Abstract:
The present invention is directed to a vortex-type mixing device for a down-flow hydroprocessing reactor. 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 hydroprocessing reactor.

Description:
FIELD OF THE INVENTION 
       [0001]    This invention is directed to a vortex-type mixing device for a down-flow hydroprocessing reactor. Such down-flow hydroprocessing reactors are used in the petroleum and chemical processing industries for carrying out catalytic reactions of hydrocarbonaceous feedstocks in the presence of hydrogen, at an elevated temperature and pressure. Exemplary reactions including hydrotreating, hydrofinishing, hydrocracking and hydrodewaxing. 
       BACKGROUND OF THE INVENTION 
       [0002]    In fixed-bed hydroprocessing reactors, gas and liquid reactants (e.g. 
         [0003]    hydrogen and a hydrocarbonaceous feedstock) flow downward through one or more beds of solid catalyst. (See, e.g. U.S. Pat. No. 4,597,854 to Penick). 
         [0004]    As the reactants flow downward through the reactor catalyst beds, the reactants contact the catalyst materials and react to produce the desired products. Gas reactants such as hydrogen are consumed, and heat is generated by the catalytic reactions. Controlling the temperature of the feedstock as it travels downward through the reactor is important to ensure the quality and quantity of product yield is maximized toward the target product(s). 
         [0005]    Cool hydrogen-rich gas can be introduced between the catalyst beds to quench the temperature rise and replenish the hydrogen consumed by the reactions. In order to maintain overall reactor performance, the temperature of the fluids within the reactor should be as uniform as possible and liquids and gases should be 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 the radial temperature differences, these differences 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 product 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 event, which can damage the catalyst, the vessel, or downstream equipment. 
         [0006]    Due to these hazards, refiners operating with poor reactor 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. 
         [0007]    Distributor 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 overhead 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. 
         [0008]    The mixing device is a key component of many distributor assemblies because it provides efficient and thorough mixing of fluids/gases and helps avoid hot spots and poor temperature distribution. 
         [0009]    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. Designs for mixing devices vary, including baffle mixer designs such as ribbon blenders and disk-and-donut type mixers that promote mixing through changing the direction of the fluid and gases. 
         [0010]    Another type of mixer is a centrifugal or vortex-type design. This type of mixer collects the liquid and gas streams flowing downward through the reactor, and introduces them into a circular chamber where they make several rotations before being passed downward through a centrally located aperture. 
         [0011]    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. In addition, lower height distributor assemblies can increase catalyst loading volume with the same reactor volume, therefore improve utilization of the reactor volume. 
         [0012]    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 have lower vertical footprint and that can be retrofitted to existing reactors which have limited interbed space are of particular necessity. 
       SUMMARY OF THE INVENTION 
       [0013]    The present invention is directed to a vortex-type mixing device for a down-flow hydroprocessing reactor. The mixing device provides a 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. 
         [0014]    The mixing device includes a horizontal top plate having an inner surface and a base plate extending parallel to the top plate. The base plate having an inner surface and a base plate aperture. 
         [0015]    A plurality of inwardly-curved vanes extend vertically between the inner surfaces of the top and base plates. A vertical weir ring extends vertically from the base plate inner surface proximal to the circular aperture. The weir ring has a weir ring top edge and a weir ring diameter. A bubble cap extends downwardly from the inner surface of the cover plate into a mixing region. The bubble cap has a bubble cap diameter and a bottom edge, the bubble cap diameter being smaller than the weir ring diameter, and the bubble cap bottom edge extends below the weir ring top edge. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0016]      FIG. 1  is a schematic view of an embodiment of the mixing device of the invention situated within a multi bed catalytic reactor. 
           [0017]      FIG. 2  is a cross-sectional diagram of the mixing device of the present invention. 
           [0018]      FIG. 3  is an isometric view of one half of the mixing device  26 , and  FIG. 4  is a top plan view of the mixing device  26 . 
           [0019]      FIG. 4  is a top plan view of the mixing device. 
           [0020]      FIG. 5  is a top plan view illustrating the layout of the vanes. 
       
    
    
     DETAILED DESCRIPTION 
       [0021]    It has been found that the vortex-type mixing device of the present invention affords benefits over vortex-type mixing devices known in the art. Such benefits include, a reduced vertical footprint in the reactor (reduction in reactor volume occupied by inter bed distributor assemblies), high throughput, enhanced mixing, lower pressure drop, and enhanced overall reactor performance. Specific embodiments and benefits are apparent from the detailed description provided herein. It should be understood, however, that the detailed description and specific examples, while indicating embodiments among those preferred, are intended for purposes of illustration only and are not intended to limit the scope of the invention. 
