Abstract:
Aromatic extraction and hydrocracking processes are integrated to optimize the hydrocracking units design and/or performance. By processing aromatic-rich and aromatic-lean fractions separately, the hydrocracking operating severity and/or catalyst reactor volume requirement decreases.

Description:
RELATED APPLICATIONS 
     This application claims the benefit of U.S. Provisional Patent Application No. 61/513,191 filed Jul. 29, 2011, the disclosure of which is hereby incorporated by reference in its entirety. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to hydroprocessing systems and methods, in particular for efficient reduction of catalyst-fouling aromatic nitrogen components in a hydrocarbon mixture. 
     2. Description of Related Art 
     Hydrocracking operations are used commercially in a large number of petroleum refineries. They are used to process a variety of feeds boiling in the range of 370° C. to 520° C. in conventional hydrocracking units and boiling at 520° C. and above in the residue hydrocracking units. In general, hydrocracking processes split the molecules of the feed into smaller, i.e., lighter, molecules having higher average volatility and economic value. Additionally, hydrocracking typically improves the quality of the hydrocarbon feedstock by increasing the hydrogen to carbon ratio and by removing organosulfur and organonitrogen compounds. The significant economic benefit derived from hydrocracking operations has resulted in substantial development of process improvements and more active catalysts. 
     Mild hydrocracking or single stage hydrocracking operations, typically the simplest of the known hydrocracking configurations, occur at operating conditions that are more severe than typical hydrotreating, and less severe than typical full pressure hydrocracking. Single or multiple catalysts systems can be used depending upon the feedstock and product specifications. Multiple catalyst systems can be deployed as a stacked-bed configuration or in multiple reactors. Mild hydrocracking operations are generally more cost effective, but typically result in both a lower yield and reduced quality of mid-distillate product as compared to full pressure hydrocracking operations. 
     In a series-flow configuration the entire hydrocracked product stream from the first reaction zone, including light gases (e.g., C 1 -C 4 , H 2 S, NH 3 ) and all remaining hydrocarbons, are sent to the second reaction zone. In two-stage configurations the feedstock is refined by passing it over a hydrotreating catalyst bed in the first reaction zone. The effluents are passed to a fractionating zone column to separate the light gases, naphtha and diesel products boiling in the temperature range of 36° C. to 370° C. The hydrocarbons boiling above 370° C. are then passed to the second reaction zone for additional cracking. 
     Conventionally, most hydrocracking processes that are implemented for production of middle distillates and other valuable fractions retain aromatics, e.g., boiling in the range of about 180° C. to 370° C. Aromatics boiling higher than the middle distillate range are also included and produced in the heavier fractions. 
     In all of the above-described hydrocracking process configurations, cracked products, along with partially cracked and unconverted hydrocarbons, are passed to a distillation column for separating into products including naphtha, jet fuel/kerosene and diesel boiling in the nominal ranges of 36° C.-180° C., 180° C.-240° C. and 240° C.-370° C., respectively, and unconverted products boiling in the nominal range of above 370° C. Typical jet fuel/kerosene fractions (i.e., smoke point &gt;25 mm) and diesel fractions (i.e., cetane number &gt;52) are of high quality and well above the worldwide transportation fuel specifications. Although the hydrocracking unit products have relatively low aromaticity, aromatics that do remain lower the key indicative properties (smoke point and cetane number) for these products. 
     A need remains in the industry for improvements in hydrocracking operations for heavy hydrocarbon feeds to produce clean transportation fuels in an economical and efficacious manner. 
     SUMMARY OF THE INVENTION 
     In accordance with one or more embodiments, the invention relates to systems and methods of hydrocracking heavy hydrocarbon feedstocks to produce clean transportation fuels. An integrated hydrocracking process includes hydroprocessing an aromatic-rich fraction of the initial feed separately from an aromatic-lean fraction. 
     In a series-flow hydrocracker configuration provided herein, an aromatic separation unit is integrated in which: 
     the feedstock is separated into an aromatic-rich fraction and an aromatic-lean fraction; 
     the aromatic-rich fraction is passed to a first stage hydroprocessing reaction zone operating under conditions effective to hydrotreat and/or hydrocrack at least a portion of aromatic compounds contained in the aromatic-rich fraction and to produce a first stage hydroprocessing reaction zone effluent; 
     a mixture of the first stage hydroprocessing reaction zone effluent and the aromatic-lean fraction is passed to a second stage hydroprocessing reaction zone to produce a second stage hydroprocessing reaction zone effluent; and 
     the second stage hydroprocessing reaction zone effluent is fractionated in a fractionating zone to produce a product stream and a bottoms stream that are separately recovered. 
