Abstract:
A divided exchange column includes a shell column having a first longitudinal axis, a cylindrical wall spaced apart from and surrounding the first longitudinal axis and defining a first interior space, a first divided wall column having a second longitudinal axis substantially parallel to the first longitudinal axis, and a second divided wall column having a third longitudinal axis substantially parallel to the first and the second longitudinal axes, where the first divided wall column and the second divided wall column are positioned in the first interior space of the shell column.

Description:
BACKGROUND 
     Traditional methods for building divided wall columns for use in heat and/or mass transfer processes required welding a dividing wall in an appropriate location directly to a pressure shell column and then proceeding with the installation of packing or distillation trays on either side of the divided wall column. Other methods for building divided wall columns required that half-cylindrical shells be welded onto the dividing wall itself. Both methods required a welder, working inside the pressure shell column, to place welds on the interior wall of the column and the dividing wall to prevent leaks between these members. 
     The previous methods of manufacture and designs of divided wall columns for smaller sized applications, however, are problematic for several reasons. 
     First, use of a divided wall column incorporating a traditional dividing wall subjects the dividing wall to both pressure and temperature differences on both sides of the divided wall column. These temperature and pressure differences arise due to the very nature of divided wall column operation. On either side of the dividing wall, different mass transfer conditions are created by the varying liquid and vapor flows through the column sections. These diverse mass transfer conditions yield different composition profiles along the length of the divided wall sections. The different composition vapors and liquids having distinct saturation temperatures thus create temperature differences on either side of the dividing wall. 
     The temperature differences create significant mechanical stresses on both the dividing wall as well as the pressure shell column, and as such, have to be carefully monitored and managed during plant operation. The pressure and temperature differences especially are significant during operating upsets, for example, during the startup or shut down of the facility. 
     These upset conditions can be especially problematic when the operating temperatures of the column system deviate substantially from ambient conditions. For example, an air separation column operates under cryogenic conditions with temperatures reaching −195° C. At that temperature, care must be taken while cooling down the columns to ensure that both sides of the columns are cooled at a uniform rate (i.e., to prevent one column from cooling at a rate much faster than the opposite side of the column). 
     To combat these mechanical stresses, support members or stiffening members have been incorporated to prevent buckling, for example. As illustrated in U.S. Pat. No. 7,357,378, incorporated herein by reference in its entirety, these support members assist in withstanding the pressure differentials and minimizing the effect of the temperature differentials. Use of these support or stiffening members, however, may increase the cost and have a detrimental effect on the efficiency of the distillation process. 
     Increasing the thickness of the dividing wall to compensate for the mechanical stresses has also been attempted, however, the increase in strength realized is minimal, especially in columns having large diameters. Further, increasing the diameter of the dividing wall also leads to complications associated with the welding of the dividing wall to the column wall. Increasing the thickness of the dividing wall also causes occupation of a greater portion of the column area leading to less efficient use of the available area. 
     Use of double (laminated) or honeycombed walls, strengthening ribs, or using distillation trays as stiffeners to strengthen the dividing wall all suffer from the same drawbacks described above. 
     Second, with the increased demand for divided wall columns to be smaller, the space available for a welder to enter the pressure shell column and weld the dividing wall to the pressure shell column or the half-cylindrical shell to the dividing wall itself is problematic. For example, welding a dividing wall in columns with smaller diameters creates a measurably “tighter” clearance for the welder and welding tools to fit and work. In addition, welding shims, support members and/or stiffening members inside these small traditional divided wall columns is increasingly difficult. 
     BRIEF SUMMARY 
     Embodiments of the present invention satisfy a need in the art by providing an apparatus and method for facilitating the use of divided wall technology with the specific advantage of providing for the production of smaller column diameters than are currently manufactured or used. 
