Patent Publication Number: US-2017361242-A1

Title: Tray assembly for gas/liquid contact tower

Description:
FIELD OF THE INVENTION 
     The present invention relates to a tray assembly for gas/liquid contact in a chemical process tower. 
     BACKGROUND OF THE INVENTION 
     Several tray designs are known for gas-liquid contactors used in chemical processes including reactions and separations. In each design, trays are situated within the towers for contact between the gas and liquid within the towers. Trays for large diameter towers are prone to liquid flow maldistribution on the trays as described, for example, by M. J. Lockett in “Distillation Tray Fundamentals” (1986). It is also known that liquid flow maldistribution on a tray affects tray efficiency—see Wijn, E. F. in “The effect of downcomer layout pattern on tray efficiency” published in The Chemical Engineering Journal, vol. 63, pages 167-180 (1996). 
     Several tray designs intended for correcting liquid flow maldistribution have been invented, of which the following are representative examples: U.S. Pat. No. 3,729,179 (1973) issued to Keller; U.S. Pat. No. 3,747,905 (1973) issued to Nutter et al.; U.S. Pat. No. 5,269,976 (1993) issued to Biddulph et al; U.S. Pat. No. 5,453,222 (1995) issued to Lee et al; U.S. Pat. No. 6,371,455 (2002) issued to Lee et al; U.S. Pat. No. 6,817,596 (2004) issued to Fischer; and U.S. Pat. No. 8,070,142 (2011) issued to Lee et al. 
     The above listed patents disclose various downcomer designs that allow the liquid to leave the downcomer and flow across an immediately subjacent tray in a more uniform pattern. However, because tray decks are not rectangular, some degree of maldistribution develops when the liquid travels some distance across a tray to an outlet downcomer. Unexpectly, it was discovered that liquid entering the downcomer does not undergo lateral mixing because there is no hydraulic gradient. In a multiple tray tower, the lack of downcomer mixing can develop a concentration gradient which propagates down the tower. All prior art patents disclose downcomer distributor designs that aim at distributing liquid at the entrance of the tray to improve liquid distribution across the tray, but none include means at the top of the downcomer for ensuring that the liquid entering the downcomer has a uniform composition. 
     In spite of previous efforts, there still exists a need to address the maldistribution issue. The present invention addresses the maldistribution issue by providing a downcomer, which incorporates both a novel liquid mixer with a novel liquid distributor. 
     SUMMARY OF THE INVENTION 
     The tray assembly of the present invention includes an improved tray design for use in a chemical process tower. The tray assembly includes a downcomer having a mixer its inlet and a distributor in its outlet. 
     In combination, the mixer and distributor effect relatively even liquid flow across the surface of a tray immediately below the downcomer, thereby avoiding the formation of stagnant regions. The net result is that there is higher tray capacity and efficiency when compared to prior art tray assemblies. 
     It has been discovered that liquid mixing in the downcomer enhances and optimizes mass transfer efficiency. This is achieved by providing a mixer in the downcomer inlet comprising at least one vertical baffle located centrally in the downcomer and spaced apart from a downcomer floor. Preferably, the baffle is substantially parallel to and downstream of a downcomer inlet weir in the direction of fluid flow. 
     In one embodiment, the mixer is in the form of a plurality of spaced apart, vertically extending, corrugated plates having ridges and grooves for directing flow outwardly toward the perimeter of the tower wall. 
     The distributor is in the form of a plurality of spaced apart flow directing plates disposed at various angles to the vertical and spaced from a tray deck immediately below, for directing fluid flow substantially evenly across a fluid receiving/distribution area of the tray below. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is an isometric view of one embodiment of a tray assembly in accordance with the present invention; 
         FIG. 2  is an end view of the tray assembly of  FIG. 1 ; 
         FIG. 3  is an isometric view on one end of the tray assembly showing details of a mixer; 
         FIG. 4  is an isometric view of a distributor used in the tray assembly of  FIG. 1 ; 
         FIG. 5  is a top view of the distributor of  FIG. 4 ; 
         FIG. 6  is an end view of the distributor of  FIG. 5 ; and 
         FIGS. 7A and 7B  illustrate time lapse sequences of comparative liquid flow tests over tray decks, wherein  FIG. 7A  shows results for a prior art system and  FIG. 7B  shows results for a tray assembly according to the invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     A tray assembly used for gas-liquid contact for use in a distillation tower is described below. The assembly includes a tray with a downcomer at one end thereof. The downcomer contains a mixer and a distributor suspended below a perforated floor by conventional means such as bolting or welding. The mixer is located in the inlet opening of the downcomer. The mixer includes at least one corrugated mixing baffle. The number of baffles depends upon the dimensions of the downcomer and operating conditions. Where two or more baffles are provided, they are spaced from one another and arranged substantially parallel to an outlet weir. The baffles are mounted vertically and extend downwardly toward a perforated bottom floor of the downcomer and are spaced therefrom. 
