Abstract:
This is a process for upgrading a petroleum naphtha fraction. The naphtha is subjected to reforming and the reformate is cascaded to a benzene and toluene synthesis zone over a benzene and toluene synthesis catalyst comprising a molecular sieve of low acid activity. The preferred molecular sieve is steamed ZSM-5. The benzene and toluene synthesis zone is operated under conditions compatible with the conditions of the reformer such as temperatures of above about 800° F. (427° C.). In one aspect on the invention, the benzene and toluene synthesis catalyst includes a metal hydrogenation component from group VII(B), specifically rhenium. In one mode of operation, the benzene and toluene synthesis catalyst replaces at least a portion of the catalyst in the reformer. The process produces a product containing an increased proportion of benzene, toluene, and/or xylenes, and a reduced portion of alkylated aromatics, as compared to reformate.

Description:
FIELD OF THE INVENTION  
         [0001]    This invention relates to a process for conversion of hydrocarbons. More specifically, the invention relates to a process for upgrading a hydrocarbon feedstock by reforming followed by hydrodealkylation. A multi-stage reforming process using a rhenium-containing catalyst in the final stage is disclosed. The final stage produces additional benzene, toluene and xylenes by dealkylating alkylated aromatics.  
         BACKGROUND OF THE INVENTION  
         [0002]    The reformate upgrading process of this invention, and its background, is more completely described in U.S. Pat. No. 5,865,986, which is incorporated by reference in this application. In the instant application, however, the catalyst of the final stage is loaded with rhenium rather than palladium or platinum, providing significant cost advantages.  
           [0003]    Catalytic reforming of naphtha feedstocks is well known in the petroleum refining industry. Most naphtha feeds contain large quantities of naphthenes and paraffins and consequently they have low octane numbers. In catalytic reforming these components go through a variety of hydrocarbon conversions resulting in a gasoline product of improved octane number. Some of the more important conversion reactions include dehydrogenation of naphthenes to aromatics and dehydrocyclization of normal paraffins to aromatics. Less desirable reactions which commonly occur include hydrocracking of paraffins and naphthenes to produce gaseous hydrocarbons such as methane and ethane. Because of these less desirable reactions, an important objective of catalytic reforming is to rearrange the structure of the hydrocarbon molecules to form higher octane products without any significant change in the carbon number distribution of the stock.  
           [0004]    The reforming reactions are, typically, catalyzed by catalysts comprising porous supports, such as alumina, that have dehydrogenation promoting metal components impregnated or admixed therewith. Platinum on alumina and more recently bimetallics such as platinum and rhenium on alumina are examples of these catalysts. Such catalysts are described in U.S. Pat. Nos. 3,415,737 and 3,953,368.  
           [0005]    U.S. Pat. No. 5,744,674 discloses the preparation of benzene, toluene and xylene from C9+ heavy aromatics using ZSM-5 loaded with rhenium, tin and platinum or palladium. This reference is not concerned with naphtha upgrading, however. There is no teaching of a multi-stage reforming process.  
           [0006]    U.S. Pat. No. 4,877,514 discloses the preparation of catalyst suitable for use in Fluid Catalytic Cracking or Reduced Crude Conversion hydrocarbon conversion operations. These catalysts may comprise zeolites and may further incorporate rhenium oxide.  
           [0007]    U.S. Pat. No. 4,855,036 discloses a process for Fluid Catalytic Cracking which employs a catalyst comprising a large pore zeolite. The zeolite is prepared by contact with a fluoroanion. U.S. Pat. Nos. 4,642,409; 4,654,457; and 4,499,321 disclose dealkylation of 1,4 dialkylbenzene with use of a zeolite catalyst such as ZSM-5. This catalyst may be modified with a Group VIIb element such as rhenium.  
           [0008]    U.S. Pat. No. 4,467,129 discloses catalytic dealkylation of ethylbenzene, where the ethylbenzene is mixed with xylene. The catalyst comprises mordenite and a zeolite such as ZSM-5. Rhenium may be added for hydrogenation purposes.  
           [0009]    None of these patents discloses the concept of increasing the yield of benzene, toluene and xylene products from a multi-stage naphtha reforming process by use of ZSM-5 loaded with rhenium in the last bed, as in the instant invention.  
           [0010]    It is known that pollution can be reduced by lowering gasoline endpoint, resulting in a product endpoint where, in a standard ASTM distillation, 90 volume percent of the gasoline distills below about 270° F. (132° C.) to 350° F. (177° C.) (T90). Based on this, there have been regulatory proposals, particularly in the State of California, to require gasoline to meet a maximum T90 specification of 300° F. (149° C.). Meeting this T90 permits only 10% of the hydrocarbons in gasoline to boil above 300° F. (149° C.). A significant boiling range conversion of heavy naphthas will be required to meet this goal.  
