Abstract:
The present invention provides a novel means to provide more effective mixing of gas and fluids in a height constrained interbed space of a catalytic reactor without increasing pressure drop. In particular, the device improves the effectiveness of an existing mixing volume in mixing the gas phase and liquid phase of two-phase systems. According to the present invention, the mixing device helps create a highly arcuate flow to incoming effluents and a high degree of mixing within a constrained interbed space of a catalytic reactor.

Description:
FIELD OF THE INVENTION 
     This invention relates to a mixing device for multi-bed hydroprocessing reactors. Specifically, the mixing device is for use in down-flow catalytic reactors which include vertically superimposed packed beds of particulate catalytic material wherein a liquid or liquid and gas mixture is processed as it flows down through the packed beds. This type of reactor is used in the petroleum and chemical processing industries for carrying out various catalytic reactions, such as hydroprocessing reactions including hydrotreating, hydrofinishing, hydrocracking, and dewaxing. 
     BACKGROUND OF THE INVENTION 
     In fixed-bed fuels and lube hydroprocessing units, gas and liquid flow downward through multiple beds of solid catalyst. Heat is released from the catalytic reactions causing temperature to increase with distance down the bed. Cool hydrogen-rich gas can be introduced between the beds to quench the temperature rise and replenish the hydrogen consumed by the reactions. In order to maintain overall reactor performance, the temperature and pressure of the fluids within the reactor should be as uniform as possible and any liquids and gases well mixed in order to maximize performance. Poor interbed fluid mixing can limit reactor operation in various ways. When interbed mixing is unable to erase temperature differences, these persist or grow as the process fluids move down the reactor. Hot spots in any bed can lead to rapid deactivation of the catalyst in that region which shortens the total reactor cycle length. Product selectivities are typically poorer at high temperatures. For example, hot regions can cause color, viscosity and other qualities to be off-specification. Also, if the temperature at any point exceeds a certain value (typically 800 to 850° F.), the exothermic reactions may become self-accelerating leading to a runaway which can damage the catalyst, the vessel, or downstream equipment. Due to these hazards, refiners operating with poor internal hardware must sacrifice yield and/or throughput to avoid the deleterious effects of poor interbed fluid mixing. Reactor temperature maldistribution and hot spots can be minimized through mixing and equilibration of reactants between catalyst beds, correcting any temperature and flow maldistributions, and minimizing pressure drops. The mixing of fluids between catalyst beds can be accomplished through the use of distributer assemblies and mixing chambers. With present day refinery economics dictating that hydroprocessing units operate at feed rates far exceeding design, optimum interbed fluid mixing is a valuable low-cost debottleneck. 
     Distributer assemblies can be used to collect, mix, and distribute fluids in the interbed region of multi-bed catalyst reactors. Distributor assemblies generally include a trough for collecting and mixing liquid and gas flowing from an overlying catalyst bed and a mixing device or chamber disposed centrally within the trough for receiving liquid from the trough and further mixing the liquid and gas. The mixing device is a key component of many distributor assemblies because it provides efficient and thorough mixing of fluids and helps avoid hot spots and poor temperature distribution. The mixing device has at least one inlet for receiving liquid from the trough and at least one outlet for directing flow toward an underlying catalyst bed. The mixing device will generally improve the mixing of liquids and gases through a turbulence or swirling effect. If present, the mixing device, is generally located in the interbed space between catalyst beds in a reactor. The interbed space in many reactors is limited due to the presence of support beams, piping, and other obstructions which occupy the interbed region. Due to these space constraints, unique hardware, such as a mixing device scaled to fit the space available, is required to perform efficient two-phase mixing in what amounts to limited volume. 
     Due to the importance of sufficient interbed fluid mixing for good catalyst lifetimes, high throughput, long cycle length, and overall reactor performance, improved mixing devices are needed. In addition, mixing devices that can be retrofitted to existing reactors which have limited interbed space are of particular necessity. 
