Patent Publication Number: US-9896627-B2

Title: Processes and systems for fluidized catalytic cracking

Description:
BACKGROUND 
     Field 
     The present disclosure generally relates to processes and systems for chemical cracking of hydrocarbons, and more specifically, to processes and systems for fluidized catalytic cracking of hydrocarbons incorporating series-reactor fluidized catalytic cracking units. 
     Technical Background 
     Crude oils are refined to produce transportation fuels and petrochemical feedstocks. Typically fuels for transportation are produced by processing and blending of distilled fractions from the crude to meet the particular end use specifications. After initial atmospheric and/or vacuum distillation, fractions are converted into products by various catalytic and non-catalytic processes. Catalytic processes are generally categorized based on the presence or absence of reaction hydrogen. Processes including hydrogen, often broadly referred to as hydroprocessing, include, for example, hydrotreating primarily for desulfurization and denitrification, and hydrocracking for conversion of heavier compounds into lighter compounds more suitable for certain product specifications. Catalytic conversion of hydrocarbons without the addition of hydrogen is another type of process for certain fractions. The most widely used processes of this type are commonly referred to as fluidized catalytic cracking (FCC) processes. A feedstock is introduced to the conversion zone typically operating in the range of from about 480° C. to about 550° C. with a circulating catalyst stream. This mode has the advantage of being performed at relatively low pressure, i.e., 50 psig or less. 
     In FCC processes, the feed is catalytically cracked over a fluidized catalyst bed. The main product from such processes has conventionally been gasoline, although other products are also produced in smaller quantities via FCC processes such as liquid petroleum gas and cracked gas oil. Coke deposited on the catalyst is burned off in a regeneration zone at relatively high temperatures in the presence of air before being recycled back to the reaction zone. 
     BRIEF SUMMARY 
     In accordance with one embodiment of the present disclosure, a light fuel fraction and a heavy fuel fraction may be cracked by fluidized catalytic cracking. The cracking process may comprise feeding the light fuel fraction and a catalyst from a catalyst regenerator into a first reactor, and cracking the light fuel fraction in the first reactor to produce an at least partially cracked light fuel fraction. The first reactor may be a fluidized bed reactor. The process may further comprise transporting the at least partially cracked light fuel fraction and the catalyst from the first reactor to a second reactor, feeding the heavy fuel fraction into the second reactor, and cracking the heavy fuel fraction and the at least partially cracked light fuel fraction in the second reactor to produce at least a product fuel and a spent catalyst. The second reactor may be a fluidized bed reactor. The process may further comprise transporting the spent catalyst to the catalyst regenerator and regenerating the catalyst in the catalyst regenerator. 
     In accordance with another embodiment of the present disclosure, a system for cracking by fluidized catalytic cracking may comprise a first reactor, a second reactor, and a catalyst regenerator. The first reactor may be a fluidized bed reactor and may comprise a catalyst inlet and a light fuel fraction inlet. The second reactor may be a fluidized bed reactor and may be in fluidic communication with the first reactor and may comprise a heavy fuel fraction inlet. The catalyst regenerator may be in fluidic communication with the catalyst inlet of the first reactor. A catalyst may circulate from the catalyst regenerator to the first reactor to the second reactor and back to the catalyst regenerator. A light fuel fraction may be disposed in the first reactor and may react with the catalyst and be transported to the second reactor. A heavy fuel fraction may be disposed in the second reactor and may react with the catalyst. 
     Additional features and advantages of the technology disclosed herein will be set forth in the detailed description which follows, and in part will be readily apparent to those skilled in the art from that description or recognized by practicing the technology as described herein, including the detailed description which follows, the claims, as well as the appended drawings. 
     It is to be understood that both the foregoing general description and the following detailed description present embodiments of the technology, and are intended to provide an overview or framework for understanding the nature and character of the technology as it is claimed. The accompanying drawings are included to provide a further understanding of the technology, and are incorporated into and constitute a part of this specification. The drawings illustrate various embodiments and together with the description serve to explain the principles and operations of the technology. Additionally, the drawings and descriptions are meant to be merely illustrative, and are not intended to limit the scope of the claims in any manner. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The following detailed description of specific embodiments of the present disclosure can be best understood when read in conjunction with the following drawings, where like structure is indicated with like reference numerals and in which: 
         FIG. 1  is a generalized diagram of a series-downer FCC reactor apparatus, according to one or more embodiments described herein; and 
         FIG. 2  is a generalized diagram of a series-riser FCC reactor apparatus, according to one or more embodiments described herein. 
     
    
    
