Abstract:
A process for reforming a hydrocarbon stream is presented. The process involves splitting a naphtha feedstream to at least two feedstreams and partially processing each feedstream in separate reactors. The processing includes passing the light stream to a combination hydrogenation/dehydrogenation reactor. The process reduces the energy by reducing the endothermic properties of intermediate reformed process streams.

Description:
FIELD OF THE INVENTION 
       [0001]    The present invention relates to a process for enhancing the production of aromatics 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. No. 3,729,409, U.S. Pat. No. 3,753,891, U.S. Pat. No. 3,767,568, U.S. Pat. No. 4,839,024, U.S. Pat. No. 4,882,040 and U.S. Pat. No. 5,242,576. These processes involve a variety of means to enhance octane number, and particularly for enhancing the aromatic content of gasoline. 
         [0004]    While there is a move to reduce the aromatics in gasoline, aromatics have many important commercial uses. Among them include the production of detergents in the form of alkyl-aryl sulfonates, and plastics. These commercial uses require more and purer grades of aromatics. The production and separation of aromatics from hydrocarbons streams are increasingly important. 
         [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. No. 4,677,094, U.S. Pat. No. 6,809,061 and U.S. Pat. No. 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]    Improved processes are needed to reduce the costs and energy usage in the production of aromatic compounds. 
       SUMMARY OF THE INVENTION 
       [0007]    The present invention is a process for improving the yields of aromatic compounds from a hydrocarbon feedstream. In particular, a preferred feedstream is a full boiling range naphtha. The increase in demand for aromatic compounds enhances the value of converting paraffins, olefins and naphthenes to aromatics. 
         [0008]    The process includes passing the hydrocarbon feedstream to a fractionation unit to generate a light stream comprising C7 and lighter hydrocarbons and a heavy stream comprising C8 and heavier hydrocarbons. The process includes passing the light stream to a hydrogenation/dehydrogenation reactor system to generate an intermediate process stream having C6 and C7 aromatics with a reduced olefin content. The heavy stream is passed to a reforming reactor system, to convert the heavier paraffins to aromatic compounds and generate a reformate stream. The reformate stream and the intermediate process stream are sent to a second reforming reactor system to generate a reformate product stream. The reformate product stream is passed to a reformate splitter to generate a reformate overhead stream comprising C7 and lighter aromatics, and lighter hydrocarbons, and a reformate bottoms stream comprising C8 and heavier hydrocarbons. The reformate overhead stream is passed to a aromatics recovery unit to generate an aromatics product stream. 
         [0009]    In one embodiment, the hydrogenation/dehydrogenation reactor system uses a metal catalyst on a support to hydrogenate the olefins present in the process stream and to dehydrogenate the naphthenes present in the process stream. 
         [0010]    Other objects, advantages and applications of the present invention will become apparent to those skilled in the art from the following detailed description and drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWING 
         [0011]      FIG. 1  is a diagram of a first process for increasing aromatics yields by separately processing and reforming light naphthenic and olefinic compounds; and 
           [0012]      FIG. 2  is a diagram of a second process for increasing aromatics yields by processing the light and heavy hydrocarbon streams separately. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0013]    There is an increased demand for aromatics. Important aromatics include benzene, toluene, and xylenes. These aromatics are important components in the production of detergents, plastics, and other high value products. With increasing energy costs, energy efficiency is an important aspect for improving the yields of aromatics. The present invention provides for understanding the differences in the properties of the different components in a hydrocarbon mixture to develop a better process. 
         [0014]    The feedstock comprises many compounds and the reforming process proceeds along numerous pathways. The reaction rates vary with temperature, and the Arrhenius equation captures the relationship between the reaction rate and temperature. The reaction rate is controlled by the activation energy for a particular reaction, and with the many reactions in the reforming process, there are many, dissimilar activation energies for the different reactions. For the different reactions, it is possible to manipulate the conversion of one hydrocarbon to a desired product, e.g. hexane to benzene. A process is best operated at isothermal conditions, and produces the highest yields if the reactions are controlled to a narrow temperature range to simulate near isothermal conditions. 
