Abstract:
This invention is directed to a process scheme in which a partial conversion hydrocracking (HCR) unit, preferably preceded by a hydrotreating unit, feeds unconverted oil to a FCC (fluid catalytic cracking ) unit. Most refineries run the FCC unit at the full capacity for optimal asset utilization. During shutdowns of Residue Desulfurization unit(s) which feed an FCC unit, it is desirable to reduce the conversion in the FCC feed hydrocracker. In this way, the feed to FCC unit is maximized. Jet and Diesel products that conform to specifications may be produced during low conversion HCR operation. Furthermore, undesirable over-saturation of the unconverted oil (UCO) from the HCR unit feeding the FCC unit can be avoided. Excess hydrogen consumption can also be avoided. Normally, further aromatic saturation of the middle distillate products from a low conversion HCR is achieved in a separate, post treatment, unit.

Description:
FIELD OF THE INVENTION 
       [0001]    This invention is directed to a partial conversion hydrocracking (HCR) unit, in which unconverted oil is fed to a Fluid Catalytic Cracking (FCC) unit. 
       BACKGROUND OF THE INVENTION 
       [0002]    In the refining of crude oil, vacuum gas oil hydrotreaters and hydrocrackers are employed to remove impurities such as sulfur, nitrogen and metals from the feed. Typically, the middle distillate boiling material (boiling in the range from 250° F.-735° F.) from VGO hydrotreating or moderate severity hydrocrackers does not meet the smoke point, the cetane number or the aromatic specification required. 
         [0003]    Removal of these impurities in subsequent hydroprocessing stages (often known as upgrading), creates more valuable middle distillate products. Hydroprocessing technology (which encompasses hydrotreating, hydrocracking and hydrodewaxing processes) aims to increase the value of the crude oil by fundamentally rearranging molecules. The end products are also made more environmentally friendly. 
         [0004]    In most cases, this middle distillate is separately upgraded by a middle distillate hydrotreater or, alternatively, the middle distillate is blended into the general fuel oil pool or used as home heating oil. Recently hydroprocessing schemes have been developed which permit the middle distillate to be hydrotreated in the same high pressure loop as the vacuum gas oil hydrotreating reactor or the moderate severity hydrocracking reactor. The investment cost saving and/or utilities saving are significant since a separate middle distillate hydrotreater is not required. 
         [0005]    There are U.S. patents which are directed to multistage hydroprocessing within a single high pressure hydrogen loop. In U.S. Pat. No. 6,797,154, high conversion of heavy gas oils and the production of high quality middle distillate products are possible in a single high-pressure loop with reaction stages operating at different pressure and conversion levels. The flexibility offered is great and allows the refiner to avoid decrease in product quality while at the same time minimizing capital cost. Feeds with varying boiling ranges are introduced at different sections of the process, thereby minimizing the consumption of hydrogen and reducing capital investment. 
         [0006]    U.S. Pat. No. 6,787,025 also discloses multi-stage hydroprocessing for the production of middle distillates. A major benefit of this invention is the potential for simultaneously upgrading difficult cracked stocks such as Light Cycle Oil, Light Coker Gas Oil and Visbroken Gas Oil or Straight-Run Atmospheric Gas Oils utilizing the high-pressure environment required for mild hydrocracking. 
         [0007]    U.S. Pat. No. 7,238,277 provides very high to total conversion of heavy oils to products in a single high-pressure loop, using multiple reaction stages. The second stage or subsequent stages may be a combination of co-current and counter-current operation. The benefits of this invention include conversion of feed to useful products at reduced operating pressures using lower catalyst volumes. Lower hydrogen consumption also results. A minimal amount of equipment is employed. Utility consumption is also minimized. 
         [0008]    U.S. Publication 20050103682 relates to a multi-stage process for hydroprocessing gas oils. Preferably, each stage possesses at least one hydrocracking zone. The second stage and any subsequent stages possess an environment having a low heteroatom content. Light products, such as naphtha, kerosene and diesel, may be recycled from fractionation (along with light products from other sources) to the second stage (or a subsequent stage) in order to produce a larger yield of lighter products, such as gas and naphtha. Pressure in the zone or zones subsequent to the initial zone is from 500 to 1000 psig lower than the pressure in the initial zone, in order to provide cost savings and minimize overcracking. 
         [0009]    Most refineries run the FCC unit at full capacity for optimal asset utilization. During planned and/or unplanned shutdown of Residue Desulfurization unit(s) feeding FCC unit, it is desirable to reduce the conversion in the FCC feed hydrocracker in order to maximize the feed to FCC unit. The patents disclosed above do not address the following issues:
       1. Production of on-specification Jet and Diesel products during low conversion HCR operation.   2. Avoidance of undesirable over-saturation of the unconverted oil (UCO) from the HCR unit feeding FCC unit and reduce hydrogen consumption. Normally, further aromatic saturation of the middle distillate products from a low conversion HCR is achieved in a separate Post Treatment unit.       
 
