Patent Publication Number: US-2021189257-A1

Title: Process to prepare feed by using dividing wall column and/or conventional column for catalytic cracking unit targeting olefin production

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application claims the benefit of U.S. Provisional Application Ser. No. 62/950,714 filed Dec. 19, 2019, incorporated herein by reference in its entirety. 
    
    
     TECHNICAL FIELD 
     The present invention relates to systems using catalytic cracking units for olefins production, and more particularly relates to such systems where a light naphtha feed is produced within the system. 
     BACKGROUND 
     Fluid catalytic cracking (FCC) is an important and well-known commercial conversion process used in petroleum refineries. It is widely used to convert high-boiling, high molecular weight hydrocarbon fractions of petroleum crude oils into more valuable gasoline, olefins, and other products. Cracking of petroleum hydrocarbons is now primarily done by catalytic cracking because it produces more gasoline with a higher octane rating than thermal cracking. It also produces byproduct gases that have more carbon-carbon double bonds, that is, more olefins, and hence more economic value than those produced by thermal cracking. 
     The breaking of the large hydrocarbon molecules into smaller molecules is technically referred to by organic chemists as “scission” of the carbon-to-*carbon bonds. Some of the smaller alkanes are then broken and converted into even smaller alkenes and branched alkenes such as the gases ethylene (CH 2 ═CH 2  or C2=), propylene (CH 3 —CH═CH 2  or C3=), butylenes, and isobutylenes (collectively C4=). Those olefinic gases are valuable for use as petrochemical feedstocks. The propylene, butylene and isobutylene are also valuable feedstocks for certain petroleum refining processes that can convert them into high-octane gasoline blending components. They may also be used as valuable chemical building blocks for higher molecular products such as polymers. 
     It is always desirable to improve catalytic cracking systems by improving reliability, controlling fractionation quality inside the battery limit, steadily producing light olefins, improving fractionation efficiency, reducing utility requirements, reducing overall energy requirements, reducing CO 2 /NOx emissions, reducing equipment footprint requirements, and/or improving the value of the products. 
     SUMMARY 
     There is provided, in one non-limiting embodiment, a catalytic cracking system configured for olefins production where the system comprises a reactor/regenerator, a main fractionator, and a vapor recovery unit (VRU). The reactor/regenerator is adapted to receive a feed comprising gas oils and/or deasphalted oil (DAO) and/or atmospheric residue in a first riser and an aromatics-free C5/C6-rich light naphtha feed in a second riser, the reactor/regenerator generating a high-value product stream comprising light and heavy paraffinic, naphthenic, aromatic and olefinic hydrocarbons. The main fractionator is configured for receiving and fractionating the high-value product stream, where the main fractionator includes a fractionator vapor product stream comprising fuel gas, C3/C4, and light naphtha; a wild naphtha product stream; and fractionator liquid products stream comprising optionally heavy naphtha, light cycle oil (LCO), and slurry. The VRU includes a primary absorber, where the VRU receives the fractionator vapor and wild naphtha product streams and separates them to give an aromatics-free C5/C6-rich product stream, where the aromatics-free C5/C6-rich product stream is directed as the aromatics-free C5/C6-rich light naphtha feed back to the reactor/regenerator. The VRU further includes two embodiments. In a first non-limiting embodiment the VRU includes a combination of a debutanizer column and a naphtha splitter column, and a debutanizer bottoms product that is split between a feed to the naphtha splitter column and recycle to the primary absorber, and where the naphtha splitter column splits out the aromatics-free C5/C6-rich product stream. In a second non-limiting embodiment the VRU comprises a dividing wall column (DWC) comprising a pre-fractionator section and a main-fractionator section where the DWC comprises a side draw that is the aromatics-free C5/C6-rich product stream, and the DWC comprises a bottoms product that is split between a gasoline product and recycle to the primary absorber. 
     There is additionally provided method for producing olefins in a catalytic cracking system, where the method includes directing a feed to a reactor/regenerator, where the feed comprising at least one component selected from the group consisting of gas oils, deasphalted oil (DAO), atmospheric residue, and an aromatics-free C5/C6-rich light naphtha feed. The method further includes the reactor/regenerator generating a high-value product stream comprising light and heavy paraffinic hydrocarbons, naphthenic, aromatics and olefinic hydrocarbons. The method additionally includes directing the high-value product stream to a main fractionator where main fractionator fractionates the high-value product stream and generates a fractionator vapor product stream comprising fuel gas, C3/C4, and light naphtha, a wild naphtha product stream, and a fractionator liquid products stream comprising at least one component selected from the group consisting of heavy naphtha, light cycle oil (LCO), and slurry. The method also includes directing fractionator vapor and wild naphtha product streams to a vapor recovery unit (VRU) comprising a primary absorber where the VRU separates the fractionator vapor and wild naphtha product streams to give an aromatics-free C5/C6-rich product stream. The aromatics-free C5/C6-rich product stream is directed to the reactor/regenerator as an aromatics-free C5/C6-rich light naphtha feed recycle. The VRU further comprises units of either of the two embodiments described above. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a non-limiting, schematic illustration of a typical FCC and gas plant block flow diagram illustrating imported naphtha feed for olefins production as described herein; 
         FIG. 2  is a non-limiting, schematic illustration of a FCC and gas plant block flow diagram illustrating internal naphtha feed generation for olefins production using two columns as described herein; 
         FIG. 3  is a non-limiting, schematic illustration of a FCC and gas plant block flow diagram illustrating internal naphtha feed generation for olefins production using a dividing wall column (DWC) as described herein; and 
         FIG. 4  is a non-limiting, schematic illustration of a more detailed view of the naphtha stabilizer DWC of the  FIG. 3  system. 
     
