Patent Publication Number: US-2021163829-A1

Title: Staged catalytic reforming process

Description:
BACKGROUND 
     Catalytic reforming is a major conversion process in petroleum refinery and petrochemical industries. Reforming is a catalytic process which converts low octane naphthas that have been distilled from crude oil into higher octane reformates used in gasoline blending and aromatic-rich reformates used for aromatic production. While thermal reforming could produce reformate with octane numbers of 65 to 80 (depending on the yield), catalytic reforming increases the octane numbers to around 90 to 95. Basically, the process rearranges or restructures the hydrocarbon molecules in naphtha feedstocks and breaks some of the molecules into smaller molecules. Specifically, low octane naphtha may be transformed into high-octane motor gasoline blending stock and aromatics rich in benzene, toluene, and xylenes, with hydrogen and liquefied petroleum gas as a byproduct. 
     There are four major types of reactions that take place during reforming processes: dehydrogenation of naphthenes to aromatics, dehydrocyclization of paraffins to aromatics, isomerization, and hydrocracking. In the catalytic reforming process, paraffins and naphthenes are restructured to produce isomerized paraffins and aromatics of relatively higher octane numbers. The catalytic reforming converts low octane n-paraffins to i-paraffins and naphthenes. Naphthenes are converted to higher octane aromatics. The aromatics are left essentially unchanged or some may be hydrogenated to form naphthenes due to reverse reactions taking place in the presence of hydrogen. A particular hydrocarbon/naphtha feed molecule may undergo more than one category of reaction and/or may form more than one product. 
     Due to dehydrogenation reactions being very endothermic, the hydrocarbon stream has to be heated between each catalyst bed. Further, dehydrogenation is the main chemical reaction that occurs in catalytic reforming, producing substantial quantities of hydrogen gas. In addition to the hydrogen gas produced in dehydrogenation, dehydrocylization also releases hydrogen. The hydrogen produced in these reaction can be used in hydrotreating or hydrocracking processes. However, an excess of hydrogen is produced, and thus catalytic reforming processes are unique in that they are the only petroleum refinery processes to produce hydrogen as a by-product. Catalytic reforming generally operates with multiple reactors (commonly three), each with a bed of catalyst. Reactors can be broadly classified as moving-bed, fluid-bed, or fixed-bed type. In semi-regenerative units, regeneration of all reactors can be carried out simultaneously in situ after three to twenty-four months of operation by first shutting down the whole process. On the other hand, in continuous reforming processes, catalysts can be regenerated in one reactor at a time, once or twice per day, without disrupting the operation of the unit. 
     Prior Art Catalytic Reforming 
     Catalytic reforming processes are conventionally conducted in one step where a feedstock is fed to a single or multiple reactors in which all reactions take place to produce an effluent product stream. In particular, catalytic reforming is conventionally carried out by feeding a naphtha (after pretreating with hydrogen if necessary to remove sulfur, nitrogen and metallic contaminants, for example) and hydrogen mixture to a furnace, where it is heated to the desired temperature of 450° to 560° C. It is then passed through catalytic reactors at hydrogen pressures of 1 to 50 bars and an LHSC in the range of 0.5 h −1  to 40 h −1 . 