     SUMMARY OF THE INVENTION 
     The invention provides a novel means to provide more effective mixing of fluids in the space between catalyst beds in a multi-bed reactor. In particular, the invention is direct to a mixing device that improves the effectiveness of an existing mixing volume in mixing the gas phase and liquid phase of two-phase systems. The device is well suited for retrofit applications due to its relatively small size and can also be scaled for new reactor designs to achieve efficient fluid mixing in the interbed space of a multi-bed reactor. 
     According to the invention, the mixing device for use in a multi-bed down-flow reactor is positioned in the interbed space in the reactor and attached to a generally annular collection plate. The collection plate collects liquids and gas, herein referred to collectively as “fluid” or “fluids” flowing from the overlaying catalyst bed and helps direct the fluids to the opening of the mixing device. Collection plates are well known in the art. The mixing device of the invention can be in a box-like chamber (mixing box) or simply disposed on top of the collection plate. The mixing device of the invention is generally cylindrical in shape with curved side walls and a planar top and bottom wall. The bottom wall of the mixing device is mounted to the collection plate in a fluid tight manner. In an embodiment, the cyclindrical side walls are mounted between the catalyst support beams. 
     The mixing device is generally cylindrical in shape with a) a generally circular top wall essentially parallel to the bottom of the first catalyst bed; b) a generally circular bottom wall essentially parallel to the top wall and comprising a generally circular centrally located outlet opening; c) a cylindrical divider wall between said top wall and said bottom wall in essentially fluid tight contact with said top and said bottom wall and wherein said divider wall has at least one opening that extends from the bottom of the divider wall to less than the full height of the divider wall; and d) at least one inlet passageway for fluid. In an embodiment, the mixing device further comprises at least one cylindrical riser wall wherein the riser wall surrounds the outlet opening and wherein the radius of the riser wall, as measured from the center of the outlet opening to the surface of the riser wall, is less than the radius of the divider wall, as measured from the center of the outlet opening to the surface of the divider wall, and wherein the riser wall is in fluid tight contact with the bottom wall but does not extend to the full height of the divider wall. As used herein “essentially parallel” means that two objects and/or components are within 5 degrees of being coplanar. The inlet passageway is formed by the outer surface of the divider wall, the divider wall opening, a curved side wall abutting the outer surface of the divider wall and encompassing the divider wall opening, and a top capping portion. The sidewall is connected to the divider wall in an essentially fluid tight manner. The side wall extends less than the full height of the divider wall and is level with the top of the divider wall opening. The capping top portion is in essentially fluid tight contact with the top of the side wall and the outer surface of the divider wall and level with the top of the divider wall opening. The divider wall, divider wall opening, side wall, and top capping portion form the at least one inlet passageway. The inlet passageway is of greater radius, as measured from the center of the outlet opening to the side wall, at the inlet passageway opening than at the sidewall divider wall opening thus forming a constricted passageway to thereby impart an arcuate flow to the entering fluid. 
     In an embodiment, as fluids enter the mixing device through the inlet, the inlet passageway volume tapers so as to increase the turbulence of the fluids within the mixing device and to direct the fluids in a swirling motion to the first mixing zone. The first mixing zone can be defined by the bottom of the mixing device, the exterior surface of the fluid stabilizer, the interior surface of the divider wall, and the exterior surface of the riser wall. As used herein, unless otherwise specified, “interior” surface refers to the surface facing a central axis which extends from the top wall through the center of the outlet opening. As used herein, unless otherwise specified, “exterior” surface refers to the surface facing the sidewall of the reactor. The riser is a cylindrical wall in fluid tight contact with the bottom wall of the mixing device and extending upward but not in fluid tight contact with the top wall of the mixing device. The exterior of the riser wall faces the interior of the divider wall. Attached to the exterior wall of the riser is the flow stabilizer which forms an interior channel within the mixing device and helps define the first mixing zone. 
