Patent Publication Number: US-2003223924-A1

Title: Gas-pocket distributor and method of distributing gas

Description:
FIELD OF THE INVENTION  
       [0001] The present invention relates to a gas-pocket distributor assembly for distributing a mixture of liquid hydrocarbons and hydrogen-containing gas(es) and the hydroprocessing method which employs this assembly.  
       BACKGROUND OF THE INVENTION  
       [0002] Hydroprocessing or hydrotreatment to remove undesirable components from hydrocarbon feed streams is a well known method of catalytically treating such heavy hydrocarbons to increase their commercial value. “Heavy” hydrocarbon liquid streams, and particularly reduced crude oils, petroleum residua, tar sand bitumen, shale oil or liquefied coal or reclaimed oil, generally contain product contaminants, such as sulfur, and/or nitrogen, metals and organo-metallic compounds which tend to deactivate catalyst particles during contact by the feed stream and hydrogen under hydroprocessing conditions. Such hydroprocessing conditions are normally in the range of 212° F. to 1200° F. (100° C. to 650° C.), at pressures of from 20 to 300 atmospheres. Generally, such hydroprocessing is in the presence of catalyst containing group VI or VIII metals such as platinum, molybdenum, tungsten, nickel, cobalt, etc., in combination with various other metallic element particles of alumina, silica, magnesia and so forth having a high surface to volume ratio. More specifically, catalyst utilized for hydrodemetallation, hydrodesulfurization, hydrodenitrification, hydrocracking, etc., of heavy oils and the like are generally made up of a carrier or base material; such as alumina, silica, silica-alumina, or possibly, crystalline aluminosilicate, with one or more promoters or catalytically active metal(s) (or compound(s)) plus trace materials. Typical catalytically active metals utilized are cobalt, molybdenum, nickel and tungsten; however, other metals or compounds could be selected dependent on the application.  
       [0003] A particularly effective reactor and method for hydroprocessing a hydrocarbon feed stream is disclosed in U.S. Pat. Nos. 5,885,534 and 5,958,220. That reactor, depicted in attached FIGS.  1 - 3 , comprises a reactor vessel  10  (see FIGS. 1 and 2) having an inlet conduit  14  (see FIGS.  1 - 2 ) adapted to introduce into the vessel a premixed stream  34  (FIGS.  1 - 3 ) of hydrogen-containing gas  36  (FIGS. 1 and 3) and a liquid hydrocarbon stream  38  (FIGS.  1 - 3 ). The vessel  10  contains an aperture support plate  16  (FIG. 1) for supporting thereabove a catalyst bed  18  (FIG. 2). The catalyst bed  18  descends in a “plug flow” manner as the stream of hydrogen-containing gas  36  (FIGS. 1 and 3) and liquid hydrocarbon stream passes upwardly through the catalyst bed. Spent catalyst is removed from the bottom of the bed  18  through a riser  28  (FIGS.  1 - 3 ), and fresh catalyst is introduced onto the top of the bed through a riser  19  (FIG. 2). Products of a reaction between the hydrogen gas and the hydrocarbon liquid are removed from the top of the vessel through a central duct  19 A (FIG. 2). A screen  19 B (FIG. 2) extends across the interior of the vessel at a location above the upper surface of the bed  18  to prevent catalyst from exiting through the duct  19 A.  
       [0004] Disposed below the catalyst support  16  (FIG. 1) is a grid-like circular plate  22  (FIGS.  1 - 3 ), whereby a plenum chamber  24  is formed between the catalyst support  16  and the plate  22 . The plate  22  has a plurality of openings  26  that communicate with a plurality of open-ended risers  28  that depend downwardly from the plate  22 . Each riser  28  contains a central bore  29  (FIG. 3) and an opening  30  (FIGS. 1 and 3). The risers  28  conduct the mixture  34  (FIGS.  1 - 3 ) of gas and liquid  38  (FIGS.  1 - 3 ) to the plenum chamber  24  (FIG. 1). A gas head  50  (FIG. 1) is formed between an underside of the plate  22  and lower ends of the risers  28 , the gas head extending to a level below the openings  30  (FIGS. 1 and 3). Thus, hydrogen containing gas  36 A (FIGS. 1 and 3) evolves into the head  50  and flows in the direction of arrow M and through the openings  30 . The presence of the gas head  50  (FIG. 1) is ensured by a suitable sizing of the openings  30 . In the absence of the openings  30  there would be a violent slugging of gas and liquid up the risers  28  at random locations because the liquid height would rapidly vary and expose the bottom ends of different risers  27 . Thus, large gas bubbles would reach the bottom of the catalyst support plate  16 , rather than a gentle, steady rise of small bubbles.  
