Abstract:
The subject invention comprises an aromatics complex to improve yields from a mixed aromatics feed. Through the use of a novel catalyst having higher activity and stability in a transalkylation unit in the complex, a higher xylene yield is obtained.

Description:
FIELD OF THE INVENTION 
     This invention relates to a process for increasing the production of xylenes from a mixed aromatics feed stream. More specifically, the invention comprises a complex of units including a novel transalkylation process and catalyst to convert lighter and heavier aromatics to yield and recover C 8  aromatics. 
     BACKGROUND OF THE INVENTION 
     Most new aromatics complexes are designed to yield benzene and para-xylene. Benzene is a versatile petrochemical building block used in many different products including ethylbenzene, cumene, and cyclohexane. Para-xylene is also an important building block primarily for the production of polyester fibers, resins, and films formed via terephthalic acid or dimethyl terephthalate intermediates. Since the relative yields of benzene and para-xylene generally do not match the proportions obtained from aromatics-generating processes such as catalytic reforming and cracking, an aromatics complex to obtain these preferred products usually comprises a variety of processes such as one or more of transalkylation, disproportionation, isomerization and dealkylation. 
     An aromatics complex flow scheme of the known art is represented by that of Meyers in the Handbook of Petroleum Refining Processes, 3rd. Edition in 2003 by McGraw-Hill. 
     The known art includes a number of processing schemes including transalkylation for converting lighter aromatics, particularly toluene, and heavier aromatics, especially C 9  aromatics, to yield C 8  aromatics in order to increase the yield of xylenes which can be processed to obtain para-xylene from an aromatics complex. Such transalkylation processes generally are limited in the extent to which they can convert aromatics heavier than C 9  to lighter products, and there is a need in the industry for more effective processes in order to improve catalyst stability with higher concomitant xylene yield. 
     SUMMARY OF THE INVENTION 
     Broadly, the invention comprises a process for producing xylenes from an aromatics-rich feed stream comprising the steps of: (a) separating the aromatics-rich feed stream and a transalkylation product stream in a fractionation zone to produce a benzene-rich stream, a toluene-rich stream, a C 8 -aromatics product and C 9 -and-heavier stream; (b) fractionating the C 9 -and-heavier stream in a heavy-aromatics fractionator to obtain a C 9 -C 11 + heavy transalkylation feed and a C 12 + residual stream; and (c) combining the toluene-rich stream and C 9 -C 11 + heavy transalkylation feed to obtain a combined transalkylation feed and contacting the combined feed in a transalkylation zone under transalkylation conditions with a transalkylation catalyst which comprises a UZM-14 aggregate material comprising globular aggregates of crystallites having a MOR framework type comprising 12-ring channels, a mesopore volume of at least about 0.10 cc/gram, and a mean crystallite length parallel to the direction of the 12-ring channels of about 60 nm or less, to produce the transalkylation product stream of step (a) having an increased concentration of xylenes relative to the combined transalkylation feed. 
     In a specific embodiment, the invention comprises a process for producing xylenes from an aromatics-rich feed stream comprising the steps of: (a) separating the aromatics-rich feed stream and a transalkylation product stream in a fractionation zone to produce a benzene-rich stream, a toluene-rich stream, a C 8 -aromatics product and Co-and-heavier stream; and, (b) combining the toluene-rich stream and C 9 -and-heavier stream to obtain a combined transalkylation feed and contacting the combined feed in a transalkylation zone under transalkylation conditions with a transalkylation catalyst which comprises a UZM-14 aggregate material comprising globular aggregates of crystallites having a MOR framework type comprising 12-ring channels, a mesopore volume of at least about 0.10 cc/gram, and a mean crystallite length parallel to the direction of the 12-ring channels of about 60 nm or less, to produce the transalkylation product stream of step (a) having an increased concentration of xylenes relative to the combined transalkylation feed. 
     A variant of the above embodiments comprises dividing the C 9 -and-heavier stream into a heavy recycle stream and a heavy-aromatics-fractionator feed stream and bypassing the heavy recycle stream directly to the transalkylation unit. 
     Another specific embodiment comprises the steps of: (a) combining the aromatics-rich feed stream with recycled heavy aromatics and a recycled toluene-rich stream to obtain a combined transalkylation feed and contacting the combined feed in a transalkylation zone under transalkylation conditions with a transalkylation catalyst which comprises a UZM-14 aggregate material comprising globular aggregates of crystallites having a MOR framework type comprising 12-ring channels, a mesopore volume of at least about 0.10 cc/gram, and a mean crystallite length parallel to the direction of the 12-ring channels of about 60 nm or less, to produce a transalkylation product stream having an increased concentration of xylenes relative to the combined transalkylation feed; (b) separating the transalkylation product stream in a fractionation zone to produce a benzene-rich stream, a toluene-rich recycle stream, a C 8 -aromatics product and C 9 -and-heavier stream; and, (c) fractionating the C 9 -and-heavier stream in a heavy-aromatics fractionator to obtain recycled heavy aromatics and a C 12 + residual product stream. 
