Patent Abstract:
An apparatus is disclosed for hydrocracking hydrocarbon feed in a hydrocracking unit and hydrotreating a diesel product from the hydrocracking unit in a hydrotreating unit. The hydrocracking unit and the hydrotreating unit shares the same recycle gas compressor. A warm separator separates recycle gas and hydrocarbons from diesel in the hydrotreating effluent, so fraction of the diesel is relatively simple. The warm separator also keeps the diesel product separate from the more sulfurous diesel in the hydrocracking effluent, and still retains heat needed for fractionation of lighter components from the low sulfur diesel product.

Full Description:
CROSS-REFERENCE TO RELATED APPLICATION 
       [0001]    This application is a Continuation of copending application Ser. No. 13/076,631 filed Mar. 31, 2011, the contents of which are hereby incorporated by reference in its entirety. 
     
    
     FIELD OF THE INVENTION 
       [0002]    The field of the invention is the production of diesel by hydrocracking 
       BACKGROUND OF THE INVENTION 
       [0003]    Hydrocracking refers to a process in which hydrocarbons crack in the presence of hydrogen and catalyst to lower molecular weight hydrocarbons. Depending on the desired output, the hydrocracking zone may contain one or more beds of the same or different catalyst. 
         [0004]    Hydrocracking is a process used to crack hydrocarbon feeds such as vacuum gas oil (VGO) to diesel including kerosene and gasoline motor fuels. 
         [0005]    Mild hydrocracking is generally used upstream of a fluid catalytic cracking (FCC) or other process unit to improve the quality of an unconverted oil that can be fed to the downstream unit, while converting part of the feed to lighter products such as diesel. As world demand for diesel motor fuel is growing relative to gasoline motor fuel, mild hydrocracking is being considered for biasing the product slate in favor of diesel at the expense of gasoline. Mild hydrocracking may be operated with less severity than partial or full conversion hydrocracking to balance production of diesel with the FCC unit, which primarily is used to make naphtha. Partial or full conversion hydrocracking is used to produce diesel with less yield of the unconverted oil which can be fed to a downstream unit. 
         [0006]    Due to environmental concerns and newly enacted rules and regulations, saleable diesel must meet lower and lower limits on contaminates, such as sulfur and nitrogen. New regulations require essentially complete removal of sulfur from diesel. For example, the ultra low sulfur diesel (ULSD) requirement is typically less than about 10 wppm sulfur. 
         [0007]    There is a continuing need, therefore, for improved methods of producing more diesel from hydrocarbon feedstocks than gasoline. Such methods must ensure that the diesel product meets increasingly stringent product requirements. 
       BRIEF SUMMARY OF THE INVENTION 
       [0008]    In an apparatus embodiment, the invention comprises an apparatus for producing diesel comprising a hydrocracking reactor in communication with one or more compressors on a make-up hydrogen line and a hydrocarbon feed line for hydrocracking the hydrocarbon stream to lower boiling hydrocarbons. A recycle gas compressor is in communication with the hydrocracking reactor for compressing a vaporous hydrocracking effluent stream comprising hydrogen to provide a recycle hydrogen stream in a recycle hydrogen line. A hydrotreating reactor is in communication with the recycle hydrogen line and the hydrocracking reactor for hydrotreating a diesel stream to produce low sulfur diesel. 
         [0009]    In an additional apparatus embodiment, the invention further comprises a warm separator in communication with the hydrotreating reactor for separating the hydrotreating effluent stream into a vaporous hydrotreating effluent stream comprising hydrogen in an overhead line and a liquid hydrotreating effluent stream in a bottoms line and the recycle gas compressor in communication with the overhead line. 
         [0010]    In a further apparatus embodiment, the invention comprises an apparatus for producing diesel comprising a make-up hydrogen stream with one or more compressors in communication with the make-up hydrogen line for compressing the make-up hydrogen stream. 
         [0011]    A hydrocarbon feed line is for carrying a hydrocarbon stream. A hydrocracking reactor is in communication with the make-up hydrogen line and the hydrocarbon feed line for hydrocracking the hydrocarbon stream to lower boiling hydrocarbons. A recycle gas compressor is in communication with the hydrocracking reactor for compressing a vaporous hydrocracking effluent stream comprising hydrogen to provide a recycle hydrogen stream in a recycle hydrogen line. A fractionation section is in communication with the hydrocracking reactor for fractionating a liquid hydrocracking effluent stream to produce a diesel stream carried in a diesel line. A hydrotreating reactor is in communication with the recycle hydrogen line and the diesel line for hydrotreating the diesel stream to produce low sulfur diesel. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0012]      FIG. 1  is a simplified process flow diagram of an embodiment of the present invention. 