     In one embodiment, a divided exchange column includes a cylindrical shell column having a first longitudinal axis and a cylindrical wall spaced apart from and surrounding the first longitudinal axis, thereby defining a first interior space between the cylindrical wall and the first longitudinal axis, wherein the cylindrical wall comprises an inner surface and an outer surface; a first divided wall column having a second longitudinal axis substantially parallel to the first longitudinal axis, a first wall, and a second wall, where the first wall of the first divided wall column is coupled to the second wall of the first divided wall column, and where the first wall of the first divided wall column is substantially flat and the second wall of the first divided wall column is substantially semi-cylindrical; a second divided wall column having a third longitudinal axis substantially parallel to the first and the second longitudinal axes, a first wall, and a second wall, where the first wall of the second divided wall column is coupled to the second wall of the second divided wall column, and where the first wall of the second divided wall column is substantially flat and the second wall of the second divided wall column is substantially semi-cylindrical; and where the first divided wall column and the second divided wall column are positioned in the first interior space of the cylindrical shell column such that the first wall of the first divided wall column is adjacent to the first wall of the second divided wall column and the second wall of the first divided wall column and the second wall of the second divided wall column are adjacent to the inner surface of the cylindrical wall. 
     In another embodiment, the first wall of the first divided wall column and the first wall of the second divided wall column define a slot therebetween and where the slot extends from the first longitudinal axis to the inner surface of the cylindrical shell. 
     In another embodiment, the first divided wall column has a first length, and the second divided wall column has a second length. The first length of the first divided wall column and the second length of the second divided wall column may not be equal, for example. 
     In yet another embodiment, the shell column may be pressurized. In another embodiment, the shell column may be under vacuum conditions. 
     In another embodiment, the thickness of the first wall of the first divided wall column may be 3 mm to 20 mm. 
     In another embodiment, the first divided wall column comprises at least one distillation tray and where the second divided wall column comprises structured packing. 
     In yet another embodiment, a divided exchange column includes a first wall, where the first wall is substantially flat, a second wall, where the second wall is substantially semi-cylindrical and where the first wall and the second wall are coupled together such that the first wall and the second wall form a first exchange column with a cross-section that is substantially semi-cylindrical, a third wall, where the third wall is substantially flat, a fourth wall, where the fourth wall is substantially semi-cylindrical and where the third wall and the fourth wall are coupled together such that the third wall and the fourth wall form a second exchange column with a cross-section that is substantially semi-cylindrical, and a fifth wall, where the fifth wall is substantially cylindrical and forms a shell around the first exchange column and the second exchange column, and where the first wall of the first exchange column is adjacent and spaced from the third wall of the second exchange column. 
     In another embodiment, a divided exchange column comprising a cylindrical shell having a longitudinal axis and divided into two or more longitudinally extending part-cylindrical internal columns is characterized in that each of said internal columns is discretely preformed and has a respective part-cylindrical wall adjacent the cylindrical shell and terminating in two longitudinally extending edges and closed between said edges with one or more side walls. 
     In yet another embodiment, a divided exchange column comprises a first wall, where the first wall is substantially flat; a second wall, where the second wall is substantially semi-cylindrical and where the first wall and the second wall are coupled together such that the first wall and the second wall form a first exchange column; a third wall, where the third wall is substantially flat; a fourth wall, where the fourth wall is substantially semi-cylindrical and where the third wall and the fourth wall are coupled together such that the third wall and the fourth wall form a second exchange column; and a fifth wall, where the fifth wall is substantially cylindrical and forms a shell around the first exchange column and the second exchange column, and where the first wall of the first exchange column is adjacent and spaced apart from the third wall of the second exchange column. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The foregoing brief summary, as well as the following detailed description of exemplary embodiments, is better understood when read in conjunction with the appended drawings. For the purpose of illustrating embodiments of the invention, there is shown in the drawings exemplary embodiments of the invention; however, the invention is not limited to the specific methods and instrumentalities disclosed. In the drawings: 
         FIG. 1  is a schematic diagram illustrating a traditional divided wall column; 
         FIG. 2  is a schematic diagram illustrating an exemplary embodiment of the present invention; 
         FIG. 3  is a schematic diagram illustrating an exemplary embodiment of the present invention; 
         FIG. 4  is a schematic diagram illustrating an exemplary embodiment of the present invention; 
         FIG. 5  is a schematic diagram illustrating an exemplary embodiment of the present invention; 
         FIG. 6  is a schematic diagram illustrating an exemplary embodiment of the present invention; 
         FIG. 7  is a schematic diagram illustrating an exemplary embodiment of the present invention; 
         FIG. 8   a  is a schematic diagram illustrating an exemplary embodiment of the present invention; 
         FIG. 8   b  is an enlarged cross-sectional schematic diagram of an exemplary embodiment of the present invention; 
         FIG. 9   a  is a schematic diagram illustrating an exemplary embodiment of the present invention; 
         FIG. 9   b  is an enlarged cross-sectional schematic diagram of an exemplary embodiment of the present invention; 
         FIG. 10   a  is a schematic diagram illustrating an exemplary embodiment of the present invention; 
         FIG. 10   b  is an enlarged cross-sectional schematic diagram of an exemplary embodiment of the present invention; and 
         FIG. 11  is a schematic diagram illustrating an exemplary embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  illustrates a known traditional divided wall column  10  where a dividing wall  20  is welded to column  30 . 