     Preferably the transalkylation catalyst in each of the above embodiments comprises one or more of the following characteristics: (1) globular aggregates have a mesopore volume of at least about 0.10 cc/gram, preferably at least about 0.13 cc/gram, and especially at least about 0.2 cc/gram; (2) the UZM-14 crystallites have at least about 1×10 19  12-ring-channel openings/gram of UZM-14 material; (3) the mean crystallite length parallel to the direction of the 12-ring channels is about 60 nm or less and preferably about 50 nm or less; (4) The Si/Al 2  ratio of the UZM-14 aggregate material generally is between about 8 and about 50, and preferably is no more than about 30. 
     Optionally, the C 8 -aromatics product is sent to a combination of a para-xylene-recovery process and a C 8 -aromatics-isomerization process to obtain a para-xylene product and a C 7 -and-lighter stream sent to step (a) for separation of a benzene-rich stream, a toluene-rich stream, and a non-aromatic product. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is an process flow diagram illustrating the invention. 
         FIG. 2  is an process flow diagram of a particular embodiment of the invention. 
         FIG. 3  is an alternative process flow diagram of a unique embodiment of the invention. 
         FIG. 4  is a process flow diagram of a variant of the invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The aromatics-rich feed stream to the process of the invention 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. 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. If the feed stream is catalytic reformate, the reformer preferably is operated at high severity to achieve high aromatics yield with a low concentration of nonaromatics in the product. The reformate also advantageously is subjected to olefin saturation to remove potential product contaminants and materials that could polymerize to heavy nonconvertibles in a transalkylation process. Such processing steps are described in U.S. Pat. No. 6,740,788 B1, incorporated herein by reference thereto. 
     The feed stream to the process of the invention comprises alkylaromatic hydrocarbons of the general formula C 6 H (6-n) R n , where n is an integer from 1 to 5 and R is one or more of CH 3 , C 2 H 5 , C 3 H 7 , C 4 H 9 , or C 5 H 11  in any combination. The feed stream also may comprise benzene and/or aromatics having from 2 to 4 rings. Suitable components of the feed stream thus generally include, for example but without so limiting the invention, benzene, toluene, ethylbenzene, meta-xylene, ortho-xylene, para-xylene, ethyl-toluenes, trimethylbenzenes, diethyl-benzenes, triethylbenzenes, propylbenzenes, methylpropylbenzenes, ethylpropylbenzenes, diisopropylbenzenes, butylbenzenes, indanes, naphthalenes, tetralins, decalins, biphenyls, diphenyls and fluorenes. The feed stream also may contain lesser concentrations of nonaromatics such as pentanes, hexanes, heptanes and heavier paraffins along with methylcyclopentane, cyclohexane and heavier naphthenes; pentanes and lighter paraffins generally will have been removed before processing in the aromatics complex. The combined transalkylation feed preferably contains no more than about 10 wt-% nonaromatics; olefins preferably are restricted to a Bromine Index of no more than about 1000, and preferably no more than about 500. 
       FIG. 1  is a simplified flow diagram of a broad embodiment of the present invention. The aromatics-rich feed stream  10  and a transalkylation product stream  26  are fractionated in a fractionation zone, comprising fractionators  11 ,  14  and  17  as illustrated in  FIG. 1 . Fractionator  11  separates a toluene-and-lighter stream overhead in line  12 , which passes to fractionator  14  which separates a benzene-rich stream overhead in line  15  from a toluene-rich stream in line  16 . A heavier-than-toluene stream in line  13  passes to fractionator  17  which recovers a C 8 -aromatics product overhead in line  18 . 
     There are a number of options in the fractionation zone, depending primarily on the nonaromatics content of the aromatics-rich feed stream and on product objectives. Benzene may be sent to the transalkylation zone, and a separation of benzene and toluene may not be necessary in this case. If a net benzene product is desired, then a high quality product may be obtained directly from the fractionator  14  as shown. However, if a net benzene product needs to be of very high purity; this can be effected by extractive distillation of the benzene-rich stream from fractionator  14 . If the toluene-rich stream contains excessive nonaromatics, the toluene-and-lighter stream in line  12  may be processed by extraction to remove the nonaromatics. A stabilizer in the transalkylation zone can provide a benzene-rich stream suitable for charging to extractive distillation. In this event, the feed stream and transalkylation product may be fractionated separately to obtain feed to fractionator  14 . 