       SUMMARY OF THE INVENTION 
       [0012]    A new process scheme has been developed to design a partial conversion hydrocracking (HCR) unit, feeding the unconverted oil to a FCC unit. The steps of this invention are summarized as follows:
       A method for hydroprocessing a hydrocarbon feedstock, said method employing multiple hydroprocessing zones within a single reaction loop, each zone having one or more catalyst beds, comprising the following steps:
           (a) passing a hydrocarbonaceous feedstock to a first hydroprocessing zone having one or more beds containing hydroprocessing catalyst, the hydroprocessing zone being maintained at hydroprocessing conditions, wherein the feedstock is contacted with catalyst and hydrogen;   (b) passing the effluent of step (a) directly to a hot high pressure separator, wherein the effluent is separated to produce a vapor stream comprising hydrogen, hydrocarbonaceous compounds boiling at a temperature below the boiling range of the hydrocarbonaceous feedstock, hydrogen sulfide and ammonia and a liquid stream comprising hydrocarbonaceous compounds boiling approximately in the range of said hydrocarbonaceous feedstock;   (c) passing the vapor stream of step (b) after cooling and partial condensation, to a hot high pressure separator where it is flashed, thereby producing an overhead vapor stream and a liquid stream, wherein the liquid stream, which comprises hydrotreated hydrocarbons in the middle distillate range, is passed to a second hydroprocessing zone;   (d) passing the overhead vapor stream from the hot high pressure separator of step (c), after cooling and contact with water, said vapor stream comprising hydrogen, ammonia, hydrogen sulfide, light gases and naphtha, to a cold high pressure separator, where hydrogen, hydrogen sulfide, and light hydrocarbonaceous gases are removed overhead, ammonia is removed from the cold high pressure separator as ammonium bisulfide in the sour water stripper, and naphtha and middle distillates are passed to fractionation   (e) passing the liquid stream from the hot high pressure separator of step (b) to a hot low pressure separator, where it is flashed to produce an overhead stream comprising gases and a liquid stream comprising unconverted oil;   (f) passing the liquid stream of step (e) which comprises unconverted oil, to a steam stripper, where lighter material is removed overhead as a vapor stream, and a liquid stream, which comprises stripped unconverted oil, is recovered.   
               
 
     
    
     