       [0005] In order to distribute the hydrogen-containing gas across the interior of the vessel, i.e., to oppose a concentration of the gas at the center of the vessel, a deflector plate  27  (FIG. 2) is disposed over the inlet  14  to deflect the gas bubbles laterally outwardly (see FIG. 2).  
       [0006] However, it has been learned that despite the presence of the deflector  27  (FIG. 2), considerable quantities of hydrogen-containing gas still manage to reach the risers  28  located in the center region of the plate  22 , especially the risers located near the outer edge of the deflector  27  from which many gas bubbles exit. That results in excess hydrogen going into the middle of the reactor, depriving other parts of the catalyst bed of sufficient hydrogen to prevent coking.  
       [0007] Therefore, it would be desirable to provide a reactor which results in a more uniform distribution of hydrogen-containing gas through the reactor, in order to minimize the occurrence of coking.  
       SUMMARY OF THE INVENTION  
       [0008] One aspect of the present invention relates to a reactor comprising a vessel containing a catalyst bed. A support is disposed within the vessel for supporting the catalyst bed. A plate member is disposed below the support and includes plate openings extending therethrough. The vessel includes an inlet for introducing a mixture of hydrogen-containing gas and hydrocarbon liquid into a bottom header formed beneath the plate. Risers communicate with respective plate openings and extend downwardly therefrom into the bottom header whereby a vapor space is formed in the bottom header between the plate and an upper surface of the mixture disposed in the bottom header. The upper surface of the mixture is disposed above bottom ends of the risers. Each riser includes a side opening disposed within the vapor space for conducting, into the riser, hydrogen-containing gas evolving from the mixture. At least some of the risers are obstructed. Each obstructed riser has a blockage extending across a bottom end thereof for opposing the entry of gas bubbles into those bottom ends. A lateral entrance is provided for each of the obstructed risers for admitting the mixture into the obstructed riser, the lateral entrance disposed beneath the vapor space.  
       [0009] Another aspect of the invention relates to a method for hydroprocessing a desired hydrocarbon feed stream in the reactor previously disclosed, comprising the steps of:  
       [0010] A. conducting a mixture of hydrogen-containing gas and hydrocarbon liquid through a reactor which comprises a vessel supporting a catalyst bed, the vessel having the following features: an inlet, a bottom header, a riser, a support, and a catalyst bed, the vessel further comprising a support below the catalyst bed and a plate member, having openings located below the support; a vapor space being maintained between the plate and an upper surface of the mixture disposed in the bottom header, the upper surface being disposed above the bottom ends of the risers, whereby hydrogen-containing gas evolves from the mixture within the vapor space; and  
       [0011] B. conducting the evolved gas into the risers from the vapor space, through side openings disposed in the risers; the bottom ends of at least some of the risers being obstructed in a manner which constrains the mixture to travel laterally to enter the obstructed risers. 
     
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
     [0012]FIG. 1 is a partial perspective sectional view of a reactor known in the art.  
     [0013]FIG. 2 is a schematic side view of the reactor shown in FIG. 1.  
     [0014]FIG. 3 is an enlarged sectional view of a riser shown in FIG. 1.  
     [0015]FIG. 4 is a sectional view taken through the lower portion of a reactor and depicting a first embodiment of the present invention.  
     [0016]FIG. 5 is an enlarged sectional view through a riser of FIG. 4, depicting the first embodiment of the invention.  
     [0017]FIG. 6 is a view, similar to FIG. 5, depicting a second embodiment of the invention.  
     [0018]FIG. 7 is a view, similar to FIG. 5, depicting a third embodiment of the invention. 
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS OF THE INVENTION  
     [0019] We now refer in detail to FIG. 4, which depicts a reactor. Parts of the invention similar to those of the previously described reactors (FIGS.  1 - 3 ) are identified by like reference numerals. The reactor includes a vessel  10  designed to react a hydrogen-containing gas  36  mixed with a liquid hydrocarbon stream  38  at a pressure of up to about 300 atmospheres (about 4500 lbs. per square inch) and up to about 650° C. (about 1200° F.). For such reaction, hydrogen-containing gas  36  and liquid hydrocarbon stream  38  are preferably premixed and introduced as a single stream (i.e., a single two-phase flow) through a conduit  14  having a concentric disposition with respect to the reactor vessel  10 .  
