Abstract:
An improved system and method for processing feedstocks in an ebullated-bed hydroprocessing reactor is provided in which hydrogen gas is dissolved in the fresh and recycled liquid feedstock by mixing and/or diffusion of an excess of hydrogen, followed by flashing of the undissolved hydrogen upstream of the reactor inlet, introduction of the feed containing dissolved hydrogen into the ebullated-bed hydroprocessing reactor whereby the dissolved hydrogen eliminates or minimizes the prior art problems of gas hold-up and reduced operational efficiency of the recycle pump due to the presence of excess gas in the recycle stream when hydrogen gas was introduced as a separate phase into the reactor.

Description:
RELATED APPLICATIONS 
       [0001]    This application is a continuation of allowed application Ser. No. 13/556,343 filed Jul. 24, 2012, which will issue as U.S. Pat. No. 8,940,155 on Jan. 27, 2015 which claims priority to provisional patent application Ser. No. 61/513,055 filed Jul. 29, 2011. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    1. Field of the Invention 
         [0003]    This invention relates to hydrocracking or hydroprocessing systems and processes that employ ebullated-bed reactors. 
         [0004]    2. Description of Related Art 
         [0005]    Common objectives of hydrocracking or hydroprocessing operations are to remove impurities such as sulfur, nitrogen and/or metals, particularly in residue feedstocks, and cracking the heavy feed into lower molecular weight hydrocarbons having lower boiling points to obtain transportation fuels such as gasoline and diesel. The reactions that occur in hydrocracking/hydroprocessing operations include hydrodesulfurization (HDS), carbon residue reduction (CRR), nitrogen removal (HDN), and cracking. 
         [0006]    In a refinery, it is desirable to minimize the down-time for replacement or regeneration of catalyst. Further, process economics generally require a versatile system capable of handling feed streams of varying contaminants levels such as sulfur, nitrogen, metals and/or organometallic compounds, such as those found in a vacuum gas oils, deasphalted oils and residues. 
         [0007]    There are three common reactor types used in the refining industry: fixed, ebullated, and moving bed. The decision to utilize a particular type of reactor is based on a number of criteria including the characteristics of the feedstock, desired conversion percentage, flexibility, run length, and requisite product quality. 
         [0008]    The ebullated-bed reactor was developed to overcome plugging problems commonly associated with fixed-bed reactors during processing of relatively heavy feedstocks and as the conversion requirements increases, e.g., for vacuum residue. In an ebullated-bed reactor, the catalyst is in an expanded bed, thereby countering plugging problems associated with fixed-bed reactors. The fluidized nature of the catalyst in an ebullated-bed reactor also allows for on-line catalyst replacement of a small portion of the bed. This results in a high net bed activity which does not vary with time. 
         [0009]    Fixed-bed technologies have considerable problems in treating particularly heavy charges containing relatively high quantity of heteroatoms, metals and asphaltenes, as these contaminants cause the rapid deactivation of the catalyst and plugging of the reactor. Multiple fixed-bed reactors connected in series can be used to achieve a relatively high conversion of heavy feedstocks boiling above 370° C., but such designs require high capital investment and, for certain feedstocks, commercially impractical, e.g. catalysts replacement every 3-4 months. 
         [0010]    Therefore, to treat these heavy charges, ebullated-bed reactors were developed and are in operation worldwide. These reactors have numerous advantages in performance and efficiency, particularly with heavy crudes. Early ebullated-bed processes and systems are described by Johanson in U.S. Pat. Nos. 2,987,465 and 3,197,288, both of which are incorporated herein by reference. 
         [0011]    In general, an ebullated-bed reactor includes concurrently flowing streams of liquids, solids and gas through a vertically-oriented cylindrical vessel containing catalyst. The catalyst is placed in motion in the liquid and has a gross volume dispersed through the liquid medium that is greater than the volume of the catalyst mass when stationary. Ebullated-bed reactors are incorporated in various refinery operations, including processes for the upgrading of heavy liquid hydrocarbons and the conversion of coal to synthetic oils. 