       FIG. 2  illustrates an embodiment of present invention where separate preformed divided wall columns  40 ,  50  may be constructed for use in a shell column  60 . The shell column  60  may be pressurized, for example. The shell column  60  may also be under vacuum conditions, for example. The diameter of the shell column  60  may vary. For example, the diameter of the shell column  60  may be designed as a function of the design pressure. The shell column  60  may be cylindrical or substantially cylindrical, for example. The divided wall columns  40 ,  50  may be semi-cylindrical or substantially semi-cylindrical, for example. 
     In  FIGS. 3-11 , elements that correspond to elements in the previously illustrated embodiments are identified by the same number. As illustrated in  FIG. 3 , the divided wall columns  70 ,  80 ,  90  may also be substantially pie-shaped or substantially sectorial, for example, such that one or more divided wall columns may be positioned inside a shell column  60 . 
     As further illustrated in  FIG. 2 , a planar or flat wall  42 ,  52  of the divided wall column  40 ,  50  may be welded to the semi-cylindrical wall  44 ,  54  of the divided wall column  40 ,  50  from outside the divided wall column  40 ,  50 . Space restrictions that occur when working from the inside of the divided wall column  40 ,  50  may be eliminated because the welding may be performed from outside the divided wall column  40 ,  50 . The thickness of the flat wall  42 ,  52  may vary. The flat wall  42 ,  52  thickness may be 3 mm to 20 mm, for example. 
     As illustrated in  FIGS. 2-4 , each divided wall column  40 ,  50  (or  70 ,  80 ,  90  in  FIG. 3 ) may have distillation trays  100  or packing  110  installed as appropriate. Distillation trays  100 , disclosed in U.S. Pat. No. 7,234,691, which is herein incorporated by reference in its entirety, may be used, for example. The trays  100  may not directly contact the interior walls of the divided wall column  40 ,  50 ,  70 ,  80 ,  90  as they may be installed as a stack of self-supporting trays that hang from the top, for example. 
     Each divided wall column  40 ,  50 ,  70 ,  80 ,  90  may be separately installed into the shell column  60 , for example. 
     In one embodiment, the divided wall column  40 ,  50 ,  70 ,  80 ,  90  may be secured to the shell column  60  at the top  120  of the divided wall column  40 ,  50 ,  70 ,  80 ,  90 , for example. In an alternative embodiment, the divided wall column  40 ,  50 ,  70 ,  80 ,  90  may be secured to the shell column  60  at the bottom  130  of the divided wall column  40 ,  50 ,  70 ,  80 ,  90 , for example. In another embodiment, a shear ring  140  may be used to affix the divided wall column  40 ,  50 ,  70 ,  80 ,  90  to the shell column  60 , for example. 
     In another embodiment, telescoping shoes or spacers  150  may be used to fix the relationship of the divided wall columns  40 ,  50 ,  70 ,  80 ,  90  to the shell column  60  and each other. 
     Manufacturing the divided wall columns  40 ,  50 , as illustrated in  FIG. 2 , for example, may diminish the effect of the mechanical stresses created by the pressure and temperature differences that typically occur on either side of the dividing wall because the two divided wall columns no longer share a common dividing wall  20 , as illustrated in  FIG. 1 . The divided wall columns  40 ,  50  may be positioned apart such that an open space or slot  170  may be defined between the substantially flat walls  42 ,  52  of each divided wall column  40 ,  50 . Use of the separated divided wall columns  40 ,  50  may allow for some freedom of movement that may be required by the temperatures and pressures that each divided wall column  40 ,  50  may experience without being directly affected by those of the other corresponding divided wall column. The independent sections are free to move independently from each other in response to whatever mechanical stresses are resulting from temperature and pressure differences. Thus, the effect of the mechanical forces due to the temperature and pressure differences is largely eliminated except for the single area where divided wall column  40 ,  50  is attached to the shell column  60  and the vapor seal is created. 