         BRIEF DESCRIPTIONS OF THE INVENTION  
         [0011]    A process has been discovered for producing benzene, toluene and xylenes while enhancing the octane value of the gasoline boiling range materials of a naphtha fraction of low octane value and high gasoline end boiling range.  
           [0012]    The process of this invention can increase the benzene production of a reformer by more than 10% while producing fewer C9+ hydrocarbons, through hydrodealkylation reactions.  
           [0013]    The invention is directed to a multi-step integrated process for upgrading a petroleum naphtha comprising the steps of  
           [0014]    (a) introducing the naphtha to a catalytic reforming zone comprising a plurality of operatively connected fixed bed or moving bed catalyst zones, the catalyst zones being maintained under reforming conditions of temperature and pressure to provide an intermediate comprising aromatics and paraffins; and  
           [0015]    (b) cascading the reaction product to a synthesis zone for mixtures of benzene, toluene and xylenes, comprising at least one fixed bed or moving bed catalytic zone operatively connected to the catalytic reforming zone, the benzene and toluene synthesis zone being maintained under conditions of temperature and pressure compatible with the reforming conditions of step (a), the reaction zone containing a catalyst which comprises a hydrogenation component from Group VII(b),and preferably comprises a molecular sieve of low acid activity, typically, as determined by an alpha value of less than about 150, more specifically, less than about 100, even more specifically, less than about 60, to provide a reaction product comprising more benzene, toluene, or xylenes than the intermediate.  
           [0016]    The hydrogenation component of Group VII(b) in step (b) is preferably rhenium.  
           [0017]    The catalytic reforming zone and the benzene and toluene synthesis zone are in series flow arrangement, preferably without intermediate separation of the reformer effluent so that the two zones are operated under compatible conditions including hydrogen circulation rate and pressure.  
           [0018]    In one embodiment of the invention, a low acidity molecular sieve can be provided by using a deactivated catalyst from another refinery process. In this respect, the other refinery process provides the catalyst treatment conditions needed to reduce catalyst acidity.  
           [0019]    Prior to the contacting with the reformate, the deactivated catalyst can be regenerated by conventional techniques such as by burning in an oxygen-containing gas to remove at least a major part of the accumulated coke from the catalyst or by hydrogen regeneration. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0020]    [0020]FIG. 1 is a simplified schematic flow diagram of the process of the invention.  
         [0021]    [0021]FIG. 2 is a simplified schematic flow diagram of an alternative embodiment of the invention. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0022]    A more detailed discussion of reforming, reformate upgrading, and catalyst composition is provided in U.S. Pat. No. 5,865,986.  
         [0023]    In the present invention a petroleum naphtha characterized by a boiling range of C5 to about 450° F. (232° C.), typically boiling up to about 400° F. (204° C.), is contacted with a reforming catalyst under reforming conditions selected to produce a reaction product comprising aromatics and paraffins. Typically, the hydrocarbon feed contains a percentage of components which boil above 300° F. (149° C.). The components boiling above 300° F. (149° C.) usually comprise at least 10% of the entire feed. In general, the feed can be further characterized by the presence of C9+ hydrocarbons which are usually present in an amount of less than about 40 wt. %, typically 25 wt. % to 40 wt. %, based on the entire weight of the feed. Yield advantages can be achieved by increasing the cut-point of the reformer feed to boost C9+ aromatics. Alternatively, a C9+ aromatic cofeed can be employed in which case the feed can contain over 40 wt. % C9+ hydrocarbons, typically, up to 50 wt. % C9+ hydrocarbons. Since C6− components are olefin precursors, yield loss is minimized by removing them from the feed. Thus, the feed can be substantially devoid of C6− hydrocarbons.  
         [0024]    The reforming process can be continuous, cyclic or semiregenerative. The process can be in a fixed bed, moving bed, tubular, radial flow or fluid bed. Typically, a hydrogen to hydrocarbon mole ratio of up to 8:1 is employed to maintain a reasonable catalyst cycle length.  
         [0025]    The conditions of reforming typically include temperatures of at least about 800° F. (427248 C) to about 1050° F. (565° C.) and pressures from about 50 psig (446 kPa) to about 500 psig (3,549 kPa), more specifically from about 50 psig (446 kPa) up to and including 450 psig (3204 kPa). It may often be preferred to employ pressures in the lower ranges e.g. 50 psig (446 kPa) to about 125 psig (963 kPa) to encourage formation of aromatics which supply precursors for the preferred reactions of the benzene and toluene synthesis zone and enhance yield of the preferred products. The hydrogen-to-hydrocarbon ratio ranges from about 0.5 to about 20 and the liquid hourly space velocity can be in the range of about 0.1 to 10, usually about 0.5 to 5.  