         [0012]    Typically, a liquid hydrocarbon phase and a gaseous hydrogen phase are passed upwardly through the bed of catalyst particles at a rate such that the particles are forced into motion as the fluids pass upwardly through the bed. The catalyst bed expansion level is, at least in part, determined by the bottoms recycle liquid flow rate, which is controlled by an ebullating-bed pump. During steady-state operation (i.e., the ebullated-bed state), the bulk of the catalyst does not rise above a certain level in the reactor which is predetermined during reactor design. This level is established to prevent the catalyst particles from leaving the reactor or to interfere with the efficient operation of the cyclones that are installed inside the reactors to separate carried-over catalyst particles from the gas-liquid mixture. More catalyst can be loaded into the reactor initially because of no gas hold-up and liquid viscosity. These design criteria are well within the routine skill in the art. A substantial portion of the product vapors and liquids pass through the upper level of the catalyst particles into a substantially catalyst-free zone and are removed proximate to the upper portion of the reactor. 
         [0013]    Substantial amounts of hydrogen gas and light hydrocarbon vapors rise through the reaction zone into the catalyst-free zone. Liquid is recycled to the bottom of the reactor and removed from the reactor as net product from this catalyst-free zone. A certain portion of the vapor is separated from the liquid recycle stream before being passed through the recycle conduit drawn by suction of ebullating pump. However, gases or vapors present in the bottoms recycle stream materially decrease the capacity of the recycle pump. The presence of vapors also reduces the liquid residence time in the reactor and limit hydrogen partial pressure. 
         [0014]    Certain reactors employed in the catalytic hydrocracking process with an ebullated-bed of catalyst particles are designed with a central vertical recycle conduit which serves as the downflow conduit, or downcomer, for recycling liquid from the catalyst-free zone above the ebullated catalyst bed to the suction of a recycle pump to re-circulate the liquid through the catalytic reaction zone. 
         [0015]      FIG. 1  schematically illustrates a system and apparatus  100  of the prior art in which liquid is recycled internally with a recycle downflow conduit. Apparatus  100  includes an ebullated-bed reactor  160  and an ebullating pump  164 . Ebullated-bed reactor  160  includes an inlet  130  for receiving a mixture of hydrogen gas and feedstock and an outlet  166  for discharging product effluent. Ebullating pump  164  is in fluid communication with the ebullated-bed reactor  160  and includes an inlet  162  for receiving effluent recycled from ebullated-bed reactor  160  and an outlet  163  for discharging the recycled effluent at an increased pressure. 
         [0016]    In the practice of system  100 , a mixture of hydrogen gas and feedstock is introduced into the ebullated-bed reactor  160  via inlet  130  for reaction that includes conversion of the feedstock into lower molecular weight hydrocarbons. Liquid reaction effluent continuously flows down in the downflow conduit located inside ebullated-bed reactor  160 , and is recycled back to the ebullated-bed reactor  160  at elevated pressure using ebullating pump  164 . Product effluent is recovered via outlet  166 . 
         [0017]    Alternatively, the recycle liquid can be obtained from a vapor separator located downstream of the reactor or obtained from an atmospheric stripper bottoms stream. The recycling of liquid serves to ebullate and stabilize the catalyst bed, and maintain temperature uniformity through the reactor. 
         [0018]      FIG. 2  schematically illustrates a system and apparatus  200  of the prior art with an external recycle system that includes an ebullated-bed reactor  260 , an ebullating pump  264  and a high-pressure separator  280 . Ebullated-bed reactor  260  includes an inlet  230  for introducing a mixture of hydrogen gas and feedstock and an outlet  266  for discharging product effluent. High-pressure separator  280  includes an inlet in fluid communication with outlet  266  for receiving product effluent, an outlet  282  for discharging a gas product stream and an outlet  284  for discharging a liquid stream. Ebullating pump  264  includes an inlet  262  in fluid communication with outlet of high-pressure separator for receiving at least a portion of the liquid stream, and an outlet  263  for discharging recycling stream at elevated pressure. 
         [0019]    In the practice of system  200 , a mixture of hydrogen gas and feedstock is introduced into the ebullated-bed reactor  260  via inlet  230  for reaction which includes the conversion of the feedstock into lower molecular weight hydrocarbons. Reaction effluent is conveyed to the high-pressure separator  280  to obtain a gas stream  282  and a liquid stream  284 . At least a portion of the liquid stream  284  is recycled as stream  288  to the ebullated-bed reactor  260  via ebullating pump  264 . The remaining portion of the stream  284  can be recovered as product stream  286  or subjected to further refinery processes. 
