Patent Abstract:
Enabling a transalkylation process to handle both C 10  alkylaromatics and unextracted toluene permits the following improvements to be realized. No longer extracting toluene allows a reformate-splitter column to be eliminated. The extraction unit can be moved to the overhead of a benzene column. No longer requiring a rigorous split between C 9  and C 10  alkylaromatics allows a heavy aromatics column to be eliminated. Such an enabled transalkylation process requires stabilization of a transalkylation catalyst through the introduction of a metal function. A further enhancement to the flow scheme is accomplished through the elimination of clay treaters in favor of selective olefin saturation at the exits of a reforming unit and an isomerization unit. These improvements result in an aromatics complex with savings on inside battery limits curve costs and an improvement on the return on investment in such a complex.

Full Description:
FIELD OF THE INVENTION 
     This invention relates to an aromatics complex flow scheme, which is a combination of process units that can be used to convert naphtha into basic petrochemical intermediates of benzene, toluene, and xylene. Based on a metal catalyzed transalkylation process that handles unextracted toluene and heavier aromatics and an olefin saturation process, the improved flow scheme removes items of equipment and processing steps, such as a reformate splitter column and a heavy aromatics column, resulting in significant economic benefits when producing para-xylene. 
     BACKGROUND OF THE INVENTION 
     Most new aromatics complexes are designed to maximize the yield of benzene and para-xylene. Benzene is a versatile petrochemical building block used in many different products based on its derivation including ethylbenzene, cumene, and cyclohexane. Para-xylene is also an important building block, which is used almost is exclusively for the production of polyester fibers, resins, and films formed via terephthalic acid or dimethyl terephthalate intermediates. Accordingly, an aromatics complex may be configured in many different ways depending on the desired products, available feedstocks, and investment capital available. A wide range of options permits flexibility in varying the product slate balance of benzene and para-xylene to meet downstream processing requirements. 
     A prior art aromatics complex flow scheme has been disclosed by Meyers in the  Handbook of Petroleum Refining Processes,  2d. Edition in 1997 by McGraw-Hill. 
     U.S. Pat. No. 3,996,305 to Berger discloses a fractionation scheme primarily directed to transalkylation of toluene and C 9  alkylaromatics in order to produce benzene and xylene. The transalkylation process is also combined with an aromatics extraction process. The fractionation scheme includes a single column with two streams entering and with three streams exiting the column for integrated economic benefits. 
     U.S. Pat. No. 4,341,914 to Berger discloses a transalkylation process with recycle of C 10  alkylaromatics in order to increase yield of xylenes from the process. The transalkylation process is also preferably integrated with a para-xylene separation zone and a xylene isomerization zone operated as a continuous loop receiving mixed xylenes form the transalkylation zone feedstock and effluent fractionation zones. 
     U.S. Pat. No. 4,642,406 to Schmidt discloses a high severity process for xylene production that employs a transalkylation zone that simultaneously performs as an isomerization zone over a nonmetal catalyst. High quality benzene is produced along with a mixture of xylenes, which allows para-xylene to be separated by absorptive separation from the mixture with the isomer-depleted stream being passed back to the transalkylation zone. 
     U.S. Pat. No. 5,417,844 to Boitiaux et al. discloses a process for the selective dehydrogenation of olefins in steam cracking petrol in the presence of a nickel catalyst and is characterized in that prior to the use of the catalyst, a sulfur-containing organic compound is incorporated into the catalyst outside of the reactor prior to use. 
     U.S. Pat. No. 5,658,453 to Russ et al. discloses an integrated reforming and olefin saturation process. The olefin saturation reaction uses a mixed vapor phase with addition of hydrogen gas to a reformate liquid in contact with a refractory inorganic oxide containing preferably a platinum-group metal and optionally a metal modifier. 
     U.S. Pat. No. 5,763,720 to Buchanan et al. discloses a transalkylation process for producing benzene and xylenes by contacting a C 9   +  alkylaromatics with benzene and/or toluene over a catalyst comprising a zeolite such as ZSM-12 and a hydrogenation noble metal such as platinum. Sulfur or stream is used to treat the catalyst. 
