Abstract:
The present invention provides a structured packing comprising a plurality of corrugated sheets and a plurality of flat, planar members alternating with and located between the sheets to inhibit turbulence in vapor ascending through the structured packing. The plurality of planar members are positioned so that at least lowermost transverse edges of the planar members and the corrugated sheets are situated at least proximal to one another as viewed when said structured packing is in use. Each of the planar members and the corrugated sheets has perforations sized to inhibit liquid and vapor flows but to allow pressure equalization. The planar members can be strip-like and positions at or near the top and bottom transverse edges of the corrugated sheets or can have the same length and width of the corrugated sheets. The size and number of perforations can be optimized for air separation applications.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to a structured packing having particular application to a method of separating air in which the packing is formed of a plurality of corrugated sheets and a plurality of flat, planar members alternating with and located between the corrugated sheets to inhibit vapor turbulence. More particularly, the present invention relates to such a structured packing in which the corrugated sheets and the flat, planar members have perforations sized to inhibit vapor and liquid flows but to allow for pressure equalization through the packing. 
     Structured packing has found wide spread use in a variety of distillations including those involved in the separation of air into its component parts. Distillations are conducted within distillation columns filled with mass transfer elements to bring ascending vapor phases into intimate contact with descending liquid phases of mixtures to be separated. As the ascending phase rises and contacts the descending liquid phase, it becomes evermore enriched in the more volatile components of the mixture to be separated. At the same time the descending liquid phase becomes ever more concentrated in the less volatile components of the mixture to be separated. In such fashion, systems of distillation columns can be used to separate various mixture components. For instance, in case of air separation, nitrogen is separated from oxygen is a double distillation column unit. Argon is then separated from oxygen in an argon column that is attached to a lower pressure column of such double distillation column unit. 
     Structured packings are widely used as mass transfer elements within distillation columns due to their low pressure drop characteristics. Structured packings generally include corrugated sheets of material in which the sheets are placed in a side by side, relationship with the corrugations of adjacent sheets crisscross-crossing one another. In use, the liquid phase of the mixture to be separated is distributed to the top of the packing and spreads out throughout the packing as a descending film. The vapor phase of such mixture rises through the corrugations contacting the liquid film as it descends. 
     There have been many attempts in the prior art to increase the efficiency of structured packings, that is, to decrease the height of packing equal to a theoretical plate. Obviously, the lower the height, the more efficient the packing. At the same time, structured packing with a low HETP inherently has an increased pressure drop over less efficient packings. One such structured packing is disclosed in U.S. Pat. No. 4,597,916 in which the corrugated sheets are separated from one another by flat, perforated sheets that extend throughout the packing. It is thought by the inventors herein that the flat perforated sheets of this packing increase efficiency both by providing additional interfacial area for vapor-liquid contact and by increasing turbulence in the vapor flow and therefore the degree of mixing between vapor and liquid phases. Transverse mixing is also increased by perforations that are specifically designed to promote liquid and vapor flow in a transverse direction of the packing. 
     As will be discussed, Applicants have designed a structured packing that unlike the prior art, is optimized not for efficiency, but rather, for smooth vapor flow. Through such optimization, the Inventors herein have found that it is possible to increase the capacity of the packing and therefore, use such packing in a more efficient cost effective manner. 
     SUMMARY OF THE INVENTION 
     The present invention provides a structured packing comprising a plurality of corrugated sheets and a plurality of flat, planar members alternating with and located between the sheets to inhibit turbulence in vapor ascending through said structured packing. The plurality of planar members are positioned so that at least lowermost transverse edges of the planar members and the corrugated sheets are situated at least proximal to one another as viewed when the structured packing is in use. Each of the planar members and the corrugated sheets has perforations sized to inhibit liquid and vapor flows but to allow pressure equalization. 
     Pairs of the planar members can be located between the corrugated sheets and spaced apart from one another so that the uppermost and the lowermost transverse edges of the planar member and the corrugated sheets are aligned. Additionally, the planar members may sized with lengths and widths equal to those of the corrugated sheets. 
     In any embodiment, each of the perforations can have a diameter in a range of between about 5% and about 40% of a channel width of corrugations of the corrugated sheets as measured between adjacent peaks or troughs of the corrugations. This diameter can be between about 5% and about 20% of the channel width. Preferably the diameter is about 10% of the channel width of corrugations. Furthermore, the perforations can constitute an open area of the planar members in a range of between about 5% and about 20% of a total area of the planar members. Such open area of the planar members can be between about 7% and about 15% of the total area. Preferably, the open area of the planar members is about 10% of the total area. 
