Abstract:
A process is presented for the increasing the yields of aromatics from reforming a hydrocarbon feedstream. The process includes splitting a naphtha feedstream into a light hydrocarbon stream, and a heavier stream having a relatively rich concentration of naphthenes. The heavy stream is reformed to convert the naphthenes to aromatics and the resulting product stream is further reformed with the light hydrocarbon stream to increase the aromatics yields. The catalyst is passed through the reactors in a sequential manner.

Description:
FIELD OF THE INVENTION 
       [0001]    The present invention relates to the process of enhancing the production of aromatic compounds. In particular the improvement and enhancement of aromatic compounds such as benzene, toluene and xylenes from a naphtha feedstream. 
       BACKGROUND OF THE INVENTION 
       [0002]    The reforming of petroleum raw materials is an important process for producing useful products. One important process is the separation and upgrading of hydrocarbons for a motor fuel, such as producing a naphtha feedstream and upgrading the octane value of the naphtha in the production of gasoline. However, hydrocarbon feedstreams from a raw petroleum source include the production of useful chemical precursors for use in the production of plastics, detergents and other products. 
         [0003]    The upgrading of gasoline is an important process, and improvements for the conversion of naphtha feedstreams to increase the octane number have been presented in U.S. Pat. Nos. 3,729,409, 3,753,891, 3,767,568, 4,839,024, 4,882,040 and 5,242,576. These processes involve a variety of means to enhance octane number, and particularly for enhancing the aromatic content of gasoline. 
         [0004]    In addition, the production of aromatics is important. Aromatics, such as benzene, are used in plastics production and the production of detergents. Increasing the yields of aromatic compounds from hydrocarbons streams increases the return, as lower value hydrocarbons are converted to higher value aromatics. 
         [0005]    Processes include splitting feeds and operating several reformers using different catalysts, such as a monometallic catalyst or a non-acidic catalyst for lower boiling point hydrocarbons and bi-metallic catalysts for higher boiling point hydrocarbons. Other improvements include new catalysts, as presented in U.S. Pat. Nos. 4,677,094, 6,809,061 and 7,799,729. However, there are limits to the methods and catalysts presented in these patents, and which can entail significant increases in costs. 
         [0006]    The increased demand generates pressure to improve the processes for increasing the production of aromatics. 
       SUMMARY OF THE INVENTION 
       [0007]    The invention comprises controlling the process and catalyst flow to improve the yields of aromatics from a naphtha feedstock. The naphtha feedstock is split into a light stream having C7 and lighter hydrocarbons and a heavy stream having C8 and heavier hydrocarbons and naphthenes. The heavy stream is passed through a first reactor system to convert the naphthenes to aromatics, and generate a first effluent stream. The light stream and the first effluent stream are passed to a second reactor system for converting C6 and C7 paraffins to aromatics. The process includes passing a reforming catalyst through the first reactor system to generate a first catalyst effluent stream. The first catalyst effluent stream is passed to the second reactor system and flows through the reactors in the second reactor system. The second reactor system is held at a substantially isothermal condition to maximize the conversion to benzene and toluene. 
         [0008]    Other objects, advantages and applications of the present invention will become apparent to those skilled in the art from the following drawings and detailed description of the invention. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0009]      FIG. 1  shows the flow scheme for parallel flow with split feed and recombination for secondary reforming; and 
           [0010]      FIG. 2  shows the flow scheme for a parallel flow with naphtha hydrotreatment of the feed and recycle. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0011]    A process is presented for addressing the need to increase the yields of benzene, toluene and xylenes from a hydrocarbon feedstream. The hydrocarbon feedstream is usually a full boiling range naphtha feedstream and the naphtha feedstream is reformed to generate C6 to C10 aromatics. The reformation process involves a catalytic reactor to selectively convert naphthenes and paraffins to aromatic compounds. In general, catalytic reforming generates unwanted byproducts, which include methane, ethane and to a lesser degree propanes and butanes. These are low value products and processes that reduce the formation of these byproducts and increase the amount of aromatics improves the economics of the reforming process. 
         [0012]    Catalytic reforming of hydrocarbons proceeds through numerous chemical reaction pathways. The reforming reaction rates vary with temperature, and the Arrhenius equation captures the relationship between reaction rate (k) and reaction temperature (T), where each reaction has an activation energy (Ea). The equation becomes: 
         [0000]        k=A *exp(− Ea/RT ), where  A  is the individual reaction rate coefficient.
 