     Unlike typical known methods, the present process separates the hydrocracking feed into fractions containing different classes of compounds with different reactivities relative to the conditions of hydrocracking. Conventionally, most approaches subject the entire feedstock to the same hydroprocessing reaction zones, necessitating operating conditions that must accommodate feed constituents that require increased severity for conversion, or alternatively sacrifice overall yield to attain desirable process economics. 
     Since aromatic extraction operations typically do not provide sharp cut-offs between the aromatics and non-aromatics, the aromatic-lean fraction contains a major proportion of the non-aromatic content of the initial feed and a minor proportion of the aromatic content of the initial feed, and the aromatic-rich fraction contains a major proportion of the aromatic content of the initial feed and a minor proportion of the non-aromatic content of the initial feed. The amount of non-aromatics in the aromatic-rich fraction, and the amount of aromatics in the aromatic-lean fraction, depend on various factors as will be apparent to one of ordinary skill in the art, including the type of extraction, the number of theoretical plates in the extractor (if applicable to the type of extraction), the type of solvent and the solvent ratio. 
     The feed portion that is extracted into the aromatic-rich fraction includes aromatic compounds that contain heteroatoms and those that are free of heteroatoms. Aromatic compounds that contain heteroatoms that are extracted into the aromatic-rich fraction generally include aromatic nitrogen compounds such as pyrrole, quinoline, acridine, carbazoles and their derivatives, and aromatic sulfur compounds such as thiophene, benzothiophenes and their derivatives, and dibenzothiophenes and their derivatives. These nitrogen- and sulfur-containing aromatic compounds are targeted in the aromatic separation step(s) generally by their solubility in the extraction solvent. In certain embodiments, selectivity of the nitrogen- and sulfur-containing aromatic compounds is enhanced by use of additional stages and/or selective sorbents. Various non-aromatic sulfur-containing compounds that may have been present in the initial feed, i.e., prior to hydrotreating, include mercaptans, sulfides and disulfides. Depending on the aromatic extraction operation type and/or condition, a preferably very minor portion of non-aromatic nitrogen- and sulfur-containing compounds can pass to the aromatic-rich fraction. 
     As used herein, the term “major proportion of the non-aromatic compounds” means at least greater than 50 weight % (W %) of the non-aromatic content of the feed to the extraction zone, in certain embodiments at least greater than about 85 W %, and in further embodiments greater than at least about 95 W %. Also as used herein, the term “minor proportion of the non-aromatic compounds” means no more than 50 W % of the non-aromatic content of the feed to the extraction zone, in certain embodiments no more than about 15 W %, and in further embodiments no more than about 5 W %. 
     Also as used herein, the term “major proportion of the aromatic compounds” means at least greater than 50 W % of the aromatic content of the feed to the extraction zone, in certain embodiments at least greater than about 85 W %, and in further embodiments greater than at least about 95 W %. Also as used herein, the term “minor proportion of the aromatic compounds” means no more than 50 W % of the aromatic content of the feed to the extraction zone, in certain embodiments no more than about 15 W %, and in further embodiments no more than about 5 W %. 
     Still other aspects, embodiments, and advantages of these exemplary aspects and embodiments, are discussed in detail below. Moreover, it is to be understood that both the foregoing information and the following detailed description are merely illustrative examples of various aspects and embodiments, and are intended to provide an overview or framework for understanding the nature and character of the claimed aspects and embodiments. The accompanying drawings are included to provide illustration and a further understanding of the various aspects and embodiments, and are incorporated in and constitute a part of this specification. The drawings, together with the remainder of the specification, serve to explain principles and operations of the described and claimed aspects and embodiments. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The foregoing summary as well as the following detailed description will be best understood when read in conjunction with the attached drawings. It should be understood, however, that the invention is not limited to the precise arrangements and apparatus shown. In the drawings the same or similar reference numerals are used to identify to the same or similar elements, in which: 
         FIG. 1  is a process flow diagram of a hydroprocessing system operating in a series-flow configuration; 
         FIG. 2  is a schematic diagram of an aromatic separation apparatus; and 
         FIGS. 3-8  show various examples of apparatus suitable for use as the aromatic extraction zone. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     An integrated system is provided for efficient hydroprocessing of heavy hydrocarbon feedstocks to produce clean transportation fuels. In general, the process and apparatus described herein for producing cracked hydrocarbons are applied to series-flow hydrocracking configurations. 
     An aromatic separation unit is integrated in a series-flow hydrocracker configuration as follows: 
     a feedstock is separated into an aromatic-rich fraction and an aromatic-lean fraction; 
     the aromatic-rich fraction is passed to a first stage hydroprocessing reaction zone operating under conditions effective to hydrotreat and/or hydrocrack at least a portion of aromatic compounds contained in the aromatic-rich fraction and to produce a first stage hydroprocessing reaction zone effluent; 