     It has been found by the inventors herein that a structured packing designed in the manner set forth above functions with a slightly higher HETP than structured packings of the prior art. This is surprising considering the fact that the packing with the intermediate planar members has a greater surface area than similar packing not incorporating such planar members. A further unexpected feature is that a packing of the present invention will flood at higher vapor rates. There are various criteria that are used to describe the flooding condition, for instance, excessive pressure drop. In all cases if HETP is plotted against F-Factor (where F-Factor is a product of the superficial vapor velocity and the square root of the vapor density) flooding is evidenced by a rapid rise of the slope of the curve. Such a rise in HETP is indicative of the vapor supporting the descending liquid thereby choking the column and disrupting the separation. This increase in the flooding point allows higher flow rates through the column and therefore for a given volume of packing, greater production. This allows for thinner columns using less packings or columns that can handle a greater throughput. The reason for such operation is that the planar member and opening design of the present invention acts to inhibit turbulence in the vapor flow ascending through the structured packing. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     While the specification concludes with claims distinctly pointing out the subject matter that Applicants regard as their invention, it is believed the invention would be better understood when taken in connection with the accompanying drawings in which: 
     FIG. 1 is a schematic view of an air separation plant utilizing a structured packing in accordance with the present invention; 
     FIG. 2 is a fragmentary view of structured packing in accordance with the present invention for use in the air separation plant illustrated in FIG. 1; 
     FIG. 3 is a fragmeentary, side elevational view of the structured packing shown in FIG. 2; and 
     FIG. 4 is an alternative embodiment of a structured packing in accordance with the present invention. 
    
    
     DETAILED DESCRIPTION 
     With reference to FIG. 1, an air separation plant  1  is illustrated. Air separation plant  1  has a double distillation column unit  10  and an argon column  12 . Although not illustrated, but as would be known by those skilled in the art, air separation plant  1  would additionally have a main heat exchanger to cool the air to a rectification temperature against warming product streams to ambient temperatures. Additionally, a main air compressor and a pre-purification unit would also be provided to compress the air and then to purify the compressed air of impurities such as carbon dioxide and moisture. 
     Air as a feed stream  14  enters a higher pressure column  16  of double distillation column unit  10  where it is rectified to produce a nitrogen rich tower overhead and a crude liquid oxygen column bottoms. Stream  18  of the crude liquid oxygen column bottoms is subcooled within a subcooling unit  20  and then expanded across an expansion valve  22 . The expansion reduces the temperature of stream  18  so that it can serve as coolant for a head condenser  24  used to form reflux for an argon column  12 . The crude liquid oxygen obtained within stream  18  is vaporized within head condenser  24  and then fed into a lower pressure column  26  (of double distillation column unit  10 ) for further refinement. The further refinement produces an oxygen-enriched column bottoms and a nitrogen vapor tower overhead within the lower pressure column  26 . 
     Reflux for both the higher and lower pressure columns  16  and  26  is provided by condensing the nitrogen-rich tower overhead within a condenser reboiler  30  to produce higher and lower pressure column reflux streams  32  and  34 . Lower pressure reflux stream  34  is subcooled within subcooling unit  20  and reduced in pressure by expansion valve  36  prior to its introduction into lower pressure column  26 . The nitrogen vapor tower overhead is-removed as a nitrogen stream  38  which serves in subcooling unit  20  to subcool stream  18  and lower pressure column reflux stream  34 . An oxygen product stream  40  may be removed as a liquid from a bottom region of lower pressure column  26 . Both the nitrogen stream  38  and the oxygen product stream  40  may be introduced into the main heat exchanger for cooling the incoming air. 
     At an intermediate location of lower pressure column  26 , an argon rich vapor stream  42  may be removed and introduced into argon column  12 . An argon rich tower overhead is produced within argon column  12 . An oxygen rich column bottoms is also produced which is returned as a liquid stream  44  back to lower pressure column  26 . An argon product stream  46  may be removed from part of the condensate of head condenser  24 . 
     In order to effectuate the distillation, ascending vapor phases and descending liquid phases must be brought into contact with one another by mass transfer elements. For instance, higher pressure column  16  is provided with transfer elements  48  which may be trays or structured packings. As vapor rises within mass transfer elements  48 , it becomes ever more rich in nitrogen until it reaches the top of higher pressure column  16 . There, the vapor is condensed and in part returned as higher pressure column reflux stream  32  to higher pressure column  16 . The nitrogen rich tower overhead, as a liquid, descends within higher pressure column  16  and becomes ever more richer in oxygen, through contact with the ascending vapor, to produce the crude liquid oxygen column bottoms. 
     Vapor rising within lower pressure column  26  passes through beds  50 ,  52 ,  56  and  58  which are formed of structured packing. The ascending vapor phase, initiated by boiling the oxygen rich liquid, rises through the column and becomes ever more rich in nitrogen to form the nitrogen vapor tower overhead. The descending liquid phase is initiated by the reflux of higher pressure column stream  34 . This liquid becomes ever more rich in oxygen as it descends. 
     Argon column  12  is provided with a mass transfer elements  60  which again, are a structured packing. The vapor phase initiated by introduction of argon rich vapor stream  42  to becomes ever more rich in argon. The reflux introduced into the top of argon column  12  becomes ever more rich in oxygen as it descends. 