     For the purpose of this simplified schematic illustration and description, 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 certain refinery operations are not included. Further, accompanying components that are in conventional refinery operations including FCC processes such as, for example, air supplies, catalyst hoppers, and flue gas handling are not shown. 
     It should further be noted that arrows in the drawings refer to pipes, conduits, channels, or other physical transfer lines that connect by fluidic communication one or more system apparatuses to one or more other system apparatuses. Additionally, arrows that connect to system apparatuses define inlets and outlets in each given system apparatus. 
     DETAILED DESCRIPTION 
     Reference will now be made in greater detail to various embodiments, some embodiments of which are illustrated in the accompanying drawings. Whenever possible, the same reference numerals will be used throughout the drawings to refer to the same or similar parts. Generally, disclosed herein are various embodiments of systems and methods for cracking a light fuel fraction and a heavy fuel fraction in an integrated FCC unit. Generally, the FCC unit includes a first reactor and a second reactor arranged in series, where the first reactor and second reactor are fluidized bed reactors. Catalyst and the light fuel fraction are fed into the first reactor, and the light fuel fraction is at least partially cracked. The at least partially cracked light fuel fraction mixed with the catalyst from the first reactor is transported to the second reactor. The heavy fuel fraction is additionally fed into the second reactor. Optionally, additional fresh catalyst may be fed into the second reactor. In the second reactor, the partially cracked light fuel fraction and the heavy fuel fraction are cracked to form a desired product. The spent catalyst is separated from the product, is regenerated, and is again fed into the first reactor and optionally the second reactor. 
     As used herein, “fuel” may include: a solid carbonaceous composition such as coal, coal derived liquids, tars, oil shales, oil sands, tar sand, biomass, wax, coke, or the like; a liquid carbonaceous composition such as gasoline, oil, petroleum, diesel, jet fuel, ethanol, or the like; and a gaseous composition such as syngas, carbon monoxide, hydrogen, methane, gaseous hydrocarbon gases (C 1 -C 6 ), hydrocarbon vapors, or the like. 
     As used herein, the “heavy fuel fraction” may be any fuel that is heavier than the “light fuel fraction.” As used herein, a fuel is heavier than another fuel if it, on average, has a higher boiling point than another fuel, and a fuel is lighter than another fuel if, on average, it has a lower boiling point than another fuel. In one embodiment, the light fuel fraction may comprise or consist essentially of straight or cracked naphthas boiling from about 36° C. to about 230° C., distillate oils boiling from about 10° C. to about 400° C., or combinations thereof. In one embodiment, the heavy fuel fraction may comprise or consist essentially of vacuum distillates, such as vacuum gas oil (VGO), boiling from about 370° C. to about 565° C., hydrotreated residues such as atmospheric distillation residues or vacuum distillation residues, visbreaking or distillation residues boiling above about 520 C, or combinations thereof. The heavy fuel fraction and light fuel fraction may come from an external supply or may be delivered from a common distillation column. For example, the bottoms fraction from a distillation column may serve as the heavy fuel fraction to the FCC unit, alone or in combination with an additional feed. A higher fraction from a distillation column may serve as the light fuel fraction to the FCC unit, alone or in combination with an additional feed. 
     As used herein, the term “downer” refers to a reactor, such as a fluidized bed reactor, where the reactant flows in a generally downward direction such as, for example, entering the top and exiting the bottom of the reactor. Downers may be utilized in embodiments of down-flow FCC reactor apparatuses described herein. Likewise, the term “riser” refers to a reactor, such as a fluidized bed reactor, where the reactant flows in a generally upward direction such as, for example, entering the bottom and exiting the top of the reactor. Downers may be utilized in embodiments of up-flow FCC reactor apparatuses described herein. 
     As used herein, “spent catalyst” refers to catalyst which has undergone reaction with fuel and is at least partially coked. Also, as used herein, “regenerated catalyst” refers to catalyst that is exiting the catalyst regenerator and is at least partially or substantially free of coke, and “fresh catalyst” refers to catalyst that is newly entering the system and is at least partially or substantially free of coke. 
     Embodiments of series-reactor FCC units and processes and methods incorporating the series-reactor FCC units will now be described. In exemplary embodiments, the series-reactor FCC unit may be a series-downer FCC unit, described below with reference to  FIG. 1 , or a series-riser FCC unit, described below with reference to  FIG. 2 . Both the series-downer FCC unit and the series-riser FCC unit may include two FCC reactors configured in series, such that a light fuel fraction is resident in a first reactor and is at least partially cracked before being transferred to a second reactor. In the second reactor, the at least partially cracked light fuel fraction may be further cracked together with a heavy fuel fraction. 
     Referring to the process-flow diagram of  FIG. 1 , a series-downer FCC unit  130  may include two downer reactors. The series-downer FCC unit  130  may be used in the processes described herein. The series-downer FCC unit  130  includes a reactor/separator unit  111  comprising a first reactor  113 , a second reactor  135 , and a separation zone  115 . The series-downer FCC unit  130  also includes a catalyst regenerator  117  for regenerating spent catalyst. Catalyst may generally circulate through the catalyst regenerator  117 , into the first reactor  113 , into the second reactor  135 , and back into the catalyst regenerator  117 . 