         [0026]    It is contemplated that any molecular sieve having a pore size appropriate to admit the bulky C9+ hydrocarbons and catalytically dealkylate the aromatics can be employed in this reformate upgrading process. More detailed information concerning appropriate molecular sieves for this invention is found in U.S. Pat. No. 5,865,986. The hydrogenation component which is preferred in this invention is rhenium, which produces results comparable to those produced using platinum or palladium but at a lower cost.  
         [0027]    The molecular sieve which catalyzes these reactions is usually an intermediate or large pore size zeolite having a silica-to-alumina mole ratio of at least about 12, specifically from about 12 to 2000. The zeolite is usually characterized by a Constraint Index of about 0.5 to 12 specifically about 1 to 12 as described in U.S. Pat. No. 4,088,605.  
         [0028]    Typically, the molecular sieve of choice is a zeolite. Zeolites contemplated include ZSM-5, ZSM-11, ZSM-12, ZSM-35, ZSM-38, zeolite beta and other similar materials. U.S. Pat. No. 3,702,886 describing and claiming ZSM-5 is incorporated herein by reference.  
         [0029]    Process Configuration  
         [0030]    In the multi-step integrated process the petroleum naphtha is catalytically reformed and the reformate is cascaded to the hydrodealkylation reaction zone.  
         [0031]    [0031]FIG. 1 is a simplified schematic flow diagram of one useful process configuration. Referring to FIG. 1, a petroleum naphtha supplied by line  10  is charged to reformer heater  12  which elevates the temperature of the feed to a temperature suitable for reforming. The heated feed is charged to a plurality of reformer reaction zones  16   a ,  16   b  and  16   c  with interstage heaters  15   a  and  15   b.    
         [0032]    Although three reformer reaction zones are shown, there can be less than three or more than three reaction zones. The bottom portion of the last reformer reaction zone  18  is loaded with the hydrodealkylation catalyst. The feed passes over the hydrodealkylation catalyst just before it exits the reformer to produce a product of increased benzene content as compared to the effluent of the last reforming catalyst zone  16   c.    
         [0033]    The hydrodealkylation catalyst of reaction zone  18  is typically isolated from the reforming catalyst to maximize its opportunity to work on the products of reforming as opposed to the reformer feed. This can be accomplished by providing a separate reactor or by segregating the catalysts within the same reactor.  
         [0034]    However, intermingling of the hydrodealkylation catalyst and the reforming catalyst will be difficult to avoid and will not be detrimental in the last part of the final reactor.  
         [0035]    Usually when the hydrodealkylation catalyst is located within the reformer, regardless of where the hydrodealkylation catalyst is located, a radial flow reactor is particularly suitable to maintain a low pressure drop. The radial flow reactor, particularly in combination with smaller particle size hydrodealkylation catalyst, contributes to improved flow distribution in the last bed of the reformer.  
         [0036]    In some operations it will be useful to employ a small particle size catalyst, typically when reactor volume is small or to alleviate pressure drop. A self bound zeolite such as self-bound ZSM-5 is specifically contemplated.  
         [0037]    Usually when the hydrodealkylation catalyst is located within the reformer, regardless of where the hydrodealkylation catalyst is located, a radial flow reactor is particularly suitable to maintain a low pressure drop. The radial flow reactor, particularly in combination with smaller particle size hydrodealkylation catalyst, contributes to improved flow distribution in the last bed of the reformer.  
         [0038]    In some operations it will be useful to employ a small particle size catalyst, typically when reactor volume is small or to alleviate pressure drop. A self bound zeolite such as self-bound ZSM-5 is specifically contemplated.  
         [0039]    [0039]FIG. 2 shows an embodiment of the invention in which the hydrodealkylation catalyst is located in a separate reactor  19  associated with switching valves  17   a  and  17   b  which, optionally, enable the catalyst zone to be removed from on-line contact during at least a portion of regeneration of the reformer catalyst. Optionally, heater  15   c  is located between the last reactor of the reformer and the hydrodealkylation catalyst reactor  19 .  
         [0040]    Referring to both FIGS. 1 and 2, after cooling, the aromatics rich product is passed to vapor/liquid separator  22  which separates a hydrogen-rich gas via hydrogen compressor  25  for recycling to the reformer via line  21 . Via line  24 , the liquid product is conveyed from separator  22  to fractionator  26  typically a series of fractionators that separate the product into C4−, C5, C 6−C 8 and C9+ hydrocarbon streams. The C9+ aromatics can be separated and recycled to the reformer or the hydrodealkylation reactor to increase yield.  