           [0013]      FIG. 2  is a simplified process flow diagram of an alternative embodiment of the present invention. 
       
    
    
     DEFINITIONS 
       [0014]    The term “communication” means that material flow is operatively permitted between enumerated components. 
         [0015]    The term “downstream communication” means that at least a portion of material flowing to the subject in downstream communication may operatively flow from the object with which it communicates. 
         [0016]    The term “upstream communication” means that at least a portion of the material flowing from the subject in upstream communication may operatively flow to the object with which it communicates. 
         [0017]    The term “column” means a distillation column or columns for separating one or more components of different volatilities. Unless otherwise indicated, each column includes a condenser on an overhead of the column to condense and reflux a portion of an overhead stream back to the top of the column and a reboiler at a bottom of the column to vaporize and send a portion of a bottoms stream back to the bottom of the column. Feeds to the columns may be preheated. The top pressure is the pressure of the overhead vapor at the vapor outlet of the column. The bottom temperature is the liquid bottom outlet temperature. Overhead lines and bottoms lines refer to the net lines from the column downstream of the reflux or reboil to the column. 
         [0018]    As used herein, the term “True Boiling Point” (TBP) means a test method for determining the boiling point of a material which corresponds to ASTM D2892 for the production of a liquefied gas, distillate fractions, and residuum of standardized quality on which analytical data can be obtained, and the determination of yields of the above fractions by both mass and volume from which a graph of temperature versus mass % distilled is produced using fifteen theoretical plates in a column with a 5:1 reflux ratio. 
         [0019]    As used herein, the term “conversion” means conversion of feed to material that boils at or below the diesel boiling range. The cut point of the diesel boiling range is between about 343° and about 399° C. (650° to 750° F.) using the True Boiling Point distillation method. 
         [0020]    As used herein, the term “diesel boiling range” means hydrocarbons boiling in the range of between about 132° and about 399° C. (270° to 750° F.) using the True Boiling Point distillation method. 
       DETAILED DESCRIPTION 
       [0021]    Mild hydrocracking reactors operate at low severity and therefore low conversion. 
         [0022]    The diesel produced from mild hydrocracking is not of sufficient quality to meet applicable fuel specifications particularly with regard to sulfur. As a result, the diesel produced from mild hydrocracking must be processed in a hydrotreating unit to allow blending into finished diesel. In many cases, it is attractive to integrate the mild hydrocracking unit and the hydrotreating units to reduce capital and operating costs. 
         [0023]    A typical hydrocracking unit has both a cold separator and a cold flash drum. It often, but not always, has a hot separator and a hot flash drum. A typical hydrotreating unit has only a cold separator. The cold separator may be operated at a lower temperature for obtaining optimal hydrogen separation for use as recycle gas, but this proves thermally inefficient as the hydrotreated liquid stream must be reheated for fractionation to obtain the low sulfur diesel. 
         [0024]    To avoid this cooling and reheating without impacting the hydrogen separation, a hydrotreating unit is utilized in parallel with hydrocracking unit, a common recycle gas compressor and a cold separator. The recycle gas splits to each unit after compression. Make-up gas can be added to the recycle gas stream upstream to the recycle gas compressor. If make-up gas is added downstream of the recycle gas compressor, it should be added solely to the hydrocracking recycle gas to improve hydrogen partial pressure in the hydrocracking reactor. 
         [0025]    The hydrotreating unit may employ a warm separator to extract a warm liquid product and then combine the vaporous hydrotreating effluent phase with the hydrocracking effluent. 
         [0026]    This arrangement allows the hydrotreating and hydrocracking units to operate at similar pressures. Additionally, the vaporous hydrotreating effluent may be sent to the cold separator to further separate hydrogen from hydrocarbon to provide recycle gas. The liquid hydrotreating effluent from the warm separator does not have to be reheated as much before fractionation. Furthermore, the liquid hydrotreating effluent comprises predominantly low sulfur diesel, so fractionation of the low sulfur diesel is simpler. 
         [0027]    The invention involves sending all makeup gas through the hydrocracking unit with recycle gas. The makeup gas addition to the hydrocracking unit is advantageous because the feedstock to the hydrocracking reactor will typically have much higher coke precursors than the diesel feed to the hydrotreating unit which leads to higher catalyst deactivation rates and shorter catalyst life. Using the make-up gas to increase the hydrogen partial pressure in the hydrocracking reactor will render the hydrocracking operation more efficient. 