         [0022]    The present invention is directed to a vortex-type mixing device for a multi-bed hydroprocessing reactor. Referring to  FIG. 1 , a cross-sectional diagram of a portion of a multi-bed down-flow reactor  10  is illustrated. The reactor  10  includes a vessel shell  12 , upper and lower catalyst beds ( 14  and  16 , respectively) containing packed catalytic extrudates. Each catalyst bed  14 ,  16  is supported on a grid screen assembly  18  (illustrated for catalyst bed  14  only) composed of a support grid, optional space cloth and screen, all of which are well known in the art. The grid screen assembly is mounted on parallel support beams  20  that are horizontally mounted to the reactor vessel inner wall  22 , and extend upwardly into the catalyst bed  14 . 
         [0023]    An interbed distribution assembly  24  is vertically interposed between the catalyst beds  14 ,  16 . The interbed distribution assembly  24  includes a vortex-type mixing device  26  of the present invention. The mixing device  26  of the invention is mounted under the catalyst bed  14 , and in fluid tight communication with a collection plate  28  adapted to receive and mix liquid and gas flowing down from the overhead. A quench gas inlet tube  30  distributes quench gas (e.g. hydrogen) into the region above the mixing device  26 . 
         [0024]      FIG. 2  is a cross-sectional diagram of the mixing device  26 ,  FIG. 3  is an isometric view of one half of the mixing device  26 , and  FIG. 4  is a top plan view of the mixing device  26 . 
         [0025]    The mixing device includes a base plate  32  having an inner surface  32   a  and mounted in fluid tight communication with the collection plate  28  ( FIG. 1 ), and a cover plate  34  having a cover plate inner surface  34   a  which extends substantially horizontally relative to the base plate  32 . The annular collection plate  28  collects fluids flowing down from the overlying catalyst bed. 
         [0026]    A plurality of staggered, inwardly-curved vanes  36  are fixedly attached to, and extend vertically between, the base and cover plate inner surfaces  32   a  and  34   a,  respectively. In one embodiment, the vanes  36  are in fluid tight communication with the base and cover plate inner surfaces  32   a  and  34   a,  respectively. 
         [0027]    As shown in  FIG. 4 , each vane has a vane external end  38  affixed proximal to the outer periphery of the top plate  34 , and internal end  40  located proximal to a mixing region (described herein below) of the mixing device  26 . 
         [0028]    The open space between the vanes  36  define a series of mixing device inlet regions  42 , each inlet region  42  being defined as area bound by neighboring vanes  36  and their respective ends  38 ,  40 . 
         [0029]    Referring again to  FIG. 2 , a weir ring  44  extends vertically from the base plate  32 , proximal to a centrally-positioned circular base plate aperture defined by edge  46 . In one embodiment, the weir ring  44  has a vertical height of one-half of the vertical height of the vanes  36 . A weir ring horizontal plate  48  extends outwardly from the weir ring upper edge  50 . The weir ring horizontal plate  48  is preferably perforated. 
         [0030]    A circular bubble cap  52  affixed to, preferably in fluid tight communication with, the cover plate inner surface  34   a  extends from the cover plate  34  downwardly into the center of a mixing region of the mixing device  26  (the mixing region being defined as the area between the plates  32 ,  34 , excluding the inlet regions  42 , or, stated differently, the region defined by the area between opposing vane internal ends  40 ). The bubble cap may be keyed as illustrated in  FIGS. 2 and 3 . 
         [0031]    The lower end  54  of the bubble cap  52  extends a distance  52   a  below the weir ring upper edge  50 . In one embodiment, wherein the bubble cap lower edge  54  is keyed, the upper edges  58  defining the keyed openings are positioned below the weir ring upper edge  50 . 
         [0032]    The collection plate  28  includes a circular aperture defined by edge  60 , and a riser tube  62  proximal to the collection plate circular aperture  60  extends vertically upward from the collection plate  28  and into the bubble cap  52 . The top edge  64  of the riser tube  62  is situated at or above the bubble cap upper edge  58  of the keyed openings. 
         [0033]    As shown in  FIG. 2 , the weir ring  44 , bubble cap  52  and collection plate aperture  60  each have a diameter  44   a,    52   a  and  60   a,  respectively, wherein the measured values of the diameters have the following relationship:  60   a &lt; 52   a &lt; 44   a.    
         [0034]    In operation, hydrocarbonaceous liquid feed rains down from the catalyst bed  14 , through the grid screen assembly  18 , and onto the annular collection plate  28 . Typically, the liquid will collect and rise to a liquid level at or above the height of the weir ring horizontal plate  48 . Gas from the upper catalyst bed  14  mixed with quench gas (e.g. hydrogen gas) introduced via the quench gas inlet tube  30  fills the void between the liquid collected on the annular collection plate  28  and the catalyst bed  14 . 
