Abstract:
A corrugated strip made of sheet material, which may be used in a packing module. The peaks and the troughs of the strip, when viewed in a side view, define lines which have a general sloping direction relative to the direction the fluid flow. Each strip includes a spanning zone and a transition zone. In the transition zone, each peak/trough line extends inside a specific area centered on a curve which extends tangentially from the peak/trough line of the spanning zone. The structured interface region between the spanning and transition zones helps to reduce head loss across the strip.

Description:
BACKGROUND  
   The present invention relates to a corrugated strip made of sheet material, especially plastically deformed sheet metal, for a packing module for treating a fluid, of the type defining flow channels for the fluid and comprising a spanning region, the channels of which define, in side view, peak/trough lines having a general direction inclined to a general flow direction of said fluid, the strip furthermore including at least one transition region adjacent to the spanning region, the direction of the peak/trough lines of which transition region progressively approaches the general flow direction of said fluid. 
   The term “packing” is understood to mean a device intended for mixing a phase and/or for bringing several phases flowing cocurrently or countercurrently into contact with one another. A heat and/or mass exchange and/or a chemical reaction may in particular take place in the packing. One particular application of the invention lies in the columns for separating gas mixtures, especially air distillation columns. 
   Mentioned in the prior art are air distillation installations comprising cross-corrugated packing modules, which are also called packs. The modules comprise corrugated strips or metal sheets placed vertically in a parallel fashion, the corrugations of which sheets are oblique with respect to a general fluid flow direction in the installation, and are inclined alternately, generally crossed at 90°, from one sheet to another. 
   The packing modules are slipped into the distillation column so that the sheets of one module are angularly offset with respect to the sheets of an adjacent module about the axis of the column, generally by 90° from one module to another. 
   During use, the gas is constrained to change direction at such an angle in order to flow from one module to another, with a corresponding head loss at this location. Such a head loss causes accumulation of liquid in the lower region of the upper module and flooding of the column at this location, whereas the central portion of the modules has not yet reached its flooding point. This phenomenon reduces the treatment capacity of the column. 
   To reduce this effect, it has been proposed in the prior art to use packing modules having a structured interface region. 
   Such a packing module is disclosed, for example, in WO-A-97/16247. 
   In this module, the packing strips comprise, in their marginal regions, corrugations with curved peaks that extend, along the edge facing an adjacent module, parallel to the general flow direction of the fluids. 
   These curved corrugations join this edge of the strip to the corrugations lying in the spanning region of the module, which lie obliquely relative to the general flow direction of the fluids. 
   However, the packing strips disclosed in the aforementioned WO-A-97/16247 still generate a substantial head loss in the interface regions of the modules. 
   SUMMARY 
   The object of the invention is to propose a packing module whose head loss is further reduced. 
   For this purpose, the subject of the invention is a corrugated strip of the aforementioned type, characterized in that each peak/trough line of the transition region, in side view, lies within a defined area centered on a curve that tangentially extends the peak/trough line of the spanning region and the radial width of which area is 10% of the corresponding radius of curvature of the curve, and in that the radius of curvature of the curve is at any point greater than 1.5 times, preferably greater than 1.6 times, the hydraulic diameter of a channel. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS  
     For a further understanding of the nature and objects for the present invention, reference should be made to the following detailed description, taken in conjunction with the accompanying drawings, in which like elements are given the same or analogous reference numbers and wherein: 
       FIG. 1  illustrates one embodiment of a packing strip, as seen in a side view; 
       FIG. 2  illustrates a larger scale rendition of the sectional view II—II of  FIG. 1 ; 
       FIG. 3  illustrates throughput measurements made for particular packing modules; 
       FIG. 4  illustrates flooding measurements made for particular packing modules; and 
       FIG. 5  illustrates a large scale rendition of another embodiment of a packing strip. 
   

   DESCRIPTION OF PREFERRED EMBODIMENTS 
   According to particular embodiments, the strip according to the invention may comprise one or more of the following features:
         the radius of curvature of the curve is greater than three times the hydraulic diameter of a channel;   the radius of curvature of the curve is less than five times the hydraulic diameter of a channel;   the curve is a circular arc;   the center of the circular arc is located on the edge of the strip or on an extension of this edge;   the curve comprises at least two circular arcs of different radii of curvature, the circular arcs being joined together in the order of increasing value of their radius of curvature starting from the edge of the strip;   the edge direction of the peak/trough lines at the point of their intersection with the edge of the strip is substantially the general direction of flow of said fluid;   the peak/trough lines of the transition region consist of at least two straight segments especially of identical lengths; and   the peak/trough lines of the transition region coincide with the curve.       