     In an embodiment, fluids overflow the flow stabilizer and collect in the interstitial space between the flow stabilizer and the riser in a second mixing zone. The riser and flow stabilizer are designed to increase swirling and turbulence of the fluids allowing more effective mixing and temperature equilibration. In an embodiment, one or more perforated plates can be attached to the riser and/or flow stabilizer. If perforated plates are present, the fluids flow over and/or through the perforated plates, increasing droplet formation and improving mixing. The fluids enter a plurality of slots of a bubble cap  85  and overflow the top of the riser before entering the annular outlet region. The bubble cap  85  is attached to the top wall  49  of the mixing device  40  and descends parallel to the divider  45  and riser  75 . The bubble cap  85  is approximately mid-way between the exterior wall of the riser  75  and the interior wall of the flow stabilizer  60 . The bubble cap  85  is disposed over and about the riser  75  such that the top of the cap is above the top of the riser so as to provide a passageway for the fluids to enter the third mixing zone. The bubble cap  85  is generally cylindrical and has a diameter larger than the riser  75  but smaller than the diameter of the flow stabilizer  60  as measured from the top of the flow stabilizer to the center axis of the mixing device. The third mixing zone is defined by the annular passageway between the outer surface of the riser and the inner surface of the bubble cap. The slots  86  of the bubble cap  85  are substantially symmetrically disposed about the circumference of the bubble cap&#39;s open end and begin at the lower extremity and ascend substantially vertically up a portion of the bubble cap. This configuration allows fluids to pass from the second mixing zone to the third mixing zone prior to egress through the annular outlet. 
     The inner wall of the cylindrical walled riser forms the annular outlet  90 . Optionally attached to the inner wall of the riser is a helicoidal perforated plate  75  which acts as a baffle to increase droplet formation and improve fluid mixing. The helicoidal perforated plate  75  does not extend the full diameter of the annular outlet  90 . The helicoidal plate forms a semi-spiral baffle that is optionally fixed to the inner wall of the riser in the outlet region of the mixing device. Fluids egress the mixing device through the outlet and discharge on to the interbed re-distributor tray  31 ,  FIG. 1 , before contact the subsequent catalyst bed. Quench gas or fluid may be injected at multiple points into the collection plate or into the mixing device as desired. 
     The mixing device is configured for disposal and fixable attachment between the inner surfaces of the walls of a multi-bed downflow reactor and for disposal between vertically stacked catalyst beds within the reactor. The mixing device can be configured to fit between the catalyst support beams in an existing reactor in a retrofit application. 
     These and other features of the invention will be more readily understood upon reading the following Detailed Description in conjunction with the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic view of an embodiment of the mixing device of the invention situated within a multi bed catalytic reactor. 
         FIG. 2  is a transparent schematic perspective view of an embodiment of the invention. 
         FIG. 3  is a top plan view of an embodiment of the invention. 
     
    
    
     DETAILED DESCRIPTION 
     In an embodiment, the mixing device of the invention is centrally disposed on top of the collection plate and between the catalyst support beams. The mixing device of the invention has at least one inlet for receiving fluids from the trough. In an embodiment, the mixing device has two inlets to receive fluids from the collection plate. The use of two inlets oriented at 180 degrees from each other with their openings situated so as to reinforce the circular, swirling motion of the incoming fluids improves the turbulence effect of the mixing device design. 
     As shown in  FIG. 1 , a multi-bed, down-flow reactor  10  has a cylindrical sidewall  11 . The section shown in  FIG. 1  is a cross section of a multi-bed catalytic reactor. Each catalyst bed,  5  and  6 , contain packed particulate catalytic material. Each catalyst bed is supported on a grid screen assembly  12  comprised of a support grid, space cloth and wire screen, all of which are well known in the art. The grid screen assembly is mounted on parallel support beams  14  and  15  that are horizontally mounted to the reactor wall  11 . The mixing device  40  of the invention is mounted between the catalyst support beams, under the catalyst bed  5 , and in fluid tight contact with the collection plate  30  to receive liquid and gas flowing down from the bottom surface of the overlying catalyst bed  5  and to mix the liquid and gas. 
       FIG. 2  is a transparent schematic perspective view of an embodiment of the mixing device  40 . A bottom wall of the mixing device  41  is mounted in fluid tight connection to the collection tray. The annular collecting tray collects fluids flowing down from the overlying catalyst bed prior to the fluids reaching inlet passageways  50  and  55  of the mixing device  40 . 