    
    
     DETAILED DESCRIPTION 
     Existing FCC units can be revamped or a new grassroots FCC unit can be designed with improved technology, including but not necessarily limited to MAXOFIN™ technology available from KBR to produce light olefins such as ethylene and propylene from a light naphtha stream. MAXOFIN™ technology is a process that enables refiners to maximize propylene production by 20% or more with significantly less ethylene than traditional steam cracking, and it provides the flexibility to operate as a conventional FCC system to produce gasoline depending on changing market demands. 
     The KBR MAXOFIN™ technology employs dual risers. The first riser processes the conventional FCC feed-stocks (gas oils, DAO and atmospheric residue) and in the second riser light paraffinic, naphthenic, or olefinic hydrocarbons are cracked to achieve the desirable high yields of the light olefins. The reactor effluent from both the risers in the MAXOFIN™ unit is fractionated in a main fractionator column. 
     The light naphtha feed, suitable to produce the light olefins by cracking in the second riser is conventionally imported from the outside battery limit (OSBL). It has been discovered that the light naphtha feed can be produced internally in the MAXOFIN™ unit. 
     A FCC unit consists of three main sections, the reactor/regenerator  10 , the main fractionator  18 , and the unsaturated gas plant section (also known as the vapor recovery unit (VRU) or gas concentration unit (GCU)).  FIG. 1  shows all three sections. The VRU comprises wet gas compressor  24 , high pressure (HP) cooler/high pressure receiver  26 , primary absorber  58 , sponge absorber  66 , stripper  30 , debutanizer (stabilizer)  40 , depropanizer  44 , and C3=splitter  50 . 
     A typical MAXOFIN™ unit block flow diagram when the light naphtha stream  12  is imported as a second riser feed from the outside battery limit is shown in  FIG. 1  to the reactor/regenerator  10 . There is also a gas oils, DAO and resid feed  14  from OSBL to a first riser of the reactor/regenerator  10 . First riser and second riser are conventional components of the catalytic cracking unit 10 and are not separately shown in the Figures but are understood to be present. 
     FCC feed (heavy atmospheric gas oils, vacuum gas oils, DAO, and/or residue)  14  is cracked into high-value products  16  in the reactor/regenerator  10  and routed to the main fractionator section  18 . The liquid product  20  from the main fractionator  18  includes heavy naphtha, light cycle oil (LCO), and slurry. Only slurry is a bottoms product; the heavy naphtha and LCO are side products. 
     The overhead products  22  from the main fractionator  18  mainly contain fuel gas (C2- and inert), liquefied petroleum gas (LPG; C3/C4), and cat-cracked naphtha are further separated in the gas plant section (VRU). Wet gas  22  from the main fractionator  18  overhead flows to the wet gas compressor  24 ; this is usually a two-stage centrifugal machine. The vapors from the first stage discharge are partially condensed in an interstage cooler and flashed in an interstage drum. The vapor discharge from the second stage of the wet gas compressor  24  is combined with the liquid from the compressor interstage drum, the primary absorber bottoms liquid  62 , and the stripper overhead vapors  36 . This combined stream flows through the high pressure (HP) cooler into the high pressure (HP) receiver, collectively designated  26 . 
     Vapor  32  from the HP separator drum flows to the primary absorber column  58 . Hydrocarbon liquid (wild naphtha)  34  from the main fractionator  18  overhead drum is pumped to the primary absorber  58  as a lean oil. The debutanizer  40  bottoms liquid  56  is also pumped to the primary absorber  58  to increase the lean oil flow, and therefore the propylene recovery. The debutanizer bottoms liquid  56  enters the primary absorber  58  at a higher point than the wild naphtha  34 . LPG (C3/C4) 62 recuperated in the primary absorber  58  bottoms is directed to the HP cooler/HP receiver  26 . The primary absorber overhead gas  64  flows to the secondary or sponge absorber  66  to recover the LPG range material by absorption into a lean sponge oil  68 . Rich sponge oil  70  is routed to main fractionator  18 . The sponge absorber overhead  72  (C2-) flows to treating and is then routed to the refinery fuel gas system. 