   The subject of the invention is also a packing module for a material and/or heat exchange column, characterized in that it comprises a stack of strips as defined above, with the general directions of their peak/trough lines of their spanning region reversed from one strip to another. 
   According to one particular embodiment of the module, the packing density is greater than 300 m 2 /m 3 , and preferably greater than 400 m 2 /m 3 . 
   The subject of the invention is also a cryogenic distillation column, especially for the distillation of air, characterized in that it comprises at least one packing module as defined above. 
     FIG. 1  shows, in side view, a packing module  2  according to the invention. The packing module  2  is intended to be mounted in a fluid treatment column with a vertical central axis (not shown). During operation of the column, fluid flows over the surface of the strip  2  in a general fluid flow direction D f , which in this case is vertical. The packing strip  2  is manufactured from a smooth sheet-metal strip and is plastically deformed by bending. 
   The packing strip  2  has a corrugated spanning region  4  to which corrugated upper  6  and lower  8  transition regions join along the direction D f . Each transition region  6 ,  8  terminates in a horizontal edge  10 ,  12 . In the mounted state of the strip  2 , the transition regions  6 ,  8  are adjacent neighboring packing modules, which consist of strips which are similar but angularly offset about the central axis of the column. 
     FIG. 2  shows a sectional view of the spanning portion  4  on the line II—II of  FIG. 1 . The strip  2  consists of a succession of flat surfaces  14 ,  16  that are inclined to the plane of  FIG. 1 , alternately to the front and to the rear. Two neighboring surfaces  14 ,  16  form a fluid flow channel  18  between them. Each channel  18  has an approximately triangular cross section and is closed along two sides and open along the third side. The surfaces of a channel  14 ,  16  make a bending angle γ, which in this case is 60°. The channels  18 , and therefore the strip, have a thickness E. The flat surfaces  14 ,  16  have curved linking areas  22 ,  24  that link two adjacent surfaces  14 ,  16 . These linking areas have a radius of curvature r. The linking areas  22 ,  24  form, in side view ( FIG. 1 ), peak  26  and trough  28  lines. In side view, two neighboring peak lines  26  or two neighboring trough lines  28  lie parallel to each other and are separated by a distance B, which is the pitch of the corrugation. The peak  26 /trough  28  lines of the spanning region  4  are straight and lie along a direction D i . This direction makes an angle δ=45° to the edges  10 ,  12  of the strip and to the direction D f . The angle δ is generally between 30° and 60°. 
   Unlike the spanning region, the channels  18  of the transition regions  6 ,  8  are curved. More precisely, the peak  26 /trough  28  lines of the channels  18  lie approximately in the direction D i  in an area joined to the spanning region  4  and progressively change their inclination toward an edge inclination direction D b  at the location of the edge  10 ,  12  of the strip. In the present embodiment, each peak  26 /trough  28  line has, in the transition regions, in side view, the shape of a circular arc A of radius Rc. The center C of each circular arc A is located on the edge  10 ,  12  of the strip or on an extension of said edge, in such a way that the direction D b  is identical to the direction D f . Each circular arc A joins a peak  26 /trough  28  line of the spanning region  4  tangentially. Each of the upper  6  and lower  8  transition regions has a height Hc, measured along the direction D f . The height Hc as a function of δ is Hc=Rc×cosδ. 
   The hydraulic diameter of each channel  18  of the spanning region  4  is: 
   
     
       
         
           Dh 
           = 
           
             
               
                 4 
                 × 
                 channel 
                 ⁢ 
                 
                     
                 
                 ⁢ 
                 cross 
                 ⁢ 
                 
                     
                 
                 ⁢ 
                 section 
               
               
                 channel 
                 ⁢ 
                 
                     
                 
                 ⁢ 
                 perimeter 
               
             
             . 
           
         
       
     
   
   If each channel  18  is defined by two V-shaped surfaces  14 ,  16 , that is to say neglecting the radius r, the hydraulic diameter of a channel ( 18 ) is:
 
 Dh=B× cos(γ/2).
 