     The mixing device  40  has cylindrical side walls  42  and  48  that are mounted in fluid-tight relation to the bottom wall  41 . A top capping portion,  46  and  47 , is mounted in fluid tight connection to the top of the side walls  42  and  48 , respectively. A divider  45  spans the full height of the mixing device and is in fluid tight contact with the top wall,  49 . The divider wall has an exterior surface,  43  and an interior surface,  44 . 
     The mixing device  40  has a generally cylindrical shape adapted to extend upwardly from the collection tray  30  between the pair of cross beams  14  and  15  that support the overlying catalyst bed. In an embodiment, the mixing device  40  has a width adapted to occupy the space between the cross beams. The mixing device external surface is formed by side walls  42  and  48  of the inlets  50  and  55 , the top wall  46  and  47  of the inlets, the top wall  49 , and the exterior surface  43  of the divider  45 . In an embodiment, the bottom wall  41  of the mixing device  40  is formed integrally with, and comprises a central portion of, the collecting tray  30 . That is, the bottom wall of mixing device  40  can be formed by the collection tray itself. In another embodiment, the bottom wall of the mixing device is a separate wall and is mounted directly to the collecting plate in essentially fluid tight contact. The collecting tray  30  is mounted under and in close proximity to the cross beams so that the mixing device of the invention  40  is disposed between the central pair of cross beams, to thereby minimize the vertical space taken up by the mixing device  40 . By minimizing the space needed for the mixing device, efficient use of overall volume of the reactor and higher catalyst fill volume can be achieved. 
     In an embodiment, the mixing device  40  has two inlets  50  and  55  extending from the outer wall  42  and  48  respectively, and a single outlet  80  in the bottom center of the bottom wall  41  (as shown in  FIG. 2  and  FIG. 3 ). The outlet opening traverses the collecting tray  30 . The inlet  50  formed by outer wall  42 , top capping portion  46 , bottom wall  41 , and the exterior surface  43  of the divider wall  45 , has an upstream end forming a rectangular opening for fluid entrance from the collecting tray  30 . The inlet  55  formed by outer wall  48 , top capping portion  47 , bottom wall  41 , and the exterior surface  43  of divider wall  45 , has an upstream end forming a rectangular opening for fluid entrance from the collecting tray  30 . The bottom edge of the divider wall  45  forms a substantially fluid-tight seal with the collecting tray  30 , and the outer surface  43  of divider wall  45  forms a generally fluid-tight seal with the top capping portions  46  and  47  of the inlets  50  and  55 , respectively. The two inlets  50  and  55  are designed to allow fluid entrance in the same direction of flow, that is the incoming fluids will reinforce the swirling direction of the fluids within the mixing device. The opening  51  and  56  ( FIG. 3 ) in the divider wall allows fluid entrance into the first mixing zone of the mixing device  40 . 
     As best seen in  FIGS. 2 and 3 , one inlet passageway is formed by the exterior surface  43  of the divider wall  45 , the divider wall opening  51 , the curved side wall  42  abutting the exterior surface  43  of the divider wall  45  and encompassing the divider wall opening, and a top capping portion  46 . The sidewall  42  is connected to the divider wall  45  in an essentially fluid tight manner. The side wall  42  extends less than the full height of the divider wall  45  and is level with the top of the divider wall opening  51 . The top capping portion  46  is in essentially fluid tight contact with the top of the side wall and the outer surface of the divider wall and level with the top of the divider wall opening. The inlet passageway is of greater radius, as measured from the center of the outlet opening to the side wall, at the inlet passageway opening  50  than at the divider wall opening  51  thus forming a constricted passageway to thereby impart an arcuate flow to the entering fluid. As shown in  FIGS. 2 and 3 , in an embodiment the mixing device has a second inlet opening  55 . The second inlet passageway is formed in a similar manner to the inlet passageway described above. The second inlet passageway is formed by the exterior surface  43  of the divider wall  45 , the divider wall opening  56 , the curved side wall  48  abutting the exterior surface  43  of the divider wall  45  and encompassing the divider wall opening, and a top capping portion  47 . The sidewall  48  is connected to the divider wall  45  in an essentially fluid tight manner. The side wall  48  extends less than the full height of the divider wall  45  and is level with the top of the divider wall opening  56 . The capping top portion  47  is in essentially fluid tight contact with the top of the side wall and the outer surface of the divider wall and level with the top of the divider wall opening. The inlet passageway is of greater radius, as measured from the center of the outlet opening to the side wall, at the inlet passageway opening  55  than at the divider wall opening  56  thus forming a constricted passageway to thereby impart an arcuate flow to the entering fluid. 
