Abstract:
A plurality of smaller mass transfer or exchange columns are provided within a larger column and are filled with packing to facilitate contact between fluids flowing within the smaller columns. The smaller columns are positioned in parallel and contacting relationship and preferably fill substantially the entire cross section of the larger column. At least some of the smaller columns share a common external wall. Structured packing fills the smaller columns to facilitate interaction between fluids flowing countercurrently within the smaller columns.

Description:
CROSS REFERENCE TO RELATED APPLICATION  
       [0001]    This application claims the benefit of provisional patent application Serial No. 60/271,584 filed Feb. 26, 2001. 
     
    
     
       BACKGROUND OF THE INVENTION  
         [0002]    The present invention relates generally to mass transfer and exchange columns and, more particularly, to apparatus used within those columns and methods of constructing and using same.  
           [0003]    In mass transfer and exchange columns, packing beds made of various types of random and structured packings are utilized to facilitate interaction between countercurrently flowing vapor and liquid streams. These packings operate by providing large surface areas across which the liquid and vapor flow to increase the area of contact between the vapor and liquid. Random packings such as rings and saddles are typically dumped onto a support provided within the column, while structured packings such as corrugated plates are frequently preassembled in bricks which are then placed on grid supports within the column.  
           [0004]    It is generally recognized that packing when used in small columns and pilot columns exhibits a greater separation efficiency than when those same types of packing are used in larger or commercial sized columns. Liquid and vapor channeling and poor lateral mixing have been reported to contribute to the reduced packing efficiency observed in larger columns.  
           [0005]    In an effort to duplicate the greater packing efficiency of small columns, a column designed for heavy water enrichment used a large number of parallel tubes filled with rings of wire gauze packing. The tubes extended vertically within a larger column and the upper ends of the tubes were welded to a tube sheet which prevented vapor and liquid from flowing in the open areas surrounding the tubes. While higher separation efficiencies could be obtained in the smaller tubes than in the larger column, the use of a tube sheet to seal around the individual tubes was undesirable for at least two reasons. First, in order for the tube sheet to retain its structural integrity, the tubes had to be spaced sufficiently apart so that the tube sheet could be formed as a single continuous piece of metal. This spacing between adjacent tubes, however, reduced the cross-sectional area that was available for fluid flow within the column. In addition to reducing the fluid flow capacity, the use of the tube sheet to secure the tubes added significantly to the time and expense required to fabricate the column because of the need to weld each individual tube to the tube sheet.  
           [0006]    In another approach which has been taken in an attempt to achieve small column efficiencies within a larger column, bricks of structured packing were oriented in a checkerboard pattern within the column. The plates in each brick were oriented so that they extended parallel to the plates in diagonally adjacent bricks and perpendicular to the plates in the sideways adjacent bricks. No walls separated the adjacent bricks and the desired small column efficiencies were not achieved.  
           [0007]    A need thus exists for a column that can more closely achieve the separation efficiency of packing used in small columns without the reduction in fluid flow capacity and installation delays and expense resulting from current approaches.  
         SUMMARY OF THE INVENTION  
         [0008]    The present invention is directed to configuring a typical commercial distillation column structured or random packing layout into multiple parallel functioning distillation columns within a single or main column structure. As a result of using these multiple parallel columns, better separation efficiencies can be obtained than would normally result from conventional commercial column packing layouts. The efficient operating range is also extended up to the flood point or maximum pressure drop that can be tolerated within the columns.  
           [0009]    In one aspect, the invention is directed to a mass transfer column comprising an external shell defining an open internal region and a plurality of vertically extending smaller columns supported within said open internal region. The smaller columns have external walls which define internal fluid passages and vapor-liquid contact packing within the internal fluid passages of at least some of the smaller columns. The external walls of at least some of the smaller columns are in contact with the external walls of adjacent smaller columns within said open internal region.  