     a mixture of first stage hydroprocessing reaction zone effluent and aromatic-lean fraction is passed to a second stage hydroprocessing reaction zone to produce a second stage hydroprocessing reaction zone effluent; and 
     the second stage hydroprocessing reaction zone effluent is fractionated in a fractionating zone to produce one or more product streams and one or more bottoms streams that can be separately recovered. 
       FIG. 1  is a process flow diagram of an integrated hydrocracking apparatus  100  in the configuration of a series-flow hydrocracking unit apparatus. Apparatus  100  includes an aromatic extraction zone  140 , a first stage hydroprocessing reaction zone  150  containing a first stage hydroprocessing catalyst, a second stage hydroprocessing reaction zone  180  containing a second stage hydroprocessing catalyst and a fractionating zone  170 . 
     Aromatic extraction zone  140  typically includes a feed inlet  102 , an aromatic-rich stream outlet  104  and an aromatic-lean stream outlet  106 . In certain embodiments, feed inlet  102  is in fluid communication with fractionating zone  170  via an optional recycle conduit  120  to receive all or a portion of the bottoms  174 . Various embodiments of and/or unit-operations contained within aromatic separation zone  140  are described in conjunction with  FIGS. 2-8 . 
     First stage hydroprocessing reaction zone  150  generally includes an inlet  151  in fluid communication with aromatic-rich stream outlet  104  and a source of hydrogen gas via a conduit  152 . First stage hydroprocessing reaction zone  150  also includes a first stage hydroprocessing reaction zone effluent outlet  154 . In certain embodiments, inlet  151  is in fluid communication with fractionating zone  170  via an optional recycle conduit  156  to receive all or a portion of the bottoms  174 . 
     First stage hydroprocessing reaction zone  150  is operated under relatively severe conditions. As used herein, the term “severe conditions” is relative and the ranges of operating conditions depend on the feedstock being processed. For instance, these conditions can include a reaction temperature in the range of from about 300° C. to 500° C., in certain embodiments from about 380° C. to 450° C.; a reaction pressure in the range from about 100 bars to 200 bars, in certain embodiments from about 130 bars to 180 bars; a hydrogen feed rate below about 2500 standard liters per liter of hydrocarbon feed (SLt/Lt), in certain embodiments from about 500 to 2500 SLt/Lt, and in further embodiments from about 1000 to 1500 SLt/Lt; and a feed rate in the range of from about 0.25 h −1  to 3.0 h −1 , in certain embodiments from about 0.5 h −1  to 1.0 h −1 . 
     The catalyst used in the first stage hydroprocessing reaction zone has one or more active metal components selected from the Periodic Table of the Elements Group VI, VII or VIIIB. In certain embodiments the active metal component is one or more of cobalt, nickel, tungsten and molybdenum, typically deposited or otherwise incorporated on a support, e.g., alumina, silica alumina, silica, or zeolites. 
     Second stage hydroprocessing reaction zone  180  includes an inlet  181  in fluid communication with aromatic-lean stream outlet  106 , first stage hydroprocessing reaction zone effluent outlet  154  and an optional source of hydrogen gas via a conduit  182 . In certain embodiments, sufficient excess hydrogen can be included in the first stage hydroprocessing reaction zone  150  so that hydrogen feed  182  can be minimized or obviated. Second stage hydroprocessing reaction zone  180  also includes a second stage hydroprocessing reaction zone effluent outlet  184 . In certain embodiments, inlet  181  is in fluid communication with fractionating zone  170  via an optional recycle conduit  186  to receive all or a portion of the bottoms  174 . 
     In general, the second stage hydroprocessing reaction zone  180  is operated under relatively mild conditions. As used herein, the term “mild conditions” is relative and the ranges of operating conditions depend on the feedstock being processed. For instance, these conditions can include a reaction temperature in the range from about 300° C. to 500° C., in certain embodiments 330° C. to 420° C.; a reaction pressure in the range of from about 30 bars to 130 bars, in certain embodiments from about 60 bars to 100 bars; a hydrogen feed rate below about 2500 SLt/Lt, in certain embodiments from about 500 to 2500 SLt/Lt, and in further embodiments from about 1000 to 1500 SLt/Lt; and a feed rate in the range of from about 1.0 h −1  to 5.0 h −1 , in certain embodiments from about 2.0 h −1  to 3.0 h −1 . 
     The catalyst used in second stage hydroprocessing reaction zone  180  has one or more active metal components selected from the Periodic Table of the Elements Group VI, VII or VIIIB. In certain embodiments the active metal component is one or more of cobalt, nickel, tungsten and molybdenum, typically deposited or otherwise incorporated on a support, e.g., alumina, silica alumina, silica, or zeolites. 
     Fractionating zone  170  includes an inlet  171  in fluid communication with the second stage hydroprocessing reaction zone effluent outlet  184 , a product stream outlet  172  and a bottoms stream outlet  174 . Note that while one product outlet is shown, multiple product fractions can also be recovered from fractionating zone  170 . 
     A feedstock is introduced via inlet  102  of the aromatic extraction zone  140  for extraction of an aromatic-rich fraction and an aromatic-lean fraction. Optionally, the feedstock can be combined with all or a portion of the bottoms  174  from fractionating zone  170  via recycle conduit  120 . 