     A C 9 -and heavier stream from fractionator  17  passes to heavy-aromatics fractionator  20 , which separates a C 9 -C 11 + heavy transalkylation feed  21  (comprising 11-carbon aromatics plus optionally a portion of 12-and-higher-carbon aromatics with atmospheric boiling points of up to about 250° to 260° C.) from a C 12 + residual stream  22  (comprising biphenyls, diphenyls, fluorenes and associated components). The C 9 -C 11 + heavy transalkylation feed favorably comprises a substantial amount of 12-and-higher-carbon aromatics; the transalkylation catalyst of the invention tolerates accompanying polycyclics which would adversely affect known catalysts. In lieu of fractionator  20 , stream  21  may be obtained as a sidecut stream from fractionator  17 . Streams  16  and  21  provide the combined transalkylation feed. The reaction in transalkylation zone  23  generally is effected in the presence of hydrogen, supplied in line  24 , and light ends are stripped from the product and removed in stream  25 . The transalkylation zone yields a product with an increased content of xylenes in stream  26  which is sent to the fractionation zone. 
       FIG. 2  is a simplified flow diagram of a particular embodiment of the present invention. The aromatics-rich feed stream  30  and a transalkylation product stream  47  are fractionated in a fractionation zone, comprising fractionators  31 ,  34  and  37  as illustrated in  FIG. 2 . Fractionator  31  separates a toluene-and-lighter stream overhead in line  32 , which passes to fractionator  34  which separates a benzene-rich stream overhead in line  35  from a toluene-rich stream in line  36 . A heavier-than-toluene stream in line  33  passes to fractionator  37  which recovers a C 8 -aromatics product overhead in line  38 . The same options for nonaromatics removal apply as for  FIG. 1 . 
     A C 9 -and heavier stream  39  from fractionator  37  passes to splitter  40 , which divides the stream into line  41  as a heavy recycle stream and line  42  to heavy-aromatics fractionator  43  which separates a C 9 -C 11 + heavy transalkylation feed  44  (comprising 11-carbon aromatics plus optionally a portion of 12-or-higher-carbon aromatics with atmospheric boiling points of up to about 250° to 260° C.) from a C 12 + residual stream  45  (comprising biphenyls, diphenyls, fluorenes and associated components). It may also be advantageous in this embodiment to take a sidecut stream from fractionator  37  which substantially contains C 9  and C 10  aromatics and send this stream directly to the transalkylation reactor, thereby reducing the size of stream  39  and therefore the size of the downstream heavy-aromatics fractionator  43 . The proportion of stream  39  which is sent directly to transalkylation as heavy recycle via stream  41  may vary generally from about 1 to about 99 mass-%, and more usually in the range of from about 10 to about 90 mass-% depending on the nature of the heaviest portion, particularly polycyclics, of stream  39 . Streams  36 ,  41  and  44  provide the combined transalkylation feed which thus includes a portion of the C 12 + since the transalkylation catalyst of the invention tolerates accompanying polycyclics which would adversely affect known catalysts. The reaction in transalkylation zone  46  generally is effected in the presence of hydrogen, supplied in line  47 , and light ends are stripped from the product and removed in stream  48 . The transalkylation zone yields a product with an increased content of xylenes in stream  49  which is sent to the fractionation zone. 
       FIG. 3  is a simplified flow diagram of a unique embodiment of the present invention. The aromatics-rich feed stream  50  and a transalkylation product stream  63  are fractionated in a fractionation zone, comprising fractionators  51 ,  54  and  57  as illustrated in  FIG. 3 . Fractionator  51  separates a toluene-and-lighter stream overhead in line  52 , which passes to fractionator  54  which separates a benzene-rich stream overhead in line  55  from a toluene-rich stream in line  56 . A heavier-than-toluene stream in line  53  passes to fractionator  57  which recovers a C 8 -aromatics product overhead in line  58 . The same options for nonaromatics removal apply as for  FIG. 1 . 
     A C 9 -and heavier stream  59  from fractionator  57  joins streams  56  to form the combined transalkylation feed which includes all of the C 12 + on the basis that the transalkylation catalyst of the invention tolerates accompanying polycyclics which would adversely affect known catalysts. The reaction in transalkylation zone  60  generally is effected in the presence of hydrogen, supplied in line  61 , and light ends are stripped from the product and removed in stream  62 . The transalkylation zone yields a product with an increased content of C 8  aromatics in stream  63  which is sent to the fractionation zone. 
     It is within the scope of the invention that the aromatics-rich feed stream, in particular the C 9 + portion, is processed directly in the transalkylation unit without fractionation to remove a residual stream; this option is illustrated in  FIG. 4 . The aromatics-rich feed stream  100 , represented here as a C 9 + stream optionally including toluene which has been separated by fractionation from a wider-range feed stream, joins recycled heavy aromatics in stream  101  and a recycled toluene-rich stream  102  as feed to transalkylation zone  103 . The reaction in transalkylation zone  103  generally is effected in the presence of hydrogen, supplied as stream  104 , and light ends are stripped from the product and removed in stream  105 . The transalkylation zone yields a product with an increased content of xylenes in stream  106  which is sent to the fractionation zone. 
     The fractionation zone comprises fractionators  107 ,  110 ,  112  and  115  as illustrated in  FIG. 4 . Fractionator  107  separates a toluene-and-lighter stream overhead in line  108 , which passes to fractionator  110  which separates a benzene-rich stream overhead in line  111  from toluene-rich recycle in line  102 . A heavier-than-toluene stream in line  109  passes to fractionator  112  which recovers a C 8 -aromatics product overhead in line  113 . The same options for nonaromatics removal apply as for  FIG. 1 . 
