Patent Publication Number: US-2015065768-A1

Title: Systems and methods for xylene isomer production

Description:
TECHNICAL FIELD 
     The present disclosure generally relates to systems and methods for producing desired isomers of xylene, and more particularly relates to systems and methods for converting non-reformed hydrocarbon streams into desired isomers of xylene. 
     BACKGROUND 
     Xylene isomers are important intermediates in chemical syntheses, and specific xylene isomers are desired for different processes. Para-xylene is a feedstock for terephthalic acid, and terephthalic acid is used in the manufacture of synthetic fibers and resins. Meta-xylene is used in the manufacture of certain plasticizers, azo dyes, and wood preservatives. Ortho-xylene is a feedstock for phthalic anhydride production, and phthalic anhydride is used in the manufacture of certain plasticizers, dyes, and pharmaceutical products. 
     Reactions that produce xylene generally produce the xylene isomers in ratios that do not match the demand, and also produce ethyl benzene which is difficult to separate from the xylene. The demand for para-xylene in particular exceeds the production ratios, and several methods have been developed for adjusting the amount of para-xylene recovered from various production processes. The isomer production ratio can be adjusted to meet commercial demand by combining xylene isomer recovery, such as by selective adsorption and/or crystallization, with isomerization to yield additional quantities of the recovered isomer. The xylene isomer recovery changes the ratio of the xylenes to a non-equilibrium value lean in the recovered isomer, and isomerization adjusts the isomer ratio back towards the equilibrium value. 
     In many xylene isomer recovery processes, aromatics compounds with 9 carbons or more (C9+ aromatics) are present in the feed stream, where xylene is a C8 aromatic. In this description, the abbreviation of “C” followed by a number indicates the number of carbons present in the molecule, and a “+” sign afterwards indicates the indicated number of carbons or more. For example, C7+ means a molecule with 7 carbons or more. A “−” sign after the number indicates the number or less, so C7− means a molecule with 7 carbons or less. The C9+ aromatics are undesirable in the xylene isomer recovery process because they decrease performance, such as by reducing catalyst and/or adsorbent life. Therefore, the feed stream is fractionated to separate the C9+ aromatics, which involves vaporizing and re-condensing, or lifting, the entire C8 aromatics portion. Lifting the entire C8 aromatics portion of the feed stream  10  is expensive because of the high energy demand. Xylene isomer recovery uses an isomer recovery unit and an isomerization unit, where the xylene flows in a loop through the two units. In many existing processes, an isomerized stream flows from the isomerization unit to the isomer recovery unit as part of the loop, and the isomerized stream is fractionated to remove C9+ compounds. The entire C8 aromatics portion is repeatedly lifted as the isomerized stream flows to the isomer recovery unit, and this requires energy that increases the operating costs. Larger equipment is required to lift larger quantities of xylene, so there are also increased capital costs to build and install larger equipment. 
     The feed stream is reformed before entering existing xylene isomer recovery processes, and a reforming process that produces large quantities of aromatic compounds is used. Other processes may also produce large quantities of aromatic compounds. Declining gasoline demand in many countries can lead to fluid catalytic cracking (FCC) units being operated in high severity mode to increase the production of propylene. FCC units operated in high severity mode also produce higher quantities of aromatic compounds with molecules having about 7 to about 10 carbons (C7-10). The FCC products are fractionated, so a C7-10 stream that is high in aromatics is available without reforming. The reforming process increases the operating costs, increases capital costs for manufacture and installation, and can reduce the total quantity of xylene. There are other processes that produce C7-10 streams that are high in aromatics, such as the production of liquid products from coal, and adding a reforming operation increases operating costs and capital costs in the same manner as for an FCC unit. 
     Accordingly, it is desirable to develop methods and systems for producing desired xylene isomers from aromatic rich feedstocks that are not reformed. In addition, it is desirable to develop methods and systems for producing desired xylene isomers without reforming the feedstock, where the xylene isomer recovery process is energy efficient. Furthermore, other desirable features and characteristics of the present embodiment will become apparent from the subsequent detailed description and the appended claims, taken in conjunction with the accompanying drawings and this background. 