     Referring to  FIG. 1 , a prior art process flow of a catalytic reforming system is illustrated. Catalytic reforming systems and processes typically include a series of reactors  10 ,  20 ,  30  and  40  which operate at temperatures of about 450° to 560° C. A feedstock  102  is introduced into a heat exchanger  45  and then to furnace  15 A to increase its temperature. The heated feedstock  102  is then treated in the reforming reactors to produce an effluent stream  104  that may be further treated at separator  50  to separate a hot product hydrogen  105  and separator bottom stream  106 . The heated feedstock  102  may optionally be sent directly to and treated in any of reactors  20 ,  30 , and  40  via  103 A,  103 B, and  103 C, respectively. Separator bottom stream  106  is fed to stabilizer  60 , in which reformate  108  may be separated from any excess hydrogen or light effluent product gases  110 . The reformate product may then be sent to the gasoline pool or to an aromatic recovery complex to recover BTX. 
     As mentioned above, the reforming reactions are endothermic, resulting in the cooling of reactants and products, and requiring heating of effluent, typically by direct-fired furnaces  15 B,  15 C and  15 D, prior to charging as feed to a subsequent reforming reactor. As a result of the very high reaction temperatures, catalyst particles are deactivated by the formation of coke on the catalyst which reduces the available surface area and active sites for contacting the reactants. 
     SUMMARY 
     This summary is provided to introduce a selection of concepts that are further described below in the detailed description. This summary is not intended to identify key or essential features of the claimed subject matter, nor is it intended to be used as an aid in limiting the scope of the claimed subject matter. 
     In one aspect, embodiments disclosed herein relate to a reforming process for upgrading a heavy naphtha feedstock that includes dehydrogenating naphthenes in the heavy naphtha feedstock to form a first effluent stream comprising aromatics. The process further includes separating the aromatics via extraction from the produced first effluent stream to produce a second effluent stream containing raffinate paraffins. The second stream may then be subjected to cyclization reactions to produce a third effluent stream comprising aromatics. The process further includes combining the first effluent stream and the third effluent stream prior to extraction. 
     In a further aspect, embodiments disclosed herein relate to a system for producing and separating aromatics from a heavy naphtha feedstock. The feedstock may include at least paraffins and naphthenes, and the system may include one or more dehydrogenation reactors for converting naphthenes in the heavy naphtha feedstocks into aromatics in a first effluent. The system may further include an aromatic extracting unit for extracting at least a portion of the aromatics from the first effluent to form a second effluent stream of raffinate comprising at least the paraffins; and one or more cyclization reactors for converting the paraffins in the second effluent stream into aromatics in a third effluent stream. 
     Other aspects and advantages of the claimed subject matter will be apparent from the following description and the appended claims. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a schematic illustration depicting a conventional catalytic single step process configuration. 
         FIG. 2  is a schematic illustration depicting a three step process in accordance with one or more embodiments of the present disclosure 
         FIG. 3  depicts a detailed schematic of a three staged reforming process configuration in accordance with one or more embodiments of the present disclosure. 
     