         [0028]    The apparatus and process  8  for producing diesel comprise a compression section  10 , a hydrocracking unit  12 , a hydrotreating unit  14  and a fractionation zone  16 . Hydrocarbon feed is first fed to the hydrocracking unit  12  and converted to lower boiling hydrocarbons including diesel. The diesel is fractionated in a fractionation section therein and forwarded to the hydrotreating unit  14  to provide lower sulfur diesel. 
         [0029]    A make-up hydrogen stream  20  is fed to a train of one or more compressors  22  and  24  in the compression section  10  to boost the pressure of the make-up hydrogen stream and provide a compressed make-up stream in line  26 . The compressed make-up stream in line  26  may join with a first recycle hydrogen split stream in line  28  to provide a hydrocracking hydrogen stream in line  30 . The hydrocracking hydrogen stream in line  30  taken from the compressed make-up hydrogen stream in line  26  may join a hydrocarbon feed stream in line  32  to provide a hydrocracking feed stream in line  34 . 
         [0030]    The hydrocarbon feed stream is introduced in line  32  perhaps through a surge tank. 
         [0031]    In one aspect, the process and apparatus described herein are particularly useful for hydroprocessing a hydrocarbonaceous feedstock. Illustrative hydrocarbon feedstocks include hydrocarbonaceous streams having components boiling above about 288° C. (550° F.), such as atmospheric gas oils, VGO, deasphalted, vacuum, and atmospheric residua, coker distillates, straight run distillates, solvent-deasphalted oils, pyrolysis-derived oils, high boiling synthetic oils, cycle oils, hydrocracked feeds, cat cracker distillates and the like. These hydrocarbonaceous feed stocks may contain from about 0.1 to about 4 wt-% sulfur. 
         [0032]    A suitable hydrocarbonaceous feedstock is a VGO or other hydrocarbon fraction having at least about  50  percent by weight, and usually at least about 75 percent by weight, of its components boiling at a temperature above about 399° C. (750° F.). A typical VGO normally has a boiling point range between about 315° C. (600° F.) and about 565° C. (1050° F.). 
         [0033]    Hydrocracking refers to a process in which hydrocarbons crack in the presence of hydrogen to lower molecular weight hydrocarbons. A hydrocracking reactor  36  is in downstream communication with the one or more compressors  22  and  24  on the make-up hydrogen line  20  and the hydrocarbon feed line  30 . The hydrocracking feed stream in line  34  may be heat exchanged with a hydrocracking effluent stream in line  38  and further heated in a fired heater before entering the hydrocracking reactor  36  for hydrocracking the hydrocarbon stream to lower boiling hydrocarbons. 
         [0034]    The hydrocracking reactor  36  may comprise one or more vessels, multiple beds of catalyst in each vessel, and various combinations of hydrotreating catalyst and hydrocracking catalyst in one or more vessels. In some aspects, the hydrocracking reaction provides total conversion of at least about 20 vol-% and typically greater than about 60 vol-% of the hydrocarbon feed to products boiling below the diesel cut point. The hydrocracking reactor  42  may operate at partial conversion of more than about 50 vol-% or full conversion of at least about 90 vol-% of the feed based on total conversion. To maximize diesel, full conversion is effective. The first vessel or bed may include hydrotreating catalyst for the purpose of demetallizing, desulfurizing or denitrogenating the hydrocracking feed. 
         [0035]    The hydrocracking reactor  36  may be operated at mild hydrocracking conditions. Mild hydrocracking conditions will provide about 20 to about 60 vol-%, preferably about 20 to about 50 vol-%, total conversion of the hydrocarbon feed to product boiling below the diesel cut point. In mild hydrocracking, converted products are biased in favor of diesel. In a mild hydrocracking operation, the hydrotreating catalyst has just as much or a greater conversion role than hydrocracking catalyst. Conversion across the hydrotreating catalyst may be a significant portion of the overall conversion. If the hydrocracking reactor  36  is intended for mild hydrocracking, it is contemplated that the mild hydrocracking reactor  36  may be loaded with all hydrotreating catalyst, all hydrocracking catalyst, or some beds of hydrotreating catalyst and beds of hydrocracking catalyst. In the last case, the beds of hydrocracking catalyst may typically follow beds of hydrotreating catalyst. Most typically, three beds of hydrotreating catalyst may be followed by zero, one or two 2 beds of hydrocracking catalyst. 
         [0036]    The hydrocracking reactor  36  in  FIG. 1  has four beds in one reactor vessel. If mild hydrocracking is desired, it is contemplated that the first three catalyst beds comprise hydrotreating catalyst and the last catalyst bed comprise hydrocracking catalyst. If partial or full hydrocracking is preferred, additional beds of hydrocracking catalyst may be used than in mild hydrocracking 
         [0037]    At mild hydrocracking conditions, the feed is selectively converted to heavy products such as diesel and kerosene with a low yield of lighter hydrocarbons such as naphtha and gas. Pressure is also moderate to limit the hydrogenation of the bottoms product to an optimal level for downstream processing. 
