Abstract:
A tray assembly is provided for improved gas/liquid contact when used in a large chemical process tower, resulting in better performance. Each tray assembly has two levels of tray decks and associated downcomers. Tray decks provide gas/liquid contact for mass transfer and the downcomers are required to clarify the liquid before entering the tray below. A novel downcomer configuration is provided for use in multipass trays. The shape of downcomer bottom chord is designed to allow for liquid to travel across any part of each tray deck at a similar distance between the inlet and the outlet weir. As a result, liquid on the tray deck travels in plug flow pattern. Benefits include a higher Murphree tray efficiency due to minimum liquid back-mixing, as well as minimum local entrainment due to uniform froth height.

Description:
FIELD 
     The present invention relates to trays for use in chemical process towers and, more particularly, to improved tray capacity and efficiency thereof through more uniform liquid distribution on a tray deck. 
     BACKGROUND 
     Several tray designs are known for gas-liquid contactors used in processes including reactions and separations. In each design, trays are situated within the towers for contact between the components of mixtures within the towers. Several tray designs are known, as described by, for example, Philip C. Wankat in “Equilibrium Staged Separations” published by Elsevier (1988), C. Judson King in “Separation Processes” published by McGraw-Hill Book Company (2 nd  edition, 1980), and Henry Z. Kister in “Distillation Design” published by McGraw-Hill, Inc. (1992). 
     In a conventional tray design as illustrated, for example, by Kister in FIG. 7.7 on page of “Distillation Design” flow of liquid across a tray tends to follow the shortest path across the tray deck from the inlet downcomer toward the outlet downcomer. A consequence for chemical process towers having a circular cross-sectional design in which there are stagnant regions develop on the areas of the tray deck near the walls of the tower. The flow pattern is shown by King in FIGS. 12-15 and 12-16 on page 614 of “Separation Processes.” FIGS. 12-16 shows that there is non-uniform flow of liquid across a plate and, in extreme cases, recirculation cells are formed. This non-uniform liquid distribution also exists on conventional multi-pass trays where the downcomer bottom chord and outlet weir are always in different lengths. 
     It is desirable to effect good mixing of all components on the tray deck. When there is highly non-uniform liquid distribution above the tray arising from extensive back-mixing, the Peclet number approaches zero. When there is very little back-mixing i.e. uniform distribution of liquid across the tray, the Peclet number approaches infinity. The variation in Peclet number for diffusion liquid-mixing is illustrated by King in FIGS. 12-19 on page 619 of “Separation Processes.” For a given point efficiency, the Murphree efficiency of a tray increases as the Peclet number increases, and so it is desirable to maximize the Peclet number. 
     The tray deck is perforated to allow gas to rise through the perforations and bubble through the liquid flowing across the tray deck, thereby effecting contact between the liquid and the gas. An outlet weir is situated toward the edge of the tray deck adjacent the outlet downcomer, so as to maintain a depth of liquid and froth across the tray deck. Froth is formed when gas rises as bubbles through the liquid. It is important that the froth height is approximately even at all locations along the length of the outlet weir, as uneven froth height causes high entrainment and thus premature flooding. 
     What is needed is a tray designed to have higher capacity and efficiency in which the distance traveled by all liquid across the tray deck is essentially similar for all paths along which that liquid flows, so as to achieve: 
     a high Peclet number due to very little or no back-mixing, and so very uniform distribution of components in the mixture above the tray, 
     no stagnant regions above the tray decks, and 
     a froth height that is even across the length of the outlet downcomer weir. 
     SUMMARY 
     The present invention relates to an improved design for trays within a chemical process tower. The trays have curved outlet weirs along curved edges of the tray decks so that the distance traveled by liquid flowing across the tray decks is substantially similar at all locations on those tray decks. As a consequence, there is uniform liquid distribution across the tray decks, very little or no back-mixing, no stagnant regions, and even froth height along the length of the outlet weir. The net result is that there is higher tray capacity and efficiency when compared with prior art trays. 
    
