Patent Publication Number: US-8524962-B2

Title: Use of olefin cracking to produce alkylate

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is a Division of U.S. application Ser. No. 11/781,497, filed Jul. 23, 2007, now allowed, which is incorporated herein by reference in its entirety. 
    
    
     FIELD OF THE INVENTION 
     This invention relates to an improved process combination for the conversion of hydrocarbons, and more specifically for the selective production of alkylate as intermediates for production of gasoline. 
     BACKGROUND OF THE INVENTION 
     Fuel quality demands and environmental concerns have led to the widespread removal of antiknock additives containing lead, and to the subsequent reformulation of gasoline. Because of the demands of modern internal-combustion engines, refiners have had to modify processes and install new processes to produce gasoline feedstocks that contribute to increasing the “octane,” or autoignition resistance. Premature autoignition causes the “knock” in internal combustion engines. Refiners have used a variety of processes to upgrade the gasoline feedstocks, including higher fluid catalytic cracking (FCC), isomerization of light naphtha, higher severity catalytic reforming, and the use of oxygenated compounds. Some of these processes produce higher octane gasoline feedstocks by increasing the aromatics content of the gasoline at the expense of reducing the content low-octane paraffins. Gasolines generally have aromatics contents of about 30% or more. 
     Faced with tightening automotive emission standards, refiners are having to supply reformulated gasoline to meet the stricter standards. Requirements for the reformulated gasoline include lower vapor pressure, lower final boiling point, increased oxygenate content, and lower content of olefins and aromatics. Aromatics, in particular benzene and toluene, have been the principal source of increasing the octane of gasoline with the removal of lead compounds, but now the aromatics content may eventually be reduced to less than 25% in major urban areas and to even lower ranges, such as less than 15%, in areas having severe pollution problems. 
     Alternate formulations for gasolines have been comprising aliphatic-rich compositions in order to maintain the octane ratings, as refiners have worked to reduce the aromatic and olefin content of gasolines. Currently, the processes for increasing the aliphatic content of gasolines include the isomerization of light naphtha, isomerization of paraffins, upgrading of cyclic naphthas, and increased blending of oxygenates. However, oxygenates are also becoming an issue as the use of methyl tertiary-butyl ether (MTBE) is being phased out, and ethanol has become the primary oxygenate for use with gasoline. 
     New technology, and processes can increase the production of alkylates for gasoline blending to reduce the aromatic content. Adding a complementary unit to process butenes to existing refinery process units provides a convenient upgrade, while improving the economic returns of a refinery with a minimal capital cost, and increases the flexibility of a refinery to shifting product demands. 
     SUMMARY OF THE INVENTION 
     The invention provides a process for increasing the amount of alkylate for use in gasoline blending. The process comprises recovering the butenes generated in an olefin cracking process and reacting the butenes with a C4+ effluent stream, comprising alkanes and alkenes, generated from a process for cracking higher molecular weight hydrocarbons. The process comprises combining an alkylation reactor with an olefin cracking process and adding the combination to a cracking process. The butenes are recovered from an olefin cracking process, while other heavier components are recycled for further cracking The operating conditions can be controlled to increase butene yields and the butenes are passed to an alkylation reactor, to react with the C4+ effluent stream from the cracking unit. The alkylation reactor generates an alkylate product stream comprising branched alkanes having from 5 to 12 carbon atoms, thereby producing a high quality product stream for gasoline blending. 
     Other objects, advantages and applications of the present invention will become apparent to those skilled in the art from the following detailed description. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a configuration for the process of the invention with a naphtha cracking unit; and 
         FIG. 2  is a configuration for the process of the invention with a fluid catalytic cracking unit. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Olefin cracking (OC) technology was developed to convert larger (C4+) olefins to ethylene and propylene. However, in the OC process, butenes are produced which can be separated out or recycled for further cracking The quantity of butenes produced in the olefin cracking process (OCP) can be has high as 40 wt % based on the total olefins fed to the OCP. Usually, butenes are recycled to be further cracked to produce ethylene. Olefin cracking technology was developed to work with other processes, such as a naphtha cracker, or a fluidized catalytic cracker (FCC), where heavier olefins, C4 to C8 olefins, were separated from the product stream and routed to the olefin cracker to increase ethylene and propylene production. 