         [0013]    Reactions with different activation energies will be affected differently by the reaction temperature and changes in the reaction temperature. In the case of catalytic reforming there are numerous parallel reaction pathways, or competing reaction pathways. With different activation energies, it is possible to manipulate the conversion rates to desired products by controlling the reaction temperatures. However, since there are a large number of parallel reactions, the practical control is limited to classes, or types, of chemical compounds being reformed, and the control is over the ability to sufficiently segregate the classes of compounds. In the case of naphtha, the catalytic reforming process is endothermic overall. For an adiabatic reaction system, there is a substantial temperature decrease and this adversely affects the rates of conversion. By segregating the most endothermic compounds, and reforming the more endothermic compounds, the temperatures of the reactions are more easily controlled, and the yields can be increased. This also has a benefit of reducing the selectivity of undesired side products. 
         [0014]    Improving the catalysts has been the main focus of improving the reforming process, however, modifying the process using non-obvious rearrangements can yield unexpected results. The present invention for reforming a hydrocarbon feedstream is shown in  FIG. 1 . The process includes passing the feedstream  8  to a fractionation system  10  to generate an overhead stream  12  and a bottoms stream  14 . The bottoms stream  14  is passed to a first reforming reactor system  20 , and is operated at a first set of reaction conditions to generate a first reactor system effluent  22 . The first reactor effluent  22  has an increased aromatics content. 
         [0015]    The first reactor effluent  22  and the overhead stream  12  are passed to a second reforming reactor system  30 . The second reforming reactor system  30  generates a second effluent stream  32  rich in aromatics. The second effluent stream  32  is passed to a reformate splitter  40  to generate a reformate overhead stream  42  comprising C7 and lighter aromatics, and a reformate bottoms stream  44  comprising C8 and heavier aromatics. The reformate overhead stream  42  is passed to an aromatics recovery unit  50  to generate an aromatics product stream  52  and a raffinate stream  54  comprising non-aromatic hydrocarbons. The raffinate stream  54  can be passed back to the second reactor system  30  for further conversion of the hydrocarbons to aromatics. 
         [0016]    In a preferred embodiment, the overhead stream  12  comprises n-hexane and lighter components. The bottoms stream  14  comprises cyclohexane and heavier components. The naphthenes in the bottoms stream  14  are processed in the first reforming reactor system  20  to process the components that have the highest endothermicity. This leads to lower energy usage to maintain the inlet temperatures in the second reactor system  30 . The first reforming reactor system  20  will have an inlet temperature less than 540° C., for the conversion of the naphthenes to aromatics. The second reforming reactor system  30  will preferably have the inlet temperatures greater than or equal to or near 540° C. Each reactor in the reactor systems will have a heating unit to bring the temperature of the reactor feed to the desired reaction temperatures. 
         [0017]    The process includes operating the second reactor system  30  in an operating regime to minimize the temperature changes within the reactor system  30 . The reforming process is endothermic, and the reactions drive the temperature down in the reactors relative to the inlet temperature. The second reactor system  30  can comprise a plurality of reactors with inter-reactor heaters. In  FIG. 1 , the plurality of reactors are shown by  30   a ,  30   b ,  30   c  and  30   d , with the heaters shown by  35   a ,  35   b ,  35   c  and  35   d.    
         [0018]    The catalyst used in this process is passed through the various reactors  20 ,  30 . Catalyst  62  is preferably passed through the first reactor system  20  to generate a first reactor system catalyst effluent stream  64 . The first catalyst effluent stream is then passed to the second reactor system  30 , where the catalyst is subject to a greater operating temperature. The catalyst passes through each reforming reactor in the second reactor system  30 , and is returned as a second catalyst effluent stream  66  to a regenerator. 
         [0019]    In a second embodiment, as shown in  FIG. 2 , comprises passing a hydrocarbon feedstream  108  to a hydrotreating unit  110  to generate a treated hydrocarbon feed  112 . With a preferred hydrocarbon feed, the hydrotreating unit  110  is a naphtha hydrotreater. The treated feed  112  is passed to a fractionation system  120  to generate a light overhead stream  122  comprising n-hexane and lighter hydrocarbons. The fractionation system  120  also generates a bottoms stream  124  comprising cyclohexane and heavier components. The bottoms stream  124  is passed to a first reactor system  130  to generate a first effluent stream  132  having increased aromatics content. The bottoms stream  122  is heated to an inlet reaction temperature for the catalytic reforming reaction in the reactor system  130 . The overhead stream  122  and the first effluent stream  132  are passed to a second reactor system  140 . The second reactor system  140  includes a plurality of reactor units  140   a, b, c, d  and reactor feed heaters  150   a, b, c, d , where the feed stream to each reactor is heated to a desired inlet temperature. The second reactor system  140  is sized and designed to minimize the temperature drops within the reactors due to the endothermic nature of the reforming reactions. 
         [0020]    The second reactor system  140  generates a second effluent stream  142  and is passed to a reformate splitter  160 . The reformate splitter  160  generates a reformate overhead stream  162  having C7 and lighter aromatic compounds, and a reformate bottoms stream  164  comprising C8 and heavier hydrocarbon compounds. The reformate overhead stream  162  is passed to an aromatics recovery unit  170  to generate an aromatics product stream  172  comprising benzene and toluene. The aromatics recovery unit  170  also generates a raffinate stream  174  comprising non-aromatic hydrocarbons. A portion of the raffinate stream  174  can be returned into the reactor system for converting the hydrocarbons to aromatics. The raffinate stream  174  is passed to the hydrotreating unit  110  to process and remove residual sulfur picked up from the aromatics recovery unit  170 . 