         [0020]    Catalyst bed expansion is an important factor in the ebullated-bed reactor. In the process, the expansion of the bed is controlled by the recycle pump speed. Certain systems include a bed characterized by a number of bed level detectors and one or more additional detectors for determining abnormally high bed (interface) level. The interface level is detected, for instance, by a density detector including a radiation source at an interior point within the reactor and/or a detection source in the reactor wall. 
         [0021]    Although ebullated-bed processes are generally used for conversion of heavier vacuum residue feedstocks, they can also be used to clean or treat a lower boiling point vacuum gas oil feedstock. Advantages of ebullated-bed processes include product quality and rate uniformity, reduced downtime and lower capital investment. 
         [0022]    The volume and length-to-diameter ratio are known to be factors in ebullated-bed reactor design that impact the catalyst load. For a given volume reactor, the greater the length-to-diameter ratio, the more catalyst that can be introduced into the reactor. 
         [0023]    Gas and liquid hold-up rates are important process characteristics that contribute to the system performance. High gas hold-up rates result in decreased liquid residence time which lowers process performance. The gas hold-up rate in an ebullated-bed reactor can be as high as 40%. 
         [0024]    Although there are numerous types of ebullated-bed reactor designs, the problems exists of providing a more efficient and effective ebullated-bed reactor process and achieving improved reactor performance that eliminates or minimizes gas hold-up in the recycle system and its adverse effect on the recycle pump. 
       SUMMARY OF THE INVENTION 
       [0025]    The above problems are addressed and further advantages are provided by the system and process for processing feedstocks in an ebullated-bed hydroprocessing reactor in which hydrogen gas is dissolved in the combined fresh and recycled liquid feedstock by mixing and/or diffusion upstream of the reactor inlet, flashing the mixture and recovering undissolved hydrogen and any light components, following which the feed containing dissolved hydrogen is charged to an ebullated-bed hydroprocessing reactor. The problem of gas hold-up that is typical of ebullated-bed reactors of traditional design is minimized. 
         [0026]    The invention is thus directed to a process for conversion of a liquid hydrocarbon feedstock into lower molecular weight hydrocarbon compounds in an ebullated-bed reactor, the process comprising: 
         [0027]    mixing the fresh and recycled liquid hydrocarbon feedstock and an excess of hydrogen gas in a mixing zone to dissolve a portion of the hydrogen gas in the liquid hydrocarbon feedstock to produce a hydrogen-enriched liquid hydrocarbon feedstock; 
         [0028]    conveying the hydrogen-enriched liquid hydrocarbon feedstock and excess hydrogen to a flashing zone in which at least a portion of undissolved hydrogen gas is flashed; 
         [0029]    passing the hydrogen-enriched liquid hydrocarbon feedstock from the flashing zone to a feed inlet of the ebullated-bed reactor for reaction including conversion of the feedstock into lower molecular weight hydrocarbons; 
         [0030]    recovering the converted lower weight hydrocarbon products from a substantially catalyst-free region of the ebullated-bed reactor; and 
         [0031]    recycling the unconverted fed for mixing with fresh feedstock for reprocessing. 
         [0032]    The process and system of the invention solves the problems related to gas hold-up and those associated with reduced efficiency of the recycle pump due to the presence of gas in the recycle stream that are typically encountered in ebullated-bed hydroprocessing reactors of the prior art. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0033]    The invention will be described in further detail below and with reference to the attached drawings in which the same or similar elements are referred to by the same number, and where: 
           [0034]      FIG. 1  is a schematic diagram of a conventional ebullated-bed reactor of the prior art with internal recycle; 
           [0035]      FIG. 2  is a schematic diagram of a conventional ebullated-bed reactor of the prior art with external recycle; 
           [0036]      FIGS. 3A-3C  are schematic diagrams of ebullated-bed processes with internal recycle and a straight recycle system ( FIG. 3A ), a combined recycle and feedstock hydrogen addition system ( FIG. 3B ) and a feedstock hydrogen addition system combined with a recycle separation system ( FIG. 3C ); and 
           [0037]      FIGS. 4A-4C  are schematic diagrams of ebullated-bed processes with external recycle and a straight recycle system ( FIG. 4A ), a combined recycle and feedstock hydrogen addition system ( FIG. 4B ) and a feedstock hydrogen addition system combined with a recycle separation system ( FIG. 4C ). 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0038]    In accordance with the process of the invention, a substantial portion of the hydrogen gas required for hydroprocessing/hydrocracking reactions is dissolved in the liquid feedstock. A hydrogen distributor apparatus is utilized to treat the feedstock upstream of the ebullated-bed reactor to dissolve at least a substantial portion of the requisite reaction hydrogen gas into the liquid feedstock to produce a combined feed/hydrogen stream as the ebullated-bed reactor influent. 