     During operation of the series-downer FCC unit  130 , a light fuel fraction is introduced as a feed into the first reactor  113  through a transfer line  119 . In some embodiments, the light fuel fraction may be introduced into the first reactor  113  with steam or other suitable gas for atomization of the feed. A quantity of heated fresh or hot regenerated solid cracking catalyst particles from the catalyst regenerator  117  may also be transferred to a withdrawal well or hopper (not shown) at the top of the first reactor  113 . Fresh catalyst may be heated from an energy source and the regenerated catalyst may be heated by oxidation reactions to remove coke. The quantity of catalyst may be sufficient to crack the light fuel fraction to a desired product. The catalyst particles may be transferred, for example, through a downwardly directed transfer line  121  such as a conduit or pipe, commonly referred to as a transfer line or standpipe. Hot catalyst flow may be allowed to stabilize to ensure the hot catalyst is uniformly directed into a mixing zone or a feed injection portion of the first reactor  113 . In some embodiments, transfer line  121  and/or transfer line  119  are oriented relative to the first reactor  113  to introduce the catalyst and light fuel fraction, respectively, into the upper portion or top of the first reactor  113 . 
     The light fuel fraction may be injected into a mixing zone of the first reactor  113 . For example, the light fuel fraction may enter the first reactor  113  through feed injection nozzles. In some embodiments, the feed injection nozzles may be situated proximate to where the regenerated catalyst particles are introduced into the first reactor  113 . In some embodiments, for example, multiple injection nozzles may be used to aid thorough and uniform mixing of the light fuel fraction and the catalyst. When the light fuel fraction contacts hot catalyst in the first reactor  113 , cracking reactions begin to occur. The reaction vapor of hydrocarbon cracked products, unreacted feed, and catalyst mixture quickly flow through the remainder of the first reactor  113  and into the second reactor  135 . In some embodiments, the second reactor  135  may be a downer. In some embodiments, both the first reactor  113  and the second reactor  135  may be downers. 
     In some embodiments, the first reactor  113  and the second reactor  135  are configured in series, whereby a first reaction is conducted in the first reactor  113 , at least a portion of the products from the first reaction are transferred to the second reactor  135 , and a second reaction occurs in the second reactor  135 . In some embodiments, configuration in series may include the first reactor  113  and the second reactor  135  being adjacent to one another, with a suitable fluidic connection allowing for fluidic communication between the first reactor  113  and the second reactor  135 , such as an outlet in the first reactor  113  that leads directly into an inlet in the second reactor  135 . In such embodiments, the first reactor  113  and the second reactor  135  may be in physical contact with each other, but need not necessarily be in physical contact with each other. For example, the first reactor  113  and the second reactor  135  may be divided by a partition in a tank, drum, vessel, or other like reactor. In other embodiments, configuration in series may include a connection line or conduit from the first reactor  113  to the second reactor  135 . In such embodiments, the first reactor  113  may be physically isolated the second reactor  135 . 
     During operation of the series-downer FCC unit  130 , the heavy fuel fraction may be injected via a transfer line  120  into the second reactor  135 . In some embodiments, the heavy fuel fraction may be mixed with steam or another suitable gas for atomization of the feed when the heavy fuel fraction is injected into the second reactor  135 . The heavy fuel fraction may be injected into the second reactor  135  by any suitable means such as through feed injection nozzles, for example. The second reactor  135  may include a mixing zone, into which the heavy fuel fraction is injected. In one embodiment, the second reactor  135  may receive additional fresh catalyst through a transfer line  122  having an inlet to the second reactor  135  near where the products of the first reactor  113  are introduced into the second reactor  135 . Separate injections of the fresh catalyst from transfer line  121 , of the at least partially cracked light fuel fraction product mixed with catalyst of the first reactor  113 , and of the heavy fuel fraction from the transfer line  120  may facilitate thorough and uniform mixing of the reaction components in the second reactor  135 . As used herein, “at least partially cracked light fuel fraction” refers to a light fuel fraction for which at least some cracking has occurred (for example, in the first reactor  113 ), but for which cracking not necessarily occurred to a desired final amount. In some embodiments, further cracking of the light fuel fraction takes place in the second reactor  135 . Once the heavy fuel fraction contacts the catalyst in the second reactor  135 , cracking reactions occur in one or both of the heavy fuel fraction and the at least partially cracked light fuel fraction. In some embodiments, transfer line  122  and/or transfer line  120  are oriented relative to the second reactor  135  to introduce the catalyst and heavy fuel fraction, respectively, into the upper portion or top of the first reactor  113 . 
     As cracking occurs in the second reactor  135 , the reaction vapors of hydrocarbon cracked products, of unreacted feed, and of catalyst mixture quickly flow through the remainder of the second reactor  135  and into a rapid separation zone  115  at a bottom portion of reactor/separator unit  111 . Cracked and uncracked hydrocarbons may be directed through a conduit or pipe  123  to a conventional product recovery section known in the art. 
     If necessary for temperature control, a quench injection may be provided near the bottom of the second reactor  135  immediately before the separation zone  115 . This quench injection quickly reduces or stops the cracking reactions and can be utilized for controlling cracking severity, for example, to increase process flexibility. 
     The reaction temperature in the first reactor  113 , i.e., the outlet temperature of the first reactor  113 , may be controlled by opening and closing a catalyst slide valve (not shown) that controls a flow of regenerated catalyst from the catalyst regenerator  117  into the top of first reactor  113 . In embodiments in which fresh catalyst is injected into the second reactor  135  through the transfer line  122 , the reaction temperature of the second reactor  135  may also be controlled by the flow rate of catalyst into the second reactor  135 . At least a portion of the heat required for the endothermic cracking reaction may be supplied by the regenerated catalyst which has acquired heat in the regeneration process in the catalyst regenerator  117 . By changing the flow rate of the hot regenerated catalyst, the operating severity or cracking conditions can be controlled in the first reactor  113  and/or second reactor  135  to produce the desired yields of fuel products such as, for example, light olefinic hydrocarbons and gasoline as products of the first reactor  113 , the second reactor  135 , or both. 