     A C 9 -and heavier stream  114  from fractionator  112  passes to fractionator  115 , which separates recycled heavy aromatics stream  101  (comprising 11-carbon aromatics plus optionally a portion of 12-or-higher-carbon aromatics with atmospheric boiling points of up to about 250° to 260° C.) from a C 12 + residual stream  116  (comprising biphenyls, diphenyls, fluorenes and associated components). Options for bypassing part or all of C 9 -and-heavier stream  114  directly to transalkylation as in  FIGS. 2 and 3  also are applicable in this case. 
     It also is within the scope of the invention as presented in any of the schemes represented in  FIG. 1 ,  2  or  4  that the C 9 -and heavier stream or the C 12 + residual 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. 
     The C 8 -aromatics product recovered as streams  18 ,  38 ,  58  and  113 , respectively, of  FIGS. 1 ,  2 ,  3  and  4  may be further processed to recover valuable isomer products such as para-xylene. Such processing via adsorptive separation, crystallization and isomerization are described in U.S. Pat. No. 6,740,788, incorporated herein by reference thereto. 
     In the transalkylation unit represented in  FIGS. 1 ,  2 ,  3  and  4  respectively as units  23 ,  46 ,  60  and  103 , the feed is contacted with a transalkylation catalyst under transalkylation conditions to obtain a transalkylation product stream having an increased concentration of xylenes. The catalyst significantly comprises a UZM-14 aggregate material having unique properties of nanocrystallinity and porosity which permit the conversion of heavy aromatic feedstocks. Preferably the UZM-14 aggregate material comprises one or more of the following characteristics: 
     (1) globular aggregates have a mesopore volume of at least about 0.10 cc/gram, preferably at least about 0.13 cc/gram, and especially at least about 0.2 cc/gram; 
     (2) the UZM-14 crystallites have at least about 1×10 19  12-ring-channel openings/gram of UZM-14 material; 
     (3) the mean crystallite length parallel to the direction of the 12-ring channels is about 60 nm or less and preferably about 50 nm or less; 
     (4) The Si/Al 2  ratio of the UZM-14 aggregate material generally is between about 8 and about 50, and preferably is no more than about 30. 
     The transalkylation catalyst employed in the schemes of  FIGS. 1 ,  2 ,  3  and  4  preferably comprises a binder preferably comprising one or more of alumina, silica and silica-alumina and a metal component comprising one or more elements selected from groups VIB(6), VIIB(7), VIII(8-10), 1B(11) and IVA(14) of the Periodic Table. Preferably the metal component is selected from one or more of rhenium, nickel, cobalt, molybdenum and tungsten. Suitable metal amounts in the transalkylation catalyst range from about 0.01 to about 15 wt-% on an elemental basis, with the range from about 0.1 to about 10 wt-% being preferred, and the range from about 0.1 to about 6 wt-% being highly preferred. The catalyst also may comprise a phosphorus component, and an optional binder comprises aluminum phosphate as described in U.S. Pat. No. 6,008,423 which is incorporated herein by reference thereto. The catalyst also preferably has been subjected to a presulfiding step to incorporate from about 0.05 to about 2 wt.-% sulfur on an elemental basis. 
     The catalyst may be formed into any shape useful in the process of the invention, including but not limited to extrudates, spheres, pills, tablets, cakes, powders and granules. A spherical catalyst may be manufactured by the well known oil-drop method, described fundamentally in U.S. Pat. No. 2,620,314. 
     The transalkylation catalyst employed in the schemes of  FIGS. 1 ,  2 ,  3  and  4  optionally may comprise an additional zeolitic component, The additional zeolite component preferably is selected from one or more of MFI, MEL, EUO, FER, MFS, MTT, MTWMWW, MAZ, TON and FAU (IUPAC Commission on Zeolite Nomenclature) and UZM-8 (see WO 2005/113439, incorporated herein by reference thereto). More preferably the additional zeolitic component consists essentially of MFI. Suitable total zeolite amounts in the catalyst range from about 1 to about 100 wt-%, preferably from about 10 to about 95 wt-%, and more preferably between about 75 and about 90 wt-%. 
     Conditions employed in the transalkylation zone normally include a temperature of from about 200° to about 540° C. The transalkylation zone is operated at moderately elevated pressures broadly ranging from about 100 kPa to about 6 Mpa absolute. The transalkylation reaction can be effected over a wide range of space velocities, with higher space velocities effecting a higher ratio of para-xylene at the expense of conversion. Liquid hourly space velocity generally is in the range of from about 0.1 to about 20 hr −1 . The feedstock is preferably transalkylated in the vapor phase and in the presence of hydrogen; if transalkylated in the liquid phase, then the presence of hydrogen is optional. If present, free hydrogen is associated with the feedstock and recycled hydrocarbons in an amount of about 0.1 moles per mole of alkylaromatics up to about 10 moles per mole of alkylaromatic. This ratio of hydrogen to alkylaromatic is also referred to as hydrogen to hydrocarbon ratio. 
     Example 1 
     A catalytic reformate feedstock was defined for yield comparisons based on component analyses performed in the laboratory and converted to a total feed of 650.05 metric tons/annum using toluene quantity as the base point: 
     