    
    
     DETAILED DESCRIPTION 
     Embodiments in accordance with the present disclosure generally relate to methods and apparatuses for a three step catalytic reforming process that upgrades a naphtha feedstock. In one or more embodiments of the present disclosure, the three general steps may include a first step of dehydrogenating naphthenes to aromatics at low temperatures; a second step of separating aromatics from the effluents; and a third step in which the unreacted paraffins and naphthenes exiting the aromatic extraction from the second step are directed to cyclization reactors to undergo cyclization. 
     The present systems and methods described herein are designed to utilize multiple reactors, controlled at different conditions to maximize paraffin/naphthene cyclization and aromatization, while also enhancing the endothermic reactions of the reforming process. 
     For the purposes of the present disclosure, the numerous valves, temperature sensors, electronic controllers and the like that are customarily employed and well known to those of ordinary skill in the art of refinery operations are not described. Further, accompanying components that are in conventional refinery operations for catalytic reforming processes that are known to one of ordinary skill in the art may not be shown or discussed herein. 
     Naphthas produced from crude oil distillation generally contain paraffins, napthenes, and aromatics. The naphtha feedstocks used in catalytic reforming processes may be “heavy” naphtha (containing more than six carbon atoms), which may also be referred to as “straight-run” naphthas. Such naphthas may generally have an initial boiling point of 60 to 150° C. and a final boiling point of 190 to 205° C. In one or more embodiments of the present disclosure, the feedstock may be heavy naphtha comprising feedstock comprising naphthenes. However, it is also envisioned that low-octane naphtha (e.g., coker naphtha) or hydrocracker naphtha that contains substantial quantities of naphthenes, or naphthas having lower boiling points could also be feeds in one or more embodiments. 
     In accordance with one or more embodiments of the present disclosure, during the catalytic reforming process, paraffins and naphthenes are restructured to produce isomerized paraffins and aromatics of relatively higher octane numbers. In particular, the catalytic reforming may convert low octane n-paraffins to i-paraffins and naphthenes, and naphthenes may be converted to higher octane aromatics. In accordance with embodiments of the present disclosure, aromatics may be extracted during the reforming, specifically between dehydrogenation and cyclization, to increase yield and reduce reverse reactions (e.g., hydrogenating to form naphthenes) that may otherwise take place in the presence of hydrogen. In particular, in a first step, naphthenes may be converted to aromatics by dehydrogenation at low temperatures compared to the reaction temperatures of subsequent cyclization reactor(s). After dehydrogenation, the aromatics may then be extracted from the first dehydrogenation reactor effluents in a second step to produce aromatic product and a raffinate comprising a second effluent, which may be mainly comprised of paraffins and unreacted naphthenes. The second effluent may then be directed to cyclization reactor(s) to undergo cyclization reactions to reform the paraffins comprised in the reformate in a final step to produce a third effluent. 
     The hydrocarbon/naphtha feed composition, the impurities present therein, and the desired products may play a role in determining the precise process parameters and the specific choice of catalyst(s), process type, and the like. A variety of chemical reactions may be targeted by specific selection of a catalyst or by altering the operating conditions to influence both the yield and selectivity of conversion of paraffinic and naphthenic hydrocarbon precursors to particular aromatic hydrocarbon structures. 
       FIG. 2  depicts an overview of a three step catalytic reforming process and system in accordance with one or more embodiments of the present disclosure. Generally, embodiments may include dehydrogenation  200  of a naphtha feedstock  1  (such as a straight-run naphtha, as described above) to provide a first effluent stream  2 . Dehydrogenation of the naphtha feedstock  1  may include dehydrogenation of the naphthenes present in the naphtha feedstock  1  to produce aromatics. Thus, the first effluent stream  2  containing such aromatics may be subjected to aromatic extraction  210  to separate aromatics  3  out of the reformer and from a second effluent stream  4 . The second effluent stream  4  may comprise unreacted paraffins and naphthenes and is directed to cyclization reactors  220  (preferably dehydrocyclization reactors, converting paraffins into aromatics, for example) to produce a third effluent stream  5 . Third effluent stream  5  may optionally be recycled back through the system, and be combined with the first effluent stream  2 , to increase yield of higher end products. 