     The series-downer FCC unit  130 , for example in the reactor/separator unit  111 , may include a stripper  131  for separating fuel from spent catalyst. After passing through the stripper  131 , the spent catalyst may be transferred to the catalyst regenerator  117 . The catalyst from separation zone  115  flows to the lower section of the stripper  131  that includes a catalyst stripping section into which a suitable stripping gas, such as steam, is introduced through transfer line  133 . The stripper  131  may include several baffles or structured packing (not shown), over which the downwardly flowing spent catalyst passes counter-currently to the flowing stripping gas. The upwardly flowing stripping gas, which is typically steam, is used to “strip” or remove any additional hydrocarbons that remain in the catalyst pores or between catalyst particles. 
     In the series-downer FCC unit  130 , the stripped or spent catalyst may be transported through transfer line  125 , for example, by lift forces from combustion air supplied through transfer line  127  and into the bottom portion of the catalyst regenerator  117 . This spent catalyst, which can also be contacted with additional combustion air, undergoes controlled combustion, through which any accumulated coke on the spent catalyst is burned off. Flue gases are removed from the catalyst regenerator  117  via conduit  129 . In the catalyst regenerator  117 , the heat produced from the combustion of the by-product coke may be transferred to the first reactor  113  and optionally the second reactor  135  through the catalyst in transfer line  121  and transfer line  122 , respectively. Thereby, at least a portion of the thermal energy required for the endothermic cracking reaction in the first reactor  113  and/or the second reactor  135  may be provided from heat produced during catalyst regeneration in the catalyst regenerator  117 . 
     Important properties of series-downer reactors (i.e., downers) in general include introduction of feed at the top of the reactor with downward flow, shorter residence time as compared to up-flow reactors (i.e., risers), and high catalyst to fuel ratio, e.g., in the range of from about 20:1 to about 30:1. 
     In general, the operating conditions for the first reactor  113  and/or second reactor  135  of a suitable series-downer FCC unit  130  include: a reaction temperature of from about 550° C. to about 700° C., in certain embodiments about 580° C. to about 630° C., and in further embodiments about 590° C. to about 620° C.; reaction pressure of from about 1 kg/cm 2  to about 20 kg/cm 2 , in certain embodiments about 1 kg/cm 2  to about 10 kg/cm 2 , in further embodiments about 1 kg/cm 2  to about 3 kg/cm 2 ; contact time (in the reactor) of from about 0.1 seconds to about 30 seconds, in certain embodiments about 0.1 seconds to about 10 seconds, and in further embodiments about 0.2 seconds to about 0.7 seconds; and a catalyst-to-feed ratio of from about 1:1 to about 60:1, in certain embodiments about 1:1 to about 30:1, and in further embodiments about 10:1 to about 30:1. 
     Referring to the generalized process flow diagram of  FIG. 2 , a series-riser FCC unit  230  and may be used in the systems and processes according to the present disclosure may include two riser reactors. The two risers may include a first reactor  233  and a second reactor  219  in series. The series-riser FCC unit  230  includes a reactor/separator  211  having the first reactor  233 , the second reactor  219 , a reaction zone  213 , and a separation zone  215 . The series-riser FCC unit  230  also includes a catalyst regenerator  217  for regenerating spent catalyst. 
     A light fuel fraction may be conveyed as a feed to the first reactor  233  via a transfer line  223 . In some embodiments, the light fuel fraction may be accompanied in the transfer line  223  by steam or other suitable gas for atomization of the feed. Atomization of the feed may facilitate admixture and intimate contact with a quantity of heated fresh or regenerated solid cracking catalyst particles sufficient for desired cracking of the light fuel fraction in the first reactor  233 . The catalyst particles may be conveyed to the first reactor  233  via transfer line  221  from the catalyst regenerator  217 . The light fuel fraction and the cracking catalyst are contacted under conditions to form a suspension that is introduced into the first reactor  233 . 
     In a continuous process using the series-riser FCC unit  230 , the mixture of cracking catalyst and light fuel fraction proceed upward through the first reactor  233 . In the first reactor  233  the hot cracking catalyst particles catalytically crack hydrocarbon molecules by carbon—carbon bond cleavage. The reaction vapor of hydrocarbon cracked products, unreacted feed, and catalyst mixture quickly flows through the remainder of the first reactor  233  and into the second reactor  219 . In some embodiments, configuration in series may include the first reactor  233  and the second reactor  219  being adjacent to one another, with a suitable fluidic connection allowing for fluidic communication between the first reactor  233  and the second reactor  219 , such as an outlet in the first reactor  233  that leads directly into an inlet in the second reactor  219 . In such embodiments, the first reactor  233  and the second reactor  219  may be in physical contact with each other, but need not necessarily be in physical contact with each other. For example, the first reactor  233  and the second reactor  219  may be divided by a partition in a tank, drum, vessel, or other like reactor. In other embodiments, configuration in series may include a connection line or conduit from the first reactor  233  to the second reactor  219 . In such embodiments, the first reactor  233  may be physically isolated the second reactor  219 . 