     Likewise, naphtha crackers are designed for producing ethylene and propylene through cracking of larger gasoline range paraffinic and naphthenic molecules to generate an olefin stream rich in ethylene and propylene, and other by-products. The by-products include butenes, butanes, and butadienes. Usually, the butanes, butenes, and butadienes, or C4s, are either directed to OCP units for further cracking, or are separated and used for various polymers using the butene or butadiene monomers, or the production of methyl tertiary butyl ether (MTBE), or even as a fuel. Yet, the C4s are useful as a precursor and can be processed to produce an alkylate. The C4 stream is also lean in aromatics, so the processing will produce a high value alkylate product for use in gasoline blending. 
     With the demand for cleaner gasoline, the production of isoalkylates can produce a cleaner gasoline while reducing the aromatic content and maintaining the octane rating. In a refinery, gasoline tends to be a primary product, and can be increased from effluent streams resulting from cracking units that are designed to produce light olefins. For example, the OCP can be altered to increase, rather than decrease, the amount of C4 olefins. The C4 olefins comprise 1-butene, 2-butene and isobutene. By removing the butenes on each reactor pass of the process stream, and only recycling C5+ olefins to be cracked into ethylene and propylene, the production of alkylate from the C4 olefins can be increased. The production of alkylate can be used to produce high quality, low aromatic gasolines. 
     The process of the present invention can be integrated into existing cracking processes and is a method of producing alkylate from an olefin cracking process. The process stream from an olefin cracking process comprises butenes, which are separated out before recycling a C5+ rich stream for recycle to the olefin cracking process. The result is to increase the quantity of high value ethylene, propylene, and C4 olefins, while reducing the low value C5+ olefins. The butene stream is passed to an alkylation unit where the butenes are reacted with an alkane stream passed to the alkylation unit. The alkylation unit generates an alkylate product stream comprising branched alkanes having 5 to 12 carbon atoms. Because the feed streams to the alkylation unit comprise mostly butenes and butanes, the alkylate can comprise a product with greater than 30 mole % branched C8 alkanes, and preferably with a product stream of greater than 40 mole % branched C8 alkanes. Preferably, the alkane stream comprises alkanes having 3 to 8 carbon atoms, and preferably the C4+ alkanes are isoalkanes. 
     The reaction conditions of the alkylation unit include temperatures between 40° C. and about 120° C., pressures between 350 kPa (50 psia) and 4.2 MPa (600 psia), and a weight hourly space velocity (WHSV) between 0.1 hr −1  and 30 hr −1 . Preferably, the WHSV is between 1 hr −1  and 10 hr −1 . Catalysts for alkylation include liquid catalysts such as sulfuric acid and hydrofluoric acid and solid acids such as chlorided alumina, aluminosilicates and aluminophosphates. 
     This process and equipment can be inserted into a refinery operation as an addition to the olefin cracking process to provide flexibility to the product mix of the refinery. 
     In one embodiment, the process is added after the OCP with a naphtha cracking unit, as shown in  FIG. 1 . An olefin cracking process  10  generates a light olefin stream  12  comprising ethylene and propylene. The OCP  10  also generates butenes and C5+ hydrocarbons. Included in the OCP  10  is a separation unit for separating butenes from C5+ hydrocarbons. The butene stream  14  is passed to an alkylation reaction unit  20 . The alkylation unit  20  produces a high quality alkylate stream  22  from the butenes and butanes that are generated from the OCP  10 . A portion of the C5+ hydrocarbon stream  16  is recycled to the OCP. The ethylene and propylene are passed to a separation unit  30  to separate propylene  32 , ethylene  34 , and other light gases  36 . The OCP also generates a heavy hydrocarbon stream  18  that can be recycled to a naphtha cracking unit  40 , passed to gasoline blending, or other processing units in the petro-chemical plant. The naphtha cracking unit  40  receives as a feed  6 , a naphtha boiling point range feedstock, and recycled streams having constituents in the naphtha boiling point range. The yield of C4 olefins can be maximized in the OCP  10  by recycling the C5+ olefins within the OCP  10 . As the olefins are depleted from the recycle stream  16 , the heavy constituents are purged and recycled back to the naphtha cracking unit  40 . 
     In an alternate embodiment, stream  14  from the OCP is not separated from stream  16 , and stream  16  is passed to the alkylation unit  20 . Stream  16  comprises butenes and pentenes, and can be reacted with light iso-alkanes to form alkylates comprising C8s in the alkylate stream  22 . 