     U.S. Pat. No. 5,847,256 to Ichioka et al. discloses a process for producing xylene from a feedstock containing C 9  alkylaromatics with the aid of a catalyst with a Is zeolite that is preferably mordenite and with a metal that is preferably rhenium. 
     SUMMARY OF THE INVENTION 
     An aromatics complex flow scheme with an enabled transalkylation process requires stabilization of a transalkylation catalyst through the introduction of a metal function. Enabling a transalkylation process to handle both C 10  alkylaromatics and unextracted toluene permits the following flow scheme improvements to be realized. By using toluene without first passing it to an extraction unit, the flow scheme omits a reformate-splitter column. The concomitantly smaller capacity extraction unit is moved to the overhead of a benzene column. By only extracting benzene, simple extractive distillation is used, since a more expensive combined liquid-liquid extraction method is only required for heavier contaminants. By using both C 9  and C 10  alkylaromatics in an enabled transalkylation unit, the flow scheme further omits a heavy aromatics column. A further enhancement to the flow scheme is accomplished through the elimination of clay treaters in favor of selective olefin saturation at the exits of a reforming process unit or of an alkylaromatic isomerization process unit. 
     Another embodiment of the present invention comprises an apparatus that is based on the process steps, which efficiently converts naphtha into para-xylene. 
     Additional objects, embodiments and details of this invention can be obtained from the following detailed description of the invention. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     The FIGURE shows an aromatics complex flow scheme of the present invention, which includes olefin saturation and a metal stabilized transalkylation catalyst. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Feed to the complex may be naphtha, but can also be pygas, imported mixed xylene, or imported toluene. Naphtha fed to an aromatics complex is first hydrotreated to remove sulfur and nitrogen compounds to less than about 0.5 wt-ppm before passing the treated naphtha on to a reforming unit  13 . Naphtha hydrotreating occurs by contacting naphtha in a line  10  with a naphtha hydrotreating catalyst under naphtha hydrotreating conditions in a unit  11 . The naphtha hydrotreating catalyst is typically composed of a first component of cobalt oxide or nickel oxide, along with a second component of molybdenum oxide or tungsten oxide, and a third component inorganic oxide support, which is typically a high purity alumina Generally good results are achieved when the cobalt oxide or nickel oxide component is in the range of about 1 to about 5 wt-% and the molybdenum oxide component is in the range of about 6 to about 25 wt-%. The alumina (or aluminum oxide) is set to balance the composition of the naphtha hydrotreating catalyst to sum all components up to 100 wt-%. One hydrotreating catalyst for use in the present invention is disclosed in U.S. Pat. No. 5,723,710, the teachings of which are incorporated herein by reference. Typical hydrotreating conditions include a liquid hourly space velocity (LHSV) from about 1.0 to about 5.0 hr −1 , a ratio of hydrogen to hydrocarbon (or naphtha feedstock) from about 50 to about 135 Nm 3 /m 3 , and a pressure from about 10 to about 35 kg/cm 2 . 
     In the reforming unit  13 , paraffins and naphthenes are converted to aromatics. This is the only unit in the complex that actually creates aromatic rings. The other units in the complex separate the various aromatic components into individual products and convert various aromatic species into higher-value products. The reforming unit  13  is usually designed to run at very high severity, equivalent to producing about 100 to about 106 Research Octane Number (RONC) gasoline reformate, in order to maximize the production of aromatics. This high severity operation also extinguishes virtually all non-aromatic aromatic impurities in the C 8   +  fraction of reformate, and eliminates the need for extraction of the C 8  and C 9  aromatics. 