         [0039]    In the practice of the process of the invention, the ebullated-bed reactor gas hold-up is less than 40 V % of the total liquid volume passing through the reactor, and preferably less than 10 V %, and most preferably less than 1 V % of the liquid volume. 
         [0040]    Also in the practice of the process of the invention, the ebullated-bed recycle stream contains less than 10 V % of vapors, preferably less than 1.0 V %, and most preferably less than 0.1 V % of vapors. 
         [0041]      FIGS. 3A-3C  depict internal-recycle type ebullated-bed reactor apparatus adapted for receiving a charge  320  including at least a substantial portion of the requisite hydrogen in solution with the feedstock. 
         [0042]    The apparatus for the process generally includes a hydrogen distributor  314 , a flash vessel  322 , an ebullated-bed reactor vessel  360 , and one or more pumps  364   a,    364   b  and  364   c.    
         [0043]    A feedstock stream  310  is mixed with an excess of hydrogen gas  312  in a distributor vessel  314  to dissolve a desired quantity of hydrogen in the liquid and produce a hydrogen-enriched liquid hydrocarbon feedstock. The hydrogen gas stream  312  includes fresh hydrogen  316  and recycled hydrogen  318 . 
         [0044]    In certain embodiments, a column is used as a hydrogen distributor vessel in which hydrogen gas is injected at one or more locations, at least one of which is toward the bottom of the column. The liquid feedstock can be fed from the bottom or top of the column. Hydrogen gas is injected through hydrogen distributors into the column for intimate mixing to promote saturation of the feedstock with dissolved hydrogen. 
         [0045]    Combined stream  320 , which includes hydrogen-enriched feedstock and excess hydrogen gas, is conveyed to the flashing zone  322  in which excess hydrogen and other gases (e.g., light feedstock fractions) are flashed off as stream  324 . A portion of stream  324 is recycled as stream  318  with the fresh hydrogen feed  316 . The remaining portion of the flashed gases is discharged from the system as a bleed stream  326 , which can be distributed or collected for other refinery and/or petrochemical applications (not shown). 
         [0046]    The hydrogen-enriched hydrocarbon feedstock stream  330  which contains the dissolved hydrogen is fed to an ebullated-bed reactor  360  for hydroprocessing reactions. The reactor effluents product stream  366  is discharged from the ebullated-bed reactor and fed into one or more separation vessels (not shown) for product recovery. 
         [0047]    Alternatively, the product stream  366  can be conveyed to one or more downstream ebullated-bed reactor systems, which can include one or more of the associated unit operations described herein, e.g., upstream flashing vessel, downstream flashing vessel, and/or one or more additional hydrogen distributor apparatus. 
         [0048]    The embodiments of  FIGS. 3A ,  3 B and  3 C can be implemented individually or in various combinations with the feedstock hydrogen addition system. An ebullated-bed process with a straight recycle system is depicted in  FIG. 3A . An ebullated-bed process with a combined recycle and feedstock hydrogen addition system is depicted in  FIG. 3B . An ebullated-bed process with a feedstock hydrogen addition system combined with a recycle separation system is depicted in  FIG. 3C . 
         [0049]    In particular, referring to  FIG. 3A , hydrogen-enriched hydrocarbon feedstock  330  from flashing vessel  322  is charged to the ebullated-bed reactor  360 . In the internal-recycle type ebullated-bed reactor, liquid recycle effluent  362   a  is drawn through the downcomer conduit in fluid communication with the catalyst-free zone above the catalyst bed by draw force of an ebullated pump  364   a.  The recycle stream  363   a  from ebullated pump  364   a  is returned to the catalyst bed, with the fluid pressure causing catalyst bed expansion. 
         [0050]    Referring to  FIG. 3B , internal recycle effluent  362   b  is passed to hydrogen gas distributor  314  and combined with the feedstock stream  310  for hydrogen saturation. The combined hydrogen-saturated stream  330  is conveyed to the ebullated-bed reactor with the fluid pressure imparted by ebullating-bed pump  364   b  causing bed expansion of the catalyst. 