         [0038]    In one aspect, for example, when a balance of middle distillate and gasoline is preferred in the converted product, mild hydrocracking may be performed in the first hydrocracking reactor  36  with hydrocracking catalysts that utilize amorphous silica-alumina bases or low-level zeolite bases combined with one or more Group VIII or Group VIB metal hydrogenating components. In another aspect, when middle distillate is significantly preferred in the converted product over gasoline production, partial or full hydrocracking may be performed in the first hydrocracking reactor  36  with a catalyst which comprises, in general, any crystalline zeolite cracking base upon which is deposited a Group VIII metal hydrogenating component. Additional hydrogenating components may be selected from Group VIB for incorporation with the zeolite base. 
         [0039]    The zeolite cracking bases are sometimes referred to in the art as molecular sieves and are usually composed of silica, alumina and one or more exchangeable cations such as sodium, magnesium, calcium, rare earth metals, etc. They are further characterized by crystal pores of relatively uniform diameter between about 4 and about 14 Angstroms (10 −10  meters). It is preferred to employ zeolites having a relatively high silica/alumina mole ratio between about 3 and about 12. Suitable zeolites found in nature include, for example, mordenite, stilbite, heulandite, ferrierite, dachiardite, chabazite, erionite and faujasite. Suitable synthetic zeolites include, for example, the B, X, Y and L crystal types, e.g., synthetic faujasite and mordenite. The preferred zeolites are those having crystal pore diameters between about 8-12 Angstroms (10 −10  meters), wherein the silica/alumina mole ratio is about 4 to 6. One example of a zeolite falling in the preferred group is synthetic Y molecular sieve. 
         [0040]    The natural occurring zeolites are normally found in a sodium form, an alkaline earth metal form, or mixed forms. The synthetic zeolites are nearly always prepared first in the sodium form. In any case, for use as a cracking base it is preferred that most or all of the original zeolitic monovalent metals be ion-exchanged with a polyvalent metal and/or with an ammonium salt followed by heating to decompose the ammonium ions associated with the zeolite, leaving in their place hydrogen ions and/or exchange sites which have actually been decationized by further removal of water. Hydrogen or “decationized” Y zeolites of this nature are more particularly described in U.S. Pat. No. 3,130,006. 
         [0041]    Mixed polyvalent metal-hydrogen zeolites may be prepared by ion-exchanging first with an ammonium salt, then partially back exchanging with a polyvalent metal salt and then calcining In some cases, as in the case of synthetic mordenite, the hydrogen forms can be prepared by direct acid treatment of the alkali metal zeolites. In one aspect, the preferred cracking bases are those which are at least about 10 percent, and preferably at least about 20 percent, metal-cation-deficient, based on the initial ion-exchange capacity. In another aspect, a desirable and stable class of zeolites is one wherein at least about 20 percent of the ion exchange capacity is satisfied by hydrogen ions. 
         [0042]    The active metals employed in the preferred hydrocracking catalysts of the present invention as hydrogenation components are those of Group VIII, i.e., iron, cobalt, nickel, ruthenium, rhodium, palladium, osmium, iridium and platinum. In addition to these metals, other promoters may also be employed in conjunction therewith, including the metals of Group VIB, e.g., molybdenum and tungsten. The amount of hydrogenating metal in the catalyst can vary within wide ranges. Broadly speaking, any amount between about 0.05 percent and about 30 percent by weight may be used. In the case of the noble metals, it is normally preferred to use about 0.05 to about 2 wt-%. 
         [0043]    The method for incorporating the hydrogenating metal is to contact the base material with an aqueous solution of a suitable compound of the desired metal wherein the metal is present in a cationic form. Following addition of the selected hydrogenating metal or metals, the resulting catalyst powder is then filtered, dried, pelleted with added lubricants, binders or the like if desired, and calcined in air at temperatures of, e.g., about 371° to about 648° C. (about 700° to about 1200° F.) in order to activate the catalyst and decompose ammonium ions. Alternatively, the base component may first be pelleted, followed by the addition of the hydrogenating component and activation by calcining 
         [0044]    The foregoing catalysts may be employed in undiluted form, or the powdered catalyst may be mixed and copelleted with other relatively less active catalysts, diluents or binders such as alumina, silica gel, silica-alumina cogels, activated clays and the like in proportions ranging between about 5 and about 90 wt-%. These diluents may be employed as such or they may contain a minor proportion of an added hydrogenating metal such as a Group VIB and/or Group VIII metal. Additional metal promoted hydrocracking catalysts may also be utilized in the process of the present invention which comprises, for example, aluminophosphate molecular sieves, crystalline chromosilicates and other crystalline silicates. Crystalline chromosilicates are more fully described in U.S. Pat. No. 4,363,718. 