     In yet another embodiment, and as illustrated in  FIG. 5 , the divided wall columns  40 ,  50  may be positioned proximate to each other such that the substantially flat walls  42 ,  52  of each divided wall column  40 ,  50  are in direct contact with each other. In another embodiment, the substantially flat walls  42 ,  52  of each divided wall column  40 ,  50  may be coupled to each other, for example. The top of each divided wall column  40 ,  50  may be seal welded, for example. 
     As illustrated in  FIG. 6 , the divided wall columns  40 ,  50  inside the shell column  60  may have different lengths. For example, the length of first divided wall column  40  may be longer than the second divided wall column  50  because the specific separation requirements for the first divided wall column  40  may require more theoretical stages, for example. The lengths of the divided wall columns  40 ,  50  may also be substantially identical, as illustrated in  FIG. 7 . As illustrated in  FIGS. 6-10   b , the divided wall columns  40 ,  50 , may incorporate distillation trays  100 , packing  110 , or various combinations thereof. As illustrated in  FIGS. 8   a - 10   b , each divided wall column  40 ,  50  may include a liquid distributor  180 . The divided wall column  40 ,  50  may also include a feed nozzle  190  and a roof  200 . 
     In another embodiment, the divided wall columns  40 ,  50  may occupy non-equal portions of the total available area, for example. As illustrated in  FIG. 11 , a first divided wall column  40  may have a smaller area than a second divided wall column  50 . 
     While aspects of the present invention has been described in connection with the preferred embodiments of the various figures, it is to be understood that other similar embodiments may be used or modifications and additions may be made to the described embodiment for performing the same function of the present invention without deviating therefrom. Therefore, the claimed invention should not be limited to any single embodiment, but rather should be construed in breadth and scope in accordance with the appended claims. 
     Aspects and embodiments of the invention include: 
     #1. A divided exchange column comprising a cylindrical shell having a longitudinal axis and divided into two or more longitudinally extending part-cylindrical internal columns characterized in that each of said internal columns is discretely preformed and has a respective part-cylindrical wall adjacent the cylindrical shell and terminating in two longitudinally extending edges and closed between said edges with one or more side walls. 
     #2. The divided exchange column of #1, wherein there are two complementary internal columns each having the part-cylindrical wall closed along its open side with a substantially flat chordal side wall. 
     #3. The divided exchange column of #2, wherein the internal columns are semi-cylindrical. 
     #4. A divided exchange column of #1, wherein there are three or more complementary sectorial internal columns each having a part-cylindrical wall closed along its open side with a pair of mutually angularly inclined substantially flat side walls. 
     #5. A divided exchange column of any preceding #, wherein opposed side walls of adjacent internal columns are spaced apart. 
     #6. A divided exchange column of any one of #1 to #4, wherein opposed side walls of adjacent internal columns are in contact. 
     #7. A divided exchange column of any preceding #, wherein at least one of said internal columns contains distillation trays that do not directly contact the interior surfaces of the column. 
     #8. A divided exchange column of #7, wherein said distillation trays are arranged in a self supporting stack that hangs from the top of said column. 
     #9. A divided exchange column of any preceding #, wherein at least one of said internal columns is secured at its top to the shell. 
     #10. A divided exchange column of any preceding #, wherein at least one of said internal columns is secured at its bottom to the shell. 
     #11. A divided exchange column of any preceding #, wherein at least one of said internal columns is secured at to the shell by a shear ring. 
     #12. A divided exchange column of any preceding #, wherein at least one of said internal columns is of smaller cross section than the at least one other said internal column. 
     #13. A divided exchange column of any preceding #, wherein at least one of said internal columns is of shorter length than the at least one other said internal column. 
     #14. A divided exchange column of any preceding #, wherein at least one of said internal columns comprises at least one distillation tray and wherein the at least one other internal column comprises structured packing. 
     #15, A method of constructing a divided exchange column of #1 comprising inserting into a cylindrical shell having a longitudinal axis two or more discretely preformed longitudinally extending part-cylindrical internal columns each having a respective part-cylindrical wall adjacent the cylindrical shell and terminating in two longitudinally extending edges and closed between said edges with one or more side walls.