     As fluid rains down at various locations into the annular collecting tray  30 , the presence of inlets  50  and  55  creates an arcuate or circular mixing pattern of the fluids. The circular flow pattern in the horizontal plane of the trough induces relatively low energy mixing of the fluid in the pool with that fluid raining down from the catalyst bed, so that the fluid entering inlets  50  and  55  has relatively uniform temperature and composition. The size of the inlets  50  and  55 , particularly, the width and height of the upstream and downstream ends thereof, is selected in view of the operating range of flow rate of liquid through the reactor  10 . It will thus be seen that a pool of liquid of substantial depth can be accumulated in the collecting tray by means of the constricted inlet passageways. This is due to the fact that the mixing device is contiguous to the collecting plate and that the mixing device comprises the only outlet  80  from the collecting plate  30 . The inlet passageway restrict the flow of fluids from the collecting plate  30  to cause fluids to accumulate on the plate and that also, by virtue of the constriction of the flow and expansion downstream thereof imparts turbulence to the liquid and gas stream exiting therefrom. 
     As may be seen in  FIGS. 2 and 3 , the inlet passageways to the mixing device  40  extend along the periphery of the mixing device and are radially offset from the axis of the reactor wall  11  and are also tangential to the circular outlet  80  formed in the bottom wall  41  of the mixing device. This orientation relative to the outlet  80 , together with the annular shape of the flow passageway within the mixing device, imparts a rotating or spiraling flow pattern to the liquid and gas that is additive. Also, the taper of the inlet passageways converges in the direction of flow of liquid and gas from the collecting tray  30  into the mixing device  40 , to thereby induce turbulence in the flow as the liquid and gas flows through the constricted downstream end of the inlet passageway into the first mixing zone of the mixing device  40 . The radius formed from the center point of the outlet  80  and the cylindrical side wall  42  varies from a maximum at the inlet opening ( 50  and  55 ) to a minimum at the junction of the side wall ( 42  and  48 , respectively) and the divider wall  45 . The changing radius of the side wall  42  and the center point of the outlet  80 , and side wall  48  and the center point of the outlet  80 , creates the taper in the inlet passageways which increases the arcuate motion of the fluids entering the mixing device  40 . 
     As fluids enter the mixing device  40  through inlets  50  and  55 , they enter the first mixing zone. The first mixing zone is bounded by divider wall  45  and riser wall  75 . Riser wall  75  comprises an exterior surface  73  and an interior surface  74 . The outer surface  73  of riser  75 , the inner surface  44  of the divider wall  45 , and the top wall  49  delimit the first mixing zone. Projecting into the mixing zone from the outer surface  73  of riser  75  is one or more perforated plates and one or more flow stabilizers. As shown in  FIG. 2 , perforated plate  62  extends outward from the outer surface  73  of riser  75  into the first mixing zone. In an embodiment, the perforated plate forms an approximately 45 degree angle with the riser. The perforated plate  62  extends into the mixing zone towards the divider wall,  45 , but does not intersect the divider wall. 