         [0041]    The C6 to C8 stream of fractionator  26  is transferred by line  28  to a paraffin separator  34  which separates the paraffins from the aromatics, typically, by solvent extraction. The aromatics extract can then be conveyed via line  35  to separation zone  36  which separates the extract into benzene, toluene and xylenes streams. An important advantage of the invention is a low consumption of hydrogen. Typically, hydrogen consumption is less than about 200 SCFB (35.6 n.l.l.&lt;−1&gt;), more typically, ranging from about 0 SCFB (0 n.l.l.&lt;−1&gt;) to about 100 S.C.F.B. (17.8 n.l.l.&lt;−1&gt;), more typically less than about 50 SCFB (8.9 n.l.l.&lt;−1&gt;). This low hydrogen consumption can be particularly advantageous when there is a need to balance a high hydrogen consumption in the reformer.  
         [0042]    The hydrodealkylation catalyst can be exposed to the conditions of the reformer during rejuvenative treatment of the reformer catalyst. Typically, the reformer catalyst is rejuvenated by oxychlorination but any rejuvenating method is contemplated.  
         [0043]    The hydrodealkylation catalyst may be reactivated by the rejuvenative treatment of the reformer catalyst. However, other methods known for reactivating the catalyst may be employed such as burning with oxygen, regeneration with hydrogen or an inert gas such as nitrogen.  
       EXAMPLES  
     Example 1  
       [0044]    A reformate was obtained which had the following composition.  
                                                                         Component   Composition   Units                                        C4−   0.06   WPCT           C5   5.68   WPCT           C6 Non-Aromatics   10.32   WPCT           C7 Non-Aromatics   5.78   WPCT           C8 Non-Aromatics   1.80   WPCT           C9+ Non-Aromatics   0.34   WPCT           Benzene   5.75   WPCT           Toluene   18.80   WPCT           Xylenes   21.47   WPCT           Ethylbenzene   3.00   WPCT           C9+ Aromatics   26.98   WPCT                      
 
       Example 2  
       [0045]    The catalyst used in this study was prepared by steaming an alumina bound ZSM-5 base (65/35) at 1200F for 15 hours. The alpha activity of this catalyst after steaming is 2.6. This steamed catalyst was then impregnated (incipient wetness impregnation) with an aqueous solution of ammonium perrhenate to yield a catalyst which contains 0.3% rhenium by weight (measured as the metal). This catalyst was then dried and calcined for one hour at 975F in a rotary calciner. This dried catalyst is herein referred to as 0.3% Re/ZSM-5.  
       Example 3  
       [0046]    The hydrocarbon mixture of Example 1 was used as feed in a fixed-bed, laboratory reactor filled with the catalyst of Example 2. The catalyst was first oxychlorided with a mixture of 1300 ppmv chlorine and 7% oxygen in nitrogen at 990° F., followed by reduction with hydrogen at 700° F. in a glass-lined, fixed bed reactor to simulate commercial reformer catalyst reactivation conditions.  
         [0047]    Five grams of the oxychlorided and reduced catalyst were then transferred to a 0.68″ ID, stainless steel tube, fixed-bed reactor operated in an adiabatic fashion. The catalyst was sulfided with 400 ppmv hydrogen sulfide in hydrogen at 750° F. prior to feeding the hydrocarbon mixture to the reactor to simulate commercial reformer catalyst preparation.  
         [0048]    The conditions for the experiment were 24 WHSV, ca. 6:1 H 2 :HC, 940° F. WABT, and ca. 300 psig. The hydrocarbon feed mixture was combined with makeup hydrogen and recycle gas to simulate the conditions present in the last reactor of a commercial catalytic reformer. The reactor product is cooled and flashed in a separator. A portion of the flash separator overhead gas is recycled to the inlet of the reactor by a compressor. On-line, gas chromatography is used to analyze the gaseous and liquid products from the flash separator and calculate yields of the various hydrocarbon molecules.  
         [0049]    After nine days on stream, the following yields were obtained:  
                                                                         Component   Composition   Units                                        C4−   0.92   WPCT           C5   5.44   WPCT           C6 Non-Aromatics   9.85   WPCT           C7 Non-Aromatics   5.55   WPCT           C8 Non-Aromatics   1.75   WPCT           C9+ Non-Aromatics   0.37   WPCT           Benzene   6.06   WPCT           Toluene   19.44   WPCT           Xylenes   21.72   WPCT           Ethylbenzene   2.90   WPCT           C9+ Aromatics   25.96   WPCT                      
 
         [0050]    Note that there are increases in benzene, toluene, and xylenes versus the feed composition.