         [0045]    By one approach, the hydrocracking conditions may include a temperature from about 290° C. (550° F.) to about 468° C. (875° F.), preferably 343° C. (650° F.) to about 435° C. (815° F.), a pressure from about 3.5 MPa (500 psig) to about 20.7 MPa (3000 psig), a liquid hourly space velocity (LHSV) from about 1.0 to less than about 2.5 hr − and a hydrogen rate of about 421 to about 2,527 Nm 3 /m 3  oil (2,500-15,000 scf/bbl). If mild hydrocracking is desired, conditions may include a temperature from about 315° C. (600° F.) to about 441° C. (825° F.), a pressure from about 5.5 to about 13.8 MPa (gauge) (800 to 2000 psig) or more typically about 6.9 to about 11.0 MPa (gauge) (1000 to 1600 psig), a liquid hourly space velocity (LHSV) from about 0.5 to about 2 hr −1  and preferably about 0.7 to about 1.5 hr −1  and a hydrogen rate of about 421 to about 1,685 Nm 3 /m 3  oil (2,500-10,000 scf/bbl). 
         [0046]    A hydrocracking effluent exits the hydrocracking reactor  36  in line  38 . The hydrocracking effluent in line  38  is heat exchanged with the hydrocracking feed in line  34  and in an embodiment may be cooled before entering a cold separator  40 . The cold separator  40  is in downstream communication with the hydrocracking reactor  36 . The cold separator may be operated at about 46° to about 63° C. (115° to 145° F.) and just below the pressure of the hydrocracking reactor  36  accounting for pressure drop to keep hydrogen and light gases in the overhead and normally liquid hydrocarbons in the bottoms. The cold separator  40  provides a vaporous hydrocracking effluent stream comprising hydrogen in an overhead line  42  and a liquid hydrocracking effluent stream in a bottoms line  44 . The cold separator also has a boot for collecting an aqueous phase in line  46 . 
         [0047]    The vaporous hydrocracking effluent stream in line  42  may be compressed in a recycle gas compressor  50  to provide a recycle hydrogen stream in line  52  which is a compressed vaporous hydrocracking effluent stream. The recycle gas compressor  50  may be in downstream communication with the hydrocracking reactor  36 . A split  54  on the recycle hydrogen line  52  provides the first recycle hydrogen split stream in line  28  in upstream communication with the hydrocracking reactor  36  and a second recycle hydrogen split stream in line  56  in upstream communication with a hydrotreating reactor  92 . 
         [0048]    As previously explained, in an embodiment, the first recycle split stream in line  28  may join with compressed make-up hydrogen stream in line  26  downstream of the recycle gas compressor  50 . However, if the pressure of the recycle hydrogen stream in line  52  is too great to admit the make-up hydrogen stream without adding more compressors on the make-up hydrogen line  20 , the make-up hydrogen stream may be added to the vaporous hydrocracking effluent stream in line  42  upstream of the recycle gas compressor  50 . However, this would increase the duty on the recycle gas compressor  50  due to greater throughput. 
         [0049]    It is also preferred that the compressed make-up hydrogen stream in line  26  join the recycle gas stream downstream of the split  54 , so the make-up hydrogen will be directed to supplying the hydrogen requirements to the hydrocracking reactor  36  not filled by the recycle hydrogen stream in line  52 . It is contemplated that the compressed make-up hydrogen stream in line  26  will join the recycle gas stream upstream of the split  54 , but this would allow make-up gas to go to the hydrotreating unit  14  as well as to the hydrocracking unit  12 . The hydrocarbon feed to the hydrocracking reactor  36  will have much higher coke precursors than the feed to the hydrotreating unit  14 . Hence, using the make-up hydrogen to increase the hydrogen partial pressure in the hydrocracking reactor  36  will enable the catalyst in the hydrocracking reactor to endure more heartily the more deleterious components in the feed. It is also contemplated, but not preferred, that at least a portion of the compressed make-up hydrogen stream in line  26  will travel through line  28  to the split  54  and be mixed with recycle gas stream in line  52  and supply make-up gas to the hydrotreating unit  14  as well as to the hydrocracking unit  12 . 