     The naphtha cracking unit  40  generates a light olefin stream  42  comprising ethylene and propylene which is passed to the light olefin separation unit  30 . In addition, the naphtha cracking unit  40  generates a by product known as pyrolysis gasoline (pygas). The pygas can be separated from the light olefins by a water quench stage. The pygas is a mixture of light hydrocarbons which is highly olefinic and includes butanes, butenes, other alkanes, olefins, diolefins, aromatics, such as benzene and toluene, and naphthenes. The pygas can be separated to generate a butane rich stream  44  comprising butanes, butenes, butadienes, and some amounts of larger alkanes and olefins, and a pygas stream  46  comprising the aromatics, naphthenes and larger alkanes and olefins. The butane rich stream  44  can comprise alkanes having from 3 to 8 carbon atoms, and preferably with isoalkanes and some olefins. 
     In an alternate embodiment of the present invention with the naphtha cracking unit  40 , the butane rich stream  44  is passed to a selective hydrogenation unit  50  to selectively hydrogenate butadienes and to isomerize butenes. The isomerization of butenes is to increase the 2-butene to 1-butene ratio. The butadienes are hydrogenated to butanes and butenes, and a hydrogenated butane rich stream  52  is passed to the alkylation reaction unit  20 , for reaction to produce the alkylate stream  22 . Depending on the amount of diolefins produced in the OCP  10 , the butene stream  14  can be partially, or entirely, passed through stream  24  to the selective hydrogenation unit  50  to hydrogenate diolefins. 
     The pygas stream  46  can be passed to a pygas selective hydrogenation unit  60  where the aromatics and the naphthenes are hydrogenated, thereby generating an intermediate stream  62  reduced in aromatics and naphthenes. The intermediate product stream  62  can be separated in a depentanizer  70  to recover a C5 stream  72  comprising pentanes and pentenes, and a recycle stream  74 . The C5 stream  72  is passed to the OCP  10  to generate more light olefins, i.e. ethylene and propylene. The light olefins are passed to a separation unit  30  for separation into product streams of ethylene  34  and propylene  32 . The OCP  10  also generates butenes which are passed in a butene stream  14  to the alkylation reactor  20 . The recycle stream  74  is passed to a secondary hydrogenation unit (not shown) for further hydrogenation of aromatics, and recycle to the naphtha cracking unit  40 . Alternately, stream  74  can be directed to recover aromatics, a valuable petrochemical byproduct for use in xylene production or alkyl-aromatics production. 
     In another embodiment, the process is added after an OCP  10  with a fluidized catalytic cracking (FCC) unit  80 , as shown in  FIG. 2 . The FCC  80  receives a gas oil feedstock  8  and generates a light olefin stream  82  which is passed to a separation unit  30  to generate an ethylene stream  34 , a propylene stream  32 , and other light gases  36 . The FCC unit also generates a C4 stream  84 , comprising butenes and butanes, and passes the C4 stream  84  to the alkylation unit  20 . In addition, the FCC unit  80  generates a C5+ hydrocarbon stream  86  comprising C5 and C6 olefins. The C5+ hydrocarbon stream  86  is passed to the OCP  10  where the olefins are cracked to produce light olefin product  12 , comprising ethylene and propylene; a butene stream  14 , comprising butenes and butanes; a recycle stream  19 , comprising C5s and C6s where a portion of the stream  19  can be recycled to the OCP, a portion of the stream  19  can be recycled to the FCC unit  80 , and a portion of the stream  19  is purged and used for gasoline blending; and a heavy hydrocarbon stream  18  that can be recycled to a FCC unit  80 , or other processing units in the petro-chemical plant. The FCC unit  80  also generates a C7+ stream  88  that can be passed to other processing units. 
     In an alternate embodiment, the C4 stream  84  can be passed to a selective hydrogenation unit  50  to selectively hydrogenate butadienes and to isomerize butenes to increase the 2-butene to 1-butene ratio. The butadienes are hydrogenated to butanes and butenes, and a hydrogenated butane rich stream  52  is passed to the alkylation reaction unit  20 , for reaction to produce the alkylate stream  22 . Depending on the amount of diolefins produced in the OCP  10 , the butene stream  14  can be partially, or entirely, passed through stream  24  to the selective hydrogenation unit  50  to hydrogenate diolefins and to isomerizes butenes. 
     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.