Abstract:
A gas-liquid contactor baffle includes a body having a first portion, a second portion and a middle portion, the middle portion being positioned between the first portion and the second portion. The middle portion is a corrugated sheet having a first face and a second face. The corrugated sheet has alternating ridges and open ended channels extending across each of the first face and the second face between the first portion and the second portion. The first portion has a first collection channel adapted to collect liquids from the open ended channels of the middle portion when flow is along the first face in a first direction. The second portion having a second collection channel adapted to collect liquids from the open ended channels of the middle portion when flow is along second face in a second direction.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
       [0001]     Not Applicable  
       STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT  
       [0002]     Not Applicable  
       FIELD OF THE INVENTION  
       [0003]     The present invention relates to a gas-liquid contactor baffle used to improve gas-liquid contact for mass transfer in a tray tower.  
       BACKGROUND OF THE INVENTION  
       [0004]     1. Field of the Invention  
         [0005]     The present invention relates to chemical process towers and, more particularly, but not by way of limitation, to a de-entrainment/mass transfer assembly for a tray tower, the trays having baffles, for increasing tray capacity and improving mass transfer efficiency therein.  
         [0006]     2. Description of Related Art  
         [0007]     It is a continuing goal to improve the efficiency of separation processes and like processes that are conducted using chemical process towers. To this end, many different approaches have been undertaken.  
         [0008]     Separation processes that can be performed in chemical process towers include distillation and absorption. The optimum design of a tower having trays ensures maximum throughput (i.e. capacity) and mass transfer efficiency. At high throughput there is a tendency for liquid to be entrained at high gas velocity. This reduces the capacity as well as efficiency, both caused by the liquid blowing to the tray above. It is an objective of the present invention to effectively remove the entrainment.  
         [0009]     Distillation and absorption towers are utilized to separate selected components from a multicomponent stream. Generally, such gas-liquid contact towers utilize either trays or packings, and sometimes combinations thereof. In the case of tray towers, any wetted solid surfaces will improve mass transfer through additional intimate contact between liquid and gas phases. There will be further advantage if the solid surface can also serve as a de-entrainment device.  
         [0010]     Distillation trays come in two configurations: cross-current (cross-flow) and counter-current (dual-flow). The trays generally consist of a solid tray or deck having a plurality of apertures perforating the deck and are installed on support rings secured within the tower. In cross-current trays, gas ascends through the apertures and contacts the liquid moving across the tray through the “active” area thereof. It is in this area liquid and gas mix and fractionation occurs. The liquid is directed onto the tray by means of a vertical channel from the tray above. This channel is generally referred to as the inlet downcomer. The liquid moves across the tray and exits through a similar channel generally referred to as the exit downcomer. It is the active area of the tray which most directly effects gas liquid contact and thus mass transfer efficiency.  
         [0011]     In the case of dual-flow trays, the tray deck covers the entire cross-sectional area of the tower. Gas and liquid flow through the same apertures, and contact in counter-current manner. Thus, there is no need for downcomers.  
         [0012]     A problem common in the art is that of entrainment of spray in the gas. When spray is entrained in the rising gas it is carried to higher trays, thus affecting the composition of the mixture at those trays, with the consequence that the efficiency of separation of the components in the process mixture is compromised. The gas flow rate may be reduced in order to reduce the entrainment effect, but a consequence is that the tray throughput is also reduced. Another detrimental effect is that the entrained liquid accumulates on the tray above and there is increased amount of liquid on that tray, thus causing premature flooding or reduction in the capacity.  
         [0013]     Improvements have been targeted for the technology of gas-liquid contact trays of the type discussed above to address throughput and mass transfer efficiency issues. Examples of this technology are seen in several prior art patents, which include U.S. Pat. Nos. 3,955,419, 4,604,247 and 4,597,916, each assigned to Glitsch, Inc. and U.S. Pat. No. 4,603,022 issued to Mitsubishi Jukogyo Kabushiki Kaisha of Tokyo, Japan. Other performance aspects are addressed in the prior art by the use of baffles, plates and de-entrainment devices. For example, U.S. Pat. No. 4,105,723 and U.S. Pat. No. 4,132,761, both assigned to Merricks Corporation, address special baffle and de-entrainment structures which are placed within a process tower.  