         [0050]    At least a portion of the hydrocracking effluent stream  38  may be fractionated in a fractionation section  16  in downstream communication with the hydrocracking reactor  36  to produce a diesel stream in line  86 . In an aspect, the liquid hydrocracking effluent stream  44  may be fractionated in the fractionation section  16 . In a further aspect, the fractionation section  16  may include a cold flash drum  48 . The liquid hydrocracking effluent stream  44  may be flashed in the cold flash drum  48  which may be operated at the same temperature as the cold separator  40  but at a lower pressure of between about 1.4 MPa and about 3.1 MPa (gauge) (200-450 psig) to provide a light liquid stream in a bottoms line  62  from the liquid hydrocracking effluent stream and a light ends stream in an overhead line  64 . The aqueous stream in line  46  from the boot of the cold separator may also be directed to the cold flash drum  48 . A flash aqueous stream is removed from a boot in the cold flash drum  48  in line  66 . The light liquid stream in bottoms line  62  may be further fractionated in the fractionation section  16 . 
         [0051]    The fractionation section  16  may include a stripping column  70  and a fractionation column  80 . The light liquid stream in bottoms line  62  may be heated and fed to the stripping column  70 . The light liquid stream which is liquid hydrocracking effluent may be stripped with steam from line  72  to provide a light ends stream of hydrogen, hydrogen sulfide, steam and other gases in an overhead line  74 . A portion of the light ends stream may be condensed and refluxed to the stripper column  70 . The stripping column  70  may be operated with a bottoms temperature between about 232° and about 288° C. (450° to 550° F.) and an overhead pressure of about 690 to about 1034 kPa (gauge) (100 to 150 psig). A hydrocracked bottoms stream in line  76  may be heated in a fired heater and fed to the fractionation column  80 . 
         [0052]    The fractionation column  80  may also strip the hydrocracked bottoms with steam from line  82  to provide an overhead naphtha stream in line  84 , a diesel stream in line  86  from a side cut and an unconverted oil stream in line  88  which may be suitable for further processing, such as in an FCC unit. The overhead naphtha stream in line  84  may require further processing before blending in the gasoline pool. It will usually require catalytic reforming to improve the octane number. The reforming catalyst will often require the overhead naphtha to be further desulfurized in a naphtha hydrotreater prior to reforming. In an aspect, the hydrocracked naphtha may be desulfurized in an integrated hydrotreater  92 . It is also contemplated that a further side cut be taken to provide a separate light diesel or kerosene stream taken above a heavy diesel stream taken in line  86 . A portion of the overhead naphtha stream in line  84  may be condensed and refluxed to the fractionation column  80 . The fractionation column  80  may be operated with a bottoms temperature between about 288° and about 385° C. (550° to 725° F.), preferably between about 315° and about 357° C. (600° to 675° F.) and at or near atmospheric pressure. A portion of the hydrocracked bottoms may be reboiled and returned to the fractionation column  80  instead of using steam stripping. 
         [0053]    The diesel stream in line  86  is reduced in sulfur content but may not meet a low sulfur diesel (LSD) specification which is less than  50  wppm sulfur, an ULSD specification which is less than  10  wppm sulfur, or other specifications. Hence, it must be further finished in the diesel hydrotreating unit  14 . 
         [0054]    The diesel stream in line  86  may be joined by the second recycle hydrogen split stream in line  56  to provide a hydrotreating feed stream  90 . The diesel stream in line  86  may also be mixed with a co-feed that is not shown. The hydrotreating feed stream  90  may be heat exchanged with the hydrotreating effluent in line  94 , further heated in a fired heater and directed to a hydrotreating reactor  92 . Consequently, the hydrotreating reactor is in downstream communication with the fractionation section  16 , the recycle hydrogen line  52  and the hydrocracking reactor  36 . In the hydrotreating reactor  92 , the diesel stream is hydrotreated in the presence of a hydrotreating hydrogen stream and hydrotreating catalyst to provide a hydrotreating effluent stream  94 . In an aspect, all of the hydrotreating hydrogen stream is provided from the recycle hydrogen stream in line  52  via second recycle hydrogen split stream  56 . 
         [0055]    The hydrotreating reactor  92  may comprise more than one vessel and multiple beds of catalyst. The hydrotreating reactor  92  in  FIG. 1  has two beds in one reactor vessel. In the hydrotreating reactor, hydrocarbons with heteroatoms are further demetallized, desulfurized and denitrogenated. The hydrotreating reactor may also contain hydrotreating catalyst that is suited for saturating aromatics, hydrodewaxing and hydroisomerization. 