     The aromatic-rich fraction generally includes a major proportion of the aromatic nitrogen- and sulfur-containing compounds that were in the initial feedstock and a minor proportion of non-aromatic compounds that were in the initial feedstock. Aromatic nitrogen-containing compounds that are extracted into the aromatic-rich fraction include pyrrole, quinoline, acridine, carbazole and their derivatives. Aromatic sulfur-containing compounds that are extracted into the aromatic-rich fraction include thiophene, benzothiophene and its long chain alkylated derivatives, and dibenzothiophene and its alkyl derivatives such as 4,6-dimethyl-dibenzothiophene. The aromatic-lean fraction generally includes a major proportion of the non-aromatic compounds that were in the initial feedstock and a minor proportion of the aromatic nitrogen- and sulfur-containing compounds that were in the initial feedstock. The aromatic-lean fraction is almost free of refractory nitrogen-containing compounds, and the aromatic-rich fraction contains nitrogen-containing aromatic compounds. 
     The aromatic-rich fraction discharged via outlet  104  is passed to inlet  151  of first stage hydroprocessing reaction zone  150  and mixed with hydrogen gas via conduit  152 . Optionally, the aromatic-rich fraction is combined with all or a portion of the bottoms  174  from the fractionating zone  170  via recycle conduit  156 . Compounds contained in the aromatic-rich fraction including aromatics compounds are hydrotreated and/or hydrocracked. The first stage hydroprocessing reaction zone  150  is operated under relatively severe conditions. In certain embodiments, these relatively severe conditions of the first stage hydroprocessing reaction zone  150  are more severe than conventionally known severe hydroprocessing conditions due to the comparatively higher concentration of aromatic nitrogen- and sulfur-containing compounds. However, the capital and operational costs of these more severe conditions are offset by the reduced volume of aromatic-rich feed processed in the first stage hydroprocessing reaction zone  150  as compared to a full range feed that would be processed in a conventionally known severe hydroprocessing unit operation. 
     The first stage hydroprocessing reaction zone effluent discharged via outlet  154  is mixed with the aromatic-lean fraction discharged via outlet  106  and passed to inlet  181  of the second stage hydroprocessing reaction zone  180 . Compounds contained in the mixture of the first stage hydroprocessing reaction zone effluent and the aromatic-lean fraction, including paraffins and naphthenes, are hydrotreated and/or hydrocracked. Optionally, the mixture is combined with hydrogen gas via inlet  182  and recycled bottoms from the fractionating zone  170  via conduit  186 . Note that the hydrogen source via inlet  182  can be obviated if there is no requirement for additional hydrogen, e.g., is provided via conduit  152  and passes unreacted to the second stage hydroprocessing reaction zone  180 . The second stage hydroprocessing reaction zone  180  is operated under relatively mild conditions, which can be milder than conventional mild hydroprocessing conditions due to the comparatively lower concentration of aromatic nitrogen- and sulfur-containing compounds thereby reducing capital and operational costs. 
     The second stage hydroprocessing reaction zone effluent is sent to one or more intermediate separator vessels (not shown) to remove gases including excess H 2 , H 2 S, NH 3 , methane, ethane, propane and butanes. The liquid effluents are passed to inlet  171  of the fractionating zone  170  for recovery of liquid products via outlet  172 , including, for instance, naphtha boiling in the nominal range of from about 36° C. to 180° C. and diesel boiling in the nominal range of from about 180° C. to 370° C. The bottoms stream discharged via outlet  174  includes unconverted hydrocarbons and/or partially cracked hydrocarbons, for instance, having a boiling temperature above about 370° C. It is to be understood that the product cut points between fractions are representative only and in practice cut points are selected based on design characteristics and considerations for a particular feedstock. For instance, the values of the cut points can vary by up to about 30° C. in the embodiments described herein. In addition, it is to be understood that while the integrated system is shown and described with one fractionating zone  170 , in certain embodiments separate fractionating zones can be effective. 
     All or a portion of the bottoms can be purged via conduit  175 , e.g., for processing in other unit operations or refineries. In certain embodiments to maximize yields and conversions a portion of bottoms  174  is recycled within the process to the aromatic separation unit  140 , the first stage hydroprocessing reaction zone  150  and/or the second stage hydroprocessing reaction zone  180  (represented by dashed-lines  120 ,  156  and  186 , respectively). 
     In addition, either or both of the aromatic-lean fraction and the aromatic-rich fraction also can include extraction solvent remains from the aromatic extraction zone  140 . In certain embodiments, extraction solvent can be recovered and recycled, e.g., as described with respect to  FIG. 2 . 
     Further, in certain embodiments aromatic compounds without heteroatoms (e.g., benzene, toluene and their derivatives) are passed to the aromatic-rich fraction and are hydrogenated and hydrocracked in the first stage, relatively more severe, hydrocracking zone to produce light distillates. The yield of these light distillates that meet the product specification derived from the aromatic compounds without heteroatoms is greater than the yield in conventional hydrocracking operations due to the focused and targeted hydrocracking zones. 
