Abstract:
The invention comprises multiple configurations of downcomers in a parallel flow multiple downcomer tray for vapor-liquid contacting processes such as the separation of chemical compounds via fractional distillation or the removal of a component of a gas stream with a treating liquid. In one embodiment, side downcomers are incorporated into a parallel flow multiple downcomer tray. In another embodiment, the downcomers have an inclined side wall that directs liquid onto the deck below the downcomer. The inclined side wall also provides additional volume above the inferior downcomer inlet to reduce pinching at this inlet without the need for a stilling deck.

Description:
FIELD OF THE INVENTION  
       [0001]     This invention relates to vapor-liquid contacting apparatus and specific features that improve the efficiency and capacity of this operation. The invention therefore relates to, for example, apparatus used as fractionation trays within fractional distillation columns. The invention may also be used in a variety of other gas-liquid contacting operations such as acid gas scrubbing or absorption processes.  
       BACKGROUND OF THE INVENTION  
       [0002]     Fractional distillation columns having a number of vertically spaced distillation trays are widely employed in the hydrocarbon processing, chemical, and petrochemical industries. Accordingly, a large amount of research, development, and creative thinking has been devoted to providing improved fractional distillation trays. Fractionation tray development has therefore provided many variations in contacting area structure, downcomer design, and overall tray structure.  
         [0003]     Vapor-liquid contacting devices are used in a wide variety of applications for separating liquid or vapor mixtures. One of the major applications of the vapor-liquid contacting devices is in the separation of chemical compounds via fractional distillation. These devices are also used to contact a gas stream with a treating liquid which selectively removes a product compound or an impurity from the gas stream.  
         [0004]     Within a column containing vapor-liquid contacting devices, liquid flows in a generally downward direction and vapor rises vertically through the column. On each vapor-liquid contacting device, liquid flows in a generally horizontal direction across the device and vapor flows up through perforations on the device. The cross flow of vapor and liquid streams on each device generates a froth for intimate vapor-liquid contacting and mass transfer.  
         [0005]     The apparatus can be used in the separation of essentially any chemical compound amenable to separation or purification by fractional distillation. Fractionation trays are widely used in the separation of specific hydrocarbons such as propane and propylene or benzene and toluene or in the separation of various hydrocarbon fractions such as LPG (liquefied petroleum gas), naphtha or kerosene. The chemical compounds separated with the subject apparatus are not limited to hydrocarbons but may include any compound having sufficient volatility and temperature stability to be separated by fractional distillation. Examples of these materials are acetic acid, water, acetone, acetylene, styrene acrylonitrile, butadiene, cresol, xylene, chlorobenzenes, ethylene, ethane, propane, propylene, xylenols, vinyl acetate, phenol, iso and normal butane, butylenes, pentanes, heptanes, hexanes, halogenated hydrocarbons, aldehydes, ethers such as MTBE and TAME, and alcohols including tertiary butyl alcohol and isopropyl alcohol.  
         [0006]     One important issue in the field of vapor-liquid contacting columns is improving the capacity of the trays to allow vapor and liquid to flow from tray to tray without flooding. A second important issue in the field is improving the efficiency of the trays for mass transfer between vapor and liquid.  
         [0007]     In a well-known classic study by W. K. Lewis in 1936, it was found that the mass transfer efficiency of vapor-liquid contacting trays could be maximized by bringing an unmixed vapor into contact with liquid flows across each successive tray in the same direction (Case 2). The Case 2 is referred to as a parallel flow, which, as used herein, refers to liquid flows on vertical adjacent or successive trays rather than to liquid flows on a single tray. Lewis&#39; Case 2 ensures that the driving force for mass transfer on a given tray is nearly the same regardless of where that mass transfer occurs on the tray. Because of this, substantial increases in efficiency can be obtained when using a tray operated according to Lewis&#39; Case 2.  