     With reference to FIGS. 2 and 3, structured packing  2  in accordance with the present invention as illustrated. Structured packing  2  consists of repeating pairs of corrugated sheets  62  and  64  which contain corrugations  66  and  68 , respectively. The repetition of corrugated sheets  62  and  64  produce either the top or lower half of a bed of packing. Corrugations  66  and  68  are inclined at an angle to the vertical, for instance 30 or 45° , or even greater in an appropriate application. Corrugated sheets  62  and  64  are positioned so that corrugations  66  and  68  criss cross one another. 
     Flat, planar members  70  and  72  alternate with and are positioned between corrugated sheets  62  and  64 . Preferably, each of planar members  70  and  72  sized with a width “W” equal to the width of corrugated sheets  62  and  64  and a height “h” less than that of corrugated sheets  62  and  64 . As illustrated, the lowermost transverse edges  71  are aligned with those of the corrugated sheets  62  and  64  and the uppermost transverse edges  73  are aligned with those of the corrugated sheets  62  and  64 . It is understood however that there might be some misalignment in the nature of 5 mm and hence, such lowermost and uppermost transverse edges in any embodiment are situated at least near or proximal to those of corrugated sheets  62  and  64 . 
     Although two planar members  72  and  70  are illustrated the present invention encompasses an embodiment in which upper planar members  70  are deleted. In such a possible embodiment, the remaining planar members  72  are positioned so that the lowermost transverse edges thereof are aligned with those of corrugated sheets  62  and  64  or at least proximally positioned thereto. 
     The foregoing structured packing may be optimized for use in any of the aforementioned columns of an air separation plant. If a channel width labeled in the drawing as “CW” is measured between the corrugations (from trough to trough or from peak to peak), then preferably, the height h of each of planar members  70  and  72  should be approximately between about 2 and about 8 times the channel width CW. 
     As illustrated, corrugated sheets  64  and  62  and planar members and  70  and  72  are provided with perforations  76 . In any embodiment of a structured packing of the present invention, perforations  76  should be sized to prevent vapor and liquid and vapor flows but to permit pressure equalization through the structured packing. In case of air separation, each of the perforation  76  can be sized to have a diameter in a range of between about 5% and about 40% of channel width CW. This diameter is more preferably between about 10% and about 25% of the channel width CW and is most preferably about 15% of the channel width CW. 
     A further optimization for air separation is to control the number of perforations  76  and therefore, their open area contribution. Preferably, perforations  76  can constitute an open area of the corrugated sheets  62  and  64  and the planar members  70  and  72  in a range of between about 5% and about 20% of a total area thereof. In case of corrugated sheets  62  and  64 , such open area is computed by multiplying the length and width of each of corrugated sheets  62  and  64 . More preferably such open area can be between about 7% and about 15% of the total area and most preferably, the open area is about 10% of the total area 
     With additional reference to FIG. 4, structured packing  3  is of the type having repeating pairs of corrugated sheets  76  and  78  separated by or alternating with a flat, planar member  80  which has top and bottom edges  82  and  84  which are coincident with the top and bottom edges of corrugated sheets  76  and  78 . Planar member  80  is sized with lengths and widths equal to those of corrugated sheets  76  and  78 . Additionally, corrugated sheets  76  and  78  and planar members  80  are provided with perforations  86 ,  88  and  90 . These perforations are again sized to prevent liquid and vapor flows and to permit pressure equalization. In such manner, smooth rather than turbulent vapor flows are promoted to produce the advantageous operation described above. Preferably, in case of air separation, the size and number of perforations  86 ,  88 , and  90  are optimized in the same manner as described above for structured packing  2 . 
     Structured packing  2  and  3  where tested against structured packing obtained from Sulzer Chemtech Ltd, Winterthur, Switzerland, as model Mellapak 500.YL. This packing has a density of about 500 m 2 /m 3 . The structured packing  2  and  3  was than fabricated out of corrugated sheets that would otherwise have had the same density but for planar members  70 ,  72 , and  80 , respectively, and therefore were of slightly greater density. A greater separation efficiency was therefore to have been expected. 
     Testing, however, showed that with the type of mixtures to be separated in an air separation plant, either in a lower pressure column, such as lower pressure column  26 , or in an argon column, such as argon column  12 , at operational ranges of F-Factor prior to flooding, structured packing  2  had an HETP of about 15% greater than the Mellapak 500.YL packing. Structured packing  3  (more dense than structured packing  2 ) had an HETP of about 25% greater than the Mellapak 500.YL packing. Moreover the flooding points of structured packings  2  and  3  where about 25% and about 40% greater than the Mellapak 500.YL packing. 
     While the present invention has been described with reference to a preferred embodiment, as will occur to those skilled in the art, numerous changes, additions and omissions may be made without departing from the spirit and scope of the present invention.