     In the above-described embodiment, the feedstock generally includes any liquid hydrocarbon feed conventionally suitable for hydrocracking operations, as is known to those of ordinary skill in the art. For instance, a typical hydrocracking feedstock is vacuum gas oil (VGO) boiling in the nominal range of from about 300° C. to 900° C. and in certain embodiments in the range of from about 370° C. to 520° C. De-metalized oil (DMO) or de-asphalted oil (DAO) can be blended with VGO or used as is. The hydrocarbon feedstocks can be derived from naturally occurring fossil fuels such as crude oil, shale oils, or coal liquids; or from intermediate refinery products or their distillation fractions such as naphtha, gas oil, coker liquids, fluid catalytic cracking cycle oils, residuals or combinations of any of the aforementioned sources. In general, aromatics content in VGO feedstock is in the range of from about 15 to 60 volume % (V %.) The recycle stream can include 0 W % to about 80 W % of stream  174 , in certain embodiments about 10 W % to 70 W % of stream  174  and in further embodiments about 20 W % to 60 W % of stream  174 , for instance, based on conversions in each zone of between about 10 W % and 80 W %. 
     The aromatic separation apparatus is generally based on selective aromatic extraction. For instance, the aromatic separation apparatus can be a suitable solvent extraction aromatic separation apparatus capable of partitioning the feed into a generally aromatic-lean stream and a generally aromatic-rich stream. Systems including various established aromatic extraction processes and unit operations used in other stages of various refinery and other petroleum-related operations can be employed as the aromatic separation apparatus described herein. In certain existing processes, it is desirable to remove aromatics from the end product, e.g., lube oils and certain fuels, e.g., diesel fuel. In other processes, aromatics are extracted to produce aromatic-rich products, for instance, for use in various chemical processes and as an octane booster for gasoline. 
     As shown in  FIG. 2 , an aromatic separation apparatus  240  can include suitable unit operations to perform a solvent extraction of aromatics, and recover solvents for reuse in the process. A feed  202  is conveyed to an aromatic extraction vessel  208  in which in which a first, aromatic-lean, fraction is separated as a raffinate stream  210  from a second, generally aromatic-rich, fraction as an extract stream  212 . A solvent feed  215  is introduced into the aromatic extraction vessel  208 . 
     A portion of the extraction solvent can also exist in stream  210 , e.g., in the range of 0 W % to about 15 W % (based on the total amount of stream  210 ), in certain embodiments less than about 8 W %. In operations in which the solvent existing in stream  210  exceeds a desired or predetermined amount, solvent can be removed from the hydrocarbon product, for example, using a flashing or stripping unit  213 , or other suitable apparatus. Solvent  214  from the flashing unit  213  can be recycled to the aromatic extraction vessel  208 , e.g., via a surge drum  216 . Initial solvent feed or make-up solvent can be introduced via stream  222 . An aromatic-lean stream  206  is discharged from the flashing unit  213 . 
     In addition, a portion of the extraction solvent can also exist in stream  212 , e.g., in the range of about 70 W % to 98 W % (based on the total amount of stream  215 ), in certain embodiments less than about 85 W %. In embodiments in which solvent existing in stream  212  exceeds a desired or predetermined amount, solvent can be removed from the hydrocarbon product, for example, using a flashing or stripping unit  218  or other suitable apparatus. Solvent  221  from the flashing unit  218  can be recycled to the aromatic extraction vessel  208 , e.g., via the surge drum  216 . An aromatic-rich stream  204  is discharged from the flashing unit  218 . 
     Selection of solvent, operating conditions, and the mechanism of contacting the solvent and feed permit control over the level of aromatic extraction. For instance, suitable solvents include furfural, N-methyl-2-pyrrolidone, dimethylformamide, dimethylsulfoxide, phenol, nitrobenzene, sulfolanes, acetonitrile, furfural, or glycols, and can be provided in a solvent to oil ratio of about 20:1, in certain embodiments about 4:1, and in further embodiments about 1:1. Suitable glycols include diethylene glycol, ethylene glycol, triethylene glycol, tetraethylene glycol and dipropylene glycol. The extraction solvent can be a pure glycol or a glycol diluted with from about 2 to 10 W % water. Suitable sulfolanes include hydrocarbon-substituted sulfolanes (e.g., 3-methyl sulfolane), hydroxy sulfolanes (e.g., 3-sulfolanol and 3-methyl-4-sulfolanol), sulfolanyl ethers (e.g., methyl-3-sulfolanyl ether), and sulfolanyl esters (e.g., 3-sulfolanyl acetate). 
     The aromatic separation apparatus can operate at a temperature in the range of from about 20° C. to 200° C., and in certain embodiments from about 40° C. to 80° C. The operating pressure of the aromatic separation apparatus can be in the range of from about 1 bar to 10 bars, and in certain embodiments from about 1 bar to 3 bars. Types of apparatus useful as the aromatic separation apparatus in certain embodiments of the system and process described herein include stage-type extractors or differential extractors. 