         [0008]     U.S. Pat. No. 5,223,183 to Monkelbaan, et al. teaches a parallel flow tray with at least one central downcomer and no side downcomers. The downcomers of each tray are aligned with the downcomers on the other trays of the column such that the downcomers on one tray are immediately below those on the tray above. The outlets of one downcomer are directly above the inlet of another. A pair of inclined liquid deflecting baffles over each downcomer connects the outlets and inlets of vertically adjacent downcomers and provides a crisscrossing liquid flow path. The downcomer baffles prevent liquid from the tray above from entering each downcomer and define the direction of liquid flow onto the tray deck. The inclined surface of the baffle also imparts a horizontal momentum to the descending liquid which tends to push the liquid and froth present on the tray towards the inlet of the outlet downcomer for this portion or zone of the tray. In certain designs of the trays there is provided a perforated anti-penetration weir on the lower end of the downcomer baffles, with the weir being perpendicular to the downcomer baffle. Further, froth flow into an outlet downcomer is pinched by the downcomer right above, which may reduce tray capacity.  
         [0009]     U.S. Pat. No. 5,318,732 to Monkelbaan, et al. teaches another parallel flow multiple downcomer type fractionation tray, which increases tray capacity by providing imperforate stilling decks that extend across the tray deck surface outward from the downcomer inlet opening together with vertical inlet weirs attached to the outer end of the stilling decks. The inlet weirs may function as pre-weirs used in addition to the conventional inlet weir formed by the upward extension of the downcomer side wall. Further, the stilling decks help reduce pinching; however they also reduce the active area of the deck.  
         [0010]     Therefore an improved high-capacity tray providing a Lewis Case  2  parallel flow pattern is needed in the art.  
       SUMMARY OF THE INVENTION  
       [0011]     Two determinants of the quality of a contacting tray are its efficiency for performing a process and its capacity in terms of liquid or vapor traffic. It is an objective of the subject invention to increase the efficiency of contacting trays with Lewis Case 2 vapor-liquid contacting arrangement. It is another objective of the invention to provide a vapor-liquid contacting apparatus with improved capacity.  
         [0012]     The invention comprises multiple configurations of a parallel flow multiple downcomer tray for vapor-liquid contacting processes such as the separation of chemical compounds via fractional distillation or the removal of a component of a gas stream with a treating liquid. In one embodiment, side downcomers are incorporated into a parallel flow multiple downcomer tray having a center baffle. In another embodiment, the downcomers have an inclined side wall that directs liquid onto the deck below the downcomer. The inclined side wall also provides additional volume above the inferior downcomer inlet to reduce pinching at this inlet without the need for a stilling deck. In a further embodiment, features of the first two embodiments are combined.  
         [0013]     More particularly, the invention comprises, in one form thereof, a vapor-liquid contacting tray that includes at least one centrally located downcomer. The vapor-liquid contacting tray further includes a means to define vertical liquid flow paths for liquid flowing through each central downcomer onto a subsequent tray that comprises an inclined downcomer baffle. A plurality of vapor-liquid contacting decks is included on the tray. Two side downcomers are included proximate to the outer perimeter of the tray. Each side downcomer has a liquid receiving portion and a liquid distributing portion wherein the receiving portion directs liquid to the distributing portion and the distributing portion is substantially sealed against fluid entering directly from a proximate contacting deck. A central baffle extends between at least two of the downcomers and intersects at least one of the contacting decks.  
         [0014]     In another embodiment, the invention comprises a vapor-liquid contacting tray that has a generally circular circumference and includes a plurality of central downcomers that are formed by a first elongate side wall and an opposing second elongate side wall extending a shorter vertical distance below a proximate contacting deck than the first elongate side wall. Each downcomer further includes a bottom plate that intersects the first elongate side wall. The vapor-liquid contacting tray further includes a means to define vertical liquid flow paths for liquid flowing through each downcomer onto a subsequent tray comprising an inclined downcomer baffle, wherein the downcomer baffle extends from the second elongate side wall, intersects the bottom plate, and extends at least to a vertical plane formed by the first elongate side wall. A plurality of vapor-liquid contacting decks is included on the tray. A central baffle extends between at least two of the plurality of downcomers and intersects at least one of the contacting decks.  