     Liquid  28  from the HP separator drum  26  is pumped to the top of the stripper column  30 . The purpose of stripper column  30  is to achieve a C2 content specification in the feed  38  to the debutanizer (stabilizer)  40 . Stripped vapor  36  containing most of the C2- and some LPG leaves the top of the stripper column  30  and is returned to the HP cooler/HP receiver  26 . The stripper bottoms liquid  38  is preheated by heat exchange with the debutanizer bottoms liquid and fed to the debutanizer column  40 . 
     The debutanizer column  40  separates the feed into LPG  42  and cat-cracked naphtha (combined streams  56  and  60 ). The overhead product  42  from the debutanizer  40  is LPG (C3/C4). Bottoms liquid (combined streams  56  and  60 ) from the debutanizer  40  is cooled against the column feed and then by air and/or a water cooler (not shown). A portion of the cooled debutanizer bottoms liquid  56  is returned to the primary absorber  58  as a super lean oil and the remainder  60  is yielded as a cat-cracked naphtha gasoline blendstock. The LPG  42  from the overhead of the debutanizer after amine and mercaptan removal treatment flows to a depropanizer  44  where it is separated into C3 and C4 products. The C3 product  48  may be further separated into propylene (C3=)  54  product and propane (C3) product  52  in a propylene splitter  50  downstream of the depropanizer column  44 . The C4 bottoms product  46  from depropanizer  44  is sent to OSBL. 
     It has been discovered that the light naphtha stream  74  can be produced inside the battery limit as the second riser feed to reactor/regenerator  10 , which is a configuration that increases olefinic LPG yield.  FIG. 2  presents a nonlimiting, schematic illustration of a reactor/regenerator  10 , main fractionator  18  and gas plant. The system and method herein relate to an unsaturated gas plant section, also called a VRU or GCU. In one non-limiting embodiment the VRU comprises two conventional columns but in a unique series—namely the debutanizer  40  and a naphtha splitter  76 . The debutanizer  40  and a naphtha splitter  76  can be used to fractionate the stripper bottoms liquid  38  into three product streams: a LPG C3/C4 product is taken from the debutanizer overhead  78  and a stabilized naphtha stream from the debutanizer bottoms is partly recycled back to the primary absorber  58  as stream  80  and the balance  82  is further fractionated in the naphtha splitter  76  to produce an aromatics-free light naphtha (C5/C6) product  74  from the splitter overhead and the gasoline range material  84  from the splitter bottoms. The overhead product from the naphtha splitter  76  is an aromatics free C5/C6 rich stream  74  which is suitable to be recycled to the reactor/regenerator  10  as a second riser feed. The bottoms product  84  from the naphtha splitter  76  is routed for further treatment to make it suitable for the gasoline blending. In this non-limiting embodiment, a portion of the C4 bottoms product  85  from depropanizer  44  may be directed as an optional recycle stream  86  to the aromatics-free light naphtha (C5/C6) product  74  feed to reactor/regenerator  10 . Other units and streams common to the prior art configuration of  FIG. 1  have like reference numerals. 
     In a different non-limiting embodiment, the VRU comprises Dividing Wall Column (DWC) technology which can be used to further improve and simplify the process flow scheme (as shown schematically in  FIG. 3 ) for olefins productions by the catalytic cracking unit. Dividing Wall Column technology delivers the performance of two conventional columns in series in a single column with reduced energy and capital requirement. Dividing Wall Column technology can be used to design a single column to replace the two columns in series system as shown in the  FIG. 2  embodiment. As presented In  FIG. 3  and  FIG. 4  a Dividing Wall Column (DWC) 88 is used to fractionate the stripper bottoms liquid  38  from the stripper  30  into three products:
         A LPG C3/C4 product  90  is taken from the DWC overhead.   An aromatics-free light naphtha stream  96  taken from the main-fractionator side of the DWC 88 is concentrated with the C5/C6 components 96 and routed to the second riser of the reactor/regenerator  10  for olefins production.   The bottoms product from the DWC 88 is divided into naphtha recycle 92 back to the primary absorber  58 , where the balance  94  is routed for further treatment to make it suitable for the gasoline blending.
 