     BRIEF SUMMARY 
     A method is provided for producing xylene. The method includes fractionating a feed stream in a feed fractionator to produce a feed bottoms stream with aromatic compounds having 8 carbons or more and a feed overhead stream with aromatic compounds having 7 carbons or less. The feed stream includes aromatic compounds and non-aromatic compounds, and more than 5 weight percent of the non-aromatic compounds have a boiling point above 105° C. at one atmosphere of pressure. The feed bottoms stream is de-ethylated in a heavy aromatics conversion zone to produce a de-ethylated aromatics stream and a light gases stream, where non-aromatic compounds are converted to light gases in the light gases stream. The de-ethylated aromatics stream is fractionated to produce a heavy aromatics stream and an intermediate aromatics stream, and a desired isomer stream is recovered from the intermediate aromatics stream and an isomerized stream in an isomer recovery process. The isomer recovery process produces an isomer raffinate stream, and the isomer raffinate stream is isomerized in an isomerization zone to produce the isomerized stream. 
     Another method is also provided for producing xylene. The method includes fractionating a feed stream in a feed fractionator to produce a feed bottoms stream and a feed overhead stream, where the feed stream has more than 30 weight percent non-aromatic compounds. The compounds in the feed overhead stream are separated into a non-aromatics stream and a first light aromatics stream, where the first light aromatics stream includes toluene. The feed bottoms stream is contacted with a heavy aromatics conversion catalyst to obtain a de-ethylated aromatics stream and a light gases stream. The de-ethylated aromatics stream is fractionated to produce a heavy aromatics stream with aromatic compounds having 9 carbons or more, and an intermediate aromatics stream with aromatic compounds having 8 carbons. The intermediate aromatics stream and an isomerized stream are subjected to an isomer recovery process to produce a desired isomer stream and an isomer raffinate stream. The isomer raffinate stream is contacted with an isomerization catalyst at isomerization conditions to produce the isomerized stream. The heavy aromatics stream and the first light aromatics stream are contacted with a transalkylation catalyst to produce a transalkylation stream. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Various embodiments will hereinafter be described in conjunction with the following figures, wherein like numerals denote like elements, and wherein: 
         FIG. 1  is a schematic diagram of an exemplary embodiment of a fluid catalytic cracking unit that produces a feed stream; and 
         FIG. 2  is a schematic diagram of an exemplary embodiment of a xylene isomer product system. 
     
    
    
     DETAILED DESCRIPTION 
     The following detailed description is merely exemplary in nature and is not intended to limit the various embodiments and the application and uses thereof. Furthermore, there is no intention to be bound by any theory presented in the preceding technical field, background, brief summary, or the following detailed description. 
     The various embodiments relate to systems and methods for producing a desired xylene isomer from a hydrocarbon feedstock that has not been reformed. Xylene production is simplified by using a feedstock that has not been reformed. The feed stream is fractionated to produce a feed overhead stream and a feed bottoms stream, and various process streams are removed from the feed overhead stream to leave a first light aromatics stream rich in toluene. The feed bottoms stream is fed to a heavy aromatics conversion zone that de-ethylates heavy aromatics, such as methyl ethyl benzene, and also converts non-aromatic compounds into smaller non-aromatic compounds that are vented in a light gases stream. The de-ethylated stream is fractionated to produce an intermediate aromatics stream rich in xylene, and a heavy aromatics stream rich in C9+ aromatic compounds. The heavy aromatics stream and the light aromatics stream are fed to a transalkylation zone that produces benzene and xylene from toluene and C9+ aromatics. The xylene from the transalkylation zone is fed into the heavy aromatics conversion zone to increase the quantity of xylene. The intermediate aromatics stream is fed into a xylene isomer recovery process to recover the desired xylene isomer. 