           [0010]    In another aspect, the invention is directed to a method of constructing a plurality of longitudinally extending smaller mass transfer columns within a larger column. The method comprises the steps of assembling together a plurality of wall panels within the larger diameter column to form external walls of a plurality of parallel extending smaller columns having internal fluid passages. At least some of the wall panels form a common portion of the external walls of adjacent smaller columns. The method includes installing packing within the internal fluid passages of the smaller columns during or after assembly of the wall panels. The packing may also be preassembled with the wall panels to facilitate formation of the smaller columns within the larger column. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0011]    In the accompanying drawings which form part of the specification and are to be read in conjunction therewith and in which like reference numerals are used to indicate like parts in the various views:  
         [0012]    [0012]FIG. 1 is a fragmentary side elevation view of a larger column with portions of the column shell broken away to show a plurality of smaller columns within the larger column;  
         [0013]    [0013]FIG. 2 is an enlarged fragmentary side elevation view of one of the smaller columns shown in FIG. 1;  
         [0014]    [0014]FIG. 3 is a plan view of the larger column taken in horizontal section to show the circular cross section of the smaller columns;  
         [0015]    [0015]FIG. 4 is a plan view of the larger column taken in horizontal section to show the hexagonal cross section of the smaller columns;  
         [0016]    [0016]FIG. 5 is a fragmentary side elevation view taken in vertical section and showing the lower end of an upper wall segment inserted within the upper end of a lower wall segment;  
         [0017]    [0017]FIG. 6 is an enlarged fragmentary plan view of the larger column showing the construction of the external walls of the smaller columns;  
         [0018]    [0018]FIG. 7 is a bottom perspective view of a packing brick;  
         [0019]    [0019]FIG. 8 is a somewhat schematic plan view of a wall panel and packing subassembly; and  
         [0020]    [0020]FIG. 9 is a side elevation view of the wall panel and packing subassembly shown in FIG. 8. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0021]    Referring now to the drawings in greater detail and initially to FIG. 1, a mass transfer column or exchange column is represented broadly by the numeral  10  and includes an external shell  12  which defines an open internal region  14  in which various column internals are located. The shell  12  has a vertically extending longitudinal axis and is generally cylindrical in configuration, but other configurations such as polygon can be utilized if desired.  
         [0022]    Column  10  is of the type utilized for processing liquid and vapor streams, including to obtain fractionation products. One or more liquid streams are directed to the column  10  through flow lines  16  for downward flow therein and one or more vapor streams are directed to the column  10  through flow lines  18  or are generated within the column  10  for countercurrent or ascending flow. After processing within the column  10 , the vapor and liquid streams are removed through overhead and bottoms flow lines  20  and  22 , respectively. Because the general structure of these types of columns is well known, only a portion of the column  10  relevant to the present invention is illustrated.  
         [0023]    A plurality of smaller columns  24  are located within the larger column  10  and are placed in side by side and contacting relationship so that they preferably fill substantially the entire cross section of the larger column  10 . Each of the smaller columns  24  is generally of similar construction, with the longitudinal axes of the smaller columns  24  being arranged parallel to each other and to the vertical longitudinal axis of the larger column  10 .  
         [0024]    As can be seen in FIG. 2, each smaller column  24  comprises a perimeter or external wall  26  defining an internal fluid passage  28  in which packing  30  is placed and through which vapor and liquid streams flow in countercurrent relationship. The external walls  26  restrict or prevent lateral flow of fluid from one smaller column  24  to another smaller column  24  and are preferably liquid and vapor impermeable. The material used to form the external wall  26  can be various metals, polymers and ceramics which are compatible with the conditions within the column  10 .  
         [0025]    The external walls  26  of the smaller column  24  can have any desired cross-sectional configuration, such as the circular shape illustrated in FIG. 3 or a polygonal shape such as the hexagonal shape illustrated in FIG. 4. Square, triangular and hexagonal shapes are generally preferred because they allow the smaller columns  24  to nest against each other without forming small voids between the smaller columns as in the case with the circular shape. The longitudinal length of the external wall  26  of some or all of the smaller columns  24  can be of a single piece construction or can be formed be two or more wall segments  32  and  34  placed end to end and joined together in any of various fashions, such as by inserting the lower end of the upper wall segment  34  within the upper end of the adjacent lower wall segment  32  as illustrated in FIG. 5. Tabs  36  formed in the end of one or both wall segments  32  and  34  may be used to limit the depth to which the end of wall segment  34  can be inserted within wall segment  32 . The external wall  26  may also be formed from two or more wall panels, such as panels  38 ,  39  and  40  which are joined or simply abutted together along their sides as illustrated in FIG. 6. Constructing the external wall  26  in this manner allows adjacent smaller columns  24  to share a common wall along a portion or all of their perimeters. If desired, a double wall can also be formed along a portion or all of the perimeters of the smaller columns  24 .  