     An example of a stage-type extractor is a mixer-settler apparatus  340  schematically illustrated in  FIG. 3 . Mixer-settler apparatus  340  includes a vertical tank  381  incorporating a turbine or a propeller agitator  382  and one or more baffles  384 . Charging inlets  386 ,  388  are located at the top of tank  381  and outlet  391  is located at the bottom of tank  381 . The feedstock to be extracted is charged into vessel  381  via inlet  386  and a suitable quantity of solvent is added via inlet  388 . The agitator  382  is activated for a period of time sufficient to cause intimate mixing of the solvent and charge stock, and at the conclusion of a mixing cycle, agitation is halted and, by control of a valve  392 , at least a portion of the contents are discharged and passed to a settler  394 . The phases separate in the settler  394  and a raffinate phase containing an aromatic-lean hydrocarbon mixture and an extract phase containing an aromatic-rich mixture are withdrawn via outlets  396  and  398 , respectively. In general, a mixer-settler apparatus can be used in batch mode, or a plurality of mixer-settler apparatus can be staged to operate in a continuous mode. 
     Another stage-type extractor is a centrifugal contactor. Centrifugal contactors are high-speed, rotary machines characterized by relatively low residence time. The number of stages in a centrifugal device is usually one, however, centrifugal contactors with multiple stages can also be used. Centrifugal contactors utilize mechanical devices to agitate the mixture to increase the interfacial area and decrease the mass transfer resistance. 
     Various types of differential extractors (also known as “continuous contact extractors,”) that are also suitable for use as an aromatic extraction apparatus include, but are not limited to, centrifugal contactors and contacting columns such as tray columns, spray columns, packed towers, rotating disc contactors and pulse columns. 
     Contacting columns are suitable for various liquid-liquid extraction operations. Packing, trays, spray or other droplet-formation mechanisms or other apparatus are used to increase the surface area in which the two liquid phases (i.e., a solvent phase and a hydrocarbon phase) contact, which also increases the effective length of the flow path. In column extractors, the phase with the lower viscosity is typically selected as the continuous phase, which, in the case of an aromatic extraction apparatus, is the solvent phase. In certain embodiments, the phase with the higher flow rate can be dispersed to create more interfacial area and turbulence. This is accomplished by selecting an appropriate material of construction with the desired wetting characteristics. In general, aqueous phases wet metal surfaces and organic phases wet non-metallic surfaces. Changes in flows and physical properties along the length of an extractor can also be considered in selecting the type of extractor and/or the specific configuration, materials or construction, and packing material type and characteristics (i.e., average particle size, shape, density, surface area, and the like). 
     A tray column  440  is schematically illustrated in  FIG. 4 . A light liquid inlet  488  at the bottom of column  440  receives liquid hydrocarbon, and a heavy liquid inlet  491  at the top of column  440  receives liquid solvent. Column  440  includes a plurality of trays  481  and associated downcomers  482 . A top level baffle  484  physically separates incoming solvent from the liquid hydrocarbon that has been subjected to prior extraction stages in the column  440 . Tray column  440  is a multi-stage counter-current contactor. Axial mixing of the continuous solvent phase occurs at region  486  between trays  481 , and dispersion occurs at each tray  481  resulting in effective mass transfer of solute into the solvent phase. Trays  481  can be sieve plates having perforations ranging from about 1.5 to 4.5 mm in diameter and can be spaced apart by about 150-600 mm. 
     Light hydrocarbon liquid passes through the perforation in each tray  481  and emerges in the form of fine droplets. The fine hydrocarbon droplets rise through the continuous solvent phase and coalesce into an interface layer  496  and are again dispersed through the tray  481  above. Solvent passes across each plate and flows downward from tray  481  above to the tray  481  below via downcomer  482 . The principal interface  498  is maintained at the top of column  440 . Aromatic-lean hydrocarbon liquid is removed from outlet  492  at the top of column  440  and aromatic-rich solvent liquid is discharged through outlet  494  at the bottom of column  440 . Tray columns are efficient solvent transfer apparatus and have desirable liquid handling capacity and extraction efficiency, particularly for systems of low-interfacial tension. 
     An additional type of unit operation suitable for extracting aromatics from the hydrocarbon feed is a packed bed column.  FIG. 5  is a schematic illustration of a packed bed column  540  having a hydrocarbon inlet  591  and a solvent inlet  592 . A packing region  588  is provided upon a support plate  586 . Packing region  588  comprises suitable packing material including, but not limited to, Pall rings, Raschig rings, Kascade rings, Intalox saddles, Berl saddles, super Intalox saddles, super Berl saddles, Demister pads, mist eliminators, telerrettes, carbon graphite random packing, other types of saddles, and the like, including combinations of one or more of these packing materials. The packing material is selected so that it is fully wetted by the continuous solvent phase. The solvent introduced via inlet  592  at a level above the top of the packing region  588  flows downward and wets the packing material and fills a large portion of void space in the packing region  588 . Remaining void space is filled with droplets of the hydrocarbon liquid which rise through the continuous solvent phase and coalesce to form the liquid-liquid interface  598  at the top of the packed bed column  540 . Aromatic-lean hydrocarbon liquid is removed from outlet  594  at the top of column  540  and aromatic-rich solvent liquid is discharged through outlet  596  at the bottom of column  540 . Packing material provides large interfacial areas for phase contacting, causing the droplets to coalesce and reform. The mass transfer rate in packed towers can be relatively high because the packing material lowers the recirculation of the continuous phase. 