     Aromatic reforming is a process that re-arranges and re-structures hydrocarbon molecules with a propensity to produce aromatic compounds. Aromatic reforming is not 100 percent efficient, so some of the hydrocarbons are broken into smaller molecules or re-arranged to form branched paraffins. There are several different reforming methods and catalysts for different purposes, and one of those methods is catalytic reforming for aromatics. Catalytic reforming is a chemical process that re-arranges or re-structures hydrocarbon molecules, and typically breaks some of the hydrocarbon molecules into smaller molecules. Aromatic reforming is used when aromatic products are desired, but aromatic reforming favors benzene (C6) or toluene (C7) over xylene or ethyl benzene (C8). In some embodiments, aromatic reforming produces about 90 percent C6 and C7 aromatics, so only a small percentage of the resulting product stream is C8 aromatics. In fact, in some embodiments aromatic reforming can actually reduce the amount of C8 aromatics, because the C8 aromatics are converted to other products, such as the C7 or C6 aromatics. In some embodiments, a feed stream is richer in C8 aromatics before reforming, so reforming reduces the total xylene available. Many embodiments of aromatic reforming also reduce the fraction of C8+ non-aromatics, typically to less than 5 mass percent of the total quantity of non-aromatics. Prior to reforming, the C8+ non-aromatics are more than 5 mass percent of the total quantity of non-aromatics, so the reforming process shifts the average molecular weight of the non-aromatics downward. In several embodiments, a hydrocarbon stream has not been reformed if the C8+ non-aromatic compounds are more than about 5 mass percent of the total non-aromatic compounds, and the feed stream has been reformed if it is less than about 5 mass percent of the total non-aromatic compounds. C8+ non-aromatic compounds generally boil at 105° C. or greater at one atmosphere of pressure, and C7-non-aromatic compounds generally boil below 105° C. at one atmosphere of pressure, so a hydrocarbon stream where more than 5 weight percent of the non-aromatic compounds have a boiling point above 105° C. at one atmosphere of pressure generally indicates a feed stream that has not been reformed. 
     Reference is now made to the exemplary embodiment in  FIG. 1 . Suitable feed streams  30  for producing a desired xylene isomer, as described in detail below, are available from many sources. For example, a fluid catalytic cracking (FCC) unit  10 , when run in high severity mode, produces a larger fraction of propylene and butylene than when run in standard operating modes. The FCC stream  12  discharged by the FCC unit  10  is fractionated in an FCC fractionator  14  to produce various fractions that are processed in different manners, such as a propylene stream  16 , a C7-C10 stream  18 , and a diesel stream  20 . In some embodiments, the C7-C10 stream  18  includes about 60 mass percent aromatic compounds or more, and 60 mass percent aromatic compounds is an aromatic rich stream. Increased demand for propylene and butylene, combined with decreased demand for gasoline, provides an incentive to operate the FCC unit  10  in high severity mode to better match production with commercial demands. The propylene and butylene fractions are commercially valuable, and it is desirable to utilize the aromatics in the C7-C10 stream  18  as a co-product. The C7-C10 stream  18  can be hydrotreated in a hydrotreating unit  22  to remove sulfur in a sour gas stream  24 , because sulfur can poison catalysts and lower the quality of the xylene products. The feed stream  30  exits the hydrotreating unit  22  prior to any reforming process, because no reforming process is used on the components of the feed stream  30 , so the feed stream  30  is derived from the FCC unit  10  without reforming. 
     There are several other possible sources for the feed stream  30  that are rich in aromatic compounds but have not been reformed. For example, certain coal liquefaction processes produce hydrocarbon streams rich in aromatic compounds, and these hydrocarbon streams are suitable for use as the feed stream  30 . Other possible sources include various petroleum refining, thermal or catalytic cracking of hydrocarbons, or petrochemical conversion processes. 
     Reference is now made to an exemplary embodiment of a xylene isomer production system  26  illustrated in  FIG. 2 . The feed stream  30  is fed to a feed fractionator  32  and fractionated to produce a feed overhead stream  34  and a feed bottoms stream  36 . The feed fractionator  32  can be operated from a pressure of about 5 kilo Pascals (KPa) to about 1,800 KPa, and a temperature from about 35 degrees centigrade (° C.) to about 360° C. The feed stream  30  includes mixed hydrocarbons with aromatic and non-aromatic compounds, and many of the hydrocarbons have from about 7 to about 10 carbons (C7-10). In some embodiments, the feed stream  30  has more than about 30 weight percent non-aromatic compounds, and in other embodiments the feed stream  30  has more than about 35 weight percent non-aromatic compounds. In still other embodiments the feed stream  30  includes non-aromatic compounds where the C8+ non-aromatic compounds are more than about 5 mass percent of the total non-aromatic compounds. Therefore, more than about 5 percent of the non-aromatic compounds in the feed stream  30  boil at a temperature greater than about 105° C. at 1 atmosphere of pressure, as described above. In yet another embodiment, the feed stream  30  has not been reformed. The feed fractionator  32  is operated such that the feed overhead stream  34  primarily includes C7− compounds, and the feed bottoms stream  36  primarily includes C8+ compounds. 
