via conduit  65  to xylene column  30  to separate C 9  and heavier materials from the isomerized C 8 -aromatics. Overhead liquid from deheptanizer  64  is sent to stripper  66 , which removes light materials overhead in conduit  67  from C 6  and C 7  materials which are sent via conduit  68  to the extractive distillation unit  20  for recovery of benzene and toluene values. 
         [0024]    There are many possible variations of this scheme within the known art, as the skilled routineer will recognize. For example, the entire C 6 -C 8  reformate or only the benzene-containing portion may be subjected to extraction. Para-xylene may be recovered from a C 8 -aromatic mixture by crystallization rather than adsorption. Meta-xylene as well as para-xylene may be recovered from a C 8 -aromatic mixture by adsorption, and ortho-xylene may be recovered by fractionation. Alternatively, the C 9 -and heavier stream or the heavy-aromatics stream is processed using solvent extraction or solvent distillation with a polar solvent or stripping with steam or other media to separate highly condensed aromatics as a residual stream from C 9+  recycle to transalkylation. In some cases, the entire heavy-aromatic stream may be processed directly in the transalkylation unit. The present disclosure is useful in these and other variants of an aromatics-processing scheme, aspects of which are described in U.S. Pat. No. 6,740,788 which is incorporated herein by reference. 
         [0025]    Turning now to  FIG. 2 , an aromatics complex and process in accordance with one aspect wherein the aromatics complex includes an integrated toluene methylation zone will be illustrated and described.  FIG. 2  is a simplified flow diagram of an exemplary aromatics-processing complex of the known art integrated with a toluene methylation unit directed to the production of at least one xylene isomer. The complex may process an aromatics-rich feed which has been derived, for example, from catalytic reforming in a reforming zone  6 . The reforming zone generally includes a reforming unit  4  that receives a feed via conduit  2 . The reforming unit will typically comprise a reforming catalyst. Usually such a stream will also be treated to remove olefinic compounds and light ends, e.g., butanes and lighter hydrocarbons and preferably pentanes; such removal, however, is not essential to the practice of the broad aspects of this disclosure and is not shown. The aromatics-containing feed stream contains benzene, toluene and C 8  aromatics and typically contains higher aromatics and aliphatic hydrocarbons including naphthenes. 
         [0026]    The feed stream is passed via conduit  10  via a heat exchanger  12  to reformate splitter  14  and distilled to separate a stream comprising C 8  and heavier aromatics, withdrawn as a bottoms stream via a bottoms outlet  15  in conduit  16 , from toluene and lighter hydrocarbons recovered overhead via conduit  18 . The toluene and lighter hydrocarbons are sent to extractive distillation process unit  20  which separates a largely aliphatic raffinate in conduit  21  from a benzene-toluene aromatics stream in conduit  22 . The aromatics stream in conduit  22  is separated, along with stripped transalkylation product in conduit  45 , an overhead from para-xylene finishing column in conduit  57 , and a light aromatic stream in conduit  88  in benzene column  23  into a benzene stream in conduit  24  and a toluene-and-heavier aromatics stream in conduit  25  which is sent to a toluene column  26 . The benzene stream in conduit  24  is passed from the benzene column  23  to the transalkylation unit  40 . In one embodiment, the transalkylation conditions may include a temperature of about 320° C. to about 440° C. The transalkylation zone may contain a first catalyst. In one embodiment, the first catalyst comprises at least one zeolitic component suitable for transalkylation, at least one zeolitic component suitable for dealkylation and at least one metal component suitable for hydrogenation. Toluene is recovered overhead from this column in conduit  27  and may be sent partially or totally to a toluene methylation unit  80  along with a methanol stream in conduit  82  as shown and discussed hereinafter. 
         [0027]    The methanol stream in conduit  82  and the toluene in conduit  27  is passed to the toluene methylation unit  80  and produces a hydrocarbon stream in conduit  84 . The hydrocarbon stream in conduit  84  is passed to column  90  which produces an overhead stream in conduit  86  and a bottoms stream in conduit  88 . The bottoms stream in conduit  88  is sent back to the benzene column  23 . In one embodiment, the toluene methylation product stream has a paraxylene to total xylene ratio of at least about 0.2, or preferably at least about 0.5, or more preferably about 0.8 to 0.95. 