     Referring now to  FIG. 3 , a detailed schematic showing a three-step catalytic reforming process of the present disclosure, in which an aromatic extraction is integrated between an initial dehydrogenation and subsequent cyclization, is shown. 
     As illustrated, a heavy naphtha stream  302  is heated in a heat exchanger  45  and is then subjected to a further heat treatment in furnace  15 A before being directed to catalytic dehydrogenation reactor  10  (which optionally may include more than one reactor). In catalytic dehydrogenation reactor  10 , naphthenes contained in the heavy naphtha stream  302  may be converted to aromatics, at temperatures ranging, for example, from 400-450° C. The dehydrogenation reactor effluents, or first effluent stream  304 , are cooled in heat exchanger  45 . Thus, heat exchanger  45  is a feed/effluent exchanger in which the feed to the dehydrogenation reactor  10  is heated by the effluent from the dehydrogenation reactor  10 . After cooling, the first effluent stream  304  is directed to separator  50 , which separates the gas-liquid phases from each other. 
     Specifically, the first effluent stream  304  is separated in separator  50  for recovery of hydrogen stream  305  and a separator bottoms stream  306 . Recovered hydrogen stream  305  may be split, and a portion of the hydrogen  305  may be fed to compressor  35  and recycled back to the heavy naphtha feedstock  302 . However, as dehydrogenation produces substantial quantities of hydrogen gas, the remaining portion of the recycled hydrogen gas  305  may be sent to other refining unit operations, such as hydro-treating and hydrocracking. The separator bottoms stream  306  is sent to a stabilizer column  60  to separate and remove any excess hydrogen  310  from a liquid reformate stream  308 . 
     The reformate  308  is sent to an aromatic extraction unit  70  to obtain aromatics  312  as an extract and a second effluent stream  314  comprising paraffins and unreacted naphthenes as raffinate. The aromatics may be subsequently sent to an aromatic recovery complex to recover, for example, benzene, toluene, and xylene (collectively referred to as BTX). The raffinate from the aromatic extraction, i.e., second effluent stream  314  is sent to cyclization reactors,  20 ,  30 ,  40 . Based on the initial dehydrogenation and aromatic extraction, at least a majority of the raffinate may be paraffins. In particular embodiments, at least 95 wt % of the raffinate is constituted by paraffins. Specifically, the initial dehydrogenation may convert at least a substantial portion of the naphthenes present in the heavy naphtha feed into aromatics. Following aromatic extraction, the remaining raffinate has unreacted naphthenes and residual aromatics; however, such components may comprise less than 5 wt % of the raffinate. In one or more embodiments, the second effluent stream containing raffinate paraffins may comprise paraffins in amount ranging from 95 to 99 wt % and residual aromatics and unreacted naphthenes in amount ranging from 1 to 5 wt %. 
     Further, as shown, second effluent stream  314  may be heated by furnace  15 B prior to feeding into reactor  20  (and heated by furnaces  15 C,  15 D, as the stream feeds into reactors  30 ,  40 , respectively). While three cyclization reactors  20 ,  30 ,  40  are shown, it is understood that any number of reactors may be present. Further, it is also understood that in addition to cyclization reactions, such reactors  20 ,  30 ,  40  may also perform dehydrogenation (in combination with cyclization i.e., dehydrocyclization, as well as a sequential reaction) and/or isomerization to convert paraffins and unreacted naphthenes into isomers (i.e., n-paraffins to isoparaffins) and/or into aromatics. However, as mentioned above, based on the initial dehydrogenation and then aromatic extraction, the second effluent stream may be primarily paraffinic, as compared to the original naphtha feedstock. Whereas dehydrogenation reactor  10  is operated at temperatures ranging from 400-450° C., as described above, the cyclization reactors  20 ,  30 ,  40  may operate at a higher temperature than the dehydrogenation reactor  10 , such as at a temperature ranging from 480-520° C. Furnaces  15 C and  15 D may be used between cyclization reactors  20 ,  30 ,  40  to maintain the temperature of the stream. The number and conditions of cyclization reactors may depend on the feedstock composition, the extent of reactions, and the targeted product properties. Further, it is also understood that reactors  20 ,  30 ,  40 , may be operated in semi-regenerative configurations, cyclic configurations or continuous catalyst regeneration configurations. 