    
     
       DESCRIPTION OF THE DRAWINGS 
       For a more complete understanding of the present invention and for further objects and advantages thereof, reference is made to the following description taken in conjunction with the accompanying drawings. 
         FIG. 1  is a perspective view of a tray assembly comprising a combination of a first tray deck level and a second tray deck level for high efficiency and uniform liquid distribution. 
         FIG. 2  is a perspective view showing stacking of tray assemblies shown in  FIG. 1 . 
         FIG. 3  is a first side view of the tray assembly shown in  FIG. 1 . 
         FIG. 4  is a second side view of the tray assembly shown in  FIG. 1 . 
         FIG. 5  is a top view of the first tray deck level shown in  FIG. 1 . 
         FIG. 6  is a top view of the second tray deck level shown in  FIG. 1 . 
         FIG. 7  is a side view of the tray assembly shown in  FIG. 3 , showing the flow of liquids and gases. 
         FIG. 8A  shows the flow paths of a 2-pass tray, where the left side and right side each show one-half of successive levels of trays. 
         FIG. 8B  shows the flow paths of a 3-pass tray, where the left side and right side each show one-half of successive levels of trays. 
         FIG. 8C  shows the flow paths of a 4-pass, where the left side and right side each show one-half of successive levels of trays. 
     
    
    