     In the reforming unit  13 , hydrotreated naphtha from a line  12  is contacted with a reforming catalyst under reforming conditions. The reforming catalyst is typically composed of a first component platinum-group metal, a second component modifier metal, and a third component inorganic-oxide support, which is typically high purity alumina. Generally good results are achieved when the platinum-group metal is in the range of about 0.01 to about 2.0 wt-% and the modifier metal component is in the range of about 0.01 to about 5 wt-%. The alumina is set to balance the composition of the naphtha hydrotreating catalyst to sum all components up to 100 wt-%. The platinum-group metal is selected from platinum, palladium, rhodium, ruthenium, osmium, and iridium. The preferred platinum-group metal component is platinum. The metal modifiers may include rhenium, tin, germanium, lead, cobalt, nickel, indium, gallium, zinc, uranium, dysprosium, thallium, and mixtures thereof. One reforming catalyst for use in the present invention is disclosed in U.S. Pat. No. 5,665,223, the teachings of which are incorporated herein by reference. Typical reforming conditions include a liquid hourly space velocity from about 1.0 to about 5.0 hr −1 , a ratio of hydrogen to hydrocarbon from about 1 to about 10 moles of hydrogen per mole of hydrocarbon feed entering the reforming zone, and a pressure from about 2.5 to about 35 kg/cm 2 . Hydrogen produced in the reforming unit  13  exits in a line  14 . 
     The reformate product from the reforming unit  13  in a line  15  is sent to a debutanizer zone  53 , which typically comprises a debutanizer column  20  that strips off the light end hydrocarbons (butanes and lighter) in a line  21 . The debutanizer zone  53  may also comprise at least one olefin saturation zone  16 , which may be placed upstream or downstream from the debutanizer column  20 . Moreover, streams from other units in the aromatics complex may also be sent to the debutanizer column  20  for stripping. These other units include the transalkylation zone, which sends a transalkylation stripper-overhead stream in a line  17 , and the isomerization zone, which sends a deheptanizer overhead stream in a line  18 . Both of these units are described in greater detail below. 
     The olefin saturation zone  16  may consist of the well-known clay treating means or other means to treat residual olefin contaminants. Preferably, the olefin saturation zone  16  comprises an olefin saturation catalyst operating under olefin saturation conditions. Catalytic olefin saturation is also a flow scheme enabler. If a catalytic process is used, the olefins are converted to useful products. For example, a C 6  olefin can be converted to benzene (if the olefin is cyclic), a C 7  to toluene, and a C 8  to xylene. If clay is used, then the olefin will be polymerized, often to C 11 +, which is not very useful for an aromatics complex. Thus, the catalytic olefin saturation helps improve the economics of the flow scheme. Suitable olefin saturation catalysts in the present invention contain elemental nickel or a platinum-group component preferably supported on a inorganic oxide support, which is typically alumina. In the case where the elemental nickel is present on a support, the nickel is preferably present in an amount from about 2 to about 40 wt-% of the total catalyst weight. One catalyst for use in the present invention is disclosed in U.S. Pat. No. 5,658,453, the teachings of which are incorporated herein by reference. Typical olefin saturation conditions include a temperature from about 20° to about 200° C., a pressure from about 5 to about 70 kg/cm 2  and a stoichiometric ratio of hydrogen to olefins from about 1:1 to about 5:1. Olefin treated reformate is shown in a line  19 . 
     The debulanized reformate comprising aromatics in a line  22  is combined with a transalkylation stripper-bottoms stream in a line  24  and sent to a benzene-toluene (BT) fractionation zone  54  via a line  23 . The BT fractionation zone  54  generally comprises at least one column, and usually comprises a benzene column  25  and a toluene column  31 . However, the benzene column  25  may be eliminated in favor of a tramalkylation stripper column, with a stabilizer section sufficient to produce a suitable benzene stream. The BT fractionation zone  54  produces a benzene-enriched stream in a line  26 , a toluene-enriched stream in a line  32 , and a xylene-plus-enriched stream in a line  33 . Typically, the benzene-enriched stream in line  26  is produced from the overhead of the benzene column  25 , with the bottom of the benzene column  25  being sent via a line  30  to feed the toluene column  31 . The toluene-enriched stream in line  32  is produced from the overhead of the toluene column  31  and sent to a transalkylation unit  36 , with the bottom of the toluene column  31  producing the xylene-plus-enriched stream in line  33 . The xylene-plus-enriched stream in line  33  from the bottom of the toluene column  31  is sent to a xylene recovery section  55  of the aromatics complex described below. 