         [0028]    The downstream process is the same as in  FIG. 1 . The C 8 -aromatics overhead provided by fractionator  30  contains para-xylene, meta-xylene, ortho-xylene and ethylbenzene and passes via conduit  31  to para-xylene separation process  50 . The separation process operates, preferably via adsorption employing a desorbent, to provide a mixture of para-xylene and desorbent via conduit  51  to extract column  52 , which separates para-xylene via conduit  53  from returned desorbent in conduit  54 ; the para-xylene is purified in finishing column  55 , yielding a para-xylene product via conduit  56  and light material which is returned to benzene column  23  via conduit  57 . A non-equilibrium mixture of C 8 -aromatics raffinate and desorbent from separation process  50  is sent via conduit  58  to raffinate column  59 , which separates a raffinate for isomerization in conduit  60  from returned desorbent in conduit  61 . 
         [0029]    The raffinate, comprising a non-equilibrium mixture of xylene isomers and ethylbenzene, is sent via conduit  60  to isomerization reactor  62 . The raffinate is isomerized in reactor  62 , which contains an isomerization catalyst to provide a product approaching equilibrium concentrations of C 8 -aromatic isomers. In one embodiment, the isomerization conditions include a temperature of about 240° C. to about 440° C. Further, the isomerization zone includes s second catalyst. In one embodiment, the second catalyst comprises at least one zeolitic component suitable for xylene isomerization, at least one zeolitic component suitable for ethylbenzene conversion, and at least one metal component suitable for hydrogenation. In one embodiment, the isomerization process is carried out in the vapor phase. In yet another embodiment, the isomerization process is carried out in the liquid phase. In one embodiment, the isomerization process converts ethylbenzene by dealkylation to produce benzene. In another embodiment, the isomerization process converts ethylbenzene by isomerization to produce xylenes. 
         [0030]    The product is passed via conduit  63  to deheptanizer  64 , which removes C 7  and lighter hydrocarbons with bottoms passing via conduit  65  to xylene column  30  to separate C 9  and heavier materials from the isomerized C 8 -aromatics. Overhead liquid from deheptanizer  64  is sent to stripper  66 , which removes light materials overhead in conduit  67  from C 6  and C 7  materials which are sent via conduit  68  to the extractive distillation unit  20  for recovery of benzene and toluene values. 
         [0031]    There are many possible variations of this scheme within the known art, as the skilled routineer will recognize. For example, the entire C 6 -C 8  reformate or only the benzene-containing portion may be subjected to extraction. Para-xylene may be recovered from a C 8 -aromatic mixture by crystallization rather than adsorption. The separation zone may also contain a simulated moving bed adsorption unit. In one example, the simulated moving bed adsorption unit uses a desorbent with a lower boiling point than xylenes, such as toluene or benzene. In yet another embodiment, the simulated moving bed adsorption unit uses a desorbent with a higher boiling point than xylenes, such as paradiethylbenzene, paradiisopropylbenzene, tetralin, or paraethyltoluene. Meta-xylene as well as para-xylene may be recovered from a C 8 -aromatic mixture by adsorption, and ortho-xylene may be recovered by fractionation. Alternatively, the C 9 -and heavier stream or the heavy-aromatics stream is processed using solvent extraction or solvent distillation with a polar solvent or stripping with steam or other media to separate highly condensed aromatics as a residual stream from C 9+  recycle to transalkylation. In some cases, the entire heavy-aromatic stream may be processed directly in the transalkylation unit. The present disclosure is useful in these and other variants of an aromatics-processing scheme, aspects of which are described in U.S. Pat. No. 6,740,788 which is incorporated herein by reference. 
       EXAMPLES 
       [0032]    The following examples are intended to further illustrate the subject embodiments. These illustrations of different embodiments are not meant to limit the claims to the particular details of these examples. 
         [0000]    
       