       BRIEF DESCRIPTION OF THE FIGURE 
         [0020]    The FIGURE illustrates the flow scheme of the current invention. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0021]    Feeds 
         [0022]    A wide variety of hydrocarbon feeds may be used in the instant invention. Typical feedstocks include any heavy or synthetic oil fraction or process stream having a boiling point above 392° F. (200° C.). Such feedstocks include vacuum gas oils (VGO), heavy coker gas oil (HCGO), heavy atmospheric gas oil (AGO), light coker gas oil (LCGO), visbreaker gas oil (VBGO), demetallized oils (DMO), vacuum residua, atmospheric residua, deasphalted oil (DAO), Fischer-Tropsch streams, Light Cycle Oil, Light Cycle Gas Oil and other FCC product streams. 
         [0023]    Products 
         [0024]    The process of this invention is especially useful in the production of middle distillate fractions boiling in the range of about 250-700° F. (121-371° C.). A middle distillate fraction is defined as having an approximate boiling range from about 250 to 700° F. At least 75 vol. %, preferably 85 vol. % of the components of the middle distillate have a normal boiling point of greater than 250° F. At least about 75 vol. %, preferably 85 vol. % of the components of the middle distillate have a normal boiling point of less than 700° F. The term “middle distillate” includes the diesel, jet fuel and kerosene boiling range fractions. The kerosene or jet fuel boiling point range refers to the range between 280 and 525° F. (138-274° C.). The term “diesel boiling range” refers to hydrocarbons boiling in the range from 250 to 700° F. (121-371° C.). 
         [0025]    Gasoline or naphtha may also be produced in the process of this invention. Gasoline or naphtha normally boils in the range below 400° F. (204° C.), or C 5  to 400° F. Boiling ranges of various product fractions recovered in any particular refinery will vary with such factors as the characteristics of the crude oil source, local refinery markets and product prices. 
         [0026]    Conditions 
         [0027]    “Hydroprocessing conditions” is a general term which refers primarily in this application to hydrocracking or hydrotreating. 
         [0028]    Hydrotreating conditions include a reaction temperature between 400° F.-950° F. (204° C.-482° C.), preferably 600° F.-850° F. (315° C.-464° C.); a pressure between 500 to 5000 psig (pounds per square inch gauge) (3.5-34.6 MPa), preferably 1000 to 3000 psig (7.0-20.8 MPa): a feed rate (LHSV) of 0.3 hr-1 to 20 hr-1 (v/v) preferably from 0.5 to 4.0; and overall hydrogen consumption 300 to 2000 SCF per barrel of liquid hydrocarbon feed (63.4-356 m 3 /m 3  feed). 
         [0029]    Typical hydrocracking conditions include a reaction temperature of from 400° F.-950° F. (204° C.-510° C.), preferably 650° F.-850° F. (315° C.-454° C.). Reaction pressure ranges from 500 to 5000 psig (3.5-4.5 MPa), preferably 1000-3000 psig (7.0-20.8 MPa). LHSV ranges from 0.1 to 15 hr-1 (v/v), preferably 0.5 to 5.0 hr-1. Hydrogen consumption ranges from 500 to 2500 SCF per barrel of liquid hydrocarbon feed (89.1-445 m 3 H 2 /m 3  feed). 
         [0030]    Catalyst 
         [0031]    A hydroprocessing zone may contain only one catalyst, or several catalysts in combination. 
         [0032]    The hydrocracking catalyst generally comprises a cracking component, a hydrogenation component and a binder. Such catalysts are well known in the art. The cracking component may include an amorphous silica/alumina phase and/or a zeolite, such as a Y-type or USY zeolite. Catalysts having high cracking activity often employ REX, REY and USY zeolites. The binder is generally silica or alumina. The hydrogenation component will be a Group VI, Group VlI, or Group VIII metal or oxides or sulfides thereof, preferably one or more of molybdenum, tungsten, cobalt, or nickel, or the sulfides or oxides thereof. If present in the catalyst, these hydrogenation components generally make up from about 5% to about 40% by weight of the catalyst. Alternatively, platinum group metals, especially platinum and/or palladium, may be present as the hydrogenation component, either alone or in combination with the base metal hydrogenation components molybdenum, tungsten, cobalt, or nickel. If present, the platinum group metals will generally make up from about 0.1% to about 2% by weight of the catalyst. 
         [0033]    Hydrotreating catalyst is typically a composite of a Group VI metal or compound thereof, and a Group VIII metal or compound thereof supported on a porous refractory base such as alumina. Examples of hydrotreating catalysts are alumina supported cobalt-molybdenum, nickel sulfide, nickel-tungsten, cobalt-tungsten and nickel-molybdenum. Typically, such hydrotreating catalysts are presulfided. 
         [0034]    In some cases, high activity hydrotreating catalyst suitable for high levels of hydrogenation, is employed. Such catalysts have high surface areas (greater than 140 m.sup.2/gm) and high densities (0.7-0.95 gm/cc). The high surface area increases reaction rates due to generally increased dispersion of the active components. Higher density catalysts allow one to load a larger amount of active metals and promoter per reactor volume, a factor which is commercially important. Since deposits of coke are thought to cause the majority of the catalyst deactivation, the catalyst pore volume should be maintained at a modest level (0.4-0.6). A high activity catalyst is at times desired in order to reduce the required operating temperatures. High temperatures lead to increased coking. 
       Description of the Preferred Embodiment 
       [0035]    Please refer to the FIGURE: 
         [0036]    In this process scheme, fresh feed (Stream  9 ) is passed to the top of fixed bed hydrotreater reactor  10 . Hydrogen passes through stream  1 . Stream  29  is a sidestream from stream  1 . From stream  29 , streams  3  and  4  add hydrogen in between the first and second beds, and second and third beds of reactor  10  respectively. Hydrotreater  10  is loaded with a high activity hydrotreating catalyst, where most of the feed impurities (heteroatoms) such as nitrogen, sulfur, etc. are removed and some degree of aromatic saturation is achieved. 