     The benzene-enriched stream in line  26  is sent to an extractive distillation zone  27  which produces a high purity benzene product stream in a line  29  and rejects a by-product raffinate stream in a line  28 . The raffinate stream may be blended into gasoline, used as feedstock for an ethylene plant, or converted into additional benzene by recycling to the reforming unit  13 . The use of extractive distillation instead of liquid-liquid extraction or combined liquid-liquid extraction/extractive distillation processes results in an economic improvement. 
     Extractive distillation is a technique for separating mixtures of components having nearly equal volatility and having nearly the same boiling point It is difficult to separate the components of such mixtures by conventional fractional distillation. In extractive distillation, a solvent is introduced into a main distillation column above the entry, point of the hydrocarbon-containing fluid mixture that is to be separated. The solvent affects the volatility of the hydrocarbon-containing fluid component boiling at a higher temperature differently than the hydrocarbon-containing fluid component boiling at a lower temperature sufficiently to facilitate the separation of the various hydrocarbon-containing fluid components by distillation and such solvent exits with the bottoms fraction. Suitable solvents include tetrahydrothiophene 1,1-dioxide (or sulfolane), diethylene glycol, triethylene glycol, or tetraethylene glycol. The raffinate stream in line  28  comprising nonaromatic compounds exits overhead of the main distillation column, while the bottoms fraction containing solvent and benzene exits below. Often the raffinate will be sent to a wash column in order to be contacted with water and thus remove any residual dissolved solvent. The bottoms stream from the main distillation column is sent to a solvent recovery column, where benzene is recovered overhead and the solvent is recovered from the bottom and passed back to the main distillation column. The recovery of high purity benzene in the line  29  from extractive-distillation typically exceeds 99 wt-%. 
     The toluene-enriched stream in line  32  is usually blended with a stream in a line  41  rich in C 9  and C 10  alkylaromatics produced by a xylene column  39  and charged via a line  34  to the transalkylation unit  36  for production of additional xylenes and benzene. In the transalkylation unit  36 , the feed is contacted with a transalkylation catalyst under transalkylation conditions. The preferred catalyst is a metal stabilized transalkylation catalyst. Such catalyst comprises a zeolite component, a metal component, and an inorganic oxide component The zeolite component typically is either a pentasil zeolite, which include the structures of MFI, MEL, MTW, MTT and FER (IUPAC Commission on Zeolite Nomenclature), a beta zeolite, or a mordenite. Preferably it is mordenite zeolite. The metal component typically is a noble metal or base metal The noble metal is a platinum-group metal is selected from platinum, palladium, rhodium, ruthenium, osmium, and iridium. The base metal is selected from the group consisting of rhenium, tin, germanium, lead, cobalt, nickel, indium, gallium, zinc, uranium, dysprosium, thallium, and mixtures thereof. The base metal may be combined with another base metal, or with a noble metal. Preferably the metal component comprises rhenium Suitable metal amounts in the transalkylation catalyst range from about 0.01 to about 10 wt-%, with the range from about 0.1 to about 3 wt-% being preferred, and the range from about 0.1 to about 1 wt-% being highly preferred. Suitable zeolite amounts in the catalyst range from about 1 to about 99 wt-%, preferably between about 10 to about 90 wt-%, and more preferably between about 25 to about 75 wt-%. The balance of the catalyst is composed of inorganic oxide binder, preferably alumina. One transalkylation catalyst for use in the present invention is disclosed in U.S. Pat. No. 5,847,256, the teachings of which are incorporated herein by reference 
     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 1 to about 60 kg/cm 2 . 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 supplied via a line  35 . 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. 