         [0015]    The reforming process is substantially endothermic, and requires a continuous addition of heat to maintain the temperature of reaction. Different components within a hydrocarbon mixture have different endothermicities during the reforming process. Separating out the components with the highest endothermicities reduces the heat load to the process. In addition, separate processing of components that take in the most heat allows for more isothermal control of the reforming process downstream. While the description herein describes the reaction temperatures in the reactors, the reaction temperatures refer to the reactor inlet temperatures. The actual reactor temperatures with fluctuate, and drop somewhat from the reactor inlet temperatures. The control of the process is to maintain a relatively constant inlet temperature, with the reactor sized and process controls directed to minimize the temperature drop within the reactors. 
         [0016]    While all of the components react differently, it would be impossible to separate out each component. But it has been found that some of the types of components have different properties which significantly affect the reaction process. Dehydrogenation is an important process for the production of aromatics. Generally, naphthenes are highly endothermic, and this requires a continuous addition of heat to the process. By separating the naphthenes from the bulk of the feedstock, and processing the naphthene rich stream separately, downstream reactors can be held in a more near isothermal operation. The process can be utilized with a variety of hydrocarbon feedstreams, but a full boiling range naphtha feedstream having a significant amount of naphthenes and aromatics provides a useful preferred source of hydrocarbons for the generation and recovery of aromatics. 
         [0017]    The present invention, as shown in  FIG. 1 , includes passing a hydrocarbon feedstream  8  to a fractionation unit  10 . The fractionation unit  10  is operated to separate the feedstream into an overhead stream  12  having C7 and lighter hydrocarbons, and a bottoms stream  14  having C8 and heavier hydrocarbons. In particular, the operation is for separating light naphtenes, such as cyclohexane, to the overhead stream  12 . The overhead stream  12  is passed to a hydrogenation/dehydrogenation reactor system  20 , to dehydrogenate the naphtenes and to hydrogenate some of the olefins, to generate a first stream  22  having C6 and C7 aromatics and with a low olefin content. The bottoms stream is passed to a bottoms, or heavy, reforming unit  30  to generate a bottoms reformate  32  having aromatic compounds. The first stream  22  and the bottoms reformate stream  32  are passed to an isothermal reactor system  40  to further convert paraffins to aromatics and to generate an aromatics process stream  42 . The aromatics process stream  42  is passed to a reformate splitter  50  to recover the lighter aromatics. The reformate splitter  50  generates a reformate overhead stream  52  having C7 and lighter aromatics, and C7 and lighter compounds such as paraffins. The reformate splitter  50  also generates a reformate bottoms stream  54  having C8 and heavier hydrocarbons. The reformate overhead stream  52  is passed to an aromatics recovery unit  60  to generate an aromatics product stream  62  comprising benzene and toluene. The remainder of the hydrocarbons from the aromatics recovery unit  60  are passed out as a raffinate stream  64  comprising paraffins. 
         [0018]    The aromatics recovery unit  60  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. 
         [0019]    The process can further include passing the raffinate stream  64  to the hydrogenation/dehydrogenation reactor  20  for further conversion of the hydrocarbons in the raffinate stream  64 . The need to pass the raffinate stream  64  to the hydrogenation/dehydrogenation reactor  20  can depend on the amount of naphthenes and olefins in the raffinate stream  64 . When the raffinate stream  64  has an olefinic content of at least 10 wt %, the raffinate stream  64  is passed to the hydrogenation/dehydrogenation reactor  20 . For a raffinate stream  64  having low naphthene content, the raffinate stream  64  can, in an alternative, be passed to the isothermal reactor system  40 . 
         [0020]    The passing of high olefinic content streams to the hydrogenation/dehydrogenation reactor system  20  removes olefins that can reduce the reforming catalyst deactivation due to the presence of the olefins in the hydrocarbon stream. 