     [0020] The reactor vessel  10  contains a catalyst bed support means, generally illustrated as  16 , for supporting a catalyst bed  18  and containing appropriate openings (not shown) well known to the artisans in the art. The catalyst bed support means  16  contained in the reactor vessel  10  may be of any suitable geometric shape, such as concentric rings, conical, pyramidal, truncated polygonal or conical, frusto-conical, etc. The catalyst bed support means  16  further may be of any type that preferably insures even and equal distribution of hydrogen-containing gas  36  and liquid hydrocarbon stream  38  across a full cross-sectional area of the catalyst bed  18 .  
     [0021] To assure maximum catalytic benefit during the hydroprocessing of the hydrogen-containing gas  36  and the liquid hydrocarbon stream  38 , it is preferred that the reactor vessel  10  contain as much catalyst as possible within the design volume of the reactor vessel  10 . Accordingly, it is preferred that the catalyst bed support means  16  for the catalyst bed  18  be placed as low as possible in the reactor vessel  10  while assuring full and adequate dispersion of the hydrogen-containing gas  36  within the liquid hydrocarbon stream  38 .  
     [0022] The upper level of the catalyst bed  18  is to be controlled such that ebullation, expansion, or fluidization of the catalyst bed  18  is minimized and that undesirable excursions from the design flow rate for hydrogen-containing gas  36  and liquid hydrocarbon stream  38  flowing upwardly through the catalyst bed  18  are avoided for the selected catalyst. U.S. Pat. No. 5,472,928, issued Dec. 5, 1995, which is fully incorporated herein by reference, discusses control of catalyst bed level in more detail. The size, shape, and density of the catalyst particles within the catalyst bed  18  are to be essentially uniform and are selected in accordance with the designed maximum rate of flow of feed streams or a mixture  34  of the hydrogen-containing gas  36  and the liquid hydrocarbon stream  38  to prevent ebullation, expansion, or fluidization of the catalyst bed  18  while the latter progressively moves down through the reactor vessel  10  in layers by plug flow.  
     [0023] The reactor vessel  10  also contains a generally circular plate member  22  (i.e., a distributor tray), that is secured to an internal generally cylindrical wall  11  of the vessel such that a plenum chamber or vapor space  24  is produced between the catalyst bed support means  16  and the generally circular plate member  22 . The distance between the level of the mixture  34  and the plate member  22  defines a static head wherein a suitable gas head  50  comprises evolved hydrogen-containing gas  36 A that has originated from the mixture  34  of the hydrogen-containing gas  36  and the liquid hydrocarbon stream  38 .  
     [0024] As was previously mentioned, the term “evolved hydrogen-containing gas” comprises: the hydrogen-containing gas  36  that is being introduced into the reactor vessel  10  along with the liquid hydrocarbon stream  38 , any hydrogen gas that has evolved from the liquid hydrocarbon stream  38  itself, and hydrogen-containing gas  36  that solutionized and/or dissolved into and/or with the liquid hydrocarbon stream  38  and which has subsequently evolved from the liquid hydrocarbon stream  38 , especially after introduction Into the reactor vessel  10 .  
     [0025] The plate member  22  has a plurality of openings  26  that respectively communicate with a plurality of hollow risers  28  that are bound to the plate member  22 . Stated alternatively, the plate member  22  includes a plurality of hollow risers  28  forming openings  26  through the plate member  22 . At least one of the risers  28  (preferably all of them) contains at least one side opening  30  located intermediate its ends and above the mixture  34  (i.e., within the vapor space). Risers  28  may alternately be referred to as tube members and are so described in the claims.  
     [0026] The lengths of the respective hollow risers  28  may be selected such that the suitable gas head  50  is formed underneath the plate member  22  and/over the level of the mixture  34  to suppress surges in the feed stream(s) entering the bottom header  40  from the conduit  14 . The hollow risers  28  receive the mixture  34  of hydrogen-containing gas  36  and liquid hydrocarbon stream  38  and pass the same through the openings  26  to enter the vapor space  24 .  