         [0014]     Chuang et al. in U.S. Pat. No. 5,262,094 teach the utilization of a bed of packing material disposed beneath a fractionation tray for de-entraimnent. Further examples of de-entrainment performed using packing below trays are described by, for example, Monkelbaan et al. in U.S. Pat. Nos. 5,554,329 and 5,707,563, and by Nutter et al. in U.S. Pat. No. 5,975,504. Several other types of de-entrainment devices are described by, for example, Mahar in U.S. Pat. No. 4,274,923, Bentham in U.S. Pat. No. 4,818,346, Stober et al. in U.S. Pat. Nos. 5,837,105 and 6,059,934, and Ross et al. in U.S. Pat. No. 5,972,171. In another example, Lee et al. in U.S. Pat. No. 5,762,668 use a structured packing assembly to reduce entrainment in a chemical process tower and improve mass transfer efficiency.  
         [0015]     However, when de-entrainment is performed using chevron types of demisting device or structured packings, the wetted surfaces of these devices are in direct contact with high velocity gas streams, resulting in re-entrainment of the liquid. This reduces the effectiveness of these devices for de-entrainment.  
       SUMMARY OF THE INVENTION  
       [0016]     According to the present invention there is provided a gas-liquid contactor baffle which includes a body having a first portion, a second portion and a middle portion, the middle portion being positioned between the first portion and the second portion. The middle portion is a corrugated sheet having a first face and a second face. The corrugated sheet has alternating ridges and open ended channels extending across each of the first face and the second face between the first portion and the second portion. The first portion has a first collection channel adapted to collect liquids from the open ended channels of the middle portion when flow is along the first face in a first direction. The second portion having a second collection channel adapted to collect liquids from the open ended channels of the middle portion when flow is along second face in a second direction.  
         [0017]     There will hereinafter be described how this liquid gas contactor baffle improves effectiveness of de-entrainment and provides additional wetted surface area for enhanced mass transfer. As a result, towers equipped with this apparatus have greatly enhanced capacity and efficiency when compared with the prior art. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0018]     These and other features of the invention will become more apparent from the following description in which reference is made to the appended drawings, the drawings are for the purpose of illustration only and are not intended to in any way limit the scope of the invention to the particular embodiment or embodiments shown, wherein:  
         [0019]      FIG. 1  is a fragmentary, vertical, cross-sectional view of a chemical process showing an illustrative example of positioning of baffles and trays within said tower.  
         [0020]      FIG. 2  is a back view of a baffle illustrated in  FIG. 1 .  
         [0021]      FIG. 3  is a side view of the baffle shown in  FIG. 2 .  
         [0022]      FIG. 4  is a top view of the baffle shown in  FIG. 2 .  
         [0023]      FIG. 5  is a bottom view of the baffle shown in  FIG. 2 .  
         [0024]      FIG. 6  is a perspective view of the front face of a baffle illustrated in  FIG. 2 .  
         [0025]      FIG. 7  is a perspective view of the back face of the baffle illustrated in  FIG. 2 .  
         [0026]      FIG. 8  is a diagram illustrating de-entrainment of spray using the assembly illustrated in  FIG. 1 . 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0027]     Exemplary embodiments of the invention, which are non-limiting, will now be described with reference to  FIG. 1  through  FIG. 8 .  
         [0028]     Referring to  FIG. 1 , an embodiment of the present invention  10  is illustrated. A chemical process tower  12  is fitted with a plurality of trays  14 , each tray having a top side  16 , a bottom side  18  and apertures  20  extending between bottom side  18  and top side  16 . Tray  14  extends for a portion of the width of tower  12 . A majority of an edge  22  of tray  14  is adjacent an inside surface  24  of walls  26  of tower  12 . A weir  28  is situated along the remaining edge  30  of tray  14 , and on the other side of said weir  28  is situated a downcomer  32 . An apparatus  33  comprising a plurality of baffles  34  is arrayed in a zone  36  below bottom  18  of tray  14 .  