         [0021]    The hydrogenation/dehydrogenation reactor system  20  uses a single catalyst. The catalyst is a non-acid catalyst and has a metal function. The preferred catalyst is a metal deposited on an inert support. The catalyst is non-chlorided. The catalyst performs two functions, while it is a single catalyst. The catalyst will hydrogenate olefins and also dehydrogenate naphthenes. In studying the reaction rates various classes of hydrocarbons and for various reactions were looked at for catalytic reactions over a catalyst with a platinum metal. For hydrogenation the reaction rates run from about 10 −2  to 10 2  molecules/site-s, and has an operating window generally from 200° C. to 450° C. Dehydrogenation has reaction rates from about 10 −3  to 10 molecules/site-s, and has an operating window generally from 425° C. to 780° C. There is an overlap of these reaction windows where both reactions occur when the temperature in the reactor is held to between 400° C. and 500° C., and preferably 420° C. and 460° C., and more preferably between 425° C. and 450° C. A wider range can be employed depending on the relative amounts of naphthenes and olefins. This allows for the simultaneous reactions of hydrogenation of some hydrocarbon components, while dehydrogenating other hydrocarbon components. In particular, olefins present can be hydrogenated while naphthenes are dehydrogenated. 
         [0022]    Preferably, the hydrogenation/dehydrogenation reactor system  20  is a fixed bed reactor system, but it is intended to include other types of reactor bed structures within this invention, including, but not limited to, moving bed systems, bubbling bed systems, and stirred reactor bed systems. 
         [0023]    The catalyst in the hydrogenation/dehydrogenation reactor system  20  is preferably a metal only catalyst on a support, where the choice of catalyst metal is from a Group VIII noble elements of the periodic table. The Group VIII noble metal may be selected from the group consisting of platinum, palladium, iridium, rhodium, osmium, ruthenium, or mixtures thereof. Platinum, however, is the preferred Group VIII noble metal component. It is believed that substantially all of the Group VIII noble metal component exists within the catalyst in the elemental metallic state. Preferably, the catalyst in the hydrogenation/dehydrogenation reactor has no acid function. 
         [0024]    Preferably the Group VIII noble metal component is well dispersed throughout the catalyst. It generally will comprise about 0.01 to 5 wt. %, calculated on an elemental basis, of the final catalytic composite. Preferably, the catalyst comprises about 0.1 to 2.0 wt. % Group VIII noble metal component, especially about 0.1 to about 2.0 wt. % platinum. 
         [0025]    The Group VIII noble metal component may be incorporated in the catalytic composite in any suitable manner such as, for example, by coprecipitation or cogelation, ion exchange or impregnation, or deposition from a vapor phase or from an atomic source or by like procedures either before, while, or after other catalytic components are incorporated. The preferred method of incorporating the Group VIII noble metal component is to impregnate the support with a solution or suspension of a decomposable compound of a Group VIII noble metal. For example, platinum may be added to the support by commingling the latter with an aqueous solution of chloroplatinic acid. Another acid, for example, nitric acid or other optional components, may be added to the impregnating solution to further assist in evenly dispersing or fixing the Group VIII noble metal component in the final catalyst composite. 
         [0026]    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. 
         [0027]    The isothermal reactor system  40  can comprise a plurality of smaller reactors operated sequentially, with inter-reactor heat exchangers between sequential reactors. This provides for maintaining the process nearer to isothermal conditions. 
         [0028]    The process can further include passing the feedstream  8  to a hydrotreater (not shown) before passing the feedstream to the fractionation unit  10 . The hydrotreater removes sulfur compounds prior to passing the hydrocarbon stream to the catalytic reactors, thereby providing protection to the catalysts by removing common catalytic poisons. 
         [0029]    The isothermal reactor system  40  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 reactor 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 or 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 . Due to the elevated temperature, the problems of potential increased thermal cracking are addressed by having a shorter residence time of the process stream in the isothermal reactor system  40 . An aspect of the process can use a reactor with an internal coating made of a non-coking material. The non-coking material can comprise an inorganic refractory material, such as ceramics, metal oxides, metal sulfides, glasses, silicas, and other high temperature resistant non-metallic materials. The process can also utilize piping, heater internals, and reactor internals using a stainless steel having a high chromium content. Stainless steels having a chromium content of 17% or more have a reduced coking ability. 