The radius Rc of each arc A is greater than 1.5 times the hydraulic diameter Dh. In practice, it is between 1.6Dh and 5Dh, and is preferably about 3Dh.
 
     FIG. 3  shows curves comparing the capacities of two packings of the prior art and one packing according to the invention. Each packing has a density of 500 m 2 /m 3 . The angle of inclination of the channels in the spanning region δ is 45°. Plotted on the x-axis is the maximum throughput per unit area of the gas, plotted on the y-axis is the head loss per unit length. 
   Curve C 1  shows the capacity of a first conventional packing, with no transition regions, namely a packing consisting of corrugated strips that are formed only by a spanning region. Curve C 2  indicates the capacity of a second packing, having two, lower and upper, transition regions, the Rc/Dh ratio of which is equal to 1. For a head loss of 10 mbar/m, a 25% increase in capacity over the conventional packing is observed. 
   Curve C 3  indicates the capacity of a packing according to the invention, the Rc/Dh ratio of which is 3. For a head loss of 10 mbar/m, the capacity is further increased by 12% over the second packing. 
     FIG. 4  shows curves comparing the behavior of the aforementioned packings. Plotted on the x-axis is the degree of flooding of the packing, while plotted on the y-axis is the head loss per unit length. 
   Curve C 4  shows the behavior of the conventional first packing. Curve C 5  indicates the behavior of the second packing and curve C 6  indicates the behavior of the third packing. For a head loss of 6 mbar/m, the capacity is increased by 8% for a packing with Rc/Dh=3, compared to a packing with Rc/Dh=1. 
     FIG. 5  shows a portion of a transition region of an alternative embodiment of a packing strip according to the invention. 
   Unlike the first embodiment, the angle of inclination δ of the peak  26 /trough  28  lines of the spanning region  4  is 30°. Consequently, they are inclined at 60° to D f . In addition, the peak  26 /trough  28  lines in the transition region  8  are made up of three straight segments  36 ,  38 ,  40  of identical length l. The successive segments  36 ,  38 ,  40  are inclined, from the spanning region to the edge, by δ 1 =39°, δ 2 =58° and δ 3 =77° to the edge  12 . 
   Each peak  26 /trough  28  line of the strip  2  lies, in the transition region  8 , within an area  42  that is defined as follows: 
   A curve in the form of a circular arc A joins the terminal point T 1  of the peak  26 /trough  28  line in question of the spanning region tangentially. 
   This circular arc A has a radius Rc of at least 1.5 times, preferably at least 1.6 times, the hydraulic diameter Dh of the channels  18  of the strip  4 . 
   At the point of intersection T 2  of the circular arc A with the edge  12 , this circular arc has a tangential direction D t  that is more inclined to the general flow direction D f  of the fluid than the peak  26 /trough  28  lines of the spanning region  4 . 
   The area  42  has a radial width Lr that is 10% of the radius Rc. The area  42  is centered on the circular arc A, in such a way that it extends by 5%×Rc on either side of the circular arc A. 
   It should be noted that this circular arc A is the ideal line of curvature for a peak  26 /trough  28  line lying within the area  42 . 
   It has been observed that a packing module manufactured from a packing strip according to the invention exhibits an increased capacity. 
   The packing modules manufactured from strips according to the invention preferably have a packing density a of greater than 300 m 2 /m 3  and preferably greater than 400 m 2 /m 3 . 
   It should be noted that the hydraulic diameter Dh can also be calculated approximately as a function of the packing density a according to the formula: Dh=4/a. 
   The modules comprising a stack of packing strips according to the invention are, for example, used in cryogenic distillation columns, especially for air distillation. 
   As a variant, the curve defining the area 42 may have a variable radius of curvature, especially one that decreases from the spanning region of the strip. In particular, it may consist of a plurality of circular arcs of different radii of curvature. For example, it may consist of two circular arcs having a radii of curvature Rc of 1.5Dh and 2Dh. Preferably, the circular arcs extend from the edge in the increasing order of their radii of curvature. 
   It will be understood that many additional changes in the details, materials, steps and arrangement of parts, which have been herein described in order to explain the nature of the invention, may be made by those skilled in the art within the principle and scope of the invention as expressed in the appended claims. Thus, the present invention is not intended to be limited to the specific embodiments in the examples given above.