         [0015]     A further form of the invention comprises a vapor-liquid contacting tray that includes at least one centrally located downcomer, a means to define vertical liquid flow paths for liquid flowing through each central downcomer onto a subsequent tray comprising an inclined downcomer baffle, and a plurality of vapor-liquid contacting decks. A bubble promoter, which comprises a perforated plate, is situated on the tray to direct liquid from the inclined downcomer baffle to one of the contacting decks. A central baffle extends between at least two of the downcomers and intersects at least one of the contacting decks.  
         [0016]     An even further form of the invention comprises a vapor-liquid contacting tray that includes a plurality of vapor-liquid contacting decks and two side downcomers proximate to the outer perimeter of the tray. Each side downcomer has a liquid receiving portion and a liquid distributing portion wherein the receiving portion directs liquid to the distributing portion. A cover is included over the liquid distributing portion that prevents liquid from a superior side downcomer from entering the liquid distributing portion. The cover also directs vapor in the liquid distributing portion to a vapor flow path outside the perimeter of the tray.  
         [0017]     A still further form of the invention comprises a vapor-liquid contacting tray that has a generally circular circumference and includes at least one centrally located downcomer, a plurality of vapor-liquid contacting decks, and two side downcomers proximate to the outer perimeter of said tray. Each side downcomer has a liquid receiving portion and a liquid distributing portion wherein the receiving portion directs liquid to the distributing portion. A central baffle extends between at least two of the downcomers and intersects at least one of the contacting decks. The central baffle comprises a bend proximate to each of the side downcomers, which increases the size of the liquid receiving portions.  
         [0018]     The invention further comprises a central downcomer for a vapor-liquid contacting tray, having a first portion and a second portion, which is substantially a mirror image of the first portion. Each of the first and second portions include a first elongate side wall, an opposing second elongate side wall extending a shorter vertical distance below the proximate contacting deck than the first elongate side wall, a bottom plate that intersects the first elongate side wall, an inclined downcomer baffle that extends from the second elongate side wall, and an extension flange. The first and second portions fit together such that each of the extension flanges overlaps with the complimentary inclined downcomer baffle and second elongate side wall. In a particular embodiment, each of the first and second portions is made from a single sheet of material. The central downcomer may further include cross braces between the first and second elongate side walls of each portion. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0019]     The above-mentioned and other features and advantages of this invention, and the manner of attaining them, will become apparent and be better understood by reference to the following description of several embodiments of the invention in conjunction with the accompanying drawings, wherein:  
         [0020]      FIG. 1  is a schematic view of a vapor-liquid contacting column having a plurality of vapor-liquid contacting trays according to the present invention;  
         [0021]      FIG. 2  is a schematic plan view of the first embodiment of the parallel flow multiple downcomer fractionation tray of the present invention;  
         [0022]      FIG. 3  is a schematic cross-sectional view of a column of trays according to  FIG. 2 ;  
         [0023]      FIGS. 4A and 4B  are schematics of a floating valve;  
         [0024]      FIG. 5  is a schematic plan view of the tray of  FIG. 2  having a swept-back side weir and a bent center baffle;  
         [0025]      FIG. 6  is a schematic plan view of the second embodiment of the parallel flow multiple downcomer fractionation tray of the present invention;  
         [0026]      FIG. 7  is a schematic cross-sectional view of a column of trays according to  FIG. 6 ;  
         [0027]      FIG. 8A  is a bottom isometric view of a strengthened central downcomer;  
         [0028]      FIG. 8B  is a top isometric view of the strengthened central downcomer of  FIG. 8A ;  
         [0029]      FIGS. 9A-14  are various schematic views of the third embodiment of the parallel flow multiple downcomer fractionation tray of the present invention; and  
         [0030]      FIGS. 15 and 16  are schematic cross-sectional views of further embodiments of the parallel flow multiple downcomer fractionation tray of the present invention. 