     In the continuous process using the series-riser FCC unit  230 , the heavy fuel fraction is injected as a feed into the second reactor  219  through transfer line  235 . In certain embodiments, the heavy fuel fraction may be injected using steam or another suitable gas for atomization of the feed. In one embodiment, the second reactor  219  may receive additional fresh catalyst through a transfer line  237  having an inlet to the second reactor  219  near where the products of the first reactor  233  are introduced into the second reactor  219 . The at least partially cracked light fuel fraction from the first reactor  233 , itself mixed with catalyst involved in the reaction that occurred in the first reactor  233 , mixes thoroughly and uniformly in the second reactor  219  with the heavy fuel fraction from the transfer line  237 . Once the heavy fuel fraction contacts the catalyst in the second reactor  219 , cracking reactions occur. Additionally, the at least partially cracked light fuel fraction (the product of the first reactor  233 ) may be further cracked in the second reactor  219 . The reaction vapor of hydrocarbon cracked products, unreacted feed, and catalyst mixture quickly flows through the remainder of the second reactor  219 . As the reaction proceeds, the reacting components are moved upward through the riser. 
     During the reactions in the first reactor  233 , the second reactor  219 , and the reaction zone  213 , as is conventional in FCC operations, the cracking catalysts may become coked. In coked catalysts, access to active catalytic sites is limited or nonexistent. Reaction products from the series-riser FCC unit  230  may be separated from the coked catalyst using any suitable configuration known in FCC units, generally referred to as the separation zone  215  in series-riser FCC unit  230 , for instance, located at the top of the reactor/separator  211  above the reaction zone  213 . The separation zone  215  can include any suitable apparatus known to those of ordinary skill in the art such as, for example, cyclones. The reaction product may be withdrawn through transfer line  225 . 
     Catalyst particles containing coke deposits from fluid cracking of the hydrocarbon feedstock pass from the reaction zone  213  and/or separation zone  215  through a transfer line  227  to the catalyst regenerator  217 . In the catalyst regenerator  217 , the coked catalyst contacts a stream of oxygen-containing gas, e.g., pure oxygen or air, which enters the catalyst regenerator  217  via a transfer line  229 . The catalyst regenerator  217  may be operated in a configuration and under conditions that are known in typical FCC operations. For instance, catalyst regenerator  217  can operate as a fluidized bed to produce regeneration off-gas comprising combustion products that is discharged through a transfer line  231 . The hot regenerated catalyst may be transferred from catalyst regenerator  217  through transfer line  221  and optionally through transfer line  237  to the bottom portion of the first reactor  233  and bottom portion of the second reactor  219 , respectively, for admixture with the hydrocarbon feedstock (i.e., the light fuel fraction or the heavy fuel fraction) as noted above. 
     In general, the operating conditions for the first reactor  233  and/or second reactor  219  of a suitable series-riser FCC unit  230  include: reaction temperature of from about 480° C. to about 700° C., in certain embodiments about 500° C. to about 620° C., and in further embodiments about 500° C. to about 600° C.; reaction pressure of from about 1 kg/cm 2  to about 20 kg/cm 2 , in certain embodiments about 1 kg/cm 2  to about 10 kg/cm 2 , in further embodiments about 1 kg/cm 2  to about 3 kg/cm 2 ; contact time (in the reactor) of from about 0.1 seconds to about 10 seconds, in certain embodiments about 1 second to about 5 seconds, and in further embodiments about 1 second to about 2 seconds; and a catalyst to feed ratio of from about 1:1 to about 60:1, in certain embodiments about 1:1 to about 10:1, and in further embodiments about 8:1 to about 20:1. 
     A catalyst that is suitable for the particular charge and the desired product may be conveyed to the fluidized catalytic cracking reactor or reactors. In certain embodiments, to promote formation of olefins and minimize olefin-consuming reactions, such as hydrogen-transfer reactions, an FCC catalyst mixture is used in the FCC unit, including an FCC base cracking catalyst and an FCC catalyst additive. 
     In particular, a matrix of an FCC base cracking catalyst may include natural or synthetic zeolites including one or more Y-zeolite, clays such as kaolin, montmorilonite, halloysite and bentonite, and/or one or more inorganic porous oxides such as alumina, silica, boria, chromia, magnesia, zirconia, titania and silica-alumina. A suitable FCC base cracking catalyst may have a bulk density of 0.5 g/mL to 1.0 g/mL, an average particle diameter of 50 μm to 90 μm, a surface area of 50 m 2 /g to 350 m 2 /g and a pore volume of 0.05 mL/g to 0.5 mL/g. 
     A suitable FCC catalyst mixture may contain, in addition to an FCC base cracking catalyst, an FCC catalyst additive containing a shape-selective zeolite. The shape selective zeolite referred to herein means a zeolite having a pore diameter is smaller than that of Y-type zeolite, so that hydrocarbons with only limited shape can enter the zeolite through its pores. Suitable shape-selective zeolite components include ZSM-5 zeolite, zeoliteomega, SAPO-5 zeolite, SAPO-11 zeolite, SAPO34 zeolite, and pentasil-type aluminosilicates, for example. The content of the shape-selective zeolite in the FCC catalyst additive is generally in the range of from about 20 wt. % to 70 wt. %, and in certain embodiments from about 30 wt. % to 60 wt. %. 
     A suitable FCC catalyst additive may have a bulk density of 0.5 g/mL to 1.0 g/mL, an average particle diameter of 50 μm to 90 μm, a surface area of 10 m 2 /g to 200 m 2 /g, and a pore volume of 0.01 mL/g to 0.3 mL/g. 
     In some embodiments, the FCC catalyst mixture may contain from 60 wt. % to 95 wt. % FCC base cracking catalyst, based on the total weight of the FCC catalyst mixture. The FCC catalyst mixture may contain from 5 wt. % to 40 wt. % FCC catalyst additive, based on the total weight of the FCC catalyst mixture. If the weight fraction of the FCC base cracking catalyst in the FCC catalyst mixture is lower than 60 wt. %, or if the weight fraction additive in the FCC catalyst mixture is higher than 40 wt. %, the yield of light-fraction olefin may not be optimal, because of low conversions of the feed fuels (i.e., the heavy fuel fraction and/or light fuel fraction). If the weight fraction of the FCC base cracking catalyst in the FCC catalyst mixture is higher than 95 wt. %, or if the weight fraction of the FCC catalyst additive in the FCC catalyst mixture is lower than 5 wt. %, the yield of light-fraction olefin may not be optimal, despite high conversion of the feed fuels. 