         
               
               
               
               
             
           
               
                 TABLE 
               
               
                   
               
               
                   
                 Comparative 
                   
                   
               
               
                 Case: 
                 Example 
                 Example 1 
                 Example 2 
               
               
                   
               
             
             
               
                 Toluene Methylation 
                 No 
                 Yes 
                 Yes 
               
               
                 Included? 
               
               
                 Xylene Isomerization Type 
                 EB 
                 EB 
                 EB 
               
               
                   
                 Dealkylation 
                 Dealkylation 
                 Isomerization 
               
               
                 Feed Flowrate, 
               
               
                 MT/yr × 1,000 
               
               
                 Reformate 
                 1703 
                 1140 
                 1159 
               
               
                 Methanol 
                 0 
                 340 
                 304 
               
               
                 Hydrogen 
                 8 
                 9 
                 9 
               
               
                 Product Flowrate, 
               
               
                 MT/yr × 1,000 
               
               
                 p-Xylene 
                 1000 
                 1000 
                 1000 
               
               
                 Benzene 
                 376 
                 0 
                 0 
               
               
                 Heavy Aromatics 
                 45 
                 42 
                 49 
               
               
                 Sulfolane Raffinate 
                 173 
                 116 
                 123 
               
               
                 Water 
                 0 
                 191 
                 171 
               
               
                 Light Ends 
                 117 
                 141 
                 129 
               
               
                   
               
             
          
         
       
     