     
    
       [0031]     Corresponding reference characters indicate corresponding parts throughout the several views. The examples set out herein illustrate several embodiments of the invention but should not be construed as limiting the scope of the invention in any manner.  
       DETAILED DESCRIPTION  
       [0032]     Referring to  FIG. 1 , there is shown an example of a vapor-liquid contacting column having a plurality of vapor-liquid contacting trays of the present invention. The details of the trays will be disclosed in the subsequent embodiments of the invention. The column  10  includes a cylindrical inner chamber  11 , a top section  12 , a bottom section  14 , and a plurality of vapor-liquid contacting trays  16  having a circular perimeter. The top section  12  collects vapor from the chamber  11  and supplies liquid to the chamber  11 . In certain applications, such as continuous fractionation, the top section  12  is in fluid communication with a condenser that condenses the vapor and adds a portion of the resultant liquid to the liquid supply to the chamber  11 . The bottom section  14  collects liquid from the chamber  11  and supplies vapor to the chamber  11 . Similar to the top section  12 , in certain applications, such as continuous fractionation, the bottom section  14  is in fluid communication with a reboiler that converts a portion of the liquid to vapor, which is added to the vapor supply. The column  10  may also include one or more intermediate feeds that adds a liquid or vapor mixture to the middle of the column  10  with some trays  16  above the feed and some trays  16  below the feed. Each tray  16  comprises a contacting deck  18 , at least one downcomer  20 , and at least one inclined downcomer baffle  22 .  
         [0033]     A particular embodiment of the invention shown in  FIGS. 2 and 3  includes a plurality of parallel flow multiple downcomer fractionation trays  100  having at least one central downcomer  102  and two side downcomers  104 . Between each two downcomers  102  and  104 , each tray  100  includes active areas in the form of a perforated deck  106 . The deck  106  is bisected by central baffle  108 .  
         [0034]     The central downcomer  102  includes side walls  110 , a bottom plate  112 , stilling decks  114 , and inlet weirs  116 . The flat, horizontal bottom plate  112  extends between the side walls  110 . A number of openings  118  are provided in the bottom plate for the exit of the liquid which accumulates within the central downcomer  102 . The purpose of the bottom plate  112  is to retard the liquid flow sufficiently that the bottom of the central downcomer  102  is dynamically sealed by liquid to the upward passage of vapor. The openings may be circular, square or elongated in either direction, that is, along the width or length of the central downcomer  102 . The sealing of the downcomer outlet to upward vapor flow could be accomplished by other structures as well. The stilling decks  114  are imperforate, and thus inactive, regions just prior to the inlets of each central downcomer  102 . The combination of the inlet weir  116  and the stilling deck  114  helps prevent pinching by providing an area near the inlet of the central downcomer  102  that doesn&#39;t add vapor to the froth.  
         [0035]     The central downcomers  102  may be supported by any conventional means such as a support ring, not shown, which is welded to the inner surface of the column wall. The deck  106  may be supported, for example, by an angle-iron support welded to the side walls  110  and the support ring welded to the column wall. The central downcomers  102  and the deck  106  are bolted, clamped, or otherwise affixed to the supports so that the central downcomers  102  and the deck  106  are kept in position during operation. The central downcomers  102  may act as the main supports for the tray  100 ; however, additional support beams may need to be included for substantially large trays. Further, strengthened central downcomers may be used.  