     The feed overhead stream  34  flows to an aromatics extraction zone  38  that produces a non-aromatics stream  40  and an aromatics stream  42 . Any suitable process for separating high purity aromatics from non-aromatics may be employed in the aromatics extraction zone  38 , including but not limited to liquid liquid extraction processes using sulfolane, crystallization extraction processes, or combinations of the two. The non-aromatics stream  40  is discharged from the xylene isomer production system  26 , and can be used for other purposes. The aromatics stream  42 , which primarily includes benzene and toluene, is routed to an aromatics fractionation zone  44 . 
     The aromatics fractionation zone  44  includes one or more fractionation units that separate the aromatics stream  42  into a benzene stream  46  and a first light aromatics stream  48 . The fractionation unit(s) in the aromatics fractionation zone  44  can be operated from a pressure of about 5 kilo Pascals (KPa) to about 1,800 KPa, and a temperature from about 35 degrees centigrade (° C.) to about 360° C. The benzene stream  46  primarily includes benzene, which is a valuable product, and the benzene stream  46  is discharged from the xylene isomer production system  26  and made available for other uses. The first light aromatics stream  48  primarily includes toluene, and is at least partially used within the xylene isomer production system  26  as further described below. In some embodiments, a portion of the first light aromatics stream  48  is optionally split off from the xylene isomer production system  26  (not illustrated). 
     Returning now to the feed fractionator  32 , the feed bottoms stream  36  is transferred to a heavy aromatics conversion zone  50 . The heavy aromatics conversion zone  50  includes a heavy aromatics conversion catalyst  52  that is tolerant of C9 aromatics. The heavy aromatics conversion catalyst  52  de-ethylates aromatic compounds with ethyl groups and changes the structure of some aromatic compounds, so ethyl benzene is converted to ethylene and benzene, toluene and methane, or a xylene. Other aromatic compounds with ethyl groups are also converted to benzene or aromatic compounds without ethyl groups. Benzene and aromatic compounds with methyl groups are generally more valuable than aromatic compounds with ethyl groups. 
     The heavy aromatics conversion zone  50  removes the remaining non-aromatic compounds from the feed bottoms stream  36  as well as de-ethylating the aromatic compounds. Smaller non-aromatic compounds, such C7−, are in the feed overhead stream  34 , so larger non-aromatic compounds, such as C8+, are present in the feed bottoms stream  36 . The heavy aromatics conversion catalyst  52  breaks non-aromatic compounds into smaller non-aromatic compounds, such as C4−. The smaller non-aromatic compounds are much more volatile than the C8+ aromatic compounds, and are vented off of the aromatics conversion zone  50  in a light gases stream  56 . The smaller non-aromatic compounds in the light gases stream  56  are removed from the xylene isomer production system  26 , and are available for other uses, so the remaining hydrocarbons are primarily aromatic compounds. 
     The heavy aromatics conversion zone  50  is operated at heavy aromatics conversion conditions in the presence of hydrogen, where the hydrogen is supplied by the heavy aromatics conversion hydrogen line  54 . Suitable heavy aromatics conversion conditions include a temperature ranging from about 200° C. to about 600° C., or from about 300° C. to about 500° C. Suitable pressures are from about 100 KPa to about 5 mega Pascals (MPa) absolute, or from about 500 KPa to about 3 MPa absolute. The heavy aromatics conversion zone  50  contains a sufficient volume of heavy aromatics conversion catalyst  52  to provide a liquid hourly space velocity with respect to an intermediate stream (described below) from about 0.5 to about 50 hr −1 , or from about 0.5 to about 20 hr −1 . Hydrogen is provided from the heavy aromatics conversion hydrogen line  54  in a sufficient volume for a hydrogen/hydrocarbon mole ratio of about 0.5:1 to about 25:1. Other compounds may be present in the hydrogen, such as nitrogen, argon, or light hydrocarbons, without adverse effect. 