         [0056]    If the hydrocracking reactor  36  is operated as a mild hydrocracking reactor, the hydrocracking reactor may operate to convert up to about 20-60 vol-% of feed boiling above diesel boiling range to product boiling in the diesel boiling range. Consequently, the hydrotreating reactor  92  should have very low conversion and is primarily for desulfurization if integrated with a mild hydrocracking reactor  36  to meet fuel specifications such as qualifying as ULSD. 
         [0057]    Hydrotreating is a process wherein hydrogen gas is contacted with hydrocarbon in the presence of suitable catalysts which are primarily active for the removal of heteroatoms, such as sulfur, nitrogen and metals from the hydrocarbon feedstock. In hydrotreating, hydrocarbons with double and triple bonds may be saturated. Aromatics may also be saturated. Some hydrotreating processes are specifically designed to saturate aromatics. Cloud point of the hydrotreated product may also be reduced. Suitable hydrotreating catalysts for use in the present invention are any known conventional hydrotreating catalysts and include those which are comprised of at least one Group VIII metal, preferably iron, cobalt and nickel, more preferably cobalt and/or nickel and at least one Group VI metal, preferably molybdenum and tungsten, on a high surface area support material, preferably alumina. Other suitable hydrotreating catalysts include zeolitic catalysts, as well as noble metal catalysts where the noble metal is selected from palladium and platinum. It is within the scope of the present invention that more than one type of hydrotreating catalyst be used in the same hydrotreating reactor  92 . The Group VIII metal is typically present in an amount ranging from about 2 to about 20 wt-%, preferably from about 4 to about 12 wt-%. The Group VI metal will typically be present in an amount ranging from about 1 to about 25 wt-%, preferably from about 2 to about 25 wt-%. 
         [0058]    Preferred hydrotreating reaction conditions include a temperature from about 290° C. (550° F.) to about 455° C. (850° F.), suitably 316° C. (600° F.) to about 427° C. (800° F.) and preferably 343° C. (650° F.) to about 399° C. (750° F.), a pressure from about 4.1 MPa (600 psig), preferably 6.2 MPa (900 psig) to about 13.1 MPa (1900 psig), a liquid hourly space velocity of the fresh hydrocarbonaceous feedstock from about 0.5 hr −1   to about 4 hr −1 , preferably from about 1.5 to about 3.5 hr −1 , and a hydrogen rate of about 168 to about 1,011 Nm 3 /m 3  oil (1,000-6,000 scf/bbl), preferably about 168 to about 674 Nm 3 /m 3  oil (1,000-4,000 scf/bbl) for diesel feed, with a hydrotreating catalyst or a combination of hydrotreating catalysts. The hydrotreating unit  14  is integrated with the hydrocracking unit  12 , so they both operate at about the same pressure accounting for normal pressure drop. 
         [0059]    The hydrotreating effluent stream in line  94  may be heat exchanged with the hydrotreating feed stream in line  90 . The hydrotreating effluent stream in line  94  may be separated in a warm separator  96  to provide a vaporous hydrotreating effluent stream comprising hydrogen in an overhead line  98  and a liquid hydrotreating effluent stream in a bottoms line  100 . The vaporous hydrotreating effluent stream comprising hydrogen may be mixed with the hydrocracking effluent stream in line  38  perhaps prior to cooling and enter into the cold separator  40 . The warm separator  96  may be operated between about 149° and about 260° C. (300° to 500° F.). The pressure of the warm separator  96  is just below the pressure of the hydrotreating reactor  96  accounting for pressure drop. The warm separator may be operated to obtain at least 90 wt-% diesel and preferably at least 93 wt-% diesel in the liquid stream in line  100 . All of the other hydrocarbons and gases go up in the vaporous hydrotreating effluent stream in line  98  which joins the hydrocracking effluent in line  38  and may be processed after heating therewith first by entering the cold separator  40 . Consequently, the cold separator  40  and, thereby, the recycle gas compressor  50  are in downstream communication with the warm separator overhead line  98 . Accordingly, recycle gas loops from both the hydrocracking section  12  and the hydrotreating section  14  share the same recycle gas compressor  50 . 
         [0060]    The liquid hydrotreating effluent stream in line  100  may be fractionated in a hydrotreating stripper column  102 . In an aspect, fractionation of the liquid hydrotreating effluent stream in line  100  may include flashing it in a warm flash drum  104  which may be operated at the same temperature as the warm separator  96  but at a lower pressure of between about 1.4 MPa and about 3.1 MPa (gauge) (200-450 psig). A warm flash overhead stream in line  106  may be joined to the liquid hydrocracking effluent stream in bottoms line  44  for further fractionation therewith. The warm flash bottoms stream in line  108  may be heated and fed to the stripper column  102 . The warm flash bottoms may be stripped in the stripper column  102  with steam from line  110  to provide a naphtha and light ends stream in overhead line  112 . The naphtha and light ends stream in line  112  may be fed to the fractionation section  16  and specifically to the stripping column  70  at an elevation above the feed point of light liquid stream in line  62 . A product diesel stream is recovered in bottoms line  114  comprising less than  50  wppm sulfur qualifying it as LSD and preferably less than  10  wppm sulfur qualifying it as ULSD. It is contemplated that the stripper column  102  may be operated as a fractionation column with a reboiler instead of with stripping steam. 