         [0035]    The liquid and gas enter the mixing device  26  via the mixing device inlet regions  42 , wherein the vanes  36  tangentially direct the liquid and gas to flow in an arcuate or circular flow pattern as the liquid and gas enter the mixing region of the mixing device  26 . The liquid travels up and over the weir ring  44  (and through the horizontal weir plate  48  if it is perforated), and intermixes with the gas as the liquid/gas flow into the bubble cap  52  under its lower end  54  and through the keyed openings, over the riser tube top end  64  and into the riser tube  62 . The intermixed gas and liquid then travel downward out of the riser tube  62 , typically to a tray containing a plurality of perforations, downcomers or nozzles, and then on to the lower catalyst bed  16 . Perforated spiral plates are installed on inner surface of the riser  62  to further improve gas/liquid mixing while flowing downward in the riser  62 . 
         [0036]    As can be appreciated by one skilled in the art, a mixing device  26  as described herein is intended for use in a large hydroprocessing reactor designed to process thousands or tens-of-thousands of barrels of feedstock per day (1 barrel=43 gal.; 164 L). Accordingly, the mixing device  26  described herein may be several feet in diameter and, because of the materials used to construct the device  26  (e.g. ¼″-½″ plate steel), weigh several hundred pounds (.lbs) when constructed. 
         [0037]    The mixing device  26  of the present invention may be constructed in place by welding or otherwise affixing the individual components together to achieve construction of the finished device  26 . However, it will be recognized that constructing the device  26  in place using this method may take several days, delaying operation of the reactor unit. In addition, where the device  26  is being employed to update or retrofit the design of an existing reactor, it is desirable to reduce the amount of assembly taking place within the reactor vessel (due to safety concerns such as possibly igniting residual hydrocarbon materials remaining in the reactor). 
         [0038]    In order to reduce the amount of time needed to construct a new reactor, or retrofit an existing reactor, portions of the mixing device  26  are preferably pre-assembled to form subassemblies, and the subassemblies are inserted into the reactor and assembled to form the completed mixing device  26 . 
         [0039]    In one embodiment illustrated in  FIGS. 2 ,  3  and  4 , the mixing device  26  consists of two mixing device subassemblies  26   a , 26   b,  each representing one-half of the mixing device  26 . Each subassembly  26   a , 26   b  is provided with one or more lifting lugs  66  and  68 , respectively. The lifting lugs  66 , 68  are provided for attaching each subassembly  66 , 68  to a hoist, crane or other device capable of lowering the subassembly into the reactor and maneuvering the subassembly into place. 
         [0040]    Each subassembly  26   a , 26   b  is provided with a mating flange  70  and  72 , respectively, containing a plurality of openings through which a nut/bolt combination (or such other appropriate affixing device) can be inserted to hold the subassemblies  26   a , 26   b  in place during operation, and further allows the subassemblies  26   a , 26   b  to be disassembled between operating periods during maintenance to allow access to the areas above and below the mixing device  26 . 
         [0041]    Referring to  FIG. 5 , the horizontal placement of the vanes  36  is illustrated. Circle R 1 , representing the outer diameter of the mixing device  26  as prescribed by process hydraulic calculations employed by those skilled in the art, along with circle R 3 , presenting the inner diameter of the mixing device inlet regions  42  also prescribed by process hydraulic calculations, are illustrated. Circle R 2  is located half the radial distance between R 1  and R 3 . 
         [0042]    Angle “A” of  FIG. 5  represents the angular offset of each corresponding vane  36 , which results in a radial overlap of a vane internal end  40  with a larger portion of a neighboring vein external end  38 . In one embodiment, A=15° for a 4 vane system, 10° for a 6 vane system, and 8° for a 8 vane system. Angle “B” represents the radial distance the vane  36  occupies within the region between R 1  and R 3 . In one embodiment, B=360°/(the number of vanes). At angle “A”, the inner surface of the vane  36  intersects with R 1 . At an angle that is the sum of angles “A” and “B” (A+B) the inner surface of the vane  36  intersects with R 3 . At an angle that equals A+B/2, the inner surface of the vane  36  intersects with R 2 . 
         [0043]    The radial overlap of the vanes  36  is defined by angle “A”. In operation, the liquid and gas enter the mixing device  26  via the mixing device inlet regions  42 , wherein the vanes  36  tangentially direct the liquid and gas to flow in an arcuate or circular flow pattern as the liquid and gas enter the mixing region of the mixing device  26 . 
         [0044]    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.