Again, the same units and streams have the same reference numbers as used in discussions of the previous Figures.
       

       FIG. 4  is a more detailed view of the naphtha stabilizer DWC 88 shown in the  FIG. 3  system. DWC 88 comprises a pre-fractionator section  98  and a main-fractionator section  99  divided by dividing wall 100. 
     It will be appreciated that the DWC can be a tray column, a packed column, or a combination of both. 
     It will also be appreciated that the systems and processes described herein will have a number of technical and commercial advantages. 
     For the non-limiting embodiment where two conventional columns are used for the debutanizer and naphtha splitter as schematically illustrated in  FIG. 2 , technical advantages include, but are not necessarily limited to:
         Improved reliability as compared to the light naphtha imported from outside the battery limit; and   Control on fractionation quality inside the battery limit.       

     Commercial advantages for this non-limiting embodiment include, but are not necessarily limited to steady production of the light olefins. 
     For the non-limiting embodiment where a DWC is used as schematically illustrated in  FIGS. 3 and 4 , technical advantages include, but are not necessarily limited to:
         Improvement on fractionation efficiency;   Improvement on primary absorber performance;   Reduced utility requirements;   Reduced overall energy requirements;   Reduced CO 2 /NOx emissions;   Opportunity to revamp existing column to get an additional product suitable for the light olefins production; and   Enhanced plant safety due to less hydrocarbon inventory.       

     Commercial advantages for the non-limiting embodiment where a DWC is used include, but are not necessarily limited to, the commercial advantage noted above for when two conventional columns are used, in addition to:
         20-30% less capital requirement as compared to the conventional column solution;   Improvement in fractionation economics;   Less plot space (footprint) requirement;   Offers advantages for plant upgrading/debottlenecking;   Overall improvement in the value of products; and   Alternative use of existing assets to improve the overall economics of the plant.       

     Stated another way, a non-limiting improvement in the  FIG. 2  embodiment over the  FIG. 1  conventional column system involves introducing a two-product naphtha splitter column downstream of a debutanizer column to produce an aromatics-free light naphtha as distillate which is recycled back for light olefins production and gasoline stream as a bottoms product. 
     Improvements in the non-limiting embodiment of  FIGS. 3 and 4  over the  FIG. 2  embodiment include using a DWC to match the separation requirement of a debutanizer column and a two-product conventional naphtha splitter column. A DWC is producing LPG as a top product, light naphtha as a side product and gasoline stream as a bottoms product. The application of DWC offers capital expenditure (CAPEX) benefits (less equipment count and plot space benefits) and operational expenditure (OPEX) benefits (lower energy requirement) as compared to the route shown in  FIG. 2 . 
     Furthermore, there are many noticeable differences between a fractionation approach such as Honeywell UOP U.S. Patent Application Publication 2008/0081937 A1 and the proposed system described herein including, but not necessarily limited to, those listed in Table I below: 
     
       
         
           
               
             
               
                 TABLE I 
               
             
            
               
                   
               
               
                 Differences Between Proposed System and ‘937 Application 
               
            
           
           
               
               
               
            
               
                   
                 ‘937 Application 
                 Proposed System 
               
               
                   
               
               
                 Feed to DWC 
                 Stabilized Naphtha 
                 Unstabilized Naphtha 
               
               
                   
                 (C5 to C9+ hydrocarbons) 
                 (C3 to C9+ hydrocarbons) 
               
               
                 DWC Operating 
                 5-15 psig (0.034-0.103 MPag) 
                 About 140 psig (0.965 
               
               
                 pressure 
                   
                 MPag) 
               
               
                 Top product 
                 C5-C6 range feed for light 
                 C3-C4 range 
               
               
                 (Distillate) 
                 olefins production 
                   
               
               
                   
                 A Total Boiling Point (TBP)  
                   
               
               
                   
                 at the 95% cut point in the 
                   
               
               
                   
                 range of about 162° F. to 
                   
               
               
                   
                 about 172° F. (about 72° C. to 
                   
               
               
                   
                 78° C.) 
                   