     The heavy aromatics conversion zone  50  is a single reactor in one exemplary embodiment, but in other embodiments it is two or more separate reactors with suitable means to ensure the desired isomerization temperature is maintained at the entrance to each reactor. The hydrocarbons are contacted with the heavy aromatics conversion catalyst  52  in any suitable manner, including upward flow, downward flow, or radial flow. The hydrocarbons may be in a liquid phase, a vapor phase, or a mixed liquid/vapor phase in the heavy aromatics conversion zone  50 . 
     The heavy aromatics conversion catalyst  52  is an aromatics complex catalyst, and can include a zeolitic component, a metal component, and an inorganic oxide. Suitable zeolites include one or more of ATO, BEA, EUO, FAU, FER, MCM-22, MEL, MFI, MOR, MTT, MTW, NU-97 OFF, Omega, UZM-5, UZM-8, UZM-14, and TON, according to the Atlas of Zeolite Structure Types. The metal component includes one or more of the base noble metals in a proportion from about 0.01 weight percent to about 10 weight percent. Suitable metals include Rhenium (Re), Tin (Sn), Germanium (Ge), Lead (Pb), Cobalt (Co), Nickel (Ni), Indium (In), Gallium (Ga), Zinc (Zn), Uranium (U), Dysprosium (Dy), Thallium (Tl), Molybdenum (Mo), Ruthenium (Ru), Rhodium (Rh), Palladium (Pd), Osmium (Os), Iridium (Ir), and Platinum (Pt). The balance of the heavy aromatics conversion catalyst  52  can be an inorganic oxide binder, such as alumina. A variety of catalyst shapes can be used, such as spherical or cylinder shaped, but other shapes are also acceptable. 
     A de-ethylated aromatics stream  58  exits the heavy aromatics conversion zone  50 , where the de-ethylated aromatics stream  58  primarily includes C10− aromatic compounds. Any non-aromatic compounds introduced to the heavy aromatics conversion zone  50  are converted to smaller non-aromatic compounds that are vented off in the light gases stream  56 , as described above. The de-ethylated aromatics stream  58  feeds a heavy aromatics fractionator  60 , which produces a second light aromatics stream  62 , an intermediate aromatics stream  64 , and a heavy aromatics stream  66 . The second light aromatics stream  62  is primarily C7− aromatics, the intermediate aromatics stream  64  is primarily C8 aromatics, and the heavy aromatics stream  66  is primarily C9+ aromatics. The heavy aromatics fractionator  60  is one, two, or more fractionators in various embodiments, and suitable operating conditions include a temperature from about 35° C. to about 360° C. and a pressure from about 5 KPa to about 1,800 KPa. 
     The intermediate aromatics stream  64  includes the xylene compounds, and is fed to the isomer recovery process  70 . The process employed to recover a particular desired isomer in the isomer recovery process  70  is not critical, and any effective recovery scheme known in the art may be used. For example, selective adsorption with a crystalline aluminosilicate adsorbent, crystallization processes, or combinations of the two can be used. The isomer recovery process  70  produces a desired isomer stream  72  and an isomer raffinate stream  74 . The desired isomer stream  72  primarily includes one of the xylene isomers. In one embodiment, the para xylene is the isomer primarily present in the desired isomer stream  72 , but in other embodiments ortho xylene or meta xylene is primarily present. The isomer raffinate stream  74  primarily includes the two xylene isomers that are not present in the desired isomer stream  72 . 
     The isomer raffinate stream  74  flows to the isomerization zone  76 , where the xylenes are isomerized to a ratio closer to the equilibrium ratio for xylene. One of the xylene isomers was removed in the isomer recovery process  70 , and the removal of one isomer shifts the composition of the isomer raffinate stream  74  away from equilibrium. The isomer raffinate stream  74  primarily includes 2 of the 3 xylene isomers, so the third isomer is produced in the isomerization zone  76  to bring the mixture closer to an equilibrium ratio. The equilibrium ratio is about 20 to 25 percent ortho xylene, 20 to 30 percent para xylene, and 50 to 60 percent meta xylene at about 250° C., and this equilibrium ratio varies with temperature and other conditions. 