     The distributor includes a plurality of plates spaced apart from one another and attached to the bottom of the perforated downcomer floor by conventional means, e.g. a bracket. The plates are spaced from a tray assembly immediately below. The purpose of the distributor plates is to divide and direct fluid flow substantially evenly across a receiving, bubbling/froth initiating area of a subjacent tray and to and guide the froth thus formed in a divergent flow pattern across the subjacent tray while achieving approximately the same liquid residence time perpendicular to the bulk flow direction on the tray. 
     The combination of mixer and distributor provides more uniform liquid concentration in the downcomer and more even liquid flow across the tray immediately below the downcomer. The integrated design of mixer and distributor reduces maldistribution of liquid flow and liquid concentration present in other designs, which in turn improves the efficiency of gas-liquid contact on the tray. 
     With reference to  FIG. 1 , a tray assembly, which is indicated generally at  10 , is installed in a cylindrical chemical process tower indicated generally at  12 . The tray assembly  10  is affixed to the inside of the tower wall  16  by a C-shaped frame assembly  19 . The tray assembly includes a deck  30  containing perforations  33 . The tray deck  30  can be made of one or several sections, and at least one downcomer indicated generally at  14  is located at an outlet edge of the deck  30  for transporting liquid-vapor mixtures between adjacent tray decks  30 . The downcomer  14  is defined by the tower wall  16 , an inclined inner wall  18 , a top inlet opening  31 , and a floor  40  containing perforations  41  defining an outlet from the downcomer. The inclined wall  18  tapers downwardly. The side edges of the inclined wall  18  are attached to the tower wall  16  by fluid-tight seals  21 . The horizontal downcomer floor  40  is attached to the inclined wall  18  by a plate  42  and rivets. The outer edge of the floor  40  is also attached to the tower wall  16  by a fluid-tight seal. The downcomers of adjacent trays are arranged in the tower diametrically opposite to one another. 
     The deck  30  is circular with a segment removed from one side thereof, the resulting opening  31  defining the inlet to the downcomer  14 . A vertical outlet weir  32  at the inlet opening  31  maintains a depth of a liquid-vapor mixture on the tray deck  30  and introduces the liquid-vapor mixture into the downcomer  14 . The weir  32  extends across the downcomer inlet opening  31  and is attached at its ends in fluid-tight manner to the support  19  and the tower wall  16  by a horizontal strip  32   a    
     A plurality of vertically spaced apart decks  30  are situated in the tower  12 . The decks  30  are oriented so that the downcomer  14  of one tray  10  is located above an inlet distribution/receiving area  30   a  of a deck immediately below. 
     The tray deck of  FIG. 1  includes an inlet panel section or area  30   a  having perforations  33  for initiating bubbles/froth formation and a downstream panel section  30   b  with perforations  33 . The deck  30  extends from the inlet distribution area  30   a  to the downcomer inlet opening  31 . Froth initiation in the area  30   a  may be effected by various conventional means such as by upwardly extending flanges (not shown) around the perforations  33 . It will be appreciated that the tray assembly  10  may include a plurality of downcomers  14 . 
     The downcomer  14  is adjacent the inner surface of the wall  16  of the tower  12 , and is bounded on one side by a portion of the inner surface of the wall  16 , and by the inclined wall  18 . The wall  18  extends downward across the length of an edge of the deck  30  toward the inlet area  30   a  of an immediately subjacent deck  30 . There is a gap between the bottom edge of the downcomer wall  18  and the subjacent tray deck  30 , the gap extending along all or the majority of the width of the downcomer wall  18 . 
     As best shown in  FIG. 3 , the weir  32  is attached to the top of the wall  18  by a horizontal plate  32   a  and rivets. The inclined wall  18  includes a bracket  41  for attaching the floor  40  to the bottom edge of the wall  18  using rivets. Other securing means such as welding can be used to interconnect elements of the assembly. 