     Further types of apparatus suitable for aromatic extraction in the system and method herein include rotating disc contactors.  FIG. 6  is a schematic illustration of a rotating disc contactor  640  known as a Scheiebel® column commercially available from Koch Modular Process Systems, LLC of Paramus, N.J., USA. It will be appreciated by those of ordinary skill in the art that other types of rotating disc contactors can be implemented as an aromatic extraction unit included in the system and method herein, including but not limited to Oldshue-Rushton columns, and Kuhni extractors. The rotating disc contactor is a mechanically agitated, counter-current extractor. Agitation is provided by a rotating disc mechanism, which typically runs at much higher speeds than a turbine type impeller as described with respect to  FIG. 3 . 
     Rotating disc contactor  640  includes a hydrocarbon inlet  691  toward the bottom of the column and a solvent inlet  692  proximate the top of the column, and is divided into number of compartments formed by a series of inner stator rings  682  and outer stator rings  684 . Each compartment contains a centrally located, horizontal rotor disc  686  connected to a rotating shaft  688  that creates a high degree of turbulence inside the column. The diameter of the rotor disc  686  is slightly less than the opening in the inner stator rings  682 . Typically, the disc diameter is 33-66% of the column diameter. The disc disperses the liquid and forces it outward toward the vessel wall  698  where the outer stator rings  684  create quiet zones where the two phases can separate. Aromatic-lean hydrocarbon liquid is removed from outlet  694  at the top of column  640  and aromatic-rich solvent liquid is discharged through outlet  696  at the bottom of column  640 . Rotating disc contactors advantageously provide relatively high efficiency and capacity and have relatively low operating costs. 
     An additional type of apparatus suitable for aromatic extraction in the system and method herein is a pulse column.  FIG. 7  is a schematic illustration of a pulse column system  740 , which includes a column with a plurality of packing or sieve plates  788 , a light phase, i.e., solvent, inlet  791 , a heavy phase, i.e., hydrocarbon feed, inlet  792 , a light phase outlet  794  and a heavy phase outlet  796 . 
     In general, pulse column system  740  is a vertical column with a large number of sieve plates  788  lacking down comers. The perforations in the sieve plates  788  typically are smaller than those of non-pulsating columns, e.g., about 1.5 mm to 3.0 mm in diameter. 
     A pulse-producing device  798 , such as a reciprocating pump, pulses the contents of the column at frequent intervals. The rapid reciprocating motion, of relatively small amplitude, is superimposed on the usual flow of the liquid phases. Bellows or diaphragms formed of coated steel (e.g., coated with polytetrafluoroethylene), or any other reciprocating, pulsating mechanism can be used. A pulse amplitude of 5-25 mm is generally recommended with a frequency of 100-260 cycles per minute. The pulsation causes the light liquid (solvent) to be dispersed into the heavy phase (oil) on the upward stroke and heavy liquid phase to jet into the light phase on the downward stroke. The column has no moving parts, low axial mixing, and high extraction efficiency. 
     A pulse column typically requires less than a third the number of theoretical stages as compared to a non-pulsating column. A specific type of reciprocating mechanism is used in a Karr Column which is shown in  FIG. 8 . 
     Distinct advantages are offered by the selective hydrocracking apparatus and processes described herein when compared to conventional processes for hydrocracking selected fractions. Aromatics across a full range of boiling points contained in heavy hydrocarbons are extracted and separately processed in hydroprocessing reaction zone operating under conditions optimized for hydrotreating and/or hydrocracking aromatics, including aromatic nitrogen compounds that are prone to deactivate the hydrotreating catalyst. 
     According to the present processes and apparatus, the overall middle distillate yield is improved as the initial feedstock is separated into aromatic-rich and aromatic-lean fractions and hydrotreated and/or hydrocracked in different hydroprocessing reaction zones operating under conditions optimized for each fraction. 
     Example 
     A sample of vacuum gas oil (VGO) derived from Arab light crude oil was extracted in an extractor. Furfural was used as the extractive solvent. The extractor was operated at 60° C., atmospheric pressure, and at a solvent to diesel ratio of 1.1:1.0. Two fractions were obtained: an aromatic-rich fraction and an aromatic-lean fraction. The aromatic-lean fraction yield was 52.7 W % and contained 0.43 W % of sulfur and 5 W % of aromatics. The aromatic-rich fraction yield was 47.3 W % and contained 95 W % of aromatics and 2.3 W % of sulfur. The properties of the VGO, aromatic-rich fraction and aromatic-lean fraction are given in Table 1. 
     