     DETAILED DESCRIPTION 
     Exemplary embodiments of the invention, which are non-limiting, will now be described with reference to  FIG. 1  through  FIG. 8 . 
     Referring to  FIGS. 1 and 2 , a tray assembly  50  for gas/liquid contact in a chemical process tower  10 , having walls  12  with an inner surface  14  to which tray assemblies  50  can be affixed, comprises a first tray deck level  16  and a second tray deck level  18 . Referring to  FIGS. 1 ,  3 ,  4  and  5 , first tray deck level  16  has at least one first tray deck  20  and at least one first downcomer  24  at a first edge  52  of first tray deck  20 . Referring to  FIGS. 1 ,  3 ,  4  and  6 , second tray deck level  18  has at least one second tray deck  22  and at least one second downcomer  26  at a second edge  54  of said second tray deck  18 . 
     Referring to  FIGS. 1 and 2 , each of first tray deck  20  and second tray deck  22  is perforated with holes  38 . Referring to  FIG. 7 , the direction of travel of liquid flowing across tray decks  20 ,  22  and descending through downcomers  24 ,  26  is shown as solid arrows  60  and the direction of gas flow is shown by open arrows  62 . Gas  62  can rise through holes  38  in tray decks  20 ,  22  from the regions immediately below one of first tray deck  20  and second tray deck  22  and bubble through liquid  60  flowing across the corresponding tray deck  20 ,  22 . 
     To illustrate the structure of tray assembly  50 , the following description and references to  FIGS. 1 through 7  show first tray deck level  16  having a single tray deck  20  and two first downcomers  24  each of which is situated between first edge  52  and walls  12 . Second tray deck level  18  has two divided parts of second tray deck  32  extending from walls  12  to a single central second downcomer  26  extending between second edges  54  of each of divided parts of second tray deck  32 . It will be recognized that each of first tray deck  20  of first tray deck level  16  and second tray deck  22  of second tray deck level  18  may be divided into several parts, with a plurality of first downcomers  24  and second downcomers  26  situated therebetween, as illustrated in  FIG. 8 , without deviating from the principles, intent and targeted benefits of the present invention.  FIG. 8  shows three different examples of possible embodiments of tray assembly, showing the relationships between portions of each of the components of successive tray deck levels  16  and  18 , the division between the portions being indicated by a dash-dot line  80 . In  FIG. 8A  a first embodiment  50   a  shows the relationships between first deck  20  and second deck  22  and first downcomer  24  and second downcomer  26  in a two-pass design. In  FIG. 8B , a second embodiment  50   b  shows a three-pass design. In  FIG. 8C , a third embodiment  50   c  shows a four-pass design. Liquid flow paths are indicated by solid arrows  60 . 
     Referring to  FIGS. 1 ,  3 ,  4  and  5 , when first downcomer  24  is adjacent walls  12  of chemical process tower  10 , it is bounded by a combination of on one side inner surface  14  of walls  12  and on the other side a first curved downcomer wall  30 . When there are a plurality of downcomers  24  at first tray deck level  16 , interior downcomers  24  are bounded at their ends by opposed portions of walls  12  and extending between them a combination of two first curved downcomer walls  30 . Referring to  FIGS. 1 ,  3 ,  4  and  6 , similarly, second downcomer  26  is bounded by one of a combination of on one side the inner surface  14  of the walls  12  of the chemical process tower  10  and on the other side a second curved downcomer wall  33 , or a combination of two second curved downcomer walls  33  extending between opposed portions of walls  12 . 
     First downcomer walls  30  extend downward from first edges  52  of first tray deck  20  toward second tray deck  22  immediately below. There is a gap  44  between a first bottom edge  56  (downcomer bottom chord) of first downcomer walls  30  and second tray deck  22  that extends along all or the majority of the length of first downcomer walls  30 . Second downcomer walls  33  extend downward from second edges  54  of second tray deck  22  toward first tray deck  20  immediately below. There is a gap  46  between a second bottom edge  58  (downcomer bottom chord) of second downcomer walls  33  and first tray deck  20  that extends along all or the majority of the length of second downcomer walls  33 . Preferably, each of first downcomer walls  30  and second downcomer walls  33  is sloped so the corresponding first downcomer  24  and second downcomer  26  narrows as it approaches corresponding first bottom edge  56  and second bottom edge  58 , as illustrated in  FIGS. 1 and 3 . 
     Referring to  FIGS. 1 ,  2  and  3 , a first inlet (receiving) area  34  on tray deck  20  is immediately below each one of second downcomers  26 . A second inlet (receiving) area  36  on tray deck  22  is immediately below each one of first downcomers  24 . 
     Preferably, a first outlet weir  28  is situated across the breadth of first tray deck  20  and is positioned at or adjacent to first edge  52 , and a second outlet weir  32  is situated across the breadth of second tray deck  22  and positioned at or adjacent to second edge  54 . 
     Optionally, a first inlet weir (not illustrated) is situated across the breadth of first tray deck  20  and is positioned adjacent to first inlet area  34 . Optionally, a second inlet weir (not illustrated) is situated across the breadth of second tray deck  22  and is positioned adjacent to second inlet area  36 . 
     First edge  52  of first tray deck  20  is curved, and second edge  54  of second tray deck  22  is also curved. First downcomer walls  30  and first outlet weir  28  are curved to match the curve of first edge  52 . Second downcomer walls  33  and second outlet weir  32  are curved to match the curve of second tray edge  54 . The curvature of each of first edge  52 , second edge  54 , first downcomer walls  30 , second downcomer walls  33 , first outlet weir  28  and second outlet weir  32  are in a plane defined mainly by tray deck  20 ,  22  of the respective tray deck level  16 ,  18 . The shapes of the curvatures are selected from among a smooth arc, a curve achieved by a sequence of short linear adjoining sections at graduated angles relative to each other, and a combination of smooth arcs and short linear sections. 
     The curvature of the inward side of first inlet area  34  is defined approximately by the curvature of second bottom edge  58  of second downcomer walls  33 . The combination of the curvatures of first edge  52  and of the inward side of first inlet area  34  defines the area of first tray deck  20  perforated by holes  38  over which liquid flows from inlet area  34  to first edge  52 . Similarly, the curvature of a side of second inlet area  36  is defined approximately by the curvature of first bottom edge  56  of first downcomer walls  30 . The combination of the curvatures of second edge  54  and the corresponding side of the second inlet area  36  defines the area of second tray deck  22  perforated by holes  38  over which liquid flows from second inlet area  36  to second edge  54 . 
     Referring to  FIG. 1 , the curvatures of each of first edge  52  and of the inward side of first inlet area  34  are such that any part of liquid flowing across any portion of first tray deck  20  travels a similar distance to any other part of said liquid, as shown by double arrows  40  and  42 . Similarly, any part of liquid flowing across any portion of second tray deck  22  travels a similar distance to any other part of said liquid. The consequence is that there is uniform liquid distribution with minimized back-mixing, and so a high Peclet number, across the entire area of both first tray deck  20  and across the entire area of second tray deck  22 . A further beneficial consequence is that there is approximately even froth height at both of first edge  52  of first tray deck  20 , and approximately even froth height at second edge  54  of second tray deck  22 , thus reducing any propensity to premature flooding resulting from high entrainment. 
     It will be recognized that there may be an odd number of tray deck levels in any chemical process tower  10 , in which case there will be one more of either first tray deck level  16  or second tray deck level  18 . 
     In this patent 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 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 of the elements. 
     It will be apparent to one skilled in the art that modifications may be made to the illustrated embodiment without departing from the spirit and scope of the invention as hereinafter defined in the Claims.