     In processes incorporating series-reactor FCC units according to embodiments herein, such as the series-downer FCC unit  130  of  FIG. 1  or the series-riser FCC unit  230  of  FIG. 2 , for example, a total residence time of the light fuel fraction includes a first average reaction time in the first reactor  113 ,  233  and a second average reaction time in the second reactor  135 ,  219 . On the other hand, a total residence time of the heavy fuel fraction includes only a single average reaction time in the second reactor  135 ,  219 . Moreover, the single average reaction time of the heavy fuel fraction in the second reactor  135 ,  219 , occurs together with the second average reaction time of the light fuel fraction, which also is being cracked in the second reactor  135 ,  219 . Thus, the total residence time of the light fuel fraction in the series-reactor FCC units is longer than the total residence time of the heavy fuel fraction. Accordingly, the series-reactor FCC units may have a residence-time ratio equal to the ratio of the total residence time of the light fuel fraction to the total residence time of the heavy fuel fraction. In various embodiments, the series-reactor FCC units according to embodiments herein may have residence-time ratio of at least 1, at least 2, at least 5, or at least 10, such as, for example, from 1 to 20, from 2 to 20, from 2 to 10, from 2 to 5, or from 5 to 20, for example. Without intent to be bound by theory, it is believed that a residence-time ratio in the ranges described above is exemplary because light and heavy fuel fractions have different reactivities. For example, naphtha may be less reactive than VGO and may require a higher residence time to react. 
     In processes incorporating series-reactor FCC units according to embodiments herein, such as the series-downer FCC unit  130  of  FIG. 1  or the series-riser FCC unit  230  of  FIG. 2 , for example, the series-reactor FCC unit during a particular process may have a catalyst-to-light fuel ratio that is greater than a catalyst-to-heavy fuel ratio. 
     The catalyst-to-light fuel ratio is the weight ratio of the FCC catalyst to the light fuel fraction. The catalyst-to-light fuel ratio is determined by dividing the total flow rate of catalyst from the catalyst regenerator  117 ,  217  into the first reactor  113 ,  233  and/or second reactor  135 ,  219  by the flow rate of the light fuel fraction entering the first reactor  113 ,  233 . For example, in embodiment where regenerated catalyst enters directly into the first reactor  113 ,  233  and second reactor  135 ,  219 , the catalyst-to-light fuel ratio is calculated as the sum of the flow rates of (1) the FCC catalyst entering the first reactor  113 ,  233  from the catalyst regenerator  117 ,  217  and (2) the catalyst entering the second reactor from the catalyst regenerator  117 ,  217  by the flow rate of the light fuel fraction entering the first reactor  113 ,  233 . In embodiments where regenerated catalyst enters only directly into the first reactor  113 ,  233 , the catalyst-to-light fuel ratio is calculated as the flow rate of the FCC catalyst entering the first reactor  113 ,  233  divided by the flow rate of the light fuel fraction entering the first reactor  113 ,  233 . 
     The catalyst-to-heavy fuel ratio is the weight ratio of the FCC catalyst to the heavy fuel fraction. The catalyst-to-heavy fuel ratio is determined by dividing the total flow rate of catalyst from the catalyst regenerator  117 ,  217  into the first reactor  113 ,  233  and/or second reactor  135 ,  219  by the flow rate of the heavy fuel fraction entering the first reactor  113 ,  233 . For example, in embodiment where regenerated catalyst enters directly into the first reactor  113 ,  233  and second reactor  135 ,  219 , the catalyst-to-heavy fuel ratio is calculated as the sum of the flow rates of (1) the FCC catalyst entering the first reactor  113 ,  233  from the catalyst regenerator  117 ,  217  and (2) the catalyst entering the second reactor from the catalyst regenerator  117 ,  217  by the flow rate of the heavy fuel fraction entering the second reactor  135 ,  219 . In embodiments where regenerated catalyst enters only directly into the first reactor  113 ,  233 , the catalyst-to-light fuel ratio is calculated as the flow rate of the FCC catalyst entering the first reactor  113 ,  233  divided by the flow rate of the heavy fuel fraction entering the second reactor  135 , 219 . 
     A unit catalyst ratio is defined as the ratio of the catalyst-to-light fuel ratio and the catalyst-to-heavy fuel ratio. In exemplary embodiments of the series-reactor FCC units described above, processes incorporating the series-reactor FCC units may have a unit catalyst ratio greater than 1, greater than 1.5, greater than 2, greater than 5, or greater than 10, such as from 1.1 to 20, from 2 to 20, from 2 to 10, from 2 to 5, or from 5 to 20, for example. Without intent to be bound by theory, it is believed that a unit catalyst ratio in the ranges described above is exemplary because light and heavy fuel fractions have different reactivities with the catalyst. For example, naphtha may be less reactive than VGO and require relatively more catalyst to react. 
     While  FIGS. 1 and 2  depict embodiments of systems that include two reactors, it is contemplated herein that the FCC units may comprise more than two reactors in series, such as, but not limited to, three, four, or even five reactors arranged in series. Additionally, it should be understood that the systems described herein are not limited to two downers or two risers is series. For example, one or more risers and one or more downers connected in series are contemplated herein, such as a riser and downer in series or a downer and riser is series. 
     EXAMPLES 
     The various embodiments of methods and systems for the cracking of a light fuel fraction and a heavy fuel fraction by fluidized catalytic cracking will be further clarified by the following examples. The examples are illustrative in nature, and should not be understood to limit the subject matter of the present disclosure. 
     Example 1 
     A light straight run naphtha (LSRN), the composition of which is given in Table 1, was cracked in a microactivity test unit (MAT unit) using ASTM method D3907 at high severity FCC conditions. The naphtha was cracked at 650° C. and 48 catalyst-to-fuel ratio. The light naphtha was converted and yielded products as shown in Table 2. 
     