         [0037]    The hydrotreated reactor effluent (stream  12 ) exchanges heat in exchanger  5  with the reactor feed (stream  2  prior to entering the exchanger  5  and stream  9  upon leaving the exchanger  5 ). Stream  12  is flashed in hot high pressure separator  40  at high temperature and pressure conditions to recover most of the unconverted oil (UCO) components in the liquid phase (stream  13 ). Vapor leaves separator  40  overhead in line  22 , and heat is exchanged with hydrogen stream  31  in exchanger  25 . Stream  22 , which is made up of more than 85 wt % diesel and lighter material, preheats the fractionator feed (not shown in the FIGURE) and generates high pressure steam. Stream  22  is finally cooled to about 200° C. in the hot high pressure separator vapor/recycle gas exchanger  25 . Stream  22  is then flashed in hot high pressure separator  50 . At these relatively high pressure and low temperature conditions, most of the hydrotreated jet and diesel range material is recovered as liquid stream  27  at high pressure, which is pumped(pump  35 ) to the feedstream (stream  11 ), which passes to hydrocracking reactor  20 , for further processing. The overhead vapor from the hot high pressure separator  50 , stream  23 , is then cooled in an air cooler (not shown) before entering a cold high pressure separator (not shown). The overhead vapor stream, stream  23 , comprises hydrogen, ammonia, and hydrogen sulfide, along with light gases and naphtha. In the cold high pressure separator (not shown) hydrogen, hydrogen sulfide, and light hydrocarbonaceous gases are removed overhead, ammonia is removed from the cold high pressure separator as ammonium bisulfide in the sour water stripper. Naphtha and middle distillates are passed to fractionation. 
         [0038]    Stream  13  passes to hot low pressure separator  60 , where it is flashed. Vapor is removed as stream  28 . The hot low pressure separator bottoms are removed as stream  73  and passed to UCO (unconverted oil) stripper  30 . The material of stream  73  is stream stripped in stripper  30  to recover any lighter material in the UCO stream. Lighter material is removed as stream  26 . Jet and diesel range material is withdrawn as a side draw  17  from the column. Side draw  17  combines with stream  19 , stripper bottoms  16  (UCO) to become stream  19 . A side stream  18  may be taken from bottoms stream  16 . Stream  19 , recycle oil, is pumped, via pump  45 , to storage drum  70 . The recycle oil exits storage drum  70  through stream  21  and is pumped, by means of pump  55 , to stream  11 . Stream  11  is heated in exchanger  15  prior to entering hydrocracking reactor  20  for further aromatic saturation. The overhead liquid stream  26  from the UCO stripper  30  is sent to the main product stripper, and the offgas is sent to fuel gas (not shown). 
         [0039]    The hydrotreated, stripped UCO (stream  16 ) from the bottom of the UCO stripper, is an excellent quality FCC feed. At this point, a part of stripped unconverted oil (stream  18 ) is sent out as FCC feed. Further saturation of the FCC feed is thus avoided. Only a limited portion of the UCO (mixed with stream 19 , is passed to hydrocracker  20  for further saturation of aromatic components and conversion to distillate products. The amount recycled back is based on the desired overall conversion level. 
         [0040]    The second stage hydrocracking reactor  20  is loaded with hydrocracking catalyst and operates under a clean environment (no heteroatoms), ideally selectively converting the UCO to desired products and further saturating the aromatic components to achieve required jet and diesel properties at different conversion levels. Stream  32  is a sidestream from stream  1 . From stream  32 , streams  7  and  8  add hydrogen in between the first and second beds and second and third beds of reactor  20  respectively. 
         [0041]    Both the hydrotreating reactor  10  and hydrocracking reactor  20  are designed for the maximum conversion desired. During lower conversion operation, the hydrotreating reaction is maintained at the same temperature as the highest conversion case in order to achieve target denitrification and desulfurization. The temperature of the hydrocracking reactor is reduced at lower conversions. 
         [0042]    The effluent (stream  72 ) of second stage hydrocracking reactor  20  is cooled (in exchanger  15 ) to preheat second stage reactor feed (stream  11 ) fractionator feed and cold low pressure separator liquid stream. Stream  72 , now renumbered stream  74 , combines with hot high pressure separator vapor stream  23  for further cooling and the removal of high pressure recycle gas. The hydrocarbon liquid from the cold high pressure separator (not shown) is sent to the fractionation section (not shown) for product recovery. 
       EXAMPLE 
       [0043]    The following table highlights the advantages of the process scheme of this invention over a conventional process scheme for a 65,000 BPOD (barrel per operating day) hydrocracking unit: The table indicates that there is no need in the current invention for post treatment in order to reach desired product specifications. Furthermore, less hydrogen is consumed in the scheme of the current invention than in the conventional case. 
         [0000]    
       
         
               
               
               
             
           
               
                   
               
               
                   
                 Conventional Process Scheme 
                 New Process Scheme 
               
               
                   
               
             
             
               
                 Process Scheme 
                 Single Stage Once-Through 
                 Targeted-Hydrogenation Hydrocracking 
               
               
                 Fresh Feed Rate, BPSD 
                 65,000 
                 65,000 
               
               
                 Overall LHSV, 1 hr 
                 0.7-0.9 
                 0.7-0.9 
               
               
                 Product Yield 
                 Base 
                 Similar to Base case 
               
               
                 Jet &amp; Diesel Quality 
                 Needs post treatment for aromatic 
                 On specification product at all desired 
               
               
                   
                 saturation at low conversion 
                 conversions: 
               
               
                   
                   
                 Jet: S &lt; 10 ppm; Smoke Point &gt; 24 mm 
               
               
                   
                   
                 Diesel: S &lt; 10 ppm; Cetane Index &gt; 50 
               
               
                 Chemical H 2  Consumption 
                 Base 
                 Base - 160 SCFB at 60% conv. 
               
               
                   
                   
                 Base - 100 SCFB at 40% conv. 
               
               
                   
                   
                 Base - 50 SCFB at 30% conv.