         [0051]    Referring to  FIG. 3C , hydrogen-enriched hydrocarbon feedstock  330  from flashing vessel  322  is charged to the ebullated-bed reactor  360 . Internal recycle effluent  362   c  is passed through a separator vessel  370  with optional hydrogen incorporation via stream  378 . Separator bottoms  374  are passed through an ebullating pump  364   c  and a recycle stream  363   c  is returned to ebullated-bed reactor  360 . The fluid pressure imparted to recycle stream  363   c  by ebullating pump  364   c  causes catalyst bed expansion. A portion of separator tops  372  including recycle hydrogen and light gases is bled from the system, and a portion  376  is recycled and mixed with recycle effluent  362   c  along with optional make-up hydrogen via stream  378 . 
         [0052]    In another embodiment depicted in  FIGS. 4A-4C , external-recycle type ebullated-bed reactors are adapted to receive a charge  420  including at least a substantial portion of the requisite hydrogen in solution with the feedstock. 
         [0053]    The apparatus for the process generally includes a hydrogen distributor  414 , a flash vessel  422 , an ebullated-bed reactor vessel  460 , a recycle separation vessel  480 , and one or more ebullating pumps  464   a,    464   b  and  464   c.    
         [0054]    A feedstock stream  410  is mixed with hydrogen gas  412  in a distributor vessel  414  to dissolve a suitable quantity of hydrogen in liquid mixture and produce a hydrogen-enriched liquid hydrocarbon feedstock. The hydrogen gas stream  412  includes fresh hydrogen  416  and recycled hydrogen  418 . 
         [0055]    In certain embodiments, a column is used as a hydrogen distributor vessel, in which hydrogen gas is injected at one or more locations, at least one of which toward the bottom of the column. The liquid feedstock can be fed from the bottom or top of the column. Hydrogen gas is injected through hydrogen distributors into the column for adequate mixing to promote formation of a feedstock containing dissolved hydrogen. 
         [0056]    Combined stream  420 , which includes hydrogen-enriched feedstock and excess hydrogen gas, is conveyed to the flashing zone  422  in which excess hydrogen and other gases, e.g., light feedstock fractions, are flashed off as stream  424 . A portion of stream  424  is recycled as stream  418  with the fresh hydrogen feed  416 . The remaining portion of the flashed gases is discharged from the system as a bleed stream  426 , which can be distributed or collected for other refinery and/or petrochemical applications (not shown). 
         [0057]    The hydrogen-enriched hydrocarbon feedstock stream  430  which contains a suitable quantity of dissolved hydrogen is fed to the ebullated-bed reactor  460  for hydroprocessing reactions. 
         [0058]    The ebullated-bed reactor effluents product stream  466  is sent to a recycle separation vessel  480  to flash-off the gas products stream  482  and recover a liquid products stream  484 , a portion of which is recycled. A portion  486  of the liquid products stream is drawn-off from the process and passed to one or more separation vessels (not shown) for product recovery. Alternatively, the product stream  486  can be conveyed to one or more downstream ebullated-bed reactor systems, which can include one or more of the associated unit operations shown herein, e.g., upstream flashing vessel, downstream flashing vessel, and/or one or more additional hydrogen distributor apparatus. 
         [0059]    The various embodiments of  FIGS. 4A ,  4 B and  4 C can be implemented individually or in various combinations with the feedstock hydrogen addition system. An ebullated-bed process with a straight recycle system is depicted in  FIG. 4A . An ebullated-bed process with a combined recycle and feedstock hydrogen addition system is depicted in  FIG. 4B . An ebullated-bed process with a feedstock hydrogen addition system combined with a recycle separation system is depicted in  FIG. 4C . 
         [0060]    In particular, referring to  FIG. 4A , hydrogen-enriched hydrocarbon feedstock  430  from flashing vessel  422  is charged to the ebullated-bed reactor  460 . In the external-recycle type ebullated-bed reactor, the effluent stream  466  is separated into a product fraction  482  and a liquid fraction  484  in the recycle separation vessel  480 . A portion  488   a  of the liquid products stream  484  serves as the external recycle stream  463   a  that is charged to the bottom of reactor vessel  460  through an ebullating pump  464   a,  with the fluid pressure causing catalyst bed expansion. 
         [0061]    Referring to  FIG. 4B , hydrogen-enriched hydrocarbon feedstock  430  from flashing vessel  422  is charged to the ebullated-bed reactor  460 . External recycle effluent  478   b  is passed to hydrogen gas distributor  414  and combined with the feedstock stream  410  for hydrogen saturation. The combined hydrogen-saturated stream  430  is conveyed to the ebullated-bed reactor with the fluid pressure imparted by ebullating pump  464   b  causing catalyst bed expansion. 