       
         
           
               
               
               
               
               
               
               
             
               
                 TABLE 1 
               
               
                   
               
               
                   
                 n- 
                 Iso- 
                   
                   
                   
                   
               
               
                 wt. % 
                 Paraffins 
                 Paraffins 
                 Olefins 
                 Naphthenes 
                 Aromatics 
                 Total 
               
               
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
               
               
               
            
               
                 C-4 
                 0 
                 0 
                 0 
                 0 
                 0 
                 0.0 
               
               
                 C-5 
                 28.8 
                 8.0 
                 0 
                 2.2 
                 0 
                 39.0 
               
               
                 C-6 
                 26.4 
                 27.7 
                 1.4 
                 3.5 
                 0 
                 59.1 
               
               
                 C-7 
                 0 
                 0.58 
                 1.3 
                 0 
                 0 
                 1.9 
               
               
                 C-8 
                 0 
                 0 
                 0 
                 0 
                 0 
                 0 
               
               
                 Total 
                 55.2 
                 36.4 
                 2.7 
                 5.7 
                 0 
                 100.0 
               
               
                   
               
            
           
         
       
     
     
       
         
           
               
               
               
             
               
                 TABLE 2 
               
               
                   
               
             
            
               
                   
                 Feedstock 
                 LSRN 
               
               
                   
                 conversion, wt. % 
                 33.47 
               
               
                   
                 C3 =, wt. % 
                 11.60 
               
               
                   
                 Total Remaining Gas, wt. % 
                 21.39 
               
               
                   
                 Gasoline, wt. % 
                 66.53 
               
               
                   
                 LCO, wt. % 
                 0.00 
               
               
                   
                 HCO, wt. % 
                 0.00 
               
               
                   
                 Coke, wt. % 
                 0.48 
               
               
                   
                 Total, wt. % 
                 100.00 
               
               
                   
               
            
           
         
       
     
     Example 2 
     A hydrotreated vacuum gas oil, properties of which are given in Table 3A and 3B, was cracked in a MAT test unit using ASTM method D3907 at high severity FCC conditions. Table 3A provides various properties of the vacuum gas oil and Table 3B reports the temperature at which a specified volume percentage of the vacuum gas oil boils. The hydrotreated vacuum gas oil was cracked at 650° C. and 5.8 catalyst-to-fuel ratio. The product yields are shown in Table 4. 
     
       
         
           
               
               
               
             
               
                 TABLE 3A 
               
               
                   
               
             
            
               
                   
                 Specific gravity 
                 0.8967 
               
               
                   
                 Sulfur, wt ppm (1) 
                 &lt;300 
               
               
                   
                 Nitrogen, wt ppm 
                 &lt;170 
               
               
                   
                 Total Aromatics, % wt (2) 
                 38.5 
               
               
                   
                 Conradson carbon, % wt 
                 &lt;0.2 
               
               
                   
                 Watson K factor (3) 
                 12.17 
               
               
                   
                 Ni (ICP), ppm wt 
                 &lt;0.1 
               
               
                   
                 V (ICP), ppm wt 
                 &lt;0.2 
               
               
                   
               
            
           
         
       
     
     
       
         
           
               
               
               
             
               
                 TABLE 3B 
               
               
                   
               
               
                   
                 Distillation % vol (ASTM D1160) 
                 Temperature 
               
               
                   
               
             
            
               
                   
                 0% (initial boiling point) 
                 364° C. 
               
               
                   
                  5% 
                 382° C. 
               
               
                   
                 10% 
                 388° C. 
               
               
                   
                 30% 
                 417° C. 
               
               
                   
                 50% 
                 444° C. 
               
               
                   
                 70% 
                 480° C. 
               
               
                   
                 90% 
                 541° C. 
               
               
                   
                 95% 
                 564° C. 
               
               
                   
               
            
           
         
       
     
     
       
         
           
               
               
               
             
               
                 TABLE 4 
               
               
                   
               
             
            
               
                   
                 Feedstock 
                 Hydrotreated VGO 
               
               
                   
                 Conversion, wt. % 
                 87.56 
               
               
                   
                 C3 =, wt. % 
                 25.49 
               
               
                   
                 Total Remaining Gas, wt. % 
                 38.60 
               
               
                   
                 Gasoline, wt. % 
                 21.83 
               
               
                   
                 LCO, wt. % 
                 8.62 
               
               
                   
                 HCO, wt. % 
                 3.82 
               
               
                   
                 Coke, wt. % 
                 1.64 
               
               
                   
                 Total, wt. % 
                 100.00 
               
               
                   
               
            
           
         
       
     
     Example 3 
     A hydrotreated vacuum gas oil (100 parts by volume) (shown in  FIG. 3 ) and straight run naphtha (10 parts by volume), properties of which are shown in Tables 1 and 3, were cracked in separate reactors in series (naphtha followed by VGO) in high severity FCC conditions. Both feedstocks were cracked at 650° C., with a catalyst-to-light fuel ratio of 50 and a catalyst-to-heavy fuel ratio of 6. The product yields are shown in Table 5. 
     
       
         
           
               
               
               
             
               
                 TABLE 5 
               
               
                   
               
               
                   
                 Feedstock 
                 Yields 
               
               
                   
               
             
            
               
                   
               
            
           
           
               
               
               
            
               
                   
                 Conversion, W % 
                 90.91 
               
               
                   
                 C 3  =, W % 
                 26.65 
               
               
                   
                 Total Remaining Gas, W % 
                 40.74 
               
               
                   
                 Gasoline, W % 
                 28.48 
               
               
                   
                 LCO, W % 
                 8.62 
               
               
                   
                 HCO, W % 
                 3.82 
               
               
                   
                 Coke, W % 
                 1.69 
               
               
                   
                 Total, W % 
                 110.00 
               
               
                   
               
            
           
         
       
     