               
               
                 Middle product 
                 C7-C8 range 
                 Aromatics free C5-C6 
               
               
                   
                   
                 range feed for light olefins 
               
               
                   
                   
                 production 
               
               
                   
                   
                 Theoretical true final 
               
               
                   
                   
                 boiling point at about 
               
               
                   
                   
                 174° F. (79° C.) 
               
               
                 Bottoms product 
                 C8+ 
                 Gasoline (Benzene, C7+) 
               
               
                 The light olefins 
                 Routed to light fraction 
                 Recycled back to the 
               
               
                 reactor feed 
                 compound cracking zone 283 
                 equivalent of fluidized 
               
               
                   
                 and products (288 and 259) 
                 reactor zone “214” and 
               
               
                   
                 shown separately from light 
                 product from the second 
               
               
                   
                 fraction compound cracking 
                 riser is mixed with the 
               
               
                   
                 zone 283 
                 products of the first riser 
               
               
                   
                   
                 before they are routed to 
               
               
                   
                   
                 the main fractionator 
               
               
                   
               
            
           
         
       
     
     In the foregoing specification, the invention has been described with reference to specific embodiments thereof. However, the specification is to be regarded in an illustrative rather than a restrictive sense. For example, equipment, columns, DWCs, processes, reactants, olefins, products, reactors, regenerators, splitters, stabilizers, absorbers, compressors, coolers, and operating conditions falling within the claimed or disclosed parameters, but not specifically identified or tried in a particular example, are expected to be within the scope of this invention. 
     The present invention may be practiced in the absence of an element not disclosed. In addition, the present invention may suitably comprise, consist or consist essentially of the elements disclosed. For instance, there may be provided a catalytic cracking system configured for olefins production where the system consists essentially of or consists of a reactor/regenerator, a main fractionator, and a VRU, where the reactor/regenerator is adapted to receive a feed comprising gas oils and/or DAO and/or atmospheric residue and an aromatics-free C5/C6-rich light naphtha feed, the reactor/regenerator comprising a high-value product stream comprising light and heavy paraffinic hydrocarbons, naphthenic, aromatics and olefinic hydrocarbons; the main fractionator is configured to receive and fractionate the high-value product stream, where the main fractionator comprises a fractionator vapor product stream comprising fuel gas, C3/C4, and light naphtha, a wild naphtha stream, and a fractionator liquid products stream comprising heavy naphtha, LCO, and slurry; and the VRU comprises a primary absorber, the VRU receiving the fractionator vapor and wild naphtha product streams and separating them to give an aromatics-free C5/C6-rich product stream, where the aromatics-free C5/C6-rich product stream is directed as the aromatics-free C5/C6-rich light naphtha feed recycle to the reactor/regenerator; the VRU further comprises units selected from the group consisting of 1) a combination of a debutanizer column and a naphtha splitter column or 2) a DWC, where a debutanizer bottoms product is split between a feed to the naphtha splitter column and feed to the primary absorber, and the naphtha splitter column splits out the aromatics-free C5/C6-rich product stream; and where if the DWC is used, it comprises a pre-fractionator section and a main fractionator section where the DWC comprises a side draw that is the aromatics-free C5/C6-rich product stream and the DWC comprises a bottoms product feed to the primary absorber. 
     The words “comprising” and “comprises” as used throughout the claims, are to be interpreted to mean “including but not limited to” and “includes but not limited to”, respectively. 
     As used herein, the word “substantially” shall mean “being largely but not wholly that which is specified.” 
     As used herein, the singular forms “a,” “an,” and “the” are intended to include the plural forms as well, unless the context clearly indicates otherwise. 
     As used herein, the term “about” in reference to a given parameter is inclusive of the stated value and has the meaning dictated by the context (e.g., it includes the degree of error associated with measurement of the given parameter). 
     As used herein, the term “and/or” includes any and all combinations of one or more of the associated listed items.