         [0062]    Referring to  FIG. 4C , hydrogen-enriched hydrocarbon feedstock  430  from flashing vessel  422  is charged to the ebullated-bed reactor  460 . External recycle effluent  488   c  is passed through a secondary separator vessel  470  with optional hydrogen incorporation via stream  478 . Secondary separator bottoms  474  are passed through an ebullating pump  464   c  and a recycle stream  463   c  is returned to ebullated-bed reactor  460 . The fluid pressure imparted to recycle stream  463   c  by ebullating pump  464   c  causes catalyst bed expansion. A portion of secondary separator tops  472  including recycle hydrogen and light gases is bled from the system, and a portion  476  is recycled and mixed with recycle effluent  488   c  along with optional make-up hydrogen via stream  478  and returned to secondary separator  470 . 
         [0063]    In general, the operating conditions for the ebullated-bed reactor include a temperature in the range of from 350° C. to 500° C., in certain embodiments from 400° C. to 450° C.; a pressure in the range of from 50 to 300 Kg/cm 2 , in certain embodiments from 100 to 250 Kg/cm 2 , and in farther embodiments from 150 to 200 Kg/cm 2 ; and a recycle-to feedstock ratio in the range of from 1:1 to 40:1, in certain embodiments from 1:1 to 20:1. 
         [0064]    For the saturation of the feedstock, a hydrogen feed rate of up to about 10,000 standard cubic feet per barrel (SCFB) of feed, in certain embodiments from 500 to 10,000 SCFB, and in further embodiments from 1,500 to 5,000 SCFB is used, the rate being determined by the nature and characteristics of the feedstock. 
         [0065]    According to the process and system of the invention, by using a hydrogen-enriched hydrocarbon feedstock which contains at least a substantial portion of the requisite hydrogen for hydroprocessing reactions (in certain embodiments a substantially single-phase combined feed and hydrogen stream) as the feed through the ebullated catalyst bed reactor, problems associated with ebullated pump apparatus related to excess gas in the recycle are alleviated. 
         [0066]    Operation of the ebullated-bed reactor is optimized as at least a substantial portion of the hydrogen necessary for hydroprocessing reactions is dissolved in the liquid phase with the feedstream. In certain embodiments, a substantially two-phase system of catalyst and liquid is provided to minimize the reactor volume requirements, increase catalyst loading and liquid volume, and reduce the gas hold-up rate. 
         [0067]    Since excess hydrogen gas in the system is minimized or substantially eliminated, the recycle stream and therefore the recycle liquid will have a reduced gas phase compared to conventional ebullated-bed hydroprocessing systems, thereby increasing the efficiency of ebullated-bed recycle pump and minimizing the need for ebullating pumps designed to handle a substantial gas phase. Further, the reduced levels of excess hydrogen will minimize the likelihood of gas hold-up, and reactor volume can be used more effectively, e.g., in certain embodiments an effective reactor volume increase of up to about 40%. 
         [0068]    One feature of ebullated-bed reactors is that catalyst addition/withdrawal occurs on a regular basis, and in certain operations, on a continuous or semi-continuous basis without interrupting the reactor&#39;s operation. Any catalyst deactivation caused by a possible lack of hydrogen as compared to conventional systems without incorporation of hydrogen in solution with the liquid feedstock is at least partially offset by the regular partial replacement of catalyst. The ebullated-bed reactor cycle length is therefore set by the refiner&#39;s inspection and turnaround schedule, not by catalyst activity. 
         [0069]    The ebullated-bed reactors operate at constant temperature, whereas the fixed-bed reactors operate over wide a temperature range. Pressure drop is relatively low in the ebullated-bed reactor as a result of ebullation. 
         [0070]    Back-mixing characteristics of the ebullated-bed reactor process provides enhanced reactant dispersing and results in a near-isothermal bed conditions. Reaction temperature is controlled by the temperature of the feed, which results in isothermal temperature operation across the reactor. This eliminates the requirement for a hydrogen gas quench in the reactor. Isothermal operation (no quench requirement) in the ebullated-bed process will increase the feedstock processing flexibility, and relatively heavier feedstock can be process in the ebullated-bed reactors according to the present system and process. 
       EXAMPLE 
       [0071]    A vacuum residue derived from Arabian heavy crude oil, the characteristics of which are given in Table 1, was hydrocracked in a single-stage ebullated-bed reactor at 440° C., 150 bars of hydrogen partial pressure, 0.3 liters of oil per liter of reactor volume and with 0.8 kg of catalyst per ton of vacuum residue. The total hydrogen gas rate is set at four times the rate at which hydrogen is consumed in the process. The ebullated-bed reactor was operated at a recycle-to-feedstock ratio of 10:1. 
         [0000]    
       