         [0061]    By operating the warm separator  96  at elevated temperature to reject most hydrocarbons lighter than diesel, the hydrotreating stripping column  102  may be operated more simply because it is not relied upon to separate naphtha from lighter components and because there is very little naphtha to separate from the diesel. Moreover, the warm separator  96  makes sharing of a cold separator  40  with the hydrocracking reactor  36  possible and heat useful for fractionation in the stripper column  102  is retained in the hydrotreating liquid effluent. 
         [0062]      FIG. 2  illustrates an embodiment of a process and apparatus  8 ′ that utilizes a hot separator  120  to initially separate the hydrocracking effluent in line  38 ′. Many of the elements in  FIG. 2  have the same configuration as in  FIG. 1  and bear the same reference number. Elements in  FIG. 2  that correspond to elements in  FIG. 1  but have a different configuration bear the same reference numeral as in  FIG. 1  but are marked with a prime symbol (′). 
         [0063]    The hot separator  120  in the hydrocracking section  12 ′ is in downstream communication with the hydrocracking reactor  36  and provides a vaporous hydrocarbonaceous stream in an overhead line  122  and a liquid hydrocarbonaceous stream in a bottoms line  124 . The hot separator  120  operates at about 177° to about 343° C. (350° to 650° F.) and preferably operates at about 232° to about 288° C. (450° to 550° F.). The hot separator may be operated at a slightly lower pressure than the hydrocracking reactor  36  accounting for pressure drop. The vaporous hydrocarbonaceous stream in line  122  may be joined by the vaporous hydrotreating effluent stream in line  98 ′ from the hydrotreating section  14 ′ and be mixed and transported together in line  126 . The mixed stream in line  126  may be cooled before entering the cold separator  40 . Consequently, the vaporous hydrocracking effluent may be separated along with the vaporous hydrotreating effluent stream in the cold separator  40  to provide the vaporous hydrocracking effluent comprising hydrogen in line  42  and the liquid hydrocracking effluent in line  44  and which are processed as previously described with respect to  FIG. 1 . The cold separator  40 , therefore, is in downstream communication with the overhead line  122  of the hot separator  120  and an overhead line  98 ′ of the warm separator  96 . 
         [0064]    The liquid hydrocarbonaceous stream in bottoms line  124  may be fractionated in the fractionation section  16 ′. In an aspect, the liquid hydrocarbonaceous stream in line  124  may be flashed in a hot flash drum  130  to provide a light ends stream in an overhead line  132  and a heavy liquid stream in a bottoms line  134 . The hot flash drum  130  may be operated at the same temperature as the hot separator  120  but at a lower pressure of between about 1.4 MPa and about 3.1 MPa (gauge) (200 to 450 psig). The heavy liquid stream in bottoms line  134  may be further fractionated in the fractionation section  16 ′. In an aspect, the heavy liquid stream in line  134  may be introduced into the stripping column  70  at a lower elevation than the feed point light liquid stream in line  62 . 
         [0065]    The rest of the embodiment in  FIG. 2  may be the same as described for  FIG. 1  with the previous noted exceptions. 
         [0066]    Preferred embodiments of this invention are described herein, including the best mode known to the inventors for carrying out the invention. It should be understood that the illustrated embodiments are exemplary only, and should not be taken as limiting the scope of the invention. 
         [0067]    Without further elaboration, it is believed that one skilled in the art can, using the preceding description, utilize the present invention to its fullest extent. The preceding preferred specific embodiments are, therefore, to be construed as merely illustrative, and not limitative of the remainder of the disclosure in any way whatsoever. 
         [0068]    In the foregoing, all temperatures are set forth in degrees Celsius and, all parts and percentages are by weight, unless otherwise indicated. Pressures are given at the vessel outlet and particularly at the vapor outlet in vessels with multiple outlets. 
         [0069]    From the foregoing description, one skilled in the art can easily ascertain the essential characteristics of this invention and, without departing from the spirit and scope thereof, can make various changes and modifications of the invention to adapt it to various usages and conditions.

Technology Classification (CPC): 2