     The effluent from the transalkylation zone  36  is sent to a transalkylation unit stripper column  52  to remove light ends, then sent to the BT fractionation zone  54  through the lines  24  and  23 . There the benzene product is recovered, and the xylenes are fractionated out and sent to the xylene recovery section  55  via the xylene plus enriched stream in line  33 . The overhead material from the transalkylation stripper column  52  is normally recycled back via the line  17  to the reforming unit debutanizer for recovery of residual benzene. Alternatively, a stabilizer section or column is placed on or after the transalkylation unit stripper column  52 . This transalkylation stabilizer section can produce a benzene-enriched stream suitable for extractive distillation, and eliminate the need for a separate benzene column in the BT fractionation section. Such a stabilizer or stripper column from the transalkylation unit is thus encompassed in the definition of the BT fractionation zone  54  when the separate benzene column is eliminated. The transalkylation unit stripper column  52  can also accept treated product from the olefin saturation zone or overhead from the alkylaromatic isomerization deheptanizer column that would normally be recycled back to the reforming unit debutanizer column  20 . 
     As noted above, the xylene plus-enriched stream in line  33  from the bottom of the toluene column  31  is sent to the xylene recovery section  55  of the aromatics complex. This section of the aromatics complex comprises at least one xylene column  39 , and generally will further include a process unit for separation of at least one xylene isomer, which is typically the para-xylene product from the aromatics complex. Preferably such a para-xylene separation zone  43  is operated in conjunction with an isomerization unit  51  for isomerization of the remaining alkylaromatic compounds back to an equilibrium or near equilibrium mixture containing para-xylene, which can be recycled around again for further recovery in a loop-wise fashion. Accordingly, the xylene-plus-enriched stream in line  33 , which may be blended with a recycle stream in a line  38  to form a steam in a line  37 , is charged to a xylene column  39 . The xylene column  39  is designed to rerun a feed stream in a line  40  to the para-xylene separation zone  43  down to very low levels of C 9  alkylaromatics (A 9 ) concentration. A 9  compounds may build up in a desorbent circulation loop within the para-xylene separation zone  43 , so it is more efficient to remove this material upstream in the xylene column  39 . The overhead feed stream in the line  40  from the xylene column  39  is charged directly to the para-xylene separation zone  43 . 
     Material from the lower part of the xylene column  39  is withdrawn as a strewn rich in C 9  and C 10  alkylaromatics via the line  41 , which is then sent to the transalkylation zone  36  for production of additional xylenes and benzene. The stream in line  41  taken as a sidecut stream on the xylene column (which eliminates a heavy aromatics column) is really enabled by the metal stabilized transalkylation catalyst. A separate column doing a rigorous split to keep coke precursors such as methyl indan or naphthalene out of the steam is no longer needed because the metal stabilized transalkylation catalyst can handle them. Any remaining C 11 + material is rejected from the bottom of the xylene column  39  via a line  42 . Another embodiment is to just send the whole xylene column bottoms stream to the transalkylation unit instead of the sidecut stream. 
     Alternatively, if ortho-xylene is to be produced in the complex, the xylene column is designed to make a split between meta and ortho-xylene arid drop a targeted amount of orthoxylene to the bottoms. The xylene column bottoms is then sent to an ortho-xylene column (not shown) where high purity ortho-xylene product is recovered overhead. Material from the lower part of the ortho-xylene column is withdrawn as a stream rich in C 9  and C 10  alkylaromatics then sent to the transalkylation unit. Any remaining C 11 + material is rejected from the bottom of the ortho-xylene column. 
     The para-xylene separation zone  43  may be based on a fractional crystallization process or an adsorptive separation process, both of which are well known in the art, and preferably is based on the adsorptive separation process. Such adsorptive separation can recover over 99 wt-% pure para-xylene in a line  44  at high recovery per pass. Any residual toluene in the feed to the separation unit is extracted along with the para-xylene, fractionated out in a finishing column within the unit, and then optionally recycled to the transalkylation unit stripper column  52 . Thus, the raffinate from the paraxylene separation zone  43  is almost entirely depleted of para-xylene, to a level usually of less than 1 wt-%. The raffinate is sent via a line  45  to the alkylaromatics isomerization unit  51 , where additional para-xylene is produced by reestablishing an equilibrium or near-equilibrium distribution of xylene isomers. Any ethylbenzene in the para-xylene separation unit raffinate is either converted to additional xylenes or converted to benzene by dealkylation, depending upon the type of isomerization catalyst used. 