         [0029]     A preferred embodiment of baffle  34  will now be described with reference to  FIGS. 2 and 3 . It will be recognized that several aspects of the illustrated embodiment may be altered without deviating from the principles or spirit of the invention, and so the following description is non-limiting. In  FIGS. 1 and 8  a cross-current tray design is illustrated. It will be recognized that apparatus  33  of the present invention can also be applied with counter-current trays, commonly known as dual flow trays.  
         [0030]     Referring to  FIGS. 2 and 3 , baffle  34  has a body  40  comprising a middle portion  42 , a first or upper portion  44  having a first collection channel  45  and a second or lower portion  46  having a second collection channel  47 .  
         [0031]     Referring to  FIG. 3 , middle portion  42  comprises a sheet of material  48  having a first face or back face  50  and a second face or front face  52 . Referring to  FIGS. 2 and 7 , sheet  48  is corrugated, as illustrated, or otherwise shaped so as to form a series of approximately parallel ridges  54  with channels  56  therebetween, with discharge holes  57  at the bottom end of channels  56 . Ridges  54  and channels  56  extend from a position adjacent top edge  58  to a position adjacent bottom edge  60  of sheet  48 , between side edges  62  of sheet  48 . It will be recognized that alternative shaping of sheet  48  may be used, including but not limited to concertina shaping, sinusoidal wave patterning, and mixtures of these and other shaping.  
         [0032]     Referring to  FIGS. 3 and 4 , first or upper portion  44  comprises an upper curved portion  64  that forms first collection channel  45  and a flange  66 . Curved portion  64  has an opening  68  extending between flange  66  and distal edge  70  along its full length. Curved portion  64  of first collection channel  45  has a plurality of holes  72  situated adjacent flange  66 , through which a liquid may exit first collection channel  45  toward channels  56  in middle portion  42 . Advantageously, the number of holes  72  matches the number of channels  56 , and each of holes  72  is situated so that liquid passing therethrough flows directly into the corresponding channel  56 .  
         [0033]     Referring to  FIG. 3 , second or lower portion  46  is shaped so as to form a second collection channel  47 . The illustrated embodiment has a curved portion  49  that has a J-shape. It will be recognized that alternative shapes may be used to form second collection channel  47 . An opening  78  extends between a first edge  80  adjacent middle portion  42  and a distal second edge  82  along the length of second portion  46 . Referring to  FIGS. 3 and 5 , a plurality of holes  86  perforate the bottom of second or lower portion  46  so that liquid can flow downward out of second collection channel  47  to the area below baffle  34 .  
         [0034]     Referring to  FIGS. 3, 6  and  7 , flange  66  of first or upper portion  44  is connected along the length of middle portion  42  at upper ends of ridges  54 . First edge  80  of second or lower portion  46  is connected along the length of middle portion  42  at lower ends of bases  84  of channels  56 .  
         [0035]     Referring again to  FIG. 1 , baffles  34  are arrayed across the breadth and width of the area of zone  36  of tower  12  below tray  14 . Typically, baffles  34  are ranked in parallel so that one baffle  34  in one rank  88  is situated immediately above or below the spacing between two other baffles  34  in another rank  88 . Typically baffles  34  are angled relative to both of the principal axis of tower  12  and the plane of tray  14 . An angle  90  of the principal plane of baffle  34  relative to the plane of tray  14  is an angle between 5 degrees and 85 degrees. Preferably, baffles  34  in one of ranks  88  are rotated at an angle of 180 degrees relative to baffles  34  in another of ranks  88  immediately above or below the one rank  88 .  
         [0036]     The method of utilization of apparatus  33  of the present invention will now be described with reference to  FIG. 8  for a cross-current tray design. It will be recognized that the operation of a duel flow tray tower having apparatus  33  installed therein is similar.  