       
         
               
               
               
             
           
               
                   
                   
               
             
             
               
                   
                 Pentanes 
                 17.0 
               
               
                   
                 Hexanes 
                 38.0 
               
               
                   
                 Benzene 
                 51.0 
               
               
                   
                 Heptanes 
                 8.0 
               
               
                   
                 Toluene 
                 100.0 
               
               
                   
                 Octanes 
                 1.0 
               
               
                   
                 C 8  Aromatics 
                 228.0 
               
               
                   
                 C 9  Aromatics 
                 161.0 
               
               
                   
                 C 10  Alkylaromatics 
                 22.0 
               
               
                   
                 C 11  Alkylaromatics 
                 4.33 
               
               
                   
                 C 12 + Alkylaromatics 
                 1.44 
               
               
                   
                 Naphthalene 
                 0.96 
               
               
                   
                 Methylnaphthalenes 
                 2.16 
               
               
                   
                 C 12 + Naphthalenes 
                 12.51 
               
               
                   
                 Biphenyls/Diphenyls/Fluorenes 
                 2.65 
               
               
                   
                   
               
             
          
         
       
     
     Example 2 
     Using data derived from laboratory tests on conversions of the various components of Example 1, yields were calculated based on the proportion of the C 9  and heavier stream that is fractionated prior to being sent to transalkylation. The total yield of benzene and C 8  aromatics is shown as the result and compared to yields according to the process of the known art: 
                                                               % Fractionated   Yield, T/A                                        Known art   100   538.3           Invention   100   548.6           Invention   75   550.6           Invention   50   553.3           Invention   25   556.9                        
The invention shows a clear advantage over the known art.
 
     The above description and examples are intended to be illustrative of the invention without limiting its scope. The skilled routineer will readily understand how to extrapolate parameters of the disclosure to other embodiments of the invention. The invention is limited only by the claims set forth herein.