       
         
               
             
               
               
               
               
               
             
               
               
               
               
               
             
           
               
                 TABLE 1 
               
             
             
               
                   
               
               
                 Properties of VGO and its Fractions 
               
             
          
           
               
                   
                   
                   
                 VGO-Aromatic- 
                 VGO-Aromatic- 
               
               
                 Property 
                   
                 VGO 
                 Rich 
                 Lean 
               
               
                   
               
             
          
           
               
                 Density at 
                 Kg/L 
                 0.922 
                 1.020 
                 0.835 
               
               
                 15° C. 
                   
                   
                   
                   
               
               
                 Carbon 
                 W % 
                 85.27 
                   
                   
               
               
                 Hydrogen 
                 W % 
                 12.05 
                   
                   
               
               
                 Sulfur 
                 W % 
                 2.7 
                 2.30 
                 0.43 
               
               
                 Nitrogen 
                 ppmw 
                 615 
                 584 
                 31 
               
               
                 MCR 
                 W % 
                 0.13 
                   
                   
               
               
                 Aromatics 
                 W % 
                 47.3 
                 44.9 
                 2.4 
               
               
                 N + P 
                 W % 
                 52.7 
                 2.6 
                 50.1 
               
               
                   
               
             
          
         
       
     
     The aromatic-rich fraction was hydrotreated in a fixed-bed hydrotreating unit containing Ni—Mo on silica alumina as hydrotreating catalyst at 150 Kg/cm 2  hydrogen partial pressure, 400° C., liquid hourly space velocity of 1.0 h −1  and a hydrogen feed rate of 1,000 SLt/Lt. The Ni—Mo on alumina catalyst was used to denitrogenize the aromatic-rich fraction, which includes a significant amount of the nitrogen content originally contained in the feedstock. 
     The aromatic-lean fraction and total reactor effluents from the aromatic rich fraction hydroprocessor were combined and sent to the second stage hydroprocessor containing a zeolitic hydrocracking base catalyst designed for nitrogen containing compounds (Ni—Mo on USY Ti—Zr inserted zeolite catalyst) at 120 Kg/cm 2  hydrogen partial pressure, 380° C., and a liquid hourly space velocity of 1.0 h −1 . No additional hydrogen was added to the second stage as the hydrogen coming from the first stage reactor was sufficient. 
     The product yields resulting from each hydroprocesser and the integrated process are given below. 
     
       
         
               
             
               
               
               
               
             
               
               
               
               
             
           
               
                 TABLE 2 
               
             
             
               
                   
               
               
                 Product Yields 
               
             
          
           
               
                   
                   
                 First Stage 
                 Second Stage 
               
               
                   
                 Property 
                 VGO-Aromatic Rich 
                 Overall 
               
               
                   
                   
               
             
          
           
               
                   
                 Stream 
                 154 
                 171 
               
               
                   
                 Hydrogen 
                 2.39 
                 3.57 
               
               
                   
                 H 2 S 
                 2.42 
                 2.87 
               
               
                   
                 NH 3   
                 0.07 
                 0.07 
               
               
                   
                 C 1 -C 4   
                 2.78 
                 4.24 
               
               
                   
                 Naphtha 
                 19.08 
                 28.90 
               
               
                   
                 Mid Distillates 
                 38.04 
                 39.68 
               
               
                   
                 Unconverted Bottoms 
                 40.00 
                 27.81 
               
               
                   
                 Total 
                 102.39 
                 103.57 
               
               
                   
                   
               
             
          
         
       
     
     The method and system herein have been described above and in the attached drawings; however, modifications will be apparent to those of ordinary skill in the art and the scope of protection for the invention is to be defined by the claims that follow.