     In one or more embodiments, a third effluent stream  324  is produced from the cyclization reactors  20 ,  30 ,  40  and may then be combined with the first effluent stream  304  coming from the dehydrogenation reactor  10 . Thus, the combined stream may then be subjected the same separation scheme described above, including cool down in exchanger  45 , phase separation in separator  50 , stabilization in stabilizer  60 , and aromatic extraction in extraction unit  70 . 
     In addition to the operational temperatures mentioned above, the processing conditions of the different reformers allows for different operational control. Additional variables that may be controlled to alter the quality of the reformed product include the space velocities, the hydrogen to hydrocarbon feed ratios, and the pressures. 
     As mentioned above, the naphtha stream  302  is reformed in dehydrogenation reactor  10  to produce a first product effluent stream  304 . In one or more embodiments, the operating conditions for the dehydrogenation reactor  10  include a temperature in the range of from 350° C. to 460° C., and in particular embodiments a temperature ranging from about 400° C. to 450° C.; a pressure in the range of from 1 bar to 50 bars, and in certain embodiments from 1 bar to 20 bars; and a LHSV in the range of 0.1 h −1  to 40 h −1 , and in certain embodiments from 0.5 h −1  to 2 h −1 . In one or more embodiments, operating conditions for the dehydrogenation reactor may also include a hydrogen to hydrocarbon ratio ranging from 4 to 8. 
     In accordance with one or more embodiments of the present disclosure, the second effluent stream  314  comprises a fractioned raffinate separated from the aromatic extraction unit  70  that may be cyclized and aromatized via dehydrocyclization reactions in one or more of the cyclization reactors  20 ,  30 ,  40 , to produce third effluent stream  324 . In one or more embodiments, the operating conditions for the cyclization reactors  20 ,  30 ,  40  include a temperature in the range of from 450° C. to 550° C., and in particular embodiments a temperature ranging from about 480° C. to 520° C.; a pressure in the range of from 1 bar to 50 bars, and in certain embodiments from 1 bar to 20 bars; and an LHSV in the range of 0.1 h −1  to 40 h −1 , and in certain embodiments from 0.5 h −1  to 2 h −1 . In one or more embodiments, operating conditions for the dehydrogenation reactor may also include a hydrogen to hydrocarbon ratio ranging from 4 to 8. In one or more embodiments, two or more, or three or more cyclization reactors may be used, in series. 
     In one or more embodiments of the present disclosure, the dehydrogenation catalyst and the cyclization reformation catalyst used may be any suitable catalyst that is known to one of ordinary skill in the art. Such catalysts include mono-functional or bi-functional reforming catalysts which generally contain one or more active metal component of metals or metal compounds (such as oxides or sulfides) selected from the Groups 8-10 of the IUPAC Periodic Table. A bi-functional catalyst has both metal sites and acidic sites. In certain embodiments, the active metal component can include one or more noble metals, such as platinum, rhenium, gold, palladium, germanium, nickel, silver, tin, or iridium, or halides. The active metal component may be deposited or otherwise incorporated on a support, such as amorphous alumina, amorphous silica alumina, zeolites, or combinations thereof. In certain embodiments, platinum or platinum alloy supported on alumina or silica or silica-alumina are the reforming catalyst. Effective liquid hourly space velocity values (h −1 ), on a fresh feed basis relative to the hydrotreating catalysts, are in the range of from about may have a lower limit of any of 0.5, 1, or 1.5 h −1 , and an upper limit of any of 2, 3, or 4 h −1 , where any lower limit can be used in combination with any upper limit. In particular embodiments, the catalysts used in the naphthene dehydrogenation step may be a conventional reforming catalyst or noble metals (or Group VIIIB) on alumina, and they may be acidic or non-acidic. The catalysts in the cyclization steps may be conventional catalytic reforming catalysts and may include alumina based or zeolitic based catalysts containing noble metals. 
     EXAMPLES 
     The following examples are merely illustrative and should not be interpreted as limiting the scope of the present disclosure. An example was provided to illustrate the impact of the three stage catalytic reforming process described in one or more embodiments of the disclosure. The resulting properties of a dehydrogenated feedstock are given in Table 1, and the properties of the resulting dehydrogenated and dehydrocyclized reformate are provided in Table 2. 
     A heavy naphtha stream was processed over a conventional catalytic reforming catalysts at 460° C., 8 bars, hydrogen to hydrocarbon molar ratio of 7 and LHSV of 1 h −1 . Table 1 summarizes feedstock composition along with yield and composition of the dehydrogenated product. As shown, 83.7 wt % of naphthenes were converted to aromatics. 
     