         
               
               
               
             
               
               
               
             
           
               
                 TABLE 1 
               
               
                   
               
               
                 Property/Composition 
                 Unit 
                 Value 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                 Density 
                 Kg/L 
                 1.04 
               
               
                 Sulfur 
                 W % 
                 5.3 
               
               
                 Nitrogen 
                 ppmw 
                 4000 
               
               
                 CCR 
                 W % 
                 25 
               
               
                 1050° C.+ 
                 W % 
                 91 
               
               
                   
               
             
          
         
       
     
         [0072]    The total material balance for the process which is configured as shown in  FIG. 3B  is given in Table 2. 
         [0000]    
       
         
               
               
               
               
               
               
               
               
               
               
               
             
               
               
               
               
               
               
               
               
               
               
               
             
           
               
                 TABLE 2 
               
               
                   
               
               
                 Component/ 
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
               
               
                 Stream 
                 310 
                 316 
                 312 
                 320 
                 324 
                 326 
                 318 
                 330 
                 366 
                 362b 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                 Hydrogen 
                   
                 93 
                 381 
                 381 
                 288 
                 0 
                 288 
                 93 
                   
                   
               
               
                 H 2 S 
                   
                   
                   
                   
                   
                   
                   
                   
                 510 
               
               
                 NH 3   
                   
                   
                   
                   
                   
                   
                   
                   
                 30 
               
               
                 CH 4   
                   
                   
                   
                   
                   
                   
                   
                   
                 85 
               
               
                 C 2 H 6   
                   
                   
                   
                   
                   
                   
                   
                   
                 86 
               
               
                 C 3 H 8   
                   
                   
                   
                   
                   
                   
                   
                   
                 150 
               
               
                 C 4 H 10   
                   
                   
                   
                   
                   
                   
                   
                   
                 120 
               
               
                 C 5 -180 
                   
                   
                   
                   
                   
                   
                   
                   
                 810 
               
               
                 180° C.-240° C. 
                   
                   
                   
                   
                   
                   
                   
                   
                 537 
               
               
                 240° C.-370° C. 
                   
                   
                   
                   
                   
                   
                   
                   
                 1,634 
               
               
                 370° C.-520° C. 
                 900 
                   
                   
                 900 
                   
                   
                   
                 900 
                 2,340 
               
               
                 520° C.+ 
                 9,100 
                   
                   
                 48,000 
                   
                   
                   
                 48,000 
                   
                 38,900 
               
               
                 Total 
                 10,000 
                 93 
                 381 
                 49,281 
                 288 
                 0 
                 288 
                 48,993 
                 6,302 
                 38,900 
               
               
                   
               
             
          
         
       
     
         [0073]    The total conversion of the hydrocarbons boiling above 520° C. was 61 W % and 92 W % of hydrodesulfurization was achieved in the process. Since hydrogen was dissolved in the feedstock, the gas phase hydrogen was eliminated in the ebullated-bed reactor which resulted in a savings of 30-40 V % of the reactor space that was previously required for gas hold-up in the three phase system of the prior art. 
         [0074]    The method and system of the present invention 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.