         [0036]     An inclined baffle  120  is situated between the bottom of a central downcomer  102  and the top of a central downcomer  102  immediately below it. It may be seen that the inclined baffles  120  extend between the central downcomers  102  in such a manner that liquid may not travel horizontally over the central downcomer  102  from one decking surface  106  to another. Liquid descending from one central downcomer  102  is prevented from falling into the next lower central downcomer  102  and must flow horizontally across the decking  106  to a different downcomer, whether it&#39;s a side downcomer  104  as shown in  FIG. 3 , or another central downcomer  102  in order to proceed to the subsequent tray. In this embodiment two inclined baffles  120  cover the inlet of each central downcomer  102 . The inclined baffles  120  have opposite slopes that deliver liquid onto deck portions  106  on different sides of the central downcomer  102  such that the liquid flows in the direction of the arrows. In this embodiment the inclined baffles  120  on one side of the tray  100  all slope in the same direction, and the inclined baffles  120  on the other side (or other half) of the column face in the opposite direction. Liquid therefore flows in the opposite direction on the two sides of any one tray  100 , but flows in the same direction (parallel flow) on all deck areas  106  on one side of each tray  100 . A perforated anti-penetration or distribution weir  122  may be situated at the bottom of each of the inclined baffles  120 . In the present embodiment, the distribution weir  122  is inclined for 0 to 90 degrees, preferably about 45 degrees, to horizontal.  
         [0037]     The side downcomers  104  are provided to improve the fluid handling at the sides of the tray  100 . Each of the downcomers  104  includes a receiving portion  124  and a distribution portion  126 . The receiving portion  124  includes a side weir  128  and an imperforate, sloped bottom plate  130 , which is oriented to direct liquid towards the distribution portion  126 . The distribution portion  126  includes a bottom plate  112  as described above with the central downcomers  102 . An inclined baffle  120  and distribution weir  122  are situated below the distribution portion  126 .  
         [0038]     The deck  106  is perforated to allow vapor to flow through the deck  106  and contact the fluid on the deck  106 . The perforations may take many forms including evenly spaced, circular holes and a number of vapor-directing slots. The slots are oriented such that the vapor rising upward through the deck  106  through these slots imparts a horizontal thrust or momentum to the liquid or froth on the tray  100  in the direction of the nearest outlet downcomer. There is therefore achieved a more rapid passage of the froth into the downcomer means and a decrease in the froth height on the tray. More importantly by proper slot arrangement liquid flows uniformly across deck  106  into downcomer means. These slots and their function may resemble those described in U.S. Pat. No. 4,499,035, which is incorporated herein by reference. U.S. Pat. No. 3,417,975 issued to B. Williams et al. provides representations of a portion of decking material having both circular perforations and flow directing slots. This patent is also incorporated herein for its teaching as to the design and usage of flow directing slots.  
         [0039]      FIG. 4A  is an enlarged schematic view of an alternate flow opening within the deck  106 . Deck  106  has valve  130  in opening  132 .  FIG. 4B  is an alternative flow opening with a venturi type opening  132 ′ created by extrusion or pressing of the deck  106 . Valve  130  is inserted into venturi opening  132 ′ of deck  106 .  
         [0040]     At the midpoint of each downcomer, a central baffle  108  rises upward from each deck area  106  of the overall tray surface. This central baffle  108  may be formed from a number of connecting plates or a single plate. The central baffle  108  prevents liquid and froth present on the two sides of the central baffle  108  from admixing. The central baffle  108  terminates a short distance below the next higher tray to provide a gap which allows pressure and vapor flow equalization. The central baffle  108  optionally includes equalization ports in the middle of the deck, remote from the downcomers as described in a subsequent embodiment with reference to  FIGS. 9A and 9B .  
         [0041]     As one can see with reference to  FIGS. 2 and 3 , the side weirs  128  are significantly shorter than the inlet weirs  116  associated with the inlets to the central downcomers  102 . This may limit the capacity of the tray  100  because of more liquid accumulation on the deck  106  proximate to the inlet of a side downcomer  104  than proximate to the inlet of a central downcomer  102 . A swept-back side weir  128 ′, a swept back central baffle  108 ′, or both, as shown in  FIG. 5 , cooperate with a side downcomer that is unequally divided between a receiving portion and a distributing portion to increase the length of the side weir thereby reducing the difference in weir loads between the side downcomers  104  and the central downcomers  102 . Also importantly, the use of swept-back weir  128 ′ and the swept-back central baffle  108 ′ increases the entrance space of the side downcomer  104 , and therefore, reduces downcomer choking tendency. Thus the capacity of the side downcomer  104  is increased so that it is substantially equal to the capacity of the central downcomers  102 . As shown in  FIG. 3  the column wall may serve as a side wall of the side downcomers. In other embodiments two side walls may define the distribution portion and/or receiving portion with one of the side walls conforming to shape of the column in an abutting or spaced-apart relationship therewith.  