       
         
           
               
               
               
               
               
             
               
                   
                 TABLE 1 
               
               
                   
                   
               
               
                   
                   
                   
                 Heavy Naphtha  
                 Dehydrogenated  
               
               
                   
                 Variables 
                   
                 Feedstock 
                 Product 
               
               
                   
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
               
            
               
                   
                 Time-on-stream 
                 h 
                   
                 7.0 
               
               
                   
                 RON 
                   
                   
                 86.43 
               
               
                   
                 n-Paraffins 
                 W % 
                 34.67 
                 10.66 
               
               
                   
                 iso-Paraffins 
                 W % 
                 28.15 
                 23.73 
               
               
                   
                 Olefins 
                 W % 
                 2.55 
                 0.00 
               
               
                   
                 Naphthenes 
                 W % 
                 19.20 
                 3.12 
               
               
                   
                 Aromatics 
                 W % 
                 13.07 
                 62.49 
               
               
                   
                 Unknown 
                 W % 
                 2.34 
                 0.00 
               
               
                   
                   
                   
                   
                 100.00 
               
               
                   
                 Molecular Weight 
                 g/mol 
                   
                 104.79 
               
               
                   
                 Specific Gravity 
                 g/mL 
                   
                 0.7902 
               
               
                   
                 C1 + C2 Yield 
                 W % 
                 0.00 
                 1.41 
               
               
                   
                 C3 + C4 Yield 
                 W % 
                 0.00 
                 5.55 
               
               
                   
                 C5 + Yield 
                 W % 
                 100.00 
                 89.09 
               
               
                   
                 Hydrogen Yield 
                 W % 
                 0.00 
                 3.31 
               
               
                   
                 Total 
                   
                   
                 99.36 
               
               
                   
                   
               
            
           
         
       
     
     Example 2: Paraffin Cyclization 
     The heavy naphtha stream in example 1 was processed and subjected to cyclization reactions over a conventional catalytic reforming catalysts at 520° C., 8 bars, and a hydrogen to hydrocarbon molar ratio of 7 with an LHSV of 1 h −1 . Table 2 summarizes yield and composition of the dehydrogenated and cyclized product. As estimated, 84.6 wt % of paraffins were converted to aromatics. 
     
       
         
           
               
               
               
             
               
                 TABLE 2 
               
               
                   
               
               
                   
                   
                 Dehydrogenated and  
               
               
                   
                   
                 Dehydrocyclized 
               
               
                 Variables 
                   
                 reformate 
               
               
                   
               
             
            
               
                   
               
            
           
           
               
               
               
            
               
                 Time-on-stream 
                 h 
                 36.0 
               
               
                 RON 
                   
                 106.49 
               
               
                 Compound Type 
                   
                   
               
               
                 n-Paraffins 
                 W % 
                 2.71 
               
               
                 iso-Paraffins 
                 W % 
                 6.93 
               
               
                 Olefins 
                 W % 
                 0.00 
               
               
                 Naphthenes 
                 W % 
                 0.49 
               
               
                 Aromatics 
                 W % 
                 89.87 
               
               
                 Unknown 
                 W % 
                   
               
               
                 Total 
                 W % 
                 100.00 
               
               
                 Liquid Properties 
                   
                   
               
               
                 Molecular Weight 
                 g/mol 
                 105.32 
               
               
                 Specific Gravity 
                 g/mL 
                 0.8450 
               
               
                 Yields 
                   
                   
               
               
                 C1 + C2 Yield 
                 W % 
                 2.3 
               
               
                 C3 + C4 Yield 
                 W % 
                 5.4 
               
               
                 C5 + Yield 
                 W % 
                 87.2 
               
               
                 Hydrogen Yield 
                 W % 
                 5.1 
               
               
                 Total 
                 W % 
                 100.0 
               
               
                   
               
            
           
         
       
     
     Thus, as evidenced in the tables above, the naphthenes in the naphtha feedstock may be primarily dehydrogenated to form aromatics. By extracting such aromatics prior to the cyclization reactions, the reaction kinetics of such downstream reactions may be improved. Additionally, the three staged catalytic reforming process may provide for less required heating of the effluent streams and reduced the reactor/catalyst volume requirements. 
     Although the preceding description has been described herein with reference to particular means, materials and embodiments, it is not intended to be limited to the particulars disclosed herein; rather, it extends to all functionally equivalent structures, methods and uses, such as are within the scope of the appended claims. In the claims, means-plus-function clauses are intended to cover the structures described herein as performing the recited function and not only structural equivalents, but also equivalent structures. Thus, although a nail and a screw may not be structural equivalents in that a nail employs a cylindrical surface to secure wooden parts together, whereas a screw employs a helical surface, in the environment of fastening wooden parts, a nail and a screw may be equivalent structures. It is the express intention of the applicant not to invoke 35 U.S.C. § 112(f) for any limitations of any of the claims herein, except for those in which the claim expressly uses the words ‘means for’ together with an associated function.