     A semi-cylindrical flow stabilizer  60  is mounted to the exterior surface  73  of the riser  75 . The flow stabilizer  60  joins the riser approximately half the full height of the riser and projects outwardly into the space between the divider wall and the riser at an approximately 45 degree angle. The flow stabilizer continues this 45 degree outward projection until it is essentially level with a downward projecting bubble cap  85  and in the space between the bubble cap  85  and the divider wall  45 . The flow stabilizer then projects upward parallel to the bubble cap  85  and the divider wall  45  until the top of the flow stabilizer is higher than the bottom of the downwardly projecting bubble cap but below the top wall  49  of the mixing device  40 . The flow stabilizer constrains the fluid flow to the region of the mixing device delimited by the interior surface  44  of the divider wall  45  and the outer surface of the flow stabilizer. The angle of the flow stabilizer is designed to increase the swirling motion of the fluids in the mixing device and to increase turbulence. Optionally, the flow stabilizer can have a perforated plate  61  attached to its top portion and projecting into the space between the top of the flow stabilizer and the divider wall  45 . The perforated plate  61  can be parallel with the top wall  49  and bottom wall  41  of the mixing device. The perforated plate  61  does not extend the full length of the space between the top of the flow stabilizer  60  and the divider wall  45 . 
     As the fluids enter the first mixing zone they flow in an arcuate motion and contact perforated plate  62 . The fluid level increases until the fluids are at the level of the top of the flow stabilizer  60 . A perforated plate  61  located at the top of the flow stabilizer further increases droplet formation and improves mixing as the fluids overflow the flow stabilizer and enter the second mixing zone. The second mixing zone is delimited by the interior wall of the flow stabilizer  60  and the exterior wall  73  of the riser  75 . The bubble cap  85 , projects downward from the top wall  49  into the space between the flow stabilizer and the riser. The bubble cap has a plurality of slots  86  located essentially symmetrically about the bubble cap and extending from the bottom of the bubble cap part way up the height of the bubble cap. The top of the slots is lower than the top of the riser  75 . Further liquid mixing and liquid-gas entrainment is provided by the combination of the perforated plate and bubble cap. The narrow bottom portion of the second mixing zone and the wider top portion help increase the swirling flow of the fluid. An optional perforated plate  63  projects into the second mixing zone to increase droplet formation. Perforated plate  63  is mounted to the exterior wall of riser  75  at an approximately 45 degree angle. The perforated plate  63  extends part way into the second mixing zone without contacting the flow stabilizer or the downward projecting bubble cap  85 . Fluids fill the second mixing zone while maintaining a swirling flow until they reach the slots  86  of the bubble cap  85 . The fluid then overflows the riser  75  and enters the third mixing zone. 
     The third mixing zone is delimited by the cylindrical interior wall  74  of riser  75 . Optionally mounted to the interior wall of riser  75  is a helicoidal perforated plate  90 . The helicoidal perforated plate  90  curves around the interior of the riser wall in the same direction as the fluid flow, further increasing the swirling motion of fluids through the mixing device. The helicoidal perforated plate  90  starts below the top of the riser  75  and curls or spirals downward in a corkscrew manner such that the height of the helicoidal plate at the end closest to the top of the riser  75  is greater than the height of the helicoidal plate and the opposite end closest to the outlet opening  80 , where the height of the helicoidal plate is measured from the outlet opening and perpendicular to the bottom wall. The helicoidal plate  90  extends from below the top of the riser and ends above the outlet opening  80 . The outlet opening  80  is formed by an annular opening in the bottom wall  41  and an annular opening in the collection tray  30 . In an embodiment, outlet  80  is a generally circular aperture formed at the center of the collector tray  30  and coaxial with the axis of the reactor side wall  11 . Fluids then exit the mixing device through outlet  80 . One or more catalyst beds are located below outlet  80 . 
     Quench gas (for example, hydrogen) can be injected through one or more tubes into the pool of fluid collected within the collecting tray  30  prior to entrance of the fluid into the mixing device. Quench gas injection at one or more locations can cause agitation of the pool of liquid at each location, to thereby enhance the mixing of liquid falling into and flowing through the trough prior to entry of the fluid into the mixing device. 
     The previous description of a preferred embodiment of the present invention is primarily for illustrative purposes, it being recognized that a number of variations might be used which would still incorporate the essence of the invention. Accordingly, reference should be made to the following claims in determining the scope of the invention.