     In the alkylaromatic isomerization unit  51 , the raffinate stream in line  45  is contacted with an isomerization catalyst under isomerization conditions. The isomerization catalyst is typically composed of a molecular sieve component, a metal component, and an inorganic oxide component. Selection of the molecular sieve component allows control over the catalyst performance between ethylbenzene isomerization and ethylbenzene dealkylation depending on overall demand for benzene. Consequently, the molecular sieve may be either a zeolitic aluminosilicate or a nonzeolitic molecular sieve. The zeolitic aluminosilicate (or zeolite) component typically is either a pentasil zeolite, which include the structures of MFI, MEL, MTW, MTF and FER (IUPAC Commission on Zeolite Nomenclature), a beta zeolite, or a mordenite. The non-zeolitic molecular sieve is typically one or more of the AEL framework types, especially SAPO-11, or one or more of the ATO framework types, especially MAPSO-31, according to the “Atlas of Zeolite Structure Types” (Butterworth-Heinemsn, Boston, Mass., 3rd ed. 1992). The metal component typically is a noble metal component, and may include an optional base metal modifier component in addition to the noble metal or in place of the noble metal. The noble metal is a platinum-group metal is selected from platinum, palladium, rhodium, ruthenium, osmium, and iridium. The base metal is selected from the group consisting of rhenium, tin, germanium, lead, cobalt, nickel, indium, gallium, zinc, uranium, dysprosium, thallium, and mixtures thereof. The base metal may be combined with another base metal, or with a noble metal. Suitable total metal amounts in the isomerization catalyst range from about 0.01 to about 10 wt-%, with the range from about 0.1 to about 3 wt-% preferred. Suitable zeolite amounts in the catalyst range from about 1 to about 99 wt-%, preferably between about 10 to about 90 wt-%, and more preferably between about 25 to about 75 wt-%. The balance of the catalyst is composed of inorganic oxide binder, typically alumina. One isomerization catalyst for use in the present invention is disclosed in U.S. Pat. No. 4,899,012, the teachings of which are incorporated herein by reference. 
     Typical isomerization conditions include a temperature in the range from about 0° to about 600° C. and a pressure from atmospheric to about 50 kg/cm 3 . The liquid hourly hydrocarbon space velocity of the feedstock relative to the volume of catalyst is from about 0.1 to about 30 hr −1 . The hydrocarbon contacts the catalyst in admixture with a gaseous hydrogen containing stream in a line  46  at a hydrogen-to-hydrocarbon mole ratio of from about 0.5:1 to 15:1 or more, and preferably a ratio of from about 0.5 to 10. If liquid phase conditions are used for isomerization, then no hydrogen is added to the unit. 
     The effluent from the isomerization unit  51  is sent via a line  47  to a deheptanizer column  48 . A bottoms stream in a line  49  from the deheptanizer  48  is treated to remove olefins, if necessary, in an olefin saturation unit  50  with the olefin saturation methods described above. An alternative is to put the olefin saturation unit  50  after the isomerization unit  51  and use the deheptanizer column  48  to remove residual hydrogen. If the catalyst used in the isomerization unit  51  is the ethylbenzene dealkylation type, then olefin saturation may not be required at all. 
     The deheptanizer bottoms stream in line  49 , after olefin treatment, is then recycled back to the xylene column  39  via the line  38 . In this way, all the C 8  aromatics are continually recycled within the xylenes recovery section of the complex until they exit the aromatics complex as para-xylene, benzene, or optionally ortho-xylene. The overhead from the deheptanizer is normally recycled back via the line  18  to the reforming unit debutanizer for recovery of residual benzene. Alternatively, the overhead liquid is recycled back to the transalkylation unit stripper column  52 . 
     Accordingly, the aromatics complex of the present invention displays excellent economic benefits These improvements result in an aromatics complex with savings on inside battery limits curve costs and an improvement on the return on investment in such a complex.

Technology Classification (CPC): 8