     The isomerization zone  76  includes an isomerization catalyst  78 , and operates at suitable isomerization conditions. Suitable isomerization conditions include a temperature from about 100° C. to about 500°, or from about 200° C. to about 400° C., and a pressure from about 500 KPa to 5 MPa absolute. The isomerization unit includes a sufficient volume of isomerization catalyst  78  to provide a liquid hourly space velocity, with respect to the isomer raffinate stream  74 , from about 0.5 to about 50 hr −1 , or from about 0.5 to about 20 hr −1 . Hydrogen may be present up to about 15 moles per mole of xylene, but in some embodiments hydrogen is essentially absent from the isomerization zone  76 . In embodiments where hydrogen is essentially absent from the isomerization zone  76 , no free hydrogen is added and residual dissolved hydrogen from prior processing is less than about 0.05 moles of hydrogen per mole of aromatic compound in the isomer raffinate stream  74 . In other embodiments, hydrogen is present at less than about 0.01 moles of hydrogen per mole of aromatic compound in the isomer raffinate stream  74 . The isomerization zone  76  may include one, two, or more reactors, where suitable means are employed to ensure a suitable isomerization temperature at the entrance to each reactor. The xylenes are contacted with the isomerization catalyst  78  in any suitable manner, including upward flow, downward flow, or radial flow. 
     The isomerization catalyst  78  includes a zeolitic aluminosilicate with a Si:Al 2  ratio greater than about 10, or greater than about 20 in some embodiments, and a pore diameter of about 5 to about 8 angstroms. Some examples of suitable zeolites include, but are not limited to, MFI, MEL, EUO, FER, MFS, MTT, MTW, TON, MOR, and FAU, and gallium may be present as a component of the crystal structure. In some embodiments, the Si:Ga 2  mole ratio is less than 500, or less than 100 in other embodiments. The proportion of zeolite in the catalyst is generally from about 1 to about 99 weight percent, or from about 25 to about 75 weight percent. In some embodiments, the isomerization catalyst  78  includes about 0.01 to about 2 weight percent of one or more of Ru, Rh, Pd, Os, Ir, and Pt, but in other embodiments the isomerization catalyst  78  is substantially absent of any metallic compound, where substantial absence is less than about 0.01 weight percent. The balance of the isomerization catalyst  78  is an inorganic oxide binder, such as alumina, and a wide variety of catalyst shapes can be used, including spherical or cylindrical. 
     An isomerized stream  80  exits the isomerization zone  76  and returns to the isomer recovery process  70 , so the xylenes make a loop and repeatedly passes between the isomer recovery process  70  and the isomerization zone  76 . The isomerized stream  80  includes more of the xylene isomer primarily present in the desired isomer stream  72  than in the isomer raffinate stream  74 , so more of the desired xylene isomer is available for recovery. In this manner, the total amount of the desired xylene isomer recovered can exceed the equilibrium value of the desired xylene isomer. An isomerization purge stream  82  is also taken from the isomerization zone  76  to remove small concentrations of ethyl benzene and other lighter and heavier compounds produced in the isomerization zone  76  to prevent the build-up of these materials in the isomerization zone  76 /isomer recovery process  70  loop. The isomerization purge stream  82  is fed into the heavy aromatics conversion zone  50 , described above. The isomerization zone  76 /isomer recovery process  70  loop operates without fractionating the xylenes, so far less energy is used than for other processes that do fractionate the xylenes in isomerization zone  76 /isomer recovery process  70  loops. 
     As mentioned above, the heavy aromatics stream  66  primarily includes C9+ aromatics. The heavy aromatics stream  66  can optionally be fractionated in a pre-transalkylation fractionator  84  to produce a C10+ aromatics stream  86  and a C9 aromatics stream  88 . The C10+ aromatics stream  86  primarily includes C10+ aromatics that are removed from the xylene isomer production system  26 , and are available for other uses. The C9 aromatics stream  88  and/or the heavy aromatics stream  66  are then introduced to a transalkylation zone  90 . The first light aromatics stream  48  and the second light aromatics stream  62 , which are both primarily toluene, are also introduced to the transalkylation zone  90 . The transalkylation zone  90  converts some of the C9+ aromatics from the heavy aromatics stream  66 , preferably in the presence of toluene from the first and second light aromatics stream  48 ,  62 , to C8 aromatic compounds. The transalkylation zone  90  further increases the yield of the desired xylene isomer by converting C9+ and C7 aromatics to C8 aromatics. 