         [0021]    A catalyst stream  182  from a regenerator is passed to the first reactor system  130  to generate a first reactor catalyst effluent stream  184 . The first reactor catalyst effluent stream  184  passes to the second reactor system  140  where the catalyst passes through the plurality of reactors and generates a second reactor system catalyst effluent stream  186 . The second reactor catalyst effluent stream  186  is returned to the regenerator. 
         [0022]    The aromatics recovery unit  170  can comprise different methods of separating aromatics from a hydrocarbon stream. One industry standard is the Sulfolane™ process, which is an extractive distillation process utilizing sulfolane to facilitate high purity extraction of aromatics. The Sulfolane™ process is well known to those skilled in the art. 
         [0023]    The first reactor system  130  is operated for the conversion of naphthenes to aromatics, and is operated at a lower temperature than the second reactor system  140 . The first reactor system  130  will experience greater temperature drops due to the higher relative concentration of endothermic compounds, such as the naphthenes, converted to aromatics before passing the first effluent stream  132  on to the second reactor system  140 . The first reactor system  130  includes an inlet temperature less than 540° C., and the second reactor system includes heaters to raise the inlet temperature of the reactor feed streams to at least 540° C. 
         [0024]    The catalyst used is a reforming catalyst, and the process is a moving bed process where the catalyst is cycled through the reactors and then regenerated. The catalyst as it passes through the reactors is partially deactivated, and the process yields and selectivities can be maintained through raising the temperature of the reaction. The process therefore passes catalyst from a regenerator to the first reactor system, and generates a first catalyst effluent stream. The first catalyst effluent stream is passed to the second reactor system and generates a second catalyst effluent stream. The second catalyst effluent stream is passed to the regenerator to return the catalyst to a regenerated state. 
         [0025]    When the second reactor system comprises a plurality of reactors, the catalyst can be passed sequentially through the reactors in a series relationship. The catalyst enters the first reactor in the series and sequentially passes through each reactor, with the catalyst reheated upon leaving a reactor and before entering the subsequent reactor to the reaction inlet temperatures. The catalyst exiting the final reactor in the series is passed to the regenerator. 
         [0026]    The isothermal reactor system, or second reactor system, utilizes a reforming catalyst and is operated at a temperature between 520° C. and 600° C., with a preferred operating temperature between 540° C. and 560° C., with the reaction conditions controlled to maintain the isothermal reactions at or near 540° C. A plurality of reactors with inter-reactor heaters provides for setting the reaction inlet temperatures to a narrow range, and multiple, smaller reactors allow for limiting the residence time and therefore limiting the temperature variation across the reactor system  40 . The process of reforming also includes a space velocity between 0.6 hr −1  and 10 hr −1 . Preferably the space velocity is between 0.6 hr −1  and 8 hr −1 , and more preferably, the space velocity is between 0.6 hr −1  and 5 hr −1 . 
         [0027]    Reforming catalysts generally comprise a metal on a support. The support can include a porous material, such as an inorganic oxide or a molecular sieve, and a binder with a weight ratio from 1:99 to 99:1. The weight ratio is preferably from about 1:9 to about 9:1. Inorganic oxides used for support include, but are not limited to, alumina, magnesia, titania, zirconia, chromia, zinc oxide, thoria, boria, ceramic, porcelain, bauxite, silica, silica-alumina, silicon carbide, clays, crystalline zeolitic aluminasilicates, and mixtures thereof. Porous materials and binders are known in the art and are not presented in detail here. The metals preferably are one or more Group VIII noble metals, and include platinum, iridium, rhodium, and palladium. Typically, the catalyst contains an amount of the metal from about 0.01% to about 2% by weight, based on the total weight of the catalyst. The catalyst can also include a promoter element from Group IIIA or Group IVA. These metals include gallium, germanium, indium, tin, thallium and lead. 
         [0028]    The first reforming reactor system uses the same catalyst, but is operated at a lower temperature and allows for greater temperature swings within the reactor. 
         [0029]    An alternative arrangement is for the catalyst to be passed in parallel to each of the reactors in the second reactor system. This provides for fresher catalyst as the process flow stream passes through each reactor in a series arrangement to increase the yields of aromatics. The catalyst, after passing through the reactors, is then passed to the regenerator. 
         [0030]    When the first reactor system comprises a plurality of reactors, the catalyst from the regenerator can be passed to the first reactor in the first reactor system with the catalyst flowing through the subsequent reactors in a series arrangement. The catalyst is not heated before entering each reactor. Optionally, the catalyst can be reheated to the reactor inlet temperatures. 
         [0031]    An alternate arrangement is for the catalyst from the regenerator to be split and passed in a parallel arrangement to the plurality of reactors in the first reactor system, with each reactor generating a first catalyst effluent stream. The catalyst in the first catalyst effluent streams are combined and routed to the second reactors in the second reactor system. 
         [0000]    
       