     Based on the foregoing, it should now be understood that various aspects of method and systems for cracking light fuel fractions and heavy fuel fractions by fluidized catalytic cracking are disclosed herein. According to a first aspect of the present disclosure, a light fuel fraction and a heavy fuel fraction may be cracked by fluidized catalytic cracking. The cracking process may comprise feeding the light fuel fraction and a catalyst from a catalyst regenerator into a first reactor, and cracking the light fuel fraction in the first reactor to produce an at least partially cracked light fuel fraction. The first reactor may be a fluidized bed reactor. The process may further comprise transporting the at least partially cracked light fuel fraction and the catalyst from the first reactor to a second reactor, feeding the heavy fuel fraction into the second reactor, and cracking the heavy fuel fraction and the at least partially cracked light fuel fraction in the second reactor to produce at least a product fuel and a spent catalyst. The second reactor may be a fluidized bed reactor. The process may further comprise transporting the spent catalyst to the catalyst regenerator and regenerating the catalyst in the catalyst regenerator. 
     In a second aspect, a system for cracking by fluidized catalytic cracking may comprise a first reactor, a second reactor, and a catalyst regenerator. The first reactor may be a fluidized bed reactor and may comprise a catalyst inlet and a light fuel fraction inlet. The second reactor may be a fluidized bed reactor and may be in fluidic communication with the first reactor and may comprise a heavy fuel fraction inlet. The catalyst regenerator may be in fluidic communication with the catalyst inlet of the first reactor. A catalyst may circulate from the catalyst regenerator to the first reactor to the second reactor and back to the catalyst regenerator. A light fuel fraction may be disposed in the first reactor and may react with the catalyst and be transported to the second reactor. A heavy fuel fraction may be disposed in the second reactor and may react with the catalyst. 
     A third aspect includes the method of the first aspect, further comprising transporting additional catalyst from the catalyst regenerator to the second reactor. 
     A fourth aspect includes the method of the first aspect or the system of the second aspect, wherein both the first reactor and the second reactor may be downers. 
     A fifth aspect includes the method of the first aspect or the system of the second aspect, wherein both the first reactor and the second reactor may be risers. 
     A sixth aspect includes the method of the first aspect or the system of the second aspect, wherein: a sum of a first average reaction time of the light fuel fraction in the first reactor and a second average reaction time of the at least partially cracked light fuel fraction in the second reactor defines a total residence time of the light fuel fraction; a single average reaction time of the heavy fuel fraction in the second reactor defines a residence time of the heavy fuel fraction; a ratio of the total residence time of the light fuel fraction and the residence time of the heavy fuel fraction defines a residence-time ratio; and the residence-time ratio is from about 1 to about 10. 
     A seventh aspect includes the method of the first aspect or the system of the second aspect, wherein the light fuel fraction comprises straight or cracked naphthas with boiling points from about 36° C. to about 250° C., distillate oils with boiling points from about 10° C. to about 400° C., or combinations thereof. 
     An eighth aspect includes the method of the first aspect or the system of the second aspect, wherein the heavy fuel fraction comprises vacuum distillates with boiling points from about 370° C. to about 565° C., residues with boiling points above 520° C., or combinations thereof, the residues being chosen from hydrotreated residues, atmospheric distillation residues, vacuum distillation residues, visbreaking residues, distillation residues, or combinations thereof. 
     A ninth aspect includes the method of the first aspect, further comprising atomizing the light fuel fraction before feeding the light fuel fraction into the first reactor, and atomizing the heavy fuel fraction before feeding the heavy fuel fraction into the second reactor 
     A tenth aspect includes the method of the first aspect or the system of the second aspect, wherein the products are light olefins (C 2 -C 4 ), and/or gasoline. 
     An eleventh aspect includes the method of the first aspect or the system of the second aspect, wherein the spent catalyst is separated from other products of the second reactor in a separation zone. 
     A twelfth aspect includes the method of the first aspect or the system of the second aspect, wherein the spent catalyst is separated from other products of the second reactor in a separation zone 
     A thirteenth aspect includes the system of the second aspect further comprising a transfer line connecting the catalyst regenerator and the second reactor. 
     A fourteenth aspect includes the system of the second aspect, wherein the light fuel fraction in the first reactor is atomized. 
     A fifteenth aspect includes the method of the first aspect or the system of the second aspect, wherein at least a portion of the catalyst in the second reactor is spent catalyst comprising coke deposits 
     For the purposes of describing and defining the present disclosure it is noted that the term “about” are utilized herein to represent the inherent degree of uncertainty that may be attributed to any quantitative comparison, value, measurement, or other representation. The term “about” are also utilized herein to represent the degree by which a quantitative representation may vary from a stated reference without resulting in a change in the basic function of the subject matter at issue. 
     It is noted that one or more of the following claims utilize the term “wherein” as a transitional phrase. For the purposes of defining the present technology, it is noted that this term is introduced in the claims as an open-ended transitional phrase that is used to introduce a recitation of a series of characteristics of the structure and should be interpreted in like manner as the more commonly used open-ended preamble term “comprising.” 
     It should be understood that any two quantitative values assigned to a property may constitute a range of that property, and all combinations of ranges formed from all stated quantitative values of a given property are contemplated herein. 
     Having described the subject matter of the present disclosure in detail and by reference to specific embodiments thereof, it is noted that the various details disclosed herein should not be taken to imply that these details relate to elements that are essential components of the various embodiments described herein, even in cases where a particular element is illustrated in each of the drawings that accompany the present description. Rather, the claims appended hereto should be taken as the sole representation of the breadth of the present disclosure and the corresponding scope of the various embodiments described herein. Further, it will be apparent that modifications and variations are possible without departing from the scope of the appended claims.