     The transalkylation zone  90  includes a transalkylation catalyst  92 , and the transalkylation zone  90  is operated in the presence of hydrogen supplied by the transalkylation hydrogen line  94  at suitable transalkylation conditions. Suitable transalkylation conditions include a temperature ranging from about 200° C. to about 600° C., for example from about 300° C. to about 500° C. Suitable pressures are from about 100 KPa to about 5 mega Pascals (MPa) absolute, for example from about 500 KPa to about 3 MPa. The transalkylation zone  90  contains a sufficient volume of transalkylation catalyst  92  to provide a liquid hourly space velocity with respect to a transalkylation stream  96  (described below) from about 0.5 to about 50 hr −1 , or from about 0.5 to about 20 hr −1 . Hydrogen is provided from the transalkylation hydrogen line  94  in a sufficient volume for a hydrogen/hydrocarbon mole ratio of about 0.5:1 to about 25:1. Other compounds may be present in the hydrogen, such as nitrogen, argon, or light hydrocarbons, with adverse effect. 
     The transalkylation zone  90  is a single reactor in one exemplary embodiment, but in other embodiments it is two or more separate reactors with suitable means to ensure the desired transalkylation temperature is maintained at the entrance to each reactor. The hydrocarbons are contacted with the transalkylation catalyst  92  in any suitable manner, including upward flow, downward flow, or radial flow. The hydrocarbons may be in a liquid phase, a vapor phase, or a mixed liquid/vapor phase in the transalkylation zone  90 . 
     In exemplary embodiments, the transalkylation catalyst  92  includes a zeolitic component, a metal component, and an inorganic oxide. Suitable zeolites include one or more of ATO, BEA, EUO, FAU, FER, MCM-22, MEL, MFI, MOR, MTT, MTW, NU-97 OFF, Omega, mordenite, UZM-5, UZM-8, UZM-14, and TON, according to the Atlas of Zeolite Structure Types. The proportion of zeolite in the transalkylation catalyst  92  is from about 1 to about 99 weight percent, or from about 25 to about 75 weight percent. The metal component includes one or more of the base noble metals in a proportion from about 0.01 weight percent to about 10 weight percent. Suitable metals include Re, Sn, Ge, Pb, Co, Ni, In, Ga, Zn, U, Dy, Tl, Mo, Ru, Rh, Pd, Os, Ir, and Pt. The balance of the transalkylation catalyst  92  can be an inorganic oxide binder, such as alumina. A variety of catalyst shapes can be used, such as spherical or cylinder shaped, but other shapes are also possible. 
     A transalkylation stream  96  is produced by the transalkylation zone  90 . The transalkylation stream primarily includes C7-10 aromatics, including many C8+ aromatics. The C8+ aromatics from the transalkylation stream  96  are fed into the heavy aromatic conversion zone  50  and contact the heavy aromatic conversion catalyst  52 . Several different embodiments can be used to transfer the C8+ aromatics from the transalkylation stream  96  into the heavy aromatic conversion zone  50 . In one exemplary embodiment, the transalkylation stream  96  is fed into the aromatics fractionation zone  44 , and an aromatics fractionation bottoms stream  98  is fed into the heavy aromatic conversion zone  50 . The aromatics fractionation zone  44  is operated so the aromatics fractionation bottoms stream  98  includes C8+ aromatics, which are introduced to the aromatics fractionation zone  44  by the transalkylation stream  96 . In other embodiments (not shown), the transalkylation stream  96  is directly fed into the heavy aromatics conversion zone  50 , and thereby transfers the C8+ aromatics as well as the C7− aromatics and any other compounds present. In yet another embodiment (not shown), a separate fractionation column is used to separate the components of the transalkylation stream  96  and feed the C8+ aromatics to the heavy aromatic conversion zone  50 . 
     Many different embodiments are possible, so it should be appreciated that a vast number of variations exist. It should also be appreciated that the embodiment or embodiments illustrated are only examples, and are not intended to limit the scope, applicability, or configuration of the application in any way. Rather, the foregoing detailed description will provide those skilled in the art with a convenient road map for implementing one or more embodiments, it being understood that various changes may be made in the function and arrangement of elements described without departing from the scope as set forth in the appended claims.