         [0033]    The Table demonstrates the benefits of having an integrated toluene methylation zone integrated within an aromatics complex. As shown in the Table, Example 1 illustrates an aromatics-processing complex for the production of paraxylene with zero benzene byproduct according to the invention as illustrated in  FIG. 2 . In this example, the xylene isomerization unit converts ethylbenzene by dealklyation. Further, as shown in the Table, Example 2 illustrates an aromatics-processing complex for the production of paraxylene with zero benzene byproduct according to the invention as illustrated in  FIG. 2 . In this example, the xylene isomerization unit converts ethylbenzene by isomerization. 
         [0034]    It should be noted that various changes and modifications to the presently preferred embodiments described herein will be apparent to those skilled in the art. Such changes and modifications may be made without departing from the spirit and scope of the present subject matter and without diminishing its attendant advantages. 
       Specific Embodiments 
       [0035]    While the following is described in conjunction with specific embodiments, it will be understood that this description is intended to illustrate and not limit the scope of the preceding description and the appended claims. 
         [0036]    A first embodiment of the invention is a process for producing paraxylene with no benzene byproduct, comprising passing a lighter aromatic stream containing benzene and a heavier aromatic stream containing C 9 -C 10  aromatic compounds to a transalkylation zone; subjecting the lighter aromatic stream and the heavier aromatic stream in the transalkylation zone to transalkylation conditions including the presence of a first catalyst to provide a transalkylation product stream having a greater concentration of toluene to C 8  aromatics; separating by fractionation from the transalkylation product stream a first boiling fraction comprising benzene, a second boiling fraction comprising toluene, a third boiling fraction comprising C 8  aromatics and a fourth boiling fraction comprising C 9+  aromatics; recycling at least a portion of the benzene from the transalkylation product stream back to the transalkylation zone; passing at least a portion of the second boiling fraction from steps c, g and i and a methanol stream to a toluene methylation zone operating under toluene methylation conditions to produce a toluene methylation product stream; separating by fractionation from the toluene methylation product stream the same fractions described in step c subjecting at least a portion of the third boiling fraction comprising C 8  aromatics of steps c, g and i to a separation zone to selectively remove a para-xylene product and provide a non-equilibrium mixture of C 8  aromatics; subjecting the non-equilibrium mixture of C 8  aromatics to xylene isomerization conditions including the presence of a second catalyst to provide an isomerization product; and separating by fractionation from the isomerization product stream the same fractions described in step c. An embodiment of the invention is one, any or all of prior embodiments in this paragraph up through the first embodiment in this paragraph, wherein the transalkylation conditions include a temperature of about 320° C. to about 440° C. 
         [0037]    An embodiment of the invention is one, any or all of prior embodiments in this paragraph up through the first embodiment in this paragraph, wherein the first catalyst comprises at least one zeolitic component suitable for transalkylation, at least one zeolitic component suitable for dealkylation and at least one metal component suitable for hydrogenation. An embodiment of the invention is one, any or all of prior embodiments in this paragraph up through the first embodiment in this paragraph, wherein the toluene methylation product stream has a paraxylene to total xylene ratio of at least about 0.2, or preferably at least about 0.5, or more preferably about 0.8 to 0.95. An embodiment of the invention is one, any or all of prior embodiments in this paragraph up through the first embodiment in this paragraph, wherein the isomerization conditions include a temperature of about 240° C. to about 440° C. An embodiment of the invention is one, any or all of prior embodiments in this paragraph up through the first embodiment in this paragraph, wherein the second catalyst comprises at least one zeolitic component suitable for xylene isomerization, at least one zeolitic component suitable for ethylbenzene conversion, and at least one metal component suitable for hydrogenation. An embodiment of the invention is one, any or all of prior embodiments in this paragraph up through the first embodiment in this paragraph, wherein the isomerization process is carried out in the vapor phase. An embodiment of the invention is one, any or all of prior embodiments in this paragraph up through the first embodiment in this paragraph, wherein the isomerization process converts ethylbenzene by dealkylation to produce benzene. An embodiment of the invention is one, any or all of prior embodiments in this paragraph up through the first embodiment in this paragraph, wherein the isomerization process converts ethylbenzene by isomerization to produce xylenes. An embodiment of the invention is one, any or all of prior embodiments in this paragraph up through the first embodiment in this paragraph, wherein the isomerization process is carried out in the liquid phase. 
         [0038]    An embodiment of the invention is one, any or all of prior embodiments in this paragraph up through the first embodiment in this paragraph, wherein all of the benzene is recycled to the transalkylation zone. An embodiment of the invention is one, any or all of prior embodiments in this paragraph up through the first embodiment in this paragraph, wherein the separation zone contains a crystallization unit. An embodiment of the invention is one, any or all of prior embodiments in this paragraph up through the first embodiment in this paragraph, wherein the sepration zone contains a simulated moving bed adsorption unit. An embodiment of the invention is one, any or all of prior embodiments in this paragraph up through the first embodiment in this paragraph, wherein the simulated moving bed adsorption unit uses a desorbent with a lower boiling point than xylenes, such as toluene or benzene. An embodiment of the invention is one, any or all of prior embodiments in this paragraph up through the first embodiment in this paragraph, wherein the simulated moving bed adsorption unit uses a desorbent with a higher boiling point than xylenes, such as paradiethylbenzene, paradiisopropylbenzene, tetralin, or paraethyltoluene. 
         [0039]    An embodiment of the invention is one, any or all of prior embodiments in this paragraph up through the first embodiment in this paragraph, further comprising segregating the C 8  aromatic fraction produced in the toluene methylation unit from the other C 8  aromatic fractions produced in the process. An embodiment of the invention is one, any or all of prior embodiments in this paragraph up through the first embodiment in this paragraph, wherein the C 8  aromatic fraction produced in the toluene methylation unit is processed in the same separation zone as one or more other C 8  aromatic fractions, but is introduced at a different feed location An embodiment of the invention is one, any or all of prior embodiments in this paragraph up through the first embodiment in this paragraph, wherein the C 8  aroamtic fraction produced in the toluene methylation unit is processed in a separation zone that is distinct from the separation zone used for the other C 8  aromatic fractions. An embodiment of the invention is one, any or all of prior embodiments in this paragraph up through the first embodiment in this paragraph, wherein the C 8  aromatic fraction produced in the toluene methylation unit is processed in a separation zone that contains a crystallization unit An embodiment of the invention is one, any or all of prior embodiments in this paragraph up through the first embodiment in this paragraph, wherein the C 8  aromatic fraction produced in the toluene methylation unit is processed in a separation zone that contains a simulated moving bed adsorption unit. 
         [0040]    A second embodiment of the invention is an apparatus for producing paraxylene, comprising a transalkylation zone in fluid communication with a toluene methylation zone, wherein the toluene methylation zone is in fluid communication with an aromatics separation zone, wherein the aromatics separation zone is in fluid communication with an isomerization zone. 
         [0041]    Without further elaboration, it is believed that using the preceding description that one skilled in the art can utilize the present invention to its fullest extent and easily ascertain the essential characteristics of this invention, without departing from the spirit and scope thereof, to make various changes and modifications of the invention and to adapt it to various usages and conditions. The preceding preferred specific embodiments are, therefore, to be construed as merely illustrative, and not limiting the remainder of the disclosure in any way whatsoever, and that it is intended to cover various modifications and equivalent arrangements included within the scope of the appended claims. In the foregoing, all temperatures are set forth in degrees Celsius and, all parts and percentages are by weight, unless otherwise indicated.