         [0042]     A second embodiment of the invention, shown in  FIGS. 6 and 7 , includes a tray  200  having at least one central downcomer  202 . Each central downcomer  202  includes a side wall  210   a,  a shortened side wall  210   b,  a bottom plate  212 , an inlet weir  216  defined by the portion of side wall  210   b  above the deck  206 , a liquid balancing box  234 , and an inclined baffle  220  with an anti-penetration weir  222 . The bottom plate  212  includes openings  218  for the exit of the liquid which accumulates within the central downcomer  202 . In this embodiment, the inclined baffle  220  is incorporated into one side wall  210   b  to form a sloped downcomer that provides additional volume above the inlet to the central downcomer  202 . The extension of the shortened side wall  210   b  below the decking can improve tray strength and aid in supporting the decking. However, extension of the shortened side wall  210   b  below the decking is not required. Thus, if the first side wall extends below the decking and the second side wall does not extend below the decking, the requirement in this embodiment that the second elongate side wall extends a shorter distance below the decking than the first elongate side wall is still satisfied. The additional volume prevents pinching of liquid and froth flow over the inlet without the need for a stilling deck  114  as shown in  FIGS. 2 and 3 . The liquid balancing box  234 , which may be located in the middle of the central downcomer  202 , facilitates liquid communication between the two portions of each central downcomer  202 , which are sloped in different directions. This feature promotes the balancing of the liquid flow in the case that one side of the central downcomer  202  has a higher liquid input or output than the opposite side. The deck portions  206  are similar to deck portions  106  described in the previous embodiment. Further, the central baffle  208  is similar to the central baffle  108  described in the previous embodiment. In another embodiment not shown, the central baffle extends beyond at least one of the downcomers towards the periphery of the tray. This extension of the central baffle ensures a more uniform residence time on the decking located adjacent the perimeter of the tray.  
         [0043]     As described in the first embodiment, strengthened central downcomers may be used for additional support for the tray  100 . The central downcomers may provide the majority of the support for the contacting tray  200  and since tray efficiency is increased with fewer central downcomers, strengthened central downcomers  202  may be needed. Strengthened central downcomers  202 , as shown in  FIGS. 8A and 8B , may be made with two pieces (one with an inclined baffle  220  slanted one way and the other slanted in the opposite direction). Each piece may be made mostly of a single sheet of material that is cut and bent into shape. Thus there are as few joints as possible. The pieces each have a flange  254  that overlaps with the shortened side wall  210   b  and the inclined baffle  220  of the opposite piece and cooperates to form a strong joint between the two pieces and also to form a modified liquid balancing box  234  that facilitates fluid transfer between the downcomer sections. The flanges  254  distribute the stress on the joint between the two downcomer pieces and include several slots  256  for liquid flow. Holes are not placed near high stress areas. The top edge of the strengthened central downcomer  202  is folded over and welded for additional strength. Further, cross braces  258  may be included to increase lateral stability of the downcomer and, thus, increase the downcomer strength. In certain embodiments with a substantially large contacting tray (such as those having a diameter of over 16-ft), structural I-beams may be used for additional support for the deck  206  while limiting the adverse affect on tray efficiency that additional downcomers may cause.  