     [0027] As the mixture  34  of the hydrogen-containing gas  36  and the liquid hydrocarbon stream  38  flows through the respective hollow risers  28 , the evolved hydrogen-containing gas  36 A within the static head (or the suitable gas head  50 ) enters or passes through the side openings  30 . As the mixture  34  flows through conduit  14  and into a bottom header  40  disposed between the plate  22  and the bottom of the vessel, evolved hydrogen-containing gas  36 A commences to evolve from the mixture  34  and the suitable gas head  50  begins to form. Continual flow of the mixture  34  into the bottom header  40  fills a volumetric portion of the bottom header  40  such as to produce the suitable gas head  50 , all as best shown in FIG. 4.  
     [0028] The suitable gas head  50  has a pressure that is greater than the pressure of the mixture  34  such that with continual introduction of the mixture  34  into the bottom header  40 , the mixture  34  commences to flow up and through each of the hollow risers  28  and out of the openings  26  and into the vapor space  24 . When the suitable gas head  50  is formed and/or begins to form, evolved hydrogen-containing gas  36 A commences to flow in direction of the arrows M through the side opening(s)  30  in the hollow risers  28 ; that is, evolved hydrogen-containing gas  36 A commences to flow towards a lower pressure zone.  
     [0029] As the mixture  34  commences to flow up each of the hollow risers  28 , some of the hydrogen-containing gas  36  evolves out of the mixture  34  such as to commingle with and/or admix with the evolved hydrogen-containing gas  36 A entering through the openings  30  in the hollow risers  28 .  
     [0030] The size of the openings  30  is carefully chosen to control the liquid level safely above the bottom of the riser  28  and also safely below the openings  30 .  
     [0031] A deflector  27  is positioned above the inlet conduit  14  to help distribute the hydrogen gas bubbles across the cross sectional area of the vessel.  
     [0032] The features described above are also found in FIGS.  1 - 3 . As previously noted, it has been found that despite the presence of the deflector  27 , considerable quantities of hydrogen-containing gas manage to reach the more centrally located risers  28 .  
     [0033] In accordance with the present invention, at least some, but possibly all, of the risers  28  are provided with an obstruction in the form of a cap  100  disposed across the bottom of the riser as shown in FIGS. 4 and 5. The cap constitutes a plate secured to the bottom of the riser and extending laterally therebeyond. Formed in each of the thus-obstructed risers is at least one, but preferably a plurality of openings  102  disposed below the level of mixture  34 . Each opening  102  is shown as being in the form of a vertical slot which defines a lateral entrance into the riser. The openings  102  could have any suitable shape. It will be appreciated that the gas bubbles will not readily travel horizontally so as to be able to pass through the openings  102 , especially since the portions of the cap  100  which project laterally beyond the riser tend to increase the lateral distance that the gas bubbles must flow in order to enter the riser. Accordingly, little gas will enter the obstructed risers at a location beneath the surface of mixture  34 , enabling the openings  30  to perform the desired gas-metering function.  
     [0034] In an alternative embodiment of the invention shown in FIG. 6, the riser obstruction is in the form of a plate  110  spaced beneath the bottom of the riser. The plate  110  could be attached by brackets  112  to the riser, or situated on a perforated screen  112  (e.g., a conventional so-called Johnson screen) extending across the bottom header  40 . The space  114  between the plate  110  and the bottom of the riser defines a lateral entrance to the obstructed riser.  
     [0035] The plate need not be flat. Instead, the plate  120  could be curved, as shown in FIG. 7, with the concave side thereof facing upwardly and spaced beneath the bottom of the riser. The outer edge  122  of the plate could be situated at a higher elevation than the bottom of the riser, requiring that any gas entering the riser travel not only laterally, but also downwardly, which makes it much more unlikely that gas bubbles can enter the riser through the lateral entrance  124  defined between the plate  120  and the riser bottom.  
     [0036] The obstruction  100 , or  110  or  120  is preferably applied to the risers that are disposed in close relationship to the deflector  27 , but if desired, all of the risers could be provided with obstructions. As a result of the obstructions, the hydrogen bubbles are distributed more uniformly across the bottom header, preventing a concentration of the bubbles at the center of the reactor.  
     [0037] While the present invention has been described herein with reference to particular embodiments thereof, a latitude of modification, various changes and substitutions are intended in the foregoing disclosure, and it will be appreciated that in some instances some features of the invention will be employed without a corresponding use of other features without departing from the scope of the invention as set forth.