         [0037]     Apparatus  33  comprising a plurality of baffles  34  is installed in an array having at least one rank  88  below trays  14  in tower  12 . When tower  12  is in operation, liquid  100  spills over weir  28  of one tray  14  and descends downcomer  32  toward another tray  14  immediately below the one tray  14 . A head of liquid  100 , an extent of which is indicated by curved lines  104 , is in downcomer  32 , and causes liquid  100  to flow across top surface  16  of tray  14  toward and over weir  28 . Gas  102  in zone  36  immediately below tray  14  rises through apertures  20  in tray  14  and bubbles through liquid  100 . A froth  106  that is formed has an extent indicated by a curved line. As bubbles  107  exit froth  106  they form a spray  108  that travels upward as indicated by arrows. Baffles  34  intercept spray  108 . Referring to  FIG. 6  through  8 , gas  102  containing entrained spray  108  impinges on front face  52  and first collection channel  45  of baffles  34 . Droplets of spray  108  coalesce to form liquid  100  on front face  52 . Said liquid  100  on front face  52  of middle portion  42  flows in a first direction toward first or upper portion  44 , and then passes through holes  72  in first collection channel  45  onto back face  50  and into channels  56  of sheet  48 . Liquid  100  descends channels  56  in a second direction toward second or lower portion  46 , flows out channels  56  via discharge holes  57 , and is collected in second collection channel  47 . Liquid  100  exits second collection channel  47  via holes  86  and falls as droplets  110  toward active area  112  below.  
         [0038]     Liquid  100  descending channels  56  is not exposed to any high velocity gas  102 , and so is not subject to re-entrainment.  
         [0039]     Droplets  110  are much larger than spray  108 , and so droplets  110  do not become entrained in gas  102 .  
         [0040]     In this manner, liquid  100  that has been formed into spray  108  is prevented from being entrained in gas  102  and ascending through apertures  20  in tray  14  above.  
         [0041]     When compared with operation of trays  14  having no baffles, the surface area of liquid  100  is increased by having liquid  100  flow along channels  56  on back face  50  of baffles  34  and fall as droplets  110 , thereby improving mass transfer between liquid  100  and gas  102 , and hence enhancing operation of tower  12 .  
         [0042]     It has been found through monitoring embodiment  10  shown in  FIGS. 1 and 8  that spray  108  generated above one tray  14  impinges on front face  52  of middle portion  42  and is directed to form liquid  100  at the back face  50  of middle portion  42  of baffles  34  below another tray  14  immediately above. When the gas rate is small, liquid  100  carried over is collected in corrugated channels  56 , flows out through discharge holes  57  downward to second collection channel  47 , and subsequently is discharged via holes  86  to fall as droplets  110  into active area  112  below. When the gas rate is high, liquid  100  carried over is pushed upwards and mostly ends up in first collection channel  45 . Liquid  100  collected in first collection channel  45  is redirected down via holes  72  and the openings at the top of corrugated sheet  48  to back face  50 . Because liquid  100  flowing down on back face  50  of baffle  34  is not subjected to gas flow, spray  108  is eventually redirected to active area  112 . As the gas rate continues to increase, some portion of the liquid  100  carried over could be pushed over to the upper rank  88  of baffles  34  which are rotated at a similar angle  90  relative to the horizontal plane but in an opposite direction to angle  90  of the lower rank  88  of baffles  34  when the upper rank  88  is rotated at an angle of 180 degrees to that of the lower rank  88 . Further ranks  88  of baffles  34 , if present, are similarly rotated relative to each other, and similarly alternate in direction of angle  90 . The spray-deflecting function repeats at the upper rank  88  of baffles  34 .  
         [0043]     Benefits found to accrue from operation of the present invention include: converting tray action from spray regime to froth regime; deflecting spray by collecting it from the front sides of the baffles and routing it to the back sides of the baffles and subsequently discharging it to the active area below, and increasing liquid surface area for additional mass transfer. These beneficial characteristics of the new invention have been shown to result in greater capacity (up to 20%) and higher tray efficiency.  
       EXAMPLES  
     Example 1  
       [0044]     It has been shown experimentally that a combination of trays  14  with baffles  34  that serve as “spray deflector” can have as much as 15 to 20% more capacity in a weir loading of 0.5 to 5.0 gpm per inch of weir  28  when compared with use of trays alone. The apparatus was tested using different gas rates (C-factor) and liquid weir loadings (WL). When WL was 4.0 gpm/in for standard valve trays separated by 610 mm, the C-factor at capacity increased from 0.39 ft/s to at least 0.46 ft/s when baffles were in place.  
         [0045]     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.  
         [0046]     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.