     A mixer is mounted in the top end of the downcomer  14 . The mixer is defined by two spaced apart, corrugated mixing plates  35  located centrally in the top of the downcomer substantially parallel to the outlet weir  32  and to each other. The plates  35  are suspended from a pin  52  attached to the inner downcomer wall  18 . As best shown in  FIG. 3 , the corrugated plates  35  include alternating ridges  35   a  and grooves  35   b  for directing fluid flow within the downcomer  24 . As shown in  FIG. 2 , on one side of the vertical center, the ridges  35   a  and grooves  35   b  are inclined outwardly away from the center, and on the other side of center the ridges and grooves are inclined outwardly in the opposite direction. The plates  35  extend vertically downwardly to about one half the height of the downcomer  14 , and direct liquid flow towards the outside of the downcomer walls. The mixing plates  35  are located centrally in the top portion of the downcomer  14  and occupy approximately 70% of the top portion of the downcomer, the remaining 30% of the top portion being open. Preferably the ridges  35   a  and grooves  35   b  are inclined by 45° to provide mixing in both x and y directions, i.e. both vertically and horizontally. Since the plates  35  extend approximately half way down the downcomer, most of the fluid directed to the perimeter area will flow back to the central area below the plates, creating a mixing effect without pressure drop. Therefore, the capacity of the downcomer is not impeded by the mixer plates. The plates can be solid or perforated. 
     A distributor indicated generally at  50  is provided at the bottom of the downcomer. As best seen in  FIGS. 4 to 6 , the distributor  50  includes a plurality of spaced apart baffles  51   a  and  51   b  attached to a frame member  52  for directing fluid flow substantially evenly across the fluid receiving area of a deck immediately below. Two central baffles  51   a  extend vertically. The outer baffles  51   b  extend at various angles to the vertical. The angles and sizes of baffles  51   a  and  51   b  are designed to provide even liquid distribution for different tray dimensions as well as at different liquid and gas flow rates. 
     Preferably, the baffles  51   a  and  51   b  are of certain dimensions i.e. a height of at least 50 to 80% of the distance between the downcomer floor  40  and the tray  30  immediately below. Their length is preferably 30 to 70% of the floor width. 
     The mixer  50  is used to optimize the mass transfer efficiency needed for enhanced liquid mixing in the downcomer. The mixer design achieves a relatively uniform liquid concentration in the downcomer without affecting its capacity. 
     In operation, either a feed or reflux liquid is supplied to the distillation tower  12  in a downcomer  14  above a first tray deck  30  and flows across the deck in the form of froth towards an outlet edge spaced apart from the tower wall and over an outlet weir  32  into a downcomer. The liquid-vapor mix flows over the weir  32  and downwardly through the baffles  35  toward and then through openings in the downcomer floor. The lower distributor  50  directs the liquid flow to the inlet portion of a subjacent tray deck. Separated effluent vapors are directed upwardly through the tower  12  and collected above the tray decks  30 , and liquid is collected below the tray decks  30 . The backpressure from the rising vapors allows liquid/froth to flow across the decks  30  to the next downcomer  14 , rather than weeping through perforations in the deck. 
     It has been found through experiment that the illustrated embodiment provides a more even flow across the tray deck  30  than prior art gas-liquid contactors as demonstrated with reference to  FIGS. 7A and 7B . The tests involved the injection of a dye into the liquid at the inlet  31  of the downcomer, and examining the pattern of flow over the area of a subjacent tray at intervals after injection of the dye.  FIGS. 7A and 7B  illustrate the results of a pilot plant test with air-water in a 1.3 m diameter column.  FIG. 7A  shows the results of a test using a conventional tray assembly.  FIG. 7B  illustrates the test results including the tray assembly of  FIGS. 1 and 2 . During the tests using the apparatus of the present invention it is seen that substantially even flow is observed over a time sequence of 10, 35, 70 and 120 seconds. Moreover, no stagnant liquid pools or retrograde flow in the tray bubbling area was observed. 
     It has been established experimentally that the advantages of the present invention are provided over a wide range of flow rates of both vapor and liquid, which is not the case for other liquid distribution control means such as baffles or deflectors arrayed across an upper surface of a tray as described in the prior art. 
     Thus the long sought goal of high tray efficiency due to more uniform liquid concentration in the downcomer and more evenly distributed liquid flow across all of deck  30  of tray assembly  10  is achieved through incorporation of a mixer in the top of the downcomer and a distributor at the bottom of the downcomer  40  above the inlet distribution area of the subjacent deck  30 . The incorporation of both the mixer and the distributor in the downcomer provides substantial advantages for use in gas-liquid contact towers when compared with prior art tray designs. 
     It will be recognized that the structure described above may be used to design trays for different types of towers. 
     The tray design allows for a relatively high capacity and efficiency, in which the distribution of volumetric liquid flow across the tray deck is essentially similar for all paths along which that liquid flows. Some of the benefits that may be achieved using the tray assembly include no stagnant regions on the tray decks, and no areas in which there is back-flow, thus enabling efficient and effective utilization of the entire area of the tray deck for mass transfer. 
     In this document, the word “comprising” is used in its non-limiting sense to mean that items following the word are included, but items not specifically mentioned are not excluded. A reference to an element by the singular indefinite article “a” does not exclude the possibility that more than one of the element is present, unless the context clearly requires that there be one and only one such element.