         [0044]     In order to improve the flow of the liquid around the sides of the tray  200  and increase tray capacity, a third embodiment of the invention combining the first two embodiments is presented. Multiple downcomer trays  300  are shown in  FIGS. 9A and 10 . The tray  300  includes at least one central downcomer  302 , each of which is similar in structure to the central downcomer  202 , and side downcomers  304 . The side downcomers  304  are similar in structure to the side downcomers  104  except that the side downcomers  304  may incorporate a sloped side wall to reduce pinching. More particularly, the tray  300  includes a central downcomer  302  with a liquid balancing box  334 , side downcomers  304 , a deck  306 , and a swept-back center baffle  308 ′ similar to the center baffle  108 ′ described in the first embodiment. The tray  300  may alternatively include a straight center baffle  308 , as shown in  FIG. 11A . The central downcomer  302 , as well as the side downcomers  304  include an inclined downcomer baffle  320  with anti-penetration weirs  322 . The central downcomer  302  and the side downcomers  304  also include a bottom plate  312  and openings  318 . The side downcomers  304  further include a receiving portion  324  and a distribution portion  326 .  
         [0045]     The center baffle  308 ′ also includes equalization ports  350  as best seen in  FIG. 9B . The equalization ports  350  allow balancing of the liquid flows between the sides of the deck  306  divided by the swept-back center baffle  308 ′. The equalization ports  350  should be kept remote from the downcomer inlets and outlets so they do not form a shortcut for the liquid to flow from an outlet to an inlet without flowing over the majority part of deck  306 . Further, the vapor-liquid mixture is similar on both sides of the baffle  308 ′ in the middle of the deck  306 , remote from the downcomers, whereas the areas proximate to the downcomers have different vapor-liquid compositions on either side of the baffle  308 ′. Particularly, one side of the baffle  308 ′ is proximate a downcomer outlet, while the opposite side of the baffle  308 ′ is proximate a downcomer inlet. The equalization ports  350  may also be used in conjunction with the straight center baffle  308 .  
         [0046]     Vapor may enter the receiving portion  324  with the liquid in the form of froth. Therefore, it may be necessary to include an outlet path for the vapor in the distribution portion  326  to prevent choking. As shown in  FIG. 10 , the outlet path above the distribution portion  326  includes a side wall  351  to prevent liquid from entering and directs the vapor between the side downcomer and the outer perimeter of the tray, that is, along the inner wall of the column. The flow path may continue along the outer wall of the column to the top of the column or the vapor may be vented below a deck portion  306  of a superior tray. A momentum dampening device  352  may be installed in the distribution portion of the side downcomers to reduce the flow momentum of liquid from the receiving portion.  
         [0047]     A variation of the side downcomers  304  shown in  FIGS. 11A and 11B  is that the top of the distribution portion  326  is sealed with a flat plate  336  to prevent the liquid from short cutting from the superior downcomer. Further, the center baffle  308  cooperates with the flat plate  336  to prevent liquid deposited on the flat plate  336  from running into the receiving portion  324 . A perforated distribution weir  340  may be included under the distribution portion  326  next to the flat plate  336  of the inferior distribution portion to improve liquid distribution. Alternatively, a bubble promoter  342  may be included instead of the distribution weir  340 .  
         [0048]     Further methods of maximizing the active area under the distribution portion  326  are shown in  FIG. 12 . The first means includes a perforated plate  344  positioned at an angle below the distribution portion  326 . A gap  345  between the deck  306  and distribution portion  326  allows vapor to enter the area under the plate  344  and the perforations allow the vapor to pass through the plate  344  and mix with the liquid deposited on the plate  344  from the superior distribution portion. Alternatively, a perforated flat plate  346 , similar to plate  344 , is used in cooperation with an inclined baffle  320 . In a further alternative, a perforated plate  348  having a combination of slots, louvers, and/or valves is used. The active area may be further increased by including a bubble promoter  342 , with gap  345 , below the outlets of the central downcomer  302 . The bubble promoters may also include an outlet weir  343 . Alternatively, bubble promoter  342 ′ is located below the outlet of the central downcomer  302 . Bubble promoter  342 ′ does not include a gap  345 , however, the portion of the contacting deck  306  under the bubble promoter  342 ′ has much more fractional perforations than the rest of the deck  306  so that the total open area of the deck  306  covered by the bubble promoter  342 ′ is equivalent to or larger than the total open area of the sloped perforated plate on the top of the bubble promoter  342 ′. This alternative provides for easier fabrication and installation.  