         [0026]    As shown in FIG. 1, liquid is individually fed into the open upper ends of the smaller columns  24  by a liquid distributor  44  so that equal amounts of liquid can be fed to each smaller column  24 . The bottoms of the smaller columns  24  are open and are preferably supported on a grid  46  that is itself supported by a support ring  48  that is secured to an inner surface of the larger column shell  12 .  
         [0027]    The packing  30  may be random packing, but is preferably structured packing such as corrugated, parallel sheets or plates  48 . As can be seen in FIG. 2, the plates  48  are vertically disposed and the corrugations extend at an angle to the vertical axis of the smaller columns  24 . The plates  48  are arranged so that the corrugations of adjacent plates extend in crisscrossing relationship and are in contact with each other. Turning to FIG. 7, the plates  48  can be held together in a brick  50  using pins, bolts, rivets, welding, soldering or preferably mesh banding  52  as is well-known in the art. The mesh banding  52  is slit and bent outwardly along its top or bottom to form wall wipe bands  54  that redirect liquid descending along the inner surface of the external walls  26  back into the packing  30 . The packing  30  can be installed within the smaller columns  24  after the external walls  26  of the smaller columns  24  have been assembled within the larger column  10 . Alternatively, the packing  30  can be installed as the external walls  26  are being assembled. For example, packing  30  can be installed in lower wall segments  32  before the upper wall segments  34  are installed. After some or all of the lower wall segments  32  have been filled with packing  30 , the upper wall segments  34  can be installed and then similarly filled with packing  30 . Likewise, when wall panels  38  and  40  or  42  are used, the packing  30  can be inserted as some or all of the perimeter of the smaller columns  24  is formed. In a still further embodiment, the packing  30  may be preassembled with portions of the external walls  26  outside of the larger column  10 . For example, as shown in FIGS. 8 and 9, one or more packing bricks  50  may be preassembled with one wall panel  38  or  40  before the wall panels  38  and  40  are assembled within the larger column  10  to form the external walls  26  of the smaller columns  24 .  
         [0028]    The packing  30  preferably fills the smaller columns  24  from top to bottom and may be arranged in a plurality of horizontal layers, each of which is in contact with and rotated 90° or other desired angle from a vertically adjacent layer. The total height of the packing  30  within the smaller columns  24  can be selected to suit particular process conditions.  
         [0029]    Without wishing to be bound by any particular theory, the improved separation efficiencies believed to be obtainable with the packing  30  in the smaller columns  24  results from: (1) eliminating the gaps that are formed between the sides and ends of adjacent packing bricks in commercial-sized columns packing is made in bricks which may cause gaps to be formed at the ends and/or sides of the bricks; (2) using wall wiper bands around each packing element to redirect liquid from the small column walls back into the packing; and (3) facilitating lateral liquid mixing because the liquid flow channels along the inclined corrugations do not extend from the top to the bottom of the element without first hitting the column wall.  
         [0030]    It is believed that liquid and/or vapor maldistribution does not occur or is substantially reduced in smaller columns  24 . Alternatively, if maldistribution should occur, for instance at the top of the smaller columns  24  due to a flooded column  10 , correction takes place within a short distance and separation efficiency recovers. The smaller column diameter also reduces or prevents channeling of liquid and distortion of the liquid/vapor ratios across the cross section of the column which causes the net effective separation to be negatively affected.  
         [0031]    From the foregoing, it will be seen that this invention is one well adapted to attain all the ends and objectives hereinabove set forth together with other advantages which are inherent to the structure.  
         [0032]    It will be understood that certain features and subcombinations are of utility and may be employed without reference to other features and subcombinations. This is contemplated by and is within the scope of the invention.  
         [0033]    Since many possible embodiments may be made of the invention without departing from the scope thereof, it is to be understood that all matter herein set forth or shown in the accompanying drawings is to be interpreted as illustrative and not in a limiting sense.