         [0030]    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. 
         [0031]    A second process for improving the production of aromatic compounds from a full boiling range naphtha is presented as shown in  FIG. 2 . The process includes passing the naphtha feedstream  8  to a fractionation unit  10  to generate an overhead stream  12  having C7 and lighter hydrocarbons and a bottoms stream  14  having C8 and heavier hydrocarbons. The overhead stream  12  is passed to a hydrogenation/dehydrogenation reactor system  20 , where a first stream  22  is generated having a low olefin content, a reduced naphthene content and an increased C6 and C7 aromatics content. The first stream  22  is passed to a light reforming reactor system  44  to generate a first aromatics stream  47 . The light reforming reactor system  44  is operated to be a substantially isothermal system. 
         [0032]    The bottoms stream  14  is passed to a bottoms reforming unit  30  for conversion of some of the hydrocarbons, including the naphthenes to aromatics, and generates a second stream  32  having a reduced naphthene content. The second stream  32  is passed to a heavy reforming reactor system  46 , thereby generating a second aromatics stream  48 . The first  47  and second  48  aromatics streams are passed to a reformate splitter  50 . The reformate splitter  50  generates a reformate overhead stream  52  having C7 and lighter aromatics and hydrocarbons, and a reformate bottoms stream  54  having C8 and heavier hydrocarbons. The reformate overhead stream  52  is passed to an aromatics recovery unit  60  to generate an aromatics product stream  62 , and a raffinate stream  64 . The aromatics product stream  62  comprises benzene and toluene, and can include small amounts of xylenes. 
         [0033]    The process can further include passing the raffinate stream  64  to the hydrogenation/dehydrogenation reactor system  20  for hydrogenating the olefins. In an alternative, if the raffinate stream  64  is sufficiently low in olefin content, the raffinate stream  64  can be passed to the light reforming reactor system  44 . 
         [0034]    The hydrogenation/dehydrogenation reactor system  20  uses a single catalyst that will perform both the function of hydrogenating olefins and dehydrogenating naphthenes. The hydrogenation/dehydrogenation reaction is operated in a relatively narrow temperature window where both reactions occur when the temperature in the reactor is held to between 400° C. and 500° C., and preferably 420° C. and 460° C., and more preferably between 425° C. and 450° C. When the catalyst contacts an olefin, it performs a hydrogenation of the olefin, but if the catalyst contacts a naphthene, it performs a dehydrogenation of the naphthene. This reactor also processes the hydrocarbon components that have the greatest amount of endothermicity in the conversion to aromatics. The conversion of these components before passing the first stream  22  on to the isothermal system  44  reduces the energy input to the light reforming reactor system  44 . The isothermal system  44  can comprise a plurality of smaller reactors with inter-reactor heaters for maintaining a substantially isothermal reaction system. 
         [0035]    The bottoms reforming unit  30  is operated at a temperature lower than the heavy reforming reactor system  46 . The heavy reforming reactor system  46  can comprise a plurality of reactors with inter-reactor heaters, and is operated as a substantially isothermal process. The preferred operating temperature range for the heavy reforming reactor system  46  is 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. The bottoms reforming unit  30  is operated at a lower temperature and a temperature range for the bottoms unit  30  is from 420° C. to 540° C., with a preferred temperature between 440° C. and 500° C. The bottoms reforming unit  30  provides for the conversion of higher endothermic components before passing the second stream  32  on to the isothermal heavy reforming reactor system  46 . 
         [0036]    In an alternate embodiment, the heavy reforming reactor system  46  is operated at a lower temperature, such as in the temperature range from 420° C. to 540° C. 
         [0037]    This process is useful for a hydrocarbon feedstream having a substantial amount of naphthenic compounds, such as a full boiling range naphtha. The naphtha feedstream  8  can be passed to a hydrotreater for removing sulfur compounds and other compounds that will act as poisons to the catalysts in the reforming reactors. 
         [0038]    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.