         
               
             
               
               
               
               
             
               
               
               
               
             
           
               
                 TABLE 1 
               
             
             
               
                   
               
               
                 Example of Yield Benefit Via Split Feed Series 
               
             
          
           
               
                   
                 Base Case 
                 Paraffin Recycle 
                   
               
               
                   
                 C6 Split Feed 
                 Case C6 Split Feed 
               
               
                   
                 0% Raffinate 
                 75% C6+ Raffinate 
               
               
                   
                 Recycle 
                 Recycle 
               
               
                   
                 3 - Isothermal 
                 3 - Isothermal 
                 % Change 
               
               
                   
                 RxRs 55% P6 
                 RxRs 55% P6 
                 (Paraffin 
               
               
                   
                 Conversion 
                 Conversion 
                 Recycle - 
               
               
                 Products 
                 (Kg/hr) 
                 (Kg/hr) 
                 Base) 
               
               
                   
               
             
          
           
               
                 H2 
                 8035.0 
                 8184.0 
                 1.9% 
               
               
                 C1-C2 
                 8053.5 
                 6889.7 
                 −14.5% 
               
               
                 C3-C4 
                 16939.9 
                 14847.7 
                 −12.4% 
               
               
                 Light 
                 8326.0 
                 8774.8 
                 5.4% 
               
               
                 Gasoline 
               
               
                 Raffinate 
                 10060.1 
                 11716.0 
                 16.5% 
               
               
                 Benzene 
                 13431.1 
                 13445.5 
                 0.1% 
               
               
                 Toluene 
                 41747.4 
                 41828.8 
                 0.2% 
               
               
                 PX 
                 9010.3 
                 8961.9 
                 −0.5% 
               
               
                 A8 
                 39452.4 
                 39228.9 
                 −0.6% 
               
               
                 (excluding 
               
               
                 PX) 
               
               
                 A9 
                 29372.7 
                 30019.2 
                 2.2% 
               
               
                 A10 
                 5810.3 
                 5978.8 
                 2.9% 
               
               
                 Total 
                 138824.3 
                 139463.1 
                 0.5% 
               
               
                 A6-A10 
               
               
                   
               
             
          
         
       
     
         [0032]    The process was divided into two reaction zones. The first reaction zone performed reformation of more highly endothermic compounds, such as naphthenes, where the temperature dropped more. The second reaction zone was controlled to simulate an isothermal system, with the temperature drops within the reactions in the second reaction zone reduced due to the reduction in the amount of highly endothermic compounds. The results from simulations of the reactions show an increase in the desired benzene and toluene, with a reduction in the amounts of light hydrocarbons in the C1-C4 range. 
         [0033]    The process shows the separation of the feed into highly endothermic compounds and unconverted paraffin compounds. The highly endothermic compounds were passed to the non-isothermal reactor system, or the first reactor system, and the less endothermic compounds were passed to the substantially isothermal reactor system, or second reactor system. The process included passing the first reactor system effluent with unconverted paraffins to the isothermal reactor system, and the recycling of unconverted paraffins to the reforming reactors. 
         [0034]    Therefore, increases can be achieved through innovative flow schemes that allow for process control of the reactions. While the invention has been described with what are presently considered the preferred embodiments, it is to be understood that the invention is not limited to the disclosed embodiments, but it is intended to cover various modifications and equivalent arrangements included within the scope of the appended claims.