         [0049]      FIG. 13  shows distribution portions  326  having inclined baffles  320  of different inclinations and different distribution weirs  322 . The inclined baffles  320  provide increased volume over the distribution portion  326  to improve vapor venting and to reduce downcomer choking, similarly to the inclined baffles associated with the central downcomers  302 . With reference to line A, one can also see that the receiving portion  324  and the distribution portion  326  of the side downcomer  304  may be designed with different depths to increase downcomer capacity.  
         [0050]      FIG. 14  shows several varying distribution portions  326  that omit the bottom plate  312  and instead have a side wall that extends to close proximity to the inclined baffle  320  directly below the distribution portion  326 . The side wall leaves a small gap above the baffle  320  in order to allow liquid to escape the distribution portion  326  and not allow vapor to enter into the side downcomer  304 .  
         [0051]     While the above described methods may have advantages there may also be advantages in particular applications to increase the active area of tray  300  by simply reducing the area of distribution portion  326 .  
         [0052]     The issue of pinching may alternatively be addressed as shown in  FIG. 15 . The Vapor-liquid contacting tray  400  includes at least one stepped central downcomer  402  and a plurality of stepped side downcomers  404 . These stepped downcomers include a stepped side wall  410  that increases the volume over the inlet to an inferior downcomer rather than extending the inclined baffle  420  vertically.  
         [0053]     Tray  500 , shown in  FIG. 16 , comprises further variations of the central downcomers  502 . Side walls  510  are inclined or stepped on both sides of the downcomer to further increase the volume over the deck portions  506 .  
         [0054]     It should be noted that although several of the figures show downcomers having differing designs on different trays in the same column, it is expected that trays of a similar design will be used in a single column or a section of a column. It should also be noted that although substantially circular contacting trays are shown and described, other shapes, such as polygonal shapes, may also be imagined.  
         [0055]     The physical size of any portion of a parallel flow multiple downcomer tray must be chosen by a skilled designer considering all aspects of the intended operation of the tray. The spacing between vertically adjacent trays will normally be between 20 and 91 centimeters (8-36 inches) and is preferably between 30-61 centimeters (12-24 inches). The total open area of the deck area is generally in the range of about 5 to about 20 percent. For the deck with sieve holes and slots, the normal hole diameter of the circular perforations may range from about 0.3 to about 2.6 centimeters (⅛-1.0 inches). A hole size of about 0.47 to about 0.64 centimeters ( 3/16-¼ inch) is normally preferred. The open area provided by slots is from about 0.25 to about 5 percent of the area of the deck. A representative thickness of the decking is about 0.19 centimeters (0.075 inches) to 0.34 cm (0.14 inches).  
         [0056]     The rectangular inlet openings of the central downcomers are normally about 6 to about 25 cm wide (2.5-10 inches). The height of a downcomer as measured from the horizontal top edge of the first side wall to the bottom edge of the first side wall is normally between about 40 to 80% of the spacing between two adjacent trays. This includes the height that the first side wall extends above the decking and below the decking. Thus, the height of the downcomer is equivalent to the overall height of the tallest side wall. The spacing between two adjacent trays is the vertical distance measured between the decking of the two trays. The height of the central liquid/vapor baffle above the decking will normally be approximately 50 to 90% of the spacing between two adjacent trays. The width of the central downcomers may be different from each other depending on their length. The side downcomer is in some embodiments sized such that its top inlet area is similar to top inlet area of the central downcomers.  
         [0057]     While the invention has been described with reference to particular embodiments, it will be understood by those skilled in the art that the tray may be designed by combining the elements disclosed above and various changes may be made and equivalents may be substituted for elements thereof without departing from the scope of the invention. In addition, many modifications may be made to adapt a particular situation or material to the teachings of the invention without departing from the scope of the invention.  
         [0058]     Therefore, it is intended that the invention not be limited to the particular embodiments disclosed as the best mode contemplated for carrying out this invention, but that the invention